Chemical Fuel Properties

Chemical properties of fuels affect their heat content and the types of emissions released during a fire (Hough 1969, Shafizadeh et al. 1977, Rundel 1981, DeBano et al. 1998). Forest fuels are composed of living plants and dead plant parts, both of which are constructed of:

• cellulose (from 35 to 55%): an important source of combustible volatiles and is made up of polymers of hexose sugar D-glucose in a linear structure. Cellulose pyrolizes rapidly, producing much of the gaseous fuel in flaming combustion.

• hemicellulose (15 to 25%): a carbohydrate polysaccharide that is similar to cellulose and is found in the cell walls of many plants.

• phenolics such as lignin and tannins (15 to 30%): lignin is an aromatic polymer that does not degrade as easily as cellulose and hemicellulose. This material gives wood its hardness and structural integrity. If heated to temperatures above 400-450° C, 50% of lignin will volatize while the rest becomes char residue or pyrolizes slowly during smoldering combustion.

• extractives (from 2 to >45% in foliage fuels): extractives consist of organic compounds such as sugars, resins, alcohols, hydrocarbons, and fatty acids. Extractives can be highly flammable and are often the first hydrocarbons to be volatilized (during pre-ignition) and consumed in combustion. Extractives that are volatile compounds, such as oils and resins can increase heat release because of their high energy content.

• mineral ash: minerals (Si, Ca, Mg, K, etc) are found in low concentrations and are not combustible.

The relative proportions of these compounds can affect a fuel’s ignition, combustion, and extinction characteristics through particle-level flammability. Woody fuels are high in cellulose, lignin, and hemicellulose, while foliage contains high amounts of extractives.