A general model is developed to describe the mass transport and heterogeneous chemistry which occurs during the gasification of porous coal char. The pore branching sequence depicts that each pore that reaches the exterior surface of the particle is the trunk of a tree. The gasification rate per pore tree is obtained analytically. The effects of Knudsen diffusion, continuum diffusion, and both adsorption and desorption kinetics are included. Each physical mechanism may limit the gasification rate within some range of pore size, particle size, gas temperature and gas pressure. The gasification rate per pore tree is integrated over a pore distribution function to determine the total gasification rate. In general, no single mechanism controls the entire gasification process. However, the known results of existing models are recovered in the extreme limits of kinetic and diffusion controlled gasification.