UC Berkeley

Wildfires have increased in frequency and severity over recent decades, resulting in serious impacts on both the environment and human lives. These fires are often fueled by dry conditions, high temperatures, and strong winds, making them difficult to contain. The consequences of these extreme wildfires are enormous, leading to the destruction of vast forested areas, the loss of biodiversity, and the release of harmful gases and particulate matter into the atmosphere. These emissions pose a serious health threat to nearby populations and firefighters who are tasked with controlling the fire.

In this study, the concentration of gaseous and particulate matter emissions resulting from flaming and smoldering combustion of different wildland fuels found across the United States is characterized. Furthermore, the effect of fuel moisture content (FMC) on these emissions is explicitly studied, which has not been considered in the literature before. To achieve this, a custom linear tube-heater apparatus is constructed, enabling the steady production of emissions in different combustion modes across a wide range of FMC. The results obtained demonstrate that smoldering combustion exhibits increased emissions of CO, particulate matter, and unburned hydrocarbons when compared to flaming combustion. Moreover, it is observed that the concentrations of CO and particulate matter in the flaming mode are significantly correlated with FMC, while FMC has little influence on emissions during smoldering mode combustion when considering the dry mass of fuel burned. These variations are observed in certain vegetative fuel species but not others, highlighting the importance of fuel type. Additionally, Fourier-transform infrared (FTIR) spectroscopy is employed to provide detailed characterization of the unburnt hydrocarbons.

Secondly, this study quantifies the limiting conditions for the smoldering-to-flaming (StF) transition, a commonly observed phenomenon during wildfire events, specifically focusing on cellulose powder, which constitutes approximately 50% of vegetative wildland fuel. It is discovered that the external heat flux required for the StF transition increases as the oxygen (O2) concentration decreases from 21% to 10% at fixed flow velocities. However, for a constant O2 concentration, an increase in flow velocity leads to a higher required heat flux due to the growing significance of convective heat losses. Under the experimental conditions, an oxidizer flow velocity of 5 cm/s is determined to be a limiting value for the StF transition. Finally, the efficacy of six different respiratory protection (RP) materials commonly worn during wildland fire events is tested against simulated wildland fire smoke. The findings indicate that cloth bandanas, currently the most commonly used RP material, offer minimal benefits in terms of reducing both particulate matter (PM) and gaseous emissions. On the other hand, all other filter materials demonstrate significant advantages. N95, P95, and P100 filters prove highly effective in removing PM and show some effectiveness in filtering certain gaseous species, particularly those with nuisance volatile organic compound (VOC) capabilities, although this effect may not be sustained over longer durations.