Fundamentals - Wildland Fuel Fundamentals and Applications - page 27

moisture stress, size). In reality, the distribution of M x in a typical fuelbed could be
quite variable and difficult to accurately quantify for operational fire management.

2.3.1.5 Mineral Content ( S e , S T )

Mineral content ( S e and S T depending on the equation in Table 2.2 ) has a profound
impact on fire behavior (Philpot 1970 ; Eqs. 2.9, 2.10). Biomass with high mineral
content, such as duff and slash fuel, will tend to burn slower and have a higher pro-
portion of burning in smoldering combustion, often resulting in reduced combus-
tion. In fact, fire retardant depends on this relationship to be effective; the primary
purpose of retardant is to retard the spread of fire by increasing mineral content,
thereby depressing fire spread (Giménez et al. 2004 ). The mineral content ( S T ) is the
percent of the total weight per unit volume of fuel particle that is inorganic material
or mineral (i.e., not composed of molecules of C, H, and O). It is usually estimated
by burning a fuel particle of known dry weight and weighing the ash that is left after
complete combustion; the weight of ash divided by dry weight of the wood is the
mineral content. The effective mineral content ( S e ) is the mineral content with the
proportion of silica removed (Table 2.2 ).
Since minerals are key nutrients needed in plant photosynthesis and respiration,
they become incorporated into biomass, and each fuel particle has its own rela-
tively static mineral content (around 5 %). Wood in woody fuels is usually 1 % min-
eral, while the bark can have ten times that amount (Ragland et al. 1991 ). Needles
have fewer minerals (0.1 %), but mineral content often increases with needle age
(Weikert et al. 1989 ). Particles near or in contact with the ground will usually have
higher mineral contents because of the diverse processes involved in decomposition
(Chap. 6); soil macrofauna break down organic material and often incorporate min-
eral soil onto the downed fuel particles. And, minerals will also tend to accumulate
in the duff layer as microbes process the organic material and leave the minerals to
collect in the duff (Chap. 6). As a result, ground fuels usually have the highest min-
eral contents, often greater than 10 %, partially explaining why ground fuels mostly
burn in smoldering combustion. Keane et al. ( 2012b ) found mineral contents were
the highest in the smallest fuel particles with litter and duff having 10-50 % mineral
contents, 1 h woody having 2-5 %, 1,000 h woody having 0.1-0.8 % for forest and
rangelands of the northern Rocky Mountains, USA. However, these mineral con-
tents varied greatly from site to site, and stand by history.

2.3.1.6 Heat Content ( h )

The heat content ( h ) is the heat yield of the fuel per unit mass (kJ kg -1 ) and, when
multiplied by loading ( W ), is used to compute fire intensity (Eq. 2.4). This is best
described as the heat released from the combustion of the gases evolved in the igni-
tion phase (see Sect. 2.1.2) and is also called the effective heat content (Shafizadeh
et al. 1977 ). The heat content of wildland fuel is mostly dependent on the chemical
composition of the material being burned. While the majority of fuel is cellulose