Unburned Carbon Loss in Fly Ash of CFB Boilers Burning Hard Coal

TSINGHUA SCIENCE AND TECHNOLOGYISSN1007-021408/21olume 8. Number 6. December 2003Unburned Carbon Loss in Fly Ash of cFB boilersBurning hard CoalU Junfu(吕俊复), WANG Qimin(王启民), LI Yong(黎永)YUE Guangxi(岳光溪),YamY.Lee2, Baldur eliasson2,SHEN Jiezhong(沈解忠)3, YU Long(于龙1. Department of Thermal Engineering, Tsinghua University, Beijing 100084. China;2. Energy and Global Change Department, ABB Corporate Research Ltd, Switzerland3. Wuxi Boiler Works, Wuxi 214028, China;4. Harbin Boiler Works co. Ltd, Harbin 150040, ChinaAbstract: The unburned carbon loss in fly ash of circulating fluidized bed(CFB) boilers, most of which areburning active fuels such as lignite or peat, is normally very low. However, most CFB boilers in China usuallyburn hard coals such as anthracite and bituminous coal and coal wastes, so the carbon content in the fly ashfrom these boilers is higher than expected. This paper investigates the source of unburned carbon in the fly ashof CFB boilers burning hard coal through a series of field tests and laboratory investigations. The char behaviorduring combustion, including fragmentation and deactivation, which is related to the parent coal, has ant impact on the carbon burnout in Cb boilers. The research shows that char deactivation occursduring char burnout in fluidized bed combustion, especially for large particles of low rank coal. The unevenmixing of solids and air in the core region of the furnace also causes poor burnout of carbon in CFB fly ash. Anindex describing the volatile content (as dry ash free basis over the heating value is proposed to present theoal rank. The coal combustion efficiency is shown to be strongly connected with this coal index. Severalchanges in the CFB boiler design are suggested to reduce the unburned carbon loss in the fly ashKey words: circulating fluidized bed (CFB): unburned carbon loss; uneven mixing coal indexIntroductionChina. There are already more than eight hundredCFB boilers operating in China. Most smallAs a clean coal technology, circulating fluidized capacity CFB boilers (35-130 t/h steam capacitybed(CFB) technology has become a desirable and some large capacity CFB boilers (over 220 t/hchoice for both industrial boilers and power station steam output )are domestic technology while someboilers in China because of its flexible fuel large capacity CFB boilers are importedequivalents and low emission control costs. CFB technology. The combustion efficiency of CFBboiler development started in the early 1980s in boilers is reportedly very high. However,inChina, many CFB boilers burning hard coals, suchReceived:2002-04-25; revised:2002-07-03as anthracite and bituminous coal and coal wastesSupported by the National Key basic Research Speciahaund (No. 200026309), the Major Research Project of explthe Tenth-Five Plan (2001-2005) of China NoYHa中国煤化工 t in the fly ash thanNMHburned carbon IJU.avA aLut efficiency, so they are2001BA401A03),and Energy and Global Change less competitive than pulverized coal boilers in theDepartment, ABB Corporate Research LtdChinese boiler market. Furthermore, the highSwitzerlandcarbon content in the CFB fly ash limits the usage兴共 To whom correspondence should be addressedEmal万忘数据 tsinghua.edu.cnof the fly ash as construction materials or as partialreplacement for cement.. Therefore, the carbonTel:86-10-62781559Tsinghua Science and Technology, December 2003, 8(6): 687-691content in the fly ash must be decreased to an basis, and Qar. net, p is the low grade heating valueacceptable level for the cement industryThe carbon content in the fly ash wcfa of theseThe carbon burnout in Chinese cfb boilers was cfb boileith the index i as sheinvestigated both in the laboratory and in the field Fig. 1. In addition, although the furnaceto identify the major factors affecting the formation temperature for anthracite combustion is higherof unburned carbon in the fly ashthan for the other coals, the carbon content in the1 Carbon Content in Fly Ash fromfly ash for anthracite is still the highest. Inaddition to these seven boilers. theCFB Boilers in Chinaexperience in China is that the low carbntentCFB boilers can be designed to burn almost any in fly ash from CFB boilers that burn anthracite iskind of solid fuel, which provides excellent fuel always suspectable. Even for some kinds offlexibility. But practical experience with Chinese bituminous, the carbon content in the fly ash isCFB boilers has shown that the carbon content in still relatively high, although the unburned carbonthe fly ash is not as low as expectedloss from the boiler is not very significant becauseData from seven selected chinese cfb boilersd from the highpresented in Tables 1 and 2. The feed coheating value bituminous coal. The lignite alwaysproperties are listed in Table 1, while the carbon burns out easily in the CFB boilers. The volatilitycontent in the fly ash and boiler operating index represents the ratio of easily burnt volatilesparameters are listed in Table 2.to combustible matter in the coal. The volatile2 Relation Between Coal Propertieslonger residence time to burn out in the CFBand Fly ash Carbon ContentThe carbon content in the fly ash is strongly 3 Cyclone Performancesrelated to the coal type. Coal in Boilers A and Glignite, coal in Boiler B is anthracite, C is low rank Fine char particles elutriated from the dense bedbituminous,D is lean coal, and coal in Boilers e are the major source of fly ash with high carbonand F is bituminous. An index I was defined to content. Most people believe that cyclonerelate the volatile content (as dry ash free basis )to performance is the key to improving carbonthe heating valueburnout. At low combustion temperatures, thecombustion rate is lower than that in pulverized(1)combustion boilers. Longer residence time iswhere wv, daf is the volatile content as dry ash freeTable 1 Properties of feed coal burned in seven Chinese CFB boilersboiler(MJ·kg)34.362.530.620.106.7844.4413.58ABcDE0.663.091.01299.108.2016.792.6323.106.1011.6924.2027.1052.917.2025.9140.2119.83Table 2 Operating parameters and carbon content in the fly ash for seven Chinese CFB boilersFurnaceECirculating ash Exhaust gas Carbon in Fluidized FurnaceBoiler capacity temperature factortemperaturevelocity(MW)(C)中国煤化工(ms-)(m920-9401.39CNMHG 4.8930950900930860-8801.358.19.8D920-930885-89627.3E850-89030-86018.0850-890895-9151.33827-85413728.5LU Junfu(吕俊复)etal: Unburned Carbon Loss in Fly Ash of CFB boilers689example in China is the Alhstrom 100 MW. CFBboiler that burns anthracite. The carbon content infly ash decreased to around 10% by recirculation ofall the fly ash from the first stage of the electrostatic precipitator (ESP). However, fly ash recirculation cannot be easily applied to all small andmedium size cfb boilers in china because of thesystem complexity and maintenance cost0010.020.03/(kgM」)Fig. 1 Relation of the carbon content in the fly ashwith the volatility index 1needed for the char particles to burn outTherefore, the cyclone collection efficiency shouldbe important, but the test data on fractionalcollection efficiency of real operating cyCfb boilers is hard to find. The size distribution o50300450600the fly ash and the circulating ash is an indicator ofcyclone performance, although the ash sizeFig. 3 Circulating ash size distribution in seven CFBdistributions are also a function of the ashformation characteristics of the coal and theattrition characteristics of the ash particles o. the4 Solid-Gas Mixingsh and the fly During the CFB boiler operation, a large numberash from seven boilers are shown in Figs. 2 and 3. of solid particles are elutriated from the dense bedand move upward. The high solid loading in thegaThe effect of solid-gas mixing on the combustionwasy measuring the oxygenconcentration in the furnace above the secondaryair in several boilers 9. The cross section of thetested furnace was 3 m (depth) by 6 m(width)The oxygen detection probe was inserted throughthe furnace side-wall lo.ll]. The measured datin difference height above the distributor areFig. 4, where Co. is the oxygeconcentration and is the distance from the leftFig. 2 Fly ash size distribution in seven CFB boilersding bulk density, DwasThe data show that the cyclone performances for also measured and is shown in Fig. 5hese boilers are basically the same. The peak sizeof the circulating ash was 110-160 um with the18.2 m above distributormajority of fly ash being less than 100 um. TheseHH11. 3 m above distributorresults are consistent with other published data 7)中国煤化工 bove distributorMost work has shown that the cut size of largeCNMHGcyclones used in CFB boiler is hardly less than100 um. Much work has been done to improve thecollection efficiency of cyclones for CFB boilers.8Jbut the effect on fine particles has been limitedThe recircLe数据 of fly ash is an effective way toimprove the fly ash carbon burnout. A typical Fig. 4 Oxygen concentration at various furtTsinghua Science and Technology, December 2003,8(6): 687-691ufficiently fadistribute the secondary air into the center of thefurnace. The oxygen distribution will thenfluence the carbon burnout in the fine particles inthe core region. The secondary air mixing wasmpacted by modifying the nozzles to increase theinjection velocity. The fly ash carbon contentd by the modified secondaair nozzles as shown in Fig. 7, which compares thefractional carbon content in different size fly ashL/mparticles before and after modification.Fig. 5 Density distribution at a furnace height of 4. 5m5 Deactivation of Char in CFBAt the center of the furnace, the solidCombustionsuspension density was relatively low while in theThe analysis of the carbon in the fly ash particleswall region, the solid suspension density wasrelatively high because of the down flow along theinto two groups according to their reactivity[2l.wall11. Surprisingly, the oxygen concentration inOne group was“ fresh” char particles with highthe center of the furnace is almost toward zero reactivity and certain amount of volatile contentWItoxygen rich zolnear the wall. This These particles probably come from the elutriatedoxygen distribution occurs at almost all levelsfine coal particles with relatively short residenceabove the secondary air nozzles. The distributiontime in the furnace. Improving the cycloneof the lean and rich oxygen zones is showncollefficiency or recycling the Esp ash teschematically in FiA lean oxygen core burn out such particles should improve the overalldeveloped in the furnace above the secondanozzles because the secondary air does notburnout. The second group of char particles hasvery low reactivity, so evenly they are fed back tohe furnace, and their burnout will not increasemuch. A series of experiments were designed toower reactivityThe coal deactivation during pylysis has beoxygenthoroughly investigated in previous researchin encoreSecondary hThe reactivity has been shown to be a function ofresidence time and pyrolysis temperature with aconstant decrease of reactivity at hightemperatureThe chaFig. 6 Lean and rich oxygen zones in a CFB furnacerapidly at first and then more slowly as thepyrolysis proceeded with approaching anasymptotic value. The reactivity was then constantSenneca et al. summarized similar results fronseveral other researchers in a plot of theasymptotic timesn the present study中国煤化工 various residence timefor 7 min at 900 CNew secondary air nozzlesCNMHUw llldt ut char reactivity decreasesduepends on the temperature. For the CFBcombustion temperature (1173 K), the dramaticFig. 7 Fly as ttbon content reduction with improved deactivation of the coal activity occurs with heatsecondary air nozzlesreatment time of 10 30 min. Therefore, charU Junfu(吕俊复)etal: Unburned Carbon Loss in Fly Ash of CFB Boilers691deactivation cannot occur during CFB combustion [4J Wang Zhiwei, Sun Xianbin, Lu Huaian, et alsince the residence time is too short. However, if aExperimental study on combustion of anthracitelarge coal particle is fed into the CFB, some of theirculating fluidized bed. Power equi pment,2001char particles will be heated for a rather long time8. (in Chi5 Muschelknautz E. Improvements of cyclones in CFBin the furnace before the large particles break intopower plants and quantitative estimations of theirfine particles. In addition, the temperature of largeeffects on the boiler solids inventory. In: WeatherpT. ed. Proceedings of the 6-th Internationalsurrounding gas temperature, which might furtherConference on Circulating Fluidized Bedshorten the heat treatment period foWurzburg: ASME, 1999: 761-768deactivation[is]. Therefore,. the inert fine char in [6] Tang Zhi, Yue Guangxi, Qian Min, et al. Thethe fly ash may be the fragments of large coalexperimental investigation on the coal ash formationCFBparticles. If true, the coal should reduce theIn: Donald W, Ceiling PEProc, of the 16-th International Conf. on Fluidizedfraction of large particle in the feed coal for theBed Combustion. Nevada: ASME, 2001CFB boiler. More investigations are needed to [71 Dallas W T. Iqbal A. An update of operatingconfirm this hypothesisexperiences burning petroleum coke in a utility scale6 ConclusionsCFB the Nisco cogeneration project. In: Preto F DS, ed. Proceedings of the 14-th FBC International1) The fly ash carbon content for hard coalConference. Vancouver: ASME, 1997:57-70.burned in CFB boilers in China is excessively high[8 Jin Xiaozhong, Lu Junfu, Yue Guangxi, et al. Gas2)The fly ash carbon content strongly dependoncentration profiles in the furnace of large CFBon the coal rank. The ratio of the volatile contenboilers with water-cooled square separator. InRuixian, ed. Proceedings of the 1st Internationalto the coal heating value correlates well with theConference of Engineering Thermophysics. Beijingfly as carbon content.Academic Press, 1999: 286-2753) The gas-solid mixing in the furnace influences9 Lu Junfu, Yue Guangxi, Liu Qing, et al. Thethe char particles combustion. The secondary airprogress of the water cooled separator CFB boiler inshould have enough momentum to penetrate deeplyChina. In. Reuther b. Nonald L Bonk. edsinto the furnace to avoid producing a lean oxygenProceedings of the 15th International Conference ofFBC. Savannah: ASME. 1999.[10 Hurt R H, Sun Jian-Kuan, Lunden M. A kinetic4)Some of the fly ash in a CFB has very lowmodel of carbon burnout in pulverized coal combuscarbon reactivity even though deactivation requirestion. Combustion and Flame, 1998.113:181 197a relatively long time. However, this deactivation [111 Zhang Jiansheng, Lu Junfu, Jin Xiaozhong, etmight occur during CFB combustion in large coalDensity distribution in the furnace in 75-t/hparticles, which would generate fine inert charcirculating fluidized bed boiler with water cooledparticles that do not easily burn out even with ESPsquare separator, Tsinghua U. (Sci. &. 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