|Particulate Controls Technologies|
Electrostatic precipitators, which have been used for particulate control since 1923, use electrical fields to remove particulate from boiler flue gas. Because precipitators act only on the particulate to be removed, and only minimally hinder flue gas flow, they have very low pressure drops, and thus low energy requirements and operating costs.
In an electrostatic precipitator, an intense electric field is maintained between high-voltage discharge electrodes, typically wires or rigid frames, and grounded collecting electrodes, typically plates. A corona discharge from the discharge electrodes ionizes the gas passing through the precipitator, and gas ions subsequently ionize fly ash (or other) particles. The electric field drives the negatively charged particles to the collecting electrodes. Periodically, the collecting electrodes are rapped mechanically to dislodge collected particulate, which falls into hoppers for removal.
In a typical electrostatic precipitator, collecting plates are arranged parallel to the gas flow, normally 9-18 inches apart, with discharge electrodes between them. Most precipitators have 3-5 independent electrical sections, i.e., sets of discharge and collecting electrodes with independent power supplies, in series. Each independent section removes a fraction of the particulate in the gas stream. This arrangement allows the use of higher voltages in the first sections of the precipitator, where there is more particulate to be removed. Lower voltages must be used in the final, cleaner precipitator sections to avoid excessive sparking between the discharge and collecting electrodes. In a precipitator with only one electrical section, the power input would be limited to the input which would cause sparking at the precipitator exit, thus limiting the performance of the entire precipitator.
Precipitator sectionalization has the added advantage that particles reentrained in the flue gas stream by rapping may be collected in downstream sections of the precipitator, thus minimizing net rapping reentrainment losses.
While several factors determine electrostatic precipitator removal efficiency, precipitator size is of paramount importance. Size determines treatment time: the longer a particle spends in the precipitator, the greater its chance of being collected, other things being equal.
Precipitator size also is related to the specific collection area (SCA), the ratio of the surface area of the collection electrodes to the gas flow. Higher collection areas lead to better removal efficiencies. Collection areas normally are in the range of 200-800 ft²/1000 acfm. In order to achieve collection efficiencies of 99.5%, specific collection areas of 350-400 ft²/1000 acfm are typically used. Some older precipitators on utility boilers are small, with specific collection areas below 200 ft²/1000 acfm and correspondingly short treatment times. Expansion of these precipitators, or their replacement with larger precipitators, can lead to greatly enhanced performance.
Maximizing electric field strength will maximize precipitator collection efficiency. Automatic voltage controllers are used to maintain an electric field strength as high as possible to ensure maximum particle charging and collection, consistent with preventing electrical breakdown of the gas and sparking between the discharge and collecting electrodes, which would extinguish the electric field. These controllers detect spark onset, and maintain voltages just below the level at which sparking would occur.
Factors limiting precipitator performance are flow non-uniformity and reentrainment. More uniform flow will ensure that there are no high gas velocity, short treatment time paths through the precipitator. Attaining flow uniformity also will minimize "sneakage," or gas flows bypassing the electrical fields.
Reentrainment of collected particles may occur during rapping. Proper rapper design and timing will minimize rapper reentrainment. Maintenance of appropriate hopper ash levels and of flow uniformity will minimize reentrainment of ash from the hoppers.
A key determinant of electrostatic precipitator collection efficiency is the resistivity of the particles to be collected. Resistivity is the resistance of particles to the flow of electric current. Particles with resistivities in the range of 107-1010 ohm-cm are amenable to collection with precipitators: these particles are easy to charge, and only slowly loose their charge once deposited on a collecting electrode. Particles with low resistivities (less than 107 ohm-cm), on the other hand, loose their charge to a collecting electrode so rapidly that they tend not to adhere to the electrode, with the result that there will be high rapping reentrainment losses. Carbon black is an example of a low resistivity material.
Particles with high resistivity (greater than 1010 ohm-cm) can be difficult to remove with a precipitator: such particles are not easily charged, and thus are not easily collected. High-resistivity particles also form ash layers with very high voltage gradients on the collecting electrodes. Electrical breakdowns in these ash layers lead to injection of positively charged ions into the space between the discharge and collecting electrodes ("back corona"), thus reducing the charge on particles in this space and lowering collection efficiency. Fly ash from the combustion of low-sulfur coal typically has a high resistivity, and thus is difficult to collect.
Electrostatic precipitator overall (mass) collection efficiencies can exceed 99.9%, and efficiencies in excess of 99.5% are common. Precipitators with high overall collection efficiencies will have high collection efficiencies for particles of all sizes, so that excellent control of PM-10 and PM-2.5 will be achieved with well designed and operated electrostatic precipitators.
Precipitator collection efficiencies will be somewhat lower for particles with diameters near 0.3 microns. The reason for a minimum in collection efficiency for 0.3 micron particles is that both particle charge and the resistance of the gas to particle motion both increase with particle size. Near 0.3 micron, the particle charge is low enough and the resistance to particle motion is high enough that particles are collected relatively poorly. In practice, however, this effect means only that a precipitator with a 99.9% overall mass collection efficiency will collect over 90% of 0.3 micron particles, and over 97-98% of all 0-5 micron particles.
As noted above, older electrostatic precipitators may show poor performance. In addition to general deterioration of the precipitators, several design factors can lead to less-than-desired performance. These can include small size and consequent short treatment time and low specific collection area, non-uniform flow, and inadequate electrical control systems.
Options for improving the performance of existing precipitators begin with simple rebuilds. These normally include the replacement of electrodes, rappers, and other internal elements, and modernization of the precipitator power supply and control system. An upgraded control system allows for improved voltage control, so that the voltage in each field may be maintained at the highest level possible without sparking. Precipitator rebuilds also include improvements to the ductwork, casing, and flow devices to improve the flow distribution, seal leaks, etc.
Replacement of electrodes typically is accompanied by an increase in the spacing between collecting electrodes from nine to 12 or 18 inches. While using a wider plate spacings lowers specific collection area, the ability to use higher operating voltages without sparking increases collection efficiency enough to more than compensate for this change.
Further performance improvements can be obtained by increasing precipitator size and specific collection area. Some older utility units have specific collection areas as low as 150 ft²/1000 acfm; increases to values near 400 ft²/acfm may be necessary in order to meet the new source performance level of 0.03 lb/MMBtu. Specific collection area and treatment time may be done by increasing plate height, which allows maximum use of the existing casing. On the other hand, there is a limit to the extent to which plate height can be increased, as precipitators should be longer than they are wide for maximum performance. Better options for increasing treatment time and collection area are adding one or more electrical fields and increasing the length of the fields. Because these options require additional construction outside of the existing casing, they are more expensive.
Finally, replacement of the precipitator with a new one is a last-resort option for improving collection efficiency.
Unlike dry electrostatic precipitators, which use rapping to remove particulate from the collecting electrodes, wet electrostatic precipitators use a water spray to remove this particulate. A typical wet configuration has (vertical) cylindrical collecting electrodes, with discharge electrodes located in the centers of the cylinders. Wet precipitators are useful in obtaining low opacities through the removal of acid gases and mists in addition to fine particulate. In addition, these devices have no rapping reentrainment losses, and no back corona.
Fabric filter collectors (baghouses) are conceptually simple: by passing flue gas through a tightly woven fabric, particulate in the flue gas will be collected on the fabric by sieving and other mechanisms. The dust cake which forms on the filter from the collected particulate can contribute significantly to collection efficiency.
Practical application of fabric filters requires the use of a large fabric area in order to avoid an unacceptable pressure drop across the fabric. To provide a large fabric area in a small space, the fabric is formed into cylindrical bags (hence the term baghouse). Each bag may be 20-30 feet long and 5-12 inches in diameter, and a baghouse for a 250 MW utility boiler may have 5,000 separate bags with a total fabric area approaching 500,000 square feet. Groups of bags are placed in isolable compartments, to allow cleaning of the bags (see below), or to allow replacement of some of the bags without shutting down the entire baghouse.
Baghouse size for a particular unit is determined by the choice of air-to-cloth ratio, or the ratio of air flow to cloth area, typically expressed in feet per minute (cubic feet per minute of flow divided by square feet of fabric area). The selection of air-to-cloth ratio depends on the particulate loading and characteristics, and the cleaning method used. A high particulate loadings will require the use of a larger baghouse in order to avoid forming too heavy a dust cake, resulting in an excessive pressure drop.
Baghouses often are capable of 99.9% removal efficiencies, and commonly can reduce utility boiler emissions to below 0.03 lb/MMBtu, and often to below 0.01 lb/MMBtu. Baghouse removal efficiency is relatively level across the particle size range, so that excellent control of PM-10 and PM-2.5 can be obtained.
Determinants of baghouse performance include the fabric chosen, the cleaning frequency and methods, and the particulate characteristics. Fabrics can be chosen which will intercept a greater fraction of particulate, and some fabrics are coated with a membrane with very fine openings for enhanced removal of submicron particulate. Such fabrics tend to be more expensive.
Cleaning intensity and frequency are important variables in determining removal efficiency. Because the dust cake can provide a significant fraction of the fine particulate removal capability of a fabric, cleaning which is too frequent or too intense will lower the removal efficiency. On the other hand, if removal is too infrequent or too ineffective, then the baghouse pressure drop will become too high.
Two major baghouse types are used in utility applications. In a reverse-air baghouse, the flue gas flows upward through the insides of vertical bags which open downward. The fly ash thus collects on the insides of the bags, and the gas flow keeps the bags inflated. To clean the bags, a compartment of the baghouse is taken off-line, and the gas flow in this compartment reversed. This causes the bags to collapse, and collected dust to fall from the bags into hoppers. (Shaking or other method may be necessary to dislodge the dust from the bags.) The cleaning cycle in a reverse-air baghouse typically lasts about three minutes per compartment. Because reverse-air cleaning is gentle, reverse-air baghouses typically require a low air-to-cloth ratio of 2 ft/min.
In a pulse-jet fabric filter, dirty air flows from the outside of the bags inward, and the bags are mounted on cages to keep them from collapsing. Dust which collects on the outsides of the bags is removed by a reverse pulse of high-pressure air. This cleaning does not require isolation of the bags from the flue gas flow, and thus may be done on-line. Because pulse-jet cleaning is harsh, the bags remain relatively clean, so that a higher air-to-cloth ratio of 4 ft/min (i.e., a smaller baghouse) may be used in utility applications.
Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitators. Fabric filters therefore may be good candidates for collecting fly ash from low-sulfur coals or fly ash containing high unburned carbon levels, which respectively have high and low resistivities, and thus are relatively difficult to collect with electrostatic precipitators.
Adding a baghouse downstream from an existing electrostatic precipitator has been explored as a means of obtaining very low particulate emissions of 0.01 lb/MMBtu or less. Because the precipitator will remove the bulk of the particulate, the baghouse can be relatively small, with an air-to-cloth ratio of 8 ft/min or higher, and thus inexpensive. One embodiment of this concept is the use of a small pulse-jet fabric filter downstream of the precipitator, which has been patented by EPRI and is known as a Compact Hybrid Particulate Collector (COHPAC). This fabric filter may be separate from the precipitator, as is being installed at a large utility plant in Texas, or may be installed into the last field of an electrostatic precipitator, as has been demonstrated at a plant in Alabama. This latter approach requires little space and relatively low capital expense.
Wet scrubbers is based on the collection of particles in liquid droplets, and scrubber design therefore is optimized for droplet creation. In venturi scrubbers, which are commonly used for particulate collection, the scrubbing liquid and flue gases accelerate through a converging section of duct into a narrow throat, and then pass through the throat into a diverging section. In the throat, very high gas velocity shears the scrubbing liquid into a cloud of very fine droplets, which collect particles. Increased pressure drop across the venturi leads to better droplet formation and particle collection, and pressure drops may exceed 30 inches of water in the most efficient units. Some scrubbers with lower pressure drops, down to about 5 inches of water, provide lower removal efficiencies which may be acceptable in some applications.
Wet scrubbers designed for 85% SO2 removal can provide control of particulate emissions to below 0.10 lb/MMBtu, with a removal efficiency greater than 90% for particles with diameters above 10 microns. Efficiencies for smaller particles will be significantly lower.
Cyclones use centrifugal force to separate particulate from gas streams, and belong to the broader family of mechanical collectors, which use a variety of mechanical forces to collect particulate. A multiple cyclone is an array of a large number of small (several inch diameter) cyclones in parallel. Multiple cyclones have overall mass removal efficiencies of 70-90%. However, cyclone collection efficiencies fall off rapidly with particle size, so that control of fine particulate (PM-2.5) is limited. While no accurate statement of collection efficiency can be made without precise details of the cyclone design and fly ash properties, cyclone removal efficiencies will be 90% or greater for 10 micron particles, dropping to perhaps 70% for 2.5 micron particles, and 50% for 1 micron particles. Addition of a second multiple cyclone in series with the first will allow for increased removal efficiency.
Note that the centrifugal force in, and hence efficiency of, a cyclone increases with the gas flow rate through the cyclone. Thus, multiple cyclones on boilers are most effective at high boilers loads, where flue gas flow rates are highest, with collection efficiency decreasing at lower loads.
Multiple cyclones have no moving parts, but do require regular cleaning to avoid plugging, and preventive maintenance to avoid leaks which would disrupt flow patterns and thus lower collection efficiency.
Resources: (below are a list of technical papers on particulate matter controls)