In the current work, a model has been successfully developed to describe the production of activated carbons from lignite char by physical activation. The mathematical model is based on the random pore model for the partial gasification undergone in a solid composed of spherical char particles placed in a fixed bed. To provide the necessary experimental input to test the model, three sets of samples have been produced by activation with carbon dioxide at different temperatures and a sequence of times. Data for the adsorption of nitrogen were obtained and analyzed to study the experimental development of the solid porosity in different ranges of pore size. With the initial textural characteristics of the char and only one previously fitted parameter from thermogravimetric analysis, the kinetic constant, the model predicts accurately not only the conversion rate but also the porosity development of the solid to an extent of 60% activation.