Croll-Reynolds Company, Inc.

Wet Electrostatic Precipitation Demonstrating Promise for Fine Particulate Control

Editor’s Note: Part l of this two-part feature on wet electrostatic precipitation (wet ESPs) outlines the need for multi-pollutant air pollution control technologies, the basics of wet and dry electrostatic precipitation, the ability of wet and dry ESPs to treat fine particulate, and the use of wet ESP technology at Northern States Power Company’s Sherco Station.

Part II will explore the key design considerations associated with wet ESP technology, compare plate and tube-type configurations, and examine the use of wet ESPs at Potomac Electric Power Company’s Dickerson Station.

Power plants are coming under increased scrutiny from regulators, the public and environmental groups. The ready availability of information about power plant emissions, along with a growing belief among regulators that acid gases,
fine particulate and toxic chemicals adversely affect the environment and human respiratory systems, are expected to force power plants to control their emissions to a much greater degree than ever before. The U.S. Environmental Protection Agency (EPA) has proposed regulating N0x, S0x, PM2.5 and possibly mercury in addition to existing regulations for PM10 and S02. The trend is apparent: EPA is seeking to control a multitude of pollutants that are comprised of smaller and harder-to-capture sub-micron particles, mists and metals.

Industrial facilities have dealt with multi-pollutant regulations for many years, and have demonstrated successful control. For almost a century, in fact, wet electrosatic precipitators (wet ESPs) have been standard technology in sulfuric acid plants to abate S03 mist, a sub-micron particle. However, wet electrostatic precipitation is a relatively unknown
technology to most industries and power producers because air regulations up to recently have not required the extremely high levels of control of submicron particulate that can be achieved by wet ESPs.

Though a typical industrial power boiler's air volume is only 50,000-150,000 cfm, and a typical power boiler has an air flow of approximately 500,000-1,500,000cfm, the chemistry of the pollutants and size of the particles being emitted are, in many applications, similar to those of a fossil-fueled generating unit.

Most industrial and nearly all power facilities already have some sort of dry technology installed to control particulate emissions, such as a cyclone, fabric filter or dry ESP. Where acid gases or moist particulate may be present in a gas stream, a scrubber or gas absorber is typically in place. However, as regulations emerge requiring more stringent control of sub-micron particulate—which includes acid mists and low and semi-volatile metals as well as soluble
gases such as certain mercury compounds and dioxins/furans—wet ESP technology is increasingly attractive as a final emissions polishing device due to its low pressure drop and high removal performance.

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