TM 5-815-1/AFR 19-6
(2) SO2. Use wet scrubbing system with a low
Emission levels generally vary considerably
pressure drop.
with the type and size of unit and are also a
(3) NOx. May be controlled by utilizing limited
function of loading.
excess-air firing; flue gas recirculation; staged
(4) In some large boilers, several operating modi-
combustion; or combinations of these.
fications have been employed for NOx
(4) Opacity. May be controlled by limiting or col-
control. In staged combustion, for example,
lecting the particulates and by properly
including off-stoichiometric firing, also called
adjusted and designed combustion controls
"biased firming," some burners are operated
with good burner and furnace designs.
fuel-rich, some fuel-lean, while others may
d. Pulverized coal-fired power boiler. NSPS cover
supply air only. In two-staged combustion,
limitations for particulates; SO2; NOx; and opacity.
the burners are operated fuel-rich (by
Methods of modifying or controlling emissions are dis-
introducing only 80 to 95 percent
cussed in the following.
stoichiometric air) with combustion being
(1) Particulates.
completed by air injected above the flame
(a) Control by use of electrostatic
zone through second-stage "NOx -ports". In
precipitator
staged combustion, NOx emissions are
(b) Control by use of fabric filters
reduced because the bulk of combustion
(c) Control by use of venturi scrubber
occurs under fuel-rich, reducing conditions.
(d) Control by combination of a mechanical
collector followed by either (a), (b), or
ticulates; SO2; NOx; and opacity. Methods of modifying
(c), above
or controlling emissions are discussed in the following.
(2) SO2..
(1) Particulate. The user should note that in most
(a) Use suitable wet scrubber (can double for
cases external pollution control devices are
both SO2 and particulates)
not required for boilers firing No.1 or No.2
(b) Use suitable dry scrubber followed by
fuel oils.
fabric filters or electrostatic precipitator
(2) SOx. Most distillates will contain sulfur quan-
(c) Selection of a wet or dry scrubbing
tities low enough so that no treatment will be
system is determined by evaluating the
necessary. However, a fuel analysis must be
economics (installation and operating
reviewed as some distillates can have as much
costs) and the disposal of the collected
as one percent sulfur. When the sulfur content
pollutant.
produces SO2 emissions in excess of the
(3) NOx. Ensure that the burner and furnace are
allowable a wet scrubbing system will be
designed for limited excess-air firing and
required.
staged combustion. In some cases it may be
necessary to have a second stage air fan
controls, and burners and furnaces designed
designated as an NOx control fan in order to
to limit NOx generation from high combustion
gain compliance.
temperatures. Usually NOx reductions are
(4) Opacity. This may be controlled by
accomplished by limiting excess air firing and
particulate removal and properly adjusted
staged combustion. Large utility system units
combustion controls. In some cases this could
sometimes also employ flue-gas recirculation
be the more stringent requirement for
in addition to the other methods.
particulate removal.
(4) Opacity. This may be controlled by proper
e. Spreader and mass feed stoker coal fired boilers
air-fuel ratios; good combustion controls;
with a traveling grate. NSPS cover limitations for par-
limiting particulate emissions; and proper
ticulates; SO2; NOx; and opacity. Methods of modifying
engineering design of the burners and furnace
or controlling emissions are discussed in the following.
chamber.
(1) Particulates.
c. Residual oil fired power boilers. NSPS cover par-
(a) Control by use of electrostatic precipitator
ticulates; SO2; NOx; and opacity. Methods of modifying
(b) Control by use of suitable fabric filter
or controlling emissions are discussed in the following.
(c) Control by use of suitable wet scrubber
(1) Particulate control.
(d) Control by a combination of a mechanical
(a) When using low-sulfur oils, cyclonic
collector followed by either (a), (b), or
mechanical collectors are usually
(c) above
adequate. On larger utility size units
(2) SO2.
electrostatic precipitators are employed to
(a) Use suitable wet scrubber (can double for
limit particulate emissions.
both SO2 and particulate).
(b) For emissions from combustion of high-
(b) Use suitable dry scrubber followed by
sulfur oils a wet scrubbing system can be
either a fabric filter or an electrostatic
used for both SO2
removal and
precipitator
particulate control.
12-3
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