Flaming Combustion

Flaming combustion is the most efficient phase of combustion producing the least amount of smoke per unit of fuel consumed. The transition from pre-ignition to flaming combustion is the transition of the overall combustion reaction from a heat absorbing (endothermic) to a heat releasing (exothermic) reaction and occurs at fuel temperatures just above 300^oC. Flaming combustion is initiated by the actual ignition of the volatile gases produced during the pre-ignition phase and located above the heated fuel. A principle characteristic of the flame is that the fuel (gases) and oxidizer (air) are initially separate and combustion occurs in the zone where the gases mix. Flames are not attached directly to the surface but are separated by a thin layer of vapor or gas. In the flame, thermal degradation of molecules occurs more completely and rapidly than in pre-ignition, with large releases of heat energy. This heat induces further volatilization of organic compounds in the solid fuel, which sustain the flame. Ignition tends to occur as a chain reaction, with the ignition of fuels at a particular location initiating the ignition of adjacent fuels. A line of fire can be considered a chain of ignition events.

The energy that maintains the flame and chain reaction of ignitions is known as the heat of combustion, which is the amount of heat released when a unit of fuel is oxidized completely. The heat of combustion for most wildland fuels is approximately 18,620 KJ/kg (8000 Btu/lb). Temperatures reached during flaming combustion (after ignition) can vary dramatically depending on the amount of fuels and weather conditions. Very intense fires have been recorded as reaching temperatures of 1650^oC, but average temperatures tend to fall within the range of 700-980^oC (Pyne et al. 1996). In the flaming phase, combustion efficiency is relatively high and emits the least quantity of pollutants compared with the mass of fuel consumed. Combustion efficiency is the fraction of fuel carbon converted to carbon as CO₂ (USDA 2002). For more information see Flame chemistry.

Products of flaming combustion

The products of flaming combustion are predominately carbon dioxide (CO₂) and water vapor. This water vapor is not the result of the heating of the fuels as in the pre-ignition phase but rather a result of the oxidation of organic molecules. Other combustion products contained in the smoke above a flame include small solid particles and multi-carbon molecules of various lengths and structures that are not fully broken down into basic carbon molecules (i.e., CO and CO₂) while passing through the flame, but are light enough to be lifted by convection above the flames. Some organic gases cool and condense without passing through the flame zone. Others pass through the flames and are only partially oxidized producing a great variety of emissions. Some compounds with high molecular weights cool and condense into tar droplets and soot (solid particulates). This particulate matter makes up part of the visible smoke, and affects ambient air quality and human health. Particles can be separated into a fine-particle mode with a mean diameter of 0.3 micrometers and a coarse particle mode with mean diameter of 10 or more micrometers. The percentage of fine particles produced ranges from 80 to 95% depending on the turbulence in the combustion zone. Fires of low intensity, where fire is barely sustained and inefficient in combustion, increase particulate matter emissions (Ward 1998).

Flaming combustion tends to follow a cycle beginning as an intense fire that causes a high rate of volatilization and ignition of surrounding materials. As the fire progresses, combustion levels off, and as the amount of fuel available for combustion decreases and the amount of carbon and ash on the surfaces of the fuels increases, combustion decreases. When the amount of volatiles released by the fuels is so reduced that it can no longer maintain a flame above the fuel surface, combustion enters the smoldering phase. Consumption of plant material usually ranges from 50-95%, depending on fuel characteristics. Dry fine fuels, such as cured grasses, tend to be more completely consumed during combustion than moist heavy fuels (see Fuel Availability and Consumption).

Extractives and other non-cellulosic compounds have different requirements for combustion, and can emit products such as hydrocarbons, alcohols, aldehydes, methane (CH₄), nitrous oxides, and sulfur oxides. Sulfur emissions such as carbonyl sulfide are essential in the synthesis of plant amino acids. Methyl chloride is produced in greater quantities during the smoldering phase than the flaming phase (inversely proportionate to the rate of heat release). Methane and other hydrocarbons are also produced in greater quantities during the smoldering phase. Products of incomplete combustion contribute to air pollution and are a concern for land resource managers.