Influence of intraparticle secondary heterogeneous reactions on the reaction enthalpy of wood pyrolysis

•Pyrolysis of a wood particle is investigated by means of Laser-Induced Fluorescence.•Reaction heat is correlated with the total fluorescence intensity of volatiles.•Char and PAH are produced when heterogeneous secondary reactions are promoted.•These secondary reactions lead to a more exothermic global heat of reaction.

Despite the technical relevance of wood pyrolysis the reaction enthalpy of this conversion step remains a matter of controversy. In this work the slow pyrolysis of dry spherical beech wood particles with 25 mm diameter was investigated by means of Laser-Induced Fluorescence of the evolving volatiles together with macroscopic thermogravimetric analysis in a particle cell. It was observed that the reaction heat is correlated with the ratio of total fluorescence intensity versus conversion rate. In exothermic regimes of pyrolysis this ratio changes significantly, while it is relatively constant in endothermic regimes. Taking into account the results of differential scanning calorimetry and the char yields at different sample sizes the results of this study can be interpreted such that when heterogeneous secondary reactions of the pyrolysis volatiles are promoted, the production of char is enhanced together with the production of polyaromatic hydrocarbons, leading to a more exothermic global heat of reaction.