A simplified model for steady, isothermal diffusion in the layer surrounding a spherical carbon particle undergoing steam gasification process is considered by way of the classical theory of reacting fluid mixtures. The model accounts for the steam – carbon reaction and the steam – carbon monoxide reaction. These two reactions appear in the model as linear boundary conditions that comprise the intraparticle effects, which are lumped at the surface. The ambient conditions are assumed to be constant. Differential equations for the calculation of steam and heat rate consumption, carbon monoxide, carbon dioxide and hydrogen rate generation and rate of gasification are also included. By means of digital simulation it was possible to study the effects of particle radius, temperature and steam concentration on the process. The kinect equations used in this paper are those of coal from Charqueadas, State of Rio Grande do Sul, Brasil.