Heating Process Characteristics and Kinetics of Biomass at Different Oxygen Concentrations

Mass transfer is very important for the combustion of biomass fuels, especially pellets. The oxygen diffusion, which results in a nonuniform distribution of oxygen in the pellets, should be considered in the modeling of pellet combustion. In this study, the effect of oxygen on thermal degradation of biomass is studied by thermogravimetric (TG) experiments and then a kinetic model considering oxygen effect is developed. The fuels investigated are spruce, sophora, wheat straw, and peanut straw. TG curves are taken under different oxygen concentrations, ranging from 0 to 20 %. Oxygen concentration has little effect on the devolatilization but is critical for the behavior of char combustion. Reaction mechanism is proposed based on the observation of decomposition at different reaction conditions. The kinetic model consists of three devolatilization reactions ( for cellulose, hemicellulose, and lignin) and one char combustion reaction. In the devolatilization steps, char and gas are formed as products and oxygen is not involved in the reactions. A power law dependence of oxygen is assumed for the char combustion stage. The model parameters are fitted by the experimental TG data and good agreement is observed between the experimental and modeled data at various oxygen concentrations and for different biomass fuels. It is expected that the developed kinetic model could be applied for the modeling of pellets combustion considering oxygen diffusion process.