Environmental Engineering Reference
In-Depth Information
To stack
Steam drum
Water
wall
Cyclone
Coal
hopper
Boiler tubes
Limestone
hopper
Fluidized
bed
Recycle
Transport
air
Grate
Combustion air
Bed drain
Removal
Figure 5.10
Fluidized bed combustor, schematic.
The capture efficiency of SO
2
in FBC is dependent on the intimate mixing of coal and limestone
and is also dependent on their quality. Calcined limestone containing CaO is found to be a better
sorbent than raw limestone. On the average, FBC with limestone injection captures only about
40-60% of the sulfur in coal.
The advantages of FBC are as follows:
•
Low-quality coal and other fuels can be used.
•
It has a thermal efficiency comparable to that of pulverized coal combustion with emission
controls.
•
Sorbent is cheap (limestone).
•
Combustion temperature is lower (800-900
◦
C) than that in pulverized coal combustion
(1900-2000
◦
C), which reduces thermal NO
x
formation.
The disadvantages of FBC are as follows:
•
Sulfur capture efficiency is only about 40-60%, and it may not meet emission standards for
SO
2
.
•
It has a constant capacity and cannot follow load.
•
It has start-up difficulties.
•
It causes corrosion and fouling of boiler tubes.
•
Because of the stringent requirement for proper mixing of fuel, limestone, and air, it cannot
be scaled to large power plant loads; it is suitable only for 10- to 100-MW-capacity boilers.
In summary, FBC is best suited for combustion of low-quality fuels, such as nonpulverizable
coal, crude oil residues, wood, and municipal and industrial waste. Its sulfur emission reduc-
