Fludised Bed Combustion Report

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FLUIDISED BED COMBUSTIONSYSTEMS FOR POWER

GENERATION AND OTHERINDUSTRIAL APPLICATIONS

Report No. COAL R188

DTI/Pub URN 00/743

by

AJ Minchener, PJ I Cross, TN Smith and MJ Fisher

CRE Group Limited

The work described in this report was carried out under contract as part of the Departmentof Trade and Industrys Cleaner Coal Technology Programme, and under the supervisionof ETSU. The views and judgements expressed in this report are those of CRE GroupLimited and do not necessarily reflect those of ETSU or the Department of Trade andIndustry.

Crown Copyright 2000First published 2000


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FLUIDISED BED COMBUSTION SYSTEMS FOR POWER GENERATION ANDOTHER INDUSTRIAL APPLICATIONS

by

AJ Minchener, PJ I Cross, TN Smith and MJ FisherCRE Group Limited

SUMMARY

During the past three decades, fluidised bed combustion (FBC) in its various forms has beenused to burn all types of coals, coal wastes and a wide variety of other fuels, either singly orco-fired with coal. FBC boilers are currently available commercially in the capacity range1MWth to 250MWe and continue to be adopted for a variety of commercial and industrial

applications, as well as by independent power producers. There are two main derivatives ofFBC, namely bubbling fluidised bed combustion (BFBC) and circulating fluidised bedcombustion (CFBC). There are also several hybrid systems and there are also pressurisedversions of both BFBC and CFBC. The status of these different systems varies, with somenow fully commercial and some still under development.

The principal aims of the work reported here are to assess the current global state ofdevelopment and application of fluidised bed combustion systems, including bubbling,circulating and pressurised systems, to discuss the likely development of these, and to identifyareas of research, development and demonstration (R,D&D) that would be of significantbenefit to the UK. The scope of the review includes all types of fluidised bed combustionsystem for power generation and other industrial applications, critical equipment, componentsand materials, fuel aspects, instrumentation and control systems, residue disposal and residueutilisation.

BFBC was originally considered as a potential technology for coal-fired power generation.However while there have been some large (up to 180MWe) demonstration units in the USAand Japan the technology has been eclipsed by CFBC. Rather it has established a niche at thesmall-medium industrial scale with the very great majority of units in the range 3-100MWthand a few in the range 150-280MWth. Although many units presently operate on coal, otherfuel types are becoming of increasing importance. Many of these are waste-derived and/or

problem feedstock that would either be sent to landfill or be otherwise disposed of. Evenwhen burning such fuels, overall energy efficiency and environmental performance can begood with a variety of techniques available to be used to minimise environmental impact.

There are numerous organisations who continue to manufacture and supply BFBC, includingKvaerner who are the market leader for the larger scale units. However, future technologydevelopment is likely to be limited to ensuring fuel flexibility on existing designs for theincreasing use of biomass and/or waste as feedstock with or instead of coal. There are nomajor industrially focused international R&D projects underway. There is however, apotential UK niche in waste to energy schemes which is referred to below.


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In contrast to BFBC, CFBC technology has become increasingly established for a variety ofapplications including steam raising, cogeneration and power generation. CFBC is recognisedas a versatile technology capable of burning a wide range of coals and other feedstock; the listof fuels utilised successfully continues to grow. Different plant configurations are now

available from major vendors. The leaders are Foster-Wheeler/Ahlstrom and Lurgi LentjesBabcock, with the maximum unit size at present 250MWe.

On a worldwide basis, the market opportunities appear good, with a potential market up to2020 of some 150GW capacity being estimated. This represents some 20% of the likelyglobal capacity increase for coal-fired power generation over that time period. The marketproposals are localised with the major opportunities being seen as China (125GW), althoughthere are concerns regarding a true market being available to international suppliers, NorthAmerica (17GW) and India (6GW).

In terms of development requirements and opportunities, there are strong driving forces to

improve competitiveness of the technology. The need is to improve overall cycle efficiency,minimise environmental impact while enhancing fuel flexibility, reduce capital costs and toensure effective scale up in order to compete over the full product range with pf units. Thereis a major US DoE programme to support US vendors in achieving these aims, while inEurope there is a nationally focussed R&D programme in France, various other national basedR&D programmes, plus opportunities for EU industry to gain some development supportfrom the European Commission.

Pressurised fluidised bed combustion (PFBC) offers the prospect of a coal/multi fuel-firedcombined cycle process of higher efficiency than CFBC and, ultimately, of pf (undercomparable steam cycle conditions). There is also scope for its use in a topping combinedcycle advanced configuration to achieve still higher cycle efficiencies. To date the majority(seven of the eight) of PFBC plants operating as commercial prototypes have been based onABB Carbon bubbling bed technology. Most have been based on their use of the smallerP200 module. However, the start of commercial operation of a plant using the scaled-up P800module is imminent.

Good environmental performance has been achieved with the present tranche of PFBC plants.Efforts are continuing to improve environmental impact further, reduce plant capital andoperating costs, and increase flexibility through initiatives that include a broadened range offuels. The R&D is driven by ABB Carbon, working with otherinstitutes and universities

where appropriate.

Overall, the uptake of bubbling bed PFBC technology is progressing slowly although thereare a number of proposals under consideration. PFBC technology is effectively excludedfrom some regions and market areas as a result of its perceived higher costs and complexitycompared to competing systems. It is believed that the success of the scaled-up unit in Japanis critical to the success of the technology, as the major potential market is seen to be Japan(3GW) over the time period to 2020. The technology may well have a narrow window ofopportunity and runs the risk of being overtaken by other combined cycle systems such asIGCC in the future.


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Pressurised Circulating Fluidised Bed Combustion (PCFBC) is the alternative technologyvariant and remains at an earlier stage of development. Several major suppliers are activelydeveloping their versions of the technology including Foster Wheeler and Lurgi LentjesBabcock. PCFBC is being championed by the US DoE through several demonstration plants

including a topping cycle concept. These should be completed before 2020 but marketopportunities at the commercial scale cannot be predicted at this time.

The prospects for UK industry in exploiting the potential market for fluidised bed combustionsystems have been considered. Although much of the pioneering R&D work on pressurisedfluidised bed combustion and on bubbling fluidised bed combustion was undertaken in the UK,this early leading position of technical excellence was not converted into a commerciallysuccessful business sector, for a variety of reasons. Amongst the most significant of these wasthe policy, while the UK electricity sector was in public ownership, of adding coal-firedcapacity in the form of large (2000MW) plant based on the proven technology of pulverisedfuel combustion, a policy which persisted during the key development years of FBC and which

must now with hindsight be recognised for stifling the commercial exploitation of FBC forutility application in the UK. Consequently, the prospects for UK industry to exploit thepotential worldwide market for FBC would appear to be severely restricted, for a variety ofreasons:

the UK does not have a strong home market for the technology with only a small number ofFBC installations and no major national programme of technology development

as a consequence, very few UK organisations have the capability to provide FBCtechnology

international competitors are now well established in the technology marketplace as can beseen from examination of the reference lists presented elsewhere in the report.

This view appears to be shared by the UK FORESIGHT Clean Coal Power GenerationTechnology Task Force that reported on R, D and D priorities for cleaner coal technology inOctober 1998. They stated that AFBC and PFBC are not current UK strengths.

That said, there would appear to be a small niche in the home market in establishing BFBCsystems for non-coal applications but the export potential is likely to be limited. In terms ofR&D and D, the UK drivers appear to be increased fuel flexibility and increased utilisation ofresidues. Although there are a limited number of UK vendors, a significant level of expertiseand experience remains vested in UK institutes and associated universities. Such organisations

can tackle the R&D issues in support of the UK vendors.

Although not strictly R, D & D, promoting technology transfer and consulting are activities thatare able to make use of the expertise that remains in the UK organisations. The key focus areawould appear to lie in advising clients of the merits of different technology options from aposition of impartiality.


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CONTENTSPage No.

1. INTRODUCTION 1

2. FLUIDISED BED COMBUSTION SYSTEMS OVERVIEW 22.1 Introduction 22.2 Bubbling Fluidised Bed Combustion (BFBC) 22.3 Circulating Fluidised Bed Combustion (CFBC) 92.4 Pressurised Fluidised Bed Combustion (PFBC) 142.5 Pressurised Circulating Fluidised Bed Combustion (PCFBC) 202.6 Capital and Operating Costs for FBC Systems 21

3 CURRENT STATUS AND ASSESSMENT OF FBC SYSTEMS 253.1 Introduction 253.2 Bubbling Fluidised Bed Combustion 25

3.3 Circulating Fluidised Bed Combustion 333.4 Pressurised Fluidised Bed Combustion 41

4. REGIONAL DEVELOPMENT AND ADOPTION TRENDS 454.1 Introduction 454.2 Regional Trends in Development and Application 454.3 R, D and D Activities at National and International Level 50

5. FUTURE TECHNOLOGY DEVELOPMENT 555.1 Bubbling Fluidised Bed Combustion 555.2 Circulating Fluidised Bed Combustion 565.3 Pressurised Fluidised Bed Combustion 63

6. MARKET POTENTIAL 656.1 Introduction 656.2 Assessment of Commercial Aspects 656.3 Prospects for UK plc 73

7. UK ACTIVITIES AND PROSPECTS 747.1 Introduction 747.2 UK Capabilities 74

7.3 UK Input to R, D & D Activities 767.4 Areas for Further R, D & D Activities in the UK 777.5 Other Prospects 78

8. CONCLUSIONS 78

9. ACKNOWLEDGEMENTS 81

10. BIBLIOGRAPHY 81

TABLES

FIGURES


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APPENDIX A Major suppliers of FBC plant and equipmentAPPENDIX B UK design and manufacturing capabilities contacts and further company

detailsAPPENDIX C Individuals and organisations that contributed to Section 5, future technology

developmentAPPENDIX D Recent CFBC technological developmentsAPPENDIX E Lists of major CFBC and PFBC plant


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FLUIDISED BED COMBUSTION SYSTEMS FOR POWER GENERATION ANDOTHER INDUSTRIAL APPLICATIONS

1. INTRODUCTION

During the past three decades, fluidised bed combustion (FBC) in its various forms has beenused to burn all types of coals, coal wastes and a wide variety of other fuels, either singly orco-fired with coal. FBC boilers are currently available commercially in the capacity range1MWth to 250MWe and continue to be adopted for a variety of commercial and industrialapplications, as well as by independent power producers. There are two main derivatives ofFBC, namely bubbling fluidised bed combustion (BFBC) and circulating fluidised bedcombustion (CFBC). There are also several hybrid systems and there are also pressurisedversions of both BFBC and CFBC. The status of these different systems varies, with somenow fully commercial and some still under development.

CRE Group Ltd undertook a review of fluidised bed combustion systems for powergeneration and other industrial applications on behalf of the UK Department of Trade andIndustry (DTI) the results of which are presented in this Report.

The principal aims of the review were to assess the current global state of development andapplication of fluidised bed combustion systems, including bubbling, circulating andpressurised systems, to discuss the likely development of these, and to identify areas ofresearch, development and demonstration (R,D&D) that would be of significant benefit to theUK. The scope of the review included all types of fluidised bed combustion system for powergeneration and other industrial applications, critical equipment, components and materials,fuel aspects, instrumentation and control systems, residue disposal and residue utilisation.

The specific objectives were as follows:

to assess objectively the current state of development and application of fluidised bedcombustion systems world-wide, including key associated equipment and components.

to assess critically the capabilities and limitations of currently available fluidised bedcombustors/systems, and to provide information on their developers, manufacturers,suppliers and major users.

to identify and discuss likely developments on the 5 year, 10 year and 20 year time scales,including expected incremental and/or step change developments necessary to bring theseabout.

to review critically R, D&D activities and future potential of UK organisations involved inthis area, in the context of the expected technology developments, prospective markets andmarket influences.

to identify and prioritise areas in which future R, D&D activities need to be focused,particularly those involving UK organisations, to meet current and future demands and toenhance the market potential for UK products/services.


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The approach adopted by CRE Group Ltd for addressing the study objectives comprised:

An initial literature review and review of other data sources, such as the InternetThe production of initial drafts of the key sections of the report, with identification of any

gaps in the basic information available

A series of interviews and discussions with FBC developers and users, internationalorganisations and governmental bodies designed to fill these gaps

The process was expedited through the use of existing databases, reviews, techno-economicstudies and testwork undertaken by CRE Group Ltd. both in-house and for clients, and onbehalf of international governmental organisations, power utilities, industry and consultants.

2. FLUIDISED BED COMBUSTION SYSTEMS OVERVIEW

2.1 Introduction

The various types of FBC system (bubbling bed, circulating bed, pressurised bed, pressurisedcirculating bed) available for power generation and other applications are reviewed brieflybelow. Each variant of the technology is considered under the headings of systemdescription, main process features, fuel flexibility, performance and environmental issues andadvantages/disadvantages. Following this, capital cost and operating cost issues areconsidered.

2.2 Bubbling Fluidised Bed Combustion (BFBC)

System Description

When a packed bed of small particles is subjected to an upward gas flow, the bed initiallyremains static but the pressure drop across it increases in proportion to the increasing gas flowrate. When the pressure drop across the bed particles equals the weight per unit area of thebed, the bed becomes suspended. The bed is then considered to be at minimum (or incipient)fluidisation. Any further increase in the gas flow rate does not significantly affect the bedpressure drop. However, gas flow in excess of the minimum will, with the size of particlesnormally considered for BFBC applications, result in the formation of bubbles. At several

times the minimum fluidisation velocity, the upwards and sideways coalescing movements ofthe bubbles provide intense agitation and mixing of the bed particles. In this state the bedparticles can transfer heat at very high rates from burning fuel to cooler surroundings.

Within the system, the only significant pressure difference is the drop from the air distributor inthe base to the top of the bed, as the weight of the bed particles opposes the rising fluidising air.For a particular bed material, as the fluidising velocity increases, the fluidisation processproceeds as follows. Bed particles are free to move around; the bed is fluidised although theparticles remain in relatively close contact and are not carried upwards to any significant degree.Thus, the bed maintains a well-defined upper surface, with air bubbles passing through the bedbursting at the surface, much as boiling water appears. Fuel can be fed into and burned in the

bubbling bed, this process being known as bubbling fluidised bed combustion.


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In a BFBC system, the fuel is delivered into or onto the fluidised bed of inert particles, andburns by virtue of the oxygen within the fluidising air and the temperature of the surroundingparticles. If the upward air flow is turned off, the particles become static ie they becomedefluidised or slumped, and settle down onto the supporting base plate, termed the airdistributor. This plate both supports the static particles and also evenly distributes the fluidising

air across the whole base area of the particle containment.

Many different refractory materials can be used to form the original bed of inert particles,although the most commonly used is graded sand, around 1 mm in mean diameter, enablingfluidising velocities in the range 1-3m/s. Alternatively, graded limestone or dolomite can beused if sulphur dioxide (SO2) capture is required. If SO2 retention is required on a continuousbasis, limestone (or dolomite) has to be fed continuously to the bed. When particles are beingfed continuously into the bed, excess material has to be drained away, either through the airdistributor base plate or via an overflow weir. This maintains the design bed depth.

When the fuel being burned has only a low ash content, or if the ash is friable, the fuel ash may

be either insignificant or will become degraded by the action of the fluidised bed, such that it issubstantially elutriated within the emergent flue gases. Alternatively, when a high ash fuel isburned, especially one that leaves behind hard particles of ash, some of the ash remains in thebed. If the ash particles are a suitable size they will fluidise and will eventually replace theoriginal inert particles. Excess fuel ash may also need to be removed in order to maintain thedesign bed depth.

In order to burn the fuel efficiently and, in the case of limestone or dolomite, successfully retainSO2, the bed particles need to be controlled in the temperature range 800-900C. During

normal operation this temperature is achieved and stabilised by the opposing effects of the heatinput from the burning fuel and outgoing heat in the flue gases and heat transferred from the bedparticles to water cooled tubing and/or containment walls. In the case of a BFBC boiler, suchtubes and walls form a part of the boiler construction. When BFBC is used for applicationsother than as a boiler, such as a hot gas furnace or incinerator, there are no such water-cooledsurfaces, so the bed temperature is stabilised by passing excess air through the bed.

In the case of a boiler or a waste/biofuel to energy plant, the emergent flue gases are constrainedto flow through or across conventional heat transfer surfaces so that they are cooled to


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height when fluidised may be between 300 mm and 1 metre. The grain size of the inert bedmaterial falls generally within the range 0.3-2 mm. BFBC boilers utilise various systems toremove excess bed material. This equipment can comprise a fully automated classifier systemthat extracts coarse particles from the bed and returns remaining useful bed material to thecombustion chamber, or simply a type of bed drain chute. Because of the low fluidising

velocities generally adopted and the low suspension density above the bed surface,conventional oil and/or gas burners can be installed in the freeboard of the furnace walls,allowing, when appropriate, full boiler output to be achieved with different types of fuels.

Fuel Flexibility and Performance

Coal has long been an important fuel source, although latterly, there has been a growingtendency to use other fuels, either alone or co-fired with coal. However, many BFBcombustors remain in use worldwide, fired on coal alone, especially in situations where otherindigenous forms of fuel are lacking. Practical experience over the past three decades hasconfirmed that BFBC technology can be well suited to the utilisation of difficult fuels.

There are three groups of fuels that can be described thus:

High moisture fuels

This group includes various materials such as moist wood bark, woodwaste and sludges frommunicipal plants, the water industry and biological treatment plants, in which moisture cannotbe reduced by conventional means. In addition, oil-derived wastes such as refinery tanksediments, some biomass fuels, and paper mill sludges fall into this category.

High ash fuels

This group of fuels has ashes that may react in an unexpected manner during combustion.Ashes may have a tendency to melt or agglomerate at the combustion temperature. Problemscaused can include bed instability, agglomeration, deposition or explosion. Fuels of this typeinclude boiler ashes and variants of municipal solid waste (MSW) and refuse derived fuel(RDF).

Low volatile fuels

This group includes anthracite, culm, graphite, power plant ash and ore coke.

BFB boilers are utilised widely for the environmentally-acceptable disposal of wastes thatwould otherwise be sent to landfill. For instance, paper mills produce various combinationsof primary sludge from the mill process, coupled with biosludges from plant waste watertreatment facilities. BFBC is a proven system for burning such wastes. Similarly, a numberof refinery sites utilise BFBC technology for the disposal of a range of wastes such as tankbottom sediments, ship bilge water and oil/water emulsions.

The relatively limited solids residence time, particularly for fine carbon particles, in BFBCsystems means that the fuel reactivity can have a significant effect on combustion efficiency.Unreactive carbon particles can be elutriated from the bed and pass through the hightemperature freeboard into the cooler sections of the boiler plant before they are completely

combusted. Less residence time and hence better burnout is needed for carbon particles frommore reactive coals and fuels. Testwork on a shallow bed BFBC boiler established that the


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combustion efficiency for low volatile coals such as anthracite was less than 85%. At thesame operating conditions, medium and high volatile coals achieved combustion efficienciesof 94-96%.

BFBC plant designed for waste or high volatile fuel incineration are less susceptible to this

effect of fuel type since they usually incorporate a large secondary combustion chamberabove the bed to ensure complete combustion of volatiles and carbon particles.

Environmental Issues

BFBCs can generate a range of pollutants. However, pollutant emissions can be controlled bya variety of means, some specific to FBC technology and others that are essentially the sameas those used in conventional combustion plant.

The main environmental issues that require consideration with BFBC operations may vary butwill include some or all of the following: NOx, N2O, SO2, CO, dioxins and furans. In

addition, control of particulates is necessary and, depending on the fuel types and theapplication, control of heavy metal release may be required.

NOx Emissions

NOx emissions from combustion are generated from two sources, oxidation of nitrogen in theair (thermal NOx) and oxidation of the nitrogen in the fuel (fuel NOx). At the temperaturesused in FBC installations, thermal NOx is negligible, thus virtually all NOx is generated fromthe oxidation of nitrogen in both the volatile matter and char present. Oxidation reactionsoccurring are rapid, forming almost exclusively NO; NO2 is formed only gradually on coolingthe flue gases and may represent


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reduction of NOx by CO. For reactive fuels such as bituminous coals, the oxidation ofvolatile nitrogen dominates, and limestone addition tends to increase NOx emissions.Conversely, for low volatile fuels, the catalytic reduction can dominate, and limestoneaddition can be beneficial in terms of NOx emissions.

Nitrous Oxide

Where nitrous oxide emissions are involved, generally, the same operating parameterstypically affect N2O in similar ways to NOx. However, the reaction pathways are morecomplex. Practical experience has confirmed that N2O emissions are reduced by increasingprocess temperature and decreasing excess air levels. Thus, a trade-off is required betweenthe two. Other FBC parameters can also influence the levels of nitrous oxide produced andlevels are decreased by:

NH3 injection increasing fuel volatile content use of staged airFuel type is also a key variable. Experience has confirmed that, in the case of high volatilecoals, relatively low levels of N2O are generated (often ~50 ppmv) whereas fuels such aspetcoke may result in emissions as high as 150-200 ppmv. Unlike coals, biomass-derivedfuels may produce only low levels of N2O. For instance, BFB combustion of de-inkingsludge tend to generate levels of


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Carbon Monoxide

Emissions of CO can be less than 100 ppm(v) as a consequence of the high mixing efficiency,hence high combustion efficiency, achieved in the bed. Although CO emissions tend to behigher than those achieved using CFBC technology, in some situations, levels detected may

be well below regulatory limits. For instance, bark-fired BFB units typically generate COlevels of


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Disposal/Utilisation of solid residues

Solid residues generated by BFB combustion systems consist primarily of a combination ofash, unburned carbon, and, where a sulphur sorbent has been used, desulphurisation productsand unreacted sorbent. In practice, such residues may be collected from several locations

within the system, such as the bed off-take and particulate control device.

Although ashes from pf-fired combustion have long been used for a variety of applications,ash from BFBC operations is sometimes sufficiently distinct to require separate evaluation ofthe utilisation and disposal options available. The chemistry of residues from FBC systems isdetermined largely by the inorganic constituents in the fuel feedstocks, the sorbent reactionproducts, and any unreacted sorbent present. Thus, the ash produced in BFBC combustioncan be different to pf ash in several important ways. The lower temperature of thecombustion processes means that the ash particles have rarely been molten and hence tend toretain their shape. The ash may also have a relatively high carbon content (up to 30% ascompared with 5% for pf combustion). The most significant difference, however, arises when

limestone has been added for sulphur capture as its presence alters the ash chemistry byintroducing several new mineral phases.

Where sulphur capture forms part of the process requirement, ash disposal options may beaffected. If no limestone has been added to the system, the ash generated is generallyclassified as being inert and as such, is suitable for disposal in a conventional landfill site.Where limestone has been used in the system, the level of residual free lime in the ash mayresult in the material being classified as a special category waste, requiring disposal in aspecialised depository. The latter category usually involves a higher landfill tax than for inertash. In some locations (eg. parts of the USA) solid waste with a pH >12.5 is classified ashazardous and requires special disposal measures to be adopted.

The high alkalinity of some such residues can create potential problems of material escapefrom the site in particulate form and/or leaching phenomena, and a number of procedures (eg.shake leaching and column leaching tests) have been developed in order to monitor potentialleaching problems.

Rather than disposal, which is cost negative, various possible options for the utilisation ofBFBC ashes have been explored (Table 2.1).Thus, where ash is presently disposed of to landfill, there are incentives for the development

of other, more cost-effective options whereby the ash is utilised in a useful manner. Thedriving force for such utilisation is a suitably structured taxation regime that encourages use,rather than disposal. This increases the incentive to find alternatives to landfilling.

Regulatory issues can impact significantly on the attractiveness of ash utilisation processes.In many countries, the majority of regulations and standards in force that govern coal ashutilisation are based on ash residue characteristics from pulverised coal combustion.However, there can be considerable differences between the two types of ashes, andregulations relating to the use of fluidised bed ash in construction and other markets are stillunder development. Standards for these materials have often yet to be agreed.


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Advantages and Disadvantages of Bubbling Fluidised Bed Combustion Technology

BFB systems are generally cited as providing some of the following advantages overcompeting systems:

environmentally acceptable disposal of many industrial and agricultural wastes that couldotherwise not be incinerated high availability is often claimed and many commercial units have operated well for

lengthy periods. high combustion efficiency can often be achieved. cost-effective operation is often cited - high availability coupled with high efficiency can

result in the generation of additional energy from the same amount of fuel. In somesituations, several fuels can be burned simultaneously.

fuel flexibility - a wide range of solid fuels has been utilised successfully in appropriatelydesigned units.

combustion can be maintained in a stable condition even during fairly significant changesin fuel characteristics.

low operating costs - costs can be relatively low as there are no moving parts in the BFBboiler. In addition, refractories are usually very durable. The lack of in-bed heatingsurfaces in some designs eliminates many potential maintenance problems.

low emissions can be achieved relatively low bed temperatures allow limestone to reacteffectively with sulphur species present. Low bed temperatures coupled with staged airminimise NOx formation.

suitable for retrofit applications - BFB units have often been used as replacements for old,inefficient alternatives such as grate-fired or small pf units.

Although BFBC units may have significant advantages over some competing forms ofcombustion technology, they may have a number of disadvantages when compared with pf-fired plant. These include:

To date, commercially proven only at relatively small scale. Range of units availablepresently limited to small-medium capacity.

Slightly lower overall generating cycle efficiencies and higher greenhouse gas emissionsper unit of power produced compared to some pf-fired technology, unless the latterutilises FGD for sulphur control purposes.

Relatively large volumes of solid residues can be generated. Some may require specialmeasures for their disposal.

2.3 Circulating Fluidised Bed Combustion (CFBC)

System Description

Circulating fluidised beds are a development of bubbling bed technology. As with the latter, airis blown through the bed. As this occurs, the air entrains with it a percentage of the solidparticles from the bed. With a bubbling bed, when operating correctly, the majority of theseparticles fall back into the bed. However, if the velocity of the fluidising air is increased abovea defined level, entrained particles are carried upwards away from the bed surface and thedistinct surface layer that characterised the bubbling bed operating at lower air velocities

disappears. The combustion chamber is then filled with a turbulent cloud of particles that nolonger remain in close contact with each other. In a simple bubbling bed, this would constitute


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a significant disadvantage. However, this phenomenon can be harnessed usefully by arrangingfor the particles to be recovered from the air flow and fed back into the lower part of thecombustion chamber. This system is known as a circulating fluidised bed and can be utilised tosustain combustion in a similar manner to a bubbling bed. Thus, in a circulating fluidised bedcombustor (CFBC) the bed solids may be heated to incandescence and fuel fed into the

combustion chamber where it burns in the fluidising air. The turbulent contact between the fuelparticles present and the bed solids stabilises the overall temperature.

The ability to capture and recirculate elutriated solids back to the combustion chamber is aninherent feature of CFBC design. However, solids collection in a CFBC system is different totechniques used in BFBC units in that the solids loading in the gas emerging from thecombustion chamber is far higher in CFBC and the hot solids themselves constitute a major heattransfer mechanism. In this respect, the emerging solids carry with them a significant portion ofthe heat released during combustion in the combustion chamber. This phenomenon has beenaddressed in a variety of ways by the different CFBC technology developers.

Main Process Features and Component Parts

CFBC units, whatever their origin, generally include some or all of the following elements.

A furnace/combustion chamber in which the coal or other fuel is injected (often withlimestone) and fluidised together with part of the recycled solids. Approximately 50% of thecombustion air is introduced below the grid plate. As a result, combustion in the lower partof the bed is reducing, thus limiting the risk of nitrogen oxide formation. Additionalcombustion air is injected as secondary air at an appropriate point above the grid plate.

A solids separation system, such as a cyclone or labyrinth separator, installed at thecombustion chamber outlet in the high temperature gases (~750-950C) which enablesmost of the solids leaving the chamber to be collected and reinjected into the system,allowing only a very small fraction of the ash produced to be carried by the discharged fluegas.

An external heat exchanger. This may be fed by fluidised solids from the bottom of thecyclone which are cooled before being fed back into the furnace or other part of the solidsrecycle loop. The distribution between hot solids and recycled cooled solids keeps thecombustion chamber temperature at the desired value.


CFBC systems have an inherent advantage in that they are designed to increase solidsresidence times by allowing for recirculation of particles into and through the hightemperature combustion zones. This means that fuels ranging from anthracites to wood canbe burnt in appropriately designed CFBC systems at high combustion efficiencies - up to99%. However, it should be stressed that any single CFBC plant or design cannot necessarilycope with a wide range of fuel properties since parameters such as fuel reactivity and particlesize will have a major effect on the heat release profile in the plant. This in turn will affectthe design and control of air distribution, fuel feed, and emission control systems.

The ability of CFBC systems to operate on a wide range of fuel types has been confirmedthrough extensive operational experience and most major suppliers have manufactured CFBC


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units that have been capable of operating with the major classes of fuel types. Although manyCFBC units currently in commercial operation utilise a single fuel feedstock, there are manyothers that regularly co-fire mixtures of fuels. The high degree of fuel flexibility thatcharacterises many designs of CFBC often allows a plant operator to select fuels on the basis ofwhat may be currently available at an economic price and where appropriate, produce a fuel

blend that combines several such elements. Often, a premium fuel may be co-fired with a low-grade feedstock such as paper mill or oil refinery wastes. Table 2.2 lists examples of fuelfeedstock currently in use in CFBC installations:

Fuel flexibility has been a significant factor in the commercial success of CFBC technology,and efforts continue to broaden the range and types of fuels combusted. In some cases, CFBCplant that was originally designed for one type of fuel feedstock has been operated successfullyon fuel combinations not originally envisaged at the plant design stage. For instance, there are anumber of LLB-built plants that are utilising such fuel combinations in varying proportions(Table 2.3).

In the case of LLB-based plants, fuel flexibility is enhanced through the use of fluidised bedheat exchangers (FBHE). Thus, differing flue gas rates and variable heat rates as a consequenceof changes in fuel feedstock, can be accommodated through the use of a system incorporating aFBHE. Modifications to the boiler section of the plant are not required although modificationsmay be required to systems that control fuel feeding and handling.

Overall, the range of fuels and fuel blends has widened significantly in recent years as CFBCtechnology has been adopted for the use of waste and/or biomass-derived feedstock. Fuelfeedstock that was previously thought of as unusable has been used increasingly, both aloneand co-fired, for a variety of CFBC applications.


Many CFBC units have proved to be capable of achieving relatively low levels of the primarypollutants, NOx, SO2, CO and particulates. Sorbent is usually added to the system in order tocontrol SO2 emissions, NOx levels are minimised through careful bed temperature control andother means, and solids passing through the system can be retained using conventionalparticulate control systems. However, to achieve acceptable overall environmentalperformance in practice, like BFBC, has often required considerable development effort bysystem developers.

Sulphur species

A major incentive for the adoption of CFBC technology is the ability to capture SO2in situusing limestone or dolomite, fed in solid form to the system. The same issues influencing theefficiency of desulphurisation apply to both BFBC and CFBC systems and are discussed ingreater detail in Section 2.2 above. However, sulphur abatement in a CFBC is enhancedcompared with BFBC as a consequence of the higher residence time and sorbent recycling.Sorbent particle size tends to be smaller in a CFBC as the recycling process results in particleattrition, exposing fresh surfaces and hence improving sorbent utilisation rates.


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Nitrogen oxides

Although NOx levels tend to be inherently low with CFBC combustion, where further reductionis required, this can be achieved through the adoption of one of the well-established controltechnologies such as Selective Catalytic Reduction (SCR) or Selective Non Catalytic Reduction

(SNCR).

Factors influencing the level of NOx formation are examined further in Section 2.2 above.

Other pollutants

Where appropriate, as with BFBC technology, other forms of pollutant emissions can becontrolled by using suitably adapted forms of control technology utilised for other combustionsystems. Particulate emissions are generally controlled through the use of variants of ESPsand bag filters.

Disposal/utilisation of solid residues

To date, disposal in landfills has been the most common means of handling ash produced byCFBC operations. As large CFBC units have been brought progressively on stream, thetonnages involved have increased significantly. This will increase further as the technology isscaled-up; hence, eventually, volumes to be accommodated could be substantial.

Ash from CFBC applications is sufficiently different to that from pf combustion to requireseparate disposal/utilisation options. Thus, compared to those generated by pf-fired plant,CFBC ashes generally have high reactivity that can lead to problems such as flash-setting,swelling and cracking in landfills, and possible long-term leaching problems. In some cases,this can limit the available utilisation options.

Efforts to develop useful markets for CFBC ashes are continuing although, as with BFBC,there are a number of technical and financial considerations that can influence the use of suchresidues. These include:

Availability of local disposal sites and restrictions on their use. Transport costs ashes are low value materials and hence transportation costs can impact

significantly on project feasibility. Characteristics and variability of the material.

The volumes available at a particular site. Competing materials. Commercial acceptance of the material.A number of studies have been carried out addressing the potential for useful application ofCFBC ashes, some of which remain on-going. Major CFBC manufacturers have initiatedsome investigative programmes. For instance, Foster Wheeler has been examining thepotential of CFBC fly ashes for a series of structural applications, this having been identifiedby the company as a major potential area for development. However, compared to pf ash, asa result of the often relatively high sulphur levels present, certain applications may beexcluded. Use has been made as a component in some types of cement- and concrete-based

formulations and where CFBC ashes are deemed to be suitable for this application, they maybe used in several ways. Thus, they may be incorporated as a replacement for cement in


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Portland cement concrete, as a pozzolanic material in the production of pozzolanic cements,or as a set retardant, along with cement, as a gypsum replacement.

Studies have also confirmed that CFBC ashes can be used for certain constructionapplications that are governed by less stringent specifications. These uses include soil

stabilisation, road base construction, structural fill, and synthetic aggregates. Whereas thehigh alkalinity of the ash is sometimes a disadvantage, conversely, for some potential uses itis an advantage. For instance, the alkalinity of the ash, the presence of residual lime, itsphysical properties and pozzolanic properties are all desirable characteristics for use in soilstabilisation and mine reclamation. In the case of the latter, work has confirmed that CFBCashes used in the form of a grout, reduced average acidity levels in areas treated by ~30%.

The alkalinity has also been utilised where CFBC ashes have been used as part of the N-Viroprocess. This is a patented process used for the alkaline stabilisation of municipal sewagesludge. Such alkaline by-products are used to raise pH >12, generate heat (52-62oC), andincrease solids content of the biosolids (50-65% solids). In total, there are 38 N-Viro facilities

worldwide that utilise alkaline residues, the actual choice being dependent on localavailability. The end product is marketed as an agricultural lime, fertiliser, and as a soilsubstitute for reclamation and horticulture.

Many potential applications for CFBC ashes are similar to those generated by BFBCoperations; issues regarding other potential applications in, for instance, products of addedvalue, are explored further in Section 2.2 which examines the disposal/utilisation options forBFBC ashes.

Advantages and disadvantages of CFBC

The advantages over competing combustion systems generally cited by proponents of thevarious CFBC technologies include:

Low levels of SO2 and NOx can often be achieved without the addition of back-end cleanupsystems.

The capital costs of a CFBC unit can be ~10% lower than those associated with, for instance,a conventional pulverised fuel-fired system of the same capacity.

Cooled combustion gases emerging from the CFBC can be cleaned of residual particlesusing conventional cleanup techniques such as bag filters or electrostatic precipitators.

A CFBC is often capable of operating on a wide range of fuel types, including those thatcannot be burned in more conventional systems. In effect, a fuel can be regarded as anymaterial whose combustible content is capable of maintaining the bed temperature, thusincreasing the range of fuel types significantly over earlier combustion systems.

For a given output, an equivalent CFBC unit is often physically smaller than a pf-firedinstallation.

The relatively long residence time of fuel particles in the system allows for the successfulcombustion of difficult-to-burn or light particles. In addition, it allows lengthy reactiontimes between limestone or dolomite and sulphur species present, thus reducing emissionproblems.


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The disadvantages of CFBC technology are viewed as:

Commercially proven only at relatively small scale compared to pf-fired systems.Although developments are in hand to scale up the technology, no

demonstration/commercial plants are yet under construction. Thermal efficiencies are limited and comparable with those of pf-fired installations.

Although supercritical steam conditions will increase efficiency, application is not yetwidespread.

Relatively large volumes of residues can sometimes be generated. Some of these residuescan require special measures for their disposal. Utilisation of residues is limited anddevelopment of further options is required.

2.4 Pressurised Fluidised Bed Combustion (PFBC)

System Description

With bubbling bed PFBC, the combustion process takes place within a pressure vessel andoccurs at a pressure higher than atmospheric. As the fluidising air is compressed, it carries agreater percentage of oxygen per unit volume and will therefore sustain a higher intensity ofcombustion within the bed. There is a requirement to feed fuel into, and remove ash from, asystem under pressure, and various arrangements have required development to meet theseneeds. Compared with competing systems, the one major advantage that PFBC has is that thehot combustion gases leave the combustor under pressure. Where this pressure can bemaintained and the gases have been cleaned, they can be fed directly into a gas turbine. Othersignificant advantages include PFBCs fuel flexibility, its modularity, and its suitability forretrofit applications. Construction times are claimed to be two years shorter than for acorresponding pulverised fuel-fired plant. For example, as a result of the ability to makeextensive use of shop fabrication techniques, it is claimed that a 70MW PFBC plant can bebuilt in 2-4 years.

Steady development over the past decade has led to the situation where PFBC is now acceptedas being essentially a commercially proven technology. A number of demonstration plantshave been operated successfully and, based on the experience gained with this first tranche offacilities, newer plant are benefiting increasingly from operational refinements and advancesmade.

In the area of PFBC based on the concept of the bubbling bed, one company, ABB Carbon,now part of ABB Alstom Energy, has supplied all but two installations, these initiallyfunctioning as demonstration units, although most now operating on a commercial basis.Thus, plants have been supplied by ABB Carbon and its licensees that include Babcock &Wilcox in the USA, and Ishikawajumi Heavy Industries (IHI) in Japan.

ABB Carbon PFBC technology comprises a coal-fired combined cycle system that generatespower through a combination of steam and gas turbines, with the combustion gases beingexpanded through the latter. This results in a higher thermal efficiency than can be achievedin conventional coal-fired steam power plant. The split of the plants electrical outputbetween the turbines is generally ~20% from the gas turbine and ~80% from the steam

turbine.


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In the case of ABB-based technology, two standard modules have been developed forcommercial applications namely the P200 and P800 (Table 2.4).

Each type of module has its corresponding gas turbine and where large capacity plant isrequired, this has been achieved by combining two modules. The fluidised bed

combustor/boiler, gas turbine with intercooler and economisers, and the fuel preparation andinternal ash handling systems are collectively referred to as the PFBC Island.


In operation, combustion air for the process enters via the gas turbine low pressurecompressor. The air is cooled in the intercooler; this is to ensure that the temperature, afterthe high pressure compressor, is kept


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raising excess air level to >50% at 30% load level. The lower bed level reduces the in-bedresidence time and increases the Ca:S ratio necessary for ~95% sulphur retention up to ~3.5.Under part load conditions, the freeboard can be kept hot by burning fuel oil or gas.

The main plant components comprise:

Combustor

The pressure vessel and its internal components are referred to as the PFB combustor. At thedesign stage, in the case of ABB Carbon, two important principles were incorporated. Thesewere the minimisation of the number of components with movable parts inside the combustor;these are restricted to a single valve used during startup. The other fundamental principle wasthat, in order to ensure safe operation, hot, dust-laden gas would always be surrounded byclean air at a higher pressure and lower temperature.

The major components housed in the pressure vessel comprise:

the bed vessel. This contains the fluidised bed and steam generator. The latter consists ofa suitable combination of evaporator tubes, superheaters and reheaters.

the startup burner, used to provide hot gas for bed warming purposes prior tocommencement of fuel feeding.

two or more stages of high efficiency cyclones, used to reduce the ash loading to a levelacceptable to the gas turbine.

the bed reinjection system. This is used to transfer material to and from the bed in orderto accommodate load changing.

Gas Turbine

The P200 PFBC module is configured around an existing gas turbine of suitable capacity, theABB Stal GT35 machine. Through utilisation of this machines compressors, the intercooledGT35P turbine was developed. This has a low pressure compressor and low pressure turbineon a freewheeling shaft, plus a high pressure compressor and turbine on the other shaft; thisrotates at constant speed and drives the electric generator. This machine is capable of varyingthe air flow over the range 40-100%, a much greater range of flows than is achievable with asingle shaft, constant speed machine.

The larger GT140P turbine is a scaled-up variant of the GT35P; it uses the compressors from

ABB Stals GT200 machine and hot gas components derived partially from ABBs larger gasturbines. As with the smaller units, the GT140P has an air flow controllable from 40 to 100%of MCR flow.

Thus, in the case of ABB-designed PFBC plant, twin shaft gas turbines are used. These havebeen designed specifically for integration with the PFBC combustor. The turbine designincorporates features to accommodate three special requirements:

the utilisation of an intercooler makes it possible to limit air temperature such thatconventional materials can be used in the construction of the pressure vessel.

the use of the twin shaft concept allows air flow control over a wide range, allowing foroptimisation of combustion and minimisation of NOx levels whilst keeping an almostconstant volume flow through the combustor.


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the number of turbine stages is increased, and the turning angles are reduced in order tominimise the risk of erosion by dust-laden gases.

Between the gas turbine and the combustor, an intercept valve is located; this is used duringstartup and under certain plant trip situations. When closed the valve isolates the gas turbine

from the combustor.


In the case of plants supplied by ABB Carbon, all have so far relied on various forms of coal-firing (Table 2.5). Thus, the Vartan plant in Stockholm is fired with high quality export coal;the Tidd unit was designed to operate with high sulphur coal from the Eastern USA; Escatronuses a black lignite with very high sulphur, ash and moisture contents; the Wakamatsu plantwas designed to be fired on high quality export coal, and Cottbus is utilising brown coal. Thenewer Karita plant will operate with a range of coal types.

The plant under construction at the Kyushu Power Companys Karita site is based on theABB P800 module; the others utilise P200s.

The performance of the P200-based PFBC plant has not varied significantly with the type ofcoal being fired (Table 2.6).

Apart from the variety of coals noted above, bubbling-bed-based PFBC has shown itself to beamenable to firing blends of coals. For instance, ENEL of Italy requested ABB Carbon tocarry out a series of tests firing blends of South African coal with low quality Italian Sulciscoal. Results were encouraging.

During the past few years, a series of other fuels or fuel combinations have also beeninvestigated by ABB Carbon using their process test facility. These have included:

petroleum coke brown coal +sewage sludge Polish coal +biomass Israeli oil shaleIn all cases, the fuels performed satisfactorily.


The first generation of PFBC plants built predominantly using ABB Carbon technology havenow been in operation for sufficient time to form firm conclusions as to their overallenvironmental performance. These are reviewed below:

NOxand N2O emissions

At the relatively low combustion temperatures encountered in PFBC operations, thermaloxidation of air-borne nitrogen is negligible and the nitrogen compounds present in the fluegas originate from the nitrogen in the fuel. Thus, NOx emissions are dependent on the type of

fuel being utilised (especially its volatility), and operating parameters that include excess airlevels and combustion pressure.


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During full load PFBC operations, NOx emissions noted for commercial plant have been low-moderate. However, where fuels containing higher levels of volatile matter are used, and/orreduced operating pressure and oxygen-rich conditions are used, NOx emissions have beenfound to increase. Thus, NOx levels have sometimes tended to increase at lower loads, mainly

as a result of the greater level of excess air used, but also as a consequence of the reducedpressure. Conversely, the adoption of air staging, with air injection above the bed,significantly reduced NOx levels. Additional downstream NOx control measures such as SCRare capable of reducing levels further.

In the case of N2O, emissions tend to be higher than in pulverised coal combustion and fallgenerally in the range 50-200 ppmv. Emissions are strongly temperature-dependent and asmall increase in temperature can lead to a significant decrease in N2O emissions. The type offuel is also an important factor and low rank coals generate lower levels of N2O. The use ofammonia or urea injection to minimise NOx levels will also result in raised N2O levels.

SO2 emissions

The successful retention of sulphur is dependent on many factors. These include the sorbentproperties, porosity and pore size, resistance to attrition and disintegration processes, specificsurface area and microstructure. Various operational parameters also influence overall levelsof sulphur capture, the most important being the calcium:sulphur molar ratio, the bedtemperature, the coal and sorbent granulometry, the residence time of the gas in the bed andthe operating pressure. In PFBC, high levels of sulphur retention can be achieved, often usinglimestone with feed rates slightly lower than those required for atmospheric FBC.

In PFBC, the calcination of the limestone is inhibited by the high partial pressure of CO2.Thus, sulphation involves direct reaction between CaCO3 and SO2 with formation of calciumsulphate (direct sulphation). The CO2 is progressively liberated from the sorbent particles,clearing the pores of the CaSO4 formed and making the residual limestone accessible. Inaddition, there appears to be no optimum temperature for desulphurisation under pressure(optimum for atmospheric FBC is 850oC) hence the boiler, if not limited by other factors,could be operated at higher temperatures.

In practice, the first generation of PFBC plants have all achieved or bettered their originaldesign levels for sulphur removal (Table 2.7).

Disposal/Utilisation of Solid Residues

There are two separate sources of ashes generated during PFBC operations: fly ash, which isseparated in the cyclones and baghouse or ESP, and bed ash, which is withdrawn through thebase of the bed via a lock hopper system. As with other forms of FBC technology, the ashgenerated is different to that from pf combustion as a consequence of the sorbent-derivedmaterials present. PFBC ash is also different to that produced by CFBC operations; in thelatter case, the limestone added for sulphur control purposes completely calcines, oftenresulting in significant levels of free lime in the ash. In PFBC, limestone sulphation proceedswithout calcination, resulting in an ash with only low levels of free lime present (~


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Ash residues may be disposed of by either wet or dry disposal methods. In the wet method,ash is slurried with water and pumped to settling lagoons. As a result of the potentialalkalinity of PFBC ash, pH levels of the lagoon effluent may require monitoring. With drydisposal, the main effluent issue requiring consideration is the potential run-off. However,exposed ash has been found to harden rapidly when moistened, as a result of the hydration of

calcium sulphate anhydrite present. Hydration also leads to a significant reduction inpermeability that tends to minimise the rate of leaching from ash deposits.

There are clear incentives to develop commercial outlets for ash residues generated by PFBCoperations. PFBC ash residues, unlike those from conventional coal combustion, are notglassy pozzolans, hence cannot be utilised in normal cements and concretes. Although directutilisation of some FBC residues in Portland cement has been demonstrated in the USA, theyare not presently used widely in this manner elsewhere.

Where PFBC ash has been incorporated into cement formulations, the low level of free limepresent can make cement products less likely to undergo secondary reactions that can result in

cracking. Where dolomite has been used for sulphur control, the magnesium carbonatepresent is converted to magnesium oxide; this can promote secondary reactions in cements,thus possibly restricting utilisation options available. However, where limestone has beenused, investigative work has confirmed that ashes can be mixed with water, vibro-compactedand used to manufacture strong concrete-like materials with low permeability. Thus, potentialareas of utilisation include fill material, road construction, synthetic gravel, stabilisation ofsoil or mine waste, sealing layers for disposal sites, or concrete manufacture.

Such uses are being made of ash from the Vartan plant in Stockholm, where none of the32000 tonnes/year of ash produced is dumped. It has proved more practical and economicalto use the ashes directly for soil stabilisation purposes and as sealing layers for the mixturethus obtained. For these purposes, residues from the nearby Hogdalen waste incinerationplant are mixed in a predetermined ratio of bed ash and cyclone ash from the Vartan plant,prior to application.

In the case of the Wakamatsu plant in Japan, an investigative programme carried out by theElectric Power Development Company into the utilisation of PFBC ashes resulted in thedevelopment of a range of decorative and functional mouldings; these are marketed under thetrademark ACEMENT.

Other uses investigated have included the use of PFBC ashes to ameliorate acidic soil

conditions. Thus, several types of PFBC ashes have been used as acid-reducing soilamendments.

Advantages and Disadvantages of PFBC

The main advantages of PFBC technology are:

Hot combustion gases leave the combustor under pressure. Once cleaned, they can be feddirectly to a gas turbine, allowing for increased overall system generation efficiency.

There is increasing evidence that fuel flexibility is high and that PFBC systems arecapable of utilising many of the fuel feedstock fired in CFBC units. To date, various fuel

types and combinations have been used successfully on a small scale. Plant modificationsrequired for firing different fuel combinations are claimed to be relatively minor.


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Systems can have a high degree of modularity, where appropriate, allowing for theconstruction of facilities based on the use of two or more individual units.

PFBC technology is suitable for retrofit applications. Plant construction times are shorter than for pf plant of comparable scale, and a higher

degree of shop assembly can be achieved with PFBC systems.

PFBC plants are generally physically smaller than CFBC and pf-fired plants of the samecapacity. Good environmental performance is achievable.The disadvantages of PFBC are viewed as being:

Long-term plant availability has not yet been fully proven, although indications are thatacceptable levels should be achievable.

Present commercially available PFBC technology is limited to two sizes, dictated by thestandardised modules available commercially.

Thermal efficiency is currently limited by the relatively low gas turbine inlet temperatureavailable.

PFBC technology remains an expensive option compared to competing systems. Plantcosts are affected by the necessity of operating various systems under pressure

2.5 Pressurised Circulating Fluidised Bed Combustion (PCFBC)

Although PFBC based on bubbling bed technology currently predominates, an alternative, inthe form of pressurised circulating fluidised bed combustion (PCFBC) is also the focus ofsignificant development efforts.


PCFBC-based combined cycle plant will generally comprise the following major components:

Fuel and sorbent feeding systems. Fuel is fed in a slurry/paste form utilising conventionalsolids handling technologies. Sorbent is fed pneumatically using conventional lock hoppersor fed in paste form along with the fuel.

Combustion chamber and hot loop. This system is contained within the pressure vessel andmakes use of developments made with atmospheric CFBC technologies.

Hot particulate collection device (eg. hot gas filter) Gas turbine and convective heat recovery section. Conventional gas turbines may be used

for recovering energy by expanding the high pressure cleaned flue gases from the filter.Between 20-30% of the plants net output would be generated by the gas turbine. The cleanexhaust gases from the turbine are cooled in the convective heat recovery section. Steamgenerated in this section and the combustion chamber will be utilised to produce 70-80% ofthe net plant output.

Status and potential

PCFBC systems are some way behind bubbling bed-based systems in terms of their overalldevelopment and commercialisation. It is expected that once developed further, PCFBCsystems, as with bubbling bed technology, have the potential to achieve very low levels of

pollutants and high process efficiencies. However, despite considerable R&D effortsexpended to date by several process developers, there remains considerable scope for


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improvements in most major plant areas; areas that would benefit from additional effortinclude examination of combustion behaviour, and improved methods for the minimisation ofSO2, NOx and plant residues. In addition, consideration will be required in order to reduceemissions of trace elements.

One particular advantage of PCFBC technology is the high level of heat release attainable,compared to competing combustion technologies (Table 2.8), suggesting that plant size couldbe significantly smaller than bubbling bed-based systems of comparable capacity.

It is claimed that potentially, PCFBC technology offers significant advantages over the presentatmospheric pressure systems and work is being carried out by several organisations with aview to developing a successful commercial-scale process.

Amongst the advantages of such pressurised systems cited are:

Smaller size and shorter construction time. High fuel flexibility. Higher plant efficiency. Lower capital costs. Lower operating costs. Reduced emissions of NOx, SO2 and CO2. Ease of operation and maintenance. Suitability for repowering applications.There are currently several variants of PCFBC at various stages of development and eachdeveloper has its own opinions on the outlook for the future of its particular system.

2.6 Capital and Operating Costs for FBC Systems

Comment on capital and operating costs of the various alternative fluidised bed combustiontechnologies including the cost of electricity is provided. Particular focus is given to coal-fired utility scale applications of the technology, since as is noted in subsequent sections theserepresent the major market for FBC.

Capital costs

Any attempt to provide an overview of capital costs for fluidised bed combustion based power

plant must begin with a number of important provisos. These include:

Only limited amounts of hard information for actual plants are ever released into thepublic domain because this type of information tends to be highly commercially sensitive.

Even in cases where headline numbers are published it is rarely possible to be certain ofthe exact extent of inclusions and exclusions.

There is a high level of variation in capital costs worldwide for what might ostensibly bethe same basic design of plant. These location-related variations arise through, forexample, variations in local labour rates, labour productivity, social charges, taxes and

duties etc. as well as through other fundamental variations such as material costs.


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The market for power plant is like any other market in the sense that prices will always beinfluenced by supply and demand, competition, and other market conditions at the time ofordering.

For a given basic technology and location, capital costs are highly dependent on plantspecification. Factors such as plant size, fuel type, site ambient conditions, fuel flexibilityrequired, most adverse design fuel specification, availability requirement, turndown andload following capability required, thermal efficiency required, environmentalperformance required etc, type of site (greenfield or brownfield), infrastructuredevelopment, proximity to grid connection, all have a significant impact.

A further important determinant of capital cost is the type of contracting arrangemententered into between the customer and supplier. This may range from a turnkey contractfor engineering, procurement, construction and commissioning against a basicrequirement specification issued by the customer, to the other extreme where the customeressentially acts as his own EPC contractor and manages many small contracts for

individual elements of the plant, taking responsibility for ensuring that all interface issuesare properly defined and managed. Since the contracting approach adopted affects theapportionment of risk between the customer and supplier, this in turn is reflected in theapparent capital cost of the plant.

With all these issues influencing capital cost it will therefore be apparent that any attempt tofocus in on a single best estimate of capital cost must be regarded with caution.Notwithstanding this, however, a number of recent publications do provide some estimateswhich are summarised below.

IEA Coal Research has recently reported on capital costs and efficiencies for clean coaltechnologies for power generation in their report entitled Competitiveness of future coal-firedunits in different countries. This report includes consideration of both circulating fluidisedbed combustion and pressurised fluidised bed combustion and presents the following capitalcosts (for the purposes of this report the quoted costs have been converted from US$ basis tosterling basis at the exchange rate prevailing at the time of writing.

These costs are stated to include turnkey construction costs, purchasers engineering andadministration costs, and interest charges during construction.

By way of comparison, the UK FORESIGHT Clean Coal Power Generation Task Force that

examined research, development and demonstration priorities for cleaner coal technology in1998 indicated that AFBC plant would cost in the range 550-560/kW and PFBC plant wouldcost in the range 545-550/kW.

It is further noted that the Jacksonville Electric Authority demonstration project on CFBC tobe undertaken under the US Clean Coal Technology Program is quoted as having a total costof $309 million, which would equate to around 700/kW based on the quoted net capacity ofthe plant. However, no breakdown of the programme cost is provided and the resulting figureshould therefore be used with caution.


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Finally results of work carried out under the Joule III programme of the EuropeanCommission by the Energy Research Centre of the University of Ulster, and completed inDecember 1998 are noted. This work addressed strategic considerations for clean coal R&Dand presented capital cost data for a wide range of coal technologies for power generation

including both CFBC and PFBC systems. The data were obtained by a process simulationapproach to establish principal material and energy flows which were then used as the basisfor equipment sizing and cost estimating using standard chemical engineering practiceencoded in the form of a further stage of computer simulation. This approach forms areasonable basis for self-consistent cost comparisons between different technologies but isless suitable for development of absolute capex data. Nevertheless, Ulster quote the resultsobtained as 695/kW for CFBC and 630/kW for PFBC, in both cases based on a 250MWunit size. These data have been converted from data quoted in Euro/kW in the original reportby converting at the exchange rate prevailing at the time of writing.

The variability of the data from these different and recent sources, which show a range of

550-820/kW for CFBC plant and 545-840/kW for PFBC, serves to illustrate the pointsmade earlier regarding the difficulties involved in establishing reliable capital cost data forpower plant generally. For the present review it is noted that capital costs of new utility scaleconstruction may currently be expected to fall somewhere within the above quoted ranges.

Operating costs and overall electricity costs

Like capital costs, operating costs for fluidised bed combustion-based power plants willexhibit a very high degree of variability depending on location, fuel type, design philosophy,guaranteed running hours etc. It is not considered realistic to state any specific numbers foroperating costs, because there is so much intrinsic variability and each individual plant will beunique.

Examples of the types of operating cost category that need to be considered in a detailedanalysis of a power project using FBC include:

Fuel Operating labour (salaries and wages) Maintenance labour (salaries and wages) Water Start-up fuel

Limestone (if used as sulphur sorbent) Chemicals Waste disposal Administration Depreciation Insurance Taxation License fees Grid access charges Financing charges Capital repairs Social costs (where applicable)


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These costs, once identified, feed in to an analysis of overall electricity costs. Although it ispossible to address this on a simplistic basis, overall electricity costs for a typical independentpower project are in reality the end result of a complex series of interlinked contractualarrangements. Figure 2.1 shows the typical structure of an independent power project in adeveloping country such as China, of the type that might well use fluidised bed combustion

technology.

With this type of structure, overall electricity costsper seare of less relevance thanestablishing the overall financial viability of the project from the point of view of all thestakeholders. Typically, financial viability will be established through the use of a financialmodel of the complete project, reflecting all of the contractual relationships implicit in theproject structure.

The overall objective of the financial modelling is to evaluate the project costs and benefitswithin a consistent framework and one that is transparent to all the parties to the project. Thegeneral methodology that is used comprises a number of steps. As a starting point, it is

necessary to consider the capital investment required for the project. The capital requirementsare broken down into foreign and local currency cost components and the anticipated phasingof disbursements during the project is estimated.

The operating costs of the plant are then assessed. Cost components that are consideredinclude those listed above. Projections of these costs in real terms over the operating life ofthe project are made. At the same time projections are made of the revenues in real termsfrom electricity and any by-product sales over the operating life of the project, thus enablingcash flow projections to be developed. Estimates of movements in working capitalrequirements are also made.

The operating cash flow projections derived in this way are combined with the capitaloutflows arising during the investment phase to produce an overall set of cash flows for theentire project cycle covering the construction period followed by an operating life of typicallytwenty years. These cash flows are then used to determine project viability, of which the keymeasures are normally (a) acceptable rates of return on equity to shareholders and (b)acceptable levels of debt service coverage for the financial institutions providing the loanelement of the finance. The results of the financial model will normally be used iteratively toestablish a set of key project parameters, including but not restricted to electricity offtakeprice, that is acceptable to all project stakeholders and is then reflected in the final versions ofthe key agreements governing the project.

It will be clear from the above that it is not considered realistic to quote specific numbers foroperating costs or for cost of electricity from FBC power plants, but rather it is consideredmore useful to identify the key issues that will affect these and to describe the methodologywhich would be used to establish them on an individual project basis.


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3 CURRENT STATUS AND ASSESSMENT OF FBC SYSTEMS

3.1 Introduction

For each principal technology covered, the current status of the technology is reviewed,including brief historical information on its development, consideration of design issues,steam conditions available, key components, installed capacity with geographical distribution,problem areas, commercial aspects, and UK capabilities in design, materials, equipment, plantand components.

3.2 Bubbling Fluidised Bed Combustion

The first successful development of FBC occurred in the 1920s, and subsequently a number ofuses were developed for different applications. In the UK, research into the practicalexploitation of FBC for coal burning commenced during the late 1950s. Much of the early

development work was carried out by the National Coal Board with a view to achievingcombustion efficiencies of ~99.5%, as was then being achieved with pf-fired plants. Thetechnology developed was known as the deep bed system and utilised crushed coal (2-3 mm)injected pneumatically into the base of a fluidised bed with a depth of up to 1 m. Workconcentrated on the utilisation of coal with a high ash content (~17%), intended for pf-firedpower generation purposes.

Crushed coal was generally used. However, crushing resulted in the formation of fines whichtended to be elutriated, resulting in significant loss of carbon. In order to minimise thisproblem, the technology came to incorporate a deep bed (up to 1 m static depth), a fluidisingvelocity of ~0.9 m/s, a freeboard height of up to 3 m, and coal feed to the base of the bed.However, even at this level of sophistication, achieving 99.5% combustion efficiency did notprove possible. Addition of limestone for sulphur control purposes achieved only modestsulphur capture levels as a consequence of loss of limestone from the bed. Recyclingviaacyclone improved this but added to the systems complexity. However, for some applicationsthis level of control was adequate and in several instances (in the USA and Japan), utility-scaleoperations were achieved (see below).

As well as efforts directed towards the development of systems aimed at power generationapplications, technology applicable as a substitute for various oil-fired industrial applicationswas also considered. As part of this programme, a 4 tph vertical shell boiler was developed,

using a static bed depth of 0.3-0.5 m and a fluidising velocity of 1-3.5m/s. However, the unitproved capable of achieving only 91% combustion efficiency, against a market requirement of~95%. Further developments resulted in the use of uncrushed, washed coal, fed over-bed bygravity, dropping onto the central surface of a 0.3 m deep fluidised bed. The coal feed systemwas thereby simplified from previous designs and pre-preparation was minimised. Using thistechnology, combustion efficiencies of ~95% were attainable, this leading on to thedevelopment of the over-bed, lump coal-fired, industrial fluidised bed combustor.

Despite the promising results, in the UK, development of FBC technology reduced in the early1970s although a number of worldwide technology licensing arrangements were entered into.Thus, various licensees constructed BFBC units for a number of coal-fired applications, but also

including the incineration of sewage sludge.


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UK Government incentives of the mid 1970s attempted to transfer some of industrys relianceon fuel oil to coal. BFBC was viewed as a prime candidate for this initiative, although in amodified (shallow bed) form. A number of variants were subsequently developed by both CREand several UK boiler manufacturers. All used the same basic approach of feeding uncrushedwashed coal, over-bed, to a fluid-bed of typically only 150 mm static depth.

Thus, the existing technology, aimed primarily at power station boiler applications, was adaptedto smaller industrial coal-burning units; these utilised uncrushed washed coal fed over-bed ontothe surface of a fluidised bed of 96%, with the more reactive coals, as were conventionallysupplied to industry. This efficiency was comparable to that obtained from industrial boilersburning bituminous coal on mechanical grates.

The fluidised combustion of high quality coals was a UK innovation, the impetus being todevelop automatic, coal-fired combustion plants, comparable in automation to that available

from contemporary oil and gas fired units. By 1976 the first generation of shallow bed BFBCprototypes was being installed at various UK premises. These included both FBC boilers andhot gas generators/furnaces. Between 1976-1985, shallow bed technology was adopted byvirtually all the commercial coal equipment suppliers within the UK and some 80 boilers and 30furnaces were installed, totalling over 1,000MWth. Individual units were generally small; manywere


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shutdown in order to prevent hot spots from forming, and by keeping the bed clear of smallagglomerates. In addition, modifications were made to the bed drain system in order tominimise problems of drain plugging with large pieces of agglomerated bed material. Otherplant outages have resulted from in-bed tube leaks, inorganic crust formation on in-bedsuperheater tubes, cyclone problems, and air heater temperatures.

The Heskett unit was originally built in 1963 as a spreader stoker travelling grate boiler. Thefuel used was North Dakota lignite supplied from a captive mine. From the start ofoperations, the boiler had been unable to achieve its original rated steam production level of650,000 lb/hr as a result of severe slag build-up on the furnace division and enclosure walls.The unit rating typically declined from 78MWe (gross) to 55MWe (gross) as slagaccumulated on the upper furnace surfaces. In co-operation with Babcock & Wilcox, theoption of replacing the travelling grate system in use with a bubbling fluidised bed wasdeveloped. The main advantage of this option was that most of the combustion and heatabsorption would occur at low temperature (816oC) in the bed. This would preclude theformation of slag on the convection tubes from high temperature combustion in the furnace.

The boiler retrofit, which was carried out successfully involved the removal of the grate,installation of the bottom-supported BFBC unit with in-bed steam generating andsuperheating surface, an in-bed feeding system, replacement of the forced draught fan,installation of screw coolers for bed material withdrawal, installation of a sand (bed material)receiving, drying, storage and metering system, replacement of the existing regenerative airheater with a tubular air heater, and installation of three boiler circulating pumps for in-bedsteam generating surfaces.

The third utility-scale BFBC repowering was carried out at TVAs Shawnee power plant. TheTVA Shawnee 160MWe demonstration project started operation in 1988 and comprised arepowering of the stations existing Unit 10. Much of the existing infrastructure, ancillarysystems and steam turbine were reused; however, initially, there were a number of problemsassociated with the reuse of some components such as the coal feeding arrangement.

The new BFBC boiler was designed to match the turbine steam conditions, the steamgenerator consisting of a balanced draft, drum-type unit with a combination of both naturaland forced circulation. The natural circulation encompassed the waterwalls of the bed area,the freeboard area and the lower convection pass. The forced circulation was through theboiler floor tubes and the three in-bed tube bundles. In order to maintain control of thecombustion in the boiler and match it with the desired steam conditions, the boiler bed area

was divided into twelve compartments. This division was actually in the air plenum, belowthe distributor plate. Each compartment had its own dedicated coal and limestone (under bed)feeding, and spent bed material removal systems.

Since the start of commercial operations, the Shawnee unit is claimed to have operated welland to have demonstrated its ability to generate electricity in a reliable and environmentally-sound manner, whilst burning high sulphur coal. Significant modifications carried out to thecoal preparation, coal and limestone feed systems, boiler internals and ash disposal systemshave resulted in significantly improved performance, compared to the start-up phase. Theoverall unit performance is comparable with its design specification. It continues to producepower for the TVA system and is operated as a normally despatched unit. Total operational

costs are claimed to be competitive with other coal-fired units operated by TVA. The plantnow regularly achieves overall availability figures of very close to 100%.


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Few other utility-scale BFBC developments took place, in part, because CFBC came torapidly dominate the market, although in Japan, in 1978, the Electric Power Development Co.Ltd (EPDC) began development of its own version of BFBC technology aimed at powergeneration applications. Initial work was carried out using a 20 t/h pilot plant, followed by

construction of a 50MWth unit at Wakamatsu. This was commissioned in 1987 and was usedto demonstrate the suitability of the technology for utility-scale operation. The configurationof the EPDC BFBC system comprised a main bed cell, used in association with a carbonburn-up cell, making the unit amenable to operation on low grade coals. Following extensivetesting, it was decided to scale-up the technology and apply it to repowering the ageing oil-fired Takehara power station. Thus, the existing boiler was replaced with a coal-fired BFBCunit with an evaporation rate of 1115 t/h. The plant entered commercial operation in 1995.However, despite its operation, there has since been little further development of this type oflarge-scale system in Japan. Development of BFBC in Japan has focused primarily on thedevelopment and utilisation of smaller capacity BFBC incinerators used for the combustion ofboth coal and MSW.

In Scandinavia, one manufacturer has continued to develop BFBC technology but primarilyfor large-scale industrial applications rather than power generation. Although some have beencoal-fired, increasing emphasis has been placed on the use of non-coal fuels such as industrialwastes generated by the pulp and paper industries. In this respect, the prime developer hasbeen Tampella, latterly part of the Kvaerner group of companies. Thus, a relatively smallnumber of large capacity BFBC units have b

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Fludised Bed Combustion Report

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