A Study of the Ignition Mechanism for Dead Pinus Palustris Needles

Abstract

Combinations of cumulative impacts of drought, invasive species, climate variability, and ever-expanding wildland-urban interface make landscapes more susceptible to devastating wildland fires. To treat the increasing risks of wildland fires, one of the ways is to mitigate the risk of ignition, which requires a solid understanding of the ignition mechanism of vegetation fuels. This can be achieved mainly through discerning the degradation stage and the ignition criteria. Scaling up the experiments for the degradation stage and evaluating the suitability of existing ignition criteria are two of the primary challenges for ignition studies on vegetation fuels. Motivated by the challenges, two series of experiments were conducted using a modified Cone Calorimeter to understand the mechanisms driving the ignition of dead Pinus palustris needles. In the first set of experiments, the ignition of pine needles was studied for varied incident heat fluxes (20-35 kW/m2) and air flow rates (buoyancy-induced - 100 l/min forced flow). In the second set of experiments, Fourier transform infrared (FTIR) spectroscopy was used to characterize the composition of the pyrolysis gases generated from the thermal degradation of pine needles when exposed to various incident heat fluxes (20-35 kW/m2) under an inert atmosphere obtained using a flow of pure nitrogen (50-100 l/min). The results for the first series of experiments show that critical mass loss rate at ignition increase with both flow rates and heat flux, while the heat release rate at ignition was only influenced by the flow conditions. From the second set of experiments, it was found that methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and water vapor (H2O) are the main constituents of the pyrolysis gases. The predominance of these compounds was found to be independent of the external heat flux while their individual concentrations are sensitive to it. The flammability of pyrolysis gas was found to increase with external heat flux. The average content of flammable species, CH4 and CO, in the pyrolysis gas account for more than 31% at 20 kW/m2 and more than 40% at 30 kW/m2.