Secondary Reactions and the Heat of Pyrolysis of Wood

Abstract

Since its first estimation in 1909, the heat of pyrolysis of biomass has been found to range from endothermic to exothermic, with the latter attributed to secondary reactions that condense secondary char. Herein, the heats of primary and secondary reaction for pine are isolated using a new method to account for the radiative heat loss discrepancy between the heating cycles of wood and char in open crucibles. The overall heat of pyrolysis is then explored as a function of the tortuous path length as volatiles escape from wood particles and the generated internal pressure which together drive the extent of the secondary reactions. Furthermore, two generalized schemes for these secondary reactions are explored, where heterogeneous and homogeneous secondary reactions either compete for the primary reactive volatiles, or apply only to dedicated fractions of them. Of the schemes, the one with dedicated fractions undergoing heterogeneous and homogeneous reactions best explains the results presented here. This work is useful to designers of biomass pyrolysis reactors, where the extent of reaction and thus the overall heat of pyrolysis can be estimated as a function of the nominal particle size and system pressure.