The sulfuric acid catalyzed pyrolysis of beech wood particles packed in a cylindrical bed, rapidly heated along the lateral surface, is carried out with special emphasis on the effects of the impregnated catalyst on the reaction heats (acid contents in wood between 0 and 11%). The complexity of the conversion process is augmented by the evaporation of the residual water not removed during the drying stage, owing to the formation of the sulfuric acid/water azeotrope and the endothermicity of acid decomposition. However, the overall exothermicity of wood decomposition is highly increased with maximum temperature overshoots, with respect to the heating temperature, that increase from 28 to 200 K and give rise, under appropriate conditions, to thermal runaway. The strong exothermicity of the decomposition process is attributable to the successive enhancement of dehydration, condensation and cross-linking reactions (char, water, and carbon dioxide formation) at the expense of depolymerization reactions (condensable organics formation). The changes in the depolymerization paths from low to intermediate acid concentrations, associated with a shift from levoglucosan formation to levoglucosenone, 1,4:3,6-dianhydro-alpha-D-glucopyranose, and furfural formation, do not appear to play a significant role in the process exothermicity.