Environmental Engineering Reference
In-Depth Information
tion capacity is limited. It is difficult to scale FBC to the large load requirements of centralized
power plants.
After Combustion
The removal of sulfur oxides from the flue gas after combustion of the fuel in a furnace or boiler
is called flue gas desulfurization (FGD). There are several methods of FGD: sorbent injection and
wet and dry scrubbers.
Sorbent Injection.
In sorbent injection (SI), a sorbent, usually dry sintered CaCO
3
or CaO, or a
slurry thereof, is injected into the flue gas in the upper reaches of the boiler, past the neck. The
sorption of SO
2
proceeds similarly as in FBC by forming a mixture of calcium sulfite and sulfate.
The capture efficiency is dependent on many factors: the temperature, oxygen and moisture content
of the flue gas, time of contact between sorbent and SO
2
, and the characteristics of the sorbent (e.g.,
sintered sorbent, porosity, admixture of other sorbing agents). The resulting particles, consisting
of hydrated calcium sulfite and sulfate and unreacted sorbent, in addition to the fly ash, need to be
captured in an electrostatic precipitator or baghouse.
Sorbent injection can be retrofitted to existing coal-fired power plants, albeit the particle
removal system may have to be upgraded to collect the considerable larger load of particles. The
sulfur capture efficiency is on the average 50%, which may be adequate for meeting emission
reduction quotas for
existing
power plants, but not enough for emission standards of
new
power
plants.
Wet Scrubber.
In a wet scrubber the flue gas is treated with an aqueous slurry of the sorbent, usually
limestone (CaCO
3
) or calcined lime (CaO), in a separate tower. A schematic of a wet scrubber is
shown in Figure 5.11. After exiting the electrostatic precipitator, the flue gas enters an absorption
tower where it is sprayed through an array of nozzles with a slurry of the sorbent. The following
sequence of reactions takes place between SO
2
and the sorbent slurry:
1
2
H
2
O
1
2
H
2
O
CaCO
3
+
SO
2
+
→
CaSO
3
·
+
CO
2
(5.5)
1
2
H
2
O
3
2
H
2
O
1
2
O
2
→
CaSO
3
·
+
+
CaSO
4
·
2H
2
O
(5.6)
The water molecules that are attached to calcium sulfite and sulfate are called water of crystalliza-
tion. Hydrated calcium sulfate is similar to natural gypsum.
The formed mixture of hydrated calcium sulfite and sulfate, together with some unreacted
limestone, falls to the bottom of the wet scrubber in the form of a wet sludge, from where it
is transferred into a funnel-shaped thickener. The top liquor of the thickener is decanted into an
overflow tank, from where it is recycled to make up a fresh slurry of limestone. The settled thick
sludge is pumped into a vacuum filtering system where it is dewatered as much as possible. Both
hydrated CaSO
3
and CaSO
4
are difficult to dewater; they form a gelatinous sludge. This sludge
may be thickened further with fly ash coming from the ESP, then disposed into a landfill.
Above the spray nozzles, a mist eliminator condenses water. The clean flue gas enters a reheater
(to add buoyancy), and then it exits through the smoke stack.
While perfected over the past decades, the wet scrubber still poses many operational problems.
The spray nozzles tend to clog; sludge often clings to the bottom and side of the absorption tower,
