A Study of the Ignition Mechanism for Dead Pinus Palustris Needles

Gong, Weixuan

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A Study of the Ignition Mechanism for Dead Pinus Palustris Needles

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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 best ways is to prevent them from happening, which requires a solid understanding of the mechanisms driving the ignition of vegetation fuels. This can be achieved mainly through describing the pyrolysis and the ignition processes. For the pyrolysis process of vegetation fuels, there is a shortage of studies on the dynamic chemical evolution of pyrolysis gases. Concerning the ignition process, it is not clear how the critical mass loss rate and the heat release rate per unit area at ignition are influenced by different external conditions. Motivated by these 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, Fourier-transform infrared spectroscopy (FTIR) was used to dynamically characterize the composition of the pyrolysis gases generated from the thermal degradation of pine needles exposed to various incident heat fluxes (20 and 30 kW/m2), and a nitrogen inflow of 50 l/min. In the second set of experiments, the ignition of pine needles was studied for varied incident heat fluxes (20 to 35 kW/m2) and air flow rates (buoyancy-induced, 50 and100 l/min forced flow). The results of the first series of experiments showed that methane (〖CH〗_4), carbon monoxide (CO), carbon dioxide (〖CO〗_2), and water vapor (H_2 O) were the main constituents of the pyrolysis gases. The predominance of these compounds was found to be independent of the external heat flux, while their concentrations were sensitive to it. The heat of combustion of the pyrolysis gas and the pyrolysis reaction rate were found to increase with increasing external heat fluxes. The results from the second series of experiments showed that the critical mass loss rate at ignition increased with both flow rates and heat flux, while the heat release rate per unit area at ignition was only significantly influenced by the flow conditions. The analysis of the results suggests a significant contribution of smoldering combustion to the flaming ignition under low heat fluxes and high airflow rate conditions.

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