Particulate Matter

Particulate matter (PM) emissions from coal-fired electric utility boilers in the United States have decreased significantly since implementation of the 1970 Clean Air Act Amendments. In 2001, ~23 million short tons of particulate matter, reported as PM10 (i.e., particles with an aerodynamic diameter <10 ^m), were emitted from inventoried point and area sources, of which ~ 190,000 short tons (or ~ 1.6% of the total) were emitted by coal-fired electric utility boilers [56]. This is a substantial decrease from a total of ~1.7 million short tons of PM10 being emitted from coal-fired power plants in 1970, especially as coal consumption for electricity generation has increased more than 150% over this period, and the reduction is due to the application of particulate control technologies. Similarly, in 2001, annual emissions of particulates smaller than 2.5 ^m (i.e., PM2.5), which is a subset of PM10, were 100,000 short tons, or less than 0.8% of the total primary PM2.5 emitted from all sources.

The application of control technologies to combustion sources is illustrated in Figures 6-1 and 6-2, which show the improvements in emissions rates from coal-fired power plants since 1970 as well as near-term projected emissions rates. As of 2000, 1020 coal-fired electric generators were equipped with particulate collectors and represented a total of more than 321,000 MW generating capacity [3].

Several particulate control technologies are available for coal-fired power plants, including electrostatic precipitators, fabric filters (baghouses), wet particulate scrubbers, mechanical collectors (cyclones), and hot-gas particulate filtration [57]. Of these, ESPs and fabric filters are currently the technologies of choice as they can meet current and pending legislation PM levels. While cleaning large volumes of flue gas, they achieve very high collection efficiencies and can remove fine particles. When operating properly, ESPs and baghouses can achieve overall collection efficiencies of 99.9% of primary particulates (over 99% control of PM10 and 95% control of PM2.5), thereby achieving the 1978 New Source Performance Standards required limit of 0.03 lb PM per million Btu [58]. The primary particulate matter collection devices used in the power generation industry—ESPs and fabric filters (baghouses)—are discussed in this section. In addition, hybrid systems under development that combine ESPs and fabric filters in a single, overall system are presented.

Electrostatic Precipitators

Particulate and aerosol collection by electrostatic precipitation is based on the mutual attraction between particles of one electrical charge and a collection electrode of opposite polarity. This concept was pioneered by F. G. Cottrell in 1910 [6]. The advantages of this technology include the ability to handle large gas volumes (ESPs have been built for volumetric flow rates up to 4,000,000 ft3/min), achieve high collection efficiencies (which vary from 99 to 99.9%), maintain low pressure drops (0.1-0.5 inH2O), collect fine particles (0.5-200 ^m), and operate at high gas temperatures (up to 1200°F). In addition, the energy expended in separating particles from the gas stream acts solely on the particles and not on the gas stream.

Electrostatic precipitators have been utilized to control particulate emissions from coal-fired boilers used for steam generation for about 60 years [7]. Initially, all ESPs were installed downstream of the air preheaters at temperatures of 270 to 350°F and are referred to as cold-side ESPs. ESPs installed upstream of air preheaters, where temperatures range from 600 to 750°F, are referred to as hot-side ESPs and use low-sulfur fuels with lower fly ash resistivity. In the early 1970s, ESPs were the preferred choice for high-efficiency particulate control devices [7]. Nearly 90% of U.S. coal-based electric utilities use ESPs to collect fine particles [59].

Operating Principles Several basic geometries are used in the design of ESPs, but the common design used in the power-generation industry is the plate-and-wire configuration. In this design (shown in Figure 6-15), the ESP consists of a large hopper-bottomed box containing rows of plates forming passages through which the flue gas flows. Centrally located in each passage are electrodes energized with high-voltage (45-70 kV), negative-polarity, direct current (dc) provided by a transformer-rectifier set [8]. Examples of various designs of rigid discharge electrodes are shown in Figure 6-16 [8]. The discharge electrode most commonly used in the United States is the weighted-wire electrode, while the rigid-frame electrode is commonly used in Europe [8]. The flow is usually horizontal, and the passageways are typically 8 to 10 inches wide. The height of a plate varies from 18 to 40 feet, and the length varies from 25 to 30 feet. The ESP is designed to reduce the flow of flue gas from 50 to 60 ft/sec to less than 10 ft/sec as it enters the ESP so the particles can be effectively collected.

The electrodes discharge electrons into the flue gas stream, ionizing the gas molecules. These gas molecules, with electrons attached, form negative ions. The gas is heavily ionized in the vicinity of the electrodes,

FIGURE 6-15. Electrostatic precipitator. (From B&W, Electrostatic Precipitator Product Sheet PS151 2M A 12/82, Babcok & Wilcox Co., Barberton, OH, December 1982.)

resulting in a visible blue corona effect. The fine particles are then charged through collisions with the negatively charged gas ions, resulting in the particles becoming negatively charged. Under the large electrostatic force, the negatively charged ash particles migrate out of the gas stream toward the grounded plates, where they collect and form an ash layer. These plates are periodically cleaned by a rapping system to release the layer into the ash hoppers as an agglomerated mass.

The speed at which the migration of the ash particles takes place is known as the migration or drift velocity. It depends upon the electrical force on the charged particle as well as the drag force developed as the particle attempts to move perpendicular to the main gas flow toward the collecting electrode [6]. The drift velocity, w, is defined as:

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