Mechanisms and kinetics of homogeneous secondary reactions of tar from continuous pyrolysis of wood chips

Abstract

The change of mass and composition of biomass tar due to homogeneous secondary reactions was experimentally studied by means of a lab reactor system that allows the spatially separated production and conversion of biomass tar. A tarry pyrolysis gas was continuously produced by pyrolysis of wood chips (fir and spruce, 10–40 mm diameter) under fixed-bed biomass gasification conditions. Homogeneous secondary tar reactions without the external supply of oxidising agents were studied in a tubular flow reactor operated at temperatures from 500 to 1000 °C and with space times below 0.2 s. Extensive chemical analysis of wet chemical tar samples provided quantitative data about the mass and composition of biomass tar during homogeneous conversion. These data were used to study the kinetics of the conversion of gravimetric tar and the formation of PAH compounds, like naphthalene. It is shown that, under the reaction conditions chosen for the experiments, homogeneous secondary tar reactions become important at temperatures higher than 650 °C, which is indicated by the increasing concentrations of the gases CO, CH 4, and H 2 in the pyrolysis gas. The gravimetric tar yield decreases with increasing reactor temperatures during homogeneous tar conversion. The highest conversion reached in the experiments was 88% at a reference temperature of 990 °C and isothermal space time of 0.12 s. Hydrogen is a good indicator for reactions that convert the primary tar into aromatics, especially PAH. Soot appears to be a major product from homogeneous secondary tar reactions.