Particle Flammability

Particle level flammability describes the intrinsic components of flammability without the influence of branch or plant structure. Major influences on particle flammability are

• moisture content,
• percent cellulose, hemicellulose, and lignin,
• volatile concentration, and
• silica-free mineral content.

Both extractive and mineral content of plants are species dependent, making these intrinsic flammability characteristics vary significantly among species. How these components influence particle flammability is explained below, followed by a short explanation of how particle flammability is measured.

Moisture content

Moisture content is the most important factor influencing particle flammability. Moisture content was highly significant in the ignition time, maximum burning rate, period of flaming combustion, and flame length of leaf material from Themeda australis, Eucalyptus viminalis, and Xanthorrhoea australis (Gill et al. 1978). However, living fuels in the palmetto-gallberry fuel complex will burn at moisture levels of 100% or more, while dead fuels may not burn at moisture levels of 20-30%; similar observations have been made in southwest chaparral ecosystems and coniferous forests throughout the U.S (Rothermel 1976). Such observations demonstrate that other factors can influence the combustion of vegetation beyond moisture content (1976). See also: Fuel Moisture.

Percent cellulose, hemicellulose, and lignin

Due to their different chemical properties, plant flammability can also be related to the proportion of cellulose, hemicellulose, and lignin in plant tissue (Rundel 1981). Lignin is thermally stable, and it volatilizes slowly with increasing temperatures, losing only 50% of weight at 500^oC (Philpot 1970). In comparison, hemicellulose undergoes combustion at 250^oC with complete volatilization at 500^oC; and cellulose undergoes rapid combustion between 300 and 400^oC (Philpot 1970). The combustion characteristics of these elements are affected by the presence of organic volatiles (discussed below), which can be extracted to test their influence on flammability.

Susott (1982) examined 43 samples from different species and locations and found that foliar material combusted more rigorously than woody material. This was attributed to higher lignin-to-cellulose ratio in woody material, as well as higher extractive concentration in foliage (Susott 1982). In a study of Pseudotsuga menziesii, Pinus ponderosa, Populus tremuloides, Ilex glabra, Arctostaphylos totula, and Serenoa repens, ether and benzene-ethanol extractives contributed up to 60% of the heat release of dried, ground foliar samples (Shafizadeh et al. 1977). Concentration of the extractives in tissues was determined to be a useful but not conclusive prediction of heat release (Shafizadeh et al. 1977). Dried foliar samples from species in a flammable fynbos (South African scrubland) ecosystem were found to have higher crude fat content (oils, fats, waxes, and terpenes) and higher energy content than dried foliar samples from species in a non-flammable forest ecosystem (Van Wilgen et al. 1990). Higher crude fat content, lower foliar moisture content, and higher energy content for fynbos species were thought to contribute, along with structural characteristics of the ecosystems, to the differences in ecosystem flammability (Van Wilgen et al. 1990).

Volatile concentration

Collectively, the presence of extractives (flavonoids, waxes, terpenes, oils, and resins) increases ignitability and combustibility. This occurs because they typically undergo combustion at lower temperatures than cellulose and lignin and are highly flammable at high temperatures (Rundel 1981). Shafizadeh et al. (1977) concluded that total extractive content likely affects flammability when it exceeds 25% of oven dry mass. Owens et al. (1998) found that a 1 mg increase of limonene per dried g of foliage in Juniperus ashei, increased flammability by as much as 30%. However, the same study determined bornyl acetate was negatively related to flammability, decreasing flammability by 2% with a 1 mg per dried g of foliage increase, illustrating that not all extractives increase flammability (Owens et al. 1998). See also: Volatile Compounds.

Mineral content

In an early analysis of the impact of mineral content on flammability, Mutch and Philpot (1970) determined that the silica portion of incombustible mineral ash does not influence plant flammability. Further study of the mineral portion of vegetation revealed that an increase in silica-free ash, as percentage of dry weight, decreased maximum combustion rates and increased residues (Philpot 1970). This indicates that the percent of mineral ash, minus silica, in plant tissue decreased the rate at which the tissue combusted.

Measuring particle level flammability

Particle level flammability is determined by reducing plant material, typically leaves, into a fine, uniform substance. Measurements at this level have been made by thermal evolution analysis (Shafizadeh et al. 1977), oxygen bomb calorimetry (Dickinson and Kirkpatrick 1985, Van Wilgen et al. 1990, Rodr\xc3\xadguez-Añ\xc3\xb3n et al. 1995, Núñex-Regueira et al. 2000, Núñex-Regueira et al. 2001, Williamson and Agee 2002), thermogravimetric analysis (Mutch and Philpot 1970, Philpot 1970, Shafizadeh et al. 1977, Gill et al. 1978, Rogers et al. 1986), thermocouple analysis (Owens et al. 1998), and evolved gas analysis (Susott 1982).