Managing the impacts of smoldering combustion during wildland fires is critical to supporting Department of Defense (DoD) missions. Smoldering combustion is significant because it can occur with fine fuel sources, damage ecosystems, contribute substantially to pollution and carbon emissions, and transition to flaming combustion. Smoldering combustion can produce far more air pollutants than flaming combustion; it is responsible for most of the carbon monoxide (CO), methane (CH₄), volatile organic compounds (VOC), and fine particulate matter (PM2.5) released during fires when smoldering fuel is present. In summary, the impacts of smoldering combustion on the immediate and surrounding areas are critical to DoD fire management decisions.

Unfortunately, current tools for predicting the ignition of smoldering combustion, the burn duration, and its impacts on air quality and soil productivity are severely limited. This hindrance results from not understanding the limiting processes in smoldering combustion. Prior research is often limited to empirical trends or results from a specific fuel type or condition range. In consideration of these needs, the overall objective of this project is to develop essential scientific understanding that can enable tools for assessing risks of ignition, spread, and emissions from smoldering combustion. The working hypothesis is that ignition, propagation, and pollutant formation are controlled by common processes, which—if understood—could be used to create a predictive model applicable for multiple fuels and environmental conditions.

A coupled approach with laboratory, field, and computational studies will be used to determine the dominant processes that control ignition, propagation, and emissions of smoldering combustion. Laboratory studies will systematically vary the fuel composition, moisture content, shape, packing density, or heat transfer mode to elucidate key physics or chemistry affecting ignition and propagation. The time to ignition, temperature distribution, and behavior of the smoldering front will be measured using visible and infrared cameras. Fuels to be considered include, but are not limited to, fuel constituents (e.g., cellulose and lignin) and litter from several varieties of conifers. Field burns will assess length-scale effects and anchor ranges to be investigated in the laboratory. Field measurements will determine the distribution of litter, duff1, and moisture content. Temperatures, heat fluxes, and both visible and infrared images will be collected of smoldering fronts. Pollutant emissions measured in laboratory and field studies will be analyzed and related to the transition from smoldering to flaming combustion. A detailed numerical code will discover the underlying physics or chemistry causing the experimental observation. This code will initially be one-dimensional and focus on solid-phase chemistry, but will be extended to two dimensions and include gas-phase chemistry to investigate other aspects of smoldering combustion.

¹Originally plant forms now not recognizable, occurring just below the litter layer.

This project will result in substantial benefits to the DoD. The physical and chemical processes that control smoldering combustion ignition, propagation, and emissions will be identified. This physics-based approach will enable the development of modeling tools that can be used for fire management for multiple fuels and fuel conditions. Furthermore, better understanding of factors leading to smoldering combustion will enable more informed choices by DoD decision-makers regarding prescribed burning versus wildfire.