The Simultaneous Drying and Pyrolysis of Single Wood Particles and Pellets Made of Peat

The simultaneous drying and pyrolysis of single wood particles and pellets made of biomass are studied both theoretically and experimentally. The combined radiation and convection boundary condition is used in the theoretical model for the particle surface. The model takes into account the transient heat conduction and the effect of the flow of steam and pyrolysis products inside the particle and makes allowances to varying heats of pyrolysis. The local heat conductivity changes with time due to disappearence of matter in the course of drying and pyrolysis. In the model it is assumed that, instead of the Arrhenius type rate equation, the rate of the pyrolysis inside the particle is controlled by the diffusion of heat. Then the local density in the particle is uniquely defined by the local temperature. This dependence of (equilibrium) density on the temperature is found experimentally for different materials by using the thermogravimetric method. The effect of the shape and the initial moisture content of the particle on the rate of the pyrolysis is examined. It is found that by keeping the particle volume/surface area ratio constant, the transient mass/initial mass curves for particles of varying shape nearly coincide. The model is also applicable for particles made of other material with high volatile content