Drying, Pyrolysis and Combustion of Biomass Particles

Abstract

The simultaneous drying, pyrolysis and combustion of biomass particles are studied both theoretically and experimentally. For large biomass particles, the model takes into account the transient heat conduction and the effect of flow steam and volatiles inside the particle. The thermal properties, for example the local heat conductivity change during the drying and pyrolysis due to disappearence of matter. The rate equation for the generation of volatiles is based on the temperature dependent asymptotic yield of volatiles instead of the usual Arrhenius type rate equation. This temperature dependent asymptotic yield of volatiles or yield of char can be measured with a thermobalance reactor in an inert atmosphere for different biomass fuels. There is the devolatilization stage only in an inert atmosphere. If the atmosphere contains oxygen, the volatiles are combusted around the particle, and there is a feed-back effect between the devolatilization rate and the combustion of volatiles due to the increase in the heat transfer. The drying and the pyrolysis take place locally in successive time periods, but when considering a large particle as a whole, the drying and the devolatilization stages are overlapping. The residual char is combusted after the devolatilization stage.