On the Experimental Evidence of Exothermicity in Wood and Biomass Pyrolysis

Reaction-induced overheating during the pyrolysis of lignocellulosic material has been reported by various authors, but it is still one of the less understood aspects of the process. This Review outlines the experimental results for a mixed kinetic-transport control in which exothermicity is displayed clearly. The thermal conditions and the feedstock properties that enhance these events are discussed. Intraparticle activity, at the microscopic level, of homogeneous and heterogeneous reactions of vapor-phase tars is the main cause of the strong exothermicity, which is more significant for hardly porous materials. Pyrolytic (thermal) runaway is also sometimes observed for fixed-bed pyrolyzers. This is initiated by the fast release rate of tarry vapors at low temperatures (in the presence of catalytically active ash and significant contents of extractives and hemicelluloses) over wide spatial extensions. Then, the acceleration of the sample heating rate triggers the degradation of the other chemical components.