Next Generation CFBC

Chemical Engineering Science 54 (1999) 5565}5571

Next generation CFBC

S. RajaramBharat Heavy Electricals Limited, Tiruchirapalli, India

Abstract

CFBC boiler can utilise all grades of coal and lignites in an e$cient and environmentally clean way. Its development was initiatedin the mid 1970s and several CFBC boilers have been installed worldwide. The largest in operation is a 250 MW plant at Gardanne,France. CFBC designs with lower auxiliary power, lower refractory content, more appropriate support systems and with super criticalsteam pressures are being evolved to optimize overall costs. Future generation of power from coal is expected to be from IntegratedGasi"cation Combined Cycle plants based on Circulating Fluidised Bed Combustion and gasi"cation. ( 1999 Elsevier Science Ltd.All rights reserved.

Keywords: CFBC; Coals; Lignites; Super-critical CFBC; CFBC based combined cycle plants; IGCC

1. Introduction

Coal has re-emerged as a major energy source forpower generation after having played a subsidiary role tooil during the mid 20th century. Today about 40% of allworld electricity is generated from coal, almost doublethat of its nearest competitor, namely hydro (Table 1).World coal production was about 4630 million tonnes in1996 (Table 2); therefore, coal is the most important fuelsource for electricity generation in the world today andwould also continue to dominate the power station fuelscenario in the foreseeable future. In Asia, coal's share ofthe total electricity production rose from 26% in 1980 to42% in 1992 and is projected to increase to about 54%by 2010. However, realising electric power from coal inan e$cient and environmentally clean way has remainedthe biggest challenge that nature has imposed on man-kind. Circulating #uidised-bed combustion (CFBC)technology is probably the best solution reached com-mercially to this challenge posed by nature. CFBCe!ectively utilises all grades of coal in an environmentallyclean and e$cient way and is therefore expected to out-last conventional coal burning technologies for powerproduction. Most coals available in the US have moder-ate to high sulphur in them and require expensive add-ongas cleaning equipment to contain SO

xwhen utilised in

conventional pulverised fuel (PF) "red boilers. CFBCwith its unique in situ SO

xcapturing ability (by the

addition of a sorbent like limestone along with coal) hasbeen able to utilise these coal e!ectively. Although coal in

India have low sulphur, their high ('45%) ash contentcauses operational and maintenance problems like coalmill outages, #ame stabilisation problems at lower loadsand erosion when used in PF boilers.

CFBC technology holds a great promise for thesehigh-ash coals since it does not require pulverisation ofthe fuel but only crushing, the required ignition energy isprovided at all loads by the huge solids inventory in theCFB combustor and the ash can be handled dry.

Petroleum coke, a solid residue from oil re"nerieswhich have installed coking units to improve the pro"t-ability of the re"ning process, is gaining increasingacceptance as an alternate fuel (Dallas, 1997). Its charac-teristics include low volatile content, high sulphur(4}8%), very low ash (containing metallic compounds)and very high heat value (about 6000 kcal/kg). Conven-tional boilers primarily require add-on desulphurisationequipment that are very expensive and also experiencefouling and corrosion of heat transfer surfaces. However,a CFB boiler * with its inherent capability to absorbSO

2in situ along with the combustion process by the

addition of limestone with the fuel, contain NOx

to verylow levels because of lower combustion temperature andalso due to staging the combustion air * can utilisePetroleum Coke in a very e$cient and environmentallyclean way.

The total lignite production is 1000 million tonnes inthe world today (Table 3). Lignite is a young coal whichhas undergone less coali"cation than bituminous coaland anthracite. These are characterised by their high

0009-2509/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 8 8 - 2

Table 1World electricity generation in 1988

Fuel Trillion Wh (%)

Coal 4287 39Oil 1271 12Gas 1394 13Hydro, etc. 2182 20Nuclear 1891 17

Total 11025 100

Note. Extracted from IEA coal research report.

Table 2Major coal producing countries * 1996

Country Production(million tonnes)

Total coal Hard! coal

China 1375 1330USA 959 878India 296 271Russia 255 163Australia 249 195Germany 240 53South Africa 208 208Poland 202 138Kazakhastan 79 75Canada 76 40Others 692 309

World 4630 3660

!Sub-bituminous, bituminous, anthracite.Extracted from IEA coal research report.

Table 3Lignite resources and use

Country Proved Recent Powerrecoverable annual prod generation(million (million (milliontonnes) tonnes) tonnes)

Australia 36,200 35 33Bulgaria 3,700 32 23.5Canada 2,418 10 9.2China 126,500 30 22Czech 2,860 103 55FRG 35,150 127 107GDR 21,000 296 188Greece 3,000 32.5 31.5Hungary 2,883 7.1 5.6India 1,581 7.7 6.9Poland 14,400 50.4 49.3Turkey 4,763 22 12.4US 32,709 57 53.5USSR 94,500 122 76.8Yugoslavia 15,000 65 65

Note. Extracted from IEA coal research report.

moisture (35}70%) and oxygen contents and their lowcalori"c value just about half of bituminous coals). Someof the lignites found in Australia, China, Germany, USA(Texas), the erstwhile Soviet Union, Yugoslavia and inthe Gujarat region of India have moderate to high sul-phur in them. As mentioned above, their e!ective utilis-ation in PF boilers would require very expensive add-on#ue gas conditioning systems while CFB boilers withtheir in situ SO

xcapturing capability make utilisation of

these fuels techno-economically very attractive.CFBC technology can thus utilise a wide range of fuels

and is therefore quite di!erent from the other coal utilis-ation technologies such as PF "ring. CFBC can e!ec-tively burn almost all combustible materials* not onlyany type of coal and lignite but also coal washery wastes,anthracite culm, and other industrial wastes.

2. The CFBC boiler system * a description

Combustion in a CFB boiler takes place in a verticalchamber called the combustor amidst "ne grained solidssuspended in an upward #owing stream of gas.

The gas is generated by a fuel combustion while intro-ducing about 40}50% of the total air as primary airthrough an air distributor at the bottom of the combus-tor and the remaining as secondary air into lower/middlesections of the combustor to e!ect complete and stagedcombustion.

The fuel and sorbent (e.g. limestone, if required) are fedinto the combustor without requiring any costly fuelpreparation/distribution system, #uidised and burned attemperatures of about 8503C. The sorbent reacts with thesulphur dioxide released from burning the sulphur in fuel,to form calcium sulphate (gypsum). Due to the "ne par-ticle size of the solids and the high gas velocities in thecombustor, a considerable portion of the solids are en-trained and separated from the #ue gas in one or morerecycling cyclones and are continuously returned to thecombustor via a recycle loop, while the gases exit toa convection backpass to be cooled and then cleaned andexhausted to the stack as in conventional boilers.

CFBC boilers can be broadly classi"ed, depending onthe arrangement of the heat transfer surfaces and thehydrodynamics in the combustor, into the following:

f heat transfer surfaces immersed in the combustor *typi"ed by Ahlstrom pyropower (now Foster Wheeler)

f external #uidised * bed heat exchanger (FBHE) *typi"ed by Lurgi.

A typical Ahlstrom pyropower CFBC (Fig. 1) boiler hasa combustor in which the heat transfer surfaces arelocated in the upper region where heat is absorbed. Thesesurfaces have tubes of special construction to face the sti!combustor environment. The heat in the hot #ue gas

5566 S. Rajaram / Chemical Engineering Science 54 (1999) 5565}5571

Fig. 1. Ahlstrom/FW CFBC arrangement. Fig. 2. Lurgi CFBC arrangement.

Fig. 3. B and W CFBC arrangement.Fig. 4. DBW CFBC arrangement.

leaving the high-temperature cyclone is recovered furtherby a convection pass containing the usual superheater,economiser and airheater surfaces.

In the Lurgi CFBC boiler (Fig. 2) a mechanical SpiessValve located on the recycle loops permits a controlled

amount of solids to be passed through external #uid-bedheat exchangers (FBHE) containing portions of SH/RH/evaporator surfaces that cool the solids before theyare returned back to the combustor. FBHE helps separ-ate fuel combustion from primary loop (combustor,

S. Rajaram / Chemical Engineering Science 54 (1999) 5565}5571 5567

Table 4Comparison of auxiliary power (kW) for a typical 210 MWe (estimated)plant

Equipment PF CFBC

PA fan 967 3670SA fan 580 1220Ball mills 3220 *

Coal feed * 108Others incld. Blowers 2462 3193

Total 7229 8191

cyclone, seal pot, FBHE) heat transfer, thereby enablingthe combustor to be maintained at an optimum combus-tion conditions over wide load ranges and for fuels withwidely varying qualities. Consequently, the combustione$ciency increases and the limestone consumption alsoreduces.

Although Foster Wheeler/Ahlstrom pyropower CFBCand the Lurgi CFBC have the majority of the CFBCmarket share, other types of CFBC with various designfeatures have been developed and are commerciallyavailable.

Some typically di!erent CFB designs from the onesdescribed above are Babcock and Wilcox's InternallyRecirculating CFB design (featuring Studsvik's CFB pro-cess) (Fig. 3) with U-Beam separator for primary particleseparation, Deutche Babcock and Wilcox's Circo#uiddesign with low gas velocities with superheater tubebundles arrayed in the upper combustor section (Fig. 4),and with gas/solid temperature leaving the combustor at300}5003C to relatively cold cyclones.

3. The present CFBC

Development of CFBC boiler technology was initiatedin the mid 1970s and it gradually increased its share inthe world FBC boiler market in the 1980s. This was dueto its advantages over bubbling FBC, e.g., better combus-tion e$ciency, fewer fuel feed points and thereforesimpler fuel feed system, lower limestone requirement,lower NO

xemissions, due to longer gas}solid contact

time. The "rst utility size CFB boiler in the worldof 96 MWe capacity with reheat was started up atStadtwerke Duisburg, Germany in September 1985 byLurgi. The unit was designed to generate 270 t/h, 145 bar,5353C of SH steam and 230 t/h, 34 bar, 5353C of RHsteam while "ring coal with 20% ash and 1.5% sulphur.Several CFB units have been installed in this capacityrange, which include 2]175 MWe CFBC plant at Texas,New Mexico, USA by ABB-CE/Lurgi and 165 MWePoint Aconi Plant at Nova Scotia, Canada, by Pyro-power which are in operation for several years, all meet-ing the design parameters and sometimes even exceedingthem. The largest CFB unit in operation is the 250 MWeProvence/Gardanne unit installed in France by GECAlstrom-Stein Industrie of Lurgi design which has beenin commercial operation since April 1996 (Pierre Lucoret al., 1997). It is designed to burn sub-bituminous Gar-danne Coal with about 3.8% sulphur.

In India, BHEL Trichy, a leading utility boiler manu-facturer, sensing the rapid increase in FBC capacitydemanded by the market, entered into a technical collab-oration for CFB boilers with Lurgi Lentjes Babcock ofGermany. The "rst CFB boiler of 175 t/h, 105 ata, 5253Cwas contracted to M/S Sinarmas Pulp and Paper (India)Limited, Poona for burning coal. The unit was commis-

sioned in March 1998 and has been in operation for over2500 h on oil. Coal "ring has just begun and the operat-ing results are very encouraging.

Gujarat Industries Power Company Ltd (GIPCL)placed an order on BHEL for 2]125 MWe power plantto be located in Mangrol, Surat to burn Gujarat Lignitewith a maximum sulphur of 2.5%. Two CFB boilersrated at 390 t/h, 132 ata, 5403C of SH steam and 350 t/h,32.5 ata, 5403C of RH steam each are nearing completionof construction. The units are expected to commenceoperation by early 1999.

4. The future CFBC

Although present CFBC boiler costs are quite com-petitive as compared to PF boilers while using medium/high sulphur coals warranting add-on emission controlequipment, certain design areas of CFBC boilers still needre"nement to outlast the conventional counterparts.

f The present boiler e$ciency levels of CFBC and PFunits are comparable. Considering the feasibility toabsorb SO

xin situ, CFBC boilers can be designed for

lower (10}153C) exit-gas temperatures thereby im-proving the e$ciency (by 1}1.5%). Although theCFBC boiler auxiliary power compares favourablywith that of PF boilers with add-on emission systems,it is more by about 10}40% without these systems.This is because of the increased power consumed bythe PA and SA fans of CFBC boilers, although the coalmills o!set it considerably (Table 4).

E!orts to reduce the power consumption of the PAand SA fans by optimising combustor parameters likeheight, velocity and particle size would have to beexamined to push the CFBC technology ahead of PFboilers.

f Refractory applied in the lower combustor, cycloneand recycle systems is another area which increasesCBFC boiler installation and maintenance costs be-sides restricting start-up and shut-down periods. De-signs are already being evolved (like compact CFB) to


Table 5Present range of gaseous emissions

Country SO2

ppm(v) NOx

ppm(v)

New Existing New Existing

Australia * * 260}420" *

Canada 250" * 356" *

Czech Rep Slovakia 175}875 175}875 320 320European Union 140}700 * 320}635 *

France 140}700 140}700 320}635 320}635Germany 140}700 140}700 100}195 100}640Korea 700}1200 250}1200 355 355Netherlands 70}245 140 50}320 320}540Poland 190}615 235}1455 45}225 45}655Sweden 55}190 55}320 40}265 65}265UK 140}1050 700}1050! 245}320 245}320USA 260}520 520! 340}480 270}300

!Also based on annual totals."Guidelines.Extracted from IEA coal research report 1994.

drastically reduce the refractory content by integratingthese into the boiler steam/water circuits. This alsoimproves their structural rigidity and eliminates manyexpansion joints on their interconnections with thecombustor. E!orts are also underway to develop ap-propriate refractories that are abrasion resistant andwith high conductivity (to facilitate heat transfer toenclosing membrane water walls) requiring least main-tenance so that cost-e!ective, maintenance-free CFBCdesigns can be o!ered.

f Considering the large weight of bed material and therefractory in the lower combustor, CFBC designs witha combination (top and bottom) of support pointshave been evolved. But this leads to the inclusion ofa large number of expansion joints between the com-bustor and its connected systems besides increasingthe erection time. CFBC designs with appropriateboiler support systems that can optimise on overallcosts are also being evolved.

f The present gaseous emission levels (Table 5) are ex-pected to be further reduced, neccesciating design up-dating. Although su$cient work has already beendone in determining the mechanism of SO

xcapture,

the sorbent's (limestone) interaction with NOx

reduc-tion is still under scrutiny. Further reduction of NO

xlevels by optimally sizing the lower combustor region,improving the secondary air admission system to-gether with spraying of NO

xabsorbents in the upper

combustor are being examined.

CFBC design development has reached a mature stage.However, constant review on the cycles adopted for con-version of coal to power are continuously made. Some ofthem are presented in the following two paragraphs:

4.1. Super critical CFB design

Super critical CFB design improves plant e$cienciesthereby reducing operating (fuel) costs of the plant. Oncethrough PF "red steam generators designed for supercritical steam conditions (241 bar/5383C) also capable ofcyclic variable pressure duty, have been available in thelast decade. Adoption of these steam cycles for CFBboiler based power plants appears feasible, especiallyconsidering the following inherent advantageous featuresof the CFB combustion process (Skowyra et al., 1995):

f Low heat yux: The plan area of a CFB combustor islarger than a PF boiler of equivalent rating. Thisresults in a lower heat input/plan area or lower heat#ux. The average net heat input/plan area (NHI/PA)for a typical CFB boiler is about one third that of thePF boiler.

f Heat yux proxle: In a CFB combustor the solids den-sity and therefore heat transfer rate is maximum at thebottom of the combustor and decreases gradually withincrease in combustor height. The peak heat #ux istherefore near the bottom of the combustor, wherehowever, the #uid temperature is the lowest as it justenters the combustor at this place.

f This situation therefore creates a very favourable con-dition from wall metal temperature considerations.Moreover, in addition to the lower temperature facedin the combustor as compared to PF boiler, the tem-peratures are also uniform along the height of thecombustor.

f Cleaner combustor wall: Due to lower combustiontemperatures, the ash does not fuse, and there are noash deposits on the combustor walls. Due to the


Fig. 5. First generation IGCC plant schematic.

cleaner walls the heat absorption is nearly uniform.The heat absorption patterns change little laterallywith load, and tends to become compressed verticallydue to lower velocities, at part loads.

Developing CFB boiler design for super critical condi-tion therefore becomes more attractive.

4.2. CFB-based combined cycle plants

The liberal allotment of oil/gas/naptha, lower capitalcosts, shorter installation schedules and higher cycle ef-"ciencies are resulting in a sudden increase in premiumfuel-based combined cycle power plants. Consideringthat these premium fuels are not expected to last beyonda few decades these combined cycle plants would have todepend on coal as its fuel. Integrated gasi"cation com-bined cycle (IGCC) power plant based on pressurisedCFB combustion and gasi"cation are therefore expectedto be the future electric power generation technologiesfrom coal. BHEL through its in-house R and D e!ortshave already commissioned a 6 MW combined cyclepower plant based on coal with a pressurised #uidised-bed gasi"er of the bubbling bed type.

The schematic of the "rst generation IGCC plant witha pressurised CFB combustor is given in Fig. 5. Coal andsorbent are introduced into the pressurised CFB com-bustor, which is fed by air from the GT compressor. Thesolids are separated in a primary cyclone, and returned to

the combustor after cooling a part of these in the FBHE.The #ue gas at about 8503C is cleaned in special "ltersbefore admission into the gas turbine (GT) for expansionand power generation. A heat recovery steam generator(HRSG) located at the back end of the GT cools the gasby producing steam which is further heated in the FBHEof the CFB boiler, before expansion in the steam turbineto complete the combined cycle power plant.

Advanced IGCC plants based on integrated CFB gasi-"cation and combustion is given in Fig. 6. Fuel andsorbent are admitted in a carbonisor along with air fromthe GT compressor, volatiles in the fuel are liberated ina controlled atmosphere, to produce fuel gas which aftercleaning in a "ler is burnt in a topping combustor beforeadmission into the GT for expansion. The residual charfrom the carboniser is tapped o! to a pressurised CFBcombustor where it is burnt along with air from theGT compressor. The primary loop of the CFB boilercollects the solids in a cyclone and these are recirculatedback into the combustor via FBHE. The hot gases leav-ing the cyclone are further cleaned before admission tothe topping combustor and GT for expansion. A heatrecovery steam generator recovers heat from the GTexhaust, as already explained, for expansion in a steamturbine generator and completing the combined cycleplant. These advanced combined cycle plants togetherwith advanced gas turbines with steam-cooled rotorblades are expected to be the future power generationtechnologies based on coal with overall e$ciencies ex-ceeding 50%.


Fig. 6. Advanced IGCC plant schematic.

5. Conclusion

Coal is expected to outlast its fossil counterparts asa fuel source for electric power generation due to itsabundance. However, its utilisation has placed innumer-able challenges in its e$cient and environmentallyfriendly utilisation for power generation. CFB boilertechnology o!ers a unique solution for coal conversionto electric power. The successful commercial operation ofthe Provence 250 MWe CFBC-based power plant atGardanne, France has demonstrated its entry into thelarge size utility market. Looking further ahead into thenext century, CFBC-based Combined Cycle Power plantutilising coal is expected to be the ultimate solution toelectricity production from coal.

Acknowledgements

The author wishes to thank the management of BHELfor having given permission to publish this paper and tothe colleagues in his group for their assistance.

References

IEA coal research reports: Lignite resources and characteristics; De-cember 1988.

IEA coal research reports: Power station coal use: Prospects to 2000;October 1991.

IEA coal research reports: Major coal xelds of the world; January 1993.IEA coal research reports: Environmental performance of coal xred FBC;

November 1994.IEA coal research reports: Coal prospects in India } perspectives; Octo-

ber 1997.Dallas, W. T. et al. (1997). An update of operating experiences burning

petroleum Coke in a utility scale CFB } The NISCO Co generationproject. International Conference on FBC * ASME.

Pierre Lucor et al. (1997). Large CFB boilers: With Provence/gardannestart-up. (This novel technology has now passed the 250 MWemark). International Conference, Green Power* the need for the 21stCentury.

Skowyra, R.S. et al. (1995). Design of super critical sliding pressurecirculating #uidised-bed boiler with vertical water walls. Interna-tional Conference on FBC * ASME.


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Next Generation CFBC