Analysis of Nox Emission Characteristics of Pulverized Coal Combustion in Different Atmosphere

Vd. 2 No. 2. June 2010amal of Measurement Science and InstrumentationSum No 6Analysis of NOx emission Characteristics of pulverizedCoal Combustion in Different AtmosphereHan-tao iu(刘汉涛), Qing-guo w(尉庆国), Tie-xiong SU(苏铁熊)(School of Mechatronic Engineering, North University of China, Taiyuan 030051, China)Abstract- Researches on the NO amission characteristics of Hr istics are still not studied sufficiently under different atmo-(Huangtai coal ), LC( Laicheng)coal are carried out in different at- sphere in the new style combustion 56). Researches on themosphere. The results show that the NO emission time is postponed NOx emission characteristics including the nitrogen releas-in O/N atmosphere mixed with aO. The releasing times o volatilenitrogen and char nitrogen approach, the amoumt of NO reduces. In ing times, the amount of NOx emission and other detailsigh O concentration atmosphere mixed with OO,, the nitrogin different atmosphere were studied in this paper.leasing times come ahead of time, NOx congregate in the beginning ofthe ambustion. On the contrary, nitrogen releases separately, and 2 Experimentthe amount of nox reduces greatly.2.1 Experimental system and materialsKey words-fued nitrogen: releasing characteristics; Pulverized analDL-1 model of double tube sulphur determination deManuscript Number:16748042(2011)02020104vice(Sheng Fang, China)was used in the experimentalsystem( Fig. 1). Silicon carbide tube was used as the heat-dio:10.3969j,isn.16748042.2010224er element in the electric furnace. The furnace temperature ranged from20℃tol600℃ was controlled and dis-1 Introductionplayed by WZK model( Sheng Fang, China)temperatureself-regulating apparatus. The temperature was measuredby platinum-rhodium thermocouple. The mock flue gas isIn order to increase the energy sources availability composed of N, O, C0, o0, two species of coal usedand reduce the pollutant emission, some new style com- and their proximate and ultimate analyses are shown in tabustion technology, such as high temperature air combus. ble 1tion( HTAC ) oxygen-rich combustion technology(OCT), emerged recently based on the normal combustion 2.2 Proceuretechnology. HTAC technology indicates that the fuel ismixed with oxidant in which the ratio of o is less thanFirst, 100 mg of the sample was put into the cruci-21%, while OCt discloses that the ratio of O is more bl, and the furnace was heated till its temperaturethan that in air, the utmost of OCT uses 100%O,. At the reached 1 200 C. At the same time, the needed atmo-present time, combustion adjustment is the primary way to sphere purged through the tube in the velocity of 1. 4 m/scontrol the NOx emission in China, among which, stage for 5 minutes in order to alleviate the effect caused by atcombustion, gas re-circulation are subordinated to low ox- mosphere diffusion. Then, the crucible was placed in theygen measures. In the process of combustion control, re- center of the tube, at the meantime, NO concentrationducing atmosphere must be occurred in the furnace. Even was analyzed by MSI flue gas analyzer( MSI Pro2, Gerthe excess air ratio is greater than 1, and most zone of the man )and the NO concentration was measured byfurmace is full of oxidizing atmosphere, meanwhile reduc- $500-S NO sensor which was displayed/saved in theing atmosphere exists in the interior of coal particles, be- computer using self- programming sof tware until the con-cause of the concentration gradient between the boundary centration became nil. All the data were handled at last inlayer and the inner of coal particles in the condition of the d中国煤化工oal combustion". But, the pollutant emission character-CNMHGRcvd:20100824ed by the Natural Science Foundation for Young Scientists of Shanxi Province(No. 2008041002)jamal o measurement science and InstrumentationN.22010Fig 1 Description of the experimental system: 1 is O, N, OO cylinder, 2 is gas mixing vessel. 3 is silica gel desiccator. 4 is sample delivering aontroller,5 is thermocouple and temperature controller, 6 is flow meter, 7 is MSI flue gas analyzer, 8 is computer. 9 is crucible. 10 is electric furmaceab. 1 Promate and ultimate analysis of the different coalsProximate analysis te(%)CoalUltimate analysis w(%)flagNQm.(M/kg)1.3016.6020.2061.9070.93.745.511.260.65hangtai1.1432.3114.4152.1458.%2.91.800.%1.820.70Tab 2 Code and composition of mixed gasFlag of mixed gas821O218amixed gas P(%)10O217o1.220.0o8.9a3.4010075N7№841.1№The other is the ratio of the integration of nitrogen3 Results and discussionreleased from the coal upon the amount of nitrogen incoal(V,) which could indicate the nitrogen releasingThe nitrogen in the coal is released in the form of tar characteristic for all the emission time.rogen and volatile nitrogen accompanying the pyrolyticC(t)v(e)didecomposition and combustion at first, while the residual×100%,nitrogen is coke nitrogen after the coal is burned outDifferent forms of nitrogen turn into NO, NO, N,O and where: V, is ratio of nitrogen releasing from the coal up-N through homogencous-heterogeneous decomposition or on the amount of nitrogen in coal, % to is start time ofoxidization)finally. In fact, just few NO is produced the experiment, s; t is sometime during the experimentin our experiment, so NOx inchuded only NO and NO ins: M, is weight of sample, mg: N is nitrogen content ofour studies. Otherwise, generated NOx consist of thermalNOx, prompt NOx and fuel NOx. Thermal NOx usually 3. 1 Nitrogen releasing characteristics under airis formatted just when the temperature exceeded1 500t. In our experiment, the temperature was lessatmospherethan 1 200 C. therefore, thermal NOx was ignored tooIn our analysis, two indexes are introduced to assess NOxNitrogen was released rapidly at the initial stage ofcoal combustion and the ratio became slow at the end.emission. One is NOx instantaneous concentration(V,) There were two clear"peak values"for different kinds ofwhich can indicate the NOx on-line concentratcoal (finA". In the early stage of coalV= C(t)v(r),中国煤化工sts in chain structureWhere: V, is NO instantaneous concentration, mg/s; or rirCNMHGtermediate productsC(t)is Nitrogen concentration corresponding to No or such as.rw,INtaNunn turned into N andNO at time t, mg/Nm,; v(e)is gas flux at time t, NOx after being oxidized/ rered. This was the forma-Nm/stion of first peak value. The second peak value formedv.2Han-tao LIU, @ing-RuD WEI, Tie-xiong SU203during coke combustion. Oxygen was absorbed to the surface of coke, and the reaction product was-CNO radi2500HT coal 4.cal, then -CNO radical was oxidized and NOx pro-duced. The nitrogen remaining in the coke made NOx turnLC coal 4nto n because of cokes reduction and basic oxides catalysis. Besides, coke particle became close-burning due tohigh temperature, which made reaction area decrease be-1000cause of pore-solids close, so the coke NOx was less25006001500-o VHT coalV LC coalV HT coal200400HT coal 4Fig 2 Profile of No released during HT and LC coal in condition 4HT coalLC coal 4"Fig. 2 indicates that the peak values appeared atLc coal I140 s, 242 s separately for HT coal, while LC coal at148 s and 266 s. The first peak values (Ht:2268×106mgs,LC:2422×10°mgs)than the second peak values(H:1397×10°mgs,LCFig3 Profiles of No released during Hr and LC coal combustion in1 728x10 mg/s)for both of the coals. The ratio of the condition I":(a)NOx instantaneous values, (b)NOx itegrated pro-integration of nitrogen released from the coal upon theamount of nitrogen in coal was 13% and 21%, respec- tion existedNO+→2N+OO(1)3.2 Nitrogen releasing characteristics in O/N at-On the other hand there existed high concentrationmosphere mixed with aoof the predecessors(e. g. NH, NOO, NH) becausehigh concentration of Co, which reacted with NOx andCoal rebuming is the primary means to control NOxchanged into N. The NOx concentration of LC coal wasin China at the present time. The fuel-rich zone containshigher, leading to much more n2 amount. Hr coal was inhigh concentration of oo gas. Brown%thought that the opposite. This was conforming with AlejandroC(O), the predecessor of CO reacted with NO acutelyMolina and chan!'s studiesSo, if c(O) changed into OO after released from carbo-coal, the reactivity would decrease. To discuss the impact3. 3 Nitrogen releasing characteristic in differentof ao, the NOx instantaneous concentration and the ratioO concentration atmosphere mixed with ooof nitrogen releasing integration from the coal upon theamount of nitrogen in coal were shown in Fig 3(a)in atIt was obvious that oxygen content affected nitrmosphere 1. The time of NOx emission brought back- releasing characteristics greatly, the releasing time of ni-ward in O/N2 atmosphere mixed with CO. Two peak trogen moved forward than 4 atmosphere because thevalues of LC coal converged into one peak value. The re- gases mixed with CO, have better heat transfer characterleasing time of nitrogen in volatile and char approached istics than air, with the result of higher temperature ofand HT coal had the same tendency. The peak valuescombustion. Coke nitrogen released in the beginningwere 0.882X 10mg/s, 1. 631 x 10 mg/s for Hr and of combustion congregately because of low volatile ingreLC coal respectively, and drop amplitudes were 33% and dientak value existed under60% to air atmospherehigh中国煤化工 er low concentration,The releasing time prolonged in atmosphere 3 volC NMHGn released separately(Fig 3(b)), and the amount of NOx reduced, drop am- which tormed two peak values. Ihe releasing time movedplitudes were 10. 9% and 14.6%separately. There were forward with the increase of O concentration. And it waswo reasons for the drop On one hand, the below reac. the reason that boosted nitrogen oxidized, and higher O2204Jamal d measurement science and InstrumentationN.22010concentration resulted in better heat transferwas gentler than that in air atmosphere around peakme of nitrowhile NOx congregated in the beginning of combustion曾5000with just one peak value appearing in high O concentra-4000tion atmosphere mixed with CO. The higher concentra-X3000tion of O2, the higher ratio of the integration of nitrogenreleasing from the coal upon the amount of nitrogen incoal if the concentration was below 80%.Else. the ratioswere equal3)The ratio was less in low O concentration atmo-sphere than that in air atmosphere. The releasing speedwas gentle, meanwhile volatile nitrogen and coke nitrogenreleased separately, with two peak values formed15References[1]H. Wang, X. w. Dong. 2002. NO formation characteristicsof coal in O/CO mixture. Jaurnal of Fuel Chemistry andTechnology, 29: 458-462[2]Z.X. Feng, L.S. An, 2007. Experimental study on pollut-ant emission characteristics during combustion of pulverizedFg 4 Profiles of NO released during HT-coal combustion in differentcoal. Joumal of Power Engineering, 2007, 27: 427-431oxygen concentration: (a) NOx instantaneous values,(b)NOx inte-[3]A G. Borrego, 2008. Coam char combustion under a a0grated profilesrich atmosphere: Implications for pulverized coal injection ina blast furnace. Fuel processing technology, 89: 1017-1024The peak values were 7. 796 x 10-3[4]H.Y. Fan, X.Y. Cao, 2003. Development of sulfur capture7145×10-3mgs,6.19×103 mg/s separately innechanism during coal combustion process in different atmo-sphere5″,6or7sphere, Jarral of Chira Caal Society, 28: 74-79[5] Chen JyhCherng, 2007. Emission characteristics of coal com-In high O concentration, the releasing time reducedbustion in different O/N, o/00 and O/RRG atmosphere.and ended about 200 s. In atmosphere 2and 3", the reJamal of hazardous Materials, 142: 266-271leasing time period ended at 356 s and 553 s. A little NOx [6] Stadler Hannes, 2009. NOx-emission from flameless coalwill generate even if O concentration was nil, because ofcombustion in air, arlO and C0/0. Proceedings of thCombustion Institute, p. 3131-3138weak bond energy of O functional group and O pro- [7] L. I. Gui, w.G. Lin,J. Z. Yao, 2006. Infuence of tem.duced. The total amount of nitrogen released from theperature and coal particle size on the flash pyrolysis of coal incoal were about 22. 5% equally in atmosphere 6 and 7a fast-entrained bed. Chemial rearch in Chinese Universi20.0% in atmosphere 5". The nitrogen releasing fromtie,2:l03110the coal were blown off before it was oxidized ioto NOr, [8] Hannes Stadler, 2009. On the influence of the char gasifica-in atmosphere 2" and 3. Because the inflow gas wastion reactions on no formation in flameless coal combustionbrown through the heating district, the total amount wereCambustion and Flame, 156: 17551763[9]T. C. Brown, B. S. Haynes, 1992. Interaction of aO withNOx, less than in high O concentration atmospherecarbon and carbon surface oxides. Energy and Fuels, 6: 154-4 Conclusion[10]Alejandro. G.E. Eric, W. David, 2003. Niric oxide de-1)The releasing time brought backward, and the to-coal combustion conditions. Cambustion and Flame, 136:tal amount of NOx reduced in O/N atmosphere mixedwith CO. Two peak values tended to coincide because of[11] Can, 1983. Kinetics of the NO- carbon reaction at fluidizedbed combustor condition. Cambustion flame, 52 37-45the delay of volatile nitrogen released. The rate of change中国煤化工CNMHG