Flaming vs. smoldering emissions of pine needles under limited oxygen and fuel moisture conditions

A linear tube heater apparatus was used to simulate the non-ideal burning commonly encountered in real wildland fire scenarios, such as limited oxygen conditions. Recently live and dead vegetative fuels were tested under oxygen concentrations varied between 0 and 21 % with applied external heat fluxes ranging from 25 to 50 kW/m. Utilizing Fourier-transform infrared spectrometry (FTIR) and a cascade impactor, both gaseous and particulate effluents produced during combustion were characterized. The results revealed a significant correlation between emission factors of various species and oxygen concentrations, external heat fluxes and fuel moisture contents. As oxygen concentration increased from 0 % to 10 %, emission factors (EFs) of CO and CO significantly increased, while CO EFs decreased under flaming conditions but slightly increased under smoldering conditions with further increases in oxygen concentration. Emission of unburnt hydrocarbons, predominantly methane (CH) and butane (CH), increased with additional available oxygen due to char oxidation at a lower heat flux, evidenced by an approximately 18 % rise in total hydrocarbon emission factors from 10.71 ± 1.5 to 12.61 ± 3.8 g/kg for dead fuels and 10.61 ± 1.0 to 14.85 ± 2.1 g/kg for recently live fuels as oxygen concentrations increased from 0 to 10 %. Accelerated thermal cracking in heavier hydrocarbons (i.e., butane) due to a stronger smoldering process and greater oxygen supply was also observed. Additionally, increasing oxygen concentrations favored the production of NO while reduced acrolein emissions under flaming conditions. TPM emissions reduced with higher heat flux and oxygen concentration, and the difference in chemical composition associated with PM size was noted. This study has important implications on efforts to model emissions from wildland fuels, as a broader understanding of the processes driving emissions production considering variations in FMC, oxygen supply concentration and combustion modes is needed to better inform future emission management and wildfire management strategies.