Environmental Engineering Reference
In-Depth Information
Transformer
Duct
Particle-laden
air in
Clean air
out to stack
Duct
Plates
Wire electrodes
Rapper
Hopper
Figure 5.8
Electrostatic precipitator, cutaway.
a structure half the size of the boiler of a power plant. The power drawn by the ESP, including the
fans that drive the flue gas through it, and the mechanical energy for rapping the plates consume
less than 0.1% of the plant's power output. The levelized cost (amortized capital and operating
cost) of running an ESP may add 6-10% to the generating cost of electricity.
8
The relatively low efficiency of small-particle collection by existing ESPs poses a problem
to the operators of existing power plants because environmental protection agencies in several
countries, notably the United States, plan to introduce new ambient standards for particles less than
2.5
m in diameter. The new standard may require retrofitting of power plants with devices that
collect more efficiently small particles, such as a fabric filter.
µ
Fabric Filter.
A fabric filter, also called
baghouse,
works on the principle of a domestic vacuum
cleaner. Particle-laden gas is sucked into a fabric bag, the particles are filtered out, and the clean
gas is vented into the atmosphere. The pore size of the fabric can be chosen to filter out any size of
particles, even submicron particles, albeit at the expense of power that is required to drive the flue
gas through the pressure drop represented by the fabric pores.
A typical fabric filter schematic is shown in Figure 5.9. Long, cylindrical tubes (bags) made of
the selected fabric are sealed at one end and open at the other end. The sealed end of the tubes are
hung up-side down from a rack that can be shaken mechanically. The particle-laden flue gas enters
through the bottom, open end of the tubes. The clean flue gas is sucked through the fabric of the
tubes by fan-induced draft or special pumps and is vented through the smoke stack. A baghouse for
a large power plant may contain several thousand tubes, each up to 4 meters high, 12.5 to 35 cm in
8
Data on emission control efficiency and cost are obtained from “Air pollution control costs for coal fired
power station,” International Energy Association Coal Research, IEAPER/17, London, UK (1995).
