Design features of the steam generatorJ.S. Goldsack, B.Sc.(Chem.Eng.)

Indexing terms: Power systems and plant, Generators

Abstract: The choice of boiler plant to satisfy best the requirements of a particular power station project isinfluenced by a combination of technical, commercial and system requirements generally unique to each newinstallation. An understanding of boiler design options and performance record is one of the major influenceson the selection of cycle parameters. The paper describes the selection of steam pressure and temperatureconditions and the consequences of the lack of indigenous fuel. The considerations bearing on the choice ofcirculation system, steam and reheat temperature control methods and the optimum arrangement of heatingsurfaces are described and the reasons for the choices given.

1 Introduction

The decision of the China Light & Power Company tofollow their Castle Peak A Power Station of four 350 MWunits with a B station of larger units had been anticipatedfrom the outset in plant design and site layout. When ins-tructions to proceed with the construction of four 660 MWunits were received in May 1981, the fuel policy for theplant design and operation had been agreed and thesystem and plant design studies to establish the options forunit size had been considered.

This paper highlights some of the main considerationsinfluencing the design of the steam generator itself.

2 Main considerations

The choice of cycle parameters, namely pressure and tem-perature, has to be made at the outset, with due regard toplant options available, economic assessments, availabilitydata and support from reference plant etc.

Fuel costs are a prime ingredient of the economicassessment and fuel quality affects the capital cost of theboiler plant and fuel and ash handling equipment.

2.1 Final steam and reheater outlet steam temperatureselection

Coal-fired plants have been ordered for UK power sta-tions with steam temperatures up to 574°C since 1948. By1959, a consistent policy of 568°C at both the superheaterand reheater outlets was being followed. The moreaggressive corrosion attack, particularly of austenitic steelsby oil ash deposits, has led the electricity supply industryin the UK and most utilities throughout the major indus-trial nations of the world to adopt a lower steam tem-perature for oil-fired plant, typically around 540°C, whichcan be achieved without the need for austenitic steels.

For firing fuel oil, China Light and Power were pre-pared to operate at reduced steam temperatures for thelimited periods of time, so that the predictable tube metalwastage would not be significant. As evidence on the avail-abilities of both temperature cycles appeared much thesame, the choice depended on economic assessment ofcapital and running costs and it was made for the lowertemperature of 538°C/538°C at turbine inlet, because theevaluation of station performance gain for the higher tem-perature cycle was not enough to justify the increased firstcost of the plant and its construction. This outcome is inline with world trends.

Paper 3363C, first read before IEE Power Division Professional Group P10, 18thMay 1983The author is with Babcock Power Ltd., 165 Great Dover St., London SE1,England

2.2 Steam pressureThe steam pressure selected at the inlet to the turbine, withaddition of main steam pipework pressure losses, results ina boiler stop valve pressure of 169 bar (gauge) [2450 lb/in2

(gauge)] which is only slightly higher than that on all the500 MW and 660 MW CEGB units, 165.5 bar (gauge)[2400 lb/in2 (gauge)].

Considerations of the long operating experience in theUK with this plant were predominant in this choice, pre-serving, as far as boiler plant is concerned, the option of adrum boiler design similar to CEGB plant.

The extra cost of higher pressure plant for only amodest gain in cycle efficiency was not a sufficient incen-tive for departing from a well established pressure level,general for the UK.

2.3 Fuel rangeThe lack of indigenous fuel in Hong Kong obliges ChinaLight and Power to obtain all fuel on the internationalmarket. Present and foreseeable relative costs between coaland oil clearly directed their choice towards coal as themain fuel. However, weather conditions in Hong Kong canbe so severe as to interrupt coal supply to the bunkers.

These considerations led to the A Station units beingdesigned to run continuously at full load on oil as well ason coal. In the case of the B Station, a capability of500 MW/unit on oil was considered enough. The durationof oil firing was considered to be limited, such that corro-sion of austenitic steels at high temperature, on oil firing,diminished very greatly in the debate regarding the choiceof final steam temperatures. Nevertheless, this still resultedin a selection of the same final steam temperature.

The properties of the coal fuels vary widely, both chemi-cally and physically, depending on the sources, and it isrequired to strike a sensible compromise between theinitial cost of the steam generator and the range of coalproperties for which it is designed. The aim is to achievethe most economic design, with a range of properties wideenough to enable the operator to purchase competitively.

Not only is the combustibility and ash content of thecoal significant but also its physical state, mechanicalproperties and, last but not least, the ash chemistry whichis influential on its slagging and fouling propensities andalso the collectability of the fly ash in the precipitators.

A typical purchasing specification is shown in Table 1,with a remarks column which indicates the likely, or pos-sible, consequences of firing fuel outside the specification.This brings out the interdependence between fuel proper-ties and plant sizing and design. For example, GCV andHardgrove limitations depend on the installed capacitychosen for the milling equipment.

It must be emphasised that this is a typical specification,

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984 261

Table 1 : Typical coal purchasing specification

Minimum Maximum Remarks

Total moisture (AR), %

Volatile matter (DMMF), %

Volatile matter (DAF), %

Ash (AR), %Total sulphur (AR), %

Chlorine (AR), %

GCV and Hardgrove (H) number, MJ/kg

Raw coal size grading, % 0 to 0.5 mm

Initial deformation temperaturereducing basis, CC

Slagging index, R

—

0.35

—

GCV23.8424.4225.5926.7527.91—

1050

—

H5049474644

(7)

25(3)2.0 (4)

1.0(5)

15(6)

—

1.25 (8)

Fouling index, lf

— 15(1) Maximum for mill 21 %

26 — Oil support at higher part load if lessthan 26

— 42 (2) Risk of spontaneous ignition onstockpile if 42 exceeded

Can be lower than lower limit ifprecipitator suitably sized

Set by chosen installed firingsystem capability; if belowminimum, possibility of higherunburned loss and shortfall inboiler output

If exceeded, risk of hang up inbunker/feeder system

If lower, cleaning difficultiespossible at convection bank inlet

If exceeded for a particular fuel,analyses to be referred to boilermaker

0.75 (8) If exceeded for a particular fuel,analyses to be referred to boilermaker

AR = as received; DMMF = dry mineral matter free; DAF = dry ash free.

Notes:(7) Often lower than maximum acceptable to mill at user's discretion(2) Limit influenced by site climatic conditions(3) Limit set by user and plant designed to suit(4) Determined by site gaseous pollution limits, in which chimney height may be a factor(5) Guide value regarding fouling(6) Guide value. For higher values special consideration to bunker design/feeders required(7) Related to design furnace exit gas temperature(8) Reached in consideration of likely fuel sources and optimisation of boiler design.

not the actual one for the A station units nor the B stationones.

3 Boiler design

The design of the boiler to satisfy the requirementsinvolves choice between a number of principal options inrelation to the evaporative circuit system design, the super-heater and reheater design including method of regulationand the adoption of appropriate physical arrangements forthe furnace. The general style and shape of the boiler isdetermined by the fuel. All these aspects can be dealt withas follows:

3.1 Furnace and combustion equipmentWhatever the fuel, the furnace must be large enough toallow full flame development and complete combustionahead of bank heating surfaces. Additionally, in the case ofplant firing pulverised coal, combustion zone dimensionsand ratings must reflect the fuel ash slagging character andsufficient effective radiant surface must be provided withinthe furnace, whether water- or steam-cooled, to lower thefurnace exit gas temperature at the entry to the convectionsurfaces to such a level that fouling of them is tolerableand that molten slagging is positively avoided. The condi-tions should be such that deposits can be suitably con-trolled by soot blowing at routine intervals.

The requirements must be met at the same time asensuring that the heat necessary to achieve the desiredsuperheater and reheater outlet steam temperatures isavailable before the gases become too cooled for effectiveheat transfer. For the B Station units, in common with all

262

high-pressure plant with steam reheating on bituminouscoal, the furnace exit gas temperature demanded by fuelash fusibility considerations is lower than that essential toachieve superheater and reheater performance, unless aradiant section of superheater or reheater surface isinstalled within the furnace itself.

Fig. 1 shows the gas temperature needed to provide allthe heat to superheat and reheat steam, with allowancesfor spray-water injection for control, for a range of differ-ent operating pressures, and at the same time maintain areasonably substantial temperature head of 200°C between

1400

1300

£1200

1100

1000

forreheatingandsuperheating

forsuperheating"only

37. S/H spray

^ ^7. S/H spray

60 80 100 120 140superheater outlet pressure, bar

160

Fig. 1 Temperature of gas needed at inlet to superheater and reheaterwith bituminous coal

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984

gas and fluid at the downstream side of the primary super-heater. Clearly, for minimum capital cost, the temperaturehead in the last part of the superheater and reheater needsto be as high as practicable, and this leads to the provisionof extensive radiant pendant superheaters within thefurnace. Radiant reheater surface is also feasible, butrequires the provision of cooling steam through a turbinebypass system during start-up, prior to synchronising theturbine generator, but more significantly would require agreater pressure drop in relation to the necessary steamspeeds for such a high heat transfer zone, than can be per-mitted within the reheat system without significant wors-ening of the cycle efficiency.

The radiant pendant superheater is suspended above thefurnace on a wide pitch, so that any collection of moltenslag which may occur at the gas temperatures prevailingwith certain of the fuels can run off, or be blown off by thesoot blowers, falling back harmlessly into the furnace fromwhere it is discharged through the hopper throat togetherwith any ash falls from the furnace walls.

The operating metal temperatures of the heating sur-faces in the gas stream beyond the furnace exit, and of theheader and pipework system which serve them, can befavourably influenced by a substantially uniform gas tem-perature across the exit of the combustion zone of thefurnace. The choice of a pattern of burners placed atregular pitches across the boiler width, with suitablychosen wing clearances compared to a corner firedarrangement, provides the designer with a wide range ofchoice in establishing the optimum physical dimensions forthe combustion chamber.

Generally speaking, single wall firing become uneco-nomic once there are more than four mills and, therefore,four rows of burners, because it leads to a tall burnerpattern and too much variation in the level of heat input,depending on which combination of mills is being used.Generally, above 300 MW, therefore, an opposed firedarrangement is more appropriate and has been adopted forCastle Peak B as was the case for Castle Peak A. The BStation units follow on essentially from the 660 MW plantsat Drax and 600 MW ones at Matla, in South Africa,which have opposed fired burners. The B station unit has 4rows of 6 in the front wall and 3 rows of 6 in the rear wall.Each row is associated with one mill group so that auniform lateral heat-input distribution to the combustionzone is obtained, whichever combination of mills is inservice. Provision is made for the possible addition, at alater date, of an eighth row supplied by an eighth mill.

All these considerations, together with firing equipmentsizes, led to 7 Babcock E-type mills being selected, eachserving a row of 6 burners. These are Babcock circularburners fitted with steam atomised Y-jet oil burners andconcentric primary air/pulverised-coal nozzles.

Although each burner is capable of firing fuel oil andcoal in a wide range of proportions, both the attainment ofminimum excess air levels and fuel strategy envisaged haveled to the adoption of an arrangement in which each groupof burners is fired either with coal or with oil, apart fromstart-up and mill changing.

3.2 Boiler circulation system selectionWith the fuel determining the overall configuration of theboiler, the circulation system may be selected from anumber of options. The style of boiler is classified as'Babcock Two Pass', the name coming from the fact thatthe heating surfaces are arranged in two gas passes with apendant surface at and immediately beyond the furnaceexit followed by a horizontal surface in a down-flow gas

pass at the rear. The name usually applies to units havingnatural circulation and corresponds to the general style ofarrangement for both the Castle Peak A and Castle PeakB Station units, thus establishing their general pedigree.However, the Babcock Two Pass unit also exists as a oncethrough type, exemplified by the 630 MW coal-fired once-through boiler in operation at Enstedvaerket PowerStation in Denmark. This unit was designed by BabcockPower and supplies steam at 199 bar and 535°C/535°C.

There are basically three options for the circulationsystem:

(i) drum-type, natural circulation(ii) drum-type, assisted circulation

(iii) once-through system.

If the pressure is supercritical or only slightly subcritical,then both drum type units are ruled out and there is noreal option regarding the circulation system, it must be aonce-through system. With the pressures chosen for CastlePeak, all three options are possible.

Many are the arguments for and against all of thesetypes and not insignificant in them are historical factors ofhow practice has developed in various countries. Broadlyspeaking in Europe, the UK has favoured drum boilers,and so has France, while Germany favoured the once-through system. In Scandinavia and Finland both systemsare widely used.

Babcock Power is a supplier and designer of bothdrum-type natural circulation units and once-throughunits of the Benson type, but has traditionally not beeninvolved with fossil-fired major utility-assisted circulationdrum boilers.

It is only possible to touch on a few of the main pointswhich need to be taken into account, from a strictly techni-cal or economic point of view.

3.2.1 Drum boiler, natural circulation (/), and drumboiler, assisted circulation (ii): There are many examplesof both systems in operation, for example, in the UK at thesame pressure conditions, but predominantly natural cir-culation.

The natural-circulation system requires that the heatedcircuits have a low resistance, so that the natural statichead in the circulating water is enough to drive the steamand water mixture at an adequate speed through theheated circuits. This requires the furnace tubes and othercircuits to be generously sized, and it leads, in fact, to asimple robust arrangement which has a number of addi-tional advantages, such as being easier to construct towithstand high explosion or implosion pressure excursions.Care is required in designing successful natural circulationsystems to take full advantage of their inherent self-stabilising features, and it is essential to have the necessaryexpertise backed by wide operating experience. Both ofthese requirements are met by Babcock Power enablingdesign criteria and margins to be reliably established,against which a particular system design is assessed.

In fact Babcock Power plants of this type firing coalcomprise 8500 MW in the capacity range 500 to 660 MW,in the UK alone, with an additional 9000 MW either inoperation or in the course of manufacture and commis-sioning overseas, with superheater outlet pressures rangingup to 174 bar (2520 lb/in2) and beyond.

The absence of circulating pumps and their associatedpower consumption is an advantage of this system.

On the other hand, with an assisted circulation unitsmaller diameter furnace tubes can be used, leading to alighter construction as the increased friction loss is then

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984 263

overcome by the head generated by the circulating pumps.The initial cost of the circulating pumps, together withtheir ancillary control equipment etc., and the capitalisedvalue of the running cost have to be set against savingspossible from a somewhat lighter furnace envelope weight,together with a corresponding saving in support steelwork,although bracing steelwork to stiffen furnace walls againstabnormal pressure excursions is not likely to be less exten-sive and possibly more extensive, than on a natural-circulation unit.

Circulating ratios for assisted circulation systems areusually slightly less than those for natural circulation,being selected to suit the purpose, bearing in mind thatany excess is reflected in extra capacity required for circu-lating pumps.

The balance of the technical arguments and otherfactors came down on the side of natural circulation, andthis is the system chosen for Castle Peak A and CastlePeak B.

3.2.2 Once-through system (///): This third possibilitywould have been technically satisfactory. Almost invari-ably, a circulating system is provided for augmenting theflow through the furnace at very low loads and start-up,with a separating vessel to ensure steam-free water into thecirculating pump. Thus, at low loads, even the once-through boiler is, in fact, an assisted or forced circulationboiler.

There are advantages in not haivng a fixed point for the

start of the superheater, and design of the superheater istherefore easier, especially for a wide range of fuel types inthe same unit. Although care has to be taken in locatingthe separating vessel, to ensure a smooth transition fromcirculation mode to once-through mode and back again,without incurring superheater outlet temperature changes.

As far as the reheater is concerned, this is a forced-flowsection in any event, and considerations regarding reheatcontrol are the same, whatever the circulation systemchosen.

To achieve adequate speeds, small bore tubing isrequired and this leads to a slightly lighter furnace con-struction than for a drum unit, with some fairly compli-cated arrangements of tubes which, in the Benson system,wind round the furnace and are not self-supporting.

This is not a problem for the operator, but the unit issomewhat less rugged and requires more precise oper-ational control.

Its most identifiable disadvantage, however, is the highpressure drop from economiser inlet to superheater outlet,compared to drum units. The increased feed-pump powerwhich results, not only requires higher pressure feedpumps to be installed at higher cost, but increases thestation heat rate. This attracts an adverse evaluation, andalso implies a requirement for a marginally increasedinstalled capacity for the station to achieve the same netoutput to the electrical network.

The general arrangement of the Castle Peak B units isshown in Fig. 2 indicating where the furnace exit plane is.

j ~ reheater

primarysuperheater

economiser

61.5m

Fig. 2 Castle Peak B—side elevation of boiler[Babcock Power Ltd.]

264 IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984

3.3 Selection of control system for HP steamtemperature and reheat temperature

HP steam temperature is to be held constant at 538°C oncoal down to 50% load and reheat temperature at 538°Cdown to 50% load, at turbine inlet. These temperatures arealso expected at the maximum oil-firing condition of500 MW.

3.3.1 HP steam temperature: There is a long-establishedpractice, with high-pressure plant of this capacity and highfeed-water purity, for HP steam temperature to be con-trolled by spray injection of feed-water interstage, betweensuperheater stages. This gives a simple and effective meansof control, and the only real question is whether thereshould be one stage or more than one stage.

Influential in this choice are the following consider-ations:

(a) The last stage of superheater must be sufficientlysmall for its speed of response with an appropriate controlsystem to be quick enough to maintain outlet temperature,within stipulated limits of variation during load change orother causes of disturbance.

(b) If only one stage of desuperheating is applied, it mustbe located to fulfil this requirement.

(c) The range of operating conditions including oper-ating with low feed temperature (which enhances super-heater performance) dictates the total amount of surfaceand spray capacity appropriate, and this determines themaximum operating steam temperature immediatelybefore the final stage of spray injection. This temperaturemay be calculated to approach or sometimes exceed thefinal steam temperature, which inevitably leads to a moreonerous condition at that point for metal temperature andmaterial choice than at the superheater outlet. In the caseof a 565°C cycle, where the general temperature level ishigh enough to require austenitic surface; anyway, aheadof a single spray arrangement as well as the final super-heater outlet, then this might not be an unacceptablearrangement technically. At the 540°C temperature level,however, it is desirable by introducing an additional stageof spray earlier in the circuit, ahead of the radiant super-heater, to retain all the surface in comparatively low-gradeferritic alloy; 2.1/4% chrome being the normal.

Consequently, the choice for the B Station, as for the AStation, is for two stages of spray.

3.3.2 Reheat steam temperature: For the control ofreheat steam temperature there are essentially threeoptions. Because the reheater is located predominantlytowards the cooler end of the unit, its characteristic ismarkedly convective; that is to say, if the unit is fired withthe same amount of excess air, over the load range, reheatsteam outlet temperature will be highest at full load andwill fall off as load is reduced. There is an immediatechoice between a design approach which seeks to fix thesurfaces for the full-load condition, and combats the inher-ent deficit at part load by boosting the performance insome way, or one which fixes the surfaces to meet thesteam temperature requirements at the control load andfinds means of countering inherent overperformance at fullload. Clearly, from a surface economics point of view, thefirst method is preferable.

Apart from the application of tilting burners in thefurnace, which can usually only give a limited range ofcontrol, needing a supplementary system as well, there areonly really the following practical options:

(a) Fix the reheater surface at the control point and

eliminate overperformance at higher load by interstagespray injection. This is expensive on heating surface andpipework. It has a bad effect on cycle efficiency. Its onlymerit is the actual simplicity of the control hardware andsystem itself.

(b) Arrange the reheater surface in one path of aparallel gas path system, locating superheater surface inthe other path and regulating the proportion of gasflowing over each by means of dampers. This system incursthe additional cost of a dividing wall and the dampers, aswell as the bank heating surface itself. Surfaces are fixed sothat nearly all the gas is passed over the reheater surface atthe control load, with very little over the primary super-heater. As the load is increased, the gas flow is progress-ively biased away from the primary superheater.

(c) The third method is to arrange the reheater surfaceto meet its full duty at full load and counteract the fall incharacteristic on load reduction by depressing the furnaceabsorption, such that more heat is available entering theconvection surfaces than before. The established techniquefor this is to recirculate flue gas through the furnace via thefurnace hopper, as indicated schematically in Fig. 3. The

J^-hot R/HL - .(Meads)

-•-cold R/H-+• (4 leads)

Fig. 3

controldamper balance damper

Reheat control by gas recirculation

gas degrades the furnace energy to a lower temperature sothat the radiant heat absorption is reduced. On coal firing,this system formerly relied on separate gas recirculationfans taking hot gas from the economiser exit with theinterposition of some gas-cleaning equipment. The unsatis-factory track record of these systems, with fan erosion asthe principal problem leading to sometimes spectacular fanfailures, caused them to fall into disrepute, but thisproblem is obviated if the recycled gas is taken after theelectrostatic precipitators. Furthermore, by taking it fromnot only beyond the precipitator but beyond the induceddraught fans as well there is no need for gas recirculationfans at all. All that is required is a damper control beyondthe take-off point to enable sufficient pressure to be main-tained in the gas-recirculation system, to allow satisfactoryflow regulation of the recirculated gas into the furnacehopper in response to deviations in reheat steam tem-perature.

This system has been in successful operation on theBabcock designed coal and oil-fired units at Studstrup and

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984 265

Fynsvaerket in Denmark, and Tahkoluoto in Finland,which provide the necessary operational backing of experi-ence to validate the system for both Castle Peak A andCastle Peak B, and, consequently, for its choice.

At high loads, the pressure difference which existsbetween the base of the stack and the furnace hopper ismore than sufficient to drive the relatively small amountsof recycled gas through the connecting flue, without anyneed to close in the control damper. Only at lower loads,with an increasing quantity of recycled gas needed, is itnecessary to elevate pressure by closing in the balancedamper.

Although the recycled gas quantity is determined by thereheater outlet steam temperature, the superheater per-formance is also enhanced, which assists in extending theload range for full steam temperature. This effect is illus-trated in Fig. 4. As load falls, the gas recirculation rate

550

3 500o<Da.££450

400

£600Q.

4/

£550

2'a 500

80

with gas recirculation

gas recirculation without spray

gas recirculation and spray-540

40 50 60 7 0 80boiler load,7o

90 100

Fig. 4 Effect of gas recirculation

progressively increases from a nominal amount at full loadon coal. Increased rates of gas recirculation for oil have theeffect of bringing the characteristics of the unit more nearlythe same for oil as for coal. The spray and gas proportion-ing systems give no opportunity for boosting performanceson oil.

The electrical power consumption associated with thissystem is low. The additional volume handled by theinduced draught fans at full load is only about 3% withoutadditional head, and the fans are operating near their peakefficiency. At lower loads, the rising quantity and addi-tional head needed require only a modest power increase.This system provides the benefits of the simplified arrange-ments of heating surface normally associated with gasrecirculation, while avoiding all the former problems of gasrecycling fans.

Another advantage of gas recirculation for reheatcontrol, especially on oil, is that the augmented flue gasquantity at full load passes through the air heater andtends to keep the temperature into the main flue system

high enough to avoid possible cold-end-corrosion prob-lems.

3.4 Arrangement of tube banksThe general sequence of surfaces is dictated by consider-ations of temperature heads for thermal efficiency and ofmetal temperatures for economic material selection; bothwithin a practical and easily accessible and cleanablearrangement. At the same time, manufacturing, transporta-tion and construction requirements have to be catered for,and all the mechanical design imperatives of wall junc-tions, corners, penetrations, casings etc. must be respected.

The temperature head diagram, Fig. 5, shows the

2000

1500

221000

500

adiabatic temp. -1900°C approx.

sat.-358

Castle Peak BBCMRcoal firing

gas recycling rate37* of live gas

[Recycled gas

furnace r»radiantS/H

final reheater prS/H r "

Fig. 5

gas path through the unit

Temperature head diagram

general progression of gas and fluid temperature throughthe unit. Noteworthy is the fact that, where temperatureheads are large, a parallel flow arrangement of surface hasa very small heat-transfer penalty compared to contraflow,but gives a marked advantage in equalising metal tem-peratures through the circuit, so that, at the fluid outlet,the excess of design metal temperature over fluid tem-perature is minimal. That applies within each heatexchange section, but, more generally, the sequences ofsurfaces associates low fluid temperatures with high gastemperatures, wherever possible, minimising metal tem-peratures throughout.

Within this overall concept, a design strategy is necess-ary which recognises that there are a very large number ofoperational variables leading to uncertainties in per-formance prediction and caters adequately for operation'off design'.

Over 20 years of experience have now been accumulatedwith this type of system involving radiant pendant surfacein furnaces for power station plant. The radiant pendantsuperheater has been proved on more than 50 installations,and only very exceptionally has actual performance beensufficiently different from predicted and catered for, thatactual physical modifications of any significance have beenrequired. The detail design of the platen surfaces and esti-mation of individual tube pick-up and metal temperaturesis a particular expertise which requires operational empiri-cism allied to theoretical predictions. Operating experience

266 IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984

has demonstrated that the radiant superheaters can bekept satisfactorily free of ash accumulations using standardhigh-temperature long-lance retractable soot blowers, eventhough at full load the gas temperature at inlet may bewell in excess of the ash fusion temperature.

fired plant is needed, and one can assert with confidencethat for any Babcock units which would contend forrenewed ordering of coal-fired plant would be based on theprinciples established and being engineered into the CastlePeak B steam generators.

4 Conclusion

It is not immediately obvious to the designer how toreconcile the requirements for the provision of fully provenplant, on the one hand, with the need and desire to imple-ment design advances, on the other.

The main aim must be to achieve the stipulated oper-ating conditions with reliable plant fit and durable for thepurpose. This is technically easy if generous margins areallowed, but not economic. Thus, adequate, not generous,margins are required. This hinges on the judgment as towhat actually is 'adequate'. Margins are usually stipulatedfor the auxiliary plant, but not for the boiler itself. It isnecessary to assess as objectively as possible what marginsto allow and minimise subjective judgements. Any truemargin beyond this only adds to the cost, with no benefitin return.

The designer has to cater for a reasonable combinationof simultaneous adverse conditions. To cater for alladverse conditions at once would lead to an investment inplant which was only justified on very rare occasions. Thisaccepts that occasionally there could be operational limi-tations. For example, if coal outside the purchasing specifi-cation is offered, then the likely consequences can beevaluated before a commitment to such a fuel consignmentis made, in comparison to the actual boiler design beingprovided.

The long experience of coal firing in the UK on drumboilers, Drax being the most recent example, and the suc-cessful experience of gas recirculation applied to coal-firedunits of Babcock Power design in Europe have beenbrought together in the design of both the Castle Peak Aand the Castle Peak B boilers.

Thus, it may be seen that a modern design which doessucceed in combining design advance with proven practicehas been achieved. Similar designs are now being offeredto major overseas utilities all the time.

For the immediate future, significant coal-fired plant forthe UK electricity supply industry seems unlikely, but thetime will eventually come when at least replacement coal-

5 Discussion

5.1 Guest Editor's comments on the paperThe paper is confined to the major design parameters ofthe steam generators, as there was not enough time avail-able in the meeting to deal with the design of the manyauxiliary systems. It is very interesting to see the pros andcons of the very large number of design considerations,such as the choice of temperature, pressure, fuel range andthe style of boiler, whether it is to be a once-through typeor drum type with natural or assisted circulation. Of parti-cular significance is the method of reheat control and thechoice of gas recirculation. It is clear that some of thesedecisions are based on the experience and history of thetechnology in a particular country or with a particularmanufacturer; there is, thus, much scope for engineeringjudgment in steam-generator design. The author alsopoints to the difficulty of reconciling proven design withadvances in design, and the need for design margins to bejustified both technically and economically. Both thesepoints are important.

5.2 At the meetingA questioner raised the issue on the degree to which plantproven in service in the UK had been used, making specialreference to the rotary air heater system for extracting heatfrom the exit gases and heating the boiler inlet air. Theauthor replied that, in general, only plant proven in servicehad been used, the air heater system is standard in the UK.However, the experience did not have to be confined to theUK and the gas-recirculation system discussed in thepaper (Fig. 3) had been proven on Babcock designedboilers in Denmark and Finland.

A questioner referred to the discussion in the paper,Section 2.3, of the wide variety of possible coals whichcould be used and why some were rejected. The authorstated that some 40 representative samples of coal wereinvestigated and, of these, only 2 or 3 were consideredmarginal in regard to ash quality, and CLP were advisedaccordingly.

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 6, SEPTEMBER 1984 267