The effects of diffusional resistance on wood char gasification

The steam gasification kinetics of wood char is studied. First, the kinetic expression of wood char gasification is derived (kinetically controlled). The modelling, uses the random pore model, has been experimentally validated. The char gasification study is performed in a tubular kiln, at various temperatures (850 °C, 900 °C, 950 °C and 1000 °C) and vapour pressures (0.2 bar, 0.5 bar and 0.7 bar). The steam gasification kinetics parameters (activation energy…), together with the physical properties of the char (porosity, specific surface…) are experimentally determined. By performing experiments with different particle sizes, steam flow rates and initial sample weights, the experimental conditions which permit the minimization of the steam penetration limitation, are found. Secondly, a 1D isothermal and non-equimolar reaction-diffusion model is developed in order to assess the diffusional effects taking place during steam gasification in a fixed bed. The model takes into account local chemical reaction rate and effective transport properties dependant on time and position within the char bed. Inter particle diffusion and structural changes of the char bed during the gasification are taken into account. The model shows a good agreement with the experimental results, obtained at various bed heights, and point out the relevant role played by diffusional effects within a fixed bed reactor. Finally, the kinetic results are used to estimate, in a down draft gasifier, the total carbon yield conversion as a function of both the mass fraction of stagnant zones within the bed char and of the characteristic length of these stagnant zones.