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Particle size dependence of coal char reactivity
81
C O M B U S T I O N A N D F L A M E 6 8 : 8 1 - 8 3 (1987)
Particle Size Dependence of Coal Char Reactivity W A Y N E F. WELLS and L. D O U G L A S SMOOT Department of Chemical Engineering, Brigham Young University, Provo, Utah
Studies of the reactivity of coal chars are limited in both scope and number, particularly for process chars from U.S. coals at high temperatures. Pioneering work in coal char combustion was conducted by Field [1]. He studied the effects of particle size on reactivity for a laboratory char prepared from a British coal. Smith [2] reviewed the extensive work performed by himself and coworkers. They have investigated the effects of size variation on reactivity for a wide variety of Australian coals. The work on U.S. chars is more limited. Among those that have been reported is the study of Goetz et al. [3] for chars derived from Illinois bituminous, Wyoming subbituminous, and Texas lignite coals. The particle size distribution for these chars was very wide ( - 2 0 0 + 4 0 0 mesh). Chen et al. [4] studied the ignition temperature for several process coal chars which were derived from U.S. coals, one of which was also examined by this study (FMC COED), but they did not conduct high temperature reactivity measurements. Gomez and Vastola [5] studied char from a Wyoming subbituminous coal with particle sizes from 850 to 1000 /~m. Due to the large particle size the reaction mechanism could be different than that for much smaller particles [6]. Young et al. [7] have presented preliminary data for a North Dakota lignite char, but have not presented any data concerning effect of particle size on reactivity. This brief communication presents new results on the dependence of char reactivity at high temperature on particle diameter for five process chars and interprets these data and companion results of a previous study [8] as they relate to reaction zone. The chars possessed very different physical Copyright © 1987 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
properties, as previously reported [8]. The parent bituminous or subbituminous coals for the five chars were all different. N2 BET surface area of the chars varied from 6 to 320 m2/g while TGA mass reactivities varied by nearly two orders of magnitude in the low temperature range of 550750K [8]. Residual char proximate volatiles content ranged from 7.3 to 20.7% (daf). In this study, tightly classified fractions of chars with mass mean particle diameters of 41 /~m or 69 /~m were collected using standard sieve screens and a fluidized bed separator. The samples were reacted in a drop-tube furnace at 18 % oxygen with particle temperatures ranging to 2100K, and were analyzed using a Perkin-Elmer C,H,N analyzer to determine the extent of reaction. The char reactivity data were reduced using standard methods [2, 8]. The new reactivity results are shown in Fig. 1, together with previously reported data. The reaction rate coefficients based on the earlier data are given in Ref. [8]. The figure shows new data for the two different size fractions for particle temperatures of 1430-2100K. The char reactivity data are presented on an external area basis. Reactivities for all five chars are essentially identical and thus independent of char size, type, or characteristics. Comparison of the area reactivities for the two size-graded fractions are illustrated in the figure. For the tests at the lowest temperature, where particle diameter was varied, the data over this small temperature range are independent of particle size, within error limits. This observed independence of external area-based char reactivity on particle size suggests that the reaction process occurs in zone II, where pore diffusion
0010-2180/87/$03.50
82
WAYNE F. WELLS and L. DOUGLAS SMOOT Particle Temperature (K) -1.0
-1.5
2100 2000 1900 1600 I
O
I
~
I
B
~
1700
1600
1500
1450
I
I
I
I
I
a ~ : s~ ' ~ " " ~
0.367
-- 0.223
"E >I--->
to
-2.0
-- 0.135 ? FMC Occidental
< W re
-2.5
z
r'l Rockwell
~
v Tosco
A
" ~ l l ~ Aa -4e.L
0.082
> I--(O < W re
Empty - 69 i.tm fraction Filled
-3.0
4.5
I
- 41 p m fraction
v
I
I
I
5 5,5 6 6.5 4 Reciprocal ParticleTemperature (10/lp, K1)
7
0.050
Fig. 1. Effect of temperature, char type, and particle size on external area reactivity.
and heterogeneous reaction control. However, the lack of variation of area-based reactivity among the very different char types suggests external diffusion control (zone III), where external area reactivity is expected to be independent of carbon type for common particle sizes [6]. To identify further the rate controlling zone, the X parameter (i.e., the ratio of the observed reactivity to the maximum possible under diffusion controlled combustion) was calculated [2]. The maximum reactivity (X = 1) is calculated by assuming that the surface 02 concentration is zero, whereas for low values of X, chemical reaction controls [9]. In this study, for the 69 #m particles, X was about 0.23 at 1500K, about 0.35 at 1800K, and about 0.50 at 2050K. For the 41 #m particles, X was approximately 0.18 at 1450K. Thus, surface reaction is an important resistance for both size fractions. Even at the highest temperature, the particles are reacting in the transition region between zones II and III, where both bulk diffusion and combined pore diffusion and surface reaction are important factors. At the lower temperature, bulk diffusion is less important and heterogeneous reaction is correspondingly more important. The true energy of activation for these chars, which was taken to be the value determined in the
low temperature range of 550-750K using thermogravimeteric analysis [10], was about the same for all the chars studied, 32 kcal/mole. The energies of activation at elevated temperatures for the five chars were about half the low temperature value (i.e., 14-17.7 kcal/mole). This result implies that the rate-controlling regime for the high temperature reactivity data is zone II. These observations lead to the conclusion that the five finely pulverized process chars were reacting in zone II or in the transition between zones II and III at the highest temperatures, with surface reaction (including near surface pore diffusion) and bulk diffusion as important effects.
This paper was based on work supported principally by the U.S. Department o f Energy, Pittsburgh Energy Technology Center, PA, with Mr. James D. Hickerson, Project Officer. Lynda Richmond and Linda Ward typed the text and figures. REFERENCES 1. 2.
Field, M. A., Combust. Flame 13:237 (1969); Cornbust. Flame 14:237 (1970). Smith, I. W., Fuel 57:409 (1978); Smith, I. W., Nineteenth S y m p o s i u m (International) on Combus-
PARTICLE SIZE DEPENDENCE
3.
4.
5. 6. 7.
tion, The Combustion Institute, Pittsburgh, Pennsylvania, 1982, p. 1045. Goetz, G. J., Nsakala, N. Y., Patel, R. L., and Lao, T. C., Paper no. 15, presented at The Second Annual Contractor's Conference on Coal Gasification, Palo Alto, California, October 20-21, 1982. Cben, M. R., Fan, L. S., and Essenhigh, R. H., 20th Symposium (InternationaO on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1984, p. 1513. Gomez, C. O., and Vastola, F. J., Fuel 64:558 (1985). Walker, P. L., Jr., Rusinko, F., Jr., and Austin, L. G., Advan. CataL 11:135 (1959). Young, B. C., McCollor, D. P., Weber, B. J., Jones, M.
83
8.
9.
10.
L., and Grow, D. T., Western States Section/Combustion Institute, Fall 1985. Wells, W. F., Kramer, S. K., Smoot, L. D., and Blackham, A. U., Twentieth Symposium (InternationaO on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1984, p. 1539. Holve, D. J., Hoornstra, J., and Fletcher, T. H., American Flame Research Committee, 1985 Fall Meeting, Sandia National Laboratories, Livermore, California, October 17-18, 1985. Radovic, L. R., Walker, P. L., Jr., and Jenkins, R. G., Fuel 62:849 (1983).
Received 5 August 1985; revised 5 December 1986
C O M B U S T I O N A N D F L A M E 6 8 : 8 1 - 8 3 (1987)
Particle Size Dependence of Coal Char Reactivity W A Y N E F. WELLS and L. D O U G L A S SMOOT Department of Chemical Engineering, Brigham Young University, Provo, Utah
Studies of the reactivity of coal chars are limited in both scope and number, particularly for process chars from U.S. coals at high temperatures. Pioneering work in coal char combustion was conducted by Field [1]. He studied the effects of particle size on reactivity for a laboratory char prepared from a British coal. Smith [2] reviewed the extensive work performed by himself and coworkers. They have investigated the effects of size variation on reactivity for a wide variety of Australian coals. The work on U.S. chars is more limited. Among those that have been reported is the study of Goetz et al. [3] for chars derived from Illinois bituminous, Wyoming subbituminous, and Texas lignite coals. The particle size distribution for these chars was very wide ( - 2 0 0 + 4 0 0 mesh). Chen et al. [4] studied the ignition temperature for several process coal chars which were derived from U.S. coals, one of which was also examined by this study (FMC COED), but they did not conduct high temperature reactivity measurements. Gomez and Vastola [5] studied char from a Wyoming subbituminous coal with particle sizes from 850 to 1000 /~m. Due to the large particle size the reaction mechanism could be different than that for much smaller particles [6]. Young et al. [7] have presented preliminary data for a North Dakota lignite char, but have not presented any data concerning effect of particle size on reactivity. This brief communication presents new results on the dependence of char reactivity at high temperature on particle diameter for five process chars and interprets these data and companion results of a previous study [8] as they relate to reaction zone. The chars possessed very different physical Copyright © 1987 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
properties, as previously reported [8]. The parent bituminous or subbituminous coals for the five chars were all different. N2 BET surface area of the chars varied from 6 to 320 m2/g while TGA mass reactivities varied by nearly two orders of magnitude in the low temperature range of 550750K [8]. Residual char proximate volatiles content ranged from 7.3 to 20.7% (daf). In this study, tightly classified fractions of chars with mass mean particle diameters of 41 /~m or 69 /~m were collected using standard sieve screens and a fluidized bed separator. The samples were reacted in a drop-tube furnace at 18 % oxygen with particle temperatures ranging to 2100K, and were analyzed using a Perkin-Elmer C,H,N analyzer to determine the extent of reaction. The char reactivity data were reduced using standard methods [2, 8]. The new reactivity results are shown in Fig. 1, together with previously reported data. The reaction rate coefficients based on the earlier data are given in Ref. [8]. The figure shows new data for the two different size fractions for particle temperatures of 1430-2100K. The char reactivity data are presented on an external area basis. Reactivities for all five chars are essentially identical and thus independent of char size, type, or characteristics. Comparison of the area reactivities for the two size-graded fractions are illustrated in the figure. For the tests at the lowest temperature, where particle diameter was varied, the data over this small temperature range are independent of particle size, within error limits. This observed independence of external area-based char reactivity on particle size suggests that the reaction process occurs in zone II, where pore diffusion
0010-2180/87/$03.50
82
WAYNE F. WELLS and L. DOUGLAS SMOOT Particle Temperature (K) -1.0
-1.5
2100 2000 1900 1600 I
O
I
~
I
B
~
1700
1600
1500
1450
I
I
I
I
I
a ~ : s~ ' ~ " " ~
0.367
-- 0.223
"E >I--->
to
-2.0
-- 0.135 ? FMC Occidental
< W re
-2.5
z
r'l Rockwell
~
v Tosco
A
" ~ l l ~ Aa -4e.L
0.082
> I--(O < W re
Empty - 69 i.tm fraction Filled
-3.0
4.5
I
- 41 p m fraction
v
I
I
I
5 5,5 6 6.5 4 Reciprocal ParticleTemperature (10/lp, K1)
7
0.050
Fig. 1. Effect of temperature, char type, and particle size on external area reactivity.
and heterogeneous reaction control. However, the lack of variation of area-based reactivity among the very different char types suggests external diffusion control (zone III), where external area reactivity is expected to be independent of carbon type for common particle sizes [6]. To identify further the rate controlling zone, the X parameter (i.e., the ratio of the observed reactivity to the maximum possible under diffusion controlled combustion) was calculated [2]. The maximum reactivity (X = 1) is calculated by assuming that the surface 02 concentration is zero, whereas for low values of X, chemical reaction controls [9]. In this study, for the 69 #m particles, X was about 0.23 at 1500K, about 0.35 at 1800K, and about 0.50 at 2050K. For the 41 #m particles, X was approximately 0.18 at 1450K. Thus, surface reaction is an important resistance for both size fractions. Even at the highest temperature, the particles are reacting in the transition region between zones II and III, where both bulk diffusion and combined pore diffusion and surface reaction are important factors. At the lower temperature, bulk diffusion is less important and heterogeneous reaction is correspondingly more important. The true energy of activation for these chars, which was taken to be the value determined in the
low temperature range of 550-750K using thermogravimeteric analysis [10], was about the same for all the chars studied, 32 kcal/mole. The energies of activation at elevated temperatures for the five chars were about half the low temperature value (i.e., 14-17.7 kcal/mole). This result implies that the rate-controlling regime for the high temperature reactivity data is zone II. These observations lead to the conclusion that the five finely pulverized process chars were reacting in zone II or in the transition between zones II and III at the highest temperatures, with surface reaction (including near surface pore diffusion) and bulk diffusion as important effects.
This paper was based on work supported principally by the U.S. Department o f Energy, Pittsburgh Energy Technology Center, PA, with Mr. James D. Hickerson, Project Officer. Lynda Richmond and Linda Ward typed the text and figures. REFERENCES 1. 2.
Field, M. A., Combust. Flame 13:237 (1969); Cornbust. Flame 14:237 (1970). Smith, I. W., Fuel 57:409 (1978); Smith, I. W., Nineteenth S y m p o s i u m (International) on Combus-
PARTICLE SIZE DEPENDENCE
3.
4.
5. 6. 7.
tion, The Combustion Institute, Pittsburgh, Pennsylvania, 1982, p. 1045. Goetz, G. J., Nsakala, N. Y., Patel, R. L., and Lao, T. C., Paper no. 15, presented at The Second Annual Contractor's Conference on Coal Gasification, Palo Alto, California, October 20-21, 1982. Cben, M. R., Fan, L. S., and Essenhigh, R. H., 20th Symposium (InternationaO on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1984, p. 1513. Gomez, C. O., and Vastola, F. J., Fuel 64:558 (1985). Walker, P. L., Jr., Rusinko, F., Jr., and Austin, L. G., Advan. CataL 11:135 (1959). Young, B. C., McCollor, D. P., Weber, B. J., Jones, M.
83
8.
9.
10.
L., and Grow, D. T., Western States Section/Combustion Institute, Fall 1985. Wells, W. F., Kramer, S. K., Smoot, L. D., and Blackham, A. U., Twentieth Symposium (InternationaO on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1984, p. 1539. Holve, D. J., Hoornstra, J., and Fletcher, T. H., American Flame Research Committee, 1985 Fall Meeting, Sandia National Laboratories, Livermore, California, October 17-18, 1985. Radovic, L. R., Walker, P. L., Jr., and Jenkins, R. G., Fuel 62:849 (1983).
Received 5 August 1985; revised 5 December 1986