Electrostatic precipitators

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. Electrostatic precipitator overall (mass) collection efficiencies can exceed 99.9% and efficiencies in excess of 99.5% are common.

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-sulphur coal typically has a high resistivity and thus is difficult to collect.

Precipitator collection efficiency

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 PM₁₀ 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.

Wet Electrostatic Precipitators (WESP)

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.