Pyrolysis and combustion behaviors of densified wood

Abstract

Densified wood (DW) is a promising functional material with excellent mechanical performance, whose applications are restricted by fire safety problem. The key to solving this problem is to understand the effects of the components and density on wood thermal decomposition and combustion behaviors while keeping other influencing properties constant. In this study, natural wood (NW), lignin modified wood (MW) and DW were comparatively studied by thermogravimetric analysis (TGA) and cone calorimetry tests. MW was prepared by delignification process to reduce the hemicellulose and lignin contents, while DW was obtained by further densification treatment which increased wood density. TGA pyrolysis showed that MW and DW presented enhanced thermal stability as they had a 7% increase in char yield and decreased peak mass loss rates. A 0D numerical model including four parallel reactions was employed to simulate the pyrolysis behaviors. Particle swarm optimization algorithm was applied to exact the pyrolysis kinetics and stoichiometric coefficients. Two different sets of kinetics were obtained for NW and MW (or DW) since the delignification changed the pyrolysis evolutions. The model predictions well agreed with the measurements for all wood samples. Results from cone calorimetry tests showed that both delignification and densification delayed ignition time. DW formed a compact char layer, which slowed the heat release rate (HRR) decline after the second HRR peak. Wood component contents and density had negligible impacts on characteristic HRR and heat of combustion. Analytical study was performed to obtain thermal inertia and critical heat flux. NW and MW were treated as thermally thin material, while DW spanned the thermally intermediate and thick regimes. DW had the highest thermal inertia due to its dense structure. The critical heat flux of DW and MW were close and higher than that of NW, which could be explained by their increased thermal stability.