TM 5-815-1/AFR 19-6
-- Smaller units required because lower gas
may, in certain cases, preclude the use of
temperatures reduce gas volume,
otherwise satisfactory equipment.
(5) Refuse disposal needs. Methods of removal
-- Less oxygen required in the effluent stream
since catalyst promotes efficient use of oxy-
and disposal of collected materials will vary
gen,
with the material, plant process, quantity
-- Lower NOx emissions due to lower flame
involved, and collector design (chap 6, 7, and
temperatures and reduced oxygen loads.
9). Collectors can be unloaded continuously,
(4) The principle disadvantages are:
or in batches. Wet collectors can require
-- High initial capital equipment costs
additional water treatment equipment and if
-- Periodic replacement of expensive catalysts
the pollutation control device uses water
(5) Absorbers. Absorption systems for odor con-
directly or indirectly, the supply and disposal
trol involve the use of selected liquid absor-
of used water must be provided for.
bents to remove odorous molecules from
12-8. Tradeoffs and special considerations
effluent gases. The gas to be absorbed should
have a high solubility in the chosen absorbent
a. Design considerations. In order to design equip-
or should react with the absorbing liquid.
ment to meet air pollution control requirements, the
Various methods are used to affect intimate
top output or maximum ratings should be used in the
contact of liquid absorbent and gaseous
selection of control equipment. The additional cost for
pollutant.
extra capacity is negligible on the first cost basis, but a
later date addition could cost a substantial sum. It
12-7.
Technical evaluation of control
should also be noted whether the dust-generating pro-
equipment
cess is continuous or cyclic, since an average dust con-
a. Given the site-specific ambient air quality
centration design may not satisfy high emissions at
requirements, and the NSPS emissions limitations, and
start-up or shut-down. Cyclic operation could also lead
then comparing them with the uncontrolled emissions
to problems in terms of equipment performance rela-
data for the combustor, it becomes possible to make a
tive to high or low temperatures and volumes. Duct-
selection of various emissions controls systems to meet
work providing good gas distribution arrangements for
the emission restraints. Required is a knowledge of the
a specific volume could cause significant problems if
various emissions control devices and their application
the gas volume were to increase or decrease.
to specific problems including their sizing and
operation.
equipment is relatively expensive, and due to the fact
b. Other factors which must be evaluated in selecting
that it is usually an integral part of the power
control equipment include: site compatibility; dis-
generation process, it is of utmost importance that the
position of the collected pollutant; installation and
equipment provide reliable service. Wrong choices of
operation costs; maintainability; and the ability to
fabric for fabric filters; wrong materials of construction
provide continuous protection during operation of the
for wet scrubbers; the wrong choice of a multicyclone
combustion units. Tables 12-1 and 12-2 offer a com-
to achieve high efficiency on fine particles; can all lead
parison of these characteristics to serve as an aid in the
to collector outages, or complete failure. Collector
final determination of the best control system for a
failures may be accompanied by a loss of production or
particular application.
by expensive replacement with new devices. Evalua-
c. Specific operating characteristics that should be
tion trade-offs should be made between costs for an
compared in evaluating suitable collection equipment
auxiliary control unit and the cost of shutting down the
are listed below. Each control device section of this
entire process due to collector failure.
c. Space allowance. Special consideration by the
manual should be consulted for specific descriptions of
various control equipment.
design engineer must be given to provide space in the
(1) Temperature and nature of gas and particles.
planned plant layout for adding more pollution control
Collection equipment must be compatible
equipment in the future. Future plant modifications will
with operating temperatures and chemical
in most cases have to meet more stringent standards
composition of gas and particles.
than the existing NSPS.
(2) Collector pressure loss. The power require-
d. Gas cooling. When high temperature (greater than
ment for gas-moving fans can be a major cost
450 degrees Fahrenheit) exhaust gasses are being
in air pollution control.
handled, a study should be made on the cost of install-
(3) Power requirement. Electrostatic pre-
ing equipment to operate at the elevated temperature
cipitators, scrubbers, and fabric filters have
versus the cost and effects of gas cooling.
e. Series operation of collectors. Dust collectors
additional electrical requirements beside fan
power.
may be used in series operation for the following
(4) Space requirement. Some control equipment
reasons:
requires more space than others. This factor
(1) A primary dust collector acts as a precleaner
12-6
Previous Page
