Role of gas-phase reaction and gas-solid thermal nonequilibrium in reverse combustion

The process of adiabatic reverse combustion when air is supplied through a packed bed of wood particles is analyzed in the oxygen-limited regime when both the gas-phase homogeneous reaction and the gas-solid surface reaction are significant. This is an extension of an earlier study where the gas-solid surface reaction was assumed dominant and the local thermal equilibrium was assumed between the gas and solid phases (i.e. the single-medium model). Here allowance is made for the local thermal and chemical nonequilibria between the phases (i.e. the two-medium model) and the model is used to describe the propagation of the reaction front through the fuel bed. The role of the air pore velocity and the homogeneous reaction rate in the front speed, the adiabatic final temperature, the degree of solid consumption and the unburnt volatiles concentration in the post gas-phase oxidation region, are examined parametrically. The volumetric solid-phase pyrolysis is considered to be the mechanism producing the volatile fuel. First-order Arrhenius kinetics are used in the single-step reaction models. It is found that the gas-phase reaction becomes dominant over the heterogeneous reaction at high air pore velocities, thus reducing the degree of solid consumption. These predictions are in agreement with the earlier experimental results and the predicted results of the single-medium treatment. (C) 1997 Elsevier Science Ltd.