A volumetric reaction model for the combustion of moist wood exposed to moderate thermal irradiances

Abstract

An unsteady one-dimensional solid-phase model is formulated and experimentally validated for the glowing combustion of moist wood. Global reactions are considered for wood decomposition and char combustion, with volumetric rates, and a kinetic rate for moisture evaporation. Good agreement is found between the predicted and measured surface temperatures and mass fluxes for thick specimens and/or high heat fluxes. The effects of the moisture content (0–0.75 mass fractions on dry basis), specimen thickness (10–40 mm) and heat flux (18–50 kW/m²) are studied. The cases of combustion, athermic combustion (Δh_c = 0) and pyrolysis are compared. Combustion may cause a pseudo steady state (simultaneous presence of the various zones propagating with roughly constant rates), comprised between short initial and final transients. The conversion time is mainly affected by the reaction heat with minor effects from surface regression. In fact, the characteristic times of athermic combustion and pyrolysis are very close. Compared with pyrolysis, the combustion heat also mitigates the influences of moisture evaporation on the conversion dynamics. The existence and duration of the pseudo steady state are enhanced by high values of specimen thickness and moisture content and external heat fluxes.