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Ind. Eng. Chem. Res. 1998, 37, 4290-4295
Kinetic Study of the Pyrolysis of Waste Wood Joaquin Reina, Enrique Velo, and Luis Puigjaner* Department of Chemical Engineering, ETSEIB, Universitat Polite` cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain
The kinetics of the weight loss for the thermal decomposition of five different types of waste wood (forest wood, wood from demolition of buildings, dismantled slot machines, old furniture, and used or broken pallets) was investigated by dynamic and isothermal experiments in a thermogravimetric analysis (TGA) apparatus. Isothermal runs were carried out at two temperature levels, one between 225 and 325 °C (low range) and the other between 700 and 900 °C (high range). Results from both kind of experiments demonstrate that the weight loss of these materials is characterized by two main stages: cracking of cellulose and lignin and cracking of residual organic compounds. Kinetic parameters for each type of waste wood are presented. It must be remarked that the individual chemical composition of each type of wood together with the compounds added to the wood for each application play a fundamental role in the kinetic behavior of their thermal decomposition. Introduction In the past decades, special attention has been paid to the exploitation of renewable energy resources and also to the minimization and treatment of waste. Biomass, which is rich in cellulose and other organic compounds with a high heating value, has been considered as an inexhaustible source for the production of energy and chemical products. On the other hand, human activities produce a huge amount of residua, whose treatment has been converted lately into a subject of interest for many countries. In this framework, new technologies are needed to minimize residua and, at the same time, recover the remaining energy. Obviously, these new technologies must respect strict environment regulations. Wood and cellulose-rich materials are an important part of the solid waste generated by human activities. Wood is extensively used as a building material (i.e., furniture, machinery, houses, and pellets), which at the end of its life cycle will be rejected and considered as a waste material. Other source of cellulosic waste are the cleaning and pruning of forests and gardens. Traditionally, these residua were burned in a fireplace or in the open country. Combustion, pyrolysis, and gasificationold industrial processes for which more efficient and cleaner technologies are under development-can be considered as ways to recover energy from waste wood. The understanding of the pyrolysis process is necessary, not only for the pyrolysis reactor design, but also for the design of wood gasifiers. From the point of view of its chemical composition, wood is a complex substance that contains essentially cellulose, polysaccharides, and lignin. The main components are cellulose, lignin, and water. According to Ko¨nig,1 the cellulose content oscillates between 40 and 60%, and the content of lignin varies between 21 and 30%. Other important components of wood are pentosanes and hexosanes. There is a remarkable difference in composition between the wood generated by * Author to whom correspondence should be addressed. E-mail:
[email protected].
different species. Coniferous or soft woods contain up to 20% pentosanes and hexosanes and leafy or hard woods contain between 20 and 30%. Because of this chemical complexity, the thermal decomposition of ligno-cellulosic materials is a complex process, where a high number of different reactions take place. Cracking, devolatilization, and depolymerization produce a number of gases and condensable products and a solid residue with a high degree of carbonization (charcoal). This complexity has led researchers to study each wood constituent separately.2-5 Cellulose is the most studied wood constituent6 because of its more simple composition and because it is the major constituent of ligno-cellulosic materials. Several researchers2,7-9 support the theory that cellulose and ligno-cellulosic materials thermally decompose in two stages. The main weight loss is attributed to the release of volatile material during the degradation and also to the decomposition to tar or coal. Stamm,10 Browne and Tang,11 Roberts,12 Shafizadeh,9,13,14 and Thurner and Man15 studied kinetics and reaction mechanisms for the pyrolysis of wood and ligno-cellulosic substances. Kinetic data reported in the literature for the pyrolysis of wood are summarized in Table 1. Important differences can be observed in the reported results. These discrepancies can be explained by the differences between each study; that is, experimental equipment, experimental methodology (isothermal or dynamic experiments), data reduction, experimental conditions, physical properties of the material, and chemical composition of the wood. It has been frequently observed that the activation energy for the overall pyrolysis of wood tends to group around discrete values. Roberts12 reported two different groups for the activation energy, one around 125 kJ/ mol and the other around 235 kJ/mol. Antal et al.2 grouped the values reported in the literature for the activation energy in three groups. The first group, between 210 and 250 kJ/mol, corresponds to experimental values in which the thermocouple was outside of the sample. For the second group, between 138 and 210 kJ/mol, the thermocouple was in contact with the gas.
10.1021/ie980083g CCC: $15.00 © 1998 American Chemical Society Published on Web 10/02/1998
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4291 Table 1. Reported Kinetic Data for Wood Pyrolysis author [ref]
range of temperature (°C)
biomass
Stamm [10] Stamm [10] Roberts and Clough [16] Brink [17] Brink [17] Thuner and Mann [15] Browne and Tang [11]
pine fir sawdust beech tree wood sawdust wood sawdust oak wood
reaction order (n)
energy of activation (kJ/mol)
preexponential factor (s-1)
1 1
123.3 108.7
5.1 × 1011 1.9 × 109
>282
1
62.7
1.51 × 103
275 °C, the activation energy is 135.76 kJ/mol, whereas for lower temperatures, the activation energy is 23.94 kJ/mol. This behavior could be explained by the presence of components that accompany the sample under study; for example, lacquer, glazes, paintings, and other organic compounds. The decomposition temperature of such substances is 0.85. Conclusions The pyrolysis of five different types of waste wood was investigated, to determine the basic kinetic parameters
Nomenclature A ) preexponential factor (s-1) E ) activation energy (kJ/mol) k ) kinetic constant (s-1) n ) reaction order R ) constant of gases t ) time (s) T ) temperature (K) X ) degree of conversion Wo ) weight of the sample at time to (mg) W ) weight of the sample at time t (mg) W∞ ) weight of the sample at time t∞ (mg) β ) heating rate (°C/min)
Acknowledgment This work was financed in part by the Junta de Residus (Generalitat de Catalunya, Project C-2364). The support of the European Commission (Project No. 7220ED-081) is also thankfully acknowledged. J.R. acknowledges the financial support from the Instituto de Cooperacion Iberoamericana. Literature Cited (1) Ko¨nig, B. Die Bestandteile des Holzes und ihre wirtshaftliche Verwertung, Mu¨nster (Westfalia), 1918. Z. Angew. Chem. 1919, 32, 155. (2) Antal, M. J.; Friedman, H.; Roger, F. Kinetic of Cellulose Pyrolysis in Nitrogen and Steam. Combust. Sci. Technol. 1980, 21, 141. (3) Bilbao, R.; Arauzo, J.; Millera, A. Kinectis of Thermal Decomposition of Cellulose. Part I. Influence of Experimental Conditions. Thermochim. Acta 1987, 120, 121-131. (4) Bilbao, R.; Millera, A.; Arauzo, J. Thermal Decomposition of Lignocellulosic Materials: Influence of the Chemical Composition. Thermochim. Acta 1989, 143, 149-159. (5) Milosavljevic, I.; Suuberg, E. Cellulose Thermal Decomposition Kinetics: Global Mass Loss Kinetics. Ind. Eng. Chem. Res. 1995, 34, 1081-1091. (6) Antal, M. J.; Varhegyi, G. Cellulose Pyrolysis Kinetics: The Current State of Knowledge. Ind. Eng. Chem. Res. 1995, 34, 703717. (7) Kilzer, F.; Broido, A. Speculation on the Nature of Cellulose Pyrolysis. Pyrodynamics 1965, 2, 151-163. (8) Arseneau, D. F. Competitive Reactions in the Thermal Decomposition of Cellulose. Can. J. Chem. 1971, 49 (4), 632-638.
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4295 (9) Shafizadeh, F. Pyrolytic Reactions and Products of Biomass. In Fundamentals of Thermochemical Biomass Conversion; Overend, R., Milne, T., Mudge, L., Eds.; Elsevier: New York, 1985; p 183ff. (10) Stamm, A. J. Thermal Degradation of Wood and Cellulose. Ind. Eng. Chem. 1956, 48, 13. (11) Browne, F. L.; Tang, W. K. Effects of Various Chemicals on TGA of Ponderosa Pine. Forest Products Laboratory Paper 6, Madison, WI, 1963. (12) Roberts, A. F. A Review of Kinetic Data for the Pyrolysis of Wood and Related Substances. Combust. Flame 1970, 18, 261. (13) Shafizadeh, F. Industrial Pyrolysis of Cellulosic Materials. Appl. Polym. Symp. 1975, 28, 153. (14) Shafizadeh, F.; Bradbury, A. G. Thermal Degradation of Cellulose in Air and Nitrogen at Low Temperatures. J. Appl. Polym. Sci. 1979, 23, 1431.
(15) Thuner, F.; Mann, U. Kinectic Investigation of Wood Pyrolysis. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 3. (16) Roberts, A. F.; Glough, G. Thermal Decomposition of Wood in an Inert Atmosphere. 9th Symposium (International) on Combustion; Academic: New York, 1963. (17) Brink, D. L.; Massoudi, M. S. A flow reactor technique for the study of Wood pyrolysis. J. Fire Flammability 1978, 9, 176182. (18) Liou, T. H.; Chang, F. W.; Lo, J. J. Pyrolysis of AcidLeached Rice Husk. Ind. Chem. Res. 1997, 36, 568.
Received for review February 10, 1998 Revised manuscript received July 14, 1998 Accepted July 20, 1998 IE980083G
