Emissions of SO2,NO and N2O in a circulating fluidized bed combustor during co-firing coal and bioma

Availableonlineatwww.sciencedirect.coma. science direct8N19Journal of Environmental Sciences 19(2007)109-11Emissions of So2, NO and N2O in a circulating fuidized bed combustorduring co-firing coal and biomassXIE Jian-jun.2, YANG Xue-min, ZHANG Lei. 2, DING Tong-li.2SONG Wen-li LIN Wei1. Institute of process Engineering, Chinese Academy of seiences, Beying2. Graduate School of the Chinese Academy of ScienceReceived 16 January 2006: revised 26 January 2006: accepted 29 March 2006AbstractThis paper presents the experimental investigations of the emissions of SO2, NO and N2o in a bench scale circulating fluidized bedcombustor for coal combustion and co-firing coal and biomass. The thermal capacity of the combustor is 30 kw. The setup is electricall yheated during startup. The influence of the excess air, the degree of the air staging, the biomass share and the feeding position of thefuels on the emissions of SO2, NO and N2O were studied The results showed that an increase in the biomass shares resulted in anincrease of the CO concentration in the flue gas, probably due to the high volatile content of the biomass. In co-firing the emission ofSO2 increased with increasing biomass share slightly, however, non-linear increase relationship between SO2 emission and fuel sulfurcontent was observed. Air staging significantly decreased the NO emission without raising the SOz level. Although the change of thelevel. Taking the coal feeding position R as a reference, the relative NO emission could significantly decrease during co-firing coal af "fuel feeding position from riser to downer resulted in a decrease in the NO emission level, no obvious change was observed for the scbiomass when feeding fuel at position D and keeping the first stage stoichiometry greater than 0.95. The possible mechanisms of theIfur and nitrogen chemistry at these conditions were discussed and the ways of simultaneous reduction of SO2, NO and N2O wereKey words: circulating fluidized bed(CFB); combustion; SO2; NO; N2O; co-firing: biomassIntroductiontechnologies (i.e. combustion, gasification and pyrolysis)Coal now contributes 70% of China total energy concombustion is the only proven technology for heat andsumption and this tendency will keep quite a long time power production(Nussbaumer, 2003).In general,therein the future(Liu, 2000). Combustion in low capacity, the is,a rapid consumption of oxygen for firing biomass,main manner to use coal in China, leads to the problem of which takes place in one phase. Whereas, for coal, thelow combustion efficiency and high level pollutant ems- acterizing a short phase for volatile combustion and asionsEmitted SO2 and NO from combustion is the main long char combustion phase(Werther et al., 2000). So thereason of acid rain in southwest China(Lin, 2000). Using following phenomena may occur when co-firing coal andcirculating fluidized bed CFB) combustor integrated within-situ calcium injection can effectively promote combusbiomass: firstly, biomass is easy to be ignited because oftion efficiency and reduce NO and SO2 simultaneously. of biomass reduces the oxygen available for coal, andsignificant amount of N2O(20-250 ppm)et aL., 1998), which is one of the greenhouse gases and alsothe coal-char concentration at suspension area will becontributes to the ozone depletion at stratosphere, emitted increased during CFB combustion with increasing biomassduring CFB combustion(Wojtowicz et aL., 1993). So, it is mass share, the possibility of reduction of No and N2Oimportant to control the emissions of NO, N2O and so2 on char surface will increase accordingly; secondly, largeduring CFB coal combustionamount of biomass will be risen to freeboard quickly whenfeeding coal and biomass at the same position becauseApproximate 54x10 t of rice husk, which can generate of lower terminal velocity of biomass. That is,biomassheat or power as part substitute of coal, are producedvery year in China(Armesto et al., 2002; Fang et aL. combustion and coal-char combustion would occur at thesame2004). Among the biomass thermo-chemical conversion中国煤化工 ginatesdevolucingif OICNMHGNH3mpositionon of China (John(No.9021101,20221603),*ComDevolatilization or gasification solid fuels like coalXIE Jian-jun er al.Vol. 19biomass results in forming a fuel gas containing highconcentrations of combustible gas with CO, H2 and CHaas its major compounds. At the same time, quite amountof NH, and HCN, the major precursors of NO and N2O,originate from the organic nitrogen in the fuels( Glarborg etaL., 2003; Leppalahti and Koljonen, 1995). Basically, therere three principal approaches to decrease the NO and N2Oconcentration during solid fuels thermal-chemical conversion process:(1)to find proper pyrolysis/gasificationconditions that the fuel-nitrogen would directly react to n2tsuka et aL. 1998 Tsubouchi et al. 2003: Wu and ohtsuka, 1997); (2)to convert the Nh3 and hcn to molecularnitrogen before combustion; ( 3)to use NH3 and HCN asreducing agent to reduce NO and N2o finally producingN2 at proper operating conditions during combustion(Lin,994). Generally speaking, the solid fuel feeding positionalways near the distributor during CFB combustion anddowner is merely act as return leg, if changing the fuelfeeding position to downer, pyrolysis or gasification solidfuels will take place before they enter the riser, the releasedNH3 and HCN may be reducing NO to N2 In principal, thereactions would be the same as in thermal De-NO process(Leckner et aL, 1991; Miller and Bowman, 1989)orreburning technology(Smoot et aL., 1998). By integratingwith calcium injection technology, simultaneously reterPrimary airtion NO, N2O and so will be reached. However there istill not enough information about reduce emissions of Noand N2o by co-firing coal and biomass and by changing Fig. 1 Schematic diagram ofThe objective of this work was to investigate thesolid mixer; (7)U-type valvepneumatic feederof different parameter such as biomass share, first(10)filter;(11)water trap:(12)gas analyzer;(13) gas concentrationacquisition station; (14) temperature acquisition station.stoichiometry and excess air on the emissions ofN2O, SO2, and co during coal combustion and co-firing measure the temperature and they are located 0. 15 m(T6)coal and rice husk in a CFB combustor. In order1.6m(T7),28m(T8),4.0m(T2),5.2m(r9and6.43out an in-situ pollutant retention measure, the comparison m(T1)above the gas distributor respectively. However,of emission performance of different coal and biomass only 3 K-type thermocouples are used to measure thetemperature in the downer. The pressures in the CFBand rice husk has also been studied in this paper. Thecombustor are measured by 10 U-type manometers, 4 forresults obtained from this study could be used to propose a the riser and 6 for the downernovel in-situ simultaneous process to reduce emissionsThere are 2 feeding ports for coal and biomass particles:NO, N2O, and soone is located at 20 mm above the gas distributor in the1 Experimentalriser(position R), the other is designed at the lower part ofsolid-solid mixer in the downer(position D). A two-stagefeeding system, which is composed of a screw feeder and apneumatic transportation feeder, was specially designedThe experiments were carried out in a 30-kw(thermal) introduce fuel particles or silica sand particles into the CFBCFB combustor as schematically illustrated in Fig. 1. The combustor. Coal or biomass particles was firstly screwedapparatus consists of a riser, with a height of 7 m and an by the screw feeder and then transported pneumaticald of 0.86 m, and a downer with a height of 3 m and an into the CFB combustor by compressed air. The feedingi d of 0.39 m. Both reactors are made of high temperature rate of coal or biomass particles was easily controlledresistant stainless steel and are externally heated by six by varying the motor rotation speed of the screw feederelectric heating elements, which can be independently The feeding coal and biomass particles were well mixedcontrolledin the transportation pipe before introduced into the CFBThe combustion air can be divided into two streamcombustorpreheated primary air is introduced below the gas distrib-中国煤化工tor and the secondary air is tangentially introduced to N20d by an ABB-A02020the combustor via a port located 1. 66 m above the gas contiCNMH Gmpling pipes from thedistributor. Thus, the two stage combustion can be realized. CFB combustor to the ABB-Ao2020 continuous gas ana-6 K-type thermocouples are installed on the riser to lyzer are made of stainless steel and Teflon. All measuredEmissions of SOz, NO and N2O in a circulating fluidized bed combustor during co-firing coal and biomasscomputer by a data acquisition system at set li ged into a riser by the U-valve, and the air ratio is about 0.15-0.20temperatures and gas concentrations werene intervals. The char and gas together with circulated solid in theTo compare the experimental data with other references, downer will be transported through the U-valve to the riserall gas concentrations have been converted to 6%O2 basis. where combustion reactions are performed. Silica sands1.2 Raw materialsash and incompletely burned char particles are transportedto the top of the riser, then separated from the flue gas byThe coal and rice husk were from Datong, Shanxi a cyclone and entered into the hot media hopper while theProvince, and Heibei Province of China. The proximate clean flue gas is exited from the ventand ultimate analysis of bituminous coal and rice husk areEach experiment run is performed more than one orlisted in Table 1. Both coal and rice husk are milled and two hours under specified operating conditions when thesieved into 0. 154-0.600 mm and then dried in an oven temperature and the flue gas concentrations only fluctuateat 383 K for at least 4 h before experiments. The size in a very narrow range. The results presented in this paperdistributions of these particles are shown in Fig. 2. The are the mean values. The operation conditions are listed insilica sand with the size of 0.224-0.280 mm is used as bed Table 2material in the CFB combustor2 Results and discussion1.3 Experimental procedureCoal is introduced into the CFB combustor at a rate of 2.1 Influence of biomass shareabout 3 kg/h at position R or D when temperature at gas Influence of rice husk mass share on emissions of NO,distributor is higher than the ignition temperature of the N2O, SO2 and CO during co-firing coal and biomass withair flow rates of 20.8 Nm/h and 5. 1 Nm/h as primaryTo compare with the emissions of SO2, NO and N20 and secondary air and fuel feeding position R are shownduring coal combustion and co-firing of coal and rice husk, in Fig 3. When mass ratio of rice husk to coal increasestwo sets of experiments have been performed. Coal firing from 0, 13 wt%, 22 wt%, 27 wt% and 30 wt%, emissionsexperiments are firstly carried out by changing excess air of NO and N2o decreases dramatically from 125 to 79and the primary air/secondary air ratio at a 2.68 kg/h coal ppm and 122 to 32 ppm, respectively. Emission of SO2feeding rate; then coal and biomass co-firing experiments is found to show a slight increase with the increase of niceare performed with rice husk mass share as 13 wt %, 22 husk mass share. The emission of Co is observed to havewt%,27 wt%and 30 wt%, whereas coal feeding ratea higher concentration with large fluctuation at higher ricekept at 2.68 kg/hhusk mass share as expectedThe fuel particles introduced at position D can be Many research results have shown that biomass additionquickly heated up to 1023-1093 K by heat exchange had a positive effect on emissions during co-firing withbetween fuel and silica sand particles in the downer. a stable total mass/energy load by increasing biomassGasification reactions of introduced fuel particles occurred mass/heat fraction as well as decreasing coal mass/heatin the downer where the atmosphere is isolated from the fraction. Because the nitrogen content in biomass is lessthan that in coal. the less emission of nitrogenous oxideS Rice buskZ Coa160sO2-o-CoWRH/WCoul+WRH)(%) WRH/(WCoa+WRH)(%Fig 3 Effect of rice husk mass share on the NO, N20, SOz and COemission during co-firing coal and rice husk. TheFig. 2 Diameter distribution of coal and rice husktwice the standard deviation associated with each data point.able 1 Chemical analysis of coal and biomass samplesProximate analysis(%)中国煤化工Rice husk1593CNMHG00633.Based on dry basis; by diffcrence: VM: volatile matter; A: ash; FC: fixed carbon112XIE Jian-jun et al.VoL. 19Table 2 Operating conditions of CFB and flue gas analysisCaseItemCoal feeding rate (kg/h)2.682.682682682682682682Fuel feeding positionRRRRRRTotal primary air(Nm3h)20.820.820.820.820.8187825918.722.824927.05.l3.100555.1Excess air(%o)16.59.035.0TI(K)11461129113911531147l6l11751159i17111731181Il8011721177116611551039101110301050102610241093103210351033113211112811412015611151100116311621167100210171050371051105810511065M0714(K)1001101102510304610511048105810651066T5(K)103210501,901071431.70.7314.581590166516.9817.7317.3617.1417.0317.78CO(ppm)4606131477111N2O(ppm)SO2(ppm)50314178Coal feeding rate (kg/h)2.682682.68682.682682682682682.68omass feeding rate(kg/h1.15tomass share(%o)Fuel feeding positionDDDotal primary air(Nm3/h)220.820.820820.820.81872825.922.8249275.15.l725.15.1Excess air(%)165904.81.69035.011421145114611471731154T6(KJ741085107910881l10761059106710381035114215511481157115211114711411115l139115311621157116311621161651151L3311T9(K)116606910751074107610751077056106T4(K1068107310751078105810481050105609373417687.349216.741582418o(ppm12265304648072.Total primary air is composed of primary air, camier gas and fluidized gas of U-valve.and sulfur dioxide during co-firing can be easily explained nitrogen species as volatiles and oxidation of nitrogen inas the dilution of N and S content in the mixed fuel(Dayton char( Glarborg et al., 2003; Johnsson, 1994). Due to theet aL., 1999; Gayan et aL., 2004; Leckner and Karlsson, density difference between coal and rice husk particles,1993: Spliethoff and Hein, 1998). However, in this study, rice husk particle has a smaller terminal velocity thanthe co-firing experiments were performed with rice husk that of coal or silica sand particles( Fang et al., 2004)mass share as 0, 13 wt%, 22 wt%, 27 wt%o and 30 wt%, Therefore the rice husk particles will be devolatilized andwhereas coal feeding rate kept at 2.68 kg/h. Hence, the combusted at higher region than coal in the riser, theenergy load in this study was increasing. This is why released volatile gases, such as CHi, HCN and NH3 fromresults from this study do not agree with reported results rice husk, can reduce generated NO from oxidation of coalwhich explained by the"diluting effect". The reason for char at higher region of the riser to form N2, just like thehat could be explained as follows: (1)O2 concentration in "reburning"mechanism reported elsew here(Smoot et al.the cFb combustor decreases with the increase of amount 199of fuel, therefore, rapid conversion of carbon in fuels toC中国煤化, the slight increase ofCO reduces oxygen reaction competition ability with N to SoCNMHGeconsidered as a resultform NO and N2o ( enkins et al, 1998) 2)the formation of fueI Se witn une Increasing rice husk masof NO from fuel nitrogen takes place via combustion of share as shown in Fig 3. In those experiments about 88%-Emissions of SO2, NO and N2O in a circulating fluidized bed combustor during co-firing coal and biomass11394% fuel S is converted to SO2, the increase tendency excess air, a lower N2O emission can be observed withand the non-linear relationship between SOz emission and the same excess air during co-firing( Fig 4b). Emission ofrice husk mass share can be considered as variation of So2 during co-firing(Fig. 4c)is about 40 ppm higher thansulfur conversion rate(Spliethoff and Hein, 1998)or the that for coal combustion, and no great fluctuation with theincrease of biomass share. It is known that there are some increasing excess air can be obtained during both firing andilkaline metal in rice husk, such as calcium and potassium co-firing. Higher SO2 emission during co-firing is due to(Armesto et aL., 2002; Natarajan et aL, 1998; Werther et aL., extra sulfur charged from biomass Leckner and Karlsson,2000), and those alkali metal elements have strong ability 1993). Emission of co decreases with the increase ofto capture SOz at CFB combustor via the formation of excess air from 0 to 9%o during co-firing(Fig. 4d). However,calcium and potassium sulfates under oxidizing conditions CO emission during co-firing is higher and more fluctuated(Cheng et aL, 2003; Knudsen et aL., 2004; Wolf et aL., than that during coal combustion. The high CO emission2005). However, the results in this paper showed the sulfur with large fluctuation is leveled off with the increasingretention of alkaline metal is of neglectableexcess air: hence excess air above 35% is of benefit toGeneral speaking, emission of Co is closely related improve combustion efficiency during co-firingwith temperature, oxygen concentration, mixing of fuelparticles with air, and residence time of gaseous prodMh here are two oxygen consumption regions in the riserduring coal combustion charactering a shorter phase for( Armesto et aL., 2002; Permchart and Kouprianov, 2004; volatile combustion and a longer phase for char combus-Spliethoff and Hein, 1998; Svoboda ef aL., 2003; Werther tion(Werther et al., 2000). However, this will be changedet aL., 2000). Armesto et aL.( 2002)measured that 200- during co-firing because volatile matters in rice husk is sig2000 mg/Nm of CO can be emitted during combustion nificantly higher than that in coal (Table 1), considerableof rice husks in a bubbling fluidized bed. The decreased amount of volatile matters can be burned at suspensionO2 concentration and high content of volatile gases of rice region in the riser during co-firing(Natarajan et al., 1998)husk in the riser should be the important reasons for high Most fuel nitrogen in rice husk can be volatilized duringco emission observed in this studpyrolysis or gasification and released as NH3 and so on, but2.2 Influence of excess aircoal retains more nitrogen in char( Glarborg et aL., 2003Leppalahti and Koljonen, 1995). No formed partly fromThe effect of excess air on emissions of NO, N2O, coal nitrogen can be easily reduced by NH, from pyrolysisSO2 and Co during coal combustion and co-firing coal of rice husk, or by CHi, whose quantity is proportionaland biomass introduced at position R is shown in Fig 4. to the amount of local CO(Nussbaumer, 2003; SalzmannThe rice husk mass share was kept at 22 wt%; excess air and Nussbaumer, 2001; Smoot et al., 1998). However, thewas changed from 11%to 53% for coal combustion and reduction of No by this route is not remarkable under lot0. 4% to 35% for co-firing by keeping the secondary air oxygen concentration condition during co-firing, becauseat 5. 1 Nm /h and changing the primary air from 18.7 to a large amount of No can be generated at higher region in27.0 Nm/h. It can be seen from Fig 4a that the emission the riser, where the formed NH3 is not available any longerof NO increase with increasing excess air during both by oxidizing of O or OH radical to NO. Furthermore, thefuel combustion processes; however, the increase tendency higher CO concentration during co-firing also indicatesduring co-firing is slightly lower than that during coal that a larger amount of char exists in the riser and thecombustion. Hence, NO emission during co-firing is lower char can reduce NO to form molecular nitrogen(Johnsson,than that during coal combustion at higher excess air.1994; Leckner and Karlsson, 1993).bvious influence of excess air on emission of Compared with coal combustion, 22 wt of rice huskN20 can be found not only for coal combustion, but share during co-firing can lead to a 30 K temperaturealso for co-firing. Although the emission of N2O during increase in the riser. Hence, the formed N2O can becoal combustion is kept almost constant with different effectively decomposed over char at higher temperatureCoal Biomass%8000000000中国煤化工CNMHGFig 4 Effect of excess air on the NO(a), N2O(b), So (c) and CO(d) emission during coal combustion without and with rice husk addition( the massshare of rice husk is stable at 22 wt%). The error bar represents twice the standard deviation associated with each data pointXIE Jian-jun et aL.during co-firing(Amand et aL., 19911994: changing feeding positions, The relative concentration ofeckner and Karlsson, 1993). Analyet al., each pollutant gas can be expressed as a percentage with1999)of the triple trade-off between Nand N2 respect to the basis of coal combustion feeding at positionshows that the major N2 produced from NO in the range R, and negative relative concentration means a decreaseof 900-1050 K is via Reaction(1)and (2), rather than of pollutant emission resulting from changing fuel feedingReaction (3)described as followspositionThe relationship between first stage stoichiometry andNO+(-CNO)→N2O+(CO(1) relative emissions of NO, N20, SOz and Co during coN2O+(C)→N2+(CO)(2) firing coal and biomass introduced at R and D are shownin Fig. 6. The relative emissions of NO, N2O, SO2 andNO+(C)→N2+(CO(3) CO during coal combustion introduced at position D arealso presented in Fig. 6. An obvious decrease of relativeThe observed slight increase of N2O with the increasing NO emission can be found with the increase of the firstexcess air(Fig 4b)may be related with char decrease in the stage stoichiometry in three studied cases. Changing fueluspension region in the riser during co-firingfeeding position from R to D will result in an obvious2.3 Influence of first stage stoichiometrydecrease of emission of No during firing in all the four testruns, hoEffect of first stage stoichiometry on emissions of NO, when the first stage stoichiometry is greater than 0.95N2O, SO2 and CO during coal combustion and co-firing during co-firing in spite of feeding position at R or DI positon40% at theFig. 5. The biomass mass share is kept at 22 wt% and total stoichiometry of 1.08 during co-firing with fuel feedingfirst stage stoichiometry on emissions of NO, N2O, SozEmission of N2O increases when the first stage stoiand Co shows almost the same tendency as the influence chiometry is greater than 0.97 during coal combustion withof excess air shown in Fig 4Emission of no during co-firing shows a smaller in-fuel feeding position D. No difference of N2O emissionsobserved during co-firing with different fuel feedingcrease with the increase of first stage stoichiometry than positions or different first stage stoichiometrythat during coal combustion. The NO emission was keptThe relative SO2 emissions during coal combustion withat a very low value when the first stage stoichiometry coal feeding at D or co-firing with fuel feeding at D andis greater than 0.95 during co-firing. These indicate that R are illustrated in Fig. 6c. The relative concentrationsno can be effectively reduced in oxygen-rich atmosphere. show an increase with the increase of the first stageAir staging does not lead to an appreciable change in stoichiometry from 0.87 to 1.24N2O emission during co-firing as shown in Fig. Sb. A littleThe relative emission of Co will reduce during coalnigher SOz emission and fluctuated CO emission are found combustion by changing the feeding position fromR to D,spectively during co-firing, and but relative emission of co is obseryedboth of them seem to be not sensitive to the first stage co-firing at two feeding positions as showed in Fig. b ngncreaseThe relationships between the relative emi2.4 Influence of feeding positionsO, N2O and the first stage stoichiometry during co-firing at two feeding positions suggest that changing fuelTo investigate influence of fuel feeding positions on feeding position may lead to a variation of fuel nitrogenemissions, two groups of co-firing experiments were transformation. In this study, air at flow rate of 4 Nm'hperformed in this study by changing feeding fuel from is applied not only as carrier gas for fuel feeding systemtraditional position R to position D. The relative con- but also as gasification agent in the downer when fuelcentration is applied to present emission variation from00%0R20050l0000,9中国煤化工13First stage stoichiometry(-)CNMHGFig 5 Effect of first stage stoichiometry on the No(a), N2O(b), So(c) and Co (d)emission during coal combustion without and with rice husk addition(the mass share of rice husk is stable at 22 wt%). The error bar represents twice the standard deviation associated with each data point.Emissions of SO2. NO and N2O in a circulating fluidized bed combustor during co-firing coal and biomass00000060First stage stoichiometry (-First stage stoichiometry(-)000000000022E如78%178%88080910111213Fig 6 Relative NO (a), N2O(b), Soz(c), CO (d)emission vs. first stage stoichiometry when changing fuel added position(expressed as a percentagewith respect to the situation only coal added at nser). The error bar represents twice the standard deviation associated with each datais introduced at position D. Chang et al.(2003)believe variation to relative N2O emissionthat oxidation of coal with 4%O in a temperature from No obvious change of relative sO2 emission is shown672 to 873 K leads to formation of HCn and NH3 due when fuel feeding position is changed to d during coalto the enhancement of H radical production. when those combustion or co-firingreducing gases including CH; are introduced into the riser When only coal is burned, the relative CO emissionthrough U-valve, they can combine easily with NO or is likely to decrease because a pre-oxidation occurs inO and OH radical depending on the local atmosphere the downer. No such a similar tendency is found during(Nussbaumer, 2003; Smoot et al., 1998). There is not co-firing. However, the violence oscillate of CO emis-enough oxygen available in the primary zone at the lower sion during co-firing indicates that a higher excess air isfirst stage stoichiometry, NH, and HCN introduced from favourable for complete combustionthe downer will be oxidized to no. This is the main reasonchy higher relative emission of no is observed during 3 Conclusionsco-firing when the first stage stoichiometry is less than Different CFB operation parameters such as the first95. However, NO reduction will take place in the primary stage stoichiometry, excess air, biomass mass share andexcess-Oxygenconditions, similar with the fuel adding position on the emissions of NO, N2O, SO2thermal De-NO process(Leckner et al., 1991; Miller and and Co during coal combustion and co-firing coal andBowman, 1989). It is well known that NH3 can effectively biomass in a CFB combustor were investigated in thisreduce NO at temperature window roughly between 1 100 study. Increasing the biomass mass share during coal-and 1400 K when molecular oxygen is present in sufficient biomass co-firing can significantly decrease emissions ofquantities. Furthermore, high concentration of hydrocar- NO and N2O. however, non-linear increase relationshipbon in the case of rice husk added during co-firing may between SO, emission and fuel sulfur content is observedlead to temperature window shift effectively to a lower Taking the coal feeding position R as a reference, it islevel. On the other hand, the obtained higher concentration observed that the effect of fuel feeding position on the Noof CO during co-firing than that of during coal combustion and N20 emission depends on the first stage stoichiometrycorresponds higher CH; in the riser, CH; may react with When the first stage stoichiometry keeps greater than 0.95fuel N to form HCN which would be oxidized to molecular the relative NO emission can significantly decrease duringnitrogen by O or OH radicals(Smoot et al., 1998)coal-biomass co-firing fed fuel samples at position D. TheCompared with co-firing, coal combustion introduced at reduce of NO and N20 during coal-biomass co-firing fedposition D emits higher relative N2O emission, as Hcn fuel samples at position D can be explained with the samereleased during coal gasification in the downer can be mecloxidized in the riser. N2O formation from Reaction(1)iH中国煤化工 d with combalso the possible route for higher relative N2O emission occiCN MH GoSt important reason toDe Soete et al., 1999). In the case of co-firing, changing redu I2v because suue of gasification productsthe fuels feeding position seems to have no appreciable such as H2, CH4, and Co can be partly burned in the116XIE Jian-jun et afVoL 19downer. The decoupling combustion(Lin, 1994), pyrolysis N2O reduction over coal chars in fluidised-bed combustion[J]combined with combustion, may be an in-situ measureEngineering Science, 53(1to reduce the NO, N20 and SOz simultaneously during Lin w, 1994. Interactions between SO and NO, emissions inCFB combustion USing pyrolysis gas as reducing gas tofluidised bed combustion of coalD. Ph. D thesis. Technicalreduce NO emission had been carried out in pulverizedUniversity of Delft, The Netherlands: 190-197oal combustion boilers(Rudiger et al., 1997; Rudiger Lin W, 2000. Research and development of clean coal technolo-t aL., 1996; Spliethoff et aL., 1996). However, applyinggies in China[J]. World Science-Technology r D, 22(4)this technology to CFB coupled with Ca-based absorbentaddition to simultaneously reduce NO, N2O and SO2 byLiu Z, 2000. Chemistry in coal energylJ]. Progress in Chemistry,12(4):45846means of in-situ is still open to discussionAcknowledgements: The authors gratefully thank Ph. DMiller J A, Bowman C T, 1989. Mechanism and modeling ofcandidate Mr Changbo Lu, Chuigang Fan, Xianwu Zhounitrogen chemistry in combustion]. Progress in Energy andCombustion Science, 15(4): 287-338e d Yong Zhang for their helpful assistance during experi- Natarajan E, Nordin A,Rao AN,1998.Overview of combustionand gasification of rice husk in fluidized bed reactors[JIReferencesBiomass and Bioenergy, 14(5/6): 533-546Nussbaumer T. 2003. Combustion and co-combustion ofAmand L E, Leckner B, Andersson S, 1991. 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