The combustion of porous carbon particles is a complex process which involves both diffusion through the particle's boundary layer and its internal pore structure as well as a heterogeneous chemical reaction governed by adsorption and desorption. To provide a general basis for the solution of the relevant Thiele/Zeldovich equation of internal diffusion with reaction, we reformulate the two-step Langmuir rate expression in the format of the one-step (n) over tilde th-order relation. The purpose of this treatment is to obtain tractable expressions, which allow adequate prediction of combustion behavior of coal char in flames, with appropriate accuracy.