where they can be removed. The units are simple and relatively inexpensive to install and maintain. Because they depend on the masses of the particles, they work best on the larger particles. Because of their low cost and relative ineffectiveness against small particles, they are often used as first-stage cleaners in connection with more efficient (and more expensive) devices.
Electrostatic precipitators consist of high-voltage discharge electrodes and grounded collection plates, between which the flue gases pass. Particles are charged by ions emitted by the discharge electrode and move toward the collection plate, where they can be collected and removed. The devices can remove as much as 99.9 percent (by weight) of the particles in the flue gas, but they too are most effective on larger particles. Capital costs of such systems for large boilers burning medium- or high-sulfur coal are about $25/kWe of generating capacity. With low-sulfur fuel, they are more expensive, because the particles produced in burning such coal tend to be electrically more resistive. To compensate, the precipitators can be enlarged, they can be placed in a hotter part of the flue gas stream (where the high temperatures reduce the particles’ resistivity), or the flue gas can be treated with some substance that reduces resistivity. The third of these options may, however, add some pollutants to the gas, and it is generally used only as a last resort.
Wet scrubbers use water to wash solid particles out of the gas stream. These devices are rather expensive to install and operate and are not widely used for particulate control, though some installations exist.
Fabric filter baghouses are the most effective of all particulate control methods against small particles, but the pressure drop involved in forcing the gas through the necessary fine filters increases operating costs. The devices have been used in industrial particulate control for many years, but the high temperatures and corrosive chemicals in coal combustion gases, among other problems, have limited their use with utility boilers. Improved, heat- and chemical-resistant filters have been developed recently, making this option more attractive to utilities. In view of the importance of small, respirable particles in the health effects of coal combustion, it is likely that filter devices will see increased use in the future, and they warrant vigorous development.
A number of new approaches to electricity generation, aimed at improved efficiency, lower costs, or reduced environmental impacts, are under development. None of those about to be described is yet available for commercial use, but all show sufficient promise to support important research and development efforts.