The aim of this work is to study the structure of the reaction front thickness and its dependence on the air mass flow rate. Thus, a few modifications are introduced in an existing fixed-bed combustor to continuously measure the position of the surface of the bed. In addition, based on the temperature measured by thermocouples, other processes are identified and analysed. The results of this study suggest that the influence of the air mass flow rate on the drying and devolatilisation phases is quite similar, and the thicknesses of the phases are almost constant until the extinction stage is achieved. The behaviour of the char reaction phase is different. Preliminary data suggest that the char thickness is the largest and that it varies with the air supplied. It has been observed that the thinnest reaction front is achieved when char stoichiometry is reached. After that, the reaction front remains invariable until the front is completely quenched. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.