Predicting Fire Emissions: Combustion Stages

At least three important stages of combustion exist when fuel particles are consumed (Mobley 1976; NWCG 1985): flaming, smoldering, and residual (also known as "glowing," "residual smoldering," or "residual combustion") (fig. 4-5). The efficiency of combustion is distinct for each stage, resulting in a different set of chemical compounds and thermal energy being released at different rates into the atmosphere.

In the flaming phase, combustion efficiency is relatively high and usually tends to emit the least amount of pollutant emissions compared with the mass of fuel consumed. The predominant products of flaming combustion are CO2 and water vapor.

During the smoldering phase, combustion efficiency is lower, resulting in more particulate emissions generated than during the flaming stage. Smoldering combustion is more prevalent in certain fuel types such as duff, organic soils, and rotten logs, and often less prevalent in fuels with high surface to volume ratios such as grasses, shrubs, and small diameter woody fuels (Sandberg and Dost 1990).

The residual stage differs from the smoldering stage in that the smoldering stage is a secondary process that occurs in fuels preheated or dried by flaming combustion, while residual is an independent process of propagation in a fuelbed unaffected by the flaming stage. This phase is characterized by little smoke and is composed mostly of CO₂ and carbon monoxide. All combustion stages occur sequentially at a point, but simultaneously on a landscape.