A general formulation based on a weighting average procedure is developed for describing the fire-induced behaviour of a multiphase, reactive and radiative medium. The complete set of the resulting equations should be used as the basic one for later studies, especially in the framework of wildland fires. For the moment, in order to demonstrate the capability of the general formulation, a simplified model, named zeroth-order model, in which some physical phenomena (such as char combustion, second-order terms, particle motion) are neglected is presented. In the frame of this simplified model, reverse and forward one-dimensional fire propagations through an heterogeneous medium composed of fixed fuel particles are studied numerically.