TM 5-815-1/AFR 19-6
CHAPTER 3
BOILER EMISSIONS
3-1.
Generation processes
(2) Residuals. Residual fuel oils (No.4, No.5,
No.6) contain a greater amount of ash, sedi-
The combustion of a fuel for the generation of steam or
ment, sulfur, and nitrogen than is contained in
hot water results in the emission of various gases and
distillates. They are not as clean burning as
particulate matter. The respective amounts and chem-
the distillate grades.
ical composition of these emissions formed are depen-
c. Gaseous fuel. Natural gas, and to a limited extent
dent upon variables occurring within the combustion
liquid petroleum (butane and propane) are ideally
process. The interrelationships of these variables do not
suited for steam generation because they lend them-
permit direct interpretation by current analytical
selves to easy load control and require low amounts of
methods. Therefore, most emission estimates are based
excess air for complete combustion. (Excess air is
upon factors compiled through extensive field testing
defined as that quantity of air present in a combustion
and are related to the fuel type, the boiler type and size,
chamber in excess of the air required for stoichiometric
and the method of firing. Although the use of emission
combustion). Emission levels for gas firing are low
factors based on the above parameters can yield an
because gas contains little or no solid residues,
accurate first approximation of on-site boiler
noncombustibles, and sulfur. Analyses of gaseous fuels
emissions, these factors do not reflect individual boiler
may be found in "Perry's Chemical Engineering
operating practices or equipment conditions, both of
Handbook".
which have a major influence on emission rates. A
d. Bark and wood waste. Wood bark and wood
properly operated and maintained boiler requires less
waste, such as sawdust, chips and shavings, have long
fuel to generate steam efficiently thereby reducing the
been used as a boiler fuel in the pulp and paper and
amount of ash, nitrogen and sulfur entering the boiler
wood products industries. Because of the fuel's rela-
and the amount of ash, hydrocarbons, nitrogen oxides
tively low cost and low sulfur content, their use outside
(NOx) and sulfur oxides (SOx) exiting in the flue gas
these industries is becoming commonplace. Analyses
stream. Emissions from conventional boilers are dis-
of bark and wood waste may be found in
cussed in this chapter. Chapter 13 deals with emissions
Environmental
Protection
Agency,
"Control
from fluidized bed boilers.
Techniques for Particulate Emissions from Stationary
Sources". The fuel's low heating value, 4000-4500
3-2.
Types of fuels
British thermal units per pound (Btu/lb), results from
a. Coal. Coal is potentially a high emission produc-
its high moisture content (50-55 percent).
ing fuel because it is a solid and can contain large
e. Municipal solid waste (MSW) and refuse derived
percentages of sulfur, nitrogen, and noncombustibles.
fuel (RDF). Municipal solid waste has historically been
Coal is generally classified, or "ranked", according to
incinerated. Only recently has it been used as a boiler
heating value, carbon content, and volatile matter. Coal
fuel to recover its heat content. Refuse derived fuel is
ranking is important to the boiler operator because it
basically municipal solid waste that has been prepared
describes the burning characteristics of a particular
to burn more effectively in a boiler. Cans and other
coal type and its equipment requirements. The main
noncombustibles are removed and the waste is reduced
coal fuel types are bituminous, subbituminous,
to a more uniform size. Environmental Protection
anthracite, and lignite. Bituminous is most common.
Agency, "Control Techniques for Particulate Emissions
Classifications and analyses of coal may be found in
from Stationary Sources" gives characteristics of refuse
"Perry's Chemical Engineering Handbook".
derived fuels.
b. Fuel oil. Analyses of fuel oil may be found in
"Perry's Chemical Engineering Handbook".
3-3.
Fuel burning systems
(1) Distillates. The lighter grades of fuel oil
a. Primary function. A fuel burning system provides
(No.1, No.2) are called distillates. Distillates
controlled and efficient combustion with a minimum
are clean burning relative to the heavier
emission of air pollutants. In order to achieve this goal,
grades because they contain smaller amounts
a fuel burning system must prepare, distribute, and mix
of sediment, sulfur, ash, and nitrogen and can
the air and fuel reactants at the optimum concentration
be fired in a variety of burner types without a
and temperature.
need for preheating.
3-1
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