The Role of Heat Release in the Trajectory and Dispersion of Fire Emissions

The consumption of biomass produces thermal energy, and this energy creates buoyancy to lift smoke particles and other pollutants above the fire. Heat release rate is the amount of thermal energy generated per unit of time. Total heat release from a fire or class of fires is a function of the heat content of the biomass, fuel consumed, ignition method and pattern, and area burned.

The early work of Anderson (1969) and Rothermel (1972) created fundamental equations for combustion energy in a variety of fuelbeds. Sandberg and Peterson (1984) adapted the combustion equations to model the temporal change in energy during flaming and smoldering combustion (Emission Production Model, EPMv.1.02). Currently, EPM provides heat release rates for most biomass smoke dispersion models (Harms and others 1997; Harrison 1995; Lavdas 1996; Sestak and Riebau 1988; Scire and others 2000a) and has been used to estimate the change in global biomass emissions patterns due to changes in land use (Ferguson and others 2000). The model, however, requires a constant rate of ignition with constant slope and wind. Such homogeneous conditions may be approximated during prescribed fires that are ignited with a deliberate pattern of drip torches or airborne incendiaries, or during portions of wildfires that experience relatively constant spread rates, both over fuelbed strata that retain a relatively consistent spatial and compositional pattern. To use EPM effectively for modeling source strength, the fire area and ignition duration are broken into space and time segments that meet the steady-state criteria.

Albini and others (1995), Albini and Reinhardt (1995), and Albini and Reinhardt (1997) do not explicitly derive temporal changes in combustion energy in their model, BurnUp, but they do assign source heat in steps of flaming and smoldering that are estimated from total fuel consumption. They have linked their model with the fire spread model, FARSITE (Finney 1998), which allows ignition rates and subsequent heat-release rates to vary over the landscape. The coupled system is computationally expensive and not yet associated with a plume rise component but may offer a reasonable approximation of the temporal and spatial varying emission rates of fires.