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Application of Wet Electrostatic Precipitation Technology (Wesp) in The Utility Industry for Multiple Pollutant Control Including Mercury
Abstract
Wet electrostatic precipitation technology can be used to control acid mists, sub-micron particulate, mercury, metals and dioxins/furans as the final polishing device within a multi-pollutant air pollution control system. Test results from coal –fired installations demonstrate >90% removal efficiencies on PM2.5, SO3, and near zero opacity. Additionally, wet ESP technology (WESP) can be used for mercury removal. How wet ESPs work, what configurations they come in and some of the design considerations are described. New developments to wet ESP technology to reduce cost, make higher performance efficiency removal in less space (single pass dual field, US Patent 6,508,861) and enhance mercury control will also be discussed.
Introduction
Power utilities are coming under increased scrutiny from regulators, the public and environmntal groups. The ready availability of information about power plant emissions, along with recognition of the effects of acid gases, fine particulate and toxic chemicals on the environment and human respiratory systems, are forcing utilities to control their emissions to a much greater degree than ever before. The U.S. Environmental Protection Agency (EPA) has issued regulations to control PM10, NOx and SO2.
New regulations to control mercury, PM2.5 and other hazardous air pollutants are being proposed. The trend is clear: EPA is seeking to control a multitude of pollutants that are comprised of smaller and harder-to-capture sub-micron particles, mists and metals.
The first ESP developed was actually a wet ESP to remove a sulfuric acid mist plume from a copper smelter designed by Dr. Cottrell in 1907. The technology has become a standard piece of process equipment for the sulfuric acid industry for over 50 years to abate SO3 mist, a sub-micron aerosol.
In the past twenty years, wet ESP technology has been employed in numerous industrial applications for plume reduction associated with PM2.5 and SO3 mist, as well as for removal of toxic metals. Unfortunately, wet electrostatic precipitation is a relatively unknown technology to most industries and utilities because air regulations up to recently have not required high levels of control of sub-micron particulate.
Most utility 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 condensable particulate may be present in a gas stream, a scrubber or gas absorber is typically in place. However, as regulations emerge requiring stringent control of sub-micron particulate—which includes acid mists, low and semi-volatile metals, mercury, and dioxins/furans—wet ESP technology is increasingly attractive due to its low pressure drop, low maintenance requirements, high removal performance and reliability as a final polishing device.
ESP Operation
Electrostatic precipitation consists of three steps: (1) charging the particles to be collected via a highvoltage electric discharge, (2) collecting the particles on the surface of an oppositely charged collection electrode surface, and (3) cleaning the surface of the collecting electrode.
As particles become smaller, gravitational and centrifugal forces become less powerful, while electrical and, to a lesser degree, Brownian forces become greater, especially for 0.1 to 0.5-micron particles.
Consequently, electrical collection is an effective method for separating those sub-micron particles from the gas stream.
Most importantly, whereas mechanical collectors exert their force upon the entire gas, ESPs exert their force only upon the particles to be collected. ESPs typically operate at around 0.5-1.0 inch pressure drop, regardless of air volume or particle size. Alternatively, a mechanical collector such as a venturi scrubber would have to operate at around 60 inches of water column to achieve 95 percent collection efficiency on 0.5-micron particles. This is a major reason why dry ESPs are predominantly used in the utility industry. Every inch of pressure drop translates into dramatically higher energy requirements for operating the ID fan. To achieve 95 percent removal efficiency on 0.5-micron particles in a 1,000,000 cfm air flow, 12,000 kW of energy is required using a venturi scrubber, while an ESP needs only 100- 200 kW of energy for I.D. fan operation.
Dry ESPs
Dry ESPs consist of a series of parallel vertical plates, which act as the collecting electrodes, with a series of discharge electrodes in between the plates spaced some distance apart. As the contaminated flue gas passes through the ESP, negatively charged ions form near the tips of the sharp points of the ionizing electrode (corona discharge). These negatively charged ions move toward the positively charged collecting electrode surface and charge the contaminated particles passing through the ESP.
These charged particles become attracted to the positively charged collection plate, where they accumulate on the surface. The collected particulate builds up on the dry collection surface and forms a layer of particles or “cake” that has insulating properties.
Dry ESPs perform best when particle deposits on the collecting plates have a resistivity greater than approximately 107 ohm-cm, but less than 2 x 1010 ohm-cm. If resistivity is less than 107, the electrostatic force holding the dust particles on to the dust layer is too low and re-entrainment of particles in the flue gas can become a serious problem, reducing efficiency. If resistivity exceeds 2 x 1010 ohm-cm, the voltage drop through the particle layer to the grounded electrode becomes significant, lowering field strength in the space between the ionizing electrode and the top of the dust layer. This can cause a breakdown in the electrical field and “back corona” can take place, again lowering efficiency. Resistivity becomes a limiting factor to the amount of electrical power that can be achieved within a dryESP. To dislodge the dust from the collecting electrode surface and into the bottom hopper, mechanical rappers or sonic horns are employed. However, portions of the particles remain suspended in air and get re-entrained in the gas stream. This secondary re-entrainment requires the use of another dry ESP field to collect the re-entrained particulate plus those particles not captured in the first field.
- Dry ESPs have been used successfully for many years in industrial and utility applications for coarse
- particulate removal. Dry ESPs can achieve 99+ percent efficiency for particles 1 micron to 10 micron in
- size. However, they have several limitations that prevent their use in all applications:
- Dry ESPs are not capable of removing toxic gases and vapors that are in a vapor state at 400F.
- Due to their low corona power levels because of resistivity of the particulate cake, dry ESPs cannot
- efficiently collect the very small fly ash particles.
- Dry ESPs cannot handle moist or sticky particulate that would stick to the collection surface.
- Dry ESPs cannot remove oxidized or elemental mercury
- Dry ESPs require a lot of real estate for multiple fields due to re-entrainment of particulate.
- Dry ESPs rely on mechanical collection methods to clean the plates, which require maintenance and periodic shutdowns.
Therefore, dry ESPs may not be the best practicable control device to meet the proposed PM2.5 standard, or as a final mist eliminator for acid gas mist on FGD systems in order to reduce opacity levels to near zero.