Page 301 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

278 10 Post-combustion Air Emission Control

Typical ESP has a cut size of 0.5 μm in diameter and airflow velocity ranges
1–1.5 m/s [22]. There has been a vast market of ESPs; however, fabric filters work
best for certain particle due to its independence of particle resistivity. Also the dust
cakes on the filters remove more pollutant gases than ESPs. ESPs are cheaper than
filter and wet scrubber due to low operational cost, but much more expensive than
cyclones. ESPs are not applicable for gaseous pollutants and require well-trained
specialized personnel to operate due to the high voltage (tens of kilovolts) required
for corona discharging.

10.2.1 Electrostatic precipitator Designs

Electrostatic precipitators (ESPs) are widely used for particulate emission control
from combustion of fossil fuels, mainly coal. As introduced in Part I, the basic
principle of ESPs involves the following three steps in particle separation:
• Charging of particles
• Particle movement relative to the gas flow
• Particle deposition on a collection surface.
In an engineering practice, we shall also consider the removal of the deposited
particles, or dust cake, from the collection surface for continuous operation. Dust
cake on the surface of the collection plates can be removed by mechanical vibration
or a knock off hammer. This usually results in re-entrainment of particles.
Alternatively wet removal by water spray can be considered. Wet ESPs have
very high efficiencies because they are operated with a stream of water that con-
tinuously removes the dust from the collector surfaces as slurry to reduce the
particle re-entrainment. Many of them are found near Japanese cities where the
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particulate emission standard is the most stringent (10 mg/m or lower at STP).
Their footprints are also smaller than regular ESPs for higher gas velocities and the
absence of the rapping devices that are required to remove the particles from cold-
side ESP collector surfaces. Disadvantages of Wet ESPs include reduced gas
temperature, corrosion concerns due to high dust and high sulfur flue gases, and
secondary water pollution to be treated.
ESPs can be installed at cold side or hot side of the flue gas. Most ESPs are
“cold-side” ones located between the air pre-heater and the flue gas desulfurization
(FGD) system, if any, where the temperature is about 120–200 °C. Alternatively,
“hot-side” ESPs are installed at upstream of the air pre-heater, where flue gas
temperature is between 300 and 450 °C. In this area, the particle resistivity is less
sensitive to the composition of the flue gas. The problem of hot-side ESPs is the
significant heat losses. In addition, they are more sensitive to temperature changes
when the furnace or boiler is at partial load.