8RuT n M

d p where M is the molecular weight of the gas, T is temperature (K), and Ru is the universal gas constant (8.31 x 103 m2/sec2/mol K).

The drift velocity is used to determine collection efficiency using the Deutsch-Anderson equation:

where w is the drift velocity, A is the area of collection electrodes, and Q is the volumetric flow rate. The units of w, A, and Q must be consistent because the factor wA/Q is dimensionless.

The ratio A/Q is often referred to as the specific collection area (SCA) and is the most fundamental ESP size descriptor [8]. Collection efficiency increases as SCA and w increase. The value of w increases rapidly as the voltage applied to the emitting voltage is increased; however, the voltage cannot be increased above that level at which an electric short circuit, or arc, is formed between the electrode and ground.

Factors that Affect ESP Performance Several factors affect ESP performance; of these, fly ash resistivity is the most important.

Fly Ash Resistivity Fly ash resistivity plays a key role in dust-layer breakdown and the ESP performance. Resistivity is dependent on the flue gas temperature and chemistry and on the chemical composition of the ash itself. Electrostatic precipitation is most effective in collecting dust in the resistivity range of 104 to 1010 ohm-cm [6]. In general, resistivities above 1011 ohm-cm are considered to be a problem because the maximum operating field strength is limited by the fly ash resistivity. Back corona, the migration of positive ions generated in the fly ash layer toward the emitting electrodes, which neutralizes the negatively charged particles, will result if the ash resistivity is greater than 1012 ohm-cm. If the fly ash resistivity is below 2 x 1010 ohm-cm, it is not considered to be a problem because the maximum operating field strength is limited by factors other than resistivity.

Examples of low- and high-resistivity fly ashes are shown in Figure 6-17, where resistivity is plotted as a function of temperature for two U.S. lignite samples from North Dakota and two subbituminous samples from the Powder River Basin [61]. The differences in fly ash resistivity are due to variations in ash composition. The low-resistivity fly ashes were produced from coals that contained higher levels of sodium in the coal ash. Higher sodium levels result in lower resistivity. Similarly, higher concentrations of iron lower resistivity. Higher levels of calcium and magnesium have the opposite effect on resistivity. This is illustrated in Figure 6-18, where the fly ash resistivities of two Texas lignites are shown along with the fly ash resistivities from the same two coals when injecting limestone for SO2 control [62]. The addition of calcium through sorbent injection resulted in increasing the fly ash resistivities.

Temperature (°F)

FIGURE 6-17. Illustration of effect of ash composition on fly ash resistivity for coals from the same geographical location. (Miller, B. G., unpublished data, 1986.)

Temperature (°F)

FIGURE 6-17. Illustration of effect of ash composition on fly ash resistivity for coals from the same geographical location. (Miller, B. G., unpublished data, 1986.)

Flue gas properties also affect fly ash resistivity. The two properties that have the most influence on ash resistivity are temperature and humidity. The effects of temperature can be observed in Figures 6-17 and 6-18. Similarly, as moisture content in the flue gas is increased, the fly ash resistivity decreases. The dome-shaped curves shown in Figures 6-17 and 6-18 are typical of fly ashes. The shape of the curves is due to a change in the mechanism of conduction through the bulk layer of particles as the temperature is varied [6]. The predominant mechanism below 300°F is surface conduction, where the electric charges are carried in a surface film adsorbed on the particle. As the temperature is increased above 300°F, the phenomenon of adsorption becomes less effective, and the predominant mechanism is

Temperature (°F)

FIGURE 6-18. The effect of limestone addition on fly ash resistivity. (From Miller, B. G. et al., Sulfur Capture by Limestone Injection During Combustion of Pulverized Panola County Texas Lignite, in Proc. of the Gulf Coast Lignite Conference, 1984.)

Temperature (°F)

FIGURE 6-18. The effect of limestone addition on fly ash resistivity. (From Miller, B. G. et al., Sulfur Capture by Limestone Injection During Combustion of Pulverized Panola County Texas Lignite, in Proc. of the Gulf Coast Lignite Conference, 1984.)

volume or intrinsic conduction. Volume conduction involves passage of an electric charge through the particles.

Other Factors The three primary mechanical deficiencies in operating units are gas sneakage, fly ash re-entrainment, and flue gas distribution [8]. Flue gas sneakage (i.e., flue gas that is bypassing the effective region of the ESP) increases the outlet dust loading. Re-entrainment occurs when individual dust particles are not collected in the hoppers but are caught up in the gas stream, thus increasing dust loading in the ESP and resulting in higher outlet dust loadings. Nonuniform flue gas distribution throughout the entire cross section of the ESP decreases the collection ability of the unit.

Many additional factors can affect the performance of an ESP, including the quality and type of fuel. Changes in coal and ash composition, grindability, and the burner/boiler system are important. Fly ash resistivity increases with decreasing sulfur content, an issue that must be considered when switching to lower sulfur coals. Moisture content and ash composition affect resistivity, as discussed earlier. Changes in coal grindability can affect pulverizer performance by altering particle size distribution, which in turn can impact combustion performance and ESP performance. Modifications to the boiler system can affect temperatures or combustion performance and thereby impact ESP performance.

Methods to Enhance ESP Performance Difficulties in collecting high-resistivity fly ash and fine particulates have led to very large units being specified, unacceptable increases in ESP power consumption, and, in extreme cases, the use of fabric filters in lieu of ESPs [8]. As a result, concepts have been developed to overcome the technical limitations and maintain competitiveness with fabric filters, including [8]:

• Pulse energization, where a high-voltage pulse is superimposed on the base voltage to enhance ESP performance during operation under high-resistivity conditions;

• Intermittent energization, where the voltage to the ESP is turned off during selected periods to provide a longer period between each energization cycle and reduce the potential for back corona;

• Wide plate spacing, which reduces capital and maintenance costs and allows for thicker discharge electrodes and increased current density.

Another approach to achieving electrical resistivities in the desired range is the addition of conditioning agents to the flue gas stream. This technique is applied commercially to both hot-side and cold-side ESPs. Conditioning modifies the electrical resistivity of the fly ash and/or its physical characteristic by changing the surface electrical conductivity of the dust layer deposited on the collecting plates, increasing the space charge on the gas between the electrodes, and/or increasing dust cohesiveness to enlarge particles and reduce rapping re-entrainment losses [8]. Over 200 utility boilers are equipped with some form of conditioning in the United States [8].

The most common conditioning agents are sulfur trioxide (SO3), ammonia (NH3), and compounds related to them, as well as sodium compounds. Sulfur trioxide is most widely applied for cold-side ESPs, while sodium compounds are used for hot-side ESPs [8]. Although results vary between coal and system, the injection of 10 to 20 ppm of SO3 can reduce the resistivity to a value that will permit good collection efficiencies. In select cases, SO3 injection of 30 to 40 ppm has resulted in reductions of fly ash resistivity of 2 to 3 orders of magnitude (e.g., from 1011 to ~108 ohm-cm) [6]. Disadvantages of SO3 injection systems include the possibility of plume color degradation. Disadvantages of sodium compounds include potential problems with increased deposition and interference from certain fuel constituents, which affect the economics of the injection [8]. Combined SO3-NH3 conditioning is used in which the SO3 adjusts the resistivity downward and the NH3 modifies the space-charge effect, improves agglomeration, and reduces rapping re-entrainment losses [8].

Wet ESPs Dry ESPs, which have been discussed up to this point, have been successfully used for many years in utility applications for coarse and fine particulate removal. Dry ESPs can achieve a 99+% collection efficiency for particles 1 to 10 ^m in size; however, dry ESPs cannot remove toxic gases and vapors that are in a vapor state at 400° F, cannot efficiently collect very small fly ash particles, and cannot handle moist or sticky particulate that would stick to the collection surface; they also require considerable space for multiple fields due to re-entrainment of particles and rely on mechanical collection methods to clean the plates that require maintenance and periodic shutdowns [63].

Wet electrostatic precipitators (WESPs) address these issues and are a viable technology to collect finer particulate than existing technology while also collecting aerosols. WESPs have been commercially available since their first introduction by F. G. Cottrell in 1907 [64]; however, they have primarily been used in small, industrial-type settings as opposed to utility power plants. WESPs have been in service for nearly 100 years in the metallurgical industry and in many other applications. They are used to control acid mists, submicron particulates (as small as 0.01 ^m with 99.9% removal), mercury, metals, and dioxins/furans when installed as the final polishing device within a multipollutant control system [63]. When integrated with upstream air pollution control equipment, such as an SCR, dry ESP, and wet scrubber, multiple pollutants can be removed when the WESP serves as the final polishing device.

Wet electrostatic precipitators operate in the same three-step process as dry ESPs: charging, collecting, and cleaning of the particles from the collecting electrode [65]. However, cleaning of the collecting electrode is performed by washing the collection surface with liquid, rather than by mechanically rapping the collection plates. WESPs operate in a wet environment in order to wash the collection surface; therefore, they can handle a wider variety of pollutants and gas conditions than dry ESPs [65]. WESPs find their greatest use where:

• The gas in question has a high moisture content;

• The gas stream includes sticky particulate;

• The collection of submicron particulate is required;

• The gas stream has acid droplets of mist;

• The temperature of the gas stream is below the moisture dew point.

WESPs continually wet the collection surface and create a dilute slurry that flows down the collecting wall to a recycle tank, never allowing a layer of particulate cake to build up [65]. As a result, captured particulate is never re-entrained. Also, when firing low-sulfur coal, which produces a high resistivity dust, the electrical field does not deteriorate, and power levels within a WESP can be dramatically higher than in a dry ESP: 2000 W/1000 scfm versus 100 to 500 W/1000 scfm, respectively. Similar to a dry ESP, WESPs can be configured either as tubular precipitators (i.e., the charging electrode is located down the center of a tube) with vertical gas flow or as plate precip-itators with horizontal gas flow [66]. For a utility application, tubular WESPs are appropriate as a mist eliminator above a flue gas desulfurization scrubber, while the plate type can be employed at the back end of a dry ESP train for final polishing of the gas.

Fabric Filters

Historically, ESPs have been the principle control technology for fly ash emissions in the electric power industry. Small, relatively inexpensive ESPs could be installed to meet early federal and state regulations; however, as par-ticulate control regulations have become more stringent, ESPs have become larger and more expensive. Also, increased use of low-sulfur coal has resulted in the formation of fly ash with higher electric resistivity, which is more difficult to collect; consequently, ESP size and cost have increased to maintain high collection efficiency [67]. As a result, interest in baghouses has increased. Baghouses offer extremely high collection efficiency (i.e., 99.9 to 99.99+%) and are capable of filtering large volumes of flue gas, and their size and efficiency are relatively independent of the type of coal burned [67]. Bag-houses are essentially huge vacuum cleaners consisting of a large number of long, tubular filter bags arranged in parallel flow paths. As the ash-laden flue gas passes through these filters, the particulate is removed. Advantages of fabric filters include high collection efficiency over a broad range of particle sizes; flexibility in design provided by the availability of various cleaning methods and filter media; wide range of volumetric capacities in a single installation, which may range from 100 to 5 million ft3/min; reasonable operating pressure drops and power requirements; and the ability to handle a variety of solid materials [6]. Disadvantages of baghouses include their large footprints, the possibility of an explosion or fire if sparks are present in the vicinity of a baghouse, and difficulties encountered when handling hydroscopic materials due to cloth cleaning problems.

The first utility baghouse in the United States was installed on a coal-fired boiler in 1973 by the Pennsylvania Power and Light Company at its Sunbury Station [67]. This baghouse, as well as the next several baghouses installed, were small, and it was not until 1978 that the first large baghouse was installed on a utility boiler. This baghouse serviced a 350 MW pulverized coal-fired boiler at the Harrington Station of the Southwestern Public Service Company. Beginning in 1978, there has been a steady increase in the installation of utility commitments to baghouse technology, and currently more than 110 baghouses are in operation on utility boilers in the United States and service more than ~22,000 MW of generating capacity [67].

Filtration Mechanisms Filtration occurs when the particulate-laden flue gas is forced through a porous, solid medium, which captures the particles. In a baghouse, this solid medium is the filter bag and/or the residual dust cake on the bag. The important filtering mechanisms are three aerodynamic capture mechanisms: direct interception, inertial impaction, and diffusion.

Electrostatic attraction may also play a role with certain types of dusts/fiber combinations [6].

Direct interception occurs if the gas streamlines carrying the particles are close to the filter elements for contact. Inertial impaction occurs when the particles have sufficient momentum and cannot follow the gas stream when the stream is diverted by the filter element and the particles strike the filter. Diffusion results when the particle mass is very low and Brownian diffusion superimposes random motion on the streamline trajectory, thereby increasing the probability of the particle contacting and being captured by the filter [67]. Particles may be attracted to or repulsed by filters due to a variety of Coulombic and polarization forces. Particles larger than 1 ^m are removed by impaction and direct interception, whereas particles from 0.001 to 1 ^m are removed mainly by diffusion and electrostatic separation [6].

The effectiveness of a filter in capturing particles is reported in terms of collection efficiency or particle penetration. Particle penetration, P, is defined as the ratio of the particle concentration (mass or number of particles per unit volume of gas), also referred to as dust loading, on the outlet of the filter (i.e., cleaned flue gas stream) to that on the inlet side of the filter (i.e., dirty flue gas stream). Collection efficiency, n, is defined as:

Typically, both penetration and collection efficiency are multiplied by 100 and reported as a percent.

The filtration process can be divided into three distinct time regimes:

(1) filtration by a clean fabric, which occurs only once in the life of a bag;

(2) establishment of a residual dust cake, which occurs after many filtering and cleaning cycles; and (3) steady-state operation, in which the quantity of particulate matter removed during the cleaning cycles equals the amount collected during each filter cycle [67]. In general, the initial collection efficiency of new filters is quite low (<99% and as low as 75-90%), whereas a conditioned bag (i.e., a bag that has retained residual particles in the fibers of the filter that cannot be removed by cleaning) may have a collection efficiency of 99.99+%. A dust cake will form on the filters, where the adhesive and cohesive forces acting between the particles and filter elements and among the particles, respectively, are sufficiently strong to allow particulate agglomerates to bridge the filter pores. The accumulated dust cake forms a secondary filter of much higher efficiency than the clean fabric. On a seasoned bag, residual dust cakes generally weigh 10 to 20 times as much as the ash deposited during an average cleaning cycle [67].

Operating Principles Baghouses remove particles from the flue gas within compartments arranged in parallel flow paths, with each compartment containing several hundred large, tube-shaped filter bags. Figure 6-19 is a cutaway view of a typical 10-compartment baghouse [67]. A baghouse on


Outlet Manifold

Clean Flue

Dirty Fl Gas Inl


Outlet Manifold

Clean Flue

Dirty Fl Gas Inl

FIGURE 6-19. Cutaway view of a typical 10-compartmentbaghouse. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

To ID Fan

FIGURE 6-19. Cutaway view of a typical 10-compartmentbaghouse. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

a 500 MW coal-fired unit may be required to handle in excess of 2 million ft3/min of flue gas at temperatures of 250 to 350°F. From an inlet manifold, the dirty flue gas, with typical dust loadings from 0.1 to 10 gr/ft3 of gas (0.23 to 23 g/m3), enters hopper inlet ducts that route it into individual compartment hoppers. From each hopper, the gas flows upward through the bags, where the fly ash is deposited. The clean gas is drawn into an outlet manifold, which carries it out of the baghouse to an outlet duct. Periodic operation requires shutdown of portions of the baghouse at regular intervals for cleaning. Cleaning is accomplished in a variety of ways, including mechanical vibration or shaking, pulse jets of air, and reverse air flow.

The two fundamental parameters in sizing and operating baghouses are the air-to-cloth (A/C) ratio and pressure drop across the filters. Other important factors that affect the performance of the fabric filter include the flue gas temperature, dew point, and moisture content, as well as particle size distribution and composition of the fly ash [68]. The A/C ratio, which is a fundamental fabric filter descriptor denoting the ratio of the volumetric flue gas flow (ft3/min) to the amount of filtering surface area (ft2), is reported in units of ft/min [8]. For fabric filters, it has been generally observed that the overall collection efficiency is enhanced (as the A/C ratio) that is, superficial filtration velocity decreases. Factors to be considered with the A/C ratio include type of filter fabric, type of coal and firing method, fly ash properties, duty cycle of the boiler, inlet fly ash loading, and cleaning method [68]. The A/C ratio determines the size of the baghouse and hence the capital cost.

Pressure drop is a measure of the energy required to move the flue gas through the baghouse. Factors affecting pressure drop are boiler type (which influences the fly ash particle size), filtration media, fly ash properties, and flue gas composition [68]. The pressure drop is an important parameter, as it determines the capital cost and energy requirements of the fans.

As the filter cake accumulates on the supporting fabric, the removal efficiency typically increases; however, the resistance to flow also increases. For a clean filter cloth, the pressure drop is about 0.5 inH2O and the removal efficiency is low. After sufficient filter cake buildup, the pressure drop can increase to 2 to 3 inH2O with a removal efficiency of 99+% [6]. When the pressure drop reaches 5 to 6 inH2O, it is usually necessary to clean the filters.

The pressure drop for both the cleaned filter and the dust cake, APt, may be represented by Darcy's equation [6]:

0 0

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