Effects of Fuels on Fire Intensity and Rate of Spread
Several physical and chemical fuel characteristics directly or indirectly affect fire intensity and rate of spread.
Fuel moisture content
Fuel moisture content is important in determining both fire intensity and rates of spread since it controls fuel availability. The more water a fuel contains, the more heat required to ignite and burn. When the moisture level is high in fuel, it may take a long time for the fuel to ignite and spread rates and energy release are low. When the moisture content is low, fuels easily ignite and burn more quickly. Prior to ignition, fuel must be heated to a minimum temperature termed the low heat of combustion. The lower the initial fuel temperature, the more energy is consumed by the fuel in order to raise its temperature enough to initiate volatilization and pyrolysis reactions (see also: Pre-Ignition Phase). This reaction is greatly affected by fuel moisture content. Heat entering the fuel raises the water temperature to 100°C, separates water from the fuel, and vaporizes the water (Whelan 1995). Until all water is vaporized temperatures cant increase further toward ignition temperatures, at which point the fuel begins to release energy in the form of heat.
High temperature, low relative humidity, and low precipitation are ideal conditions for drying of fuel. To see an example of how heat release and fire intensity vary under different fuel moisture regimes, see Heat release rates under different fuel and weather conditions.
Dead fuel moisture is a function of weather conditions and depends on the local environment. As the temperature and humidity change through the day or season, the dead fuel moisture varies. Live fuel moisture depends on the season and correlates with spring flushes and fall curing. Live fuels can serve as a heat sink to slow the spread of fire. However, in times of drought, live fuel that would otherwise not burn becomes a major available fuel source (Reinhardt et al. 1991).
Fuel arrangement
Fuel arrangement is an important factor in both fire intensity and rate of spread. If fuels are arranged too densely, there will not be enough oxygen to maintain combustion, no matter what the fuel size. The higher the packing ratio (tightness of fuels), the lower the combustion efficiency and heat release, and the less fuel is consumed (DeBano et al. 1998).
The horizontal arrangement and distribution of fuels may be continuous, patchy, or broken up by barriers, with significant impacts on the rate of spread. If the fuel exists in clumps, it may be difficult for the fire to spread without strong winds or spotting. Horizontal continuity is especially important for the spread of crown fires (Pyne et al. 1996).
Vertical arrangement describes how fuels are distributed between surface fuels and treetops. A fuel ladder (of shrubs, vines covered with pine needles, and pine crowns) can spread fire from the ground up to the canopy. Old pastures and grass prairies in the southeast are examples of a single fuel stratum; in the Lower Coastal Plain, the palmetto-gallberry-southern pine fuels are at least three strata – a forest floor mat of leaves, debris and pine needles in different stages of decay along with other living forbs, the shrub layer, and the pine canopy (Johnson and Miyanishi 2001).
Fuel size and shape
Fuel size and shape also affects combustion, heat release, and rates of spread. Fine fuels are important for the initial ignition and spread of fires. Due to their large surface area-to-volume ratio, fine fuels ignite more easily and release heat more quickly than large diameter fuels (DeBano et al. 1998). Small fuels are also needed to ignite larger fuels, which are potentially significant heat sources. In contrast, large fuel consumption requires fire to be “in residence” longer in order to penetrate the surface. Because of this requirement, large fuels are primarily consumed by smolderingglowing combustion after the passage of the active flaming front and are less important in determining rates of spread. The density of the large fuels can also affect heat release rates. For example, a rotten log ignites more easily and releases heat at a faster rate than a sound log (DeBano et al. 1998).
Fuel load
Available fuel load (weight of fuel/unit area) is one of the most important factors controlling fire intensity. Fire intensity is directly proportional to a fuels heat of combustion, the amount of fuel consumed, and a fires rate of spread. The total energy released in the fire is proportional to the amount of carbon stored in the fuel that is consumed. This relationship is represented in the fireline intensity equation, where the weight of fuel consumed per unit area in the active flaming zone (W) is directly related to rate of heat release.
Fuel loads are dependent on forest type, life stage of the forest (older, over-mature forests may have an accumulation of large woody debris), and time since last fire (DeBano et al. 1998). The proportion of this total fuel load that is consumed is influenced by fuel availability, which in turn is determined by moisture content, chemical characteristics, and size. Fire type also determines the amount of fuel consumed: a surface fire that consumes only needles and grasses consumes much less fuel than a stand-replacing crown fire. To see an example of how heat release and fire intensity vary under different fuel types, see Heat release rates under different fuel and weather conditions.
Fuel chemical composition
The chemical composition of fuels can affect their flammability, influencing both fire spread and energy output. For example, high mineral element concentrations in woody tissue and leaves reduce the flammability of these fuels, reducing total energy released (DeBano et al. 1998; see Particle Flammability). High concentrations of phosphorus in fuels have been shown to reduce a fire’s rate of spread (Lindenmuth and Davis 1973). Phosphorus reduces flammability to such a great extent that di-ammonium phosphate has been used as a fire retardant in fire fighting operations (Foster 1976).
In contrast to mineral content, fuels with high concentrations of oils, resins, or other volatiles can greatly increase their flammability and heat output due to their high energy content (Whelan 1995). For example, the heat of combustion for oak wood is 19.33 MJ/kg whereas the heat of combustion for pine pitch is 35.13 MJ/kg, an increase of over 80% (McArthur and Cheney 1972). See: Plant flammability.
Fuel type or model
Fuel types vary in their fuel moisture content, physical properties, and chemical properties. Therefore rates of spread will also vary according to fuel type, often defined by fuel models.
Fuel type effects on rate of spread are illustrated in the figure for three southern fuel models: open pine with grass understory (FM 2), Coastal Plain shrub communities under pine (FM 7), and logging slash (FM 11). Spread rates were modeled using Behave Plus 2.0, a fire behavior prediction system based on Rothermel’s equation. Total dead fuel loads in the three models are 3.5, 4.5 and 11.5 tons per acre, respectively, with FM 2 having twice as much 1-hour fuels (dead grass and pine needles) as FM 7. The higher fuel load of fine fuels in FM 2 leads to the much higher spread rates in that fuel type, especially as winds increase.
To see an example of how heat release and fire intensity vary under different fuel types, see Heat Release Rates under Different Fuel and Weather Conditions.
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