Thermomechanics of the flow through a granular medium with subsequent release of chemical compounds from the grains of solid fuel is addressed. The modified pyrolysis model, based on the functional groups, is developed for investigation of the devolatilisation process of coal particles. The Lattice Boltzmann method (LBM) is applied with additional distribution functions used to model the evolution of temperature and chemical species. First, as a validation case, the gas and tar release rates from a single grain are found to compare reasonably with experimental data and other models known from the literature. Then, the reactive granular medium, composed of coal grains, is described at the level of a representative element of volume. The three-dimensional, meso-scale thermomechanical model is applied, aiming to simulate the first stage of the coking process. Qualitative results for the yield of light gases and tar are reported and discussed. (C) 2017 Elsevier Ltd. All rights reserved.