Energy & Fuels 2001, 15, 1347-1353
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Study on Morphology of Chars from Coal Pyrolysis Xinqian Shu* and Xuchang Xu Department of Thermal Engineering, Tsinghua University, Beijing 100084, P. R. China Received September 18, 2000
Pyrolyses under different temperatures were carried out upon Yangquan anthracite, Huainan bituminous coal of medium volatile matter, Shenfu bituminous coal of high volatile matter, and various lithotypes from Yangquan and Shenfu coals. It shows that yields of the pyrolysis rise with the increase of pyrolytic temperature, rising especially rapidly from 673 to 773 K, resulting in a distinction between stages I and II of the pyrolysis. Afterward, they keep a mild increase at high temperatures above 1173 K. Kinetic parameters k0 and E have been reached on the basis of mathematical calculation and numerical stimulation. The k0 of the coals obtained in higher temperature varies between 4.86 × 104 and 15.16 × 104 s-1, and E lies between 9.03 × 103 and 22.85 × 103 J mol-1. Chars have been obtained at each of the pyrolytic temperatures. The morphology of chars was studied with a petrographic microscope. Furthermore, the specific surface area and pore volume of chars from various coals and lithotypes pyrolyzed at 1323 K were measured with an Auto-scan 33 Quantochrome.
Introduction In general, features of superficial shape, size, pore type, pore radius, and porosity of chars are always taken as char morphology, which have significant effects upon coal combustion. Accordingly, many investigators have done research into char morphology. Lightman1 first studied the morphology of pyrolyzed chars from a drop tube furnace with a scanning electron microscope and an optical microscope. Four types of chars were distinguished: a thin-walled hollow cenosphere, a thickwalled hollow cenosphere, a lacy cenosphere with many internal partitions, and a solid particle. Shibaoka2 used a Griffin-Telin hot-stage microscope to study the behaviors of different macerals. He concluded that vitrinite, sporinite, and cutinite became opaque on pyrolysis, and many vesicles appeared within them. Both sporinite and cutinite took a longer time to become opaque than vitrinite did, while resinite expanded explosively to form large vesicles. Fusinite became friable and fragmented in the course of carbonization. Nandi3 studied the morphology of chars, classifying such chars as cenosphere, honeycomb, dense, etc. Hamilton4,6 investigated the char morphology and its change with an instrument heated electrically and a microscope. Shibaoka7 researched the morphology of unburnt chars in fly ashes from power plants and four types of chars: vesicular, dense, mixed, and mineral-rich were distinguished by raw materials, pore features, particle sizes, etc. Jones8 * Corresponding author. Present address: Instituto Nacional del Carbo´n, CSIC, Francisco Pintado Fe, 26, 33011, Oviedo, Spain. Fax: 34-98-5297662. E-mail:
[email protected]. (1) Lightmam, P; Street, P. J. Fuel 1968, 47, 7. (2) Shibaoka, M. Fuel 1969, 48, 285. (3) Nandi, B. N.; Brown, T. D.; Lee, G. K. Fuel 1977, 56, 125. (4) Hamilton, L. H.; Arlying, A. B.; Shibaoka, M. Fuel 1979, 58, 873. (5) Hamilton, L. H. Fuel 1980, 59,112. (6) Hamilton, L. H. Fuel 1981, 60, 909. (7) Shibaoka, M. Fuel 1985, 64, 263.
gave an insight into the morphology of pyrolyzed chars and classified three kinds of them, cenosphere, honeycomb, and unfused chars by porosity, pore size, and the like. Tsai9 utilized fusibility, porosity, shape, and optical textures to identify chars as coal particle, solid, cenosphere, and fragment. Bailey10 recognized about 11 kinds of chars, as tenuisphere, crassisphere, tenuinetwork, mesosphere, fragment, inertoid, solid, fusinoid, mixed porous, mixed dense, and mineroid by means of the shape, pore volume, vesicles, pore shape, and wall thickness of chars. Bend11 made differences of the four chars, cenosphere, cenospheropore, network, and solid on the basis of optical texture, shape, porosity, wall thickness, etc. Thomas12,13 derived a conclusion of three kinds of chars, fused, unfused, and mixed, according to fusibility of raw coals during pyrolysis. Rosenburg14 categorized seven types of chars as tenuisphere, crassisphere, tenuinetwork, crassinetwork-mixed network, inertoid, fusinoid-solid, and mineroid according to the raw material, fusibility, pore features, etc. Zheng15,17 did a study on morphology of pyrolyzed and unburnt chars in fly ashes from some Chinese coals. To sum up the present situation in the investigation of char morphology, it can be concluded that there still exist some ambiguities to be understood. At least the following three should be noticed: (1) classification terminology is more or less in confusion; (2) there is still some room for improvement in the classification criterion; (3) there (8) Jones, R. B.; McCurt, C. B.; Morley, C.; King, K. Fuel 1985, 64, 1460. (9) Tsai, C. Y.; Scaroni, A. W. Fuel 1987, 66, 200. (10) Bailey, J. G.; Tate, A, Diesel, C. F. K. Fuel 1990, 69, 225. (11) Bend, S. L; Edwards, I. A. S.; Marsh, H. Fuel 1992, 71, 493. (12) Thomas, C. G.; Shibaoka, M.; Gawronski, E. Fuel 1993, 72, 907. (13) Thomas, C. G.; Shibaoka, M.; Gawronski, E. Fuel 1993, 72, 913. (14) Rosenberg, P.; Petersen, H. I.; Thomsen, E. Fuel 1996, 75, 1071. (15) Zheng, Y. J. Chin. Coal Soc. 1990, 15 (2), 80 (in Chinese). (16) Zheng, Y.-S.; Gong, X.-A.; Zhang, J.-Y. J. Fuel Chem. Tech. 1992, 20 (3), 225 (in Chinese). (17) Zheng,Y.-S.; Wang, Z.-J. Fuel 1996, 75, 1434.
10.1021/ef000202g CCC: $20.00 © 2001 American Chemical Society Published on Web 10/31/2001
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Energy & Fuels, Vol. 15, No. 6, 2001
Shu and Xu
Table 1. Proximate of Samples (wt %)a samples Mad YQ YQV YQK HN NT NTV NTC NTF NTD SM
Aad
Vad
Ad
Vd
Vdaf
FCad
FCd
0.97 57.26 8.53 57.82 8.61 33.24 33.57 0.92 7.25 7.06 7.32 7.13 7.69 84.77 0.35 84.19 84.49 1.36 34.48 26.59 34.90 26.96 37.57 38.09 7.12 2.83 34.70 3.05 37.36 38.54 54.95 59.16 9.86 2.33 35.38 2.59 39.25 40.30 52.43 58.17 8.01 2.66 33.21 2.89 36.10 37.18 56.12 61.00 7.23 3.72 29.34 4.01 31.63 32.95 59.71 64.36 7.86 2.94 32.32 3.19 35.08 36.23 56.88 61.74 4.39 6.92 32.25 7.24 33.73 36.36 56.44 60.64
stage II. Further disintegration, repolymerization, and resolidification are usually simultaneous in stage III. As substantiated by many researchers, coal devolatilization basically abides by the Arrhenius equation.18,25 Subsequently, the kinetics, like pre-exponential factor k0, and apparent activation energy E for three coals have been reached according to the following calculation and simulation, which are shown in Table 3. At first, pyrolytic conversion is defined as
x ) W/W0
(1)
a
M, moisture; A, ash; V, volatile matter; FC, fixed carbon content; ad, proximate basis; d, dry basis; daf, dry ash free basis.
is a need for an overall comprehension of diversified char characteristics in the study. Hereby, three Chinese coals, namely, Yangquan anthracite(YQ), Huainan bituminous coal with medium volatile matter(HN), and Shenfu bituminous coals with high volatile matter(SF) (one is from the Ningtiaota coal mine, NT, and the other, is from the Shenmu Coke-Making Plant, SM), were pyrolyzed to produce chars by a muffle. Moreover, char morphology was studied with a XP3A optical microscope. Specific area and pore volume were further probed with an Auto-scan 33 Quantochrome by the method of mercury injection in order to make a comprehensive insight into the char morphology. Coal Pyrolysis Samples for studying were first mixed and aliquotted for specimen according to standard methods. Furthermore, two lithotypes from YQ, vitrain (YQV) and minerite (YQM), and four lithotypes from NT, vitrain (NTV), clarain (NTC), durain (NTD), and fusain (NTF), were separated. The separation was done under inlet atmosphere. Table 1 shows the proximate of these specimen. It indicates that samples for studying are different from each other not only in volatile matter but also in ash content, varying from low to high ash, which means the samples chosen are fairly representative. They were pyrolyzed in a XL-1 muffle in correspondence with China National Standard GB 212-87 under a series of temperatures, from 373, 473, 573, 673, 773, 873, 973, 1073, 1193 to 1323 K for 7 min. Chars were picked out under each of pyrolytic temperatures. The yields of the pyrolysis at different temperatures were obtained and given in Table 2. Three distinctive stages for release of volatile matter seem to be identified: slow devolatilization at low temperature (stage I), fast devolatilization at intermediate temperature (stage II), and mild devolatilization at high temperature (stage III). However, it is worth mentioning that outset of the change from one stage to the other is different from one another in three coals. It looks to increase with the raise of coal rank. For example, it happens at 573-673 K in SF, about 673 K in HN, and 773 K in YQ to change from stages I to II. Transition between stages II to III always takes place in the temperature above 1173 K. Analyzed by the pyrolytic course, it can be concluded that the presentation of the main reaction in stage I is related to dewatering and devolatilization, whereas reactions such as devolatilization, secondary disintegration and decomposition, and repolymerization may take place in
where W is the yield of the pyrolysis (wt %) by the time of t and W0 is the total yield during the whole pyrolysis. A mathematical model can be set up to describe devolatilization if we suppose the reaction to be a first order one26
dx/dt ) k(1 - x)
(2)
where t is pyrolytic time and the relation between t and the rate of temperature increase can be related as φ ) dT/dt, whereas
k ) A exp(-E/RT)
(3)
where A is a constant, R is the gas constant (8.3144 J mol-1 K-1), and T is pyrolytic temperature, in K. Making an integration to eq 2 and then taking a logarithm gives:
ln[-ln(1 - x)/T2] ) ln[(AR/φE) × (1 - 2RT/E)] - E/RT (4) Relating between ln[-ln(1 - x)/T2] and 1/T gives a line. E can be obtained by the slope (S) of the line (E ) R|S|) and k0 by the intercept (I) (k0 ) exp(I)). It is worth mentioning that E and k0 obtained at lower temperatures are only for reference because correlation is not so strong in each of the coals. It is just about 0.8. The results obtained at higher temperatures show that E increases with the raise of coal rank; however, k0 decreases. Pyrolytic curves were correspondingly drawn and given in Figures 1 and 2. It discloses that yield of chars rises with the increase of coal rank. Moreover, it shows an increase from vitrain, clarain, durain to fusain in the different lithotypes. Morphology and Characteristics of Chars Classification of Char Morphology. Basic Principles for Classification of Char Morphology. At first, it is necessary to define the principles for the rational (18) Bodzioch, S.; Hawksley, D. P. W. Ind. Eng. Chem. Prov. Des. Dev. 1970, 9, 521. (19) Kobayashi, H.;Howard, J. B.; Sarofim, A. F. 15th Symp. (Int.) Combust. 1975, 521. (20) Anthony, D. B.; Howard, J. B. AIChE J. 1976, 22, 625. (21) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405. (22) Fu, W-B; Zhang, Y.-P.; Han, H.-Q.; Wang, D.-F. Fuel 1989, 68, 505. (23) Griffin, T. D.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 591. (24) Radulovic, P. J.; Grani, M. U.; Smoot, L. D. Fuel 1995, 582. (25) Adesanya, B. A.; Phem, H. N. Fuel 1995, 74, 896. (26) Xie, K.-C.; Zhang, Y.-F.; Li, C.-Z.; Ling, D.-Q. Fuel 1991, 474.
Morphology of Chars from Coal Pyrolysis
Energy & Fuels, Vol. 15, No. 6, 2001 1349 Table 2. Results of Pyrolysis(wt %) pyrolysis temp, K
samples
373
473
573
673
773
873
973
1073
1193
1323
YQ YQV YQK HN NT NTV NTC NTF NTD SM
0.36 2.13 0.90 0.37 4.22 2.73 4.12 3.45 5.26 2.36
1.26 2.70 0.94 1.43 7.84 8.26 7.29 6.15 7.61 6.50
1.11 1.69 1.16 1.74 9.35 9.79 8.84 7.17 8.10 6.85
1.13 2.38 1.34 3.31 11.22 20.43 17.71 8.99 13.27 9.84
1.90 2.34 2.92 14.97 26.02 32.17 28.22 19.71 25.07 21.06
5.73 4.51 7.35 22.38 31.11 37.25 34.56 25.31 30.74 29.19
7.25 6.29 9.30 24.16 34.85 40.62 37.77 28.81 34.96 33.09
8.64 7.44 10.22 26.68 38.25 43.07 39.35 32.03 37.17 35.89
10.17 9.89 11.33 28.29 39.92 44.96 41.55 34.67 38.35 38.24
10.20 9.93 12.37 28.85 41.42 45.45 43.09 35.68 39.19 38.28
Table 3. Kinetics of Coal Pyrolysis and Devolatilization kinetics at higher temp, >773 K coals YQ HN SF
k0,
s-1
4.86 × 9.79 × 104 15.16 × 104 104
E, J
mol-1
22.85 × 13.12 × 103 9.03 × 103 103
at lower temp, 5-8 >10 >10 changeable spherical to oblate partly spherical, irregular >10
