The problem of the diffusion of the oxidizing agent to the reaction walls of a channel is of major theoretical and practical interest for the combustion and gasification of coal. This problem has been treated previously. In this study, approximate formulas for calculating the amount of oxidizing agent reaching the reaction surface of a cylindrical coal channel and the distribution of the oxygen concentration within the channel are given. The formulas are convenient for analyzing the influence of certain factors on underground gasification of coal in a channel.