Price (excluding VAT) in Luxembourg: EUR 20 KI-NA-23869-EN-S The quality of lignite extracted in various deposits differs considerably in part. The impact of changed major coal quality parameters on steam generator operation became evident in the form of increased deposit formation on the boilers’ heating surfaces. Successful countermeasures were launched, but the bottleneck of the problem continues to be the cleaning of the first convection heating surfaces downstream of the furnace. As the cleaning facilities available so far (soot blowers) have not proved sufficient for these areas of application despite all optimisation efforts made, new processes have to be developed. In contrast to the methods applied so far, these processes do not use the common cleaning medium steam but work on the basis of alternative cleaning media. Within the scope of the Ligpower project, such alternative cleaning methods were selected and subjected to extensive testing on commercial utility boilers. Since a more intensive cleaning of superheater tubes involves increased stress of the tube material, we devel- oped new, more resistant, superheater designs and installed these in a 600 MW unit. The transferability of the findings made was ensured by a comparison of the lignites used. To permit the cleaning device to be controlled as needed, we developed a programme that analyses the degree of fouling of the boiler and generates suggestions for its cleaning. In addition, a tube-fin superheater design was calculated and optimised in terms of fluid dynamics. The extensive tests showed which cleaning methods have suitable approaches to cleaning. By testing them over several years, we gained important information about equipment design. The superheater design could be tested only to a limited degree. Thus, we obtained additional findings in tests using a test heating surface that was mounted on a 300 MW boiler. EC More efficient cleaning concepts for stepping up availability of lignite-fired power plants (Ligpower) EUR 23869 More efficient cleaning concepts for stepping up availability of lignite-fired power plants (Ligpower)
Transcript
Price (excluding VAT) in Luxembourg: EUR 20
KI-N
A-23869-E
N-S
The quality of lignite extracted in various deposits differs considerably in part. The impact of changed major coal quality parameters on steam generator operation became evident in the form of increased deposit formation on the boilers’ heating surfaces. Successful countermeasures were launched, but the bottleneck of the problem continues to be the cleaning of the first convection heating surfaces downstream of the furnace.
As the cleaning facilities available so far (soot blowers) have not proved sufficient for these areas of application despite all optimisation efforts made, new processes have to be developed. In contrast to the methods applied so far, these processes do not use the common cleaning medium steam but work on the basis of alternative cleaning media.
Within the scope of the Ligpower project, such alternative cleaning methods were selected and subjected to extensive testing on commercial utility boilers. Since a more intensive cleaning of superheater tubes involves increased stress of the tube material, we devel-oped new, more resistant, superheater designs and installed these in a 600 MW unit. The transferability of the findings made was ensured by a comparison of the lignites used. To permit the cleaning device to be controlled as needed, we developed a programme that analyses the degree of fouling of the boiler and generates suggestions for its cleaning. In addition, a tube-fin superheater design was calculated and optimised in terms of fluid dynamics.
The extensive tests showed which cleaning methods have suitable approaches to cleaning. By testing them over several years, we gained important information about equipment design. The superheater design could be tested only to a limited degree. Thus, we obtained additional findings in tests using a test heating surface that was mounted on a 300 MW boiler.
EC
M
oreefficientcleaning
concep
tsforstep
ping
upavailab
ilityoflig
nite-firedp
ow
erplants(Lig
po
wer)
EU
R23869
More efficient cleaning concepts for stepping up availability of lignite-fired
power plants (Ligpower)
Interested in European research?
RTD info is our quarterly magazine keeping you in touch with main developments (results, programmes, events, etc.). It is available in English, French and German. A free sample copy or free subscription can be obtained from:
Directorate-General for Research Information and Communication Unit European Commission B-1049 Brussels Fax (32-2) 29-58220 E-mail: [email protected] Internet: http://ec.europa.eu/research/rtdinfo/index_en.html
EUROPEAN COMMISSION Directorate-General for Research Research Fund for Coal and Steel Unit
How to obtain EU publications Publications for sale: • via EU Bookshop (http://bookshop.europa.eu);
• from your bookseller by quoting the title, publisher and/or ISBN number;
• by contacting one of our sales agents directly. You can obtain their contact details on the Internet (http://bookshop.europa.eu) or by sending a fax to +352 2929-42758.
Free publications: • via EU Bookshop (http://bookshop.europa.eu);
• at the European Commission's representations or delegations. You can obtain their contact details on the Internet (http://ec.europa.eu/) or by sending a fax to +352 2929-42758.
European Commission
Research Fund for Coal and SteelMore efficient cleaning concepts for stepping
up availability of lignite-fired power plants (Ligpower)
G. Wiechers, B. WesselRWE Power AG
Stüttgenweg 2, 50935 Köln, Germany
S. GoudanisPublic Power Corporation
Chalkokondili Street 30, 104 32 Athens, Greece
F. KlugerAlstom Power Systems GmbH
Augsburger Straße 712, 70329 Stuttgart, Germany
G. RileyRWE npower plc
Windmill Hill Business Park, Whitehill Way, SN5 6PB, Swindon, United Kingdom
Contract No RFCP-CT-2003-00002 1 September 2003 to 31 December 2007
Final report
Directorate-General for Research
2009 EUR 23869 EN
LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information.
Europe Direct is a service to help you find answers to your questions about the European Union
Freephone number (*):
00 800 6 7 8 9 10 11
(*) Certain mobile telephone operators do not allow access to 00 800 numbers or these calls may be billed.
Table of contents
1 FINAL SUMMARY......................................................................................................................... 5 2 OBJECTIVES OF THE PROJECT............................................................................................. 15 3 COMPARISON OF INITIALLY PLANED ACTIVITIES AND WORK ACCOMPLISHED 17 4 TECHNICAL IMPLEMENTATION .......................................................................................... 19
4.1 Task 1: Co-ordination ............................................................................................................... 19 4.2 Task 2: Selection and testing of more efficient cleaning facilities and techniques................... 19
4.2.1 Compilation and selection of efficient cleaning equipment to test and determine the position of testing in the boiler......................................................................................................... 21 4.2.2 Description and testing, monitoring and assessment of the selected cleaning concepts.... 27 4.2.3 Wearing protection ............................................................................................................ 37 4.2.4 Summary, analysis and evaluation of the results and assessment of the consequences in view of future applications............................................................................................................... 37
4.3 Task 3: Platen heating surface .................................................................................................. 40 4.3.1 Specification, manufacture and installation....................................................................... 41 4.3.2 Trial of platen heating surfaces.......................................................................................... 51 4.3.3 Summary, analysis and evaluation of the results and assessment of the consequences in view of future applications............................................................................................................... 56
5 CONCLUSIONS ............................................................................................................................ 59 6 LIST OF FIGURES AND TABLES............................................................................................. 61
6.1 List of figures............................................................................................................................ 61 6.2 List of tables.............................................................................................................................. 62
The lignite found in the Rhenish mining area is extracted in various deposits the qualities of which differ considerably in some cases. In particular with the transition to the Hambach opencast mine – developed in the 1980s – significant changes in major coal quality parameters were recorded in the 1990s. Depending on the particular depth, calorific value, alkali contents, and the hardness of the raw lignite developed towards ever higher average values.
The impact on steam generator operation has become more and more evident since the late 1990s in the form of increased deposit formation on the boilers’ heating surfaces.
The countermeasures launched in the field of boiler technology and coal supply showed success, which allowed the above development to be controlled. The bottleneck of the problem, however, continues to be the cleaning of the superheater banks, which – as the first of the downstream convective surfaces – are in the flue gas temperature range of 1,100°C to 1,300°C (furnace outlet) and approximately 800°C.
As the cleaning facilities available so far (soot blowers) have not proved sufficient for these areas of application despite the optimization efforts made, new processes need to be developed to solve the problem. The processes are characterized by the fact that – by contrast with the methods applied so far – they do not use the common cleaning medium steam but work on the basis of alternative cleaning media.
Under the terms of this project, more efficient cleaning equipment and new easier-to-clean heating surfaces for lignite-fired power plants are to be trialled on a demonstration scale. The object of the measure is to enhance the availability and competitiveness of this energy source, which is found extensively in Europe and is a low-cost and an important contributor to the energy supply.
Task 1: Co-ordination:
Project coordination was executed by RWE Power AG. Initially Mr. Renzenbrink was the coordinator and since 2005 Mr. Wiechers has coordinated the project.
Regular project meetings were held and all required reports were presented to the EU Commission. In addition, two expert meetings were conducted. One expert meeting brought the sootblower model experts of the participating project partners together to swap experience gained in the development and operation of sootblower models. In a second expert meeting, the project partners' material experts discussed and evaluated the results of the tests carried out on Test Rig B.
All tasks were executed as planned in the original contract except for a part of Task 3, where a change was necessary. This change in procedure was agreed with all those involved in Amendment No. 3. To allow reliable results to be obtained here as well, an adequate time of operation was necessary. For this reason, the term of the Project was extended until December 2007 by Amendment No. 3.
Task 2: Selection and testing of more efficient cleaning facilities and techniques
The heating surfaces of steam generator plants are exposed to various stresses. The ‛basic stress’ results from mechanical strains on the pipes caused by internal pressure and external forces that have to be absorbed by the pipes at a particular temperature.
In addition to this, the pipes are exposed to stresses exerted on the outside of pipes used for conventional water tube boilers. In the case of the coal boilers primarily investigated in the Ligpower
5
project, these stresses result from corrosion, erosion, the formation of a protective layer, and thermal shock stress.
Corrosion problems are a well-known and common phenomenon in coal-based plants with a wide variety of different causes and working mechanisms. The examination of corrosive effects is not part of the Ligpower project’s scope of work.
Erosion on the outer surfaces of pipes in coal-fired steam generators is caused by coal and ash particles streaming past the heating surfaces. If flue gas velocities are normal and the particles contained in the flue gas evenly distributed, erosion problems are generally minor. Day-to-day operations show that, on the one hand, a local increase in the concentration of ash particles leads to increased erosion, while, on the other hand, a local – possibly temporary – increase in velocity significantly compounds erosion. Heating surface cleaning facilities, which today are mostly operated with steam, cause precisely this effect and lead to local erosive effects. Nowadays, so-called shields are normally used to counteract this.
The formation of a protective layer in itself does not constitute any stress for the pipe; rather, as the name implies, it protects the pipe. The protective layers on the outer surfaces of flue gas-heated boiler tubes consists of magnetite (Fe2O3), an iron oxide formed from the base metal of the tube (iron) and oxygen. Thus, the formation of a protective layer results in the iron contained in the base metal of the tube being consumed. With normal protective layer thicknesses of 300-400μm, this does not lead to substantial tube wall emaciations. In combination with effects that continuously strip the mineral protective layer off the base metal of the tube, the formation of protective layers becomes a serious strain for the pipes. Apart from erosion, the spraying of pipes with water is a suitable method for removing the exterior protective layers of boiler tubes. When the protective layer has been removed, the metal tube surface is exposed to the flue gas again, so that a new protective layer forms. Since the protective layer forms particularly quickly when it is thin, this also entails accelerated tube erosion.
Boiler tubes are also exposed to thermal shock stresses when they are sprayed with water from the outside. When water hits the outer surfaces of tubes, the tube material abruptly cools down, causing residual stresses to develop in the tube wall, which, if repeated frequently, lead to cracks. These cracks grow from the surface of the tube to the inside. Viewed from the surface, they form a kind of honeycomb pattern, also known as ‘orange peel’ (Appendix Figure 7-9). If left to grow unhindered, these thermal shock cracks may lead to boiler tube failure.
The effects described above – erosion, formation of a protective layer, and thermal shock –have to be taken into account when selecting and testing more effective cleaning facilities and processes. If they were not, and thus damage to and even failure of boiler tubes risked, the particular cleaning processes would have to be considered unsuitable for permanent use in boilers.
A closer investigation of lignite from Greece and from the German Rhine area shows an much higher fouling index of the German lignite in comparison with the Greek lignite. Thus the selection of more efficient cleaning facilities should regard especially the problems of rhenish lignite that are being seen in the first superheater banks after the furnace. Nevertheless the facilities should be able to be adapted to other steam generators and other coal qualities.
In general, there is a wide range of cleaning facilities suitable for cleaning boiler heating surfaces during operation. The facilities known on the market differ according to the process principles used.
Jet techniques using steam, water, or air as cleaning medium are most commonly used today. They remove heating surface deposits by means of the impulse of the emerging jet, aided by the temperature change (usually cooling) of the deposits when they come into contact with the jet.
6
In addition, other cleaning processes that have a direct or indirect effect on deposits are known, e.g. cleaning methods that achieve the cleaning effect by exciting the flue gas (either acoustically or using explosions or detonations). Acoustic techniques are quite widespread due to their simple setup.
Other cleaning techniques, e.g. rapping devices or so-called steel shot plants, are used at least occasionally. In principle, further methods of cleaning heating surface deposits during operation are conceivable.
Criteria:
Cleaning intensity
The problem of finding effective cleaning techniques for the fouling-prone region of steam generator plants needs to be specified when dealing with lignite plants in that processes need to be found that allow deposits even in higher flue gas temperature regions to be removed. For experience gained with conventional cleaning methods (steam jet blowers) shows that it is not possible to remove these deposits, particularly in plants using Hambach lignite as fuel. More precisely, it has become apparent that though a considerable portion of deposit can be removed in one cleaning step, some of it is left on the tubes, causing a steadily growing deposit that cannot be removed to develop. This reduces the operating periods of the plants to periods far shorter than those they were originally designed for. More specifically, plants that should have 2 to 3-year availability have to be shut down after 3-5 months to clean them during the outage.
Hence, the main criterion applied to the processes to be selected is the degree of cleaning intensity they can achieve. The locations in which these techniques are to be used are characterized by a flue gas temperature of 800-1,100 (1,300)°C, i.e. the region of the first convective surfaces downstream of the furnace.
Based on operating experience and tentative tests, it may be concluded that jet cleaning processes using water as cleaning medium have the highest intensity. This theory is supported especially by the fact that, as a rule, the cleaning of furnaces using one of the above-mentioned facilities is successful. The cleaning plants employed in the Rhenish lignite-fuelled power plants all use water as cleaning medium.
Avoiding damage to the boiler plant
The second criterion of equal importance applied in selecting techniques is that the use of a particular cleaning facility does not damage the boiler plant or affect its operability in any other way. The damage that must be mentioned in particular is boiler damage due to tube leakages. Based on what we know today, these may be caused by thermal shock or protective layer removal. Erosion, a problem encountered with steam blowers as well, must also be considered as a possible damaging mechanism.
Apart from the boiler tubes themselves, the so-called support lugs of the sling tubes must also be taken into account. Support lugs are high temperature steels welded to the sling tubes to support the heating surface tubes. These components are hardly cooled or not cooled at all and hence much hotter than the tubes themselves. Thus, it seems reasonable to assume that spraying these components with water may lead to damage much quicker than would be the case with the boiler tubes themselves.
7
Use in boiler environment
The third criterion is how well the cleaning techniques can be employed in boilers, or rather in the cold-end sections of boilers. The challenge is to place the cleaning facilities in the boilers in such a way that all tubes to be cleaned can actually be reached. It must be borne in mind that the cross-sections of the plants in question range between 20x20m (old plants) and 26x26m (new plants). This entails considerable requirements regarding mechanical stress that must also be fulfilled in the given circumstances. The ambient conditions cleaning facilities are exposed to when advanced are decisive. They are primarily characterized by high temperatures (> 1,000°C) and dust-laden flue gas.
The processes were selected on the basis of technical availability. The above-mentioned criteria had to be met, or it had to be apparent that the processes have the potential to fulfil the criteria. Two water-based techniques were selected in the end:
• Two-fluid blower
• High pressure water jet cleaning
These were complemented by two processes working on the basis of explosions or detonation, namely
• Shock wave cleaning
• Explosive cleaning
Testing of the selected processes was carried out in 600MW and 300MW units of the Niederaussem power plant. This plant is supplied with lignite from the Rhenish mining area. In order to make sure that the results can be transferred to the conditions prevailing in power plants fired with Greek lignite, a comparative evaluation was made. Moreover, a fouling and sootblower monitoring model was developed to allow the cleaning facilities to be used in line with requirements, i.e. neither too often nor too rarely. Furthermore, a literature study was conducted to develop a better understanding of the damage behaviour.
Two of the selected innovative cleaning methods use water as a cleaning medium (two-fluid blower and high pressure water jet cleaning). One technique generates shock waves that are used for cleaning (shock wave cleaning). In addition, tests were carried out to determine how heating surfaces can be cleaned during ongoing operations by means of explosive charges (online explosive cleaning).
Both the two-fluid blower and high pressure water jet cleaning use water as the cleaning medium. It is well-known, however, that water can also damage heating surface tubes. Therefore, a literature study was performed with the aim of developing a better understanding for the relevant damage mechanisms. Despite in-depth research no quantitative descriptions were found for this specific case. However, a plausible qualitative description of the damage mechanisms was elaborated.
In order to avoid damage to the heat exchanger surfaces, the cleaning facilities should be used as required. "As required" means in this case that they should be used as often as necessary to keep the boiler in a reasonably clean condition but as infrequently as possible to avoid possible damage. A programme was developed by npower to calculate the fouling condition of the individual heating surfaces on the basis of current boiler data and derive proposals from the fouling condition for the use of cleaning facilities. The programme was successful installed at Tilbury power station.
8
Alstom and RWE Power have experience with similar models. Subsequent to the Ligpower meeting an experts meeting was conducted to share experiences in development and operation of these models. The experts discussed the different models with respect to the calculatory bases, data acquisition and data management, user interface, application to control sootblowers and the implemented cleaning strategies. The programmes and their utilisation are comparable in principle. The discussion made sure that the best strategies are implemented and cleaning is executed when needed.
All cleaning facilities investigated within the scope of the project were installed in the Niederaussem power station. This plant is fired by lignite from the Rhenish mining area. To ensure that results obtained in this power plant can be transferred to other power plants where Greek lignite is combusted, a comparative evaluation of various lignites was made. One main result of the investigation was that the Greek lignite from the mines in Northern Greece generally shows a lower deposit formation tendency than Rhenish lignite. The application of the investigated cleaning concepts and superheater designs in Greek power plants therefore seems to be feasible in a demonstration step after the first evaluations have been made.
The two-fluid blower can be operated both with steam and water as a cleaning medium. As a rule, it should be operated with steam to clean the heating surfaces. If, however, fouling proves to be too difficult to remove, water can be used since it is a more aggressive cleaning medium. As a result, this cleaning facility offers the advantage that either careful or more aggressive cleaning can be chosen to meet requirements. A two-fluid blower prototype was developed and tested on the 600 MW G Unit. After several optimization steps, this prototype showed a very good cleaning behaviour. Based on these positive results, the power plant operator decided to install another three two-fluid blowers on the same superheater bank (independently of the LIGPOWER Project). During operation of these four two-fluid blowers, a large number of different failures occurred. A major cause of failures in this complex system is the great temperature difference between the water-cooled inner tubes, the steam-cooled lance and ambient temperatures reaching more than 1,000°C in the boiler. The failures included cooling problems, difficulties with seals and control errors. These failures made continuous operation in the water mode impossible. Due to the comprehensive experience gained in two-fluid blowers, this development will not be continued.
The working mechanism of high pressure water jet cleaning consists in directing a comparatively small water jet under very high pressure at the deposit and breaking it up. This principle calls for a device that moves the water nozzle as closely as possible to the area to be cleaned. After various concepts had been evaluated, we decided to pursue a technical solution that advances the nozzle head into the boiler using a flexible hose. The nozzle head is positioned vertically by unreeling the hose and horizontally by a cantilever that can be introduced in the boiler. The critical components were tested before the construction of a prototype was started. Since experience gained with the two-fluid blower showed that the repair of a device already installed in the boiler can be subject to considerable delay due to ongoing operations, great importance was attached to the execution of preceding tests when high pressure water jet cleaning was developed. These tests were first conducted in the workshop and later in a full-scale test set-up in the Niederaussem power plant. A multitude of the functions required was implemented successfully. By the end of the project term, however, not all problems were solved, so that no date has been fixed for installation in the boiler yet.
The two-fluid blower and high pressure water jet cleaning were developed for the final superheater stage of a 600 MW lignite-fired power plant unit. On the one hand, this involves working conditions in a flue gas atmosphere with temperatures of 1,100 – 1,300°C and, on the other, very large geometric dimensions since e.g. the cantilever must have a length of 10 m to reach the boiler centre. In addition, cleaning operation using water had to meet exacting requirements and a large number of boundary
9
conditions in order to minimize possible damage associated with water cleaning. These requirements resulted in completely new, complex cleaning facilities that were immediately tested in a commercial utility boiler. This big development step seemed feasible but it became clear that a staged procedure could have had advantages.
In shock wave cleaning, an ignitable mixture of gas and air is produced outside the boiler and ignited. A carefully directed explosion generates a shock wave that is conducted through pipelines to the boiler area to be cleaned. This cleaning facility was installed and trialled at a 300 MW unit of the Niederaussem plant. Although cleaning intensity was continuously increased in the course of the trials, no cleaning effect was observed. Since the progress that could be reached by a modification of the plant was also expected to be minor, the tests to investigate this cleaning facility were discontinued.
A further possibility to clean fouled boiler areas during operation is seen in the use of explosive charges. During boiler outages, explosive charges are ignited to accelerate boiler cleaning in particularly fouled areas. The particular difficulty lies in introducing explosive charges into the intended area during ongoing operations and igniting them there. In a 300 MW unit, explosive charges were introduced by a cooled, slewable device into the boiler and ignited there. Handling this equipment proved to be extremely difficult, and the boiler area reached was very limited. Since, in addition, no cleaning effect was visible, the decision was taken to stop pursuing this line of development.
The results of the shock wave and online explosive cleaning tests are evident: These methods do not show a sufficient effect and are not suitable for regular use in a commercial-scale plant.
The results of the water-based cleaning process tests are less evident. Initially the two-fluid blower produced a very good cleaning effect, but the further course of the tests showed that—due to the system's structural complexity—it did not withstand the harsh operating conditions in the final superheater section of a 600 MW boiler.
When the entirely new high pressure water jet cleaning process was developed, great importance was attached to conducting preceding tests due to the experience gained with the two-fluid blower. This process, too, places heavy demands on process equipment implementation. Many functions were implemented as intended, but the cold tests revealed improvement demand in several respects. Many improvements were implemented, but some items are still open.
The high efficiency of water as a cleaning medium was demonstrated by the two-fluid blower. Owing to the high requirements to be met by water cleaning, a staged development programme should be used for future developments. Moreover, the complexity of future equipment should be as low as possible.
Task 3: Platen heating surfaces
As mentioned in the chapter before, a solution to remove the increased growing fouling and slagging deposits could be the cleaning of the heating surface with water instead of steam. The use of steam blowers in the superheater region is common practice today, but they are not sufficient for changed lignite qualities. Positive and long term experience with water cleaning is available for the furnace walls (tube-fin construction). The furnace is an evaporator and has significant lower steam parameters and therefore other tube materials then the superheaters. Water cleaning devices for the application in the superheater region are in the development phase (e.g. two fluid blower etc.).
Due to lack of experience with the application of water as a cleaning medium for bundle type heat exchangers in the superheater region the following questions arose:
• Behaviour of superheater when frequently cleaned with water (especially connections between superheater tubes and supporting tubes)
10
• Effect of heat exchanger with separate tubes compared with the water wall (tube-fin construction)
• Erosion problems, especially in the area of supporting tube / heating surface tube)
Different options can be considered to solve the issues related to the water cleaning of the superheater region. The options are as follows:
• Alternative design for the supporting tubes with the objective to avoid uncooled parts (thermal shock)
• Alternative superheater design with the objective to avoid supporting tubes
• Coatings on the superheater and supporting tubes could be a measure against erosion
Alstom Power Systems (APS) has developed an alternative design for the construction of the connection of the supporting tube with the heating surface as well as an alternative design for the first two superheater stages as a platen superheater. Due to the high steam temperature of modern supercritical boilers, austenitic materials are necessary for the superheater heating surface. Mid-term experience of water cleaning of these austenitic heating surfaces needs to be collected.
In summary three different solutions (TEST RIGs) were proposed, developed and pursued by APS in the course of this project.
• Test Rig A: Tube bundle with cooled supporting tubes made out of one piece with the main heating surface
• Test Rig B: Austenitic superheater panel without any supporting tubes
• Test Rig C: Coated super heater tubes
All Test Rigs were planned to be installed in the 600MW unit G of Niederaußem power plant.
For the proposed design of the superheater panel Test Rig B a numerical evaluation and optimization was conducted.
The Test Rig A is a newly developed design for supporting tubes. The design considers cooled supporting tubes. The cooled supporting tube surface will lower the thermal stress of the material compared to the conventional design in case water cleaning is applied.
The cooled supporting tube is made out of one piece with the main heating surface. The test rig A was designed, constructed and manufactured by APS. It was installed in July 2005 in the 600 MW Niederaussem Unit G.
The sum of operating hours of the test rig A at the end of the project reporting period was in total approx. 21.000 h.
During the Test Rig A operation in the period from July ’05 until Dec ’07 no water cleaning was performed due to the fact that the installation and operation of the water cleaning device was not successful. Only the conventional sootblowing technology with steam was applied for cleaning the heating surface of test rig A.
The visual inspections results in the conclusion that the Test rig A is in very good conditions, there are no incidents in terms of erosion and thermal shock that could be related to the general operation or especially to the cleaning procedure with conventional sootblowing.
11
The application of the conventional sootblowing procedure at the Test Rig A (“tube-fin-tube” design) compared to the conventional tube design observed with the installed furnace camera results in the conclusion, that there are no remarkable differences between the two designs in terms of cleaning efficiency by applying conventional sootblowing technology.
The Test Rig A is still in operation and will stay in operation during the coming months and years and will be further observed in order to collect additional operational experience.
The Test Rig B was developed for the validation of the super heater concept that considers an austenitic platen heating surface “tube-fin-tube” design as super heater panels.
The platen heating surface design is based on a self supporting concept. No conventional hanger supporting tubes are required, the panels are supporting each other by spacing web. The occurred forces can be absorbed by the walls. This design avoids supporting tubes, and hence the critical connection between superheater tube and supporting tube. Furthermore this design is intended to be easier to clean than designs with separate tubes. An austenitic material was chosen for the heat exchanger to gain experience with this material under water cleaning conditions.
The Test Rig B is a prototype of the platen heating surface (“tube-fin-tube” design) and was specified and manufactured by APS taking into account the boundary conditions of the existing 600 MW lignite fired power plant Niederaussem Unit G.
Useful experience was gained during the manufacturing of the austenitic platen heating surfaces with respect to the welding procedure and the selection of appropriate filler material.
Based on a risk analysis the power plant operator (RWE) decided from a safety operational point of view not to install the platen heating surface with a connection to the water steam side of the power plant.
Instead an alternative solution was proposed. All partners involved agreed to the changes with Amendment No.3.
Knowledge about the differences in the cleaning behaviour between of tube-fin design and a conventional design with separate tubes was achieved with Test Rig A, which was in operation for 21.000 hours and was frequently cleaned with sootblowers. This design showed no advantages with respect to cleaning behaviour compared with the conventional design. The tube-fin design is not easier to clean than separate tubes.
Knowledge about the material behaviour of austenitic tubes was gained with the additional Test Rig B’.
The alternative solution for Test Rig B, called Test Rig B’, consists of heat exchanger tubes, which are not connected with the water/steam system of the boiler, but are cooled independently by water and air. These heat exchanger tubes are installed in the superheater region of a 300 MW unit. The tubes are cleaned by an additional cleaning facility, able to clean with steam and water. Both the heat exchanger and the cleaning facility can be installed and uninstalled during boiler operation. In this way the ambient conditions at the furnace exit of a 300MW boiler can be used with a maximum of independence from the boiler operation and therefore good flexibility for test operation.
Austenitic as well as ferritic material tested with the Test Rig B’. A total of 14 material samples were used over different spans of time. The tested austenitic samples show no signs of damage caused by blower action. A robust statement on long-term behaviour, however, requires the samples to be used over a longer period of time. The samples of ferritic material showed serious changes in the
12
microstructure. The reason was that the samples were exposed to massive thermal overload in the beginning of the test due to insufficient cooling. No statement can be made on the blower's effect due to the untypical test conditions. Similar to the conclusions derived from the austenite material trials, the conclusion drawn here is that the test periods of this material should also be extended in subsequent trials.
The results obtained with Test Rig B’ were subject of a meeting of material experts of the project partners. The tests conducted, the findings achieved and a new test programme were discussed in detail.
The design proposed in Test Rig B was also numerically evaluated by PPC and its subcontractor NTUA. A one dimensional calculation tool is developed, which calculates the total super heater heat flux based on the flue gas temperature and its specific geometry. For validation purposes design data of the superheater III section of PPC’s Amyntaion power plant is used. The effect of the fins in a tube - fin - tube heat exchanger concept is simulated and accordingly optimized finned superheater designs are calculated. The new designs achieve material savings by keeping the heat flux constant.
The Test Rig C consists of three different kinds of tube surfaces, uncoated tubes as reference tubes, plasma coated tubes (Häusser) and wire flame spray coated tubes (Steinführer). The tubes of the test rig used for the analysis were exposed in the superheater region (SH3) of the Niederaussem G unit for approximately 17.000 h at steam parameters of about 480°C and 170 bar. For the removal of ash deposits from the heating surfaces the installed steam blower equipment was applied.
The base material for all the tubes in the Test Rig C is 13CrMo4-4 (Material-No.1.7355)
All the tubes, references tubes as well as coated tubes, were affected by steam blowing, especially in the intermediate section of the tubes
As a concluding remark for the approach with coatings it could be mentioned, that the coatings were already damaged by applying the conventional steam blowing. Due to the fact that the conditions at the tube surface are harsher with respect to thermal stress, erosion and corrosion by applying water cleaning compared to steam cleaning, it is expected that in case of applying water as cleaning medium, coating is not a sufficient measure to protect the tubes against erosion and corrosion.
13
2 Objectives of the project
The lignite found in the Rhenish mining area is extracted in various deposits the qualities of which differ considerably in some cases. In particular with the transition to the Hambach opencast mine – developed in the 1980s – significant changes in major coal quality parameters were recorded in the 1990s. Depending on the particular depth, calorific value, alkali contents, and the hardness of the raw lignite developed towards ever higher average values (Figure 2-1).
0
400
800
1.200
1.600
Garzweiler
Hambach
Inden
Sodium [ppm]Sodium [ppm]
1990 2000 20100
400
800
1.200
1.600
Garzweiler
Hambach
Inden
Sodium [ppm]Sodium [ppm]
1990 2000 2010
6.000
8.000
10.000
12.000
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Heating value [kJ/kg]Heating value [kJ/kg]
Garzweiler
Hambach
Inden
1990 2000 2010
12.000
10.000
6.000
8.000
6.000
8.000
10.000
12.000
1990
1995
2000
2005
2010
2015
2020
2025
2030
2035
2040
Heating value [kJ/kg]Heating value [kJ/kg]
Garzweiler
Hambach
Inden
1990 2000 2010
12.000
10.000
6.000
8.000
Figure 2-1: Development of coal quality parameters
The impact on steam generator operation has become more and more evident since the late 1990s in the form of increased deposit formation on the boilers’ heating surfaces (Appendix Figure 7-8).
The countermeasures launched in the field of boiler technology and coal supply showed success, which allowed the above development to be controlled. The bottleneck of the problem, however, continues to be the cleaning of the superheater banks, which – as the first of the downstream convective surfaces – are in the flue gas temperature range of 1,100°C to 1,300°C (furnace outlet) and approximately 800°C.
As the cleaning facilities available so far (soot blowers) have not proved sufficient for these areas of application despite the optimization efforts made, new processes need to be developed to solve the problem. The processes are characterized by the fact that – by contrast with the methods applied so far – they do not use the common cleaning medium steam but work on the basis of alternative cleaning media.
Under the terms of this project, more efficient cleaning equipment and new easier-to-clean heating surfaces for lignite-fired power plants are to be trialled on a demonstration scale. The object of the measure is to enhance the availability and competitiveness of this energy source, which is found extensively in Europe and is a low-cost and an important contributor to the energy supply.
15
3 Comparison of initially planed activities and work accomplished
All tasks were executed as planned in the original contract except for a part of Task 3, which was fulfilled with Amendment No.3. Task 3 was intended to test three heating surfaces designed to better withstand the exposure to water cleaning (Test Rigs A, B and C). While Test Rigs A and C were installed as planned, Test Rig B required a modification.
A prototype of the Test Rig B was specified and manufactured by ALSTOM, taking into account the boundary conditions for an installation in the superheater section of an existing 600 MW lignite fired power plant and a connection to the water/steam side of the boiler. The manufacture of the platen heating surface was completed in May 2004, and it was delivered to RWE’s power plant site at Niederaußem in June 2004.
In September 2004 fouling and slagging tests were performed by RWE and ASLTOM at Unit K of the Niederaußem power plant with Hambach coal. The test results provided new information on the fouling and slagging behaviour of Hambach coal, especially for deposit formation in the superheater section where a rapid deposit growth with a high deposit density was experienced. Due to this newly gained experience the installation of the platen heating surface was reconsidered for safety reasons and postponed in a first step.
In order to decide how to proceed with the installation of the platen heating surface a detailed risk analysis regarding the platen heating surface installation was performed (for details see chapter 4.3.1.2).
Based on the risk analysis the power plant operator (RWE) decided, from a safety point of view, not to install the platen heating surface with a connection to the water steam side of the power plant, but offered the possibility, as an alternative solution, to install an austenitic heating surface with a separate independent cooling system not connected to the water steam side of the power plant. This alternative solution allows the risk of an unscheduled power plant shutdown to be minimized by obtaining maximum knowledge on the required operational and cleaning behaviour as specified in the project.
This change in procedure was agreed with all those involved in Amendment No. 3.
To allow reliable results to be obtained here as well, an adequate time of operation is necessary. For this reason, the term of the Project was extended until December 2007 by Amendment No. 3.
17
4 Technical Implementation
4.1 Task 1: Co-ordination
Project coordination was executed by RWE Power AG. Initially Mr. Renzenbrink was the coordinator and since 2005 Mr. Wiechers has coordinated the project.
Regular project meetings were held and all required reports were presented to the EU Commission. In addition, two expert meetings were conducted. One expert meeting brought the sootblower model experts of the participating project partners together to swap experience gained in the development and operation of sootblower models. In a second expert meeting, the project partners' material experts discussed and evaluated the results of the tests carried out on Test Rig B’.
4.2 Task 2: Selection and testing of more efficient cleaning facilities and techniques
The heating surfaces of steam generator plants are exposed to various stresses. The ‛basic stress’ results from mechanical strains on the pipes caused by internal pressure and external forces that have to be absorbed by the pipes at a particular temperature.
In addition to this, the pipes are exposed to stresses exerted on the outside of pipes used for conventional water tube boilers. In the case of the coal boilers primarily investigated in the Ligpower project, these stresses result from corrosion, erosion, the formation of a protective layer, and thermal shock stress.
Corrosion problems are a well-known and common phenomenon in coal-based plants with a wide variety of different causes and working mechanisms. The examination of corrosive effects is not part of the Ligpower project’s scope of work.
Erosion on the outer surfaces of pipes in coal-fired steam generators is caused by coal and ash particles streaming past the heating surfaces. If flue gas velocities are normal and the particles contained in the flue gas evenly distributed, erosion problems are generally minor. Day-to-day operations show that, on the one hand, a local increase in the concentration of ash particles leads to increased erosion, while, on the other hand, a local – possibly temporary – increase in velocity significantly compounds erosion. Heating surface cleaning facilities, which today are mostly operated with steam, cause precisely this effect and lead to local erosive effects Figure 4-1. Nowadays, so-called shields are normally used to counteract this.
19
Figure 4-1: Erosion in the area of supporting tubes
The formation of a protective layer in itself does not constitute any stress for the pipe; rather, as the name implies, it protects the pipe. The protective layers on the outer surfaces of flue gas-heated boiler tubes consists of magnetite (Fe2O3), an iron oxide formed from the base metal of the tube (iron) and oxygen. Thus, the formation of a protective layer results in the iron contained in the base metal of the tube being consumed. With normal protective layer thicknesses of 300-400μm, this does not lead to substantial tube wall emaciations. In combination with effects that continuously strip the mineral protective layer off the base metal of the tube, the formation of protective layers becomes a serious strain for the pipes. Apart from erosion, the spraying of pipes with water is a suitable method for removing the exterior protective layers of boiler tubes. When the protective layer has been removed, the metal tube surface is exposed to the flue gas again, so that a new protective layer forms. Since the protective layer forms particularly quickly when it is thin, this also entails accelerated tube erosion (Figure 4-2).
Pro
tect
ive
laye
r thi
ckne
ss
Small protective layer at the beginning
Pronounced protectivelayer after a longerperiod of operation
No protective layer afterspalling caused byintensily cleaning
Loss of wall thickness
Time
Pro
tect
ive
laye
rthi
ckne
ss
Small protective layer at the beginning
Pronounced protectivelayer after a longerperiod of operation
No protective layer afterspalling caused byintensily cleaning
Loss of wall thickness
Time Figure 4-2: Formation and spalling of protective layer
20
Boiler tubes are also exposed to thermal shock stresses when they are sprayed with water from the outside. When water hits the outer surfaces of tubes, the tube material abruptly cools down, causing residual stresses to develop in the tube wall, which, if repeated frequently, lead to cracks. These cracks grow from the surface of the tube to the inside. Viewed from the surface, they form a kind of honeycomb pattern, also known as ‘orange peel’ (Appendix Figure 7-9). If left to grow unhindered, these thermal shock cracks may lead to boiler tube failure.
The effects described above – erosion, formation of a protective layer, and thermal shock –have to be taken into account when selecting and testing more effective cleaning facilities and processes. If they were not, and thus damage to and even failure of boiler tubes risked, the particular cleaning processes would have to be considered unsuitable for permanent use in boilers.
4.2.1 Compilation and selection of efficient cleaning equipment to test and determine the position of testing in the boiler
In general, there is a wide range of cleaning facilities suitable for cleaning boiler heating surfaces during operation. The facilities known on the market differ according to the process principles used.
Jet techniques using steam, water, or air as cleaning medium are most commonly used today. They remove heating surface deposits by means of the impulse of the emerging jet, aided by the temperature change (usually cooling) of the deposits when they come into contact with the jet.
In addition, other cleaning processes that have a direct or indirect effect on deposits are known, e.g. cleaning methods that achieve the cleaning effect by exciting the flue gas (either acoustically or using explosions or detonations). Acoustic techniques are quite widespread due to their simple setup.
Other cleaning techniques, e.g. rapping devices or so-called steel shot plants, are used at least occasionally. In principle, further methods of cleaning heating surface deposits during operation are conceivable.
Criteria:
Cleaning intensity
The problem of finding effective cleaning techniques for the fouling-prone region of steam generator plants needs to be specified when dealing with lignite plants in that processes need to be found that allow deposits even in higher flue gas temperature regions to be removed. For experience gained with conventional cleaning methods (steam jet blowers) shows that it is not possible to remove these deposits, particularly in plants using Hambach lignite as fuel. More precisely, it has become apparent that though a considerable portion of deposit can be removed in one cleaning step, some of it is left on the tubes, causing a steadily growing deposit that cannot be removed to develop. This reduces the operating periods of the plants to periods far shorter than those they were originally designed for. More specifically, plants that should have 2 to 3-year availability have to be shut down after 3-5 months to clean them during the outage.
Hence, the main criterion applied to the processes to be selected is the degree of cleaning intensity they can achieve. The locations in which these techniques are to be used are characterized by a flue gas temperature of 800-1,100 (1,300)°C, i.e. the region of the first convective surfaces downstream of the furnace.
21
Based on operating experience and tentative tests, it may be concluded that jet cleaning processes using water as cleaning medium have the highest intensity. This theory is supported especially by the fact that, as a rule, the cleaning of furnaces using one of the above-mentioned facilities is successful. The cleaning plants employed in the Rhenish lignite-fuelled power plants all use water as cleaning medium.
Avoiding damage to the boiler plant
The second criterion of equal importance applied in selecting techniques is that the use of a particular cleaning facility does not damage the boiler plant or affect its operability in any other way. The damage that must be mentioned in particular is boiler damage due to tube leakages. Based on what we know today, these may be caused by thermal shock or protective layer removal. Erosion, a problem encountered with steam blowers as well, must also be considered as a possible damaging mechanism.
Apart from the boiler tubes themselves, the so-called support lugs of the sling tubes must also be taken into account. Support lugs are high temperature steels welded to the sling tubes to support the heating surface tubes. These components are hardly cooled or not cooled at all and hence much hotter than the tubes themselves. Thus, it seems reasonable to assume that spraying these components with water may lead to damage much quicker than would be the case with the boiler tubes themselves.
Use in boiler environment
The third criterion is how well the cleaning techniques can be employed in boilers, or rather in the cold-end sections of boilers. The challenge is to place the cleaning facilities in the boilers in such a way that all tubes to be cleaned can actually be reached. It must be borne in mind that the cross-sections of the plants in question range between 20x20m (old plants) and 26x26m (new plants). This entails considerable requirements regarding mechanical stress that must also be fulfilled in the given circumstances. The ambient conditions cleaning facilities are exposed to when advanced are decisive. They are primarily characterized by high temperatures (> 1,000°C) and dust-laden flue gas.
Selected processes
The processes were selected on the basis of technical availability. The above-mentioned criteria had to be met, or it had to be apparent that the processes have the potential to fulfil the criteria. Two water-based techniques were selected in the end:
• Two-fluid blower
• High pressure water jet cleaning
These were complemented by two processes working on the basis of explosions or detonation, namely
• Shock wave cleaning
• Explosive cleaning
4.2.1.1 Effects on heating surface components
In addition to the above developments of the actual heating surface cleaning facilities, the effects these facilities will have on the superheater banks will constitute a critical issue for the overall success of
22
these developments. Theoretical preliminary considerations revealed that thermal shock damage is not expected for the actual tube walls but likely to occur to the uncooled sling tube lugs on which the actual heating surface tubes are supported. The scaling off of the outer protective magnetite layers was found to be the third type of damage caused by the application of water. The resulting quicker formation of new protective layers leads to enhanced consumption of the base material and, hence, to increased wall loss.
The effects of the facilities being developed so far could not yet be observed. However, observations made at other points where steam soot blowers have been converted for operation using water have shown that tube damage occurs to an increasing extent. First investigations led to the conclusion that most of the damage is caused by erosion which, combined with the scaling off of the protective layer, resulted in considerably accelerated tube wall consumption.
To get a better understanding of the damage behaviour, Ruhr University Bochum, Chair of Materials Technology, Prof. Dr.-Ing. Gunther Eggeler, was therefore charged with carrying out a literature study.
The object of this study was to collect information about the damage mechanisms of low-alloy chromium steel superheater tubes during high-temperature operation and when subjected to frequent sequences of slag formation, oxidation, water-jet cleaning and creep.
Despite the in-depth literature search, it was not possible to find a detailed description of the damage behaviour under exactly these conditions.
Nonetheless, it appears plausible after this study that an excessively intensive water jet does not only bring the desired effect of removing the slag layer but also causes spalling of the oxide layer. New oxide forms. If this sequence (intensive cleaning, spalling of the oxide layer, formation of new oxide) repeats frequently, metal is consumed, the tube wall thickness decreases, the creep stress increases and, ultimately, the tube ruptures as a result of creep. This is easy to understand in quality terms. On the basis of the data available was impossible, however, to identify the problem in terms of quantity.
4.2.1.2 Comparison of lignite from Greece and lignite from German Rhine area
The cleaning processes selected within the scope of the project were tested in RWE Power AG's Niederaussem power station. A comparison of the Greek and the Rhenish lignite is performed in order to estimate the slagging and fouling potential of both types and therefore determine the transferability of the results and the experiences regarding the operation of PPC and RWE power plants.
This comparison comprises the following:
• Gathering of lignite samples of various Greek mines and historical data of lignite analyses during the previous 6 years and data for Rhenish lignites.
• Determination of various slagging and fouling factors and ash melting points and comparison with data given by RWE.
Characterisation of lignite
Generally, the lignite of Northern Greece is characterised as low rank coal with a Heating Value (LHV) ranging from 5,100 to 5,400 KJ/kg on average and a high percentage of both moisture and ash content. The low heating value is reflected in the proximate and ultimate analysis, which clearly indicates that there are high oxygen content and low hydrocarbons. Data from PPC’s laboratory at Kardia (Unit 1) have been gathered for three different but successive periods of time. The average values for the
23
proximate and ultimate analysis samples K1 (Ptolemais lignite, year 2005), K2 (Ptolemais lignite, year 2006) and K3 (Ptolemais lignite, first three quarters of 2007) are presented in Table 1.
C %ww 18.15 17.64 18.13 H %ww 1.38 1.39 1.37 N %ww 0.56 0.49 0.49 S %ww 0.35 0.38 0.37 O %ww 7.60 7.31 7.47
LHV KJ/kg 5,357 5,137 5,385
Table 1: Proximate and ultimate analysis of Greek lignite, samples K1, K2 and K3
Eight German lignites, originating from the Rhenish area, have also been studied for comparison. Lingites R1 to R6 come from the Hambach mine, R7 from Inden and R8 from Garzweiler. In contrast to the Greek lignites, Rhenish ones are classified as high rank brown coals, with a LHV between 8,500 and 10,500 KJ/kg and ash content that varies from 2 to 5 w.t.%. Typical data for those fuels were supplied by RWE and presented in Table 2.
Table 2: Ash content and low heating values of Rhenish lignites by type of deposition formation
Ash composition
The chemical analysis of coal ash is reported as the mass percentage of the equivalent oxide of its major constituents and is generally expressed as shown in Table 3. The oxide P2O5 is omitted because it is present in insignificant quantities. The ash tendency for deposit formation is quantified with the calculation of a number of indices from the chemical analysis. However, the formation of ash deposits does not depend solely on the fuel used but also on the temperature profile in the boiler, the structure of boiler and the tube arrangement, especially in the area where the superheaters and other convection sections are found. Previous analytical investigations reveal that the unwanted ash depositions on surfaces are usually a combination of all the above factors.
Table 3: Chemical analysis of ash for Greek and Rhenish lignites
A detailed description of the comparison and the necessary classifications is shown in the Appendix A.
Results – Conclusions
Greek lignite ashes are mainly characterized by their high calcium content which indicates significant slagging potential. On the other hand, the presence of alumina and the relatively high dolomite percentage counterbalance the effect of calcium. Therefore, the overall slagging potential is expected to be moderate. This is also verified by the value of the fouling index.
Figure 4-3: Classification of ash based on their major oxides
The most important issue concerning the Greek lignites is the high ash content, which, coupled with the low heating value, results in the production of large quantities of ash. This in itself may intensify the effect of slagging phenomena. The low alkali content (compared with the Rhenish lignites) suggests
25
that the fouling phenomena on the heating surfaces are negligible. Therefore, fouling problems will occur mainly due to the high ash quantity and the turbulent flow field produced by the heating surfaces geometry.
0,000
0,200
0,400
0,600
0,800
1,000
1,200
1,400
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
Foul
ing
Inde
x R
f
Figure 4-4: Fouling Index (Rf) for Greek and German lignite samples
Based on the investigation the main results are the following:
The Rhenish lignites are characterized by lower ash content and higher heating value. Although this means that the ash produced for a given amount of energy is lower compared to power plants running with Greek lignites, the high heating value corresponds to increased furnace temperatures, which may exceed the melting temperatures of the ash oxides. Lignites R5 and R6 present the danger of slagging; both have high LHV while the B/A ratio for R6 is in the critical range and the S/A ratio for R5 is extremely high. However, they can be used relatively safely in boilers with minor thermal loads. Lignites R1and R2 also exhibit high heating values which may lead to increased slagging, while lignites R3 and R4 remain uncritical although their combined use with other lignite types should be carefully examined in order not to exceed the recommended silica or iron contents. R7 and R8 have the lowest heating values of the German lignites and, judging from their ash content, their slagging behavior is uncritical.
One of the major differences between the Greek and German lignites is that the latter are characterized by a much higher alkali content, which leads to increased fouling potential. Lignites R1 and R2 exhibit the highest fouling potential; taking into account their high heating value, the deposits on the heating surfaces may sinter and harden. The increased presence of SO3 intensifies the fouling phenomena.
Overall, the Greek and German brown coals examined in this study exhibit differences in their slagging and fouling behaviour, although some general tendencies, e.g. a moderate slagging potential, can be discerned for both groups. Therefore, technologies and technical expertise from the operation of German power plants to Greek ones is possible with further investigations.
26
4.2.2 Description and testing, monitoring and assessment of the selected cleaning concepts
4.2.2.1 Two-fluid blower
The two-fluid process is based on the idea of retrofitting a steam jet soot blower with a water jet option. This enables the blower to clean using either water or steam.
A flue gas cleaning device of this kind has been designed in principle by the company Clyde-Bergemann. A first blower of this type, named ‘Smart Lance’ by the manufacturer, was installed in a 600-MW lignite-fired unit of the Niederaußem power plant (unit G) in 2003. It was mounted below superheater 3, which, as the first heating surface bank downstream of the furnace, is exposed to the highest flue gas temperatures except for the furnace walls. Judging by what we know today, the installation of such an ‘intensive cleaning facility’ in this location and at the two heating surfaces above it – superheater 4 and reheater 3 – is beneficial because these surfaces foul irreversibly when conventional steam-based cleaning devices are employed. This means they determine boiler running hours, hence limiting the availability of the unit in otherwise trouble-free plant operation.
The basic concept of such a two-fluid blower is to install one or several water lines inside a steam blower that supply the water nozzles that produce the water jet required for the intensive cleaning of the heating surfaces near the steam nozzles at the soot blower lance tip. This intensive cleaning should not be carried out continually (5x daily) as with steam blowing, but merely as required, i.e. 1x per day maximum when fouled, in order to avoid too intensive cleaning, which might cause damage to the base metal of the tube. These considerations are based on the observation that cleaning using water is much more intensive than cleaning with steam. Aside from the impulse forces, this is due in particular to the fact that deposits are ‘quenched’ with water. The resulting ‘residual stresses’ in the deposits are supposed to make them brittle, causing them to fall off. In addition, the concept of water seeping into the deposits and abruptly evaporating, thus destroying the deposits from the inside, supports the models used to explain the better efficiency of water as a cleaning medium.
The design of such a blower is illustrated in Figure 4-5, which shows the following basic elements:
• Inner and outer soot blower lance tube for transporting steam. The inner one fixed, the outer one flexible to permit forward and rotary motions. In addition, there are steam nozzles at the soot blower lance tip. These elements are also incorporated in conventional steam blowers.
• Water pipes for supplying the four water nozzles at the lance tip. These elements are new compared to pure steam blowers. They are arranged in the annular space between the inner and outer steam pipe.
• Driving mechanisms for forward and rotary motions. In two-fluid blowers, separate drives are used as opposed to a joint one as in conventional blowers.
• Steam and water supply lines, plus various seal air and scouring air lines.
• Carriage as mechanical system
27
Figure 4-5: Two Fluid Blower
The new components required for water cleaning were tested in the ambient conditions of a commercial scale plant. The following test results were obtained:
Blower operation in a cold environment (Figure 7-10):
During the start-up work of the plant it became evident that the required and preset motions of the blower could be carried out as desired. The challenge is to synchronize the forward and rotary motions in such a way that the blower is able to precisely move along the heating surface sections to be cleaned (Figure 7-11). The reason for this is that the sling tubes have to be avoided when blowing water to prevent the water jet hitting the support lugs, which are made of brittle SiCro material and are much hotter than the boiler tubes through which steam flows. As far as we know, thermal shock cracks would then be inevitable. Apart from a precise and coordinated adjustment of the advance path and rotation angle, exact positioning requires that the blower bend as little as possible even when advanced. All three criteria were satisfied after the cold start-up.
Jet formation:
To avoid the sling tubes precisely, the jet used in water cleaning has to be equally precise. This precision failed to be achieved in the first few tests using water. But we succeeded in optimizing it in two steps. Firstly, cold tests were conducted with the aid of which we found special nozzles that
28
produced a well-bundled jet when retrofit with the relevant mechanisms. This problem is caused by the short distances the water has to travel on its way to the nozzle, which generally hinders the formation of a well-directed jet (Figure 7-12). Secondly, the water jet had to be optimized during hot operation. With the aid of a camera mounted near the installation location of the blower we observed that the water jet breaks apart at the normal steam-side blow pressure of approximately 22 bar. (By way of explanation: The parallel operation with steam is necessary to cool the exterior steam pipe, which is fully exposed to a flue gas temperature of about 1,000°C.) The tests showed that this effect no longer occurs at a reduced pressure of approximately 12 bar. This is most likely due to the fact that less heat is transferred to the water pipes situated in the steam space of the advancing exterior tube when steam-side pressure is lower. Thus, the water is heated less and – presumably – does not reach the boiling point anymore, which leads to the formation of bubbles causing the jet to break apart (Figure 7-13).
Cleaning in steam mode:
The two-fluid blower was installed in lieu of a regular steam blower and had to reliably fulfil the functions of the latter. After overcoming the initial start-up problems, it did, so that the two-fluid blower was usually employed as steam blower.
Cleaning effect in water mode: According to the concept, tests using water as cleaning medium cannot be carried out on a clean boiler, i.e. not shortly after outages during which it was cleaned either. Due to a high number of boiler outages during the testing period of the blower, there were only few opportunities to test the water cleaning mode. But in addition to jet quality, it was also possible during the tests to verify whether the heating surfaces of superheater 3 had been cleaned successfully by means of an advanceable camera. Figure 4-6 shows that the cleaning procedure was successful and that at the same time the blower had kept the required safe distance to the sling tubes.
Figure 4-6: Successful cleaning in water blowing mode
Since the blower proved successful during these crucial tests, we decided to procure another three two-fluid blowers of the same design independently of the Ligpower project and install them in mid-2005 to promote the development of devices that can be employed reliably during operation. But a variety of substantial equipment-related problems became apparent during operation, rendering regular, permanent operation in the water mode impossible. Apart from initial control- and sealing-related problems, problems due to expansion occurring between the hot exterior steam pipe and the cold water pipes on the inside, and cooling problems contributed to the decision against pursuing the development of two-fluid blowers. Figure 7-14 shows one of these blowers in the boiler after it was damaged.
29
With respect to its assessment, the high potential this technique has in principle remains one of its advantages. But in the actual implementation and with only limited opportunities to experiment, it was not able to satisfy the requirements of a large-scale production plant with difficult ambient conditions to a sufficient degree.
4.2.2.2 High pressure water jet cleaning
High pressure water jet cleaning is based on the concept of breaking up deposits and washing them away with a fine high-pressure water jet. This requires a device that brings the nozzle producing this jet as close as possible to the heating surfaces to be cleaned. Since these surfaces may be up to 5 m high (total height of upper and lower parts), only a small portion of the sections to be cleaned on which the lance tip of a conventional blower usually moves along on a straight line can be reached from a ‘short distance’. A number of potential solutions were investigated taking account of the actual boiler geometries, two of which were looked at more closely. The first solution involves a hose held by a cantilever that is introduced into the boiler and lowered in a suitable location. The nozzle head, which includes a positioning device, is situated at the tip of the hose. As second solution a concept involving a ‘water telescope’ was shortlisted, its advantage over the ‘hose solution’ being that the nozzle head can be positioned in several different locations. But in light of the outlays its implementation would likely require and possible problems, such a more complex concept will not be considered as start variant. In view of the boundary conditions determined by the mounting location in a 600-MW lignite-fired plant (Niederaußem H), the hose solution seems to be most suitable.
Before making a decision on the construction of a 600-MW production plant, preliminary component tests were performed. On the one hand, the hose, which would be subjected to multiple motions in operation, was tested. On the other hand, at least the nozzle head, heavily packed with numerous components, was to be designed prior to manufacturing a complete prototype. The results of these two key steps were the following:
The test of the hose to be used was conducted in 2004. The selected hose entered into and retracted from the boiler about 7500 times in boiler conditions. The findings showed that the expected fouling deposited on the outside of the hose in the area of the bend did not entail any ‘inflexibilities’ of the hose. If possible, motionless hose sections should be prevented. Further information required for the construction of a prototype could be deduced for the mouth piece of the carriage from which the hose runs out of. In addition, it was possible to deduce from cooling water measurements that the heat input is approximately 100 KW/m of hose length in trial operation. The hose permanently had only the few leakages that had been expected. Viewed as a whole, the design concept of the selected hose met the requirements, so that the next development step could be tackled.
The nozzle head of the prototype has numerous functions:
• Accommodation of two rotating nozzles. Two nozzles built into the head opposite of each other are necessary to ensure that the nozzle head, hanging from the hose, is not moved out of its position.
• Slewing motion of the nozzles, so that the water jet moves along the heating surface not only in a straight line from top to bottom, but can wet the whole heating surface.
• Accommodation of an observation device to monitor the effect achieved. This requirement was met by installing two video cameras permitting the heating surface areas to be cleaned to be observed in both jet directions.
30
A nozzle head designed according to these requirements is shown in Figure 4-7 . It is supported by the hose, which also supplies it with cooling and HP cleaning water. The cooling water must be discarded and flows down and out of the head at zero pressure to evaporate in the furnace below. The head appears suitable to be installed in the prototype.
Figure 4-7: High Pressure Water Jet Cleaning (HWJ) nozzle head
Next, the complete plant is designed. This requires the following main components to be designed, constructed and assembled into one plant:
• Carrier including cooling system
• Control system
• Pumps and connections
• Hose with interior hose and all necessary cables
• Hose reeling device
Figure 7-15 shows the design of the prototype and its main components.
Taking account of the experience gained beforehand when using the two-fluid blower in boiler conditions, we decided to test the prototype in cold tests first to prevent any defects that might occur from having to be corrected in the more difficult conditions prevailing in a commercial scale plant. The tests were carried out in the Niederaußem power plant in August 2007 (Figure 4-8) and furnished the following findings:
31
Figure 4-8: HWJ test with full scale heating surface
The following prototype functions have been implemented successfully, i.e. in accordance with the requirements: moving and positioning of the carrier and head, control, viewing angle for camera shots.
We were able to accurately record the mechanically inevitable bending of the carrier and the head capacity curve of the HP pump.
In other respects, however, problems occurred that have not allowed the prototype to be installed in a commercial-scale plant so far.
When the hose was reeled and unreeled, an unexpected elongation of this loose lined hose occurred and impaired precise positioning and, due to a hose socket, caused reeling problems. This prevented the nozzle head from moving to its final position. Moreover, if the nozzle head was advanced into the boiler, it would strike against the boiler wall and be damaged.
In the present concept, no possibility has been found yet for reducing the cooling water quantity to such a level that the water squirting out in a horizontal to diagonal direction does not hit the superheater pipes in an uncontrolled way before the head has reached its suspended working position and the cooling water can run off without hitting the superheaters. The solution to this problem is still open. Besides a reduction in the water quantity that may be possible, atomization of the ejected water is also being considered.
It was already noted during the cold test that the nozzle head does not hang vertically. This makes controlled slewing of the nozzles difficult. The present approach consisting in providing a reinforcing wire wrapping has not yet been able to eliminate the problem.
The final solution to the problems described here and to further problems (spray pattern of the stability nozzle, retractability of the blower during power failures) is currently not in sight, so that the date of installation into the commercial plant could not be fixed yet.
32
4.2.2.3 Shock wave cleaning
While the above mentioned methods use water to obtain an improved cleaning effect, shock wave cleaning applies an entirely different approach. In this process, shock waves are generated outside the boiler and conducted through pipes into the boiler. This is where the shock waves are intended to develop their cleaning effect. This cleaning technique offers the ad-vantage that the risks involved in water cleaning of the superheater tubes can be avoided.
Shock wave cleaning is based on research work performed by Prof. Dr.-Ing. K. Hanjalic at the University of Sarajevo, Bosnia-Herzegovina. The research activities were aimed at using the shock waves produced by the detonation of combustible gas mixtures for cleaning fouled areas in industrial plants and equipment such as those occurring in coal-fired steam generators, industrial boilers or chemical plants.
The plant that was erected and operated in Niederaußem is comparable with a structure that was developed by the University of Sarajevo and installed and operated successfully at two boilers of the Kakanj power plant.
The shock wave cleaning facility is installed at the 300 MW D unit and has the following key components
• gas mixing station
• shock wave generator
• connecting pipes
• control cabinet
The gas mixing station produces the mixture of gas and air that is fed to the shock wave generator. A shock wave is a pressure wave in the combustion products with supersonic speed – it amounts to > 2,000 m/s. In the generators, a shock wave is produced by the special structure of the ignition device, which accelerates the flame front in order to convert the deflagration (decomposition reaction of an explosive that, contrary to a detonation, takes place at subsonic speed) of the air/gas mixture, during the combustion process, into a detonation through interaction with compression shocks and turbulences. The maximum pressure produced in the generator during detonation of the air/gas mixture amounts to approx. 13.8 bar in the present facility.
The generated shock waves are conducted via two connecting pipes to the superheater area of the boiler (Figure 7-16).
Operation of the shock wave facility is semi-automatic. Starting is by manual activation, operation is fully automatic. After manual system start, shock waves are generated at regular intervals.
The flow of air through the gas mixing station is continuous. During system operation, a required gas amount is injected into the continuous air flow in the gas mixing station at a particular moment. As a result of the mixing process in the mixer and the downstream pipes, an explosive air/gas mixture is produced which is then ignited electrically in the shock wave generators. If the explosive gas mixture does not ignite, the system stops automatically after a misfire and changes into failure condition, so that the gas valves do not open further. Un-controlled accumulation of explosive gas mixtures inside or outside the boiler plant is thus excluded.
The shock wave cleaning facility was tested and its efficiency compared to the cleaning effect of the neighbouring conventional soot blowers. Since the cleaning efficiency of the shock wave facility was
33
initially much lower than that of the soot blowers, attempts were made to enhance its efficiency. To this end, the cleaning frequency was increased, in particular. Up to 35 cycles per cleaning process were started. Despite the increased cleaning intensity, neither a visual inspection nor the evaluation of the values measured confirmed any cleaning efficiency.
Since these results were clear and the proposed system modification was only expected to bring minor improvements, the development of the shock wave cleaning facility was discontinued.
4.2.2.4 Online explosive cleaning
Another option considered adequate for cleaning the superheater surfaces is igniting explosive charges in the fouling area. Tests to that effect were performed at the 300 MW unit D of the Niederaußem power plant. An “explosive lance” is used to introduce the explosive charges into the boiler and ignite them there.
It is a tube with a connection for cooling water and a trigger cable attached on one side and a swivel joint on the other side. An aluminium tube is attached to the swivel joint which contains the explosive charge. The cooling water leaves the lance at the tube end.
The cooled explosive lance is introduced manually through the boiler opening into the heating surface area where the explosion is triggered. To assess the efficiency of the process, photos were taken before and after explosive cleaning.
Six explosive charges were introduced into the boiler and ignited with 40 grams of explosive agents each. No cleaning effects were observed with the selected explosive charges. Cooling of the lance involved difficulties. It was not sufficient when lance aligning in the boiler lasted longer and caused defects.
As the explosive lance has to be introduced into the heating surface area through existing boiler openings and the length is restricted for handling reasons, the reachable boiler area is limited.
The results obtained – no visible cleaning effect, restricted cleaning area, cooling difficulties, high demand on the physique of the on-site staff – led to the decision to discontinue the development of this cleaning process.
4.2.2.5 Fouling and Sootblower Monitoring Model
In order to be able to use the cleaning facilities effectively in a boiler, we must have the most comprehensive picture possible of the current fouling situation. Our objective is to use the cleaning systems as often as required for preventing fouling of the heat exchangers on the one hand and, on the other, to use them as rarely as possible to avoid damage. For this reason, RWE npower developed an online monitoring system that calculates the fouling condition of the individual heating surfaces and makes suggestions for which heating surfaces should be cleaned when.
The fouling and sootblower model has been implemented at Tilbury Power Station. It monitors boiler fouling and provides an advisory screen to operators that shows current levels of slag and ash build up on the heat transfer sections of the boiler. This system is currently set up to run on Unit 8.
Data is retrieved using ‘PI-Datalink’ developed by OSI Software Inc. Using this technology, plant data is stored on a network server, and can be retrieved via an add-in that is installed in Excel. Most data from the power station is logged using this system (each piece of instrumentation that is connected to
34
the system is assigned an alpha-numeric ‘tag’ for identification) and is available for access from any computer connected to the corporate network.
The model is based on a set of mass and energy balances around the boiler, and uses temperature and pressure data at the inlet and outlet of each heat transfer section to perform the calculations. Closer to the furnace, where gas temperatures cannot be accurately measured, an estimate can be made via the heat pick-up on the steam side of each heat transfer bank.
Due to the age of the station, some temperature instrumentation is not functioning. Where possible the damaged thermocouples have been repaired / replaced. Figure 7-17 shows the location of the thermocouples at Tilbury power station. Thermocouples in these areas are normally installed in pairs – if one of the two has a reasonable reading, then this value is used. Where both are reading incorrectly (an upper and lower check is included along with a standard deviation check to ensure the value is moving), an estimate for the temperature at that point is used, to allow the model to continue. The front-end of the model includes an advisory for the operator stating if there are tag problems.
Fuel flow is used along with oxygen levels at the exit of the boiler (at the economiser exit) to estimate combustion gas flow through the boiler. Fuel composition used in the calculation can be selected for different scenarios (e.g. high biomass throughput), as this would effect overall gas weight through the boiler and possibly adiabatic flame temperatures.
Water & steam flows are taken from station instrumentation.
Information on each heat transfer bank is inputted into the model. This includes tube lengths, tube pitches, depth of bank flow direction, inner and outer tube diameters, and metal conductivity. An example input screen is shown in Figure 7-18
In order to optimise steam temperatures, Tilbury Power Station has been designed with a split back-pass so that gas flow over the banks can be altered via a series of dampers. Gas flow over the two passes can either be calculated using an energy balance over the reheater or superheater banks (with an iterative calculation), or via an estimation of the gas flow split based on damper position. Due to ongoing issues with reliability of some of the tags in this area, the latter option has been adopted. If, and when, thermocouple reliability in this area improves, then the model can be switched back to calculating the gas flows based on the energy balance.
To carry out the iterative process detailed above, it is necessary to link the modules together in a certain order. A snapshot of this screen is shown in Figure 7-19
As stated above, information on each heat transfer bank is inputted into the model. Using this, an ideal heat transfer coefficient can be calculated. By comparing this to the actual performance of the back (using real-time data), a fouling factor can be calculated. This is compared to what is ‘standard’ for that bank of tubing, and it is this percentage that is seen by the operator.
Due to each heat transfer area of the boiler being defined separately in the model, if a biomass is used that has ash emissivity properties that are far from normal coal, these can be entered into the model and selected by the operator under fuel type. Data that is currently in the model for high burn biomass originates from work carried out by Imperial College within this project scope.
Sootblower advisory front end
The operator is shown the screen in Figure 7-20.
This display can be broken down into several sections (Figure 4-9)
35
Visual display of fouled areas of the boilerVisual display of fouled areas of the boiler
Graphical display showing change in fouling / slagging
Graphical display showing change in fouling / slagging
Graphical display showing thermocouple status.
Red = both faulty amber = 1 faulty green = both ok.
Graphical display showing thermocouple status.
Red = both faulty amber = 1 faulty green = both ok.
Bar chart advisory for areas requiring sootblowing
Bar chart advisory for areas requiring sootblowing
Fuel type selection (either Normal Coal, Wet Coal, Ultra Low Sulphur Coal, or High Proportion Biomass)
Fuel type selection (either Normal Coal, Wet Coal, Ultra Low Sulphur Coal, or High Proportion Biomass)
Figure 4-9: Sections of advisory front end
As the fouling factor increases on each section (and ultimately as sootblowing is required), operators will be shown this via a visual aid (colours over each section change colour).
Furnace sootblow (Group 1):
(The furnace sootblowers (short travel) are located on the rear wall between the burner belt and the nose, and in the top cavity)
Sootblowing was carried out at the start of the chart, and as can be seen in Figure 7-21, the furnace fouling factors decreased.
Longstrokes & Furnace sootblow (Group 2)
(The longstroke sootblowers (long travel) are located in the radiant and platen superheater)
As can be seen in Figure 7-22, as the furnace and the superheater surfaces are sootblown (carried out near the start of the chart), the fouling factor decreases.
Tilbury U8 has recently run on Ultra Low Sulphur Coal (ULS). This is relevant to studies into High Burn Biomass and Lignite combustion, as ULS demonstrates chemical properties between coal & biomass, is similar to lignite, and will create similar fouling deposits. Figure 7-23 and Figure 7-24 show the relevant parameters of the different fuels.
The sootblower model has been run over the time ULS was being burnt. Figure 7-25 (taken from the front end of the sootblower model) shows the fouling increase over time.
36
The sootblower model was run for three time periods during the ULS trial, which are highlighted by the different colours in the chart below.
Experts meeting
The project partners Alstom and RWE Power have experience with similar models. In order to share experiences in the development and the operation of these models, an experts meeting was conducted.
The experts from npower, Alstom, and RWE Power presented and discussed their models with respect to:
• Calculatory bases
• Data acquisition and data management
• User interface
• Application to control sootblowers
• Cleaning strategies
It was concluded that the structure and the function of models are comparable in principle, with differences in the details. Special attention has to be turned on cleaning strategy, which has to be well adapted.
System Requirements for the npower model:
• Microsoft Windows XP
• Microsoft Office
• PI-Datalink, connected to server tik2401
• Optigen Boiler Modelling Software
• SteamTab, ChemicaLogic Corporation
4.2.3 Wearing protection
The wearing protection tests were carried out within the scope of work performed on the material test rig (Test Rig B) and are therefore described in chapter 5.3.2.2.
4.2.4 Summary, analysis and evaluation of the results and assessment of the consequences in view of future applications
Based on fundamental considerations on the cleaning and damage behaviour, promising new cleaning concepts were chosen and trialled in Task 2 "Selection and testing of more efficient cleaning facilities and techniques". The tests were carried out in the Niederaussem power plant. This plant is supplied with lignite from the Rhenish mining area. In order to make sure that the results can be transferred to the conditions prevailing in power plants fired with Greek lignite, a comparative evaluation was made. Moreover, a fouling and sootblower monitoring model was developed to allow the cleaning facilities to be used in line with requirements, i.e. neither too often nor too rarely. Moreover, a literature study was conducted to develop a better understanding of the damage behaviour.
Two of the selected innovative cleaning methods use water as a cleaning medium (two-fluid blower and high pressure water jet cleaning). One technique generates shock waves that are used for cleaning
37
(shock wave cleaning). In addition, tests were carried out to determine how heating surfaces can be cleaned during ongoing operations by means of explosive charges (online explosive cleaning).
Both the two-fluid blower and high pressure water jet cleaning use water as the cleaning medium. It is well-known, however, that water can also damage heating surface tubes. Therefore, a literature study was performed with the aim of developing a better understanding for the relevant damage mechanisms. Despite in-depth research no quantitative descriptions were found for this specific case. However, a plausible qualitative description of the damage mechanisms was elaborated.
In order to avoid damage to the heat exchanger surfaces, the cleaning facilities should be used as required. "As required" means in this case that they should be used as often as necessary to keep the boiler in a reasonably clean condition but as infrequently as possible to avoid possible damage. A programme was developed by npower to calculate the fouling condition of the individual heating surfaces on the basis of current boiler data and derive proposals from the fouling condition for the use of cleaning facilities. The programme was successful installed at Tilbury power station.
Alstom and RWE Power have experience with similar models. Subsequent to the Ligpower meeting an experts meeting was conducted to share experiences in development and operation of these models. The experts discussed the different models with respect to the calculatory bases, data acquisition and data management, user interface, application to control sootblowers and the implemented cleaning strategies. The programmes and their utilisation are comparable in principle. The discussion made sure that the best strategies are implemented and cleaning is executed when needed.
All cleaning facilities investigated within the scope of the project were installed in the Niederaussem power station. This plant is fired by lignite from the Rhenish mining area. To ensure that results obtained in this power plant can be transferred to other power plants where Greek lignite is combusted, a comparative evaluation of various lignites was made. One main result of the investigation was that the Greek lignite from the mines in Northern Greece generally shows a lower deposit formation tendency than Rhenish lignite. The application of the investigated cleaning concepts and superheater designs in Greek power plants therefore seems to be feasible in a demonstration step after the first evaluations have been made.
The two-fluid blower can be operated both with steam and water as a cleaning medium. As a rule, it should be operated with steam to clean the heating surfaces. If, however, fouling proves to be too difficult to remove, water can be used since it is a more aggressive cleaning medium. As a result, this cleaning facility offers the advantage that either careful or more aggressive cleaning can be chosen to meet requirements. A two-fluid blower prototype was developed and tested on the 600 MW G Unit. After several optimization steps, this prototype showed a very good cleaning behaviour. Based on these positive results, the power plant operator decided to install another three two-fluid blowers on the same superheater bank (independently of the LIGPOWER Project). During operation of these four two-fluid blowers, a large number of different failures occurred. A major cause of failures in this complex system is the great temperature difference between the water-cooled inner tubes, the steam-cooled lance and ambient temperatures reaching more than 1,000°C in the boiler. The failures included cooling problems, difficulties with seals and control errors. These failures made continuous operation in the water mode impossible. Due to the comprehensive experience gained in two-fluid blowers, this development will not be continued.
The working mechanism of high pressure water jet cleaning consists in directing a comparatively small water jet under very high pressure at the deposit and breaking it up. This principle calls for a device that moves the water nozzle as closely as possible to the area to be cleaned. After various concepts had been evaluated, we decided to pursue a technical solution that advances the nozzle head into the boiler using a flexible hose. The nozzle head is positioned vertically by unreeling the hose and horizontally by a
38
cantilever that can be introduced in the boiler. The critical components were tested before the construction of a prototype was started. Since experience gained with the two-fluid blower showed that the repair of a device already installed in the boiler can be subject to considerable delay due to ongoing operations, great importance was attached to the execution of preceding tests when high pressure water jet cleaning was developed. These tests were first conducted in the workshop and later in a full-scale test set-up in the Niederaussem power plant. A multitude of the functions required was implemented successfully. By the end of the project term, however, not all problems were solved, so that no date has been fixed for installation in the boiler yet.
The two-fluid blower and high pressure water jet cleaning were developed for the final superheater stage of a 600 MW lignite-fired power plant unit. On the one hand, this involves working conditions in a flue gas atmosphere with temperatures of 1,100 – 1,300°C and, on the other, very large geometric dimensions since e.g. the cantilever must have a length of 10 m to reach the boiler centre. In addition, cleaning operation using water had to meet exacting requirements and a large number of boundary conditions in order to minimize possible damage associated with water cleaning. These requirements resulted in completely new, complex cleaning facilities that were immediately tested in a commercial utility boiler. This big development step seemed feasible but it became clear that a staged procedure could have had advantages.
In shock wave cleaning, an ignitable mixture of gas and air is produced outside the boiler and ignited. A carefully directed explosion generates a shock wave that is conducted through pipelines to the boiler area to be cleaned. This cleaning facility was installed and trialled at a 300 MW unit of the Niederaussem plant. Although cleaning intensity was continuously increased in the course of the trials, no cleaning effect was observed. Since the progress that could be reached by a modification of the plant was also expected to be minor, the tests to investigate this cleaning facility were discontinued.
A further possibility to clean fouled boiler areas during operation is seen in the use of explosive charges. During boiler outages, explosive charges are ignited to accelerate boiler cleaning in particularly fouled areas. The particular difficulty lies in introducing explosive charges into the intended area during ongoing operations and igniting them there. In a 300 MW unit, explosive charges were introduced by a cooled, slewable device into the boiler and ignited there. Handling this equipment proved to be extremely difficult, and the boiler area reached was very limited. Since, in addition, no cleaning effect was visible, the decision was taken to stop pursuing this line of development.
The results of the shock wave and online explosive cleaning tests are evident: These methods do not show a sufficient effect and are not suitable for regular use in a commercial-scale plant.
The results of the water-based cleaning process tests are less evident. Initially the two-fluid blower produced a very good cleaning effect, but the further course of the tests showed that—due to the system's structural complexity—it did not withstand the harsh operating conditions in the final superheater section of a 600 MW boiler.
When the entirely new high pressure water jet cleaning process was developed, great importance was attached to conducting preceding tests due to the experience gained with the two-fluid blower. This process, too, places heavy demands on process equipment implementation. Many functions were implemented as intended, but the cold tests revealed improvement demand in several respects. Many improvements were implemented, but some items are still open.
The high efficiency of water as a cleaning medium was demonstrated by the two-fluid blower. Owing to the high requirements to be met by water cleaning, a staged development programme should be used for future developments. Moreover, the complexity of future equipment should be as low as possible.
39
4.3 Task 3: Platen heating surface
Operational experience with Hambach lignite proved conventional soot blowing of heat exchangers to be ineffective above 900 °C. As a result the fouling and slagging deposits grow and reduce the heat flux. The superheater outlet temperature can occasionally not be achieved and the flue gas temperature increases in the subsequent heating surfaces, leading thus to a continuation of the irreversible slagging/fouling process.
As mentioned in the chapters before, a solution could be the cleaning of the heating surface with water instead of steam. The use of steam blowers in the superheater region is common practice today, but they are not sufficient for changed lignite qualities. Positive and long term experience with water cleaning is available for the furnace walls (tube-fin construction). The furnace is the evaporator and has significant lower steam parameters and therefore other tube materials then the superheaters. Water cleaning devices for the application in the superheater region are in the development phase (e.g. two fluid blower etc.).
Due to lack of experience with the application of water as a cleaning medium for bundle type heat exchangers in the superheater region the following questions arose:
• Behaviour of superheater when frequently cleaned with water (especially connections between superheater tubes and supporting tubes)
• Effect of separate tubes compared with the water wall (tube-fin construction)
• Erosion problems, especially in the area of supporting tube / heating surface tube)
Figure 7-26 shows the conventional design of supporting tubes with uncooled pieces. In the photo further a typical malfunction of the conventional design of the supporting tubes is shown. The reason for the malfunction is cracking as a result of thermal stress caused by cleaning steam.
Different options can be considered to solve the issues related to the water cleaning of the superheater region. The options are as follows:
• Alternative design for the supporting tubes with the objective to avoid uncooled parts (thermal shock)
• Alternative superheater design with the objective to avoid supporting tubes
• Coatings on the superheater and supporting tubes could be a measure against erosion
Alstom Power Systems (APS) has developed an alternative design for the construction of the connection of the supporting tube with the heating surface as well as an alternative design for the first two superheater stages as a platen superheater. Due to the high steam temperature of modern supercritical boilers austenitic materials are necessary for the superheater heating surface. Mid-term experience of water cleaning of these austenitic heating surfaces needs to be collected.
In summary three different solutions (TEST RIGs) were proposed, developed and pursued by APS in the course of this project.
• Test Rig A: Tube bundle with cooled supporting tubes made out of one piece with the main heating surface
• Test Rig B: Austenitic superheater panel without any supporting tubes
• Test Rig C: Coated super heater tubes
40
4.3.1 Specification, manufacture and installation
4.3.1.1 Test Rig A
In order to prevent the utilisation of an uncooled supporting tube construction the tube bundle with the cooled supporting tube is made out of one piece with the main heating surface (Figure 4-10). The Test Rig A was designed, constructed and manufactured by APS.
Figure 4-10: Picture of Test Rig A taken in the workshop
The drawing of the Test Rig A produced by APS for the manufacturing and installation is included in the Appendix B.
The tube bundle is designed and manufactured as a “tube-fin” construction and is approximately 3000 mm wide and consist of 24 tubes with a division of 60 mm. The tubes with diameter of 38x4 mm consist of the material 13CrMo44.
The Test Rig A (“tube-fin” design) was connected after the installation with the existing heating surface (conventional tube design). This arrangement allows a direct comparison of the two different designs in terms of cleaning efficiency. In the “tube-fin” design the ash deposit growth on the heating surface is limited between the single tubes by the fin.
The tube bundle was delivered in June 2004 to RWE and was installed during a scheduled outage in June 2005 in the Niederaussem G plant. Since July 2005 the test rig A is in operation.
4.3.1.2 Test Rig B
A prototype of the platen heating surface (“tube-fin” design) was specified and manufactured by APS taking into account for an installation in the superheater region the boundary conditions of the existing 600 MW unit G of lignite fired Niederaußem power plant.
41
For the design of the platen heating surface it was considered that the panel should completely replace the lower half of one of the superheater 3 banks of the Niederaussem G plant and should be self supporting. The dimension of the platen heating surface is therefore 21 m wide and 1.4 m high. The austenitic material TP347HFG was used for the manufacturing of the platen heating surface; the austenitic material was delivered by Sumitomo.
The panel was manufactured in four pieces consisting of 24 tubes with a tube diameter of 38x4 mm (Figure 4-11).
Figure 4-11: Picture of Test Rig B taken in the workshop
The arrangement concept of the super heater panels for a lignite fired boiler is shown in Figure 7-27.
The drawing of the Test Rig B produced by Alstom for the manufacturing and installation is included in the Appendix B.
During the manufacturing of the platen heating surface fundamental experience was gained with respect to the welding procedure of austenitic materials, especially concerning welding additives and welding parameters (Figure 7-28). The manufacturing of the platen heating surfaces was finished in May 2004 and it was delivered in June 2004 to the RWE power plant site Niederaußem. Before delivering to RWE the panel was equipped with several thermocouples in order to determine the effect of water cleaning. In addition the panel was x-rayed and metallurgical tested.
Cancellation of the Test Rig B installation
In September 2004 fouling and slagging tests were performed by RWE and ALSTOM at power plant Niederaußem K with Hambach coal. The test results gave new information on the fouling and slagging behaviour of Hambach coal, especially for the deposit formation in the superheater region where a rapid deposit growth with a high deposit density was experienced. Due to this newly gained experience the
42
installation of the Test Rig B was reconsidered for safety operational reasons and was postponed in a first step.
In order to decide how to proceed with the installation of the platen heating surface, a detailed risk analysis about the platen heating surface installation was performed. A recalculation of the performance with the existing design, considering the recent fouling and slagging experience shows that the allowable deposit thickness is drastically reduced from 14 cm down to 5 cm on each side of the heating surface. Additionally different supporting options for the platen heating surface were examined in order to allow thicker deposit layers on the heating surfaces, but, due to the fact that there is no possibility to lead forces into the surrounding superheater tubes, there is no suitable supporting solution for an installation of a single platen heating surfaces available.
The other way to prevent the platen heating surface from mechanical malfunction is to keep the deposit thickness below 5 cm, but that is not acceptable because the cleaning effort is very high and the operation under such conditions will not result in representative scientific findings for the water cleaning impact.
Based on the risk analysis the power plant operator (RWE) decided from a safety operational point of view not to install the platen heating surface with a connection to the water steam side of the power plant, but offers the possibility as an alternative solution to install an austenitic heating surface with a separate independent cooling system not connected to the water steam side of the power plant. This alternative solution allows minimizing the risk of an unscheduled shut down of the power plant by obtaining maximum knowledge on the required operational and cleaning behaviour as specified within the project.
The implementation of the alternative solution is described in the following chapter.
4.3.1.3 Test Rig B’
To solve the problems that had become apparent with regard to the installation of an unsupported platen element, a test rig was designed and mounted at the Niederaußem power plant. The purpose of the test rig is to provide us with the means to conduct experiments to investigate the effects of soot blowers on boiler tubes.
The basic idea of the test rig is to reproduce the conditions prevailing in boilers as realistically as possible. Thus, it seemed reasonable to install the test rig at a utility boiler in the actual temperature and ambient conditions of the plants. A crucial prerequisite was that the operation and, hence, the availability of the plant were not jeopardized. In addition, there had to be suitable devices for monitoring the heating surfaces and it had to be possible to employ the blower in a flexible manner independent of plant operation.
Taking these aims and boundary conditions into account, the Test Rig B’ was designed.
Concept and structure of the test rig
Based on the above idea, a test rig consisting of the key components "test heating surface" and "test soot blower" was designed and installed at unit F of the Niederaußem power plant. It had the following features:
• Installation in the lignite-fired steam generator, i.e. under realistic conditions in terms of flue gas atmosphere and fouling
43
• Installation at the furnace outlet, i.e. under realistic conditions in terms of thermal load acting on final superheaters
• Cooling of the test heating surfaces by means of air, i.e. no influence is exerted on the water/steam cycle of the unit and, as a result, operations are not jeopardized by the failure of a test heating surface
• Installation of three heating surfaces connected in parallel; i.e. three tests differing in materials, dimensions and temperatures can be carried out in parallel
In a first development step, the heating surfaces were arranged as air-cooled chorded tubes by analogy with previous installations in a pilot combustion plant (Figure 7-54 and Figure 7-55). However, thermal loads at the place of installation, i.e. the furnace outlet of unit F, were so high that they caused the test heating surfaces to overheat and fail, so that the concept had to be revised.
In a second development step it was modified, so that the main cooling effect now comes from a water-cooled carrier; only the actual probe body with a length of some 50 cm continues to be cooled by air. (Figure 7-56).
The test blower with a penetration depth of some 1.5 m, corresponding to the length of the test heating surfaces, was designed right from the start as a combined steam and water blower with interchangeable nozzles and has remained unchanged.
4.3.1.4 Test Rig C
The Test Rig C is equipped with uncoated tubes and with coated tubes. The uncoated tubes are used as reference tubes. The coating is applied to superheater tubes as well as to supporting tubes.
APS ordered the coating of nine superheater tubes and four supporting tubes by the company Häuser (plasma coating) and the coating of 5 superheater tubes by the company Steinführer (wire flame spraying). Details about the different coating procedures are given in the sections below.
As a basis material for the superheater tubes 13 CrMo 44 was selected, the superheater tubes (diameter 38x4) considered for the coating have a length of 3000mm and the coating is performed on a length of 2900mm.
The drawing of the Test Rig C produced by Alstom for the manufacturing and installation is included in the Appendix B.
The Test rig C was delivered to RWE in June 2004 and was installed in June 2005 in Niederaußem G. Since July 2005 the test rig C is in operation.
Plasma Coating (Häuser Company)
In the front casing part of the plasma gun (Figure 7-29), an electric arc stabilised by the supply of gas is ignited between a tungsten cathode and a nozzle-shaped copper anode surrounding it. The gas fed in via an injector and heated up in the process is argon to which, for processing special materials, helium, hydrogen and nitrogen can be added. If argon is used as a plasma gas, the heating of the gas brings about ionisation processes producing an electrically conductive gas known as plasma. The ionisation process brings about an increase in the volume of the gas of about one hundred times and, as a result of the given geometry of the nozzle-shaped copper anode, the plasma jet reaches velocities of up to twice the speed of sound. The temperature of the plasma jet, if argon is used, is about 18000 degrees Centigrade.
44
To ensure that the plasma gun is not destroyed at this high temperature, intensive cooling with 25 litres of water per minute at a pressure of 18 bar is necessary. The water is supplied via the front casing part and fed back to the heat exchanger via the cathode holder located in the central casing part.
The powdered material to be applied is fed into the plasma leaving the gun as a jet axially via an argon gas flow, is melted by it and accelerated to the surface to be coated. The plasma recombines directly after leaving the gun and passes into the surrounding atmosphere as normal gaseous argon.
Wire Flame Spraying (Steinführer Company)
The wire flame spraying (Figure 7-30), is a spray process in which the feed stock is in wire or rod form (2-3 mm diameter).
The metal wire is molten in a combustion gas mixture of acetylene and oxygen or propane and oxygen. A stream of cleaned and oil free compressed air surrounding the flame atomized and propelled the liquefied metal. Process continuation depended on feeding the wire at a controllable rate so it melted and was propelled in a continuous stream. The annular compressed air flow accelerates the particles up to supersonic velocity towards the substrate. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating. This process carried out correctly is called a "cold process" (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material
The total thickness of the flame sprayed coat should be approximately in between 600 – 800 µm and consists of three different layers.
4.3.1.5 Evaluation of super heater design Test Rig B
The concept of Test Rig B accommodates the cleaning of the surfaces by water blowers or dual blowers (water and steam) instead of the conventional steam blowers, which is the common practice today. In this way a more efficient cleaning would be possible, especially in the super heater sections, in high temperature regimes above 900˚C, where slagging and fouling processes disturb the normal operation of the power plants. To achieve a deeper knowledge and in order to evaluate the design of a new super heater section for a Greek power plant, a one dimensional calculation methodology is developed in cooperation with PPC’s subcontractor NTUA.
The original data is taken from the section 3 superheater of PPC’s Amynteon power plant. For the reference case the total heat transfer coefficient is initially calculated and compared with the data obtained from the thermal balance of the super heater. In the next step new formulations for the tube-fin-tube panel construction are proposed. The new expressions take into account the effect of the welded fins in the heat transfer. By adding the fins the total surface area of the super heater and the heat transfer coefficient increases. Thus, in order to keep constant the total heat flux of the super heater, the distance between the tubes or the number of banks or lanes (Figure 4-12 decreases resulting to material savings. Goal of these calculations is to provide a quantitative estimation on the potential of these savings and to propose initial values for the new design parameters - height, width of the fins (Figure 4-13).
45
The validation of the 1D calculation results by CFD analysis is afterwards performed. One bank of the investigated superheater is simulated in 2D and the flow, thermal field and total heat transfer to the heat exchanger are calculated for the reference and the platen heat exchanger concept. The obtained results are in good agreement with the predictions of the one dimensional calculations.
Figure 4-12: vertical cross section of a heat exchanger
Figure 4-13: vertical cross section of the tube-fin-tube concept
Width of fin (t)
• •
• • • • • • • •
N - banks
Distance between tubes
• •
Σ-lanes
• •
• •
flue gas • •
• •
• •
• •
• •
• •
• •
• •
• •
• •
Height of fin / distance between the tubes (Lw)
Methodology of the 1D analysis
Calculation of the existing superheater design
The calculation of the heat transfer coefficient for the super heater is based on the determination of the overall thermal resistances taking into account the convection in the inner and outer side of the tubes and the conduction through the tube walls. The total heat transfer coefficient U is given by equation 1, where hc is the convection coefficient for the inner tube surface, Rw the conduction resistance and hout the convection coefficient describing the flue gas flow through the tube bundles.
inin
outWout
out AhA
RAh
U
⋅+⋅+
=1
1
Eq. 1
The convection coefficient for the inner part of the tubes, hc is obtained by a special Nusselt formulation (Eq. 2, 3)
DNuhin
λ⋅=
and
Eq. 2
Eq. 3
46
⎟⎟⎠
⎞⎜⎜⎝
⎛−⋅⎟
⎠⎞
⎜⎝⎛⋅+
⋅⋅=
1Pr8
7.1207.1
PrRe8
325.0f
fNu
and Rw is calculated by the thermal resistances of all the different steels applied in the specific super heater.
4321
11111
WWWW
W
RRRR
R+++
= Eq. 4
In order to calculate hout, the laminar and turbulent Nusselt numbers and the overall Nusselt number for one tube are calculated (Equations 5-7).
3
max PrRe664.0 glamNu ⋅⋅=
⎟⎟⎠
⎞⎜⎜⎝
⎛−⋅⋅+
⋅⋅=
− 1PrRe443.21
PrRe037.0
32
1.0max
8.0max
g
gturbNu
3.022 ++= turblam NuNuNu
Eq. 5
Eq. 6
Eq. 7
The overall Nusselt number for the tube bundle is then obtained by Equation 9 after taking into account the geometrical parameters of the bundle.
DNuh bundleout
λ⋅=
( ) 25.0
PrPr11
⎟⎟⎠
⎞⎜⎜⎝
⎛⋅⋅
Σ⋅−Σ+
=gsgNufNu A
bundle
Σ: number of banks
Af : form factor depending on the heat exchangers geometry
Eq. 8
Eq. 9
The obtained result is compared with value obtained from the alternative calculation based on the thermal balance and the measured heat flux from the specific super heater (Equation 10). The discrepancy between the two methods is lower than 1% indicating the high precision of the applied methodology.
lmAQ
Uout
SH
ΔΤ⋅= Eq. 10
47
Prediction of the effect of ash deposition on the heat exchangers performance
The 1D model of the conventional heat exchange concept was further extended in order to simulate the effect of ash deposits on the total heat transfer coefficient and the total heat flux of the reference heat exchanger. The two main parameters regarded were the width of the deposits and their thermal conduction coefficient k. Following ranges were taken into account, which are representative according to the literature:
• width of the deposits: 0,1mm - 5mm
• thermal conduction coefficient k: 0 - 2 W/mK.
The results are presented in the Figures 10 and 11. A high decrease of the initial total heat transfer coefficient (U=57 W/mK) is noticed, when the deposition thickness is increased or when the thermal conductivity is decreased. The same behaviour was also observed for the total heat flux. The presented plots indicate in conclusion the effect of the two parameters on the efficiency of the heat exchanger.
The platen heat exchanger concept
In order to optimize the geometry of the new platen heat exchanger concept proposed by Alstom, two geometry parameters are optimized, the height (Lw) and the width of the fin (t). In the calculation of the new concept a formulation for the efficiency rate for the fins is included in the given equations above, which incorporates their effect on the heat transfer. For the new geometry the optimum values of Lw and t are calculated. The constraint in this optimization procedure is the total heat flux of the heat exchanger, which has to remain constant, equal with the one of the reference case (Equation 10). The total heat flux of the superheater is however expected to increase by adding the fins, because of the increased surface area and the increased convection coefficient. The required heat flux can therefore be achieved with a decreased number of banks or lanes, which enables the reduction of the number of banks or lanes in the superheater section. In order to keep the flue gas velocity through the superheater section constant, it was decided not to change the number of banks and to decrease accordingly the number of lanes. The possible material savings of the new platen heat exchanger as well as its new dimensions can be then estimated.
Results of the 1D calculation
The investigated cases include following number of lanes: 16 (reference case), 14, 12, 10 and 8. The second parameter taken into account was the width of the fins. Five different values were tested ranging from 1mm to 5mm, although the favourable fin thickness from the construction point of view is 2 to 3 mm. The optimum fin length was calculated for each pair, and furthermore the total height and weight of the heat exchanger was determined. The main results are concluded in the following:
• For a moderate decrease of the total number of lanes (16, 14, 12) the solution is independent from the width of the fins (region A), meaning that the width can be freely chosen according to structural parameters (Figure 7-33)
• By decreasing further the number of lanes (10, 8) the necessary length of the fin becomes unrealistic (existing distance between lanes: 0,08m)
48
• Regarding the calculated total height of the new superheater a substantial decrease (present value 1.958m) can be achieved even by a moderate decrease of the total number of banks or lanes. (Figure 7-34)
• Regarding the calculated total weight the opposite picture compared to the previous results can be found. A low decrease of the number of lanes (16 or 14) leads to a higher total weight than the one of the reference case, which is from structural reasons not acceptable. The region B is therefore preferable, where the number of lanes is decreased to 12, 10 or 8. (Figure 7-35)
• The optimum solution is finally the intersection between regions A, B leading to an optimum number of lanes between 10 and 12.
Another parameter taken also into account is the efficiency of the fins, which is a function of their geometric data (length, width). The efficiency of one fin and the overall fin efficiency of the tube bundle are calculated. A strong dependence from the geometry is found concluding to the following results (Figure 7-36 and Figure 7-37)
• The optimum fins have design parameters in the Region C: low height Lw (<10mm) and increased width (t>3mm)
• The efficiency decreases drastically when the design parameters are chosen outside from the region C
• The overall efficiency of the fins referred to the whole tube bundle n0 is higher than the efficiency of one fin nf
Summing up, a one dimensional analysis tool was developed in cooperation with the subcontractor NTUA, in order to analyse and evaluate the possible effect of the tube – fin – tube concept on the performance of a superheater section. Geometrical and operational data was obtained from the section 3 superheater of PPC’s Amyntaion power plant. The predictions of the model found to be in good agreement with the design data of the super heater. Different configurations are furthermore investigated and the optimum solution meeting the geometrical and thermodynamical constraints is found. The developed methodology can therefore be used as a time and cost effective analysis tool, which can propose initial layouts for new heat exchanger concepts. A final decision on a new heat exchanger design has then to be taken after taking into account a number of additional parameters, like data regarding the applied materials and structure (ability of welding, materials’ strength) and other operational parameters as deposition tendency, existing cleaning concepts.
Methodology of the CFD analysis
In order to evaluate further the effect of the fins on the heat exchanger’s efficiency and thermal behaviour the problem was investigated also by CFD analysis. More specific, the validation case based on the geometric data of the examined superheater was set up. A commercial CFD code is used for the solution of the coupled fluid dynamics – heat transfer problem. The calculations are carried out in a 2D domain, which includes one bank out of the 60. In Figure 7-38, Figure 7-39 and Figure 7-40 different versions of the initial numerical grids are presented. Periodic boundary conditions are applied on the left and right side of the computational domain. Flue gas inlet temperature and volume flow and tubes’ wall surface are the boundary conditions to be defined. The coupled fluid dynamics-heat transfer problem is solved without taking into account the effect of particles and the total heat flux of the superheater is calculated in a post processing step. An alternative method for the heat exchanger simulation is to solve the coupled problem by applying heat flux boundary conditions on the wall.
49
Furthermore, in order to assure grid independence different numerical grids ranging from 25.000 up to 230.000 computational cells have been examined.
Results of the CFD analysis
Grid dependency
The result of the comparison between different grid types, grid sizes and boundary conditions is shown in Figure 7-41. The relative difference between the heat fluxes simulated by CFD and the reference heat flux calculated from the 1D model value is given for comparison. The main comments are concluded in the following:
• For numerical grids with more than 100.000 cells the solutions become grid independent
• The triangular grids show better behaviour than the quadrilateral ones, by getting solutions closer to the reference.
• The direct use of the mass flow as a boundary condition did not improve the accuracy of the results, probably due to the specific flue gas density calculation approach used in Fluent.
• The second order discretization schemes examined did not provide more accurate solutions compared with the first order discretization schemes.
Evaluation of the reference superheater – comparison with the design values
As a next step the flue gas inlet temperature is varied in four different values ranging from 630 up to 900 ˚C. For the nominal operating flue gas temperature of 838˚C the calculated total heat flux was found to be in good agreement with the designed data of the superheater. The results are presented in Figure 7-42. Based on the calculated points a characteristic curve can be obtained from linear regression analysis, which estimates the total heat exchanger heat flux under different inlet temperature conditions. Similar results are obtained by varying the tube surface temperature, while a change in the flue gas composition does not appear to have a major effect on the calculated heat flux.
Evaluation of the platen superheater – comparison with the design values
In order to simulate the platen heat exchanger case four different designs are taken into consideration. The designs are chosen according to the predictions of the 1D model, which have to be validated.
• 12 lanes concept, 38 mm distance between the tubes (= fin length), 3mm fin width
• 12 lanes concept, 38 mm distance between the tubes (= fin length), 5mm fin width
• 16 lanes concept, 16 mm distance between the tubes (= fin length), 3mm fin width
• 16 lanes concept, 16 mm distance between the tubes (= fin length), 5mm fin width
The numerical meshes for the different designs are given in Figure 7-43 and Figure 7-44. The results of the simulations are presented in Figure 7-45. They seem once again to be in good agreement with the predictions of the 1D model. A higher deviation is shown for the cases of the thicker fins, which may be justified by the changes in the flow field caused by the different geometry.
50
Comparison between the superheater concepts
In this part the comparison of the results between the two concepts is presented. As mentioned previously two different boundary condition types are used for the scope of the simulations. In the first case heat flux boundary conditions are applied on the tube walls and the tube surface temperature is obtained by the CFD simulations, whereas in the second case the wall temperature is determined and the heat flux is calculated at the end as a post processing step. The main results of the first case indicate an almost constant distribution of the tube surface temperature with small variations, less than 10 degrees, which remains also during the simulations with the fins.
The overall results of the second case for given surface temperature 538˚C are presented in Figure 7-46 and Figure 7-47. In Figure 7-46 the difference in the heat flux distribution between the reference and the two platen heat exchanger concepts is shown, while in Figure 7-47 the comparison between the heat flux on tube and fin surfaces is given. Following observations are included in the results:
• The placing of fins in the heat exchanger’s structure leads to a smoother heat flux distribution across the height of the heat exchanger.
• By using fins the heat flux to the tubes decreases. Therefore, in order to calculate the overall heat flux to the heat exchanger the flux obtained by the fins has to be also taken into account.
• It is possible, as calculated in the 1D model, to decrease the number of tubes and keep constant the total heat to the heat exchanger. In this way the overall heat flux per tube increases.
• The thickness of the fins seems to influence the heat transfer to a small extend. More specific thicker fins seem to show decreased heat transfer coefficient and therefore the total heat transfer slightly decreases.
It has to be also noticed that the scope of these specific calculations is to show some trends, by simulating the heat transfer problem using two opposite types of boundary conditions. In the reality neither the temperature nor the heat flux is constant along the tube perimeter, but follows a specific distribution according to the local heat transfer parameters on each tube of the heat exchanger. Due to the difficulty, however to simulate the steam flow inside the tubes for the whole geometry of the superheater and to couple in this way the heat transfer problem between the steam and the flue gas side, it was decided to keep constant the values of heat flux and temperature along the heat exchanger’s height and examine the effect of the fins on heat transfer through specific parametric investigations. Finally in the Figure 7-48, Figure 7-49 and Figure 7-50 contours of temperature, velocity and kinetic energy are presented for the reference case superheater and two platen heat exchanger concepts.
4.3.2 Trial of platen heating surfaces
4.3.2.1 Test Rig A and C
The Test Rigs A and C are in operation since July 2005 in the superheater region of the Niederaussem Unit G. The exact installation locations of the Test Rigs in the superheater region are shown in the corresponding drawings, see Appendix.
The Test Rigs are installed with respect to the location very close together so that the operational conditions in terms of flue gas composition, flue gas velocity, flue gas temperature, fly ash particle load and the heating surface cleaning are quite similar.
During the operation several visual inspections of the Test Rig’s took place. Table 4 gives an indication about the number of performed inspections and the inspection dates.
51
The sum of operating hours of the Test rig A and C at the end of the project reporting period were in total approx. 21.000 h.
2005 2006 2007 2008 Test Rig A and C Operation
Test Rig A and C Inspection
Test Rig A Dismantling
Test Rig C Dismantling (partly)
Table 4: Overview about operation and inspection of the test rigs A and C
A test program was drafted by Alstom and finally agreed between the partners, but the original foreseen test program with respect to water cleaning of the test rigs could not be applied due to technical problems with the water blower.
In the reporting period the test rig cleaning from deposits took place with the soot blowing equipment (Table 5). The frequency of soot blowing in the test rigs region was in average 5 times a day.
2005 2006 2007 2008 Test Rig A and C Steam Blowing
Test Rig A and C Water Blowing
Not performed
Table 5: Overview about cleaning device utilisation
For further observation of the heating surfaces a furnace camera was used to obtain impressions about the ash deposit growth and the effectiveness of removing ash deposits from the heating surfaces.
Test Rig A and C Inspections
As shown in Table 4 several inspections of the installed heating surfaces took place during the reporting period. The performed inspections were used for a visual check of the conditions of the installed heating surfaces with respect to incidents caused by erosion and thermal shock.
During two inspections the conditions of the different Test Rigs were documented with photos. For this purpose the boiler and therefore the heating surfaces of the Test Rigs were lavishly cleaned for a safe access to the boiler and a better possibility to check the test heating surfaces. With the cleaning device utilised for inspection the built-up layer could easily be removed, but the initial layer with a red colour is heavily attached to the heating surface and could not be removed.
Test Rig A
The examination during the different inspections shows that the Test Rig A is in very good conditions and performed very well. Figure 7-51 shows a picture of the Test Rig A taken during an inspection. As already mentioned during the project duration only the application of the conventional sootblowing cleaning device took place no water cleaning was performed, therefore, as expected, the inspections of the test rig A went without incidents.
52
Due to the very good conditions and due to the fact that no water cleaning was performed it was decided that the Test Rig A will not be dismantled and will be further operated in order to collect additional long term experience with the developed design of cooled supporting tubes.
Test Rig C
The Test Rig C consists of three different kinds of tube surfaces, uncoated tubes as reference tubes, plasma coated tubes (Häusser) and wire flame spray coated tubes (Steinführer). The coatings were considered as a tube protection measure against erosion that occurs during the cleaning of the heating surfaces with steam or water as cleaning medium.
In Figure 7-52 the arrangement of the different tubes of the test rig C are shown, a more closer view of parts of coated tubes are given in Figure 7-53. Beside the attached ash deposit layers the shiny metal surface of the coated tubes can be seen.
The visual inspection of the Test Rig C allows only a general statement about the conditions of the different tube surfaces.
It is not clear whether the coating on the different tube surfaces is damaged or not. A detailed analysis after dismantling of tube samples were performed at the end of the project. The summary of the results from the analysis is given in the next section.
Analysis of coated tube samples
The tubes of the test rig used for the analysis were exposed to the superheater region (SH3) of the Niederaussem G unit for approximately 21.000 h at steam parameters of about 480°C and 170 bar. For the removal of ash deposits from the heating surfaces the installed steam blower equipment was applied.
The base material for all the tubes in the test rig C is 13CrMo4-4 (Material-No.1.7355)
The following tube samples were cut out for the detailed analysis:
• Tube section, tube No. 5 from below, between the 4th and 5th supporting tube row Ø 38,0x4,0 mm, length of 3,0m, with coating from Häuser company
• Tube section, tube No. 12 from below, between the 4th and 5th supporting tube row Ø 38,0x4,0 mm, length of 3,0m, with coating from Steinführer company
• Tube section, tube No. 17 from below, between the 4th and 5th supporting tube row Ø 38,0x4,0 mm, length of 3,0m, without coating
As an example Figure 4-14 shows the photo documentation of the tube No. 5 when it was delivered to the laboratory. The same photo documentation was performed for the tube No. 12 and 17.
53
Figure 4-14: Photo documentation of the tube 5
The tube samples were analysed with an optical microscope and a scanning electron microscope with microprobe for the element analysis (semi quantitative).
Tube No. 5 Häuser coating
The analysis of the tube No. 5 shows that in the intermediate section of the tube the coating was heavily damaged, only about 30% of the tube surface area covered by the original coating is left. For the coating on the tube surface of the other sections a thickness of about 300 up to 600 μm was detected, the coating in these areas is well attached to the base material.
Tube No. 12 Steinführer coating
The analysis of the tube No. 12 gave the same results, the intermediate section was heavily damaged the other tube sections are still covered with the coating. The coating from Häuser Company compared to Steinführer coating seems to be in slightly better conditions.
Tube No. 17 without coating
The reference tube without coating confirmed that the intermediate section of the tube is more effected by the steam blower than the other tube sections, the wall thickness in the intermediate section shows the lowest values.
4.3.2.2 Test Rig B’
The operation of the test rig helped us gain experience with regard to the installation situation at the furnace outlet. It is characterized by high external loads in the form of thermal stress and the flue gas environment.
54
• Very high heat input into the test heating surfaces. This observation led to the adjustment to the concept (see chapter 5.3.1.3).
• Unreliable cooling of the probe carrier by cooling water from the units' cooling water system. In connection with maintenance measures executed in this system, damage occurred at the test heating surfaces and caused the tests to be stopped. Remedial action was taken by stepped-up organizational efforts.
After these problems were eliminated, the availability of the test rigs was good and allowed the test to be executed. (Figure 7-57)
Another consequence of the severe heat exposure of the test heating surfaces was detected in the course of the tests and their evaluation: There are great local differences in the test probe's material temperature. This meant that only a relatively small tube section each could be used as a sample for evaluation in the material investigations. In addition, it became clear that temperature control was too slow during water blowing on the test heating surfaces, so that short-term excess temperatures were reached.
Ash deposits dropping from superheater 6 that is arranged above the test rig did not impair test operations but calls for increased attention and can require interim manual cleaning of the test heating surface.
Results
During the 2007 test period, a total of 14 material samples were used over different spans of time. These 14 samples included 5 so-called reference samples that were exposed to identical conditions, not in the test rig, however, but in its immediate vicinity. They differ from the actual test probes in that they are not cleaned by soot blowers, so that no effects caused by water or steam should show. The overall results of the investigations can be broken down by the material used:
Austenitic materials
The tests included the materials DMV 310N (51 x 4.4mm) and Super 304H that, with a Cr content of 25% and 17% resp., are used in the final superheater stages of modern utility steam generators. All 7 samples tested were exposed to a wall temperature of 580°C. After a maximum of 46 days, they did not reveal any problems at all.
• As expected, the microstructure of all samples used corresponded to the structure of the zero samples that had not been installed.
• The structure showed no signs of thermal shock cracks.
• No wall thickness erosion was noted.
• Oxide layer thickness varied locally between 10 and 24 μm, with no difference between the blower impact area and the rest of the tube.
Two conclusions can be drawn from these investigation results: Firstly, all investigations show that the samples were used as intended and that they did not overheat, in particular. This means that the test rig is suitable for trialling this material. Secondly, the tested samples show no signs of damage caused by blower action. A robust statement on long-term behaviour, however, requires the samples to be used over a longer period of time.
55
Low-alloy, ferritic material
The material used was a 44.8 x 4 mm test sample of the widely used 10CrMo910. All of the five samples showed signs of thermal overload in the form of serious changes in the microstructure, in particular. Moreover, after only 28 days of operation, one sample showed significant wall thickness reduction that was found to roughly the same extent both on the blower side and the side that was not exposed to the blower. This reduction in wall thickness was accompanied by external oxide layers of up to 800 μm on the outside tube wall and oxide layer thicknesses of up to 250 μm on the inside tube wall. As a result, all indicators suggest that the tube samples were exposed to massive thermal overload.
When looking into the causes of this thickness reduction, we found that overheating probably took place at the beginning of the various tests. Heat absorption by the still completely unsoiled tube was highest at this point of time and cooling obviously insufficient. Cooling must therefore be improved for further tests of this material. No statement can be made on the blower's effect due to the untypical test conditions. Similar to the conclusions derived from the austenite material trials, the conclusion drawn here is that the test periods of this material should also be significantly extended in subsequent trials.
Tube shield material
To test shield materials, another two samples were deposited on the tubes as in normal boiler operation. Since this material could not be cooled to the same extent as the subjacent tube materials it was actually exposed to higher temperature than recorded in the measurements. The materials used included Sicromal 1.4713 containing 7% Cr that was exposed to 530°C and Sicromal 1.4742 containing 18% Cr exposed to 580°C. The structural examinations revealed that the low-Cr material is not suitable for the selected place of installation. The higher-Cr material 1.4743, by contrast, showed no changes in microstructure, so that we can assume that it was used as intended.
The results of the other material investigations also indicated that no blower effects, i.e. no irregularities in protective layer thickness and no thermal shock cracks, were visible after 25 days.
4.3.3 Summary, analysis and evaluation of the results and assessment of the consequences in view of future applications
In Task 3 different more resistant superheater designs, suitable for cleaning on water basis, were specified, manufactured, installed and tested in a 600 MW- and a 300 MW unit of Niederaußem power plant.
The Test Rig A is a newly developed design for supporting tubes. The design considers cooled supporting tubes. The cooled supporting tube surface will lower the thermal stress of the material compared to the conventional design in case water cleaning is applied.
The cooled supporting tube is made out of one piece with the main heating surface. The test rig A was designed, constructed and manufactured by APS. It was installed in July 2005 in the Niederaussem G unit.
The sum of operating hours of the test rig A at the end of the project reporting period was in total approx. 21.000 h.
56
During the Test Rig A operation in the period from July ’05 until Dec ’07 no water cleaning was performed due to the fact that the installation and operation of the water cleaning device was not successful. Only the conventional sootblowing technology with steam was applied for cleaning the heating surface of test rig A.
The visual inspections results in the conclusion that the Test rig A is in very good conditions, there are no incidents in terms of erosion and thermal shock that could be related to the general operation or especially to the cleaning procedure with conventional sootblowing.
The application of the conventional sootblowing procedure at the Test Rig A (“tube-fin-tube” design) compared to the conventional tube design observed with the installed furnace camera results in the conclusion, that there are no remarkable differences between the two designs in terms of cleaning efficiency by applying conventional sootblowing technology.
The Test Rig A is still in operation and will stay in operation during the coming months and years and will be further observed in order to collect additional operational experience.
The Test Rig B was developed for the validation of the super heater concept that considers an austenitic platen heating surface “tube-fin-tube” design as super heater panels.
The platen heating surface design is based on a self supporting concept. No conventional hanger supporting tubes are required, the panels are supporting each other by spacing web. The occurred forces can be absorbed by the walls. This design avoids supporting tubes, and hence the critical connection between superheater tube and supporting tube. Furthermore this design is intended to be easier to clean than designs with separate tubes. An austenitic material was chosen for the heat exchanger to gain experience with this material under water cleaning conditions.
The Test Rig B is a prototype of the platen heating surface (“tube-fin-tube” design) and was specified and manufactured by APS taking into account the boundary conditions of the existing 600 MW lignite fired power plant Niederaussem Unit G.
Useful experience was gained during the manufacturing of the austenitic platen heating surfaces with respect to the welding procedure and the selection of appropriate filler material.
Based on a risk analysis the power plant operator (RWE) decided from a safety operational point of view not to install the platen heating surface with a connection to the water steam side of the power plant.
Instead an alternative solution was proposed. All partners involved agreed to the changes with Amendment No.3.
Knowledge about the differences in the cleaning behaviour between of tube-fin design and a conventional design with separate tubes was achieved with Test Rig A, which was in operation for 21.000 hours and was frequently cleaned with sootblowers. This design showed no advantages with respect to cleaning behaviour compared with the conventional design.
Knowledge about the material behaviour of austenitic tubes was gained with the additional Test Rig B’.
The alternative solution for Test Rig B, called Test Rig B’, consists of heat exchanger tubes, which are not connected with the water/steam system of the boiler, but are cooled independently by water and air. These heat exchanger tubes are installed in the superheater region of a 300 MW unit. The tubes are cleaned by an additional cleaning facility, able to clean with steam and water. Both the heat exchanger and the cleaning facility can be installed and uninstalled during boiler operation. In this way the ambient conditions at the furnace exit of a 300MW boiler can be used with a maximum of independence from the boiler operation and therefore good flexibility for test operation.
57
Austenitic as well as ferritic material tested with the Test Rig B’. A total of 14 material samples were used over different spans of time. The tested austenitic samples show no signs of damage caused by blower action. A robust statement on long-term behaviour, however, requires the samples to be used over a longer period of time. The samples of ferritic material showed serious changes in the microstructure. The reason was that the samples were exposed to massive thermal overload in the beginning of the test due to insufficient cooling. No statement can be made on the blower's effect due to the untypical test conditions. Similar to the conclusions derived from the austenite material trials, the conclusion drawn here is that the test periods of this material should also be extended in subsequent trials.
The results obtained with Test Rig B’ were subject of a meeting of material experts of the project partners. The tests conducted and the findings achieved were discussed in detail. Testing with Test Rig B’ will be continued. A test programme was discussed.
The design proposed in Test Rig B was also numerically evaluated by PPC and its subcontractor NTUA. A one dimensional calculation tool is developed, which calculates the total super heater heat flux based on the flue gas temperature and its specific geometry. For validation purposes design data of the superheater III section of PPC’s Amyntaion power plant is used. The effect of the fins in a tube - fin - tube heat exchanger concept is simulated and accordingly optimized finned superheater designs are calculated. The new designs achieve material savings by keeping the heat flux constant.
The Test Rig C consists of three different kinds of tube surfaces, uncoated tubes as reference tubes, plasma coated tubes (Häusser) and wire flame spray coated tubes (Steinführer). The tubes of the test rig used for the analysis were exposed in the superheater region (SH3) of the Niederaussem G unit for approximately 17.000 h at steam parameters of about 480°C and 170 bar. For the removal of ash deposits from the heating surfaces the installed steam blower equipment was applied.
The base material for all the tubes in the Test Rig C is 13CrMo4-4 (Material-No.1.7355)
All the tubes, references tubes as well as coated tubes, were affected by steam blowing, especially in the intermediate section of the tubes
As a concluding remark for the approach with coatings it could be mentioned, that the coatings were already damaged by applying the conventional steam blowing. Due to the fact that the conditions at the tube surface are harsher with respect to thermal stress, erosion and corrosion by applying water cleaning compared to steam cleaning, it is expected that in case of applying water as cleaning medium, coating is not a sufficient measure to protect the tubes against erosion and corrosion.
58
5 Conclusions
Within the scope of the Ligpower project alternative cleaning methods were selected and subjected to extensive testing on 600MW and 300 MW commercial utility boilers. Since a more intensive cleaning of superheater tubes involves increased stress of the tube material, we developed new, more resistant superheater designs and installed these in a 600MW unit. The transferability of the findings made was ensured by a comparison of the lignites used. To permit the cleaning device to be controlled as needed, we developed a programme that analyzes the degree of fouling of the boiler and generates suggestions for its cleaning. In addition, a tube-fin superheater design was calculated and optimized in terms of fluid dynamics.
The extensive tests showed which cleaning methods have suitable approaches to cleaning. By testing them over several years, we gained important information about equipment design. The superheater design could be tested only to a limited degree. Thus, we obtained additional findings in tests using a test heating surface that was mounted on a 300MW boiler.
The testing of the shock wave cleaning facility led to a clear result. It was installed and tested on a 300 MW unit. Although cleaning intensity was continuously increased in the course of the trials, no cleaning effect was observed. For this reason we came to the conclusion that this method is not appropriate for the cleaning of fouling and slagging deposits in lignite fired power plants.
A further possibility to clean fouled boiler areas during operation is seen in the use of explosive charges. The particular difficulty lies in introducing explosive charges into the intended area during ongoing operations and igniting them there. In a 300 MW unit, explosive charges were introduced by a cooled, slewable device into the boiler and ignited there. Handling this equipment proved to be extremely difficult, and the boiler area reached was very limited. Since, in addition, no cleaning effect was visible, the decision was taken to stop pursuing this line of development.
The cleaning methods of the Two Fluid Blower and the High Pressure Water Jet use water as a cleaning medium. The very high cleaning efficiency of water was proven with the Two Fluid Blower. The use of water bears the risk of damaging heat exchanger. For this reason special areas in the superheaters were not to be cleaned with water but only with steam to minimize the risk for the commercial operated utility boiler. This led to high demands in the controls of the facilities using water as a cleaning medium. The Two Fluid Blower demonstrated that the demanded precise water cleaning is possible.
But the high demands of working in ambient conditions of a particle loaded flue gas with temperatures between 1100 and 1300°C and large geometric sizes of 10m to reach the boiler center, in combination with the high demands on the controls for a precise cleaning resulted in completely new, very complex cleaning facilities. The structural complexity of these cleaning systems caused a lot of problems. This big development step seemed feasible, bit it became clear that a staged procedure could have had advantages. Moreover, the complexity of future equipment should be as low as possible to withstand the harsh conditions of a commercial power plant.
In order to avoid damage to the heat exchanger surfaces, the cleaning facilities should be used as required. A programme was developed by npower to calculate the fouling condition of the individual heating surfaces on the basis of current boiler data and derive proposals from the fouling condition for the use of cleaning facilities. The programme was successful installed at Tilbury power station. Experts of the project partners discussed the model with respect to the calculatory bases, data acquisition and data management, user interface, application to control sootblowers and the implemented cleaning strategies. The discussion made sure that the best strategies are implemented and cleaning is executed
59
when needed. The model has to be fine tuned for different fuel qualities. Its operation will be continued in Tilbury power station.
All cleaning facilities investigated within the scope of the project were installed in the Niederaussem power station. This plant is fired by lignite from the Rhenish mining area. To ensure that results obtained in this power plant can be transferred to other power plants where Greek lignite is combusted, a comparative evaluation of various lignites was made. One main result of the investigation was that the application of the investigated cleaning concepts and superheater designs in Greek power plants seems to be feasible.
Different new superheater concepts were investigated.
A new tube-fin design was proposed. One of the advantages of this concept is, that the fins avoid the deposits to grow together between the tubes like this happens in conventional superheaters with separate tubes. Thus the cleaning of the superheater is expected to be easier. Test Rig A is a tube-fin superheater. The expected easier cleaning of the design can not be confirmed.
Test Rig A was also designed to avoid uncooled parts in the connection between superheater tubes and supporting tubes, because the uncooled parts of a conventional superheater are the first to be damaged by cleaning with water or steam. The test rig was in operation for 21.000 hours. No damages occurred. The periodic cleaning was only done by steam due to problems with the water cleaning facility. The testing of this test rig will be continued.
A self supporting tube-fin superheater panel was designed and manufactured. Useful experience was gained during the manufacturing of the austenitic platen heating surface with respect to the welding procedure and the selection of appropriate filler material.
The tube-fin superheater concept was evaluated and optimized numerically. The effect of the fins in a tube - fin - tube heat exchanger concept is simulated and optimized finned superheater designs are calculated. Material savings are achieved by the new design while keeping the heat flux constant.
With an additional test rig the behaviour of different materials periodicly cleaned with water was investigated. This test rig is installed in a 300MW unit of Niederaußem power plant, but it is not connected to the water/steam side of the boiler. Cooling of the heat exchanger and cleaning of the tubes is independent of the boiler operation and allows a maximum of flexibility while using the ambient conditions of a real utility boiler. The austenitic material showed no damages after the test period. The reliability of the results has to be improved by extending the period of testing. During an expert meeting the partners agreed that this Test Rig is a good solution for the investigation of the effect of water cleaning superheater tubes. The testing will be continued.
A clear result was obtained with test rig C. Heat exchanger tubes were coated by a plasma gun and by wire flame spraying in order to be more resistant against fluid cleaning media. Both coatings did not withstand the regular sootblower cleaning.
60
6 List of figures and tables
6.1 List of figures Figure 3-1: Development of coal quality parameters .............................................................................. 15 Figure 5-1: Erosion in the area of supporting tubes ................................................................................ 20 Figure 5-2: Formation and spalling of protective layer........................................................................... 20 Figure 5-3: Classification of ash based on their major oxides ................................................................ 25 Figure 5-4: Fouling Index (Rf) for Greek and German lignite samples .................................................. 26 Figure 5-5: Two Fluid Blower................................................................................................................. 28 Figure 5-6: Successful cleaning in water blowing mode......................................................................... 29 Figure 5-7: High Pressure Water Jet Cleaning (HWJ) nozzle head ........................................................ 31 Figure 5-8: HWJ test with full scale heating surface............................................................................... 32 Figure 5-9: Sections of advisory front end .............................................................................................. 36 Figure 5-10: Picture of Test Rig A taken in the workshop...................................................................... 41 Figure 5-11: Picture of Test Rig B taken in the workshop...................................................................... 42 Figure 5-12: vertical cross section of a heat exchanger........................................................................... 46 Figure 5-13: vertical cross section of the tube-fin-tube concept ............................................................. 46 Figure 5-14: Photo documentation of the tube 5 ..................................................................................... 54 Figure 8-1: Classification of ash based on their major oxides ................................................................ 64 Figure 8-2: Base to Acid (B/A) Ratio for Greek and German lignite samples........................................ 65 Figure 8-3: Silica to Alumina (S/A) Ratio for Greek and German lignite samples ................................ 66 Figure 8-4: Iron to Calcium (IC) Ratio for Greek and German lignite samples...................................... 67 Figure 8-5: Iron to Dolomite (ID) Ratio for Greek and German lignite samples.................................... 68 Figure 8-6: Dolomite Percentage (DP) for Greek and German lignite samples...................................... 68 Figure 8-7: Fouling Index (Rf) for Greek and German lignite samples .................................................. 70 Figure 8-8: Fouling and slagging deposits on platen heating surface / superheater 3 ............................. 73 Figure 8-9: Thermal shock ...................................................................................................................... 73 Figure 8-10: Two Fluid Blower; Nozzle tests during boiler outage........................................................ 74 Figure 8-11: Pendulum movement of water jet cleaning the heating surface ......................................... 74 Figure 8-12: Nozzle test on test rig ......................................................................................................... 75 Figure 8-13: Optimization of water jet .................................................................................................... 75 Figure 8-14: Damaged Two Fluid Blower .............................................................................................. 76 Figure 8-15: Design for High Pressure Water Jet Cleaning .................................................................... 76 Figure 8-16: structure of shock wave cleaning facility ........................................................................... 77 Figure 8-17: Location of the thermocouples at Tilbury power station.................................................... 77 Figure 8-18: Input screen ........................................................................................................................ 78 Figure 8-19: Screen for iterative process................................................................................................. 78 Figure 8-20: Sootblower advisory front end ........................................................................................... 79 Figure 8-21: Furnace sootblow................................................................................................................ 79 Figure 8-22: longstroke sootblow............................................................................................................ 80 Figure 8-23: Parameters of different fuels............................................................................................... 80 Figure 8-24: Parameters of different fuels II ........................................................................................... 81 Figure 8-25: Fouling increase over time ................................................................................................. 81 Figure 8-26: Cracked uncooled supporting piece from the supporting tube due to thermal stress, only
conventional sootblowing was applied. ........................................................................................... 82 Figure 8-27: Convective part of a lignite fired boiler with super heater surfaces designed as platen
heating surfaces................................................................................................................................ 82 Figure 8-28: Welding attempt with the filler material UTP 6170Co ..................................................... 83 Figure 8-29: Plasma gun.......................................................................................................................... 83 Figure 8-30: Wire flame spraying gun .................................................................................................... 84 Figure 8-31: Total heat transfer coefficient of the heat exchanger Uh (W/mK) ..................................... 84 Figure 8-32: Total heat flux of the heat exchanger Q (MW)................................................................... 85 Figure 8-33: Calculated length of fins for different fin thicknesses (t) and lane numbers (Σ) ................ 85 Figure 8-34: Calculated total superheater height for different fin thicknesses (t) and lane numbers (Σ) 86 Figure 8-35: Calculated total superheater weight for different fin thicknesses (t) and lane numbers (Σ)86
61
Figure 8-36: Calculated fin efficiency for different fin length and thickness ......................................... 87 Figure 8-37: Calculated overall fin efficiency of the tube bundle for different fin length and thickness 87 Figure 8-38: overall view of the calculation domain............................................................................... 87 Figure 8-39: Triangular grids with 20.000 and 100.000 cells ................................................................. 87 Figure 8-40: Quadrilateral grids with 20.000 and 60.000 cells ............................................................... 87 Figure 8-41: Relative difference between simulated and experimental heat flux for mesh types........... 88 Figure 8-42: Total heat flux to the heat exchanger under varying flue gas inlet temperature................. 88 Figure 8-43: tube fin concept: 12 lanes with 3mm and 5mm fin thickness............................................. 89 Figure 8-44: tube fin concept: 16 lanes with 3mm and 5mm fin thickness............................................. 89 Figure 8-45: Total heat flux to the platen heat exchanger under varying flue gas inlet temperature ...... 89 Figure 8-46: Heat flux on tubes for the base-line and the platen heat exchanger configurations ........... 90 Figure 8-47: Heat flux on tubes and fins for the base-line and the platen heat exchanger configurations
.......................................................................................................................................................... 90 Figure 8-48: Contours of temperature for three different cases: a) 16 lanes, baseline, b) 16 lanes, 3mm
fins, c) 12 lanes, 3mm fins ............................................................................................................... 91 Figure 8-49: Contours of velocity for three different cases: a) 16 lanes, baseline, b) 16 lanes, 3mm fins,
c) 12 lanes, 3mm fins ....................................................................................................................... 91 Figure 8-50: Contours of turbulence kinetic energy for three different cases: a) 16 lanes, baseline, b) 16
lanes, 3mm fins, c) 12 lanes, 3mm fins............................................................................................ 92 Figure 8-51: Inspection of test rig A ....................................................................................................... 92 Figure 8-52: Inspection of Test Rig C – Overview ................................................................................. 93 Figure 8-53: Inspection of Test Rig C - Detail........................................................................................ 93 Figure 8-54: Test Rig B’, air cooled tube loop design ............................................................................ 94 Figure 8-55: picture of Test Rig B’ air cooled loop ................................................................................ 94 Figure 8-56: Revised concept with water cooled carrier ......................................................................... 95 Figure 8-57: Availability of the test rig ................................................................................................... 96 Figure 8-58: Microstructure graphs DMV 310 N, exemplary for sample F............................................ 97 Figure 8-59: Measurement of the oxide layer thickness of 10CrMo910, exemplary for sample B ........ 97 Figure 8-60: Drawing of Test Rig A ....................................................................................................... 99 Figure 8-61: Drawing of Test Rig B...................................................................................................... 100 Figure 8-62: Drawing of Test Rig C...................................................................................................... 101
6.2 List of tables Table 1: Proximate and ultimate analysis of Greek lignite, samples K1, K2 and K3 ............................. 24 Table 2: Ash content and low heating values of Rhenish lignites by type of deposition formation ....... 24 Table 3: Chemical analysis of ash for Greek and Rhenish lignites ......................................................... 25 Table 4: Overview about operation and inspection of the test rigs A and C........................................... 52 Table 5: Overview about cleaning device utilisation .............................................................................. 52
62
7 Appendices
7.1 Appendix A
Comparison of coals: additional information
In general, the acid oxides constituents found in coal ash are considered to produce high melting temperatures, while a high percentage of basic oxides results in lower melting temperatures. Coal ash is a mixture of both acid and basic oxides; therefore its melting temperature lowers proportionally to the relative amount of basic oxides.
According to the data shown in Table 3, Greek lignites are characterized by high CaO, which increases the danger of slagging. On the other hand, the relatively high SiO2 and Fe2O3 presence tends to characterize non-critical behaviors in regard to slagging. The SO3 present in the ash tends to develop difficult to clean fouling, while the low content of Na2O affects inversely the development of hardened deposits and thus decreases the sintering intensity.
In order to classify the coal ash according to its propensity for depositions, the following expressions, based on the inequality between the main basic oxides and a strong acid oxide representative are used:
Ash: Bituminous – type : if (CaO + MgO) < Fe2O3
Lignitic – type : if (CaO + MgO) > Fe2O3
It is evident from the data of Table 3 that the coal ashes of Greek and German lignites are classified as lignitic-type ash. The definition of the ash type is essential due to the fact that different indices are used for each ash type. In this study, the ash parameters used are best fitted for the lignitic-type.
An additional qualitative classification of coal ash based on its chemical composition can be found in the work of Roy et al., who classified coal ash based on the ‘intersection’ in a ternary diagram of three major groups of oxides: (SiO2+Al2O3+TiO2), calcic (CaO+MgO+Na2O+K2O) and ferric (Fe2O3+MnO+SO3+P2O5). Depending on the mass percentage of these three groups, coal ashes are classified into a number of intermediate subgroups, such as ferrosialic, calsialic, ferrocalsialic, ferric, calcic and ferrocalcic. The results obtained from the triangle are informative and provide insight into the relationships between ash properties.
Figure 7-1 presents the ternary diagram for the Greek and Germans lignite ashes examined in this project. It is evident that the Greek ashes are classified into the calcic type, due to their high CaO content, while the German ones belong either to the ferrosialic or the ferrocalcic subgroups, owning to their high content in ferric oxides.
63
Figure 7-1: Classification of ash based on their major oxides
Deposit-forming propensity based on properties of lignite ash components
This section deals with the aforementioned ash parameters (indices), which constitute formulas that quantify the tendency of lignite ash to form depositions. Their comparative study is necessary, since all indices offer indications as to the slagging potential of a coal lignite ash. The mathematical expressions presented in the following paragraphs are those appropriate for lignite type ashes.
Base to Acid ratio of ash components (B/A)
The B/A ratio is calculated from the following mathematical expression:
BAFe O CaO MgO K O Na O
Al O SiO TiO=
+ + + ++ +
2 3 2 2
2 3 2 2
As was previously mentioned, a high percentage of basic oxides generally lead to low melting temperatures. Therefore, the B/A ratio reflects the potential of the different metals contained in the ash to combine in the combustion process and produce low melting salts. However, this ratio is not always an accurate measure for slagging propensity since its value may be the same for ashes with totally different chemical compositions and slagging behavior. Moreover, it does not take into account the properties of individual oxides which may lower or raise the fusion temperature, nor does it recognize the interaction between. B/A ratios in the range of 0.4 to 0.7 are generally considered to be indicative of low ash fusibility temperatures and high slagging tendency.
64
The B/A ratio for most lignite ashes studied in this project is quite high, as can been seen in Figure 7-2, although not in the range of 0.4 to 0.7 (except for R6). Therefore, according to the aforementioned criterion, although fusion temperatures are expected to be lower than more acidic lignite ashes, their slagging potential is not critical.
0,00
1,00
2,00
3,00
4,00
5,00
6,00
7,00
8,00
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
B/A
Rat
io
Figure 7-2: Base to Acid (B/A) Ratio for Greek and German lignite samples
Silica / Alumina Ratio (SA)
The silica to alumina ratio is given by the following mathematical expression:
SASiOAl O
= 2
2 3
Since both oxides are acidic, their presence in coal ash is expected to result in higher melting temperatures. However, the silica oxide is more likely to form lower melting salts (silicates) with basic oxides than alumina. Therefore, an increase of the SA ratio leads to increased slagging potential.
In the present work, the SA ratio ranges between 0.4 and 8 (Figure 7-3). The majority of the lignites examined lack a high percentage of silica oxides, with the exception of R5. Therefore, the effect of the silica oxides is equalized by alumina, indicating less slagging potential.
65
0,00
1,00
2,00
3,00
4,00
5,00
6,00
7,00
8,00
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
S/A
Rat
io
Figure 7-3: Silica to Alumina (S/A) Ratio for Greek and German lignite samples
Iron / Calcium Ratio (IC)
Iron and calcium are the most important constituents of coal ash, because they make up the largest amount of the basic oxides. The ratio is formulated as follows:
ICFe OCaO
= 2 3
A decrease in the IC ratio results in a higher tendency for slagging, due to the higher content of calcium oxides which form alkaline earth metals and present low melting temperatures. The IC ratio for the Greek lignites is significantly lower then the German ones, due to an increased content of calcium. This is presents the possibility of increased slagging potential. The IC ratio for the lignite samples examined in this work is presented in Figure 7-4.
66
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
IC R
atio
Figure 7-4: Iron to Calcium (IC) Ratio for Greek and German lignite samples
Iron / Dolomite Ratio (ID) and Dolomite percentage (DP)
These ash parameters are attributed to the following mathematical expressions
IDFe O
CaO MgO=
+2 3
, 1003222
xOFeOKONaMgOCaO
MgOCaODP++++
+=
The ID ratio is essentially identical to the IC ratio and is mainly used when the MgO content is high. The ratio is used to classify coal ashes as lignitic or bituminous. A MgO percentage of almost 5% wt for the Greek lignites and 5 – 15% wt for the German ones leads to corresponding ID ratio values of below 1. Therefore, according to this criterion, all the examined ashes are of the lignitic variety.
On the other hand, the dolomite percentage is used when the basic oxides in coal ash is above 40%, which is the vase for all the examined ashes apart from the ferrosialic ones (R5, R6, R7). A higher DP results in increased fusion temperatures and raised ash viscosity. The DP is higher for the Greek lignite ashes than for the German ones, which indicates a counterbalancing factor to their high calcium content. The ID ratio and the DP values for the examined samples are presented in Figure 7-5 and Figure 7-6.
67
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
ID R
atio
Figure 7-5: Iron to Dolomite (ID) Ratio for Greek and German lignite samples
0,00
10,00
20,00
30,00
40,00
50,00
60,00
70,00
80,00
90,00
100,00
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
Dol
omite
Per
cent
age
(%)
Figure 7-6: Dolomite Percentage (DP) for Greek and German lignite samples
Slagging index (Rs)
The slagging index Rs is a weighted average of the maximum hemispherical temperature (HT) and the minimum initial deformation temperature (IT) ASTM Standard D 1857 and is given by the following mathematical expression:
5)min(4)max( ITHTRs
⋅+=
68
The slagging index differs from the previously described indices since it relates the slagging potential of a lignitic type ash with experimental measures of a quantity directly related to slagging phenomena, eg melting temperatures of ash. The following table presents the effect of Rs on the expected slagging intensity:
Rs > 1340oC Weak slagging tendency
1250oC < Rs <1340 oC Moderate slagging tendency
1150 oC < Rs <1250 oC High slagging tendency
Rs < 1150oC Severe slagging tendency
The average slagging index for the K1 samples is 1266oC and for the K2 1273.5oC (no data are available for the K3 samples or the Rhenish lignites), which indicates that only moderate slagging potential is expected. It should be noted that the expression of the slagging index used in this work holds only for lignitic type ashes. A slagging index which relates the B/A ratio to the sulfur content of the coal is used in the case of bituminous ash.
Total Alkaline metals (TA), Characteristic Alkali number (A) and Fouling index (Rf)
In general, the alkaline metals produce low fusibility temperatures. Two formulas are commonly used:
TA Na O K O= +2 2 , OKONaA 22 659.0 ⋅+=
The rate of deposition forming and the ability to control them effectively depend on the amount of sodium present. The fouling index for the lignitic – type ash is formulated by the following equation:
62ONaR f =
Rf < 0.2 Weak
0.2 < Rf < 0.5 Moderate
0.5 < Rf < 1 High
Rf > 1 Severe
As can been seen in Figure 7-7, Greek lignite ashes have a Rf value of less than 0.06, so their fouling potential is very weak. On the contrary, German lignites, and especially R1 and R2, exhibit a much higher Fouling Index, showing a greater tendency to heating surfaces fouling. For those Rhenish lignites with a high heating value, the increased sodium content may produce hardened deposits and sintering phenomena.
69
0,000
0,200
0,400
0,600
0,800
1,000
1,200
1,400
K1 K2 K3 R1 R2 R3 R4 R5 R6 R7 R8
Lignite samples
Foul
ing
Inde
x R
f
Figure 7-7: Fouling Index (Rf) for Greek and German lignite samples
Time Variation of fuel and fuel ash properties
In order to examine the change of the fuel properties and the ash slagging potential of the Greek lignites with time, a more extensive database of fuel properties is used. The database covers detailed fuel and ash analysis from the beginning of the year 2000 to the end of 2006. Characteristic diagrams of the most important fuel qualities regarding slagging potential and the most relevant ash indices are presented in the appendix.
The examination of these diagrams reveals that the fuel and ash properties may vary considerably even between successive months. However, some general trends are apparent. For example, the lower heating value appears to drop as time increases, while the ash quantity grows. This reflects the exploitation of successively “worse” lignite deposits. It appears that, although the B/A ratio is not considered critical according to the criterion established in the previous paragraphs, the gradual increase of the basic constituents of coal ash leads to higher slagging potential, which is in this case represented by the slagging index. Indeed, this is not only a general trend, since an increase of the B/A ratio for a month leads to a corresponding increase of the Rs. Overall, both the heating quality and the ash properties (regarding slagging potential) of the lignite from the Kardia mines worsen with time. Special emphasis should be placed on the development of corrective measures aiming to lessen the impact of slagging phenomena and increasing the efficiency of the coal plant.
Comparison of Greek and German lignites regarding their slagging and fouling potential
Based on the previous discussion, it is possible to draw some conclusions regarding the slagging and fouling potential of the Greek and Rhenish lignites examined in the present work. The Greek lignite ashes from the Kardia mining area are mainly characterized by their high calcium content. As indicated, this presents a significant slagging potential. On the other hand, the presence of alumina and the relatively high dolomite percentage counterbalance the effect of calcium. Therefore, the overall slagging potential is expected to be moderate. This is verified by the calculation of the slagging index Rs, which confirms that the previous conclusion.
70
The most important issue concerning the Greek lignites is the high ash content, which, coupled with the low heating value, results in the production of large quantities of ash. This in itself may intensify the effect of slagging phenomena. The low alkali content (compared with the Rhenish lignites) suggests that the fouling phenomena on the heating surfaces are negligible.
The Rhenish lignites are characterized by lower ash content and higher heating value. Although this means that the ash produced for a given amount of energy is lower compared to power plants running with Greek lignites, the high heating value corresponds to increased furnace temperatures, which may exceed the melting temperatures of the ash oxides. Lignites R5 and R6 present the danger of slagging; both have high LHV while the B/A ratio for R6 is in the critical range and the S/A ratio for R5 is extremely high. However, they can be used relatively safely in boilers with minor thermal loads. Lignites R1and R2 also exhibit high heating values which may lead to increased slagging, while lignites R3 and R4 remain uncritical although their combined use with other lignite types should be carefully examined in order not to exceed the recommended silica or iron contents. R7 and R8 have the lowest heating values of the German lignites and, judging from their ash content, their slagging behavior is uncritical.
One of the major differences between the Greek and German lignites is that the latter are characterized by a much higher alkali content, which leads to increased fouling potential. Lignites R1 and R2 exhibit the highest fouling potential; taking into account their high heating value, the deposits on the heating surfaces may sinter and harden. The increased presence of SO3 intensifies the fouling phenomena. Overall, the Greek and German brown coals examined in this study exhibit differences in their slagging and fouling behavior, although some general tendencies, e.g. a moderate slagging potential, can be discerned for both groups. Therefore, technologies and technical expertise from the operation of German power plants to Greek ones is possible with further investigations.
71
7.2 Appendix B
Figure 7-8: Fouling and slagging deposits on platen heating surface / superheater 3
Figure 7-9: Thermal shock
73
Figure 7-10: Two Fluid Blower; Nozzle tests during boiler outage
Figure 7-11: Pendulum movement of water jet cleaning the heating surface
74
Figure 7-12: Nozzle test on test rig
Figure 7-13: Optimization of water jet
75
Figure 7-14: Damaged Two Fluid Blower
Figure 7-15: Design for High Pressure Water Jet Cleaning
76
+ 51.0 m
+ 45.0 m
new platform at+ 44.75 m
NIA PP, Unit Dleft side of boiler Figure 7-16: structure of shock wave cleaning facility
Figure 7-17: Location of the thermocouples at Tilbury power station
Gas side thermocouples Water / steam side thermocouples
77
Description of data requiredDescription of data required
First Fouling Model Run (just after ULS trial commenced)
Second Fouling Model Run (mid-way through ULS trial)
Third Fouling Model Run (towards the end of the ULS trial)
Figure 7-25: Fouling increase over time
81
Figure 7-26: Cracked uncooled supporting piece from the supporting tube due to thermal stress, only
conventional sootblowing was applied.
Figure 7-27: Convective part of a lignite fired boiler with super heater surfaces designed as platen heating
surfaces
82
Cracks
Figure 7-28: Welding attempt with the filler material UTP 6170Co
Figure 7-29: Plasma gun
83
Coating of supporting tubes in the sootblower area
Coating of supporting tubes in the sootblower area
Figure 7-30: Wire flame spraying gun
0 0.5 1 1.5 20
10
20
30
40
50
60
Uh k 0.0001,( )
Uh k 0.0005,( )
Uh k 0.001,( )
Uh k 0.0025,( )
Uh k 0.005,( )
k
Increasing deposition thickness
Figure 7-31: Total heat transfer coefficient of the heat exchanger Uh (W/mK)
84
0 0.5 1 1.5 20
20
60
40
Q
Q
Q
Q k 0.001,( )
Q k 0.002,( )
Q k 0.005,( )
k
k 0.0001,( )
k 0.0002,( )
k 0.0005,( )Increasing deposition thickness
Figure 7-32: Total heat flux of the heat exchanger Q (MW)
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0 0,001 0,002 0,003 0,004 0,005 0,006
fin thickness - t (m)
fin le
ngth
- Lw
(m)
Σ=16 Σ=14 Σ=12 Σ=10 Σ=8
Reference value 0.08m
Region A
Figure 7-33: Calculated length of fins for different fin thicknesses (t) and lane numbers (Σ)
85
0
0,5
1
1,5
2
2,5
3
0 0,001 0,002 0,003 0,004 0,005 0,006
fin thickness (m)
heig
ht o
f S/
(m)
Region A
Reference value 1.95m
H
Σ=16 Σ=14 Σ=12 Σ=10 Σ=8
Figure 7-34: Calculated total superheater height for different fin thicknesses (t) and lane numbers (Σ)
4
5
6
7
8
9
10
0 0,001 0,002 0,003 0,004 0,005 0,006
width of the fin (m)
wei
ght i
n 10
^3 tn
Σ=16 Σ=14 Σ=12 Σ=10 Σ=8
Reference value 8.04 103 tn
Region B
Figure 7-35: Calculated total superheater weight for different fin thicknesses (t) and lane numbers (Σ)
86
Figure 7-36: Calculated fin efficiency for different fin length and thickness
Figure 7-37: Calculated overall fin efficiency of the tube bundle for different fin length and thickness
Figure 7-38: overall view of the calculation domain
Figure 7-39: Triangular grids with
20.000 and 100.000 cells
Figure 7-40: Quadrilateral grids with
20.000 and 60.000 cells
0.1 0.2 0.30
0.2
0.4
0.6
0.8no Lw 0.001,( )
no Lw 0.002,( )
no Lw 0.003,( )
no Lw 0.004,( )
no Lw 0.005,( )
Lw
Height of the fin0.1 0.2 0.3
0
0.2
0.4
0.6
0.8nf Lw 0.001,( )
nf Lw 0.002,( )
nf Lw 0.003,( )
nf Lw 0.004,( )
nf Lw 0.005,( )
Lw
Region C
Height of the fin
Region C
Parameter: width of the fin t (in m)
87
0
5
10
15
20
25
0 50 100 150 200 250
Number of c ells in mes h (in 10^3 c ells )
Rel. differ ence between sim
ulat ed
and exper iment al h
eat flux (in %
)
T riangular meshes
Quadrilateral meshes
T riangular meshes withBC mas s flow
Figure 7-41: Relative difference between simulated and experimental heat flux for
mesh types
y = 0,1649x ‐ 93,691
R 2 = 0,9976
0
10
20
30
40
50
60
600 650 700 750 800 850 900 950F lue gas inlet temperature in ˚C
hea
t flux (M
W)
60.000 cells
100.000 cells
147.000 cells
q reference T in=838˚C , Ttube=530˚C
L inear R egres s ion (data s et 147.000 cells )
Figure 7-42: Total heat flux to the heat exchanger under varying flue gas inlet temperature
88
Figure 7-43: tube fin concept: 12 lanes with
3mm and 5mm fin thickness
Figure 7-44: tube fin concept: 16 lanes with
3mm and 5mm fin thickness
0
10
20
30
40
50
60
70
700 750 800 850 900 950
F lue gas inle t tempera ture (˚C )
total he a
t flux (M
W)
12 lanes , 5mm fins
12 lanes , 3mm fins
16 lanes , 3mm fins
16 lanes , 5mm fins
q reference Tin 838˚C , Ttube 530˚C
Figure 7-45: Total heat flux to the platen heat exchanger under varying flue gas inlet temperature
89
1800,00
2300,00
2800,00
3300,00
3800,00
4300,00
4800,00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16tube number
heat
flux
(W)
q-tubes, Reference case q-tubes, 12lanes, 3mm fins q-tubes, 12lanes, 5mm finsq-tubes, 16lanes, 3mm fins q-tubes, 16lanes, 5mm fins
Figure 7-46: Heat flux on tubes for the base-line and the platen heat exchanger configurations
0,00
1000,00
2000,00
3000,00
4000,00
5000,00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16tube number
heat
flux
(W)
q-tubes, Reference case q-tubes, 12lanes, 3mm fins q-fins, 12lanes,3mm fins q-tubes, 16lanes, 3mm fins q-fins, 16 lanes, 3mm fins
Figure 7-47: Heat flux on tubes and fins for the base-line and the platen heat exchanger configurations
90
a) b) c)
Figure 7-48: Contours of temperature for three different cases: a) 16 lanes, baseline, b) 16 lanes, 3mm fins, c) 12 lanes, 3mm fins
a) b) c)
Figure 7-49: Contours of velocity for three different cases: a) 16 lanes, baseline, b) 16 lanes, 3mm fins, c) 12 lanes, 3mm fins
91
a)kmean=5.2119 b)kmean=4.0195 c)kmean=4.0729
Figure 7-50: Contours of turbulence kinetic energy for three different cases: a) 16 lanes, baseline, b) 16 lanes, 3mm fins, c) 12 lanes, 3mm fins
Figure 7-51: Inspection of test rig A
92
Figure 7-52: Inspection of Test Rig C – Overview
Figure 7-53: Inspection of Test Rig C - Detail
93
T1
T2
Cooling air inletwith temperaturecontrol
Thermocouple for tubewall temperaturemontoring and control
Boiler building Boiler
Cooling air outletwith temperaturecontrol
Figure 7-54: Test Rig B’, air cooled tube loop design
Figure 7-55: picture of Test Rig B’ air cooled loop
94
Figure 7-56: Revised concept with water cooled carrier
95
Figure 7-57: Availability of the test rig
96
Sample F, set temperature 580 °C Zero probe II, not installed
Thermal overheating has not taken place, as the material has a greater resistance to oxide deposition (scaling) and the air cooling is sufficient, because of bigger inner tube diameters.
Thermally unstrained microstructure: normal austenitic microstructure with single carbide segregations within the matrix
Figure 7-58: Microstructure graphs DMV 310 N, exemplary for sample F
Flue gas side:
Image taken from light microscopy of sample B on the blower side
Maximum oxide layer thickness of 800 µm!
Thick oxide deposits
105504510530 °C, 58 days, unit E, reference probeE532032341530 °C, 42 days, unit FD
105504510450 °C, 28 days, unit E, reference probeC30095800150530°C, 28 days, unit FB1203027030530 °C, 28 days, unit E, reference probeAMax.Min.Max.Min.
Flue gas sideBlower sideFlue gas side oxide layer thickness in µm
samplesample-number
105504510530 °C, 58 days, unit E, reference probeE532032341530 °C, 42 days, unit FD
105504510450 °C, 28 days, unit E, reference probeC30095800150530°C, 28 days, unit FB1203027030530 °C, 28 days, unit E, reference probeAMax.Min.Max.Min.
Flue gas sideBlower sideFlue gas side oxide layer thickness in µm
samplesample-number
Figure 7-59: Measurement of the oxide layer thickness of 10CrMo910, exemplary for sample B
97
7.3 Appendix C
Figure 7-60: Drawing of Test Rig A
99
Figure 7-61: Drawing of Test Rig B
100
Figure 7-62: Drawing of Test Rig C
101
European Commission
EUR 23869 — More efficient cleaning concepts for stepping up availability of lignite-fired power plants (Ligpower)
G. Wiechers, B. Wessel, S. Goudanis, F. Kluger, G. Riley
Luxembourg: Office for Official Publications of the European Communities
