Charring pyrolysis of wood in fires by laser simulation

Abstract

Utilizing a 250 watt CO 2 -laser radiation source, wood pyrolysis at fire-level surface heat flux was investigated through the measurement of decomposition rates, solid temperatures and thermal properties, pyrolysis gas compositions and pressures, and crack formation. Heats of reaction calculated from these measurements show that at an incident heat flux of 0.76 cal/cm 2 -sec applied parallel to the wood grain direction, the pyrolysis layer (≈1cm thick) can be divided into three zones: (i) an endothermic primary decomposition zone at temperatures T <250°C, (ii) an exothermic partial char zone at 250°C< T <340°C, and (iii) an endothermic surface char zone at 340°C< T <520°C. The overall mass weighted effective heat of reaction is endothermic to the extent of −146 cal/g. At 2.0 cal/cm 2 -sec applied parallel or perpendicular to grain direction, pyrolysis reaction proceeds as a charring wave of ≈0.5 cm thickness advancing into the wood. The charring wave (250°C< T <450°C) is characterized by active overall exothermic reactions accompanied by rapid solid density change. In front of the wave is an endothermic primary decomposition zone, and behind the wave is an exothermic char layer ( T <800°C) of relatively constant density. The overall heat of reaction at the higher heat flux is exothermic, being greater for perpendicular heating (260–410 cal/g) than for parallel heating (25–94 cal/g). For parallel heating at both heat fluxes, macroscopic cracks (≈1mm wide) occur in the pyrolysis region, whereas for perpendicular heating no cracks are observed. Present results show that the wood pyrolysis process is dependent upon the external heating rate, the total time of heating, and the anisotropic properties of wood and char relative to the internal flow of heat and gas.