Fundamentals - Wildland Fuel Fundamentals and Applications - page 15

This is a chemical representation of the process of combustion, often considered a
chemical chain reaction because the heat produced by combustion acts as a catalyst
which further increases the rate of reaction. Byram ( 1959 ) presented the following
chemically balanced oxidation reaction for complete combustion of plant biomass:

4 ( CHO 25O ) + + [ 0.322M HO 94N + →+ ] 18 HO 24CO

694 2 2 2 2 2

9
11.6 10 JHeat]

(2.3)

+ [ 0.322M HO 94N + ][ + ×

2 2

Note that 4 kg moles of plant material (C 6 H 9 O 4 ) yields about 11.6 billion joules
of heat, and dividing this heat by the mass gives the heat content of the fuel (see
Sect. 2.3.1.1).

2.1.2 Scales of Combustion

The physical process of combustion is quite complex and occurs in at least four
overlapping phases (Zhou and Mahalingam 2001 ; Bebi et al. 2003 ). In the pre-igni-
tion phase, unburned fuel ahead of the advancing flame front is heated and raised to
its ignition temperature in a series of endothermic (requiring heat) reactions domi-
nated by dehydration and the volatilization of organics. Water is vaporized in the
cell structure, then driven to the surface of the fuels, and vented to the atmosphere.
The second phase of combustion known as pyrolysis begins as fuel temperature
rises and cellulose and other compounds begin to decompose to release combustible
organic gases and vapors, thereby converting biomass into volatiles, tars, char, and
ash. Cellulose is pyrolyzed between 280 and 400 °C through dehydration and depo-
lymerization, while lignin is pyrolyzed at temperatures of 280-500 °C because it is
more complex and thermally stable (Liodakis et al. 2002 ). The combustion phase
occurs when the burning process becomes exothermic (generating heat) in the pres-
ence of oxygen giving off energy in the form of heat and light, and the start of com-
bustion is often termed ignition. Flaming combustion occurs when volatized gases
are oxidized and flames are generated, usually occurring when the temperature of
the volatiles reach 450-500 °C. Combustion without flames is called smoldering
combustion, which is the surface oxidation of char, which provides just enough heat
to continue pyrolysis. In general, the smoldering combustion phase occurs when
the concentration of combustible vapors above the fuel is too small to support a
persistent flame, so gases and vapors condense, appearing as smoke. Once most
volatile gases have been driven off, the glowing combustion phase occurs, where
only embers and smoke are visible and there is little smoke; the carbon remaining
in the fuel is oxidized to continue to produce significant heat.
This complex combustion process is often simplified so that it can be taught
to fire specialists using the famous “fire triangle” (Fig. 2.1 ). At the finest spatial
scale, a combination of three elements is needed for a wildland fire to burn: heat,
oxygen, and fuel (Countryman 1969 ). The heat source for ignition can be from
lightning, matches, drip torches, or, mostly, the fire itself. Oxygen is in great sup-
ply in the earth's atmosphere, but sometimes the combustion process itself may use