1454
Ind. Eng. Chem. Res. 1987,26, 1454-1458
Knox, J. H.; Gilbert, M. T. J . Chromatogr. 1979, 186, 405. Koizumi, M.; Usui, Y. Mol. Photochem. 1972, 4, 57. Lanni, F.; Ware, B. R. Photochem. Photobiol. 1981, 34, 279. Lindqvist, L. Ark. Kemi 1960, 16, 79. McGuffin, V. L.; Zare, R. N. Appl. Spectrosc. 1985, 39, 847. Sugarman, J. H.; Prud'homme, R. K.; Langhorst, M. A,; Stanley, F. W., Jr. J . Appl. Polym. Sci. 1987, 33, 693. Sugarman, J. H.; Prud'homme, R. K. J . Chromatogr. 1987,389,236. Tijssen, R.; Bleumer, J. P. A.; Van Kreveld, M. E. J . Chromatogr. 1983,260, 297.
Weimer, W. A.; Dovichi, N. J. Appl. Opt. 1985, 24, 2981. Yang, F. J. J. High Resolut. Chromatogr., Chromatogr. Commun. 1981, 4, 83. Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size Exclusion Liquid Chromatography. Practice of Gel Permeation and Gel Filtration Chromatography; Wiley: New York, 1979. Zarrin, F.; Dovichi, N. J. Anal. Chem. 1985, 57, 1826. Receiued for reuiew April 9, 1986 Accepted March 17, 1987
Structural Variations and a Deactivation Model for Gasification of Coal Suna Balci, Giilgen Dogu,+and Timur Dogu* Department of Chemical Engineering, Middle E a s t Technical Uniuersity, Ankara, Turkey
Variations in pore structure are shown to provide a very useful tool in the analysis of the kinetics of coal gasification with carbon dioxide. The porosity and the surface area of devolatilized coal samples d o not show a considerable change below about 900 "C;they decrease significantly as the reaction proceeds above this temperature. The porosity shows a linear increase with carbon conversion during gasification with COz. It is concluded from the pore structure data that the product of the number of pores and the effective pore length remains constant during gasification. The rate of decrease of the active area was shown to be proportional to the active area itself. The values of the deactivation parameter, p, obtained from the conversion-time data and also from the variations in pore structure show reasonably good agreement. The activation energy was found to be 29 800 cal/mol and transport limitations are not significant for this reaction. Proper understanding of coal gasification and successful design of a gasifier require detailed kinetic information. Gasification reactions are typical examples of noncatalytic gas-solid reactions. For such reactions, solid undergoes structural changes as the reaction proceeds. Variations in pore structure cause changes in the active surface area and also may increase or decrease the transport limitations. The initial pore structure of the solid reactant also has a very important effect on the reaction mechanism and the reactivity of the solid. Hashimoto and Silveston (1973) modeled the gasification reaction on a microscopic scale by considering the pore growth, initiation of new pores, and coalescence of adjoining pores. The effect of intraparticle diffusion was also taken into account. Wen and Wu (1976) developed a one-parameter volume reaction model by a simplified analytical approach for the carbon dioxide gasification. Another model is proposed by Srinivas and Amundsen (1980) for the gasification of a single char particle in the presence of steam, hydrogen, carbon dioxide, carbon monoxide, and methane. Johnson (1974) performed gasification experiments with a thermobalance. His proposed gasification model consists of three consecutive stages: devolatilization, rapid methane formation, and slow gasification. The proposed rate expression depends upon the surface area and carbon conversion. Jensen (1975), Chin et al. (1983), and Edward (1984) reported experimental results for steam gasification. Jensen (1975) assumed that an unreacted core model approximates the kinetics of the reaction. On the other hand, Chin et al. (1983) assumed uniform reaction of steam with carbon within the porous particles. Edward (1984) fitted his data into equations describing continuous and un*Present address: Faculty of Sciences, University of Ankara, Ankara, Turkey. +Presentaddress: Department of Chemical Engineering, Gazi University, Ankara, Turkey.
Table I. Proximate and Ultimate Analyses and Physical ProDerties of the Coal (Zonauldak Coal) ultimate analysis proximate analysis (ash-free basis) moisture, wt % volatile matter, wt % fixed C, wt % ash, wt 70 higher heating value, cal/mol porosity, e av pore radius, pm apparent density, g/cm3 av particle radius, cm
0.46 30.20 55.96 13.38 7100 0.02 0.43 1.42 0.26
C, wt % H, wt % N + 0, wt % S, wt 9'0
71.54 4.26 23.70 0.53
reacted core models. Dutta et al. (1977) investigated the reactivities in a thermogravimetric analyzer for carbon dioxide gasification. They observed that the dimensions of particles remain practically constant below about 80% conversion. Although there are numerous studies concerned with the modeling and kinetics of gasification reactions, in none of these studies have progressive changes in pore structure and its implications for the controlling mechanism been investigated. The rapid increase in porosity during devolatilization is expected to increase the active area and, consequently, the instantaneous reactivity of the solid. The variations in pore structure during gasification also cause changes in the active area. Information about the effect of temperature on the structure of the solid is also very important. The major objective of this work is to utilize pore structure data as a tool in the analysis of kinetics of gasification of coal with carbon dioxide. Experimental Work In the first stage of this study, devolatilization experiments were conducted at different temperatures, and the effect of temperature changes on the pore structure was investigated. In the second stage, gasification reaction with
0888-58851871 2626- l454$01.50/0 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1455
r
Figure I. Schematic diagram of the experimental setup.
C02 was investigated. The chemical analysis and the physical properties of the coal used are given in Table I. The coal samples are obtained from Zonguldak area. The coal was crushed and sieved. Particles near 0.26 cm in equivalent radius were used during the experiments. The system shown in Figure 1 was used for both devolatilization and gasification experiments. The reactor was made of quartz and was inserted into a tubular electrical furnace. The inner diameter of the reactor was 2.36 cm. Coal particles of about 0.3 g were placed in a platinummesh sample holder; then it was hung in the reactor, which was preheated to the desired temperature. Nitrogen gas flowed through the reactor at a velocity of 2.3 cm/s during devolatilization experiments. On the other hand, carbon dioxide flowed through the reactor at desired intervals at the same velocity (2.3 cm/s) during gasification runs. Devolatilization of volatile matter was completed in much less than a minute. Therefore, 1 min after the insertion of the sample into the reactor, the reactor was removed from the furnace and the devolatilized sample was taken out, after cooling in a nitrogen atmosphere. These experiments were conducted at temperatures between 750 and 1100 "C. Weight losses and pore structures of devolatilized coal samples were examined. During the gasification runs, experiments were conducted for different times of contact of carbon dioxide with the coal sample. After a certain period, carbon dioxide flow was stopped and nitrogen was introduced into the reactor. Meanwhile, the reactor was taken out of the furnace and cooled. With this procedure, samples with different degrees of conversion were obtained. The pore volume distributions of the samples were determined by an Aminco Digital-Readout 30 000-psi mercury-intrusion porosimeter.
Pore Structure of Devolatilized Coal The initial porosity of the coal samples was very small. However, devolatilization of coal was very fast and took place prior to other reactions in the gasifier. As a result of the evolution of volatile matter in early stages, the porosity of coal increased sharply. Therefore, it is natural that the pore structure of the devolatilized coal has an important effect on the gasification reactions which follow. Considering these factors, the pore structures of the samples devolatilized at different temperatures were investigated in the first stage of this work. Although the total weight loss during devolatilization was nearly constant in the temperature range between 750 and 1100 "C, the pore structure of the samples changed considerably. The pore size distributions of the coal samples devolatilized at different temperatures are shown in Figure 2. This figure shows that devolatilized coal has a monodispersed pore structure. The porosity of devolatilized samples is nearly constant in the temperature range from 750 to 900 "C (Figure 3). Its value is about 0.31. A t higher tempera-
Pore R a d i u s , micron
Figure 2. Pore size distributions of devolatilized coal a t different temperatures.
03
I
II
-
I
I
I
I
I
w
-
h
;r 0 2
E L
01
0
-
1 Devalottlizoiion Temperature
I
,'C
Figure 3. Variation of porosity with devolatilization temperature. Table 11. Physical Properties of Devolatilized Coal at Different Temperatures apparent av pore int surface devolatilization density radius, area from kg/m3 X porosity ium X lo6 ea 1. m2/ke temp, "C 750 1.055 0.315 2.06 290 800 1.051 0.323 2.39 260 850 1.019 0.316 2.38 265 900 1.063 0.296 2.34 236 1000 1.081 0.257 2.51 190 1050 1.093 0.223 3.31 124 1100 1.115 0.167 2.58 116 ~~
tures, a considerable decrease in porosity is observed, probably due to the sintering of the metalloorganic components in the ash. In parallel to the decrease of porosity, the internal surface area also shows a decrease at high temperatures. Physical properties of the devolatilized coal samples are given in Table 11. The internal surface area values reported in Table I1 were evaluated by using
This equation is derived assuming cylindrical pores of uniform size (Smith, 1970).
Gasification Experiments In the second stage of this study, gasification experiments with COPare conducted at different temperatures following the experimental procedure outlined previously. Gasification reactions provide a case of severe structural changes. Therefore, both total weight loss and pore size distribution of the samples were measured successively at definite time intervals until about complete conversion was achieved. However, as soon as the samples were placed in the reactor at desired conditions, devolatilization took place prior to gasification with high weight loss in the early
1456 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987
I
2
3
4 GoIIfIIPtiPn
5 T i m e , hr
6
7
8
Figure 4. Variation of carbon conversion with gasification time a t different temperatures. I
2 21
0
02
I 06 08 Carbon Conversion, X c 04
I 10
Figure 7. Variation of average pore radius with carbon conversion a t different temperatures. 061
- 0 2
Figure 5. Pore size distributions of coal a t different conversions at 800 "C.
0
I
02
I
I
04
06
08
IO
Carbon Conversion, X c
Figure 8. Variation of porosity with carbon conversion a t different temperatures.
0w rr.-7--r-
0 00 0
02
04 06 08 Corbon C o n v e r s i o n , X c
IO
Figure 9. Dependency of porosity to square of average pore radius ratio on carbon conversion. Pore Rodiur , micron
Figure 6. Pore size distributions of coal a t different conversions a t 800 O C (differential distribution).
-
stages of the reaction. The conversion of fixed carbon to carbon monoxide, C + COP 2C0, was evaluated from the weight loss data after the subtraction of the weight loss due to devolatilization. Experimental values of the fractional conversion of fixed carbon at different times are given in Figure 4 for gasification temperatures of 800,900, and 1000 "C. The pore size distributions of the samples a t different conversion levels of fixed carbon were measured. Cumulative and differential pore volume distribution curves obtained a t 800 "C are given in Figures 5 and 6. As the reaction proceeded, the porosity and the average pore radius increased as expected. The qualitative behavior of the pore size distributions did not change much. Coal particles showed monodispersed pore size distributions throughout the gasification until complete conversion. The variation of average pore radius with carbon conversion a t different temperatures is given in Figure 7. The increase in porosity with carbon conversion shows a linear
behavior (Figure 8). This result is consistent with the theoretical prediction of the equation proposed by Ulkutan et al. (1982). t = 60 + (1 - t o ) ( l - y ) X , (2) In this equation, eo corresponds to the initial porosity, which is the porosity of the devolatilized samples for this reaction. The major assumption of the equation is that the apparent volume of the particles does not change during reaction. Experimental results, showing that e vs.-X, data are linear, indicate that the apparent volume of the particles did not change during gasification for the coal samples used and a t the temperatures investigated. The value of the parameter, y,which is the ratio of molar volumes of solid product to solid reactant, (3)
is found from the slope of Figure 8 to be 0.7. As shown in Figure 9, the ratio of porosity to the square of the average pore radius remained essentially constant
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1457 10
Table 111. Deactivation Parameter Values from Reaction Rate Data and from Pore Structure B predictions gasification @ from kinetic from pore structure temp, O C data (eq 91, s-l (eq 17), 1000 3.1 x 10-4 3.3 x 10-4 900 1.6 x 10-4 1 . 7 ~10-4 800 2.9 X 4.4 x 10-5
8
6 5 4
7-
-
3
P m
-..
2
2
Y)
5 1
at different conversion levels and at different temperatures. The values obtained a t 1000 "C were slightly lower than the values obtained a t 800 and 900 "C. This ratio is proportional to the product of the number of pores per unit of pellet volume and the average pore length.
P
08
Q
06
c
-2 0 5
04
4"
03
O i l
6
This result indicates that the number of pores and the effective pore length, or a t least their product, remained constaht during the gasification. The rate of gasification is expected to be proportional to the active surface area of the sample. ra = ka~pSg,aC~~:! (5)
It is known that the reactivity of the coal samples changes with the progress of the reaction, basically due to the variations of the active area. It was shown by Dogu (1981) and Orbey et al. (1982) that the rate of change of the active surface area of the solid reactant should be proportional to the active area itself, for the reaction of SOz with calcined limestone.
I
7
I
8 (I/T)x104
I
9
,
I
10
O K - '
Figure 10. Temperature dependency of apparent rate constant and deactivation rate constant, (3.
@-vs.-l/Tcurve is expected to be twice the slope of the 12,-vs.-1/T curve. This is predicted by the equations derived by Do& (1981) and also observed for reactions with large diffusional resistance, such as the reaction of SOz with limestone. The effect of intraparticle diffusion resistance was also tested by using the criterion originally proposed by Weisz and Prater (1954) and recently generalized by Do& (1985). Under reaction conditions with negligible intraparticle heat-transfer resistance, Dogu's criterion reduces to where
This approach is similar to the uniform deactivation of catalyst pellets. Here, @ corresponds to a deactivation rate constant. Assuming that a similar approach can be applied t.o the gasification reaction, integration of eq 6 gives (7)
for negligible intraparticle diffusion and external masstransfer resistances. In this equation, the Biot number is defined as
By regression analysis of the conversion-time data, the initial values of the gasification rate constant and the deactivation parameter, 0, are evaluated a t different temperatures. In Figure 4 the solid lines correspond to the calculated conversions from eq 9, using the estimated parameters from the regression analysis. The variation of the initial apparent rate constant, k, = kgpoq,g,with inverse temperature is shown in Figure 10. $he activation energy of the gasification reaction is 29 800 cal/mol. This value is in the range of activation energies reported for this reaction (Wen and Wu, 1976). The values of @ obtained from this analysis are given in Table 111. The variation of 0 with inverse temperature is also given in Figure 10. It is interesting to see that the slope of the P-vs.-l/T curve is essentially the same as the slope of the k,-vs.-1/T curve. As is shown by Dogu (1981), this behavior corresponds to the case when diffusional resistance is negligible. For reactions under the control of diffusion, the slope of the
Large values of Bi correspond to small external masstransfer limitation, compared to the intraparticle diffusion resistance. By use of the pore structure data, the initial effective diffusion coefficient of COPis predicted to be on the order of magnitude of 0.07 cmz/s from the model of Wakao and Smith (1962). By use of the mass-transfer correlation reported by Wakao et al. (1958), the external mass-transfer coefficient and the Biot number were estimated. The value of Bi changed from 30 to 50 in the temperature range between 800 and 1000 "C. This shows that the relative significance of external mass-transfer resistance is much less than the intraparticle diffusion resistance. If the observed rates are evaluated from the slopes of carbon conversion-vs.-timecurves (Figure 4), the effect of diffusional resistances was tested by using eq 10. It is found that at temperatures of 900 and 1000 "C, some diffusion effect is expected (Balci, 1985) only in the initial stages of the reaction. Pores get larger as the conversion increases, which increases the effective diffusion coefficient. Also, the reaction rate decreases as conversion increases. As a result, diffusional limitations become negligible, even at high temperatures. This result is in agreement with the conclusions reached from the slopes of the @- and ha-vs.-1/T curves in Figure 10.
PpSg,a
= ~0pSi,ae-'~
The rate of gasification can then be expressed as
Integration of eq 8 gives
1458 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987
shown in Table 111, the values of predicted from the variations in pore structure are in reasonably good agreement with the p values calculated from the analysis of the conversion time data using eq 9.
I
2
3
4
5
6
7
Gorificatian T i m e , h i
Figure 11. Estimation of deactivation rate constant, simeter data.
0,from poro-
The effect of intraparticle temperature gradients on the rate of reaction is negligible if 4P*r* < 3/4 (13) where
p* =
-AHDeCco, XeTO
(14)
and
Nomenclature a = pore radius d = average pore radius Bi = Biot number defined by eq 12 Cco2 = bulk concentration of C 0 2 De = effective diffusion coefficient of C02 Ea = activation energy k , = mass-transfer coefficient k , = surface reaction rate constant 1 = effective pore length Mash = average molecular weight of ash M , = molecular weight of carbon Mdev = average molecular weight of devolatilized coal n = number of pores per pellet volume r, = observed rate R, = universal gas constant Ro = particle radius = total surface area per unit mass = active surface area per unit mass s': = initial active surface area per unit mass $ = external surface temperature w, = weight fraction of f i e d carbon in the devolatilized sample X,= fractional conversion of fixed carbon
3
Greek Symbols
(Anderson, 1963; Dogu and Do&, 1984). The heat of reaction is AH = -41 200 cal/mol. If the effective thermal conductivity is estimated as A, = 1.5 X cal/(cm O C s) (Weast, 1978) and if the initial rates are used, the value of the product, @P*y*, is found to change from 0.1 to 0.5 in the temperature range from 800 to lo00 "C. This result shows that neglecting the intraparticle temperature gradients is a reasonable assumption.
Prediction of p from Pore Size Distributions The rearrangement of eq 7 gives (16) Here ppSg,ais the active surface area at any conversion and is expected to change with the conversion level. A reasonable approach is to assume that the ratio of the active surface area to the total surface area a t a specific conversion level is proportional to (1- X c ) . With this assumption and if eq 1is combined with eq 16, p is expressed as
The parameter, p, can also be written as
by combining eq 17 with eq 2. The variation of In [ [(tdo)/(q@)](l- X,)] with respect to time is given in Figure 11at different temperatures. As shown in this figure, these curves show a reasonably linear behavior, as predicted by eq 17. The values of the parameter, p, are estimated from the slopes of these curves and reported in Table 111. As
p = deactivation rate constant (eq 6) p* = dimensionless group defined by eq 14 y = dimensionless parameter defined by eq 3 y* = dimensionless group defined by eq 15 pa& = absolute density of ash Pdev = absolute density of devolatilized coal p = apparent density of the particle pp B = initial value of the apparent density (for the devolatilized coal) e = porosity eo = initial porosity (for the devolatilized coal) C$= dimensionless parameter defined by eq 11 LW = heat of reaction he = effective thermal conductivity Literature Cited Anderson, J. B. Chem. Eng. Sci. 1963,18,147-148. Balci, S.M.S. Thesis, M.E.T.U., Ankara, 1985. Chin, G. S.; Kimura, S.; Tone, S.; Otake, T. Int. Chem. Eng. 1983, 23,105-120. D o h , T. Chem. Eng. J . 1981,21,213-221. Do&, T. Can. J . Chem. Eng. 1985,63,37-42. D o ~ u ,T.; DO& G. AIChE J . 1984,30, 1002-1004. Dutta, S.;Wen, C. Y.; Belt, R. 3. Ind. Eng. Chem. Process Des. Deu. 1977,16,20-30. Edward, F. Can. J. Chem. Eng. 1984,62,257-266. Hashimoto, K.; Silveston, P. L. AZChE J. 1973,19,259-267. Jensen, G.A. Ind. Eng. Chem. Process Des. Deu. 1975,14,308-314. Johnson, J. L.Adu. Chem. Ser. 1974,131,145-178. Orbey, N.; Do&, G.; Dogu, T. Can. J . Chem. Eng. 1982,60,314-318. Smith, J. M.Chemical Engineering Kinetics; McGraw Hill: New York, 1970. Srinivas, B.; Amundsen, N. R. AZChE. J. 1980,26,487-496. Ulkutan, S.; Do&, T.; Do&, G. ACS Symp. Ser. 1982,196,515-525. Wakao, N.;Oshima, T.; Yagi, S. Chem. Eng. Jpn. 1958,22, 780. Wakao, N.;Smith, J. M. Chem. Eng. Sci. 1962,17,825-834. Weast, R. C.,Ed. CRC Handbook of Chemistry and Physics, 48th ed.; CRC: Boca Raton, FL, 1978. Weisz, P. B.; Prater, C. D. Adu. Catal. 1954,6 , 143-196. Wen, C.Y.;Wu, N. T. AIChE J . 1976,22, 1012-1021.
Received for review October 18, 1985 Accepted March 9, 1987
