In forest fires, the two products of complete oxidation– Carbon Dioxide (CO2) and water vapor – make up over 90 percent of the mass emitted. The other 10 percent contains most of the problem compounds. The major products are: Carbon Monoxide, Particulate Matter, Hydrocarbons and other organic compounds. The more intense the fire, the more complete the combustion process and a bigger percent of the smoke will be carbon dioxide and water vapor.
Carbon Dioxide is an odorless and colorless gas formed abundantly in nature by the decomposition of organic material including wood. It is exhaled by man and absorbed by plants from the air in the process of photosynthesis. It is also use to make carbonated drinks.
The amount of water is important because it interacts with the other combustion products reducing the efficiency of the combustion process. It can also reduce visibility for a short distance.
Carbon monoxide is an odorless and colorless gas that is highly toxic and regulated by EPA. Fairly high concentrations are produced but it is very light and is rapidly diluted in the open. As a result, it is not a problem.
Hydrocarbons and other organic compounds are not produce in sufficient quantities to be a problem.
Particulate matter is complex mixture of small solid or tar (liquid) particles. The size, shape, density and other physical properties are highly variable but the individual particles are too small to be seen with the naked eye. The small pieces of partially or unburned wood that fall out fairly rapidly are not included since they cause no visibility problem but they can become firebrands .
These small particles can remain suspended in the atmosphere for long periods, even weeks. They drastically reduce visibility. The small size also penetrates into the lungs, much the same as tobacco smoke, and can aggravate breathing problems such as emphysema. Particulate matter is the major problem with smoke due to the reduction in visibility. These small particles absorb and scatter light washing out the contrast. They can also act as nuclei for the formation of fog but there are generally enough material in the air already to act as nuclei.
combustion process results in a rapid release of stored energy.
The three broad phases are: pre-ignition,
flaming, and residual (smoldering).
The amount and type of smoke produced will vary during each phase.
The amount and type of smoke produced will vary during each phase.
|1.||During pre-ignition, heat is being absorbed by the fuel. The fuel’s temperature is being raised to ignition temperature. At the same time, all the moisture is being evaporated and driven off. Small amounts of white smoke is produced--predominately water vapor.|
|2.||The flaming phase begins when the fuel reaches ignition
temperature and erupts into flames. The
fuel is now going through a chemical change with the release of large amounts of
heat and gases. When mixed with
oxygen, the heated gases ignite, rapid oxidation occurs,
amounts of smoke is produced.
The smoke is lifted off the ground by the heat that is being produced. The combustion process is very efficient during this phase. Smoke is mostly carbon dioxide and water vapor.
|3.||The residual (smoldering) phase occurs when the flaming front has
moved on and some of the unconsumed fuel is still going through the combustion
process--slowly, with little heat being released. However, a large amount of smoke is produced.
Some small amounts of flaming may be occurring in scattered areas but not
enough heat is being generated to continue the smoke column.
Little fuel is being consumed even though a lot of smoke is being produced.
darker because more particulates are being produced. This phase is caused when heat, oxygen, and flammable gases are reduced
to the extent that the flaming phase is not sustained.
Reduced efficiency of the combustion process allows the temperature to
drop. With insufficient heat to produce a convective smoke column,
the smoke is concentrated close to the ground.
|4.||The glowing phase is when no visible smoke is seen but the fuel is
glowing similar to charcoal. Very
little of this phase occurs in open forest type burning and it is not an
Question: How does heat lift smoke?
the amount of fuel that is consumed by fire is doubled (and all other factors
remain the same), the amount of smoke produced will generally double.
However, there are many other physical and chemical factors of fuel that
affect the amount of smoke.
Prescribed fire should burn off litter but not any of the humus. WHY? (Protects soil and would produce more smoke)
Moisture is the most important of all fuel factors. Enough heat must be present to drive off all the moisture in
the fuel. If a lot of moisture is
present, much of the heat energy is used in converting the moisture to steam.
Consequently, the combustion process lacks intensity. More
smoke will be produced because more of the fuel will be consumed during the
A. High Moisture Content
Uses a lot of energy driving out the moisture
Less intense fire--less heat left for the combustion process
Higher concentration of particulates due to less intense fire
Shorter flaming phase and more residual phase
More smoke and not lifted as high
B. Low Moisture Content
Less energy used
Reaches ignition temperature quicker
More intense fire
Longer flaming phase
shape of fuel has a decided effect on the combustion process.
Flat-shaped fuel has a high surface-to-volume ratio.
This allows the moisture in the fuel to escape more rapidly. Consequently, the fuel will be drier. It will ignite and burn readily, producing a
minimal amount of smoke since a larger percentage of the fuel will be consumed
during the flaming phase. Round
fuel has a lower surface-to-volume ratio. Consequently,
it will be “wetter” due to less surface area for the moisture to escape. A larger percentage of the fuel will be consumed
during the residual phase, producing more smoke than flat-shaped fuel.
size of fuel affects burning intensity and smoke production in much the same way
as shape. Small fuel has a higher
surface-to-volume ratio than larger fuel that allows the moisture to escape more rapidly and
results in being drier than large fuel under the same conditions.
of fuel affects smoke production by changing both the rate of drying and the
availability of oxygen required for the combustion process.
Compacted fuel dries out slowly and results in a wetter fuel.
The result is a low intensity fire with most of the fuel
being consumed during the residual phase.
Compacted fuel reduces combustion efficiency
to the slow moving front, backing fires consume almost all of the fuel during
the flaming phase. Although less
intense, more of the heat is held close to the ground and if low fuel, will generally consume
as much of the fuel as a heading fire.
On the other hand, a heading fire will consume approximately half of the
fuel during the flaming phase and the other half during the residual phase on
a typical prescribed burn. The net result: Heading
fires produce about three (3) times as much smoke and particulates as backing fires!
Spot and flanking fires will produce somewhere between heading and backing
fires, depending on the type and condition of fuel.
|Smoke Emissions Are Affected By:|
Size and Shape
|Oils and Tars
|More Intense Fire Result In:|
More fuel consumed
More burned during flaming phase
|Smoke lifted higher
Burning completed earlier
Less residual smoke