Material Selection for Wet Precipitators

Wet precipitators (WESPs) are finding increasing use in air pollution control because they remove small droplets such as sulfuric acid and oil mist.  A number of coal fired power plants in Japan have added WESPS to meet very stringent particulate limits.  Siemens–Wheelabrator has recently sold several large systems for power plants in the U.S. WESPS are also used for various other combustion applications such as wood fired boilers and sewage sludge incinerators.  Their traditional market has been in smelters and chemical plants where acid mist is a major pollutant.

The materials of construction  for WESP’s  were originally  lead and lead-lined  steel  but today most units are constructed of FRP or stainless steel. The selection of the material of construction must take into account its chemical resistance  mechanical and electrical properties. We asked Steve Jaasund to provide a summary of material options. He is now at Lundberg Associates (Geoenergy) but was involved in the AES Deepwater project which was the pioneering project for the power industry. Here is his analysis: “The selection of materials of construction for wet ESPs is a challenging problem.  With metals, over specifying the alloy can make the project non-economical while selecting an insufficient alloy may threaten the longevity of the equipment.  To further complicate matters vendors of wet ESPs cannot be expected to offer long-term warranties with respect to corrosion but this is exactly what end users want; reasonable assurance that the installed equipment will hold up to the application environment."

Plastic materials can be a solution to this problem but are certainly no panacea.  Plastics are subject to damage due to excessive temperature.  They are also subject to arc damage from high voltage sparking.  Finally, even with plastic designs the high voltage electrodes must be metal so the same challenge of alloy selection is still present.

Clearly the best approach to alloy selection is to rely on past performance.  For example, Type 304 stainless steel has been shown to provide almost indefinite life in wood drying and in many biomass-fired boiler applications.  However, in many applications such longstanding experience may not be available.  In these cases, designers must rely on the published data for particular alloys plus a complete understanding of their particular process.  In this situation, it is extremely important to thoroughly understand the vectors that may transport the corrosive elements (chlorides, fluorides, etc.) to the wet ESP.  For example, a low pH gas stream with HCl gas present may appear to demand a high nickel alloy such as C-276.  However, if the upstream quench ahead of the wet ESP unit has removed the soluble HCl gas and the entrained mist has been effectively eliminated the threat of chloride attack may have been effectively eliminated allowing a lesser alloy.

As a final note, when selecting alloys for wet ESPs it is important to note that just because an existing wet scrubber of a given alloy has given good service that does not necessarily mean that construction of a downstream wet ESP of the same alloy will be the right choice.  In contract to wet scrubbers, wet ESPs are not well irrigated and can exhibit a concentrating effect on the unirrigated surfaces.  Also, the wet ESP may be collecting a different spectrum of emission from the upstream scrubber.  Once again care must be taken to fully understand the process and all of the transport vectors at work. 

Plastics and FRP have been used extensively as a material of construction for WESP’s  since the 1970s.  A WESP constructed from plastic/FRP is light and compact, which reduces installation cost substantially.  Plastics and FRP have the advantage that they are resistant to most of the compounds that will be typically present in the gas or liquid.  FRP will require a synthetic corrosion barrier (e.g. Nexus Veil) if there are fluorides present in the gas or liquid.  As well, additives must be added to FRP to make it UV resistant.

One major problem with plastic/FRP materials is they are generally non-conductive.  To get around this problem, some designs incorporate a conductive carbon veil in the collecting tubes surface or rely on a continuous thin liquid film on the collecting tube surface.  Another problem is the temperature limitation. However,  normal WESP temperatures are well below the design limit for FRP. Even so, excursions on furnace and kiln applications can do considerable damage.

Stainless steel and alloys are being used more extensively in sulphuric acid plants to address corrosion and maintenance problems.  WESP’s constructed of stainless steel were first placed into service in the 1970’s.  Various alloy steels are used for most power plant applications of WESPS.

The choice of stainless steel or alloy to be used will be primarily based on its corrosion resistance in the gas and liquid environment.  316L stainless steel is  suitable for many applications . However, it is not satisfactory for  other applications where the conditions are more corrosive. 2205 has been selected for several big power plant WESPS just completed in the U.S.  Industeel has analyzed various conditions relative to power plant WESP and scrubber applications and says that each application must be considered individually due to all the variables. This also involves price considerations. As can be seen the cost varies considerably as one increases the corrosion protection.

Here are the selections for a number of WESP installations:

At the New Brunswick Power Colson Cove Power Plant, over 14,000 feet of N08367 box beams were furnished for use in a WESP. These beams were drawn to an overall size of 1.0 in. x 6.0 in. x 0.120 in. average wall for the application.  Over 300,000 lbs of N08367 alloy sheet was also supplied for use as collector plates, which were hung from the box beams.

Southern Environmental  has supplied a unique combination of materials for the RockTenn plant in Stevenson, Alabama. The collection electrodes are polypropylene membranes which have not shown any signs of deterioration in 5 years.  The WESP casing at the  plant is fabricated using 1/8-in. thick 316L stainless steel with 304 stainless steel stiffeners. The support system for the discharge electrode is 904L.

Insert photo

SEI replaced an old lead type WESP at Climax Molybdenum with the membrane WESP in 2009. The discharge electrodes are C-276 and the casing is FRP. 

The unit on pet coke firing at AES Deepwater has been operation for several decades. In 1999, all of the original collection plates in one of the 12 modules were removed and replaced with new alloy collector plates made of 6 percent Mo stainless steel. After over five years of operation on this retrofitted module, the 6 percent Mo collector plates have shown no corrosion.

MHI argues that corrosion is less of an issue for wetted surfaces. Their design provides a continuous atomizing system rather than periodic washing. As a result they have had success with 316L. the First MHI application of a WESP downstream of an FGD system was 1975. Since then 26 plants have been equipped with their WESPS.

Dakota Gasification is of interest because coal is converted to synthetic natural gas. A number of coal to gas plants are now under consideration around the world.  this plant uses the WESP to capture fume created by the ammonia scrubbing system. There  is one common casing of 904 L stainless steel.

Contributors to this article:

Michael Johnson, Southern Environmental mjohnson@sei-group.com

Steve Jaasund, Lundberg Associates (Geoenergy), Steve.Jaasund@lundbergassociates.com

John Grocki, Consultant, arc_jmg@yahoo.com

Richard Garvin, Industeel, richard.garvin@arcelormittal.com

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