KINETICS OF STEAM GASIFICATION OF BROWN COAL AND VOLATILE-CHAR INTERACTIONS

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1 INETICS OF STEAM GASIFICATION OF BROWN COAL AND VOLATILE-CAR INTERACTIONS Shro ajtan,, u-lng Tay, Shu Zhang, Chun-Zhu L * Curtn Centre for Advanced Energy Scence and Engneerng, Curtn Unversty of Technology, erth, WA 6845, Australa Energy Engneerng Research Laboratory, Central Research Insttute of Electrc ower Industry (CRIEI), Yoosua, anagawa 4-96, Japan *Emal: chun-zhu.l@curtn.edu.au ABSTRACT Vctoran brown coal has hgh reactvty for gasfcaton because of catalyss of nherent AAEM speces. It was, however, found recently that the nhbtory effects of the volatle-char nteractons on char gasfcaton are not neglgble. In ths study, the effects of volatles were examned wth a novel fludsed-bed/fxed-bed reactor, and the mechansms and netcs model of the char gasfcaton and volatle-char nteractons were dscussed to descrbe quanttatvely the nhbton of volatles about char gasfcaton. Vctoran brown coal sample was gasfed wth steam at 8 C at varous feedng rates. As a result, char yelds became hgher at hgher feedng rate at the same feedng tme because of the volatle-char nteractons. The elementary reactons of the char gasfcaton and volatle-char nteractons were proposed and the L- type reacton rate equatons for brown coal gasfcaton were newly derved, and the netcs model was verfed by the comparson wth experments. The expermental data of the char yelds and the concentratons of Na n gasfed char were able to be estmated by the proposed netcs model. Ths model would be useful to estmate the performance of the gasfer for Vctoran brown coal. eywords: Vctoran brown coal, char, gasfcaton, catalyst, reactvty INTRODUCTION Coal gasfcaton has been consdered as one of the most mportant clean coal technologes. Gasfcaton technology s partcularly sutable for the utlsaton of lowran coal, such as Vctoran brown coal, due to ts hgh reactvty for gasfcaton because of catalyss of nherent alal and alalne earth metallc (AAEM) speces. It was, however, found recently that the nhbtory effects of the volatle-char nteractons on char gasfcaton are not neglgble. Volatles from coal pyrolyss are responsble for the nhbton of steam gasfcaton (Bayarsahan et al., 5), and enhance the volatlsaton of the monovalent AAEM speces, such as Na, to decrease the overall char gasfcaton rate (Wu et al., ). The volatle-char nteractons are most lely the reactons between char and free radcals formed by the thermal cracng and/or reformng of volatles.

2 In ths study, the effects of volatles were examned wth a novel fludsed-bed/fxedbed reactor. The mechansms and netcs model of the char gasfcaton and volatlechar nteractons were dscussed to descrbe quanttatvely the nhbton of volatles about char gasfcaton. The proposed netcs model would be useful to estmate the performance of the gasfer for Vctoran brown coal. EXERIMENTAL Coal Samples Loy Yang (Vctora, Australa) brown coal was used n ths study. The sample was partally dred at the temperature lower than 35 C, then pulversed and seved to obtan a sample of partcle sze between 63 5 μm. Ths coal sample was termed as a raw coal sample. The propertes of the coal sample are: C, 7.4;, 5.4; N,.6; S,.8; Cl,.; O, 3. and VM, 5. wt% (daf) together wth ash,. wt% (db) and mosture, 3 wt% (ad). Na s nown as the major element of AAEM speces n Vctoran brown coal (Quyn et al., ), and the concentraton of Na n the sample was. wt% (db). A -from coal sample was prepared by washng raw coal sample wth sulfurc acd to remove nherent AAEM speces by the same procedure descrbed elsewhere by Sathe et al. (999). Steam Gasfcaton Coal samples were gasfed usng a novel fludsed-bed/fxed-bed quartz reactor (Quyn et al., ) shown n Fg.. Two frts were nstalled n the reactor body. Slca sand was put onto the lower frt and was fludsed by the gasfyng agent. The reactor was heated up wth an external electrcal furnace and ept at 8 o C n ths study. A LC pump was used to delver water drectly nto the reactor to generate steam as the gasfyng agent. The concentraton of steam n total suppled gas was 5%. The total gas flow was l/mn at the standard ambent temperature and pressure, and the gas velocty at reacton temperature was approxmately 5.6 m/mn. Coal partcles entraned n a coal feeder were fed through a water-cooled njecton probe nto the heated sand bed at certan feedng rates to acheve rapd partcle heatng rates. Char was formed n the sand bed and was elutrated out of the sand bed due to ts lghtness. The char formed a fxed bed underneath the top frt. The confguraton of the reactor allowed the volatle-char nteractons durng char gasfcaton. After coal feedng was stopped, char was gasfed wthout the volatle-char nteractons. The reactor was fnally lfted out of the furnace and cooled down. The char yeld was determned by the dfference n the weghts of reactor and coal/char before and after each experment. The reproducblty of char yelds should be wthn ±%. Char was collected after the gasfcaton experment and the concentratons of AAEM speces n char was quantfed by the same procedure descrbed elsewhere by L et al. (). The reproducblty of AAEM analyss should be around ±5% (Ouno et al., 5).

3 Fg. : Schematc dagram of a fludsed-bed/fxed-bed reactor (Quyn et al., ) RESULTS AND DISCUSSION netcs Model The observed gasfcaton rate dx/dt was assumed to be descrbed as a sum of the overall rate constant for non-catalytc gasfcaton nc and that for catalytc gasfcaton c n ths study, because Bayarsahan et al. (5) showed that non-catalytc gasfcaton and catalytc gasfcaton too place n parallel durng steam gasfcaton of Vctoran brown coal. dx dt nc c () where X s the converson rato of char and t s tme. The mechansms and reacton rate equatons would be dscussed as follows. Non-Catalytc Gasfcaton Some evdences of dssocaton of steam to and O durng steam gasfcaton at char surface were shown by Blacwood & McTaggart (959) and Tay et al. (9). The followng dssocatve adsorpton model (Long & Syes, 948, Blacwood & McTaggart, 959 and Laurendeau, 978) s adopted for non-catalytc gasfcaton of Vctoran brown coal char n ths wor. 3

4 nc O C Cf 3 C C O C C f C O nc C O C nc5 Cf O nc6 nc 7 CO Where C f denotes the actve ste of carbon n char and nc s the rate constant for each elementary reacton for non-catalytc gasfcaton. The followng adsorpton step of free radcal from volatles, especally radcal (Wu et al., ), s proposed to descrbe the nhbton of volatles. The partal pressure of radcal should be proportonal to that of volatles and also coal feedng rates ( VM Feedng Rate ). Reacton 6 descrbes the volatle-char nteractons. nc8 C C() (6) f Assumng steady-state concentratons of C(), C(O) and C(O), nc s gven by: nc O nc (7) where nc O n n nc4 nc t nc nc nc ( ) C n nc5 nc7 nc6 n nc5 () (3) (4) (5) nc8 nc4 and n t s the nc5 densty of the total actve stes of carbon on an ntal char bass (m -3 -ntal char). C s the carbon densty n ntal char. Fg. shows the results of steam gasfcaton of -form coal char. An approxmate amount of.5 g -form coal was fed nto the fludsed-bed/fxed-bed reactor for 7 mnutes (88 mg/mn). Coal feedng was fnshed at holdng tme n the fgure and char was gasfed wthout the volatle-char nteractons for each holdng tme. The char converson rato X can be converted to the char yeld Y on a daf bass by the correlaton: Y Y ( X ). Y s ntal char yeld durng prmary pyrolyss of coal. The value of Y was unable to be determned drectly from these experments but the sutable value could be estmated. Eq. 7 descrbed exactly the expermental results as the sold lne n Fg. when nt n t, ( X ) and Eq. 7 s therefore gven by: nc where ( nc, X ) (8) nc, nc, O nc O n nc4 nc nt, nc, and subscrpt, C denotes the ntal value. The value of nc n 5 % steam at 8 o C was determned from the analyss of these data. 4

5 5 Char Yeld, Y [wt% db ] oldng Tme, t-t F [mn] Fg. : Char yelds form the gasfcaton of -form coal char n 5 % steam at 8 o C as a functon of holdng tme. The feedng tme t F was 7 mn. (Dots: expermental results. Lne: netcs model.) Catalytc Gasfcaton The dssocatve adsorpton model of steam to and O s also proposed for catalytc gasfcaton of Vctoran brown coal char, the same as for non-catalytc gasfcaton. c O M M M O 3 C M O M C c C O M c5 M O c c6 7 CO where M denotes the catalyst whch s AAEM speces, especally Na. And c s the rate constant for each elementary reacton. The followng adsorpton step of radcal from volatles s proposed to descrbe the nhbton of volatles. Two mechansms of the loss of the catalytc actvty are also proposed. One s the volatlsaton due to the volatlechar nteractons and the other s the evaporaton unrelated wth the volatle-char nteractons. c M 8 M() (3) M() c9 C - M gas (9) () () () (4) 5

6 M c M gas (5) Although the behavours of AAEM speces are complcated, Reactons 3 and 4 should descrbe the major reactons of volatle-char nteractons, and the successve reacton of Reactons 3 and 4 s the same mechansm of the exchange of Na for radcal whch s descrbed elsewhere by Sathe et al. (999) and L et al. (): CM - Na CM - Na Assumng steady-state concentratons of M(), M(O) and C(O), c and the overall rate constant for the loss of the catalytc actvty loss are gven by: c c O (6) c O c4 loss n tm loss loss c O c4 (7) tm dn dt c ( O c4 ) where n c tm c c c( ) C nt c5 c9 c6 c5 c9 c8 c4 c5 c9 c ntm c9ntm c loss loss c and n tm s the densty of the total ntm ntm c5 c9 actve stes of catalyst on an ntal char bass (m -3 -ntal char). Eqs. 6 and 7 suggest that loss should be proportonal to c when gas composton s steady. Ths correlaton descrbed by Eq. 8 agrees qualtatvely wth the expermental results of Bayarsahan et al. (5 and 6). loss c loss loss ( c O c4 ) (8) c O When t can be assumed that c s constant value durng char gasfcaton, loss, and c, should also be constant. Therefore Eqs. 7 and 6 become: tm tm, loss, n n e t (9) loss, t c c,e () where c, c, O c O c4 n c tm c, C loss, loss, loss, ( c O c4 ) loss, c, loss, c9 c O c4 6

7 4 Char Yeld, Y [wt% db ] oldng Tme, t-t F [mn] Fg. 3: Char yelds form the gasfcaton of raw coal char n 5 % steam at 8 o C as a functon of holdng tme. The feedng tme t F was 7.5 mn. (Dots: expermental results. Lne: netcs model.).6 Normalsed Concentraton of Na n Char [wt%] Char Yeld, Y [wt% db ] Fg. 4: Normalsed Na concentratons (Na Y / Y ) n char form the gasfcaton of raw coal char n 5 % steam at 8 o C as a functon of char yelds. (Dots: expermental results. Lne: netcs model.) Fgs. 3 and 4 show the results of steam gasfcaton of raw coal char. An approxmate amount of.5 g raw coal was fed nto the fludsed-bed/fxed-bed reactor for 7.5 mnutes (86 mg/mn). Coal feedng was fnshed at holdng tme n Fg. 3 and char was gasfed wthout the volatle-char nteractons for each holdng tme. Assumng n tm s proportonal to Na concentraton n char, analysed concentraton Na was normalsed as Na Y / Y on an ntal char bass to compare wth the char yeld. Eqs., 7, 6 and 7 7

8 were able to descrbe the expermental results of char yelds and Na concentratons as the sold lnes n Fgs. 3 and 4. The values of c and loss wthout the volatle-char nteractons n 5 % steam at 8 o C were determned from the analyss of these data. The char gasfcaton rate, however, seems to have dropped at the char yeld of 8 % db n Fg. 3.There are two possble reasons. One reason could be the change of the char structure durng steam gasfcaton (Guo et al., 8 and Tay et al., 9). The other reason could be the low reactvty of coe formed by the thermal cracng from volatles. Volatle-Char Interactons The volatle-char nteractons would nhbt the adsorpton of steam and enhance the volatlsaton of Na to decrease the overall char gasfcaton rate. The expermental results have been reported n the prevous paper (Shu et al., 7). Raw coal was contnuously fed nto the fludsed-bed/fxed-bed reactor at a coal feedng rate of, 5 or 5 mg/mn for each feedng tme wth no holdng tme n 5 % steam at 8 o C. Fg. 5 shows that the observed char yelds ncreased wth ncreasng the coal feedng rate at the same feedng tme. Na concentratons n char were also analysed and normalsed concentratons are shown n Fg. 6. Na concentratons decreased wth feedng tme and char yelds. The proposed netcs model can also be used to smulate these contnuous feedng experments. Char has a dstrbuton of resdence tme because of contnuous feedng. The average char converson rato X s gven by: X t t Xdt Y Y ( X ) () The normalsed concentraton of catalyst n char M c ( ntm ) s also gven wth Eq. (9) by: M c loss,t loss, () t e M cdt M c, t (3) t When sutable values for nc4, c4 and loss are estmated, the correlatons between feedng tme, char yelds and Na concentratons almost agreed wth the expermental results as the sold lnes n Fgs. 5 and 6. If there s no volatle-char nteracton durng contnuous feedng ( = ), char yelds and Na concentratons would be on the broen lnes wth ths model, regardless of the feedng rate. Then t s obvous that the dfferences between the feedng rates depend on the volatle-char nteractons whch are manly caused by free radcals formed by the thermal cracng and/or reformng of volatles. It can be sad that the volatle-char nteractons were descrbed quanttatvely n the proposed netcs model. Shu et al., (7) suggested the char structure changed after the feedng tme of 5mn. The char structure should be also ncluded n the netcs model n the future. 8

9 4 Char Yeld, Y [wt% db ] 3 mg/mn 5 mg/mn 5 mg/mn wthout volatle-char nteractons Feedng Tme, t [mn] Fg. 5: Char yelds form the gasfcaton of raw coal wth contnuous feedng n 5 % steam at 8 o C as a functon of feedng tme. (Dots: expermental results by Shu et al., 7. Lnes: netcs model.).6 Normalsed Concentraton of Na n Char [wt%]..8.4 wthout volatle-char nteractons mg/mn 5 mg/mn 5 mg/mn 4 3 Char Yeld, Y [wt% db ] Fg. 6: Normalsed Na concentratons (Na Y / Y ) n char form the gasfcaton of raw coal wth contnuous feedng n 5 % steam at 8 o C as a functon of char yelds. (Dots: expermental results. Lnes: netcs model.) 9

10 CONCLUSION The netcs model of the catalytc gasfcaton, the non-catalytc gasfcaton and the volatle-char nteractons were dscussed to descrbe quanttatvely the nhbton of volatles about char gasfcaton. The dssocatve adsorpton model of steam to and O was adopted for steam gasfcaton. The adsorpton of radcals from volatles and the exchange of Na for radcals were proposed for the nhbton of volatles. The nherent Na also evaporates unrelated wth the volatle-char nteractons. The L- type reacton rate equatons for steam gasfcaton of brown coal and the volatlsaton of catalyst were newly derved. Raw coal char and -form coal char of Vctoran brown were gasfed wth steam at 8 C usng a novel fludsed-bed/fxed-bed reactor, and netc parameters for the catalytc gasfcaton, the non-catalytc gasfcaton and the evaporaton of Na was determned. The gasfcaton experments of raw coal wth contnuous feedng, that s, wth the volatle-char nteractons, were smulated wth the proposed netcs model and the netcs parameters for the volatle-char nteractons were estmated. It can be sad that the netcs model s vald and the volatle-char nteractons was descrbed quanttatvely n the proposed netcs. Ths model would be useful to estmate the performance of the gasfer for Vctoran brown coal. REFERENCES Bayarsahan, B, ayash, J, Shmada, T, Sathe, C, L, C-Z, Tsutsum, A & Chba, T 5, netcs of steam gasfcaton of nascent char from rapd pyrolyss of a Vctoran brown coal, Fuel, vol. 84, pp Bayarsahan, B, Sonoyama, N, osoa, S, Shmada, T, ayash, J, L, C-Z & Chba, T 6, Inhbton of steam gasfcaton of char by volatles n a fludzed bed under contnuous feedng of a brown coal, Fuel, vol. 85, pp Blacwood, J D & McTaggart, F 959, Reactons of carbon wth atomc gases, Aust. J. Chem., Vol., pp Guo, X, Tay, -L, Zhang, S & L, C-Z 8, Changes n Char Structure durng the Gasfcaton of a Vctoran Brown Coal n Steam and Oxygen at 8 C, Energy Fuels, vol., pp Laurendeau, N 978, eterogeneous netcs of coal char gasfcaton and combuston, rog. Energy Combust. Sc., vol. 4, pp. -7. L C-Z, Sathe, C, ershaw, J R & ang, Y, Fates and roles of alal and alalne earth metals durng the pyrolyss of a Vctoran brown coal, Fuel, vol. 79, pp Long, F J, Syes, W 948, The Mechansm of the Steam-Carbon Reacton, roc. R. Soc., vol. A93, pp Ouno, T, Sonoyama, N, ayash, J, L C-Z, Sathe, C & Chba, T 5, rmary Release of Alal and Alalne Earth Metallc Speces durng the yrolyss of ulverzed Bomass, Energy Fuels, vol. 9, pp

11 Quyn, D M, Wu, & L, C-Z, Volatlsaton and catalytc effects of alal and alalne earth metallc speces durng the pyrolyss and gasfcaton of Vctoran brown coal. art I. Volatlsaton of Na and Cl from a set of NaCl-loaded samples, Fuel, vol. 8, pp Sathe, C, ang, Y & L, C-Z 999, Effects of eatng Rate and Ion-Exchangeable Catons on the yrolyss Yelds from a Vctoran Brown Coal, Energy Fuels, vol. 3, pp Tay, -L, ajtan, S & L, C-Z 9, Changes n Vctoran brown coal char reactvty durng the gasfcaton n a mxture of O, CO and steam, roceedngs of The Thrd Internatonal Symposum on Novel Carbon Resource Scences: Advanced Materals, rocesses and Systems toward CO Mtgaton, Fuuoa, Japan, -3 November Zhang, S, Tan, F-J, ayash, J, Camps, A & L, C-Z 7, Further nvestgaton of the effects of volatle-char nteractons on char reactvty durng steam gasfcaton, roceedngs of Internatonal Conference on Coal Scence & Technology 7, aper No. 9C4, Nottngham, U, 8-3 August. Wu,, Quyn, D M & L, C-Z, Volatlsaton and catalytc effects of alal and alalne earth metallc speces durng the pyrolyss and gasfcaton of Vctoran brown coal. art III. The mportance of the nteractons between volatle and char at hgh temperature, Fuel, vol. 8, pp BRIEF BIOGRAY OF RESENTER Dr. Shro ajtan s a research scentst n CRIEI n Japan. e receved hs doctor degree on Engneerng from yoto Unversty n Japan n 7. e was a senor vstng research scentst n Curtn Unversty and ths wor was conducted n Curtn Unversty.