Comparison of semi-global mechanisms for primary pyrolysis of lignocellulosic fuels

Abstract

The predictions of several semi-global mechanisms, which take into account the formation of char, tar and gas, are compared for the primary pyrolysis of cellulose and biomass under chemical reaction control. The cellulose mechanisms examined are: (1) the Broido mechanism with two different sets of kinetic constants (A. Broido, M. Weinstein, Proceedings of the 3rd International Conference on Thermal Analysis, Wiedemann (Ed.), Birkhauser Verlag: Basel, 1971, p. 285; R.K. Agrawal, Can. J. Chem. Eng. 66 (1988) 413), (2) the Shafizadeh mechanism (A.G.W. Bradbury, Y. Sakai, F. Shafizadeh, J. Appl. Polym. Sci. 23 (1979) 3271) and (3) a modified Broido–Shafizadeh mechanism with two sets of kinetic constants (G. Varhegyi, E. Jakab, M.J. Antal, Energy Fuels 8 (1994) 1345). For wood/biomass, a mechanism which combines the contributions of cellulose, hemicellulose (xylan) and lignin is considered, as evaluated in (C.A. Koufopanos, G. Maschio, A. Lucchesi, Can. J. Chem. Eng. 66 (1988) 403), where char and volatiles (tar and gas) formations compete. The widely used three-reaction mechanism (F. Shafizadeh, P.P.S. Chin, ACS Symp. Ser. 43 (1977) 57), leading to the formation of char, tar and gas, is also examined, with three sets of kinetic constants (F. Thurner, U. Mann, Ind. Eng. Chem. Process Des. Dev. 20 (1981) 482; W.R. Chan, M. Kelbon, B.B. Krieger, Fuel 64 (1985) 1505; R. Font, A. Marcilla, E. Verdu, J. Devesa, Ind. Eng. Chem. Res. 29 (1990) 1846). Product yields and conversion times are evaluated as the temperature and the heating rate are varied. Differences among the reaction mechanism are pointed out and suggestions are provided for their correct application in the modelling of thermochemical conversion units.