Heat Content (Heat Yield, Caloric Value, Heat of Combustion)

Heat content is an important aspect of fuel chemistry influencing fire behavior (Miller 1994). A fuel’s heat content (also termed heat yield, energy content, caloric value, or heat of combustion) is the potential heat energy of the particle or the reaction heat resulting from complete combustion, measured in kilojoules per gram (or KJ/g or Btu/lb). A forest fuel with a higher heat content will burn at a higher temperature and more rapidly than a low heat content fuel.

While standard values of heat contents are often used (18,620 KJ/kg), forest fuels vary in their heat contents (see Table: Plant flammability). The presence of volatile compounds in some live fuels increases heat content, and thus flammability (Miller 1994). For example, resinous pine heartwood has almost twice the heat content of oak wood. Foliage heat content is strongly affected by extractive content, so can vary by species and by season. Heat contents change with age, with some species increasing contents with aging and others decreasing with time (Burgan and Susott 1991, Hough 1969). Heat contents are important inputs into Rothermel’s fire spread equations and several fire models (e.g., BEHAVE).

Adjustments and reductions for heat of combustion

Total, gross, or high heats of combustion describe the caloric content of a fuel as measured by an oxygen bomb calorimeter and expressed as calories per gram of dry fuel weight. The high heat of combustion sets a theoretical maximum on the amount of potential energy available for combustion. The average value for wildand fuels is 4500 cal/g (Pyne 1984). Since ideal burning conditions are seldom approached in the field, the high heat of combustion is usually adjusted or reduced to account for fuel moisture, radiation, and incomplete combustion (Alexander 1982). The reduced value is usually called heat yield. Although heat of combustion is often used interchangeably with heat yield, the heat yield for a particular fuel will vary with the heat of combustion (Pyne 1996).

The first reduction of high heat of combustion is for the latent heat expended in evaporating adsorbed water (Byram 1959). Since this latent heat cannot be spent in pyrolysis and combustion, it reduces the amount of energy returned for the heat invested (Pyne 1984). The high heat of combustion reduced by this standard amount, 1263 kJ/kg, then becomes the low heat of combustion (Alexander 1982). In practice, low heat of combustion varies so little from fuel to fuel (roughly 10%) that a basic value of 18,620 kJ/kg has been used as a constant (Van Wagner 1973, Albini 1976). A second reduction, for fuel moisture content, is 24 kJ/kg per moisture content percentage point (Van Wagner 1972b).

Although Byram (1959) also adjusted heat of combustion for radiation losses in his equation for fireline intensity, there are two arguments against this reduction: 1) there is no sound basis available for estimating radiation heat as a proportion of the total energy output of individual fires of different intensities and, 2) radiation is not really a loss, but contributes greatly to fire behavior (Van Wagner 1973). This reduction is suggested if some special purpose requires an estimate of only convective heat output (Van Wagner 1972b).

Another possible reduction is for incomplete combustion or char formation. Heats of combustion that have been adjusted to account for these heat losses are also called effective heat yields. Effective heat yields can range from 34-78% of high heat yields (Pyne 1984). Since incomplete combustion is so variable and difficult to measure, use of effective heat yields remain a matter of subjective judgement (Alexander 1982).