Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 69

Chapter 5
Fuel Moisture

Water is the driving force of all nature
Leonardo da Vinci

5.1 Background

Moisture content is the most dynamic feature of most wildland fuel types and it
influences nearly all fire processes, especially ignition, combustion, and smolder-
ing. The amount of moisture in the fuel is called the fuel moisture content (FMC)
defined as the mass of water per unit mass of dry material and is often expressed as
a percent. FMC is a major factor determining the heat produced by a wildland fire
(see Table 2.2). High fuel moistures slow the rate of burning and fuel consumption
in a number of ways. Heat from the fire must first volatilize the water bound in the
fuel by boiling it off as gas before it can ignite the organic fuel (Simard 1968 ). This
steals heat away from ignition and combustion processes, and if there is not enough
heat to drive off a sufficient amount of moisture, then ignition will not occur. High
moisture values may also cause increased particle thermal conductivity (how fast
heat can pass through an object) and volumetric heat capacity (heat output per unit
volume of fuel) that may further dampen ignition and combustion (Nelson 2001 ).
Fuel moisture also reduces flame temperature thereby increasing smoldering com-
bustion and the production of char while retarding the rate of consumption. Because
moisture reduces both ignition potential and combustion temperatures, there is a
corresponding increase in the time it takes a fuel particle to burn (residence time).
Water vapor created from the heat of combustion may surround the fuel particle and
dilute the available oxygen thereby also retarding combustion (Simard 1968 ). These
complex interactions contribute to a lack of ignition, or if ignited, to lower heat
emissions and longer burning times. This, in turn, can result in less flaming combus-
tion, decreased fuel consumption, and increased smoldering. This is why every fire
behavior model has fuel component FMCs as input parameters (Matthews 2013 ).
Three significant fuel characteristics affect moisture of live and dead fuel par-
ticles (Nelson 2001 ). The chemical composition of the fuel dictates its ability to
attract and hold water molecules from the surrounding environment; a fuel property