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Energy & Fuels 1996, 10, 1099-1107
1099
Flash Pyrolysis of Brown Coal Modified by Alcohol-Vapor Explosion Treatment Jun-ichiro Hayashi Center for Advanced Research of Energy Technology (CARET), Hokkaido University, N13, N5, Kita-ku, Sapporo 060, Japan
Takao Mori, Shinobu Amamoto, Katsuki Kusakabe, and Shigeharu Morooka* Department of Chemical Science and Technology, Kyushu University, Fukuoka 812-81, Japan Received June 20, 1995. Revised Manuscript Received May 29, 1996X
Morwell brown coal was swollen in a pressurized vapor of methanol, ethanol, propanols, and butanols at 260 and 280 °C. The pressure was then explosively released, and the coal was rapidly quenched. FT-IR spectra and elemental analyses of treated coals revealed that alcohols were chemically incorporated into the coal mainly in the form of aryl alkyl ethers and that a part of hydrogen bonds of the coal were irreversibly disrupted. The effect of explosion on thermal degradability of the coal was examined from the distribution of pyrolysates obtained by flash pyrolysis at 764 °C. Explosion with methanol and 2-propanol increased the tar yield to 31 and 33 kg per 100 kg of original coal, respectively, from 17 kg of the original coal. The tar yields were equivalent to those from coals modified by base-catalyzed O-methylation and O-isopropylation. Swelling in alcohol vapor, instantaneous pressure release, and rapid quenching were essential for the enhancement.
Introduction Coal is a macromolecular solid composed of various covalent and noncovalent linkages. Net degradation of coal in flash pyrolysis is decided by relative rates of (1) stabilization of fragment radicals generated by covalentbond breaking; and (2) cross-linking by radical recombination and thermal condensation among functional groups. To increase the liquid yield in the flash pyrolysis, coal fragment radicals should be stabilized with hydrogen and methyl radicals before cross-links are formed. Hydrogen bonds among oxygen-containing groups such as phenolic hydroxide and carboxyl groups participate strongly in forming the three-dimensional network structure and enhance low-temperature crosslinking among oxygen-containing groups.1-7 Thus the control of hydrogen bonds is an indispensable factor to promote degradation of coal. Thermal treatment at 150-300 °C breaks a portion of the hydrogen bonds and increases the tar yield in subsequent flash pyrolysis.4,6 Solvent swelling is more effective than thermal treatment for disrupting hydrogen bonds. Mae et al.7 treated a brown coal with gaseous pyridine, a strong hydrogen-bond breaking * To whom all correspondence should be addressed. FAX +81-92651-5606. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Suuberg, E. M.; Lee, D. E.; Larsen, J. W. Fuel 1985, 64, 1668. (2) Solomon, P. R.; Serio, M. A.; Deshpande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42. (3) Miura, K.; Mae, K.; Asaoka, S.; Yoshimura, T.; Hashimoto, K. Energy Fuels 1991, 5, 340. (4) Miura, K.; Mae, K.; Sakurada, K.; Hashimoto, K. Energy Fuels 1992, 6, 16. (5) Joseph, T. Fuel 1991, 70, 139. (6) Hayashi, J.-i.; Matsuo, Y.; Kusakabe, K.; Morooka, S. Energy Fuels 1995, 9, 284. (7) Mae, K.; Hoshika, N.; Hashimoto, K.; Miura, K. Energy Fuels 1994, 8, 868.
agent, and conducted a flash pyrolysis of the modified coal. Pyridine left in the coal at a concentration of 3-7 wt % effectively suppressed cross-linking reactions and increased tar evolution. Joseph5 swelled bituminous coals in tetrahydrofuran. Liquefaction reactivity of the coals was improved even after most of the solvent was evaporated. However, he found that the preswelling was not effective for lower-rank coals. Ineffective swelling has been reported by other authors.8-10 Hydrogen bonds broken by swelling in polar solvent were often reconstructed when the swelling solvent was removed. Warzinski8 found that microporosity of a bituminous coal was actually decreased by swelling with pyridine followed by drying under vacuum. Larsen et al.9,10 reported that extractability and swellability of bituminous coals were reduced after treatment in pyridine. The unfavorable structural changes were caused mainly by rearrangement of the coal network in the pyridine removal stage. On the other hand, steam explosion treatment has been applied to degrading biomass such as lignin and cellulose.11 Water retained in organics is rapidly vaporized by an instantaneous pressure release and causes “mechanochemical” cleavage of covalent bonds. The swollen structure of the organic matrix is withheld during the steam explosion, and adiabatic cooling suppresses retrogressive reactions among oxygen-containing radicals. Minowa et al.12 steamed chips of a Japanese red pine at 2 MPa and the pressure was released explosively. The oil yield was 1.5 times higher (8) Warzinski, R. P.; Holder, G. D. Fuel 1992, 71, 993. (9) Larsen, J. W.; Azik, M.; Korda, M. Energy Fuels 1992, 6, 110. (10) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (11) Chum, H. L.; Johnson, D. K.; Ratcliff, M.; Posey, F.; Black, S.; Goheen, D. M.; Baldwin, R. U.S. DOE Report 1985, 53. (12) Minowa, T.; Yokoyama, S.; Ogi, S.; Dote, T. Kagaku Kogaku Ronbunshu 1991, 17, 816.
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1100 Energy & Fuels, Vol. 10, No. 5, 1996
Hayashi et al.
Table 1. Properties of Alcohols Used for Explosion Treatment alcohol
boiling point (°C)
critical temp (°C)
critical press. (MPa)
methanol ethanol 1-propanol 2-propanol 1-butanol 2-butanol tert-butyl alcohol
64.6 78.3 97.2 82.2 111.7 99.5 82.4
239.4 243.0 263.5 235.1 289.7 262.8 233.0
7.84 6.18 5.00 4.61 4.28 4.06 3.84
than that obtained by slow cooling after steaming. This rapid pressure release in solvent vapor is hereafter referred to as “explosion”. Ogo et al.13 treated a bituminous coal explosively using a mixed vapor of water and cyclohexanol at 400 °C as a predegradation step for liquefaction and increased both conversion and oil yield. From an economical viewpoint, however, the treatment temperature should be lowered. If “coalphilic” solvents like lower alcohols are employed, swelling of coals will occur at moderate temperature. Previous studies on coal liquefaction with subcritical and supercritical methanol14,15 suggest that methyl groups are transferred from methanol to coal as aryl methyl ethers, carboxymethyl esters, and aryl methyl groups. Thus, the methanol vapor explosion will change the chemical and physical structure of coal and improve the degradability of coal in pyrolysis as does O-alkylation.16-20 The promotion of pyrolysis by alkylation is attributed to (1) elimination of hydrogen bonds by replacing hydroxyl hydrogens with alkyl groups and (2) introduction of alkyl groups that function as efficient hydrogen and hydrocarbon radical donors. Explosion treatment of coal with alcohol vapor is thus expected to be a practical and effective method for coal alkylation. In the present study, a brown coal was heat-treated at 260 and 280 °C under a vapor of lower alcohols at 1.5-5.8 MPa, and the pressure was released explosively. The effect of the explosion on thermal degradability of the coal was evaluated by means of flash pyrolysis with a Curie-point pyrolyzer. Physical and chemical changes in the coal structure were examined by FT-IR and elemental analyses. Experimental Section Coal Sample and Solvents. Morwell brown coal was pulverized and sized to 0.037-0.074 mm and dried under vacuum at 70 °C for 24 h. Samples were sealed in brown glass ampules until use. The elemental composition was C 63.6, H 4.7, N 0.6, O + S (by difference) 29, and ash 2 wt % on dry basis. Functional groups in the original and treated coals were evaluated by Fourier transform infrared spectroscopy (FT-IR, Perkin-Elmer Paragon 1000). Spectra were obtained under predried nitrogen or air by KBr pellet method and diffuse (13) Ogo, Y.; Mizuno, K.; Banjoya, A. Proc. Int. Conf. Coal Science, Newcastle 1991, I-238. (14) Garcia, R.; Moinelo, S. R.; Snape, C. E. Fuel 1993, 72, 427. (15) Kuznetsov, P. N.; Sharypov, V. I.; Beregovtsova, N. G.; Rubaylo, A. I.; Korniyets, E. D. Fuel 1990, 69, 911. (16) Ofosu-Asante, K.; Stock, L. M.; Zabransky, R. F. Fuel 1989, 68, 567. (17) Chatterjee, K.; Stock, L. M.; Zabransky, R. F. Fuel 1989, 68, 1349. (18) Rose, G. R.; Zabransky, R. F.; Stock, L. M.; Huang, C.-b.; Srinivas, V. R.; Tse, K.-t. Fuel 1984, 63, 1339. (19) Chu, C. J.; Cannon, S. A.; Hauge, R. H.; Margrave, J. L. Fuel 1986, 65, 1740. (20) Ofosu-Asante, K.; Stock, L. M.; Zabransky, R. F. Energy Fuels 1988, 2, 511.
Figure 1. Apparatus for steaming and explosion. reflectance method. In the case of KBr pellet method, each coal sample was diluted with finely pulverized KBr to a mass ratio of 1/100, dried under vacuum at 70 °C for 6 h, and pelletized. Reproducibility was confirmed by comparing spectra of samples prepared separately from the same coal. The surface morphology was observed with a field-emission scanning electron microscope (SEM, Hitachi S-900). Vapor Explosion Treatment. Water-free methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and tert-butyl alcohol were used to modify the coal. Their boiling points, critical temperatures, and critical pressures are listed in Table 1. Figure 1 schematically shows the apparatus used for alcohol vapor explosions. Dried coal (1.0 g) was wrapped in an aluminum foil and placed in an upper stainless-steel tube cell whose inner volume was 10.2 cm3. The cell was purged with nitrogen, and alcohol was injected into the cell with an HPLC pump at a rate of 1 mL/min. The cell was heated at a rate of 10 °C/min to 260 or 280 °C and kept at the final temperature for 60 min. The temperature of coal was measured with a thermocouple inserted into the cell. The pressure releasing valve was instantaneously opened, and the coal was exploded, transferred to an evacuated chamber that was kept at ambient temperature, and quenched rapidly. Treated coals were vacuum-dried at 30 °C to remove residual alcohol and stored in sealed glass bottles. Samples for elemental analyses were prepared by further vacuum drying at 70 °C for 24 h. To examine the role of explosion, the coal was treated in alcohol vapor at 260 or 280 °C for 60 min and gradually cooled in the bomb without releasing the pressure. This treatment is hereafter referred to as “steaming”. To check the effect of the rapid quenching, a stainless-steel mesh was placed just above the pressure releasing valve. The coal sample was cooled slowly on the mesh in the upper cell after the explosion. Heat treatment in nitrogen under atmospheric pressure was also carried out. Gaseous products (CO, CO2, and CH4) produced during these treatments were collected in an impermeable bag and were analyzed by gas chromatography. Water was collected in a cold trap kept at -70 °C. It was diluted with dry methanol and analyzed by Karl-Fischer titrimetry. In the treatments with alcohol, water is produced by (1) water-forming reactions of coal itself; (2) dehydration condensation between coal and alcohol (O-alkylation); and (3) bimolecular condensation of alcohol. The water yield by scheme 1 is estimated from changes in coal mass and elemental com-
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Flash Pyrolysis of Brown Coal
Energy & Fuels, Vol. 10, No. 5, 1996 1101
Table 2. Elemental Compositions of Original Coal, O-Alkylated Coals, and Their Reference Coals
treatment
Ha
Ca
Na
none HCl washing TBAH treatment O-methylation O-isopropylation
4.87 4.89 5.12 5.31 5.57
64.7 65.4 65.4 66.0 66.1
0.60 0.53 0.69 0.56 0.69
a
O+
Sa
ash yield (wt %, db (kg/100 kg of sample) original coal)
29.9 29.5 28.9 28.1 28.9
1.6
