Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

SECRET
or burning of evolved gases, will continue so long as the needle or twig is producing hydrocarbons at a sufficient rate. This
is the flaming phase of combustion.
Meanwhile, the charcoal surface of the twig is also burning-in a very simple pair of reactions where solid carbon
changes to carbnn monoxide right at the charcoal surface, and the monoxide is further oxidized to carbon dioxide as soon
as additional oxygen becomes available. The latter reaction takes place very close to the surface: the blue flame right next
to the logs in the fireplace is the CO -CO 2 zone. This is the glowing phase of combustion. Since the initial reaction requires
the direct contact of oxygen at the solid charcoal surface, glowing is a slower process than flaming and continues
long after the flames have died out.
From the preceding discussion, several fuel characteristics are obviously important:
I
1. Moisture content. Fuel moisture is far and away the most important factor in determining whether or not a piece
of wood will bum. Moisture absorbs heat, increases conductivity, dilutes flame gases and generally reduces flammability
for all three phases of combustion. Fuel moisture relationships will be discussed in detail later.
2. Thickness. The thinner the fuel, the sooner heat is conducted all the way through, and the more rapidly surface
layers will reach the decomposition temperature. Thin fuels bum sooner and more rapidly than do thick fuels because they
Fequire less energy to reach the temperature where fiammable gases are emitted.
3. Surface - Volume Ratio. Since all heat is originally received at the fuel surface, the surface area of the fuel determines
the heat input. Since this heat is eventually distributed through the whole mass of fuel, the volume of fuel
determines how fast the temperature will rise in the surface layer. If all fuels were the same shape, the effect would be
adequately determined by fuel thickness. But forest fuels vary in shape from smooth cyclinders to rough plates, so thickness
alone is not an adequate ruide to ignitibility or heat content. The higher the ratio of fuel surface to fuel volume, the
higher the rate of combustion, once the fuel particle is ignited. Table 6 lists the surface volume ratios of some common
forest fuels and fuel mixtures.
I
!
I
SECRET 38