Fundamentals - Wildland Fuel Fundamentals and Applications - page 28

and lignin, there can be other chemical constituents of fuel that affect heat content.
Biomass with high mineral contents, for example, will have lower heat contents
(Susott et al. 1975 ). However, there are many chemical compounds in biomass that
may increase heat content. Oils, resins, and proteins may increase heat contents in
foliage and other parts of the plant (Philpot 1969 ). Moisture content also governs
the amount of heat given off from burning fuels (Chap. 5) because heat must be
used to vaporize the free and bound water in live and dead fuel particles.
Wildland fuel heat content values are quite different within and among fuel types,
season, and the intensity of the fire when it is burned. Foliage usually have higher
heat contents (20-21 MJ kg -1 or 8700-9400 BTU lb -1 ) than twigs and stems (18-20
MJ kg -1 8300-8700 BTU lb −1 ), but this relationship is quite different across spe-
cies, age, and dead versus live fuels (Philpot 1969 ). Kelsey et al. ( 1979 ) found that
the heat content of wood ranged from 19.3 to 22.5 MJ kg −1 (8300-9700 BTU lb −1 ),
while bark heat content values were substantially higher ranging from 20.2 to
25.3 MJ kg −1 (8700-10,900 BTU lb −1 ), and foliage heat contents were in between
20.1 and 22.4 MJ kg −1 (8700-9700 BTU lb −1 ). And, the heat content might change
over the course of a fire season. Philpot ( 1969 ) found that the heat content for
chamise shrub leaves were lowest in the spring (~ 21 MJ kg −1 or 9100 BTU lb −1 ) and
increased to 23.5 MJ kg −1 (10,100 BTU lb −1 ) in the autumn. And last, the heat con-
tent of fuels burned under flaming combustion might be quite different than when
fuels are burned under smoldering combustion (Susott et al. 1975 ). Yet despite this
high variability, most fire models use a constant value for heat content. As an ex-
ample, a constant value of 18 MJ kg −1 (8000 BTU lb −1 ) has been assigned to all but
two of the Scott and Burgan ( 2005 ) fire behavior fuel models.
Heat content is usually measured using a bomb calorimeter using a method
where a standardized measure of fuel is placed into a constant volume calorimeter
and electrical energy is used to ignite the fuel. As the fuel is burning, it heats the sur-
rounding air, which expands and escapes through a tube that heats water outside the
tube. The change in the temperature of the water allows for calculating the amount
of heat generated from the fuel.

2.3.1.7 Other Important Particle Properties

There are several other important fuel particle properties that are not directly used
in fire behavior modeling, but they are still important in fuel science and manage-
ment. Particle shape or the general geometry of a fuel particle is important because
it is used to define a geometric form for which an equation can be used to calculate
volume that is then used to estimate density and mass. Shape is also used to classify
particles into fuel components and to parameterize fuel components for modeling.
Particle shape is also important in fuel moisture dynamics, ignition processes, and
combustion.
Particle thermal conductivity is a physical measure of the heat conduction po-
tential of fuel or how fast heat can travel through fuel. Thermal conductivity has
the complex units of W m −1 °K −1 or Joule sec −1 m −1 °K −1 or kg m sec −3 °K −1 , there-
fore having representations of energy, mass, time, length, and temperature. Thermal