Effect of porosity on ignition and burning behavior of cellulose materials

Porosity is one of the important factors to determine the fuel efficiency and the risk of forest combustible fire. In this work, we explore the porosity effect on the ignition phenomena, autoignition limit, and extinction of flaming combustion. The ignition experiment of pine needle and wood powder fuel bed with various porosity was conducted under external radiation at a constant heat flux of 5-60 kW/m2. Several ignition phenomena including the cone-piloted flaming ignition, smouldering, smouldering to flaming transition, and no ignition can be obtained. And the pine needle could be burnt out in the flaming state, but the wood powder was easily ignited and presented a short flaming period, such as only 10% mass loss in flaming combustion and 70 -80% mass loss in smouldering combustion for an 84.67% porosity fuel under the radiant heat flux of 35 kW/m2. The ignition of a larger porosity fuel bed is more difficult, requiring a higher critical heat flux. The measurements suggested that the porosity was important to determine smouldering to flaming transition, but weak affected the smouldering ignition with the critical surface temperature 350 degrees C to pine needle and 280 degrees C to wood powder. The porosity has a slight influence on the ignition time and critical mass flux, with an order of magnitude lower than incident heat flux. The fuel with low porosity is more likely to be self-extinguished due to limited oxygen supply, and responds to a higher critical mass flux of flaming self-extinguishing. The proposed heat-transfer analysis explains and well predicted the ignition limit and delay time. This research helps understand the development of biomass burners and forest fire, and the interaction between flaming and smouldering fires.