Fossil-Fueled Power Plants - Energy and the Environment - page 101

is calculated from the Deutsch equation

η = 1 − exp ( − w A / Q ) (5.1)

where is the migration speed, A is the total area of the plates, and Q is the volumetric flow rate.

The migration speed is approximated from

w

05 d p ms − 1

w ≈ 0 . when the particle diameter d p is

given in is a function of the voltage and the electrical resistivity of the particles.

Ironically, the higher the sulfur content of the coal, the lower the resistivity of the fly ash and the
higher the migration speed. However, the higher the sulfur content of the coal, the higher acid gas
emissions, so there is a trade-off.
For example, consider a 1000-MW coal-fired power plant with a thermal efficiency of 36%
using coal with a heating value of 12,000 Btu lb − 1 (7.75 kWh kg − 1 ). This plant would consume
coal at a rate of 99.6 kg s − 1 . Assuming that the atomic composition of the combustible part of coal
is CH and the coal is burned with 20% excess air, the combustion generates flue gas at the rate of
Q

µ m. Actually, w

= 1260 m 3 s − 1 . If particles of 1- µ m diameter are to be collected with 95% efficiency, we need a

plate area A 5 m, and two sides of the plate are collecting

particles, we need 377 plates. Figure 5.7 gives the collection efficiency of an ESP with the above
specifications as a function of particle diameter. It is seen that for particle diameters greater than
1

= 75 , 500 m 2 . If each plate measures 20 ×

m, the efficiency approaches 100%, but submicron particles are not collected efficiently at all.
A cutaway drawing of a large ESP is shown in Figure 5.8. Typically the overall width of an
ESP is about 20-30 m, and the overall length is 18-20 m. Typical plate dimensions are 20

µ

5m,
plate spacing is 0.25 m, and the number of plates are in the hundreds. After exiting from the boiler,
the flue gas is transferred to the ESP in large ducts. From the ESP, the cleansed flue gas enters the
smoke stack. The ESP, together with its housing, transformers, hoppers, and truck bays, may occupy

×

1.0

0.8

0.6

0.4

0.2

0

0 1 2 3

Particle diameter d p , m

Figure 5.7 Particle collection efficiency of electrostatic precipitator as a function of particle diameter
calculated from the Deutsch equation (5.1). Flue gas volumetric flow rate Q = 1260 m 3 s − 1 , plate area
A = 75 , 500 m 2 , and drift velocity w = 0 . 05 d p ms − 1 , where d p is given in micrometers.