TM 5-815-1/AFR 19-6
service regarding fuel selection, such as AR 420-49 for
there is some optimum value for these three
the Army's use.
requirements within the boiler's operating range which
must be met and maintained in order to minimize
3-7. Emission factors
emission rates. The optimum values for time,
temperature, and fuel-air mixing are dependent upon
Emission factors for particulates, SOx and NOx, are
the nature of the fuel (gaseous, liquid or solid) and the
presented in the following paragraphs. Emission factors
design of the fuel burning equipment and boiler.
were selected as the most representative values from a
b. Fuel type.
large sampling of boiler emission data and have been
(1) Gaseous fuels. Gaseous fuels burn more readily
related to boiler unit size and type, method of firing
and completely than other fuels. Because they
and fuel type. The accuracy of these emission factors
are in molecular form, they are easily mixed
will depend primarily on boiler equipment age,
with the air required for combustion, and are
condition, and operation. New units operating at lower
oxidized in less time than is required to burn
levels of excess air will have lower emissions than esti-
other fuel types. Consequently, the amount of
mated. Older units may have appreciably more. There-
fuel/air mixing and the level of excess air
fore, good judgement should accompany the use of
needed to burn other fuels are minimized in gas
these factors. These factors are from, Environmental
combustion, resulting in reduced levels of
Protection Agency, "Compilation of Air Pollutant
emissions.
Emission Factors". It should be noted that currently
(2) Solid and liquid fuels. Solid and liquid fuels
MSW and RDF emission factors have not been estab-
require more time for complete burning
lished.
because they are fired in droplet or particle
a. Particulate emissions. The particulate loadings in
form. The solid particles or fuel droplets must
stack gases depend primarily on combustion efficiency
be burned off in stages while constantly being
and on the amount of ash contained in the fuel which
mixed or swept by the combustion air. The size
is not normally collected or deposited within the boiler.
of the droplet or fired particle determines how
A boiler firing coal with a high percentage of ash will
much time is required for complete combus-
have particulate emissions dependent more on the fuel
tion, and whether the fuel must be burned on a
ash content and the furnace ash collection or retention
grate or can be burned in suspension. Systems
time than on combustion efficiency. In contrast, a
designed to fire solid or liquid fuels employ a
boiler burning a low ash content fuel will have particu-
high degree of turbulence (mixing of fuel and
late emissions dependent more on the combustion effi-
air) to complete combustion in `the required
ciency the unit can maintain. Therefore, particulate
time, without a need for high levels of excess
emission estimates for boilers burning low ash content
air or extremely long combustion gas paths. As
fuels will depend more on unit condition and operation.
a result of the limits imposed by practical boiler
Boiler operating conditions which affect particulate
design and necessity of high temperature and
emissions are shown in table 3-2. Particulate emission
turbulence to complete particle burnout, solid
factors are presented in tables 3-3, 3-4, 3-5 and 3-6.
and liquid fuels develop higher emission levels
b. Gaseous emissions.
than those produced in gas firing.
(1) Sulfur oxide emissions. During combustion,
sulfur is oxidized in much the same way carbon
3-6. Fuel selection
is oxidized to carbon dioxide (CO2). Therefore,
almost all of the sulfur contained in the fuel will
Several factors must be considered when selecting a
be oxidized to sulfur dioxide (SO2) or sulfur
fuel to be used in a boiler facility. All fuels are not
trioxide (SO3) in efficiently operated boilers.
available in some areas. The cost of the fuel must be
Field test data show that in efficiently operated
factored into any economic study. Since fuel costs vary
boilers, approximately 98 percent of the fuel-
geographically, actual delivered costs for the particular
bound sulfur will be oxidized to SO2, one per-
area should be used. The capital and operating costs of
cent to SO3, and the remaining one percent
boiler and emission control equipment vary greatly
sulfur will be contained in the fuel ash. Boilers
depending on the type of fuel to be used. The method
with low flue gas stack temperatures may pro-
and cost of ash disposal depend upon the fuel and the
duce lower levels of SO2 emissions due to the
site to be used. Federal, state and local regulations may
formation of sulfuric acid. Emission factors for
also have a bearing on fuel selection. The Power Plant
SOx are contained in tables 3-3, 3-4, 3-5, and
and Fuel Use Act of 1978 requires that a new boiler
3-6.
installation with heat input greater than 100 MMBtu
(2) Nitrogen oxide emissions. The level of nitrogen
have the capability to use a fuel other than oil or
oxides (NOx) present in stack gases depends
natural gas. The Act also limits the amount of oil and
upon many variables. Furnace heat release rate,
natural gas firing in existing facilities. There are also
temperature, and excess air are major variables
regulations within various branches of the military
3-3
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