A two-dimensional, variable property, mathematical model of the steady smoldering combustion of a porous solid material is presented. Chemical processes account for the oxidation reaction between the carbonaceous residue and atmospheric oxygen, and the amount of carbonaceous residue accompanying the oxidation. Physical processes are described by energy and mass balances for the porous medium and the highly thermally conductive insert material. The combustion front presents a multidimensional structure, affected by the oxygen transport to the combustion zone, the physical properties and the size of the thermal insert. As the radius of the thermal insert increases the peak combustion temperature decreases and the shape of the combustion front shrinks.