During the process of gasifying thin coal layers underground. an open channel structure might exist. In this channel, chemical reactions will have an effect on both the temperature and the concentration distribution. The resulting temperature and concentration gradients give rise to opposing buoyant forces. The resulting flow field determines the heat and mass transfer and the product gas composition. Results for the two-dimensional numerical prediction of the flow field and the product gas composition are given for a number of situations. The amount of air injected as well as the radiation extinction coefficient of the gas are varied. The predictions show that the process will produce useful gases for certain air inlets. The solution of the equations is time dependent for some situations : its periodicity is presented and discussed in this paper.