TM 5-815-1/AFR 19-6
o. Dry furnace injection of limestone. In this system,
calcined to lime. The lime reacts with the SO2 present
in the combustion gases to form calcium sulfate and
dry ground limestone is injected into the boiler where
calcium sulfite. As the gas passes through a wet scrub-
it is calcined and reacts with the 502 formed during
ber, the limestone, lime, and reacted lime are washed
combustion of the fuel. The flue gases containing the
with water to form sulfite. As the gas passes through a
sodium sulfate, sodium sulfite, unreacted limestone,
wet scrubber, the limestone, lime, and reacted lime are
and fly ash all exit the boiler together and are captured
washed with water to form a slurry. The resulting
on a particulate collector. The cleaned flue gases pass
effluent is sent to a settling pond and the sediment is
through the filter medium and out through the stack
disposed by landfilling. Removal efficiencies are below
(fig 10-1a).
p. Magnesium oxide (MgO) scrubber This is a
50% but can be reliably maintained. Scaling of boiler
tube surfaces is a major problem.
regenerative system with recovery of the reactant and
k. Scrubber injection of limestone. In this FGD sys-
sulfuric acid. As can be seen in figure 10-2 the flue gas
tem limestone is injected into a scrubber with water to
must be precleaned of particulate before it is sent to the
form a slurry (5 to 15% solids by weight). The
scrubber. An ESP or venturi scrubber can be used to
limestone is ground into fines so that 85% passes
remove the particulate. The flue gas then goes to the
through a 200-mesh screen. CaCO3 absorbs SO2 in the
MgO scrubber where the principal reaction takes place
scrubber and in a reaction tank where additional time
between the MgO and SO2 to form hydrated magne-
is allowed to complete the reaction. Makeup is added
sium sulfite. Unreacted slurry is recirculated to the
to the reusable slurry as necessary and the mixture is
scrubber where it combines with makeup MgO and
recirculated to the scrubber. The dischargable slurry is
water and liquor from the slurry dewatering system.
taken to a thickener where the solids are precipitated
The reacted slurry is sent through the dewatering sys-
and the water is recirculated to the scrubber.
tem where it is dried and then passed through a recov-
Limestone scrubbing is a throwaway process and
ery process, the main step of which is calcination. High
sludge disposal may be a problem. At SO2 removal
reliability of this system has not yet been obtained. SO2
efficiencies of about 30%, performance data indicate
removal efficiencies can be high, but scaling and corro-
that limestone scrubbers have a 90% operational
sion are major problems.
q. Wellman Lord process. Sodium sulfite is the
reliability. Plugging of the demister, and corrosion and
erosion of stack gas reheat tubes have been major
scrubbing solution. It captures the SO2 to produce
problems in limestone scrub-hers. Figure 10-1 shows
sodium bisulfite, which is later heated to evolve SO2
and regenerate the sulfite scrubbing material. The SO2
a simplified process flow-sheet for a typical limestone
rich product stream can be compressed or liquified and
scrubbing installation.
l. Scrubber injection of lime. This FGD process is
oxidized to sulfuric acid, or reduced to sulfur. Scaling
similar to the limestone scrubber process, except that
and plugging are minimal problems because the
lime (Ca(OH)2) is used as the absorbent. Lime is a
sodium compounds are highly soluble in water. A
more effective reactant than limestone so that less of it
Wellman-Lord unit has demonstrated an SO2 removal
is required for the same SO2 removal efficiency. The
efficiency of greater than 90 percent and an availability
decision to use one system over the other is not clear-
of over 85 percent. The harsh acid environment of the
cut and usually is decided by availability.
system has caused some mechanical problems (See
m. Post furnace limestone injection with spray dry-
figure 10-3).
r. Catalytic oxidation. The catalytic oxidation pro-
ing. In this system, a limestone slurry is injected into a
spray dryer which receives flue gas directly from the
cess uses a high temperature (850 degrees Fahrenheit)
boiler. The limestone in the slurry reacts with the SO2
and a catalyst (vanadium pentoxide) to convert SO2 to
present in the combustion gases to form calcium
SO3. The heated flue gas then passes through a gas heat
sulfate and calcium sulfite. The heat content of the
exchanger for heat recovery and vapor condensation.
combustion gases drives off the moisture resulting in
Water vapor condenses in the heat exchanger and com-
dry particulates exiting the spray dryer and their
bines with SO3 to form sulfuric acid. The acid mist is
subsequent capture in a particulate collector following
then separated from the gas in an absorbing tower. The
the spray dryer. The particulates captured in the
flue gas must be precleaned by a highly efficient par-
collector are discharged as a dry material and the
ticulate removal device such as an electrostatic pre-
cleaned flue gases pass through the filter to the stack
cipitator preceding the cat-ox system to avoid
(fig 10-la).
poisoning the catalyst. The major drawback of this
n. Dry, post furnace limestone injection. Ground dry
system is that it cannot be economically retro-fitted to
limestone is injected directly into the flue gas duct prior
existing installations (fig 10-4).
s. Single alkali sodium carbonate scrubbing. In
to a fabric filter. The limestone reacts in the hot
order to eliminate the plugging and scaling problems
gases and is deposited on the filter bags as sodium sul-
associated with direct calcium scrubbing, this FGD
fate and sodium sulfite. The dry particulate matter is
system was developed. As shown in figure 10-5, the
then discharged to disposal and the cleaned flue gases
process is a once through process involving scrubbing
pass through the filter medium to the stack (fig 10-lb).
10-4
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