  We present a generalized kinetic model for gas-solid heterogeneous reactions
taking place at the interface between two phases. The model studies the
reaction kinetics by taking into account the reactions at the interface, as
well as the transport process within the product layer. The standard unreacted
shrinking core model relies on the assumption of quasi-static diffusion that
results in a steady-state concentration profile of gas reactant in the product
layer. By relaxing this assumption and resolving the entire problem, general
solutions can be obtained for reaction kinetics, including the reaction front
velocity and the conversion (volume fraction of reacted solid). The unreacted
shrinking core model is shown to be accurate and in agreement with the
generalized model for slow reaction (or fast diffusion), low concentration of
gas reactant, and small solid size. Otherwise, a generalized kinetic model
should be used.
