A Kinetic Study of the Nonisothermal Pyrolysis of Wood

Abstract

Results of a kinetic study of the pyrolysis of woody biomass (Quercus robur) under conditions of continuous heating to a temperature of 873 K at a constant rate of 1.25, 2.5, 5, and 10 K/min have been discussed. An integral method has been used to describe the reaction mechanism and determine the macrokinetic parameters. It has been found that, from a phenomenological point of view, the averaged woody biomass pyrolysis reaction under test conditions corresponds to a three-dimensional diffusion model (1.25 K/min), a model described by the third-order reaction equation (2.5, 5 K/min), and a one-dimensional diffusion model (10 K/min). In this case, the relative standard deviation of the conversion values calculated using this equation from the test data is <11.2%. The division of the averaged reaction into three stages (first stage is completed at a temperature of 390 K; the second, at 579 K; the third, at the completion of the conversion process) leads to agreement between the calculated degree of degradation of the studied biomass samples and the test values in a range of the degree of degradation of 0–1. Although the relative standard deviation does not exceed 3.5%, the division of the averaged reaction into stages does not exclude discrepancies in the values of the determined macrokinetic parameters. It has been shown that the choice of the model that provides the best description of the wood conversion process depends on heating rate during the tests. Owing to this dependence, the macrokinetic parameter values significantly differ from each other. Taking this fact into account, it can be concluded that the selected models and calculated macrokinetic parameters are of a formal nature and cannot be thought of as physicochemical characteristics that are universal with respect to the subject of research. Discrepancies in calculations of macrokinetic parameters that are caused by the effect of heating rate can be eliminated by studying the kinetics of conversion under isothermal conditions (at a constant temperature).