A general model for devolatilization of large coal particles

There have been several studies on the devolatilization of large coal particles under laboratory conditions simulating large-scale fluidized or fixed-bed combustion conditions, all of which demonstrate that the process of devolatilization of large
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  Tm~cnv-Siuth y~nposi,l~ll I~ltcr~iationalj n CulnbustionlThe Colribustion Institute, ISSC,/pp. 31153151 GENER L MODEL FOR DEVOL TILIZ TION OF L RGE CO L P RTICLES YUXIN ZZHAO, MICHAEL A. SERIO AND PETER R. SOLOMON Atlcc~nccd Fi1c.1 Hesr c~rcl~, nc 87 Clnlrch Strcct Eo,t Hnrtford, CT 06108, USA There ha\-? I~een everal studies on the tlevolatilization of'l~~rge oal particles under la1)oratoryconditions sim~llating arge-scale flrlidizetl or fixctl-hed coii11)ustion conditions. all of which de~nonstrate hat the proccss of devolatilization of large coal particles occurs over a 111ucl1 onger tiinescale than that of pul\~erized coals un~lcr he same conditioiis. This paper reports the develop~nent f a model for large ~(131 article dcvolatilizatior~ nder fl~~idized nd fixed-bed con~bustion onditions. The modcl combines a heat transfer lnodel with a gencral and co~nprchensive oal devolatilization model, FG-DVC. It inherits the generillity of FG-D\'C and can be applied to coals of v;~rious Gpes \*ithout cxtensi\,e prior kno\\rledge of the coals. This no el iilcl~~des detailed treatmcnt of'co;~l yrolpis reactions includir~g he yields of individual gas species, tar .ields, and tar lnolecular n~eigllt distrih~itions. The cl~anges n coal physical properties are inod(~1cd ith the applic;~tion f hl?rlick's coal physical propel-ty sl~l~models or specific heat and therlnal conductivity. Both the convective and r;idiati\re heat tra~isfcr ~et\veen lic gas pliasc ;~nd oal particles are considered and Gl~nn's orrelation for the surface heat transfer coefficient is used. Model predictions are co~npared with the data of large coal particle de\-olatilization ineasurrtl in fluidized beds and tubular reactors, for particle thermal response, total \\,eight loss. and individual gas cvolt~tion ates. 111 general, the agreement \vith the data is w y good for the particle center tc~n~perat~ire nd weight loss curves and is fair for tlle indi\~idual as evolution cllives. Introduction There liave been several studies on tlie de\.olutil- ization of large coal particles under laboratory con- ditions sinlulating large-scale fluidized or fixed-bed combustion contlitions, all of wvliicli demonstrate that the process of devolatilization of large coal par- ticles occlurs over a ~nticli onger timescale than that of pulverized coals ~mder he same conditions. Soine studies [l-31 have addressed tlie tiniescale of the dew.olatilization process for large particles and have estnl)lished tliat the tinlescale is proportiorla1 to cP, wit11 ranging from 0.32to 2.6 and t lleing tlie cliaineter of tl~e article. The large variation of in tlie reported correlations reflects the clifficlilty in dt.- ternlining the termination point of the tlevolatiliza- tion llrocess. More rece~it tudies 1iaw.e ~.e~ortc,cl Inore detailed measurt~n~ents f particle te~l~pc,ra- ture, particle weiglit loss, and gas species coiicentra- tions as a functio~i f time [4-71. Tlies? data sets were collected under relatively well-deiined condi- tions and can be used I;,r nod el \.crification. It is ge~~erall~ ccepted tliat tr;insport ollieat mid Illass lro111 the interior of particles to their s11rfac.e controls tlie timescale ol' large coal piirticle devola- tilization. Besides thct efkct on tlit, de\-olatilization rates, there are speculations as to wllether the vol- atile~ ndergo secondaq reactions within lxu-ticles as they exit. These reactions would include tar crack- iiig. gasification reactions with char, and gas-phase reactions. However, there is no solid evidence sup- porting claims that these reactions are significant enough to be modeled [S]. Experiments have shown onlv moderate, at most. cllanges in )ields with changing particle size. Tliese clinges may be clue primarily to tlie indirect effects of heat transfer (i.e., tliat larger particles have slower thermal response). An exception is tlie for~natioil of HCN and NH,]. Recent exl?erimental results llave indicated tliat large lxtrticle size enhances the formation of NH,? at the expense of IICN [9]. Although mass transport seeins not to cause significant secondary reactions for tlie ~najor olatile gas species, there is at)undant e~idence hat it plays an important role on the pri- maiy devolatilization reactions determining the )ields of tlie tar. Solo~non t al. [8] have given a de- tailed review and pointed out that longer residence ti~iies f tar could cause its precursor to be relmly- merizetl illto the char str~ictlire, eading to lower tar yields. This view is incorporated in a coal pyrolysis model, FC-DVC [8,10], which accounts for the mass transport limitations associated with tar evolution. Models have been developed to model tlie devol- atilization of large coal particles. Aganval et al. [11,12] and Hajaligol et al. [13] developed large coal particle devolatilization models that combine
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