I
C. W. ZIELKE and EVERETT GORIN Research and Development Division, Pittsburgh Consolidation Coal Co., Library, Pa.
Kinetics of Carbon Gasification An empirical correlation for predicting differential rates of carbon gasification and methane formation can b e used for
1 Interpreting pilot runs 1 Extrapolating pilot design to E x F E R m m T . 4 L L v o m oil kinetics o f interaction between hvdrogen-steam mistures and low temperature char has been extended to temperatures of 1.500" arid 1700° F. These data, and thosc previous]!. reported for hydrogen-steam and ) for p u i ' t ' mixtures a t 1600" F. ( ~ 4 hydrogen at 150@', 16@Oo,and l'OOo F. (72)>have been integrated into a singlr empirical correlation for predictinq total differential rates of carbon gasification and inethane formation over the range of conditions studied. This information should be useful in interprrting and extrapolating [o commercial design. pilot plant data on char gasification h-ew data are also prrsented on surface area and reactivity of char as affected b!, treatment ivith steam and hydrogen. These and other data are of interest in relation to the mechanism of xnethanr formation i n the hydrogen-stram system.
Brunauer-Emmett-TelIer (l3E.I') irwthod ( 2 ) using low temperature nitrogen adsorption. Experimental Results Method of Processing Data. D r y rshaiist-gas rates and gas cornpositions
commercial scale
obtainrd as a function of gasifvinc Lime at rach set \.ariabIrs for, iiiict gas composition arid total tiiwsiirr. 'l'hese ed as previously (,;, - I ) t o g i i r iiitrgral qasificatii)~~riitcs h r three ditfvwnt initial bed Lvrights. '1-hrsr f a i r s i i - r r c liet ti n r r a p o l a r c d t o ivere
Range of Conditions Studied
10 10 25
0 -50 0-50 0-100
30 I- 30 1- 30
1500 1600 1700
90 to 100 90 t o 100 75 t o 100
1200
Experimental Technique T h e same 1.5-inch diameter L;niIoy reactor (3) was used \vith two slight modifications. T h e probe holding the micrometallic gas-distribution grid \vas lengthened from 12 to 19.5 inches to provide more preheat to the gas. r i l s o , a conical stainless steel screen was inserted into the top of the reactor; this acted as a filter and eliminated small elutriation loss of fines from the bed. T h e feed, 65 X 100 Tyler mesh Disco char previously used, was pretreated for 1 hour by fluidizing in nitrogen a t the temperature subsequently investigated. Thus, pretreatment temperature varied with that studied, Analytical methods for solids and gases were the same ( 3 ) ; surface area measurements were made on gasification residues by the standard
396
0
io
20
PER
30 CENT
40
50
60
70
eo
CARBON QASlFlED
Figure 1 . Surface areas of char residues vs. per cent carbon gasified at 1700" F. and 30 atrn.
INDUSTRIAL AND ENGINEERING CHEMISTRY
INDUSTRIAL GASES
*
zero bed weight (3) to obtain differential rate. Thus, the rate was obtained by extrapolating the integral rate curves to this common convergence point. A modified procedure was employed to obtain differential rate data at 100% carbon burn off. Theoretically, the integral rate curves should converge to a common point a t l O O ~ carbon , burn off and this value represents the differential rate. Thus, the rate was obtained by extrapolating the integral rate curves to this common convergence point. Graphical and Tabular Results. Only one pressure (30 atm.) level was investigated at 1500 F. because the rate at lower pressure levels was too low for commercial interest (Table I). On the other hand, data at 1700' F. are more comprehensive than those previously obtained at 1600' F.; carbon burn offs obtained over the full range of 0 to 100% instead of 0 to So%, were possible because of the higher gasification rate at 1700' F. The range of gas compositions studied was shifted toward higher hydrogensteam ratios at 1700' F. because steep-
PER CENT CARBON GASlFlEO
Figure 2. ReactiviG of steam-activated char compared with standard char. Direct hydrogenation conditions, 1700" F. at 29.9 atm.
Table 1. Rates for Differential Total Gasification (R,,,) Exact Operating Nominal Conditions Conditions , Gas Total Total Compn. % pressure, pressure, Hz Hz0 atrn. 0 10 20 30 atm. Hz/HzO Rtot.l,
10 25 50
90 75 50
30 30 30
0.10 0.33 0.96
29.9 29.9 29.9
25
75
50 50" 50
50 50. 50
1 6 30 1
67
33
0.32 0.33 0.32 1.10 1.00 0.99 1.02 2.00 1.95
1.00 6.07 29.9 1.00 1.00 6.07 29.9 1 .oo 29.9
and Methane Formation (RcH)
yo C Gasified 40
60
60
Lb. Atoms C Gasifled/Min./Lb. Atom C X IO4 Temp., 1500° F.
66 28.7
55 40.0 19.1
145 230 350 42 47 94 176 23.4 123
145 226 338 42 46 92 171 23.1 122
50.0
39 28.0 12.9
32 22.5 9.4
20
70
80
90
100
... ... ...
...
...
...
28 19.0 7.8
23 17.5 7.0
. . ... ..
144 210 294 40 44 82 146 21.8 108
143 202 276 39 43 78 136 21.2 101
141 194 255 38 42 73 124 20.7 92
138 180 230 37 40 68 110 20.1 82
134 165 202 35 38 62 96 19.4 72
127 147 174 33 36 55 82 18.8 61
119 130 145 31 34 47 68 18.0 48
9.8
... ...
...
...
... ...
... ...
... ...
14.6 36 82 8.4 8.8 24.1 65 6.3 59
14.9 35 73 8.3 8.7 23.0 59 6.3 53
15.0 33 62 8.3 8.7 21.5 52 6.4 46
15.0 31 51 8.3 8.7 20.0 44 6.4 39
... ...
... ...
... ...
Temp., 1700' F.
1
6 30 1 30
145 221 324 41 46 89 164 22.8 119
145 216 310 41 45 86 157 22.3 114
RcH,,Lb. Moles CH4 Formed/Min./Lb, Atom C X 104 Temp., 1500° F. 10 25 50
90 75 50
30 30 30
0.10 0.33 0.96
29.9 29.9 29.9
18.0 18.2 20.0
13.5 15.4 12.3
25
75
I
0.32 0.33 0.32 1.10 1.00 0.99 1.02 2.00 1.95
1.00 6.07 29.9 1.00 1.00 6.07 29.9 1 .oo 29.9
2.7 17 91 2.3 2.3 15.0 a6 2.0 75
5.3 25 93 4.5 4.6 20.7
11.4 12.8 8.5
10.5 10.8 6.9
10.2 9.2 5.9
10.0 8.1 5.5
... ...
11.8 36 94 7.3 7.6 25.5
13.1 37 93 7.9 8.3 25.4 77 6.1 69
14.0 37 89 8.3 8.7 24.9 72 6.2 65
...
...
Temp., 1700' F. 6 30
50 SOa 50
50 50' 50
67 67
33 33
1
1 6 30 1 30
a6
4.2 76
7.6 30 94 5.6 5.7 23.5 85 5.2 76
9.8 34 95 6.5 6.7 24.8 83 5.7 75
ai
5.9 73
To carry out the correlation, experimental rates with H2/H1O = 1.10 are adjusted t o Ha/HzO = 1.00 with a semilog plot a Not experimental. of rate os. Hg/HaO with carbon burn off as parameter.
VOL. 49, NO:3
e
MARCH 1957
397
ness of the extrapolation, ivhen less than 25% of hydrogen isas employed, severely restricted accurac)- Lvith ivhich differential races could be obtained. Extrapolation was rather difficult. even in the 25:; hydrogen in steam series at 1 ami. This difficulty \\'as solved by obtaining integral rate data at smaller bed iveights than rvere usuall!, empIo!-ed. BET surface area measui'rnients ~vere made of char rcsidues obtained by gasification at 30 atm. and 1-00' F. with inlet gas compositions of 1005%,h>-drogen and 25% hydrogen in steam (Figure 1 and Table 11). The integral rates of hydrogenation for char pretreated in the standard \vay arid that gasified to 20yc carbon burn off using the same hydrogensteam mixture, are compared in Figure
2.
Discussion of Results Qualitative Properties of Rate Data at 1500' and 1700' F. The qualitative \,ariation for differential rates of total carbon gasification ( Rtor2*l)and of methane formation (RcH,)Mith the variables of the system is substantiall>. the same as previously reported a t 1600" F. (.I, 5). Rbut0, generally decreases as carbon burn off increases, bur rhe higher the temperature, the lower the rate of decrease; Rt,,,;,l is higher the loiver the hydrogen concentration and the higher the pressure and trrnperature. Behavior of ivith variation in carbon hurn off is more complicated. T h e rate increases ivith burn off at low pressures but this effect disappears as pressure increases; rates at 30 atm. generally decrease slightly ivith burn off. T h e higher the pressure and the loiver the hydrogen concentration, the higher the methane rates. Empirical Correlation of Differential Rate Data. Rates at 1:OO' F'. can be fitted ivith the same type of ernpirical expressions previously used to fit data for hydrogen-steam (5)at 1600' F'. and pure hydrogen (72) at 15OO", 1600", and 1700' F. That is. both R,,t,l and ReR4 a t 1700" F. may be correlated as a function of pressure T a t constant burn off and gas composition b>-thc equations,
JI
.
1001
P
&
I
1
1
I
2
l
3
TOTAL PRESSURE
4
6
(n),
1
0
IO
20
30
ATMOSPHERES
Figure 3. Correlation of differential total gasification rates for o mixture of hydrogen in steam at 1700' F.; porameter, per cent of carbon gasified
gasification rate \- no parr in conrrolling the reaction rate. T h e role of diffusion can be estimated by the method of \$-heeler ( I 7 ) . Since pore size distribution was not measured. it \.vas necessary to base the computations on the measured average pore radius given in Table 11. Csing this surface area data, we find that for 2 drogen in steam at 1700' F.. 30 atm.; and 23c;C carbon burn off, Kt,,,,l-HcIi4 converted to a reaction rate constant h on a unit area basis equals '.32 X 10-" mole per sq. cm. per second. Using LVheeler's formula. L = OV'TX where a is average particle diameter. average pore length, L. is calculated to cm. Under these conbe 3.5 X ditions, the proper diffusion constants. DR2and DHI,, correspond to those calculated for Knudsen flow because the mean free paths of hydrogen and steam are approximately 10 and 5 times larger, respectively, than the pore radius. Then assuming that the reaction rate is constant a t all points on the pore \call. the
400
INDUSTRIAL AND ENGINEERINGCHEMISTRY
14
12
10
8
0 6
4
2
PER
Figure 8.
Constant
CENT
HYDROGEN
D in R L H = ~ Da'"vs. per cent hydrogen at 1700- F.
zI \
100 $ 0
80
W Z
0
o
5
60
g5 $
40
m
30
15
83
8.5
8.7
89
91
10' T, 'K.
K. for the data on the 50% hydrogen in steam Figure 9. R t c , t : ~vs. ~ 1/ T mixture at 30 atm.; parameter, per cent carbon gasified
INDUSTRIAL OASES Table
II.
Pore Volume and Average Pore Size of Char (Residue from 25% H~-75% HzO series at 30 atm. at 1700' F.) Surface %C Area, Pore vel.", Av. Pore Gasified Sq. M./G. CC./G. Radiusb, A. 23.1 71.7
362 601
0.232 0.428
12.8 14.3
a From nitrogen adsorption isotherms at a relative pressure of 1. Calculated from total pore volume and BET surface area.
hydrogen within the pore relative to the main gas stream. I t is clear then that diffusion likewise is not rate-controlling a t least under the assumption of uniform pore size. I t is possible, of course, that if true pore-size distribution were taken into account, diffusion resistance into the smaller pores would be of sufficient magnitude to explain the variation of activation energy with temperature. No theoretical significance can be attached to the observed activation energy. It is complicated by a number of factors, including change in thermal treatment of char with temperature. Activation energies are useful, however, for practical correlation purposes. More recently, Walker and others (70) have measured the pore size distribution developed during gasification of carbon rods. They found that a high percentage of surface area is attributable to pores having a radius of 350 A. or greater. At first, it appears that the break in activation energy that Walker (8) observed could also not be due to relatively slow internal diffusion. The reaction rate he observed a t the break point-at about 1100' C.-is greater on a unit surface-area basis than for char gasification a t 1700' F. previously discussed. The corresponding figures are 6 X and 7 X 10-14, respectively. T h e specific rate is thus larger in Walker's case and his diffusion path is also considerably greater. Therefore, it is possible that the break in the temperature coefficient in his case is caused by the onset of a diffusion-controlled reaction. Mechanism of Reaction. If the partial pressures of hydrogen and steam are held constant, reaction orders at 1700' F. of RCot,l - RCH4and R,, for steam and hydrogen, respectively, can be calculated on the basis of the correlation. Previously this was done at 1600° F. The order of Rto$d- Rep, for steam was 1.5 and strongly negative with respect to hydrogen. T h e order of RCH, was 0.3 to 0.6 for steam and about zero order for hydrogen. These orders are similar a t 1700' F. with the exception of RCHI for hydrogen where it becomes strongly
8.3
8.1
8.7
8.9
9.1
I0'
m. ~ 1 / T ' K. for data on the mixture of 25% hydrogen in steam Figure 10. R C H vs. at 30 atm.; parameter, per cent carbon gasified positive a t 1700' F. and constant steam partial pressure as the pressure of hydrogen is increased. The mechanism of methane formation at 1700' F. follows a pattern similar to that observed a t 1600' F. (5). For example, the differential formation rate is higher in the presence of steam than a t the same partial pressure of hydrogen in the pure hydrogen runs; RCH, has a positive order with respect to steam of about 0.3 to 0.6 a t constant hydrogen pressure. Also, the ratio, RCa/Rtatai RCH, has a value close to zero when extrapolated to a hydrogen-steam ratio
-
of zero (pure steam)-this indicates that methane is not produced by a direct reaction between carbon and steam but, as was stated previously, that the presence of hydrogen is required. T h e accelerating effect of steam on the methane formation rate is illustrated in Table 111. For the activation effect, shown in Figure 2, char was gasified a t 1700' F. to 20% burn off with a 25% hydrogen in steam mixture a t 30 atm.; the resultant char was then gasified in 30 atm. of hydrogen. The value of RCH4 for direct hydrogenation of steam activated char rose to a maximum
Figure 1 1. RtOhl vs. 1/ T ' K. for data on the 25% hydrogen in steam mixture at 30 atm.; parameter, per cent carbon gasified VOL. 49, NO. 3
MARCH 1957
401
70
z 0
6o
z w
30
Figure 12. Apparent activation energy for Rc~i4 at 30 atrn. as a function of per cent carbon gasified
of about twice the value of K,,l in a routine 30-arm. r u n a t 1700' F.. using uure hydrogen as the inlet gas. Acceleration of the methane rate a t high steam concentrations relative to the rate in pure hydrogen is too high to be explained by steam activation of the surface. O n the other hand. a t high hydrogen concentrations, methane is probably formed largely by direct hydrogenation accelerated by steam activation of the surface. An attempt was made to obtain further insight into the mechanism of methane formation. Integral methane formation rates were determined in the 50% hydrogen in steam series a t 1700' F~ and 6 atm. Lvith the addition of 0. 0.83; and 2,71%, methane to the inlet gas (Figure 15). T h e integral methane rate decreased markedl!, on adding
methane. No quantitative measure of the methane inhibition is possible. since we are dealing \vith integral rather than differential rates. T h e inhibition is less than rhat predicted by retardation due to approach to mrthane equilibrium because the equilibrium concentration was actually esceeded when 2,7i5i, of methane \vas added to the inlet gas. LIethane equilibrium \vas not reached in the other two runs. Surface Area M e a s u r e m e n t s . Initially%it \vas thought thar the activation effect of steam \vas caused by its uniqite propert)- in developing a high surfacc area. Therefore, mcasurcmcnts of surface area as a function of burn off were made o n char residues from gasification both by pure hydrogen and by a 257; hydrogen in steam mixture a t 30 atm. and 1700" F. (Figure 1 ) . Because of the
w
LT W
a -1
U
0
Y
z
0 Ik 0
U
LL
0
PER CENT CARBON GASIFIED Figure 13. Apparent activation energy for Rtotaland Rt,,l - R C H as ~ a function of per cent carbon gasified; inlet gas, 50% hydrogen in steam a t 30 atm.
402
INDUSTRIAL AND ENGINEERING CHEMISTRY
fineness of pores; (Table 11) surface areas obtained by the BET inulrilaycr adsorption method using IOW temperature nitrogen adsorption (Figure 1 ) are subject to sonic uncertainty. Ho\vever, relative to each other, the plots should be approsimately correct so as not to invalidate the discussion which folio\\ s. Substantially thc same surfacc area is developed. Lvhether hydrogcnstearn mixtures or p u r e hydrogen is used. steam lherelbt-r, the dctivatinn effrct cannot be explained on the basis of surface area nieasuinnents alone. There is a n aiiotnal>. tiet\veen thr observed rapid increase of surface ut's with burn off and the drcrrasr in reactivity o f char. T h e main discrc,panc!is in the low burn off region from 0 to 20:;; \vliere the multiplication of surfaw is great. The original thermally pretreated char has a surface area o f less than 10 s q . meters 1x1,cyani. 'I'lie high sui.face arra developed by hydrogen or steaIn must rncan, ho\\cvcr. [list the reaction of carbon ivith hydrogt.n and steam cannot initially be confincd to rhc BET surfart:. T h e individual crysrallitcs I I I I I S ~ have sufficient imperfections in the \vay of fissures. cracks. and distcirttd lattice strurture t o permit prnetration b!- a form of activated difliision. The ditT'~isitigsliecies niay nor lie niolrcular steam (11- hydroger) but mobile sirrracc coniplcxes of carbon arid oxyqrn hydroqrn at(.inis. . i t a n y ratr. thesc imperfections are too small ro prrniit adsorption uf nitrogen at liquid nitroqen teinpraturt's and, thrrcfore. d o not SIIOM. u p in thr convc3ntional surface arra mcasurements. T h e cracks ai'e uiidoubtcdly trljt!ncd sufficiently ac rclarivcly I o ~ v tiurn on's 10 shoiv u p i n the B E T tnrasurctiieiits. 'Therefore. it is possiblc tliat aftri a brirn off of 1 0 to ZOcL, the principal arra o f attack is through the surface evidenced by BET ~ n r a s u r r ~ n e n ~Ts h. e reason for the decrease in reaction rate tieyond tliis point. in spite (if the incrrase i n surfact,. is clear xvhrn Tve recall ho\v rhc activation energ!- changes ivith b u r n oil'. Figure 12 slio\vs l i o w thc. activation 'ases w i t h burn dl fur Ri 11, hydrogen in strain seriths a t similar c u r w is ohtaincd lor se curves arc rcmarkat)ly similar to that giving surface a r r a LLS a function of bur11 off in Iigiire 1 for t h e same series. .i fcw siiiiple calculations \vi11 show thac t h c incrcasc in activation energy is inore tlian sufficient to explain the decrease in rcaction rate iii the b u r n off range above 10 to 205;. Application of Correlation of Rates. T h e correlations given previously arc strictly empirical and can be applied only within the range studied cxperimentally. T h e Arrhenius equation can be applied for interpolating temperature with 50% only for RtOt,l and hydrogen in steam and fnr l?,,,,ivith
I N D U S T R I A L GASES 25% hydrogen in steam both series at 30 atm. These are the only conditions where both data are available and the Arrhenius equation fits. Activation energies given in Figures 12, 13, and 14 may be used. I n other cases, it should be sufficiently precise to use a linear interpolation between the points of either the 1500' and 1600' F. or the 1600' and 1700' F . temperature ranges on a semilog plot of the rates versus
1/T.
0
IO
20 30 40 50 PER CENT CARBON GASIFIED
Figure 14. Average apparent activation energy for in steam series vs. per cent carbon gasified
Rtotsl
60
70
for the 25% hydrogen
Similarly, the correlation does not apply for interpolation between the 10 and 100% hydrogen points a t 1500' F. and 30 atm. However, a graphical interpolation procedure would be adequate. Caution is needed in applying the correlation in a practical situation. At methane equilibrium which is rapidly approached in a pressure gasification system, the appropriate rate to use is RCH,. To take into account, Rtotal in general, the retardation caused by this approach, the relationship
-
Table 111.
Comparison of Methane Rates in Pure Hydrogen and in HydrogenSteam at Same Partial Pressure of Hydrogen [T.,1700°.F.; burnoff, 30%] Series Compn., % Methane Rate" Hz Hz0 P H ~ oAtm. , PH,,Atm. Ha/HaO Pure Hab 0.25 9.8 0.076 25 75 0.75 4.5 1.5 34 0.99 10.0 22.5 7.5 95 0.50 6.7 0.20 50 50 0.50 3.0 24.8 2.67 3.0 15.0 15.0 83 24.6 0.67 5.7 0.31 67 33 0.33 20.0 75 37 67 33 10.0 Lb. atoms C/min./lb. atom C X 10'. Calculated from correlation for rate of direct hydrogenation (12).
Total carbon gasification rate = RCH,
should be used, where XI and K Zare the equilibrium constants' for the carbonhydrogen and carbon-steam reactions, respectively. However, this equation will give a conservative value for the total gasification rate, since, as shown here, methane equilibrium for the reaction, C 2Hz = CH?, can be exceeded.
+
1
I
40
20
0
PER CENT
H20
(1) Binford, B. S., Eyrin H., J . Phys Chem. 60, 486 (19565; (2) Brunauer, S., Emmett, P. H., Teller, E., J.Am. Chem. Soc. 60, 309 (1938). (3) Goring, G. E., Curran, G. P., Tarbox, R. P., Gorin, E., IND.ENG.CHEM. 44, 1051 (1952). (4) Ibid., p. 1057. (5) Goring, G. E., Curran, G. P., Zielke, C. W., Gorin, E., Ibid., 45, 2586 (1953). (6) Hougen; 0. A., Yang, K. H., Chem. Eng. Prog. 46, 147 (1950). (7) Hunt, E. B., Mori, S., Katz, S., Peck, R. E., IND.ENG. CHEM.45, 677 (1953). (8) Pilcher, J. M., Walker, P. L., Jr., Wright, C. C., Ibid., 47, 1742 ( 19c5). (9) Thiele, E. W., Ibid., 31, 916 (1939). (10) Walker, P. L., Rusinko, F., Raats, E., J. Phys. Chem. 59, 245 (1955). (11) Wheeler, A., "Advances in Catalysis," Coll. vol. 111,p. 249, Academic Press, New York, 1951. (12) Zielke, C. W., Gorin, E., IND.ENG. CHEM. 47, 820 (1955).
3.04 3.07 2.94
RECEIVED for review September 17, 1956 ACCEPTED January 7, 1957
60
CARBON GASIFIED
Figure 15. Effect of methane addition on integral methane formation rate; temperature, 1700' F.; initial bed weight, 0.012 Ib. atoms C CH4
0
0 A
0 0.051 0.18
Inlet Gas Partial Pressure Ht
3.02 3.02 3.04
literature Cited
VOL. 49, NO. 3
MARCH 1957,
403
