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A Study of Cellulose Pyrolysis Chemistry and Global Kinetics at High Heating Rates Alexander L. Brown,†,‡ David C. Dayton,*,† and John W. Daily‡ National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, MS 3322, Golden, Colorado 80401, and University of Colorado-Boulder, Department of Mechanical Engineering, Center for Combustion and Environmental Research, Boulder, Colorado 80309 Received April 10, 2001. Revised Manuscript Received June 29, 2001
Cellulose pyrolysis has been studied in a laminar entrained flow reactor (LEFR). As described in the previous companion paper, the reactor is capable of high heating rates (∼104 K/s) and has been characterized in detail to ensure that pyrolysis of lignocellulosic materials occurs under kinetic control at the conditions of the reactor. The extent of cellulose pyrolysis in the LEFR was monitored by sampling the gas phase products with a molecular beam mass spectrometer system, and independently by sampling and weighing residues on a filter paper. Varying the reactor furnace temperature controls the pyrolysis severity. Quantitative cellulose pyrolysis data have been compared to several published reaction rates. Published models that involve low activation energy rates from other high temperature experiments described in the literature best approximate the results obtained in this study. Factor analysis of the mass spectral data requires two principal components to interpret the gas phase product composition. This suggests that the primary cellulose pyrolysis products were involved in subsequent secondary reactions that directly compete with the primary release of products at the conditions in this reactor. A rate is presented that describes the observed thermal destruction of primary pyrolysis products.
Introduction Cellulose is the primary constituent in most biomass materials and is found at about 40% abundance in most woods.1 It can be harvested and separated and is an important component of many processes, such as papermaking, waste and resource recovery, charcoal and activated carbon production, textiles, food processing, and fire research. Hence, cellulose has been studied extensively. It has been theorized that global biomass pyrolysis models can be adequately formed by superimposing the pyrolysis rates of the three primary constituents (cellulose, hemicellullose, and lignin).2-5 Despite considerable research, there is still extensive debate on the global pyrolysis mechanism for cellulose. According to Grønli and Melaaen,6 “kinetic modeling is very complicated and after nearly 30 years of research...there is still no consensus concerning the kinetics of wood and cellulose pyrolysis.” The classic cellulose * Corresponding author. Fax: (303) 384-6363. E-mail:
[email protected]. † National Renewable Energy Laboratory (NREL). ‡ University of Colorado-Boulder. (1) Parham, R. A.; Gray, R. L. In The Chemistry of Solid Wood; Rowell, Roger, Ed.; ACS Advances in Chemistry Series 207, American Chemical Society, Washington, DC, 1984; pp 3-56. (2) Koufopanos, C. A.; Maschio, G.; Lucchesi, A. Can. J. Chem. Eng. 1989, 67, 75-84. (3) Maschio, G.; Lucchesi, A.; Koufopanos, C. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie Academic and Professional: New York, 1994; Vol. 2, pp 746-759. (4) Miller, R. S.; Bellan, J. Combust. Sci. Technol. 1997, 126, 97137. (5) Di Blasi, C.; Russo, G. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie Academic and Professional: New York, 1994; Vol. 2, pp 906-921. (6) Grønli, M. G.; Melaaen, M. C. Energy Fuels 2000, 14, 791-800.
model, also known as the Broido-Shafizadeh model, has for some time been considered the standard.7 It involves two consecutive or “serial” reactions. The first reaction did not produce any visible product, but was included to numerically correspond to measurements of decreasing degree of polymerization of the solid material in lowtemperature experiments. Recent studies question the validity of the classic model.8,9 Thermogravimetric analysis (TGA) is the most common technique used for low temperature cellulose pyrolysis kinetic studies. These experiments are typically conducted with small (0.1-10 mg) samples at very low heating rates (0.5-10 °C/min) to minimize heat transfer effects. Kinetic rates derived from higher heating rate data indicate a slower reaction. The reasons for this shift in the apparent pyrolysis rate have not been established. Several theories expounded in the recent literature provide an interesting background on this topic. Milosavljevic and Suuberg found through an extensive survey of existing data and some original TGA experimental data that there is an apparent shift in the measured first-order kinetics from a high activation energy (>200 kJ/mol) regime to a low activation energy regime (around 140 kJ/mol) around 600 K.9 They speculate that variations in sample origin and purity contribute to the spread in rates published in the literature. Follow-up studies suggest that the slower (7) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1979, 23, 3271-3280. (8) Va´rhegyi, G.; Jakab, E.; Antal, M. J. Energy Fuels 1994, 8, 13451352. (9) Milosavljevic, I.; Suuberg, E. M. Ind. Eng. Chem. Res. 1995, 34, 1081-1091.
10.1021/ef010084c CCC: $20.00 © 2001 American Chemical Society Published on Web 08/28/2001
Cellulose Pyrolysis Chemistry
high temperature rate could be caused by interparticle diffusion or evaporation limitations, as the latent heat of vaporization for pyrolysis tars (levoglucosan) is around 140 kJ/mol.10,11 Reynolds and Burnham attribute much of the shift in the rate of cellulose pyrolysis to non first-order effects from their studies of various cellulose based materials with TGA and Pyromat micropyrolysis.12 They evaluate their data based on various kinetic schemes and conclude that a three-parameter nucleation scheme best fits their data. Antal and Va´rhegyi, along with several co-workers, have extensively studied this topic. Using TGA techniques, they indicate that the appropriate global activation energy for cellulose pyrolysis is around 240 kJ/mol.13,14 Their data indicate a gradual shift in the observed rate at increasing heating rates up to about 80 °C/min. They attribute the shift in apparent rate primarily to an increasing roll of heat transport limitations at higher heating rates.13-15 Diebold has also developed an elaborate seven-step global kinetic scheme for cellulose pyrolysis16 motivated by the need for a global pyrolysis model capable of predicting cellulose pyrolysis behavior for a broad range of conditions. In summary, it is difficult to make an accurate judgment on an appropriate global model or pyrolysis rate for cellulose pyrolysis based on the current state of the literature. Fast pyrolysis studies are not nearly as prevalent in the literature as slow pyrolysis studies. Published fast pyrolysis experiments of cellulose involve heated grids,10,17,18 tubular reactors,19-21 fluidized beds,20 and radiant heating techniques.22,23 Some of these studies do not report rates of reaction, because some of the experiments are not specifically designed to derive reaction rate information. Furthermore, it is difficult to make accurate measurements for rapid, high temperature processes. A new experimental apparatus has been designed for the controlled study of fast biomass pyrolysis at high heating rates.24 For this study, cellulose pyrolysis occurs in a fully characterized laminar entrained flow reactor (LEFR) coupled to a molecular beam sampling mass spectrometer (MBMS) system. Gas phase cellulose pyrolysis products are monitored with the MBMS as a (10) Suuberg, E. M.; Milosavljevic, I.; Oja, V. 26th Symp. (Int.) Combust, 1996, 1515-1521. (11) Milosavljevic, I.; Oja, V.; Suuberg, E. M. Ind. Eng. Chem. Res. 1996, 35, 653-662. (12) Reynolds, J. G.; Burnham, A. K. Energy Fuels 1997, 11, 8897. (13) Antal, M. J.; Va´rhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703717. (14) Grønli, M.; Antal, M. J.; Va´rhegyi, G. Ind. Eng. Chem. Res. 1999, 38, 2238-2244. (15) Narayan, R.; Antal, M. J. Ind. Eng. Chem. Res. 1996, 35, 17111721. (16) Diebold, J. P. Biomass Bioenergy 1994, 7, 1-6, 75-85. (17) Lewellen, P. C.; Peters, W. A.; Howard, J. B. 16th Symp. (Int.) Combust. 1977, 471-1480. (18) Hajaligol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 457-465. (19) Cullis, C. F.; Hirschler, M. M.; Townsend, R. P.; Visanuvimol, V. Combust. Flame 1983, 49, 235-248. (20) Scott, D. S.; Piskorz, J.; Bergougnou, M. A.; Graham, R.; Overend, R. A. Ind. Eng. Chem. Res. 1988, 27, 8-15. (21) Graham, R. G.; Bergougnou, M. A.; Freel, B. A. Biomass Bioenergy 1994, 7, 1-6 and 33-47. (22) Lanzetta, M.; Di Blasi, C.; Buonanno, F. Ind. Eng. Chem. Res. 1997, 36, 542-552. (23) Boutin, O.; Ferrer, M.; Le´de´, J. J. Anal. Appl. Pyrolysis 1998, 47, 13-31. (24) Brown, A. L.; Dayton, D. C.; Nimlos, M. R.; Daily, J. W. Energy Fuels 2001, 15, 1276.
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function of reactor temperature. The mass spectral results collected are reduced to determine the extent of cellulose conversion to gas phase products. Originally, the intention was to use cellulose as a standard for further characterizing the LEFR with a solid material. However, in preliminary evaluations of several of the existing models, disagreement was found between the models and the rate of cellulose pyrolysis at our LEFR conditions. This paper presents the results of kinetically controlled, high heating rate, cellulose pyrolysis experiments using a new technique, and discusses the implications of the data relative to the global modeling of cellulose pyrolysis. Experimental Section Many of the techniques and experimental systems used in this study are described in a companion paper on the design and characterization of the LEFR.24 Detailed experimental descriptions in this paper will therefore be limited to aspects that have not been previously presented. The biggest challenge in obtaining quantitative cellulose pyrolysis kinetic data was achieving steady particle feed-rates that were repeatable over an extended time period, returned to the desired feed-rate after the particle flow was stopped, and had the appropriate quantitative output for the reactor. The ideal particle flow rate for the nominal 50-µm diameter cellulose particles is between 0.1 and 0.5 g/h in 10 sccm to 300 sccm of helium. This desired rate of solid feed was arrived at experimentally after extensive testing and is a compromise to provide a high enough feed-rate to yield sufficient gas phase products to be detected with the MBMS, while maintaining a low enough feed-rate to maintain appropriate reaction control. Feed-rates that are too high are manifest in several ways. First, if the particle flow rate is too high, the bulk mass flow can exceed the ability of the furnace to maintain the temperature of the reactor walls while heating the cellulose. A slight (
