An expression for the propagation velocity of a reaction front in a porous solid material is obtained by using a one-dimensional energy conservation equation. The reaction is divided in two distinct fronts.an ignition front where gasification of large part of the fuel occursand a flamingfront where gas phase oxidation takes place. Forced flow and reaction move in opposite directions, so, from a frame fixed to the front, fuel and air will enter the reaction from one side and products will leave through the other side. The propagation velocity obtained from this analysis is applicable to fuels with a high fraction of volatile materials, such as those characteristic of municipal waste products. The analysis is divided in two parts, the calculation of the propagation velocity and the determination of a critical ignition temperature. For the first part, heat transfer towards the virgin fuel, by convection, conduction and radiation, controls the propagation of the reaction front and an expression for the propagation velocity is obtained as a function of a characteristic ignition temperature. For a control volume at the ignition front the Semenov thermal explosion theory is used to determine the characteristic ignition temperature. The expressions are correlated with experimental data obtained from burning solid mixtures that are characteristic of municipal waste products. The materials used are, therefore, highly heterogeneous in both their composition and geometrical characteristics. Experiments were conducted with cardboard, wood and a mixture of cardboard, wood and plastic in a counter-flow packed bed reactor and it was observed that the results correlated well with the theoretical expressions. The good agreement of a fundamental theory and experiments conducted with materials of practical importance, not only clarifies the mechanisms controlling this type of combustion but also contribute to determine the validity of such an approach when applied to heterogeneous materials.