Multi-Pollutant Control with Dry-Wet Hybrid ESP Technology

Authors: Dr. Ralph Altman
The Electric Power Research Institute, Suite 513, 5726 Marlin Rd., Chattanooga, TN 37411 

Wayne Buckley
Croll-Reynolds Clean Air Technologies, PO Box 668, Westfield, NJ 07091

Dr. Isaac Ray
Croll-Reynolds Clean Air Technologies, PO Box 668, Westfield, NJ 07091


Wet electrostatic precipitation technology is a well-established technology for control of acid mists and sub-micron particulate as a final polishing device within a multi-pollutant air pollution control system. Typically, wet ESPs are employed after a FGD system where the flue gas is saturated. This paper presents the concept of installing a wet ESP field directly after a dry ESP in an unsaturated flue gas condition for control of PM2.5, SO3 mist, as well as some SO2 and mercury. Test results have demonstrated that a wet ESP can remove >90% PM2.5, SO3 mist, 20% SO2 as well as oxidize some elemental mercury and collect the oxidized mercury with similar efficiency as that for PM2.5. Opacity can be reduced to 10% or less.


Of the 1,100 coal-fired power plants in the US, approximately 75% have dry ESPs installed and 25% have Flue Gas Desulfurization (FGD) systems. Many plants switched to lower sulfur sub-bituminous and lignite coals over the past decade to avoid the cost of installing FGD systems to meet EPA mandated SO2 standards.

The issue that is now confronting those plants with only dry ESPs installed burning sub-bituminous and lignite coal is that these lower sulfur coals typically have higher levels of elemental mercury concentrations, which pass through the dry ESP and out the stack. Based on the EPA’s Information Collection Request (ICR) data, elemental mercury emissions are four times higher from sub-bituminous coal than eastern bituminous coal ( 4.0 lb/Tbtu vs. 1.0 lb./Tbtu).

Secondly, where low NOx burners have been installed and there is a high level of unburned carbon in the fly ash, these sub-micron particles are difficult to capture. Opacity may have increased creating permit and good neighbor issues. Finally, EPA standards for PM2.5 are due out 2005 along with more stringent State regulations for opacity that will require plants to address capturing PM2.5 and acid aerosols, the primary causes of visible emissions.

Proposed Concept-Hybrid Dry-Wet ESP

A proposed solution is to add a wet ESP field after the dry ESP. A hybrid dry/wet ESP offers the

ability to collect multiple pollutants within the same footprint of the existing dry ESP, should the last field of the dry be retrofitted with a wet field. While the dry ESP section will remove PM10 with high efficiency (>90%), the wet ESP last field can remove PM2.5, SO3 and toxic metals with high efficiency (>90%), while providing some trim control (20%-50%) of SO2 and other gases in addition to removal of mercury.

Historical Background

The first ESP developed in this country 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.

Wet ESP Technology

Wet ESPs operate in the same three-step process as dry ESPs—charging, collecting and finally cleaning of the particles from a collection surface. However, in a wet ESP, cleaning of the collecting electrode is performed by washing the collection surface with liquid, rather than mechanically rapping the collection plates. Because there is no particulate buildup on the collection surface, there is no re-entrainment of collected particulate and much higher power levels can be achieved, allowing for charging of sub-micron particles and mists with collection in the liquid slurry. Typically, a wet ESP follows a wet FGD where the flue gas is saturated and is used to collect PM2.5, SO3 (H2SO4) and liquid droplets remaining in the flue gas.

Wet ESP technology is primarily meant as particulate control device for PM2.5 and SO3 mists. While dry ESPs can typically achieve 100mg/nm3 removal of PM2.5, and FGD systems can achieve levels of 30mg/nm3, wet ESP technology can achieve emission levels down to 5mg/nm3. A wet ESP can also provide some scrubbing efficiency for acid gases such as SO2 and based upon limited test data, some mercury removal. HgCl, which is water-soluble, can be removed in either a FGD system or a wet ESP device. HgO and HgS are insoluble but can be captured in a wet ESP. In addition, a wet ESP may also provide some oxidation effect on elemental mercury from ozone generation due to high voltage corona discharge to increase total mercury.

Utility Application

Coal fired plants are starting to recognize the need for this technology to reduce opacity related to PM2.5 and SO3 concentrations in the flue gas as States start to mandate more stringent opacity limits.
A coal-fired power plant with two 750 MW boilers installed the first full-scale wet ESP in the country to reduce opacity. Twelve modules/unit of tubular, upflow wet ESP technology were installed after a FGD system and opacity was reduced to less than 10%. New proposed coal-fired plants with FGD systems are starting to specify wet ESP technology in their bid documents to avoid costly retrofit in the future to meet proposed Regional Haze and PM2.5 regulations.In several instances where plants have installed SCR technology with ammonia injection on high sulfur coal and FGD systems are installed, increased plume resulted from higher levels of SO3 and fine ammonia salt concentration in the flue gas. These plants are considering installing a wet ESP after the FGD system to capture the additional SO3 and ammonia salts, which cannot be captured in the FGD system due to their sub-micron nature. Therefore, for the 25% of coal fired plants that burn high sulfur bituminous coal that have FGD systems installed, installing a wet ESP is the next logical step to meet opacity, PM2.5 SO3 and mercury standards.However, the majority of coal-fired plants do not have a wet FGD system installed. Many are considering installation of fabric filters with injection of activated carbon for mercury control to meet new proposed mercury standards as well as removal of PM2.5. A viable, cost-effective alternative that offers multi-pollutant control is needed. Retrofit of a dry ESP last field with a wet field offers the potential to remove PM2.5, SO3 mist, SO2 and mercury without the drawbacks of activated carbon injection/fabric filter technology.For those plants with undersized dry ESPs, retrofit of a dry ESP with a wet field offers the ability to meet opacity requirements with minimal cost and impact.

Test Results

The Electric Power Research Institute tested the concept of adding a wet ESP after a dry ESP in an unsaturated condition during 1994-1995 at pilot scale level on coal-fired flue gas, primarily for opacity abatement. Those results indicated that a single wet field could achieve very high collection efficiency, greater than 90 percent.

In terms of outlet emissions, the tests indicated that a dry ESP emitting more than 0.1 lb/MMBtu before conversion would emit less than 0.03 lb/MMBtu after conversion to a wet field. This high efficiency results from the high power levels possible when fly ash electrical resistivity is no longer a controlling factor. Furthermore, the water wash system in the wet field eliminates the need for traditional mechanical rapping and thus virtually eliminates all re-entrainment losses. The EPRI tests also established that a wet ESP could be successfully operated with the flue gas temperature well above the moisture dew point. This method of operation means that equipment downstream of the converted ESP will not have to be operated under saturation conditions.

Finally, the EPRI tests demonstrated that the conversion to wet operation partially reduced SO2, HCL and HF along with some oxidized mercury. Measured results from the pilot showed the following removal levels across the wet ESP.

  • Particulate matter: 95%
  • Sulfur dioxide: 20%
  • Hydrogen chloride: 35%
  • Hydrogen fluoride: 45%
  • Oxidized Mercury: 50%
  • Total mercury 30%*

(*Outlet elemental mercury was measured at higher levels than inlet Hgo. It is estimated that some of the oxidized mercury degassed back to elemental mercury in the water solution due to improper pH control/water chemistry.)

In 2000-2001, EPRI participated in a full-scale demonstration of this concept at Mirant’s

Dickerson Station for opacity reduction. While performance testing reported high collection efficiency on PM2.5 and SO3 with opacity as low as 10%, mechanical issues associated with the wet ESP design, which could have been corrected if non-technical issues had not limited the length of the project, prevented continued implementation of this approach. No mercury testing was performed at this site.

Croll-Reynolds has since designed and installed a horizontal, plate wet ESP at slipstream pilot scale that overcomes the mechanical issues experienced at Dickerson. A patented water header system prevents splashing, a unique plate design assures thorough water coverage to avoid dry spots and a drainage system eliminates arcing and mist carryover downstream.

Mercury Control

Most mercury research has focused on sorbent injection followed by a baghouse for those plants without an FGD system. Tests of wet ESP technology at a coal-fired plant indicate a wet ESP can remove both oxidized and particulate forms of mercury, along with some partial oxidation of elemental mercury,
Croll-Reynolds installed a pilot wet ESP at a major coal-fired plant in 2001. This plant burns 3% bituminous coal and has a FGD system installed for PM10 and SO2 control. A 5,000 acfm slipstream, tubular pilot wet ESP was installed for PM2.5 and SO3 control, the two primary contributors to stack plume. Speciated mercury testing was also performed to measure collateral benefits of installing wet ESP technology.An initial series of tests completed during Sept. of 2001 were performed in a single electrical field at approximately 8,000-cfm. Removal achieved at this higher than designed for airflow was 79% for PM2.5 and 76% for SO3. Mercury testing during this time period showed 64% for particulate, 77% for oxidized and 44% for elemental. Removal levels for particulate and oxidized mercury were similar to that for PM2.5 and SO3. Most importantly, 44% removal of elemental mercury was measured at the highest inlet concentration. Mercury levels were extremely low because most of the particulate and water soluble HgCl was already removed in the upstream FGD system. It is estimated that at higher inlet levels, higher removal efficiencies would be expected.In order to improve removal efficiency within the wet ESP pilot, the electrical system was subsequently retrofitted from a single to a two-field configuration. New test results on PM2.5 and SO3 improved to 96% and 91% respectively. If particulate and oxidized mercury are removed at similar levels as PM2.5 and SO3, potentially 90% of particulate/oxidized species of mercury could be removed and 50% of elemental mercury within a wet ESP device.Additional elemental mercury collection may be achieved through collection of unburned carbon in the fly ash as PM2.5, which will adsorb some fraction of elemental mercury. Collection of sub-micron particulate is ten times more effective for toxic vapor removal than collection of course particulate. One gram of 0.1-micron particles has 10 times the surface area as a gram of 1.0-micron particles (60 m2 vs. 0.6 m2). Toxic vapors condense uniformly on the surface area of all particles, especially the smallest particles because of equilibrium vapor pressure. Therefore, the capture of a gram of 0.1-micron ash particle is 10 times more effective at reducing toxic emissions than the capture of a gram of 1.0-micron ash particles.Where Selective Catalytic Reduction equipment is installed for NOx emissions, there may also be some oxidation effect on any elemental mercury in the flue gas across the catalyst. The oxidized mercury can then be removed in the wet ESP field along with ammonia salts.


Adding a wet ESP field behind an existing dry ESP or retrofitting the last field offers several benefits to traditional methods for removing PM2.5, SO3 mist and some fraction of SO2 and mercury:

  • Less pressure drop than a fabric filter or FGD system. Typically ½” w.c. pressure drop across a wet ESP, similar to that of a dry ESP field.
  • There are no moving parts since a wet ESP is self-cleaning, reducing maintenance.
  • Less real estate is required. A wet ESP field can be retrofitted to an existing dry ESP, thereby avoiding the problem of trying to find space for new equipment within the confines of a plant.
  • The wet ESP field can operate above the saturation point, thereby avoiding costly upgrading of downstream duct and stack materials of construction. The wet ESP section would be constructed of a high-grade stainless steel.
  • Capital costs are significantly less than a fabric filter or full FGD system, typically in range in the $25-$35/kw depending upon difficulty of the installation.
  • Operating costs are minimal with less than 3 watts/cfm-power usage within the wet ESP.
  • There is no impact on existing upstream air pollution control equipment or process.
  • Multiple pollutant control is possible. While primarily a particulate control device, similar to a fabric filter, a wet ESP retrofit offers the collateral benefit of being able to abate acid mists (SO3), oxidized mercury, some acid gas (SO2, HCL) as well as some elemental mercury.
  • Opacity can be reduced to 10% or less due to removal of PM2.5 and SO3 mist.
  • There is no contamination of fly ash. The wet ESP section is located downstream of the dry ESP.
  • Fuel blending/switching can be accommodated to minimize fuel costs while meeting emission standards.
  • There is no sorbent injection system. Installation, handling, injection of sorbent plus disposal issues of mercury–laden carbon are eliminated.
  • Any mercury collected in the wet ESP slurry can be treated in a wastewater treatment system to remove the mercury from the water, concentrating the waste and reducing handling and disposition of a hazardous waste.
  • Wastewater from the wet ESP can be recycled, minimizing water-related issues.

New Pilot Demonstration

Because this approach offers so many potential benefits and there has not been sufficient testing of the concept and resolution of mechanical issues associated with the approach, the Electric Power Research Institute and Croll-Reynolds will construct a mobile 5,000 cfm slip-stream pilot wet ESP for installation at Southern Company’s Alabama Power’ Plant Miller. This plant burns PRB coal with high elemental mercury emissions has already committed to being a host. The plant is installing SCR technology for further NOx abatement. Testing for PM2.5, SO3, SO2 and mercury removal will be performed.

The pilot wet ESP consists of stainless steel plates with a patented water feed system that promotes uniform and thorough water coverage while minimizing the potential for plugging, splashing and mist carryover. Custom designed ionizing electrodes are configured to suit the electrical profile required for various flue gas concentrations.

Incorporates Plasma ESP Technology

Based upon laboratory results with up to 80% removal of elemental mercury within an ESP, EPRI and CRCAT will incorporate CR’s patented Plasma ESP technology into the pilot Hybrid ESP. The plasma ESP technology injects a proprietary reagent through the discharge electrode that oxidizes elemental mercury. The oxidized mercury can then be captured in the collecting section of the wet ESP field at similar rates as that for PM2.5. Enhancing the Hybrid ESP with the plasma technology is cost-effective, simple and avoids carbon injection, fly ash contamination and baghouse installation.


Wet ESP technology is a proven, well-known technology that can achieve very high removal of mists, particles and aerosols with low pressure drop and minimum maintenance, if properly designed and built. Whereas it has traditionally been installed after a FGD system in a saturated flue gas, this application reflects a minority of coal-fired plants. The potential benefits to retrofit a an existing dry ESP with a wet ESP field operating above the dew point merits continued investigation.Successful demonstration of the technology will offer the majority of coal-fired plants using sub-bituminous or lignite coal a reliable, a viable alternative to installing fabric filters with injection of activated carbon for mercury control as well as the co-benefits of high removal efficiency for PM2.5 and SO3 mist with some SO2 and HCL trim control.