A numerical model is presented, designed to simulate the kinetic and thermal behaviour of a porous pellet in which any gas-solid reaction is taking place. Its novelty consists in the fact that it can deal with reactions whether they are exothermic or endothermic, whether they are equimolar or not, whether they are reversible or irreversible and further reactions in the transient regime and even the possible presence of inert gases and solids can be treated. The numerical scheme is based on the finite volume method in an implicit formulation, with a specific treatment of the thermal source term for strongly exothermic reactions. The model was validated by comparison with analytical and numerical solutions from the literature and was used to simulate the exothermic reaction involved in the oxidation of zinc sulphide.