Environmental Protection

Wet Electrostatic Precipitators

Is it time for the sulfuric acid industry to consider some new ideas?

Fredrick Cottrell first used wet electrostatic precipitators (ESPs) in 1910 to control acid mist emissions at a smelting operation in Selby, Calif. Since then, wet ESPs have been a mainstay of the sulfuric acid mist manufacturing industry, used at pyrometallurgical smelters and acid regeneration plants worldwide.

A considerable amount of work has occurred in other process industries with wet ESPs, particularly since the Clean Air Act was passed in 1970. These efforts have led to many important developments in the technology that could be of benefit to the sulfuric acid industry.

State-of-the-art
Wet ESPs typically have housings made of a corrosion- resistant industrial polymer with lead or lead-lined electrostatic sections. Polymeric electrostatic sections (for example, polyvinyl chloride or polypropylene collecting tubes) also are used. More specifically, this means gas-exposed housings using monolithic fiber reinforced plastic or polymer lining of an outside structure (either carbon steel or FRP) with lead and/or lead-coated electrostatic internals. Wet ESPs are arranged two units in series, each in an independent housing. The high-voltage power supplies have undergone only minor technology improvements over the past 50 years.

This version of wet ESP technology has a lot of drawbacks. First, it is costly to fabricate. As corrosion resistant as it may be, lead is dated and costly because fewer companies and people are willing to work with it. The peculiarities of lead construction also make installation costs much higher than they need to be. Finally, its mechanical properties make the operating life of lead wet ESP internals limited, requiring periodic major maintenance change outs. Plastic collectors are not much better; spark damage eventually necessitates tube replacements.

Possible Improvements in Sulfuric Acid Wet ESPs

Ā 

Current Design

Improvement

Benefits

Material of construction

Lead

Alloy

Mechanical stability, longevity, shop fabrication

Discharge electrodes

Weighted wire

Rigid mast

Simplicity, performance

Power supplies

Single phase, 60 Hz

Three Phase, High Frequency

Performance

Arrangement

Independent units in series

Single, multi-pass unit

Reduced cost

Other applications
Wet ESPs treating millions of cubic feet per minute have been installed and operated successfully in other process industries over the past 30 years. The know-how gained in these applications should be of great value to the sulfuric acid industry.

For example, wet ESP technology is now widely recognized as state of the art in the manufacture of composite wood panelboard products such as plywood, particleboard, and oriented strand board. Wet ESPs also are used to control emissions from waste incinerators, biomass dryers, and pulp and paper and industrial boilers. In some of these applications, sulfuric acid mist is specifically targeted; for example, pulp mill non-condensable gas incinerators. The technology has even made it to the power industry -- several large units have been installed and several more are in planning, engineering, or fabrication stages.

Improvements in wet ESP technology have come in materials, power supplies and controls, process adaptations, and mechanical design.

A wide array of alloys that can resist the typical wet ESP environment is now available. Some of the highest grade materials, such as the high nickel alloys, are expensive and probably not economical. However, the nature of the process environment does not necessarily require such "bullet-proof" materials. In fact, published corrosion data show that much more economical materials, such as the super-austenitic family of alloys or even the new, "super duplex" stainless steels, can provide acceptable life, for example, greater than 10 years, for the high-voltage components of a wet ESP.

The electrostatic precipitator industry recently has seen a big change in the design of high-voltage power supplies. Before the mid-1990s, these units were exclusively based on 60 Hz, single-phase power. While robust and highly reliable, this high voltage supply could not operate continuously at the maximum possible voltage for a given electrode arrangement.

This flow has been largely overcome with the advent of high frequency, three-phase power supplies. Using power and microprocessor technology adapted from other electrical applications, these units now allow electrostatic precipitators to operate continuously at the peak attainable voltage. This technology results in a significant increase in achievable performance or, alternatively, a smaller wet ESP for the job.

In sulfuric acid manufacturing, multi-pass wet ESPs are necessary to achieve the gas cleanliness necessary for downstream gas processing. At most plants, this process requires the use of two, independent wet ESP modules operating in series. Thus, a single process step requires foundations and structural supports for two machines each with their own inlet/outlet ductwork and plenums.

A more economical way of achieving the same performance would be to have the multi-pass wet ESP integrated into a single vessel. This approach currently is being used in many other industries, including the utility industry.

Several wet ESP design improvements could be adapted to the sulfuric acid industry. If alloy construction is accepted in lieu of lead, rigid mast discharge electrodes can be used. This type of electrode can be configured without bottom supports or connections, which eliminate a source of operating problems and reduce cost. In addition, alloy construction can be fabricated in a machine shop and would require less field labor. The resulting project cost savings can be significant. Finally, wet ESP performance could be enhanced by improved discharge electrode emitter design. Lead-coated wires do not allow operation at the maximum possible voltage, for various reasons. Adapting a better emitting electrode design used in many other applications will, once again, improve performance and reduce cost.

Perhaps it is time for the sulfuric acid industry to look for new ideas in the design and operation of wet ESPs. The benefits in cost savings, performance, and operability may come as a pleasant surprise.

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