This paper is to suggest a simple flame model for the analysis of an internal flame of rotary kilns and to present the application cases. Reaction rates in the multi combustion stages of the selected solid fuel were calculated considering the reaction rates with the Arrhenius type equations. In addition, primary and secondary air flow arrangement were considered. As a simple application case, the combustion trends according to the different solid fuels were described. Improved operating conditions as related with the fuel characteristics were shown to be important for the stable combustion characteristics and the performance of the reactors as defined by the exit temperature of the solid materials.