The objective of this paper is to present a numerical simulation method to calculate the gasification of porous particles of different sizes and composition which takes into account morphological changes of the inner pore structure. Different relationships for the evolution of the inner surface dependent on the degree conversion are assessed. In a first approach the gasification mechanism is represented by a one-step reaction. The method operates on the one-dimensional and transient conservation equation for mass and energy for spherical particles with external heat and mass transfer. The solution yields the distribution of temperature, inner surface, porosity, concentration of solid material and gaseous components versus radius and time. One of the main novelties of this approach is that it covers the entire reaction regime between reacting and shrinking core model. Satisfactory agreement is obtained between measured and calculated results.