FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

1. Combustion Process.
a. Chemistry of combustion. The following is a summary of
information provided by Byram (1959). Wood is a chemically complex
substance, composed primarily of cellulose and lignin, of which carbon,
hydrogen, and oxygen are the primary constituents. When wood burns
in a completely efficient manner, it combines with atmospheric oxygen,
and produces water, carbon dioxide, and energy. Some of the water
that results from combustion is evaporated from the fuel, but a larger
proportion is a product of the chemical reaction.
b. Phases of combustion. The four phases of the combustion process
are described in Chapter II.B.2, this Guide. Some strategies for smoke
management rely on manipulation of the amount of fuel consumed in
each combustion phase. The types of emissions and factors regulating
their production will be discussed with respect to the phases of
combustion. For a more complete discussion of the phases of
combustion, see Sandberg et al. (1978).
(1) Pre-ignition phase. During the pre-ignition phase, gases, vapors,
tars and charcoal are produced. The proportions and amounts vary
widely according to the conditions under which pyrolysis occurs. If rapid
heating occurs during pyrolysis, less charcoal, a lot of tar, and highly
flammable gases are produced. Slow heating during the pyrolysis
process results in the production of more charcoal, little tar, and lower
amounts of flammable gases (Sandberg et al. 1978).
(2) Flaming phase. The following is from Ryan and MacMahon (1976 in
Sandberg et al. 1978). The principal chemical by-products of flaming
combustion are carbon dioxide and water. However, some pyrolyzed
substances cool and condense without passing through the flaming
zone. Other substances are only partially oxidized as they pass through
the flames, and many combustion by products are produced. Low
molecular weight organic compounds may remain as gases and are
dispersed by wind. Tar droplets and particles of soot result from the
cooling and condensation of compounds with higher molecular weights.
Visible smoke consists of mostly tar, soot, and water vapor.
(3) Smoldering phase. The lower temperatures of the smoldering
phase allow some gases to condense as visible smoke. Smoldering
fires produce at least twice the emissions of flaming fires. Heat release