Combustion and Flame

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1 Combuston and Flame 159 (2012) Contents lsts avalable at ScVerse ScenceDrect Combuston and Flame journal homepage: Multphyscs modelng of carbon gasfcaton processes n a well-strred reactor wth detaled gas-phase chemstry L Qao a,, Jan Xu a, Anup Sane b, Jay Gore b a School of Aeronautcs and Astronautcs, Purdue Unversty, West Lafayette, IN 47907, Unted States b School of Mechancal Engneerng, Purdue Unversty, West Lafayette, IN 47907, Unted States artcle nfo abstract Artcle hstory: Receved 5 Aprl 2011 Receved n revsed form 17 August 2011 Accepted 3 December 2011 Avalable onlne 7 January 2012 Keywords: Coal gasfcaton Carbon gasfcaton Detaled chemstry Heterogeneous surface reactons Radaton Mult-physcs numercal modelng Fuel synthess through coal and bomass gasfcaton has the potental to provde a soluton to the ncreasng demand for energy and transportaton fuels. To theoretcally understand the complex chemcal processes n a gasfer and to dentfy the most nfluental parameters for syngas producton, we developed a multphyscs model to smulate the gasfcaton processes n a well-strred reactor. Ths model s the frst of ts knd and consders detaled gas-phase chemstry, partcle-phase reactons, radatve heat transfer, as well as full couplng between the two phases at varous scales for mass, speces, and energy exchange. The gas-phase reactons use the detaled chemstry GRI-Mech 1.2, ncludng 177 elementary reactons and 31 speces, as well as varable thermodynamc and transport propertes. Four surface reactons were consdered and the reacton rates were smulated by the dffuson-knetcs model wth consderaton of boundary layer dffuson. A random pore model was used to account for the evoluton of the char porous structure and ts mpact on gasfcaton rates. A numercal code was developed to solve the gas-phase and the partcle-phase governng equatons. Numercal smulatons were conducted to understand the gasfcaton process and the effects of partcle sze, porous structure, radatve heat transfer, pressure, O 2 concentraton, and H 2 addton on gasfcaton performance. Ó 2011 The Combuston Insttute. Publshed by Elsever Inc. All rghts reserved. 1. Introducton Fuel synthess through coal gasfcaton offers a potental soluton to the problem of ncreasng demand for energy and transportaton fuels. The understandng of the complex chemcal processes n coal gasfcaton through expermental and computatonal means has generated ncreasng nterest over recent years. In terms of modelng coal gasfcaton processes, prevous works have focused manly on three areas: sngle coal partcle gasfcaton [1 3], one-dmensonal coal gasfcaton [4 7], and computatonal flud dynamcs (CFD) of coal gasfcaton reactors [8 12]. CFD modelng of entraned flow reactors s extremely complex, nvolvng gas-phase turbulent flow and partcle-phase turbulent flow, as well as partcle-gas-phase couplng, whch s beyond the scope of the present study and thus wll not be dscussed here. The modelng of a sngle char partcle offers a fundamental understandng of the gasfcaton process. Srnvas and Amundson [1] developed a smple model for gasfcaton of a sngle char partcle. It solves the partcle s mass and energy conservaton equatons wth the Stefan-Maxwell relatons assumng constant Correspondng author. Address: School of Aeronautcs and Astronautcs, Purdue Unversty, 701 W. Stadum Ave., West Lafayette, IN 47907, Unted States. E-mal address: lqao@purdue.edu (L. Qao). transport and thermodynamc propertes. Haynes [2] proposed an mproved model that calculated dffusvtes for dfferent components. Hs model also has the capablty to ncorporate multple reactons and components. Samulov et al. [3] developed a model that emphaszes the effects of a porous structure of the char and surface reacton knetcs for a sngle carbon partcle n a CO 2 envronment. It used the Laugmur Hnshelwood descrpton of the porous structure, the dffuson processes, and the gasfcaton processes. All these models, however, dd not consder detaled devolatlzaton knetcs or char-surface reactons. Moreover, nteractons between partcles and between gas phase and partcle phase were modeled n a smple way. Govnd and Shah [4] developed a 1-D mathematcal model to smulate the Texaco downflow entraned bed gasfer that used coal-water slurres as the feedstock. The unreacted-core shrnkng model was used to estmate the sold gas reacton rates. Three crucal parameters, the coal-feedng rate, the oxygen to coal rato, and the steam to coal rato, were nvestgated, and ther effects on the exhaust gas composton for the gasfer and the fnal carbon converson were determned. N and Wllams [5] developed a multvarable model for an entraned flow coal-oxygen gasfer, whch consdered one-step devolatlzaton knetcs and one char surface reacton and assumed the gas-phase reactons to be at equlbrum. The effects of coal-oxygen-steam ratos, temperature, and pressure /$ - see front matter Ó 2011 The Combuston Insttute. Publshed by Elsever Inc. All rghts reserved. do: /j.combustflame

2 1694 L. Qao et al. / Combuston and Flame 159 (2012) Nomenclature A p partcle surface area (cm 2 ) B transfer number B Bot number B k prefactor for surface reacton rate (g s 1 cm 2 atm 1 ) C p;g mean specfc heat capacty of mxture (erg g 1 K 1 ) C p,p specfc heat capacty of carbon partcle (erg g 1 K 1 ) C s total gas concentraton at the flm at partcle surface temperature (mol cm 3 ) D dffusvty (cm 2 s 1 ) D m molecular dffusvty at the flm temperature (cm 2 s 1 ) d p partcle dameter (cm) E k actvaton energy (erg mol 1 ) e nternal energy of the bulk gases (erg g 1 ) f RPM factor accounts for the pore surface evoluton because of carbon converson h convectve heat transfer coeffcent (erg s 1 cm 2 K 1 ) h specfc enthalpy of speces (erg g 1 ) K g thermal conductvty of gas mxture (erg s 1 cm 1 K 1 ) K m mass transfer coeffcent (mol s 1 cm 2 ) K k surface reacton rate constant (g s 1 cm 2 atm 1/n ) K p thermal conductvty of the partcle (erg s 1 cm 1 K 1 ) m mass (g) _m C surface reacton rate of reacton j (g s 1 ) N p partcle number densty (cm 3 ) N u Nusselt number _n C;k carbon molar reacton rate per unt area (mol s 1 cm 2 ) _n speces molar flux of speces (mol s 1 cm 2 ) P pressure (atm) Q C heat of surface reacton (erg cm 3 s 1 ) Q con convectve heat transfer between a partcle and the bulk gases (erg cm 3 s 1 ) Q p,rad radatve heat transfer between a partcle and the wall (erg cm 3 s 1 ) R C, speces generaton rate resultng from surface reactons (g s 1 ) Re Reynolds number R gas constant (erg mol 1 K 1 ) r p partcle radus (cm) Sc Sherwood number Q h enthalpy transferred from a partcle to the bulk gases as a result of mass transfer because of surface reactons (erg cm 3 s 1 ) Sh Sherwood number T temperature (K) t tme (s) W mean molecular weght of mxture (g mol 1 ) W C molecular weght of carbon (g mol 1 ) W molecular weght of speces (g mol 1 ) w producton rate of speces because of heterogeneous surface reactons (mol cm 3 s 1 ) X,s mole fracton of the speces at partcle surface X,1 mole fracton of the speces n bulk gases x carbon converson rato Y mass fracton of speces n the gas mxture Z transfer number Greek Letters / emprcal parameter q g densty of the gas mxture (g cm 3 ) q p densty of the partcle (g cm 3 ) x speces generaton rate resultng from gas-phase reactons (mol cm 3 s 1 ) e partcle surface emssvty r Stefan Boltzmann constant, (erg s 1 cm 2 K 4 ) pore structure parameter w 0 Subscrpts g gas phase the number of speces k the number of surface reactons p partcle phase W wall on gasfcaton products and steam producton were estmated. Later, Vamvuka and Woodburn [6] developed a 1-D steady-state entraned flow reactor model, whch s based on mass and energy conservaton equatons, ncludng sold-phase reactons and assumng gas-phase reactons at equlbrum. The temperature, reacton rate, and composton profles were calculated to determne the effects of dfferent operatng parameters on gasfer performance. These models, however, consdered rather smple heterogeneous surface reactons, neglectng detaled devolatlzaton knetcs and also the effects of a porous char structure on the dffuson process. Moreover, for gas-phase reactons only a few (up to 4) reactons were consdered wth a one-step overall reacton rate, and some reactons were assumed to be n equlbrum. A more detaled 1-D plug-flow reactor model was developed by Lu et al. [7] for a pressurzed entraned flow gasfer, whch emphaszed the nfluence of hgh pressure, reacton knetcs, and char structure on gasfcaton performance. The senstvty analyses show that reacton knetcs and char structure are both crucal for predctng coal gasfcaton processes. Also, low-pressure gasfcaton knetcs (.e., pressure order) cannot be extrapolated to hgh-pressure condtons. Recently, Sane et al. [13] developed a multphase well-strred reactor model to smulate coal gasfcaton. The model consders boundary layer gas dffuson reactons, two partcle-phase surface reactons, and water gas-shft reacton n equlbrum n the gas phase. The results showed the effects of pressure, temperature, partcle sze, H 2 O/coal rato, and external H 2 addton on the carbon converson and CO 2 emsson rates. In summary, prevous studes have shown that several factors, ncludng the detaled devolatlzaton knetcs, gas-phase reactons, char structure (through dffuson process), and char-surface reactons, can all nfluence the gasfcaton process, especally at hgh pressures. The models n lterature have mostly used smple gas-phase knetcs or reactons, and some reactons were assumed to be at equlbrum. The reacton rate has been mostly expressed n terms of a one-step overall reacton rate, whch may not be suffcently accurate for broader operatng condtons. Furthermore, multphyscs nteractons between gas phase and partcle phase were not thoroughly consdered n these models. Some nteractons that account for the mass and energy exchange between the two phases were even neglected. These studes ndcate that a more detaled model s needed, one that ncludes reacton dffuson processes, char structure, surface reactons, and nteractons between the two phases at the boundary. Lastly, gas-phase homogenous reactons and transport, whch have an mportant mpact on the gasfcaton behavor, should be better descrbed by the use of detaled chemstry, varable thermodynamc propertes, and varous mult-phase transport propertes. Motvated by ths, we developed the present multphyscs model wth detaled gas-phase chemstry and a numercal code

3 L. Qao et al. / Combuston and Flame 159 (2012) to smulate the complex carbon gasfcaton processes n a perfectly strred reactor. The model ncludes gas-phase and partclephase reactons as well as a couplng that ncludes mass, speces, and energy exchanges between the two phases at varous scales. The gas-phase reactons used the detaled chemstry GRI-Mech 1.2 [14], ncludng 177 elementary reactons and 31 speces, and varous transport propertes and varable thermodynamc propertes n CHEMKIN format. For the partcle-phase, four surface reactons were consdered. The surface reacton rates were smulated by usng the dffuson-knetcs model wth consderaton of boundary layer mass and energy dffuson. Numercal smulatons and parametrc studes were conducted to understand the gasfcaton process at varous operatng condtons. Whle we recognze that multple choces exst for the selecton of varous reacton mechansms, chemcal and physcal propertes, and phase dagrams, representatve results allowng conclusons that are qualtatvely ndependent and quanttatvely change only nsgnfcantly, wth specfc model selecton, are presented. 2. Model descrpton Fgure 1 shows a schematc of a well-strred reactor for whch the multphyscs model wth detaled chemstry descrbed n the prevous secton was developed to smulate carbon gasfcaton processes. Although the depcton n Fg. 1 s sphercal, the well strred reactor can be of any well defned geometrc shape. Carbon partcles wth dameter d p are unformly dstrbuted nsde the reactor together wth gaseous speces. The reactor s pressure remans constant, whch means that durng the gasfcaton process the volume ncreases as a result of thermal expanson; thus the number densty of coal partcles decreases, but the total number s conserved. It s assumed that ntense mxng occurs nsde the reactor so that all gas-phase propertes n the gas-phase bulk of the reactor, wth the excepton of the small boundary layers surroundng the partcles, are unform or spatally ndependent. As a result of ths assumpton, the temperature and number densty of the partcles can be assumed to be unform at the bulk scale of the reactor. Mass, speces, and energy exchanges between ndvdual partcles and surroundng gases cause local nonequlbrum n the boundary layers surroundng each of the partcles. These nteractons are modeled on the partcle scale. Moreover, the model developed for a sngle partcle represents all partcles nsde the reactor. For the gas-phase reactons, detaled knetcs and varable thermodynamc propertes are consdered. The governng equatons of mass, speces, and energy conservaton for the gas phase and the partcle phase are coupled to account for mass, speces, and energy exchanges between the two phases. The transent gasfcaton process s computed untl 99% of the coal partcle s gasfed. Addtonal assumptons that are of mmedate convenence but do not mpact the conclusons of the present study nclude unformty of temperature wthn the partcle phase as a result of the small sze and large thermal conductvty of partcles and sphercally symmetrc gradent dffuson heat and mass transfer to the partcle surface from the bulk gas phase. In partcular, the Bot number was found to be very small (10 4 ) for the present smulatons. Note the Bot number s defned as B = hd p /k p, where h s the convectve heat transfer coeffcent, and k p s the thermal conductvty of the partcle. The small Bot number mples that heat conducton nsde the partcle s much faster than the heat convecton away from ts surface, and thus temperature gradents are neglgble nsde of the partcle. Equal bnary dffuson coeffcents are consdered applcable for mult-speces dffuson and the bulk gas propertes are modeled usng deal gas law. The governng equatons n the Euleran coordnate system for the gas and partcle phases resultng from the above assumptons are descrbed n the followng secton Gas-phase equatons The conservaton equatons of mass, speces, and energy for the gas phase are dm g ¼ m g q g X w W ð1:1þ dy q g þ Y X K w k W k ¼ðx þ w ÞW k¼1 q g C P;g dt g þ X h ðw þ x ÞW ¼ N p ðq h þ Q con;g Þ Addtonally, the equaton of state for perfect gas s: ð1:2þ ð1:3þ 1 P ¼ qrt=w wth W ¼ P Y ð1:4þ =W In Eq. (1.1), q g and m g are the densty and mass of all gas-phase speces; w s the producton rate of speces because of surface heterogeneous reactons; W s the molecular weght of speces. In Eq. (1.2), Y s the mass fracton of speces ; x s the producton rate of speces because of gas-phase reactons. In Eq. (1.3), T g s the gas-phase temperature; h s the enthalpy of speces ; N P s the partcle number densty; Q h represents the enthalpy transferred from a partcle to the bulk gases as a result of mass transfer because of surface reactons; and Q con,g s the convectve heat transfer between a partcle and the bulk gases. Detaled dervaton of Eqs. (1.1) (1.3) s lsted n Appendx. The convectve heat transfer Q con,g between a partcle and the gases s defned as Q con;g ¼ ha P ðt g T p Þ ð1:5þ where h s the convectve heat transfer coeffcent, and A p s the reactve surface area of a partcle. The coeffcent h can be expressed as [15] h ¼ Nuk g d p B expðbþ 1 ; B ¼ _m pc p;g pd p Nuk g ð1:6þ Carbon partcles d P dameter N P number densty Gases Constant pressure valve where d p s the external dameter of partcles, and Nu s the Nusselt number. In the present low Reynolds flow, a value of 2 was chosen for the Nusselt number. The enthalpy transfer between one partcle and the bulk gas, Q h, can be expressed as Q h ¼ X w h 0 ð1:7þ Fg. 1. Coal gasfcaton n a well-strred reactor. Note f the speces s the gaseous reactant of the heterogeneous reactons, the value of h 0 s determned usng the gas phase

4 1696 L. Qao et al. / Combuston and Flame 159 (2012) temperature T g. If the speces s the gaseous product of the heterogeneous reactons, the value of h 0 s determned usng the partcle temperature T p. A detaled gas-phase reacton mechansm, GRI-Mech 1.2, s ncorporated nto the model, whch ncludes 177 elementary reactons and 31 speces. The gas-phase speces are H 2,H,O,O 2, OH, H 2 O, HO 2, H 2 O 2, C, CH, CH 2, CH 2 (S), CH 3, CH 4, CO, CO 2, HCO, CH 2 O, CH 2 OH, CH 3 O, CH 3 OH, C 2 H, C 2 H 2,C 2 H 3,C 2 H 4,C 2 H 5,C 2 H 6, HCCO, CH 2 CO, HCCOH, and N 2. Varous transport propertes and varable thermodynamc propertes were adopted based on the CHEMKIN format. GRI-Mech 3.0 mechansm was also used, and the results are essentally the same as those of GRI-Mech Partcle-phase equatons The partcle mass m p, densty q p, dameter d p, number densty N p, and temperature T p are the fve varables to solve. The governng equatons are: dm p ¼ P w W N p ¼ _m c ð2:1þ q P ¼ q P;0 m p m p;0 d P ¼ d P;0 N p ¼ N p;0 q g m g m p C p;p dt p ¼ Q con;p þ Q C Q p;rad ð2:2þ ð2:3þ ð2:4þ ð2:5þ where q P,0, m P,0, d P,0, and N P,0 are the ntal densty, mass, dameter, and number densty of each partcle at t =0s. _m C s the carbon consumpton rate because of heterogeneous surface reactons; C p,p s the heat capacty of partcles; Q con,p s the convectve heat transfer between a partcle and the bulk gases, expressed as Q con,p = ha P (T g T p )= Q con,g (see Eq. (1.3)); and Q p, rad s the radatve heat transfer between a partcle and the wall, whch can be expressed as Q p;rad ¼ erpd 2 p ðt4 W T4 p Þ ð2:6þ where e, r and T W are partcle surface emssvty, Stefan Boltzmann constant and the wall temperature, respectvely. Prevous studes have shown n gasfers gas phase radaton are much less mportant than partcle phase radaton. Thus here we neglected the radatve heat transfer between the hot gases and the wall, as well as the radatve heat transfer among partcles whch s a reasonable assumpton for dlute to moderate partcle number denstes. To help understand Eqs. (2.1) (2.6), the assumptons, models, and mechansms used for carbon gasfcaton are presented n the followng. It s well known that the physcal structure of a carbon or char partcle changes durng converson as a result of surface reactons. Emprcal correlatons have been developed for partcle dameter and densty to descrbe the transformaton. For example, the Carbon Burnout Knetcs (CBK), a knetcs package that descrbes char converson developed by Sanda Natonal Laboratores [16], assumed: a 1=3 1=3 m ; ð2:7þ q q 0 ¼ m m 0 d P d P0 ¼ q q 0 m 0 where subscrbe 0 denotes the ntal value. The value of a s estmated to be between 0.95 and 1 for both entraned flow gasfcaton and fludzed bed gasfcaton [16]. In the present model, we used 1 for a, whch results n a lnear relatonshp between q and m, as shown by Eq. (2.2), and a constant external dameter d P, as shown by Eq. (2.3). Note Eq. (2.4) descrbes the change of partcle number densty as a result of change of volume under the assumpton of constant pressure. Furthermore, char surface area evolves durng gasfcaton, and usually results n a porous structure. The Random Pore Model [17,18] has been wdely used to quanttatvely descrbe the evoluton. The present work adapted the Random Pore Model by mposng a factor f RPM nto the gasfcaton rate [16]. Ths factor accounts for the pore surface evoluton because of carbon converson: qffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff f RPM ¼ 1 w 0 lnð1 xþ ð2:8þ where x s the carbon converson rato; w 0 s a structural parameter, wth an emprcal value n a range of [16] for most chars. Here, a mean value of 4.6 as suggested n Ref. [16] s used. In Eq. (2.5), Q C s the gross thermal energy released by all surface reactons, whch can be wrtten as Q C ¼ X _m C;k Q C;k _m C ¼ X _m C;k ð2:9þ ð2:10þ where _m C;k and Q C,k are the carbon consumpton rate and the net heat of reacton of carbon surface reactons A, B, C, or D. Here we assume all heat from surface reactons s absorbed by partcles because ther thermal conductvty s much hgher than that of the gases. These surface reactons and the rate constant as well as gas transport n the boundary layer are descrbed n the followng Carbon surface reactons Four heterogeneous reactons are assumed to take place on the partcle surfaces: C þ H 2 O! CO þ H 2 C þ CO 2! 2CO C þ 2H 2! CH 4 C þ 1=/O 2! 2ð1 1=/ÞCO þð2=/ 1ÞCO 2 Reacton D s the carbon oxygen reacton, whch can produce both CO and CO 2. The rato of CO to CO 2 depends on partcle sze and temperature. The emprcal parameter / n Reacton D s obtaned followng [4]: 8 ð2z þ 2Þ=ðZ þ 2Þ for d p 6 0:005 cm >< ½ð2Z þ 2Þ Zðd p 0:005Þ= / ¼ 0:095Š=ðZ þ 2Þ for 0:005 cm < d p 6 0:1 cm >: 1:0 for d p > 0:1 cm ð3:1þ Z ¼ 2500 expð 6249=TÞ T ¼ðT p þ T g Þ=2 ðaþ ðbþ ðcþ ðdþ ð3:2þ ð3:3þ The global rate of each reacton was smulated usng the dffusonknetc model [19], whch s of the frst order for reactons A, B, and D, and of the second order for reacton C. The carbon reacton rate can be wrtten as _m C;k ¼ A p K k ðpx k;s Þ n ð3:4þ where subscrpton k denotes reactons A, B, C, or D; K k s surface reacton rate constant; X k,s s the mole fracton of the gaseous reactant at partcle surface. The surface reacton rate constant s expressed n Arrhenus form as

5 K k ¼ B k exp E k RT P ð3:5þ where B k s the prefactor; and E k s the actvaton energy. The knetc constant and the references from whch they were obtaned are lsted n Table 1. The transport rate of reactant gases to carbon surface was determned by bulk dffuson through an external boundary layer. Pore dffuson through an ash layer that could form over the char surface durng later stages of gasfcaton was neglected. The mpact of the nternal porous structure on the surface reacton rate was accounted by the Random Pore Model, as dscussed n the prevous secton. From the mass-based carbon reacton rate Eq. (3.4), the carbon molar reacton rate per unt area can be expressed as _n C;k ¼ K k ðpx k;s Þ n =W C ð3:6þ where W C s the molecular weght of carbon. Then molar flux of gaseous speces at the partcle surface can be expressed as _n H2 O ¼ _n C;A ð3:7:1þ _n CO2 ¼ _n C;B ð2=/ 1Þ _n C;D ð3:7:2þ _n H2 ¼ _n C;A þ 2 _n C;C ð3:7:3þ _n CO ¼ _n C;A 2 _n C;B 2ð1 1=/Þ _n C;D ð3:7:4þ _n O2 ¼ 1=/ _n C;D ð3:7:5þ _n CH4 ¼ _n C;C ð3:7:6þ The mole fracton of the reactant gases at partcle surface s related to the molar flux and mass transfer coeffcent by the followng transport equatons n the boundary layer that surrounds the partcle as [20] _n X ;s X _n ¼ k m ðx ;1 X ;s Þ ð3:8þ where the subscrpts s and 1 denote partcle surface and ambent; s the gaseous speces nvolved n the surface reactons; X,s and X,1 are the mole fracton of speces on partcle surface and n bulk, respectvely; k m s the mass transfer coeffcent, whch can be obtaned from the Sherwood number correlaton for spheres n a convectve flow [20] as Sh ¼ k md p C s D m ¼ 2 þ 0:6Re 1=2 Sc 1=3 2 ð3:9þ where Sh s the Sherwood number, and a value of 2 was chosen for the present low Reynolds flow. C s s the total gas concentraton at the flm at partcle surface temperature, D m s the molecular dffusvty of speces at the flm temperature. Gven the mole fracton of H 2 O, CO 2,H 2 and O 2 n the gas phase, Eqs. (3.6) (3.9) form a closed nonlnear system wth the unknowns beng the surface mole fracton of the gaseous reactants, X,s. The nonlnear equaton system s solved usng the DNEQNF solver n Table 1 Reacton rate constants and heat of reacton. Reacton k s = B exp ( A/T p ) Q C (10 7 erg/g) [28] B A A [29,30] , B [29,30] ,060 13,310 C [10,30] , D [10,30] ,967-2(1-1//) 10,260-2(2// -1)33,830 a a / s a parameter determned by partcle dameter and the mean temperature of the partcle and the gas. L. Qao et al. / Combuston and Flame 159 (2012) the IMSL lbrary [21]. The solver uses a modfed Powell hybrd algorthm and a fnte-dfference approxmaton to the Jacoban. Once the surface mole fractons of these speces are obtaned, the consumpton rates of carbon from each reacton can also be determned by Eq. (3.4) Numercal method The gas-phase and partcle-phase governng equatons, whch form a closed ODE system, were solved usng FORTRAN package DASPK3.1 [22]. DASPK was desgned to solve large-scale Dfferental Algebrac Equaton (DAE) systems. After the ntal condton for every varable and the convergence crtera were specfed, DASPK ntegrated the equatons over tme. The tme step sze and the order of temporal dscretzaton were dynamcally determned by the solver tself. 3. Results and dscussons 3.1. Model valdaton To valdate the model and the numercal code, we compared the smulaton results wth expermental data n the lterature. Gregg et al. [23] conducted a seres of experments to gasfy sub-btumnous coal, actvated carbon, coke, and a mxture of coal and bomass n a 23 kw solar furnace. The sunlght comng through a reactor wndow was focused drectly on the coal bed beng gasfed. Steam was passed through the solar-heated coal bed where t reacted wth the coal and thus formed a combustble product gas. Among the many expermental coal gasfcaton studes n lterature, ths experment s most representatve of the partcle scale processes wthn the mult-scale reactor model developed n the present study. The expermental data for valdaton used here are for gasfcaton of actvated carbon wth steam. The composton of the actvated carbon nclude 93.4% C, 0.6% H, 1.5% S, 0.2% ash, 0.2% acd-evolved CO 2, and 0.2% mosture [23], whch s close to the carbon used n the present work. The ntal sze of the carbon partcles n the experment was 5 mm and the reactor wall temperature was mantaned around 1050 K. In the experment, the energy used for gasfcaton was provded through admsson of estmated solar flux of W/m 2 through a large wndow. Other expermental condtons are lsted n Table 2. A comparson of the computed results and the expermental data for dry based concentratons of major speces s shown n Fg. 2. The results show good agreement between the computed and the expermental data for major speces. For comparson, results based on the calculatons of Sane et al. [13], who assumed bulk gas-phase equlbrum for the water gas shft reacton, are also shown Typcal gasfcaton process wthout oxygen Numercal smulatons were conducted at varous operatng condtons (e.g., ntal T, P, and reactant composton) to gan a fundamental understandng of the gasfcaton processes and to dentfy the most nfluental parameters for gasfcaton performance, ncludng carbon converson tme, syngas producton and composton, and CO 2 emsson. We frst consdered a basc case for whch no oxygen was ntally present n the reactant mxture. To provde the heat needed for the endothermc surface reactons, a constant wall temperature of 1100 K was assumed. The reactor s flled wth unformly dstrbuted carbon partcles and steam, both at a temperature of 1200 K and a pressure of 10 atm. The ntal H 2 O/C rato s 2, the partcle sze s 100 lm, and the ntal and fnal partcle

6 1698 L. Qao et al. / Combuston and Flame 159 (2012) Table 2 Intal condtons for the valdatng case. Intal gas temperature T g = 1050 K Intal partcle temperature T p = 1050 K Wall temperature T w = 755 K Solar energy densty J = W/m 2 Gas pressure P = 1 atm Densty of partcles q p = 0.9 g/cm 3 Intal water concentraton X H2O ¼ 1:0 Intal partcle dameter d p =5mm Intal H 2 O/C molar rato H 2 O/C = 3.0 Fg. 2. Profles of the computed and measured speces concentratons as a functon of tme. Expermental data are from Ref. [23]. Fg. 3. Profles of the gas and partcle temperatures as functons of tme for the basc case wthout O 2. Intal condtons are lsted n Table 3. Table 3 Intal condtons for a typcal gasfcaton process wthout oxygen. Intal gas T g = 1200 K Intal partcle T p = 1200 K temperature temperature Wall temperature T w = 1100 K Densty of partcles q p = 1.3 g/cm 3 Gas pressure P = 10 atm Intal partcle d p = 100 lm dameter Intal water X H2O ¼ 1:0 Partcle number N p = 896/cm 3 concentraton densty Intal H 2 O/C molar rato H 2 O/C = 2.0 Fg. 4. Comparson of the source terms n the gas-phase energy equaton for the basc case wthout O 2. Intal condtons are lsted n Table 3. number densty s 896/cm 3. These representatve parameters used n the present smulaton are lsted n Table 3. Fgure 3 shows the temperature profles of T p and T g as a functon of tme. The partcle and the gas have nearly the same temperatures durng the gasfcaton process. Both temperatures decrease to 1095 K at t = 4 s, from 1200 K at t =0s. The decrease n temperature s because of the endothermc nature of the surface reactons (C + H 2 O=CO+H 2 and C + CO 2 = 2CO). After t = 4 s, the temperatures reman almost constant. To understand energy converson durng the gasfcaton process, we compared the source terms n the gas-phase energy equatons, whch nclude the total heat produced by the gas-phase reactons, convectve heat transfer, and enthalpy transfer because of the mass transfer resultng from the surface reactons, respectvely. These terms plotted as functons of tme, are shown n Fg. 4. In the frst four seconds, these terms change rapdly. Especally, the convectve heat transfer shows a negatve spke. In the later stage (t > 4 s), the terms reman almost constant. The convectve heat transfer s nearly zero because the dfference between T p and T g s neglgble. Furthermore, we examned the energy balance of the 177 detaled elementary reactons and dentfed fve that have the hghest energy release, as shown n Fg. 5. These reactons are OH + H 2 =H+H 2 O, OH + CH 4 =CH 3 +H 2 O, H + CH 4 =CH 3 +H 2, OH + CH 2 O = HCO + H 2 O, and H + CH 2 O = HCO + H 2. Among them, the energy release from OH + H 2 =H+H 2 O reacton s a few magntudes larger than that from the others, ndcatng that t s the most nfluental elementary reacton n the gas phase. The energy release rates change n the frst 40 s and reman almost constant at a later stage of the gasfcaton process. Fgure 6 shows the concentraton profles of fve stable speces ncludng H 2,H 2 O, CO, CO 2, and CH 4. The carbon converson rate as functons of tme s also shown. The overall converson tme s about 160 s. Note that the smulaton ends when 99% of the carbon

7 L. Qao et al. / Combuston and Flame 159 (2012) Fg. 5. Profles of the heat release rate as a functon of tme of fve man gas-phase elementary reactons for the basc case wthout O 2. Intal condtons are lsted n Table 3. Fg. 7. Computed rato of concentratons of product and reactant gases and theoretcal equlbrum constant of water gas-shft reacton as a functon of tme for the basc case wthout O 2. Computed ratos of concentratons are based on detaled chemstry calculatons; theoretcal K p are calculated usng the polynomal expresson n Ref. [24]. Intal condtons are lsted n Table 3. Fg. 6. Profles of speces concentratons and carbon converson rate as functons of tme for the basc case wthout O 2. Intal condtons are lsted n Table 3. partcles are gasfed. The concentratons of ntermedate and mnor speces are not shown, whch are much lower than those of the stable speces. At the end of the gasfcaton process, 34.3% H 2, 30.8% CO, and 1.5% CO 2 are produced n the wet gas mxture. The water gas-shft reacton (CO + H 2 O=CO 2 +H 2 ) s often assumed to be at equlbrum n most modelng studes. To evaluate ths assumpton, we calculated the rato = X CO2 X H2 /(X CO X H2O )by usng the smulaton results (n equlbrum ths rato s equal to the equlbrum constant K p ), where the concentratons of H 2 O, CO, H 2, and CO 2 are obtaned from the present detaled-chemstry calculatons. Fgure 7 shows the rato = X CO2 X H2 /(X CO X H2O ) as a functon of tme. The concentraton rato s compared to the equlbrum constant obtaned usng the an emprcal expresson from Ref. [24]. The K p has a polynomal dependence on temperature expressed as: ln(k p )= (10 3 /T g ) (10 3 /T g ) 2. The expresson was obtaned by nonlnear regresson based on the data n the JANAF Thermochemcal Table [25], and the accuracy of the resultng value of K p s better than 1%. The comparson of Fg. 7 clearly shows that the rato of concentratons s not equal to the K p value over the entre tme, ndcatng that the water gas-shft Fg. 8. Profles of carbon consumpton rates of three surface reactons as functons of tme for the basc case wthout O 2. Intal condtons are lsted n Table 3. reacton s not n equlbrum. The reason for the nonequlbrum mght be the extreme temperature senstvty of the elemental reactons at the relatvely low temperatures (below 1200 K), as suggested by Gregg et al. [26]. Ths extreme senstvty to temperature n the relatvely low temperature range s smlar to the extreme senstvty to temperature n the gnton regme of combuston processes. The results, pont to a need for future expermental and theoretcal nvestgatons of the threshold gasfcaton temperature. A better understandng of the relatve mportance of the three surface reactons s reached when we vew n Fg. 8 the rates of reactons A, B, and C, based on a sngle partcle plotted as functons of tme. The absolute values of the reacton rates decrease rapdly soon after the gasfcaton reactons start. Later the absolute values of the reacton rates retan almost constant very low values bearng resemblance to analogous extncton regme n combuston processes. The rate of C + H 2 O? CO + H 2 reacton s much faster than the rates of C + CO 2? 2COandC+2H 2? CH 4 reactons. However,

8 1700 L. Qao et al. / Combuston and Flame 159 (2012) all three rates are sgnfcantly slower than those durng the ntal transent. Fgure 9 shows the source terms n the partcle-phase energy equaton, based on a representatve sngle partcle out of the 895 dentcally gasfyng partcles per cm 3 n the reactor, ncludng heat release from reactons A, B, and C, as well as convectve heat transfer between the two phases and the radaton heart transfer between the sngle partcle and the wall n the presence of an sotropc mxture of dentcal partcpatng partcles. Because of the endothermc nature of the three surface reactons, the partcle temperature and the gas temperature both decrease after the reactons start. Of the three surface reactons, reacton A (C + H 2 O? CO + H 2 ) consumes the most energy. Moreover, the radaton energy exchanged between the partcle and the wall s sgnfcant at the later stages of the gasfcaton process. Dependng on the wall temperature, the radaton exchange may provde the energy needed for the endothermc surface reactons and prevent ceasng of gasfcaton observed n the present example Typcal gasfcaton process n the presence of oxygen Because the carbon-steam reacton absorbs energy, practcal gasfers need to be heated to mantan a hgh-temperature envronment so that the gasfcaton reactons can proceed. The heat sources can be electrc, partal oxdaton of coal (combuston of coal), or oxdaton of an auxlary fuel such as natural gas n the gasfer. Partal oxdaton of coal usng externally njected oxygen s a more common practce because t s cost-effectve. We emulated partal oxdaton of coal by ncludng a small amount of oxygen n the ntal mxture. The carbon oxygen reactons durng the ntal stage are to be desgned to provde suffcent energy to the system. Smlar to the prevous example, we assume an adabatc process and use dentcal ntal temperature, pressure, partcle dameter, and H 2 O/C molar rato. The only dfference s that the reactant mxture now contans 20% O 2 and the wall temperature T W s 500 K. The ntal condtons are summarzed n Table 4. Fgure 10 shows the temperature profles of T p and T g as a functon of tme. The partcle and gas temperatures ncrease rapdly to a maxmum (T p = 1850 K and T g = 2460 K). Durng the ntal perod, T p s hgher than T g (t < 0.02 s), but t becomes lower durng the rest of the gasfcaton process. The peak temperatures occur at the nstant of complete oxygen depleton, as can be seen n Fg. 11, whch also shows the concentraton profles of the sx man stable gaseous speces as a functon of tme. Fg. 9. Comparson of the source terms n the partcle-phase energy equaton for the basc case wthout O 2. Intal condtons are lsted n Table 3. Also shown n Fg. 11 s the carbon converson rate as a functon of tme. The gasfcaton process needs about 0.1 s to be complete. Durng the nterval, 0 s < t < s, CO concentraton frst ncreases slghtly, then decreases to zero. Durng the nterval, 0s<t < s, oxygen concentraton approaches neglgble levels, whle CO 2 concentraton ncreases to a maxmum (18%). Durng ths perod, the carbon surface oxdaton reacton (C + 1//O 2? 2(1 1//)CO + (2// 1)CO 2 ) and the gas-phase reactons are domnant and consume most of the oxygen. For t > s when O 2 s consumed and the peak temperature has been acheved, carbon surface reactons A, B, and C become more mportant. Especally the carbon-steam reacton A, whch causes the concentratons of CO and H 2 to ncrease and the concentraton of H 2 O to decrease, and the surface reacton B whch reduces CO 2 whle removng a C atom from the carbon surface to produce two molecules of CO are mportant. In the ntal perod, oxdaton reactons of both the bulk gas phase and the partcle surface and gas phase occur. Gas-phase oxdaton reactons manly nclude the elementary steps OH + CO=H+CO 2, H+O 2 +H 2 O=HO 2 +H 2 O, OH + HO 2 =O 2 +H 2 O, H+O 2 = O + OH, OH + H 2 =H+H 2 O, 2OH = O + H 2 O and 2OH+M=H 2 O 2 + M. The reacton rates of these seven steps are plotted as functons of tme n Fg. 12. The rates of the gas-phase reactons are much faster than those of the sold gas reactons. The gas-phase oxdaton reactons are domnant n the presence of O 2. The gas-phase temperature T g reaches a peak value that s hgher than the peak partcle-phase temperature T p because of the large energy release rate of the gas-phase oxdaton reactons. Later n the gasfcaton process, after the oxygen s completely consumed, T p and T g both begn to decrease because of the endothermc nature of the surface reactons. Fgure 13 shows the reacton rates of reactons A, B, C, and D as functons of tme. Durng the ntal perod, the rate of C + 1/ /O 2? 2(1 1//)CO + (2// 1)CO 2 reacton s much hgher (100 tmes) than the rate of C + H 2 O? CO + H 2 reacton. After the oxygen s depleted, the carbon-steam reacton becomes domnant, wth a rate about 10 or more tmes hgher than the rates of C+CO 2? 2 CO and the C + 2 H 2? CH 4 reactons. Based on Fgs , one can dvde the gasfcaton process nto three stages: (1) carbon oxdaton, (2) gas-phase oxdaton, and (3) carbon gasfcaton, as noted n Fgs The carbon oxdaton reacton D can be consdered as a combnaton of two reactons, 2C + O 2? 2CO and C + O 2? CO 2. To fnd the relatve mportance of these two reactons, the rato of 2(1 1//)/(2// 1)s plotted as a functon of tme n Fg. 14. It shows that the rato of CO to CO 2 changes n the range of Ths means that the concentraton of CO 2 s much less than CO and the C + O 2? CO 2 reacton could be neglected for hgh temperature carbon oxdaton. Char porosty plays an mportant role n gasfcaton, and s a key factor that mpacts reacton rates. In the present smulatons, a random pore model [17,18] was used to count for the pore surface evoluton as well as ts mpact on the gasfcaton rates. Fgure 15 shows the profle of the factor f RPM as a functon of tme. Note f RPM represents the rato of the nternal surface area of the pores to the constant outer surface area of the partcle. Ths factor was mposed nto the gasfcaton rate to account for the ncrease of nternal surface area durng carbon converson. Fgure 15 shows the factor gradually ncreases wth tme. At the end of the gasfcaton process, ths factor reaches a value of 5.6, ndcatng that the pore structure does play an essental role n enhancng the reacton rates and the gasfcaton process. The partcle and the gas phases have not only energy transfer, but also mass transfer resultng from the surface reactons that consume and produce gaseous speces. Ths alters the gas-phase composton by the dffuson process. Fgure 16 shows the net

9 L. Qao et al. / Combuston and Flame 159 (2012) Table 4 Intal condtons for a typcal gasfcaton process wth presence of oxygen. Intal gas temperature T g = 1200 K Intal partcle temperature T p = 1200 K Wall temperature T w = 500 K Densty of partcles q p = 1.3 g/cm 3 Gas pressure P = 10 atm Intal partcle dameter d p = 100 lm Intal water concentraton X H2O ¼ 0:8 Intal oxygen concentraton X O2 ¼ 0:2 Intal H 2 O/C molar rato H 2 O/C = 2.0 Partcle number densty N p = 828/cm 3 Fg. 10. Profles of gas and partcle temperature as functons of tme for the basc case wth O 2. Intal condtons are lsted n Table 4. Fg. 12. Profles of rate of progress of man gas-phase elementary reactons for the basc case wth O 2. Intal condtons are lsted n Table 4. Fg. 11. Profles of speces concentraton and carbon converson rate as functons of tme for the basc case wth O 2. Intal condtons are lsted n Table 4. Fg. 13. Profles of carbon consumpton rates of four surface reactons as functons of tme for the basc case wth O 2. Intal condtons are lsted n Table 4. producton rates of fve stable speces as functons of tme. The CO producton rate from surface reactons s determned by reactons 2C + O 2? 2CO, C + O 2? CO 2,C+H 2 O? CO + H 2, and C + CO 2? 2CO, especally the frst three reactons. Ths explans the fact that the CO concentraton versus tme curve has a peak at t = s, whch s between the peak of the O 2 curve (t = s) and the peak of the H 2 O curve (t = s). CO 2 s frst produced from the carbon oxdaton reacton and then s consumed n reacton C + CO 2? 2CO n gas-phase oxdaton stage and carbon gasfcaton stage. As dscussed earler, the gas temperature changes because of heat release from gas-phase reactons, energy transfer because of the mass transfer from partcle surface reactons, as well as convectve heat transfer between the two phases. The partcle temperature changes because of heat release/absorpton from surface heterogeneous reactons, convectve heat transfer and radaton. The source terms n the gas and partcle-phase energy equatons are dscussed below to mprove our understandng of the energy couplng between the two phases and to dentfy the most nfluental parameters n the gasfcaton process. Fgure 17 shows the source terms n the partcle energy equaton, ncludng heat release from reactons A, B, C, and D, the convectve heat transfer between the two phases and radaton between a partcle and the wall. As shown n Fg. 17, the heat released by reacton D (the carbon oxdaton reacton) and the

10 1702 L. Qao et al. / Combuston and Flame 159 (2012) Fg. 14. The rato of CO to CO 2 n reacton D as a functon of tme for the basc case wth O 2. Intal condtons are lsted n Table 4. Fg. 16. Net producton rate of fve speces resultng from surface heterogeneous reactons for the basc case wth O 2. Intal condtons are lsted n Table 4. Fg. 15. Profle of f RPM as a functon of tme for the basc case wth O 2. f RPM s the rato of the nternal surface area of the pores to the constant outer surface area of the partcle. Intal condtons are lsted n Table 4. Fg. 17. Comparson of the source terms n the partcle-phase energy equaton for the basc case wth O 2. Intal condtons are lsted n Table 4. convectve heat transfer between a partcle and surroundng gases are most mportant n the ntal stages. The former ncreases the partcle temperature, and the latter ncreases the gas temperature by convecton. The energy absorbed by reacton A (C + H 2 O? CO + H 2 ) and the convectve heat transfer from the gas phase to the sold surface are more mportant post-o 2 consumpton n comparson to the heat absorbed by the reactons A, B (C + CO 2? 2CO) and C (C + 2H 2? CH 4 ). The effects of radaton heat transfer declne as the partcle surface and gas phase temperatures are reduced by the post-o 2 endothermc processes. Fgure 18 shows a comparson of three source terms n the gas phase energy equaton, ncludng the total heat generated by the gas-phase reactons, convectve heat transfer, and enthalpy transfer because of mass transfer from the surface reactons. In the ntal stages, the total energy released by the gas-phase reactons s domnant. A peak of the sensble energy generated by the gasphase reactons occurs around t = s. Later durng the gasfcaton stage, the three source terms change very lttle. Fg. 18. Comparson of source terms n the gas-phase energy equaton for the basc case wth O 2. Intal condtons are lsted n Table 4.

11 L. Qao et al. / Combuston and Flame 159 (2012) Lastly, we evaluated the assumpton of the water gas-shft reacton to be at equlbrum. The rato representng the multplcaton of product concentratons dvded by the multplcaton of the reactant concentratons of the water gas shft reacton (product of the concentratons of H 2 O and CO dvded by the product of the concentratons of CO 2, and H 2 ) obtaned from the detaled chemstry calculatons was compared to theoretcal equlbrum constant. The results are shown n Fg. 19. It can be seen that the values have reasonable agreement n the later phases of the gasfcaton process, ndcatng that the water gas-shft reacton s at least near equlbrum state. Ths was not observed for the low temperature gasfcaton case dscussed earler n the paper and depcted n Fg. 7. Ths s consstent wth prevous studes that suggested that at hgh temperatures the water gas-shft reacton can be assumed to be at equlbrum [24,27] Effect of partcle sze Parametrc studes were conducted to understand the effects of process condtons on the gasfcaton processes. Frst, the effect of partcle sze on carbon converson rate was examned. Fgure 20 compares the total converson tme of four mxtures contanng carbon partcles of varous szes n the range lm. Note we kept the carbon mass the same for all four mxtures, whch means the partcle-number denstes are dfferent but stll wthn the ndependent partcle regme. Other ntal condtons were the same as dscussed n Sesson 3.3 (see Table 4). The results show that as expected the carbon converson tme s sgnfcantly reduced wth a decrease of partcle sze. The reason can be seen from Fgs. 21 and 22, whch compare the profles of the gas and partcle temperature (Fg. 21) and the carbon consumpton rate (Fg. 22) for d p = 100 lm and 70 lm, respectvely. Fgure 21 shows that for smaller partcles, T p and T g reach ther peak values more rapdly. Ths means the heat transfer by means of conducton and convecton s more effectve n rasng the temperature of smaller partcles. The resultng rapd surface reacton rates lead to shorter Fg. 19. Computed rato of concentratons of product and reactant gases and theoretcal equlbrum constant of water gas-shft reacton as a functon of tme for the basc case wth O 2. The rato of concentratons of product and reactant gases are based on detaled chemstry calculatons; theoretcal K p are calculated usng the polynomal expresson n Ref. [24]. Intal condtons are lsted n Table 4. Fg. 21. Profles of gas and partcle temperature for d p = 100 lm and 70 lm for the basc case wth O 2. Other ntal condtons are lsted n Table 4. Fg. 20. Carbon converson rato as functons of tme for d p = 100 lm, 90 lm, 80 lm, and 70 lm for the basc case wth O 2. Other ntal condtons are lsted n Table 4. Fg. 22. Carbon consumpton rate for d p = 100 lm and 70 lm for the basc case wth O 2. Other ntal condtons are lsted n Table 4.

12 1704 L. Qao et al. / Combuston and Flame 159 (2012) converson tmes as shown n Fg. 22. Lastly, partcle sze has no mpact on the fnal CO 2 emsson Effect of radaton Radatve heat transfer can be a strong energy transport mechansm n real gasfers. Radaton heat loss from partcles can be sgnfcant at hgh temperatures. In the followng, we wll dscuss the effect of radatve heat transfer between the partcles and the wall on the gasfcaton process usng an example dscussed n Secton 3.3 (the basc case wth O 2, see Table 4). Note the wall temperature was assumed to be 500 K. Fgure 23 compares the temperature profles calculated wth and wthout consderaton of radaton. It can be seen the peak T p and T g are about 50 K lower when radaton s ncluded. The dfference, however, s small at later stages when T p and T g both decrease. Because of the lower temperatures resultng from radaton heat loss, the reacton rates are lower. As a result, the carbon converson tme s longer: the total converson tmes are s (wthout radaton heat loss) and s (wth radaton heat loss) Effect of pressure Fg. 24. Carbon converson rato as functons of tme for 10 atm, 12 atm, 14 atm, and 16 atm for the basc case wth O 2. Other ntal condtons are lsted n Table 4. Smulatons were conducted for the same reactant mxture at varous pressures to understand the effect of pressure on the gasfcaton process and the carbon converson rate. Fgure 24 shows the carbon converson rato as a functon of tme at pressures of 10, 12, 14, and 16 atm, respectvely. Other ntal condtons are lsted n Table 4. The dfferences n the carbon converson tme (0.05 s at 16 atm vs s at 10 atm) ndcate that pressure has a sgnfcant nfluence on the gasfcaton process. Fgure 25 shows a comparson of the reacton rates of the two surface reactons at 10 atm and 14 atm. It can be seen that the peak rates at 14 atm are several tmes larger than those at 10 atm, thus reducng the overall tme requred for gasfcaton. Ths s because the reacton rate constant s proportonal to the pressure. Also, the speces mole fractons at the partcle surface are hgher at hgh pressures, leadng to correspondng ncreases n the surface reacton rates Effect of oxygen concentraton Smulatons were conducted for reactant mxtures contanng varous concentratons of O 2 based on n the example dscussed Fg. 25. Carbon consumpton rate of two surface heterogeneous reactons as functons of tme for 10 atm and 14 atm for the basc case wth O 2. Other ntal condtons are lsted n Table 4. n Sesson C. Fgure 26 depcts the carbon converson rates as functons of tme for varous O 2 concentratons. The results show that the O 2 concentraton affects carbon converson tmes sgnfcantly (0.153 s at X O2 = 18% vs s at X O2 = 24%). At hgher O 2 concentratons, O 2 s depleted faster. The heat released by the exothermc oxdaton reactons results n hgher partcle and gas temperatures, whch consequently and subsequently ncrease the carbon converson rate. Durng the gasfcaton process, the peak CO 2 concentraton ncreases wth ncreasng O 2 concentraton. However, by the end of the gasfcaton process, the fnal CO 2 concentraton remans almost the same for all four cases Effect of hydrogen addton Fg. 23. Profles of gas and partcle temperature for the case wth radaton and wthout radaton for the basc case wth O 2. Intal condtons are lsted n Table 4. Coal gasfcaton technology s beng explored as a means to produce lqud fuels for the transportaton sector. However, the gasfcaton process also releases CO 2, whch can be a concern. Agrawal et al. [28] proposed a hybrd hydrogen-carbon (H2CAR) process for the producton of lqud fuels, n whch there s no