Clive M. Countryman

“ Thus, moisture in wildland fuels increases their ignition time and decreases their burning rate.
The thermal conductivity of most wildland fuels increases as the density of the fuel increases. When the thermal conductivity is high, the available heat is transferred through a deeper layer of fuel in a given time than when the thermal conductivity is low. And there is more material to heat per unit of volume in the dense fuels. As a result, fuels with low density tend to ignite more readily, and often burn more rapidly than do high-density fuels. ”

“ In all types of combustion, however, fuel ignition requires that the fuel temperature be raised to some minimum level by the application of heat. And the heat must also be applied long enough to raise the temperature of a fuel layer deep enough to permit the pyrolysis to become self-sustaining—some quantity of heat is required for ignition. If the time the heat is applied is too short, the necessary quantity of heat cannot be supplied, and the fuel will not ignite regardless of the temperature of the heat source. ”

“ Wildland fuels do not burn directly If we look closely at a piece of burning wood, such as a lighted match, we see that the flames are not attached directly to the wood surface, but are separated from it by a thin layer of vapor or gas. This happens because solid organic materials do not burn in flaming combustion directly, but must first be decomposed or pyrolyzed by heat and chemical reactions into various gases—some combustible and some not. ”

“ The combustible gases do not contain enough oxygen to burn when emitted from the fuel, and must first mix with the surrounding air before a flammable mixture is produced. If the pyrolysis is slow, not much gas is generated and the flames are short and intermittent. But when large amounts of fuel are burning rapidly, the volume of gas is large and some must move a considerable distance from the fuel before the mixture becomes flammable. ”

“ Heat conduction and wildland fire Wildland fire involves both chemical and physical processes. When wildland fuel bums, its stored chemical energy is converted to thermal energy or heat through complex chemical reactions. But for the reactions to be started, heat must be physically transferred from a firebrand to the fuel, and heat transfer to unburned fuel must continue if the fire is to keep on burning and spreading. ”

“ Ignition depends on temperature and heat quantity The chemical reactions of heated fuels vary with the temperature; hence the ignition temperature depends on the stage of pyrolysis at which the fuel is considered to be actually ignited. Charring can begin at relatively low fuel temperatures, and once started can continue if there is little heat loss. This sometimes occurs in deep layers of compact, fine dead fuel. Glowing combustion requires higher fuel temperatures to begin, and may proceed without flames in some types of fuel. ”

“ Many of our wildland fuel beds in their natural state are made up of a mixture of fuel sizes. These sizes may range from a small fraction of an inch to 4 or 5 inches in diameter in brush fuels, or to several feet in diameter in timber types. But because of their relatively slow burning rate, the large fuels usually contribute little to heat production in the early stages of a fire. For example, in dead and fairly dry large fuels, the pyrolysis characteristically penetrates at the rate of 1 to 2 inches per hour into the fuel. A 2-inch limb may require up to 2 hours to burn completely. ”

“ In suppressing a wildland fire, we can remove the fuel in a band or firebreak around the fire to deprive it of the fuel ingredient, or we can put water, chemicals, or dirt on the burning fuel to reduce the amount of heat and to decrease the supply of oxygen. ”

“ We can halt fire spread by reducing or preventing heat transfer by convection or radiation. But to extinguish a fire completely, we must eliminate heat transfer by conduction.
Small fuels are most important Anyone who has started a fire is well aware that small or thin fuels ignite more readily than do larger fuels, and such "kindling" is usually needed to start the larger fuels burning at all. The reason lies in the rapidity with which small fuels can be heated and ignited. All of the heat that gets into a piece of fuel must go through the surface. ”

“ Not all of the water vapor moves out of the burning fuel immediately. Some of it diffuses toward the interior and condenses in the unburned part—the moisture content of the fuel tends to increase as the fuel burns. Although the heat absorbed in vaporization is released when the moisture condenses, the increasing moisture content slows the temperature rise of the unburned fuel. As a result, the pyrolysis rate tends to slow as the fuel burns. ”

“ Small fuels also burn more rapidly than large fuels—a given weight of fuel in small-sized material releases its heat in a shorter time and produces a more intense fire. The more rapid burning is due largely to the greater ratio of surface area to volume. However, char formation is also important. Only a thin layer of char can form on small fuels, and its effect is small compared to the thick char layer on large fuels. ”

“ The pyrolysis layer moves more slowly into the moist fuel, reducing the rate of heat production. Also, the released water vapor must pass through the hot char, where it can absorb part of the heat that otherwise would contribute to pyrolysis. Outside the fuel, the water vapor dilutes the combustible gases and delays the production of a flammable mixture, or prevents it entirely. When flaming is established, the water vapor reduces flame temperature through heat absorption and by reducing the flammability of the air and gas mixture. ”

“ Ground-up grass, compact duff, peat, and decayed wood often chars and glows until consumed without ever bursting into flame. So does tobacco. High fuel temperatures are needed to initiate flaming, but flaming combustion also produces the greatest amount of heat and the highest fire temperatures. ”

“ In fuel types where there are large as well as small fuels in the fuel bed, much of the large fuel stops burning once the small fuels have been consumed, since the additional heat needed to keep the large fuels burning is no longer available. But in the usual wildland situation, some of these fuels continue to burn because they are close enough together to supply heat to each other, or are lying in hot coals or ashes. ”

“ As long as the slow-burning fuels are producing heat, they pose a threat of fire escape, and must be extinguished to assure fire control. Because of their long burning time, it is not often feasible to allow them to burn out, and stopping the pyrolysis by cooling or other means of heat removal is necessary. ”

“ Thus, when we construct a firebreak around a fire, we prevent heat transfer to unburned fuel. To determine the firebreak width, we must consider the amount of heat being produced and the amount of heat that must be transferred to ignite the particular unburned fuel. ”

“ Similarly, fuels in hot coals and ashes can be moved, or the hot material can be scraped away to deprive them of heat.
Water is highly efficient as a cooling agent because of its great heat-absorbing capacity, in both liquid and vapor form. ”

“ There are several ways to cool large fuels below the temperature needed for pyrolysis, or to deprive them of the heat required. Fuels lying close together frequently stop burning if they are simply moved farther apart. ”

“ More frequently, however, the fuel is scraped away to form a firebreak around the fire and the fine fuel within the break allowed to burn out. Since the burning time of these fuels is short, heat production is soon drastically reduced, and the chance for further fire spread greatly diminished. ”