Effect of moisture content on thermal decomposition and autoignition of wood under power-law thermal radiation

Abstract

Pyrolysis and autoignition of beech wood, with moisture content (MC) from 0% to 38%, exposed to power-law thermal radiation is investigated. An experimental facility capable of irradiating time-varying heat flux was utilized to conduct bench-scale tests. An analytical model was proposed to estimate the ignition time, surface and in-depth temperatures. Meanwhile, a transient numerical solver, FireFOAM, was employed to simulate the experimental measurements. The results show that surface and in-depth temperatures rise more quickly with larger heat flux and lower MC. Although the analytical model overestimates surface temperature slightly due to the ignoring of pyrolysis, both the analytical and numerical predictions are at acceptable levels. In-depth temperatures cannot be accurately predicted due to the recondensation of the migrated water vapor beneath the evaporation layer. No substantial discrepancy is found between the 25% and 38% MC surface temperatures since significant cracks emerge on surface upon heating. The average measured ignition temperature is 395 °C which is used as the ignition criterion. Ignition time increases with lower heat flux and larger MC when MC < 25%, but no further increase is observed for MC > 25% due to the crack effect. The predicted ignition times match the experimental results relatively well despite some minor deviations.