Caught in the Avalanche

By creating large volumes of ions that charge particulate matter and allow it to be captured for removal, wet electrostatic precipitators help facilities exceed strict air quality standards

• By Rodney L. Pennington, PE
• Oct 01, 2004

An existing consent decree (CD) provides that the U.S. Environmental Protection Agency (EPA) was required to issue a final particulate matter (PM) CD no later than December 19, 2003, and that EPA had to sign for publication notices of proposal and final rulemaking concerning its review of the PM national ambient air quality standards (NAAQS). 

Leading scientific evidence indicates direct and strong support for standards related to particulate matter that is 2.5 microns or less in diameter (PM_2.5), which provide at least the level of protection afforded by the current primary (health-based) standards. Based upon this evidence that clearly supports the current standards, many experts think that EPA should consider taking the following additional actions:

• Revising both the annual and 24-hour standards to provide additional health protection
• Selecting the annual PM_2.5 standard from a range extending from the current level of 15 down to 12 micrograms per cubic meter (µg/m³)
• Revising the current 24-hour PM_2.5 standard (65 µg/m³) to a level selected from a range of 50 to 30 µg/m³

Implementation of these proposed stricter PM standards will increase the requirements for more efficient particulate control devices such as the wet electrostatic precipitator (WESP). A WESP is a device that captures particulates from a gas process exhaust stream. High-voltage electrodes impart a negative charge to the particles entrained in the exhaust gas. These negatively charged particles are then attracted to a wet positive grounded collecting surface. The particulate is flushed away by the liquid film on the collection surface and removed from the system. The clean gas is exhausted from the system up to 99.9 percent cleaner than when it entered. WESP units are presently meeting PM emissions requirements and have the ability to meet projected future requirements.

Typically, a complete WESP system is purchased, supplied, installed, and started up by a single company, such as Western Pneumatics Environment. Each unit has the capacity to remove particulate and smoke emissions from more than 1,000,000 pounds per hour of air from multiple sources. The 240,000-standard-cubic-feet-per-minute (scfm) WESP includes two quench and four cyclone pre-treatment systems prior to the six individual high-voltage, high-frequency, down-flow round tube collector assemblies. Each collector assembly is controlled by an individual transformer/rectifier unit to maximize the performance of the system. The air discharge from the emission source is pulled through the WESP system by four exhaust fans and variable frequency drive motors. The clean air is then discharged through the exhaust stack.

Advantages of WESP Systems
WESP systems are ideally suited as ultra-high-efficiency mist and particulate eliminators that provide the ability to achieve compliance with increasingly stringent maximum achievable control technology (MACT) requirements under the Clean Air Act. WESP systems overcome many of the problems associated with other air pollution control technologies, such as dry ESPs, fabric filters, and high-energy scrubbers. Other WESP system advantages include:

• WESP systems are unaffected by condensable or sticky particulates. In contrast, fabric filters and dry ESPs are extremely sensitive to hydroscopic particles, which tend to become sticky and solidify on filter bags and dry ESP collection plates. The collection of organic material in filters or dry ESPs not only reduces the performance, but also increases the potential for fires.
• WESP systems operate at much lower temperatures in a cooled, saturated gas stream, thus eliminating high-temperature thermal-expansion problems. They also become improved collection devices since many of the hazardous air pollutants (HAPs) in the exhaust stream that are still in a vapor state will eventually condense and then be collected in the WESP system.
• WESP systems do not require a high pressure drop to provide associated high collection efficiency. Pressure drops can be as high as 30 inches water column ("w.c.) or higher for a scrubber, 6 to 8 "w.c. in a fabric filter as compared to 1 to 2 "w.c. with a WESP system.
• WESP systems do not have rapping re-entrainment losses or bleed-through losses, which are associated with dry ESP or filter type units. Resistivity is a term used to describe the resistance of a medium to the flow of an electrical current. It is an important consideration in measuring the efficiency of pollution-control technology. By definition, resistivity, which has units of ohm-centimeters (ohm-cm), is the electrical resistance of a dust sample with a 1 square centimeter (cm²) cross sectional area and 1 cm thickness. Resistivity levels are generally broken down into three categories: low (under 1 x 10⁵ ohm-cm), medium (1 x 10⁵ to 1 x 10¹¹ ohm-cm), and high (above 1 x 10¹¹ ohm-cm). If the particulate resistivity is less than 10⁷ ohm-cm, the electrostatic force holding the particulate on the collection surface is very low and re-entrainment becomes a problem with dry ESP rapping. In the case of WESP systems, the resistivity of the water film is the governing factor.

Overview of the WESP Process
The high collection efficiency of WESP systems comes from the electrostatic precipitation process, which is made possible by the corona discharge that develops from the high-voltage field established in the round tube assembly. Through an effect known as the avalanche process, the corona discharge provides a simple and stable means of generating the ions to electrically charge and collect suspended air particles, mist, or smoke. Once charged, the particles are driven to the ground surface, in this case the round tube walls. The particles are continuously rinsed from the vertical down-flow collection tube surface with water or an irrigating liquid to the bottom sump where they are collected and removed from the system.

The free ions form a space charge or cloud of the same polarity at the discharge electrode. By restricting emissions of high-speed electrons, the space charge stabilizes the corona and sets up a charged field. The particles, mist, or smoke in the process-gas stream become negatively charged by this field of ions and are driven to the positive (oppositely charged) electrode or tube walls along with the water drops. The saturated process-air stream and the fine water droplets generated in the pre-conditioning section provide a continuous collection of water at the top of the tube entry. This water flushes the collected particulate down the tube walls to the bottom sump for removal from the system. The continuous water provides a thin film of self-cleaning liquid, resulting in self-irrigation of the tube wall. An additional online wash-down system at the entry to the precipitator section can be added and periodically activated as needed to ensure effective cleaning of the tube walls. These unique features are not present with a conventional up-flow WESP system.

Down-flow Systems vs. Up-flow Systems
In contrast to the up-flow system, the down-flow WESP system offers:

• A higher secondary voltage that results in less collection area and fewer tubes and electrodes
• Gravity provides the driving force for self-cleaning of the tubes and minimizes or eliminates downtime caused by the build up of solids on the collection surface
• The flow of water required to clean the internal parts of the system is minimized
• No external expensive heat exchangers or cooling systems are required to produce the self-irrigating feature

In addition to the precipitator section, the system also includes a quench and preconditioning section. A sub cooler can be added to further lower the temperature to the point where most of the contaminants in the process stream have condensed to droplet form, which can then be collected in the precipitator section. The quench and preconditioning section saturates the process stream to the dew point and removes any larger particulate before entering the precipitator section.

WESP startup and testing has been completed and is providing the environmental control essential for the future operation of the facility.

This article originally appeared in the 10/01/2004 issue of Environmental Protection.