Thermal runaway in the pyrolysis of some lignocellulosic biomasses

The thermal decomposition of a cylindrical fixed bed consisting of agricultural residues (hazelnut shells, olive pomace, straw pellets) or softwood pellets, uniformly heated along the lateral surface, is investigated for heating temperatures in the range 473-800 K, and a comparison is made with results previously obtained for beech wood pellets. Although dependent on the external heating conditions, exothermic reaction heat effects are evident for all the biomasses, giving rise to maximum temperature overshoots of 225 K (hazelnut shells), 170 K (olive pomace), 78 K (straw), and 53 K (softwood pellets) (versus 86 K for beech wood pellets). For the first two materials and mild/moderate heating conditions, the entire bed volume experiences large temperature overshoots, so that the qualitative features of the pyrolytic conversion are those of a thermal runaway. Explanations are given, based on the different chemical and physical properties of the samples, for the different exothermicity level, and its implications in the practical application of torrefaction and pyrolysis are discussed.