Electrostatic Precipitators or ESPs have been used in industry for over 60 years. They can collect particles sized 0.1 to 10 microns very efficiently. They are generally more efficient at collecting fine particles than scrubbers or cyclones.
Electrostatic precipitators take advantage of the electrical principle that opposites attract. A high voltage electrode negatively charges airborne particles in the exhaust stream. As the exhaust gas passes through this electrified field, the particles are charged. Typically 20,000 to 70,000 volts are used. A large, grounded flat metal surface acts as a collection electrode. Microscopic particles are attracted to this surface where they build-up to form a dust cake. Periodically, a rapper strikes the plate to knock the dust cake into a collection hopper.
Because no filters are used, ESP's can handle hot gases from 350 to 1,300 degrees Fahrenheit.
A shell or house contains the electrodes, exhaust gases and rapper. The shell must be well built with a rigid frame to hold the components in their proper place. Hot temperatures inside the shell can vary greatly from subzero Iowa winter temperatures outside the shell. Such tremendous temperature differences can cause expansion and contraction to stress joints and welds. Often shells are insulated to minimize temperature differences and prevent gases from condensing into corrosive liquids.
To ensure ESPs work well, engineers must figure out a number of things. "Resistivity" means the particles have some resistance to electricity. This can reduce the effectiveness of an ESP in cleaning exhaust gas. Sometimes changing the gas temperature or changing the water vapor in the gas can reduce resistivity. To make these changes, the correct amounts of water or steam must be added. Sometimes "conditioning agents" such as sulfuric acid, ammonia, and soda ash can be added. Each option can require professional study and review.
The DNR's air quality engineers review air pollution permit applications to ensure the pollution control equipment is the correct size and many other factors. For example, the collection plates must be large enough to clean the volume of gas placed through the ESP. Many technical items must be calculated and agreed upon by DNR engineers and industry professionals before the DNR issues an air quality permit.
ESPs can handle large volumes of hot exhaust gases - beneficial for high-temperature exhausts found at Portland Cement plants, steel industry furnaces and industrial and utility boilers found across Iowa.