|By K. Sampath Kumar,|
|Albert L. Moretti,|
|and Meville C. Hedges,|
|Babcock & Wilcox Power Generation Group|
Dry Electrostatic Precipitators (ESPs) have long been the weapon of choice for utility and industrial particulate control needs. But a recent trend has seen more customers, particularly in industry, move toward the installation of wet ESPs for use as a polishing device because of their greater effectiveness in achieving ultra-low fine particulate emissions, as well the added benefits of removing acid mist and some heavy metals.
In the U.S., wet ESPs are becoming an important tool for complying with the industrial boiler MACT rule, enacted in 2013.
Paper mills in the U.S. are required to meet emissions levels as low as 0.035 lbs/mm BTU. Many of these plants have a wet particulate scrubber already in place which is not suitable for fine particulate removal or the even finer sulfuric acid mist that can be generated when firing high sulfur coal or oil. Since wet ESPs are designed to operate in a saturated gas stream, they can effectively be installed as a polishing device after the wet particulate scrubber to easily achieve the new emission standards.
Overseas demand for wet ESPs is growing as well, including demand for utility applications. The Chinese government is aggressively ratcheting down Pm2.5 emissions to improve the ambient air quality. As a result, utilities are evaluating wet ESPs as a solution to achieve lower Pm2.5 emissions from their fleet of coal fired power plants. There are several plants in China that are in the process of installing wet ESPs and many others are performing feasibility studies on implementing wet ESPs downstream from a wet flue gas desulfurization unit in order to meet stricter emissions standards.
There are several configurations of wet ESP designs now in service that are proven in commercial practice. Wet ESP internal configurations can use either tubular or parallel-plate type collecting electrodes. Whiletubular configurations will have only vertical gas flow orientation, the plate-type designs can have either horizontal gas flow or vertical gas flow orientation.
Wet ESPs for power plant applications tend to be mostly of the plate type design with multiple electrical sections. It is more common to find tubular collector bundles, often upflow or downflow, for industrial boiler applications. The collector tubes are circular, hexagonal or rectangular shape. While many applications involve just a single field wet ESP, it is more common in today’s market to design wet ESPs with at least two fields in series. These multi-field designs not only provide additional treatment time, they provide more reliability and performance margin for meeting emission regulations.
The material of construction for a wet ESP needs to be suitable for corrosive duty because it operates in a saturated acid gas environment. Carbon steel will not work for this purpose. Materials such as 316 SS, 2205 or even higher grade alloys such as 6 percent Mo, or C-276 may be needed. Several suppliers are considering FRP or polypropylene as collector electrodes instead of the more expensive alloys. The selection of these materials for wet ESP application is site specific and is primarily a function of chloride content and pH in the wet ESP section, which is, in turn, often a function of FGD mist eliminator performance, water quality, coal type and other process factors.
Experience suggests that wet ESP performance is determined by the corona power that can be received by the wet ESP. This is typically expressed as watts/1000 acfm of gas flow. Many factors determine how much corona power can be sustained by a given field. These include corona discharge electrode geometry, ash build-up potential, design of wash systems to remove tenacious ash, and the degree of electrical sectionalizaton of the total ESP fields.
A phenomenon known as the space charge effect, or corona suppression can surface during wet ESP operation. Corona suppression is associated principally with the presence of large amounts of ultra-fine particulate. This large amount of particulate can severely suppress operating corona current in the wet ESP which will result in low power and an associated decrease in collection efficiency in the wet ESP. To effectively deal with anticipated corona suppression, proper and effective designs of collection and discharge electrode geometries must be made.
The industry has seen its share of underperforming wet ESP units as a result of several factors which include: a) undersized wet ESPs with insufficient treatment time, b) too few electrical sections in series, c) insufficient wash systems for tenacious ash build-up, d) poor design of corona discharge electrodes that are susceptible to corona quenching by high degree of fine particulates, e) improper material selection for the acidic environment, and f) flow misdistributions at the wet ESP inlet.
An experienced designer of wet ESPs can take all the above factors into account and can properly deliver the needed performance and reliability.
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