TM 5-815-1/AFR 19-6
construction for the collector. Temperature may also
Resistivity must be determined if an electrostatic pre-
cipitator is to be selected to control particulate emis-
limit use of certain collectors. For instance, tem-
sions. As a general guideline, resistivity above 1010
peratures above 550 degree Fahrenheit rule out the use
ohm-cm normally rules out the use of electrostatic pre-
of fabric filters.
cipitation unless provisions are made for particulate
d. Pressure. Carrier gas pressure must be known or
electrical conditioning.
calculated to determine the structural requirements for
e. Handling characteristics. Particle-handling
the collector under operating and upset conditions.
characteristics influence dust-handling systems (duct-
e. Viscosity. Gas viscosity is a measure of molecular
work, collector structure, hoppers, conveyors) and
activity in a gas stream. The greater the gas viscosity,
materials of construction. Dust-handling characteristics
the greater the resistance to particle migration across
include flow properties, abrasiveness, hygroscopicity,
the stream normal to gas flow. Since gas viscosity
moisture content, agglomerating tendencies. These
increases with gas temperature, it is an important factor
properties, including specific gravity and bulk density
in the performance of dry particulate collection
should be evaluated in the design of a dust-collecting
devices. viscosity effects can be minimized if equip-
system.
ment is properly specified.
f. Moisture content. Moisture content affects the
particulate affects materials of construction and design
performance of collection equipment and the choice of
of the collector and ash disposal equipment as does
construction materials. It is important to know the dew
carrier gas composition.
point of the exhaust gas, as temperatures below dew
point allows acid vapors to condense and attack struc-
12-4.
Application of emission control sys-
tural surfaces. This is a particular concern with boiler
tems for boilers.
flue-gas which often contains a significant amount of
sulfuric acid vapor.
As a result of current, stringent, stack emission regula-
tions, applications of certain conventional emissions
primarily affects the choice of construction materials
control systems have evolved that provide satisfactory
for a collector. Collectors must be suitably protected to
performance when properly sized and specified. Refer-
handle corrosive gases.
enced are CFR40 part 60 for new source performance
h. Toxicity. Handling of toxic gases requires special
standards (NSPS) only, as ambient regulations have
treatment and equipment and must be reviewed on an
wide variation from site-to-site requiring investigation
individual basis. This manual does not address incin-
for each location. Following is a brief description of the
eration of toxic or hazardous wastes.
most common combustion sources, fuels, and control
devices employed:
12-3.
Particulate properties
a. Natural gas fired power boiler. NSPS cover par-
a. Particulate properties that must be determined for
and opacity.
control equipment selection and design are described
(1) External devices are not usually required.
below. Appropriate test methods are listed in table 5-1.
Properly adjusted combustion controls,
b. Concentration (loading). Particulate loading is a
burner(s), furnace designs, and gas monitor-
measurement of particulate concentration in flue gases
ing are sufficient to meet the performance
(see this manual, chapters 2 and 3) expressed in grains
standards.
per cubic foot. Particulate loading is used as a criteria
(2) Even though natural gas is a relatively clean
to design and select applicable collection equipment.
fuel, some emissions can occur from the
Fluctuations in loading (for example: soot blowing in
combustion reaction. For example, improper
boilers) must be noted and maximum, minimum, and
operating conditions, including items such as
average values should be recorded. High grain loadings
poor mixing and insufficient air, may cause
may require a series system of control devices to meet
large amounts of smoke, carbon monoxide,
particulate emissions and air quality standards. For
and hydrocarbons to be produced. Moreover,
instance, a cyclone followed by an electrostatic pre-
because a sulfur-containing mercaptan is
cipitator or baghouse.
added to natural gas for detection proposes,
c. Particle size. The particle size analysis affects the
small amounts of sulfur oxides will also be
collection efficiency for each control device. Fine par-
produced in the combustion process.
ticulate collection requires high-efficiency equipment
(3) Nitrogen oxides are the major pollutants of
such as venturi scrubbers, electrostatic precipitators, or
concern when burning natural gas. Nitrogen
fabric filters.
dioxide emissions are a function of the tem-
perature in the combustion chamber and the
factor in the design of electrostatic precipitators.
rate of cooling of the combustion products.
12-2
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