Energy & Fuels 2000, 14, 127-130
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Gasification of Biomass Chars in Air - Effect of Heat Treatment Temperature T. Ganga Devi and M. P. Kannan* Department of Chemistry, University of Calicut, 673 635, Kerala, India Received April 14, 1999
Three biomass materialsscoir dust (CD), wheat straw (WS), and potato pulp (PP)swere pyrolyzed under nitrogen, and the resulting chars were gasified in air at 400 °C. In all cases, gasification rate of the chars showed the “normal” dependence on heat treatment temperature (HTT), i.e., a decrease with increasing HTT, only up to a certain value of HTT (500, 550, and 750 °C for CD, WS, and PP, respectively), above which the rate dramatically increased. This contrasts with the monotonic decrease in reactivity reported for coal chars. The unusual HTT effect is suggested to be due to a change in the chemical state of the indigenous potassium species at the HTT inflection temperature.
Introduction The gasification reactivity of a char depends on various parameters such as heat treatment temperature (HTT),1-5 heat treatment time or residence time (Rt),2,3 the nature of the reactant gas,6 the nature and quantity of the metals (indigenous or added) present in the char,7 gasification temperature (GT),8-10 etc. To a large extent these factors are interdependent. Their role in controlling the char reactivity is complex and, at present, only poorly understood. More information is, therefore, desirable for optimizing the process conditions. We have recently studied the individual catalytic effectiveness of potassium11 and calcium species12 in the air gasification of cellulosic chars. The reactivity of potassium-containing chars first decreased and then dramatically increased with increasing HTT, whereas that of calcium-containing chars decreased with an increase in HTT. The diverse catalytic effects shown by potassium and calcium make it important to study the * Author to whom correspondence should be addressed. (1) Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1973, 52, 288-293. (2) Radovic, L. R.; Steczko, K.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849-856. (3) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983, 82, 382-394. (4) Smith, W. R.; Polley, M. H. J. Phys. Chem. 1956, 60, 689-691. (5) Kasaoka, S.; Sakata, Y.; Shimada, M. Fuel 1987, 66, 697-701. (6) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. In Advances in Catalysis, Vol. 11; Eley, D. D., Selwood, P. W., Weisz, P. Z., Eds.; Academic Press: New York, 1959; pp 133-221. (7) Mckee, D. W. In Chemistry and Physics of Carbon, Vol. 16; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker: New York, 1981; pp 1-118. (8) Marsh, H.; Adair, R. R. Carbon 1975, 13, 327-332. (9) Fernandes-Morales, I.; Lopez-Garzon, F. J.; Lopez-Peinado, A.; Moreno-Castilla, C.; Rivera-Utrilla, J. Fuel 1985, 64, 666. (10) Devi, T. G.; Kannan, M. P.; Richards, G. N. Fuel 1990, 69, 1441-1447. (11) Kannan, M. P.; Richards, G. N. Fuel 1990, 69, 999-1006. (12) Devi, T. G.; Kannan, M. P. Fuel 1998, 77, 1825-1830. (13) DeGroot, W. F.; Kannan, M. P.; Richards, G. N.; Theander, O. J. Agric. Food Chem. 1990, 38, 320-323. (14) Kannan, M. P.; Richards, G. N. Fuel 1990, 69, 747-753. (15) Menon, K. P. V.; Pandalai, K. M. The coconut palm, a monograph; Indian Central Coconut Committee, Ernakulam, 1960; p 341.
behavior of these metals when they are present together. Biomass materials, which generally contain a wide spectrum of metal species with K and Ca being the most predominant,13,14 provide naturally occurring cellulosic systems suitable for such study. This paper describes some observations on the dependence of the air gasification reactivity of some biomass chars on HTT. Experimental Section Materials. Three agricultural residuesscoir dust (CD), wheat straw (WS), and potato pulp (PP)swhich are of low or negative economic value, were selected in this study. The indigenous inorganic components of these materials, determined by inductively coupled argon plasma emission spectroscopy (ICAPS), and ash contents, determined by thermogravimetry (TG) by burning the samples in oxygen at 550 °C, are presented in Table 1 on a dry-matter basis. The values of the lignin content available in the literature13,15 are also included in the table for easy reference. Preparation of Char and Measurement of Char Reactivity. About 10 mg of biomass sample was heated under flowing nitrogen to the desired HTT at a heating rate of 100 °C/min and maintained at this temperature for Rt ) 15 min. The resulting char was then gasified in air at 400 °C for 30 min. The gasification time was set to 30 min on the basis of our previous experience that a significant or measurable extent of gasification of most cellulosic chars occurred during this period. The pyrolysis and gasification processes were carried out in succession in a programmable Perkin-Elmer TGS-2 thermobalance as described elsewhere.10,11,14 The rate of gasification Rg at any time t is given by
Rg ) dF/dt ) - (1/Wo) dW/dt where F is the fractional conversion of the char at time t, Wo the initial weight (dry basis) of the char, and W the weight at time t. The maximum rate (Rg (max)) is then determined from the rate-time data prepared at an interval of one minute (i.e., dt ) 1 min) and is expressed on a “daf” (dry ash free) basis using the expression
Rg (max,daf) ) Rg (max) [% char/(% char - % ash)]
10.1021/ef990067b CCC: $19.00 © 2000 American Chemical Society Published on Web 12/20/1999
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Table 1. Inorganic Components, Lignin Contents, Ash Yields, and Pyrolysis Peak Temperatures of the Biomass Samples Coir Dust (CD), Wheat Straw (WS), and Potato Pulp (PP) and Potassium- and Calcium-Exchanged Carboxymethyl Cellulose Samples (KCMC and CaCMC-3) concentration of inorganic component (ppm) Mg Ca Al Fe Co Cu
biomass (dry)
Na
K
CD WS PP KCMC CaCMC-3
6165 25 95 0 0
19500 8600 18300 21925 0
a
1390 445 560 0 0
1645 2355 965 0 12812
1920 135
