Coal is one of the most popular sources of energy. However, it has a relatively low energy efficiency due to high humidity and a greater release of harmful substances during combustion. On the other hand, the coal reserve on earth is estimated at 500 years and the cost is relatively low. This causes the search for new ways of processing coal. One way to efficiently process coal, while reducing the humidity and content of harmful components is microwave treatment. The basic information for microwave exposure to coal is the temperature field. In this paper, an approximate-analytically nonlinear mathematical model for heating a flat coal mass is studied, provided that the absorbed microwave energy is removing by heat radiation and convection simultaneously.