Investigation of thermal degradation of three wood species as initial step in combustion of biomass

Abstract

In this work, the thermal degradation of three different wood species (hornbeam, walnut, and scots pine) is investigated by the analysis of thermogravimetric/mass spectrometric pyrolysis data. The knowledge of the kinetics of the devolatilization of wood is essential because the evolution of gaseous products affects all consecutive steps in combustion, and pyrolysis is always the first step in any gasification or combustion process. A suitable set of overall kinetic parameters (k0, Ea, and n) for wood pyrolysis is important for plant design of technical applications. An overall model for the wood pyrolysis incorporating the separate decomposition kinetics of the main components (lignin, cellulose, and hemicellulose) is presented. Inorganic salts show a large effect on pyrolysis: therefore, washing wood with water is essential for the determination of meaningful formal kinetic parameters. Formal kinetic parameters (k0, Ea, and n) for the hemicellulose, cellulose, and lignin degradation are presented. The lignin degradation is studied by monitoring specific ion fragments from lignin degradation products. These ion fragments were determined from mass spectra obtained from fractionated pyrolysis under dynamic conditions followed by gas chromatography/mass spectrometry. It has been found that differences in wood species only results in different degradation rates for the lignin and in the early stages of the hemicellulose degradation, whereas the cellulose degradation and the degradation of the hemicellulose that decomposes at higher temperatures are similar. These differences can be explained by a different chemical composition of these macrocomponents for deciduous and coniferous trees. With the presented set of parameters, it is possible to describe the pyrolysis of the three wood species.