Decomposition of Wood Particles in Fluidized Beds

High-temperature biomass pyrolysis is the first step of the thermochemical process taking place in a fluidized bed gasifier; it influences strongly the final product gas composition, specifically hydrogen content, as well as tar (heavy organics) production. In this work, the devolatilization of wood particles of controlled size and the combustion/gasification of the remaining char have been studied, as functions of time, in bubbling nitrogen-, steam-, and air-fluidized beds of sand. The influence of particle size, moderate moisture content, bed temperature, and fluidization severity has been investigated experimentally. A semiempirical model, to account for chemical reaction and mass transfer phenomena, has been developed for the devolatilization step. This considers heat transfer to and through the wood particles, and the global wood conversion process described in terms of a single apparent activation energy and a preexponential factor which varies with biomass size. Despite its extreme simplicity, the model is able to provide a remarkably good description of the empirical behavior over the whole range of particle size (5-20 mm) and bed temperature (560-740 °C) explored in the experimental study; its reliable predictions of the overall particle devolatilization time suggest its applicability over a substantially wider interval of variation of these parameters.