As environmental requirements become tighter, the demand for better gas cleaning technology has been rising as well. This trend has pushed the development of a relatively old technology, the wet electrostatic precipitator (Wet ESP).

First utilized in 1910 at a smelter in California for the collection of sulfuric acid mist, the wet ESP led to the development of the dry ESP, which has become the dominant version of precipitation technology used on thousands of boilers and furnaces around the world.

Until the advent of the Clean Air Act in 1970, wet precipitation technology has remained a mainstay for acid mist control. The regulatory push mandated by this landmark legislation has rejuvenated interest in wet ESPs. The result has been a number of innovative improvements in wet ESPs that have led to opportunities in applications never anticipated.

Technical advantages

Enlarge this picture Figure 1: Operational concerns of fine particle control technologies.

The obvious distinction between dry and wet precipitation is that wet ESPs operate on gas streams that are 100-percent saturated with water vapor. This simple difference affords wet precipitation certain technical advantages over dry precipitation and other non-electrostatic methods of particulate collection.

The wet precipitation process is inherently useful on a variety of gas-cleaning applications. This is because wet ESPs are relatively immune to many limitations of other particulate separation alternatives, including fabric filters, dry precipitators and even wet scrubbers. Factors such as particulate resistivity and chemistry, while of great concern in dry ESPs, are not a factor in wet ESPs; the particulate is collected on a wet, electrically grounded surface. Also, temperature is normally not a concern because the gas stream must first be quenched to the wet bulb temperature, normally below 170°F. Operation at this low temperature reduces gas volume, which allows lower capital investment and saves energy.

Wet systems also offer several mechanical advantages over their dry cousins. When constructed as a vertical flow, tube-type system wet ESPs have none of the gas sneak-by issues confronting plate-type, dry ESPs. Also they do not need to be mechanically vibrated to dislodge the collected particulate matter. As a result, re-entrainment losses, a large part of the inefficiency of a dry system, are not a problem.

Wet precipitators are ideal for treating gas streams with condensable particulate matter. Such condensable matter may be in the form of acid mist or condensed organic compounds, but cannot be collected as particles until they form as a result of cooling. Dry ESPs and fabric filters, limited to operating at elevated temperatures, cannot do this job. Finally, wet ESP technology is attractive when fire is a concern and when impediments to gas flow (e.g. plugging) must be avoided.

Recent advancements

Since the days when wet ESPs were strictly used for acid mist control, there have been many significant developments that have enhanced the technology. Some of the more important ones are discussed below:

Tube cooling. Most wet ESPs are configured as vertical flow, tube-type systems, although plate-type units are still occasionally applied. The tube design is a ready-made heat exchanger that allows the design engineer to utilize the outside of the collecting tube for heat transfer. This approach has two benefits. First, it encourages condensation of water on the inside of the collecting tube, enhancing particulate collection and making it easier to remove the collected particulate matter. Second, it provides a source of warm air that can be used for a variety of purposes, including purge air for the support insulator compartments, which saves energy.

High-frequency power supplies. The development of high-frequency power supplies has been beneficial to all electrostatic precipitators. However, the benefits to wet ESPs may be the greatest because of the costly materials of construction in wet units and because of the tendency of wet units to be applied to gas streams with high concentrations of fine particulate.

As seen in Figure 2, the high-frequency power supply outperforms the standard 60-Hz, single-phase design. The additional voltage means that the precipitator can be smaller for the same efficiency or, alternatively, perform better at the same size.

Multiple, vertical stages. Multiple electric fields in series are nothing new in dry ESPs; units with three or more fields are the rule in coal-fired boiler applications. However, vertical fields in series in wet ESP designs are a new development, one which has delivered some surprising performance improvements. These multi-field installations have shown outlet particulate concentrations that rival levels achieved by the best fabric filtration systems. Also, field testing has shown that for a given level of efficiency, the total required collecting area of a multi-field wet ESP is significantly less than a single-field unit.

New materials. When Congress passed the 1970 Clean Air Act, wet ESPs for acid mist service were made exclusively of lead. In a few other applications, carbon and 300-series stainless steels were used. Today, modern alloys ranging from duplex stainless steels all the way up to high nickel alloys are commonplace. Also, fiberglass, PVC and other polymeric construction have been employed.

New applications

Enlarge this picture Figure 2: High-frequency and 60-Hz waveforms.

Perhaps the most important new application for wet precipitators is as a final gas-cleaning step for coal-fired utility boilers. Presently there are five utility-scale wet ESP systems operating in North America and several more in engineering and construction. These units are mostly intended to control the sulfuric acid mist that is emitted downstream of a wet flue gas desulfurization (FGD) scrubber. The need for an acid mist-control step is further amplified by the effect of upstream selective catalytic reduction systems designed to reduce NO_X emissions. This is because these systems tend to add SO₃ to the gas stream, which ultimately hydrates to H₂SO₄ in the FGD scrubber.

A related application presently being explored is directed toward cleaning coal-fired utility boiler flue gas prior to entering a system designed to remove CO₂. The role of the wet ESP is critical in this process if the CO₂ is to be absorbed in a highly alkaline liquor such as ammonia. In such a process it is extremely important to remove the acid mist before the gas enters the CO₂ absorber. Otherwise, the recirculated alkaline reagent will be wasted as it reacts with the acid mist.

Another innovative application of wet ESPs has been in the biofuels industry. In the manufacture of ethanol for fuel, the solid material left behind from the fermentation tanks (distillers grain) is normally dried so it can be stored and shipped as animal feed. Because the wet distillers grain contains a significant quantity of volatile organic compounds (VOCs) regenerative thermal oxidizers (RTOs), designed to incinerate the VOCs, are normally required. These units can have trouble handling the wet, sticky emissions from the dryers because of plugging and fouling. Wet ESPs are ideally suited for pre-cleaning the gas stream prior to entering the oxidizer.

There are many ethanol and other plants in operation throughout North America and many more are planned. The continued drive to replace fossil fuels with green alternatives will mean a strong future for wet ESPs in these industries.

Recently, the EPA promulgated new standards requiring the installation of maximum achievable control technology (MACT) to be installed on many solid-fuel fired boilers (Ed Note: As of Sept. 2008, the present MACT standards are being re-written by the EPA due to a recent court decision). These regulations have significantly lowered the allowable particulate emissions from many operating boilers and in some cases have forced operators to install new emission control equipment. In many cases, the technology of choice is the wet ESP.

A move to wet technology to meet MACT requirements may, however, have a silver lining. Because of the inherent flexibility of this technology, boilers with wet ESPs often can be fired with alternative, low-grade fuels such as heavy oil, petroleum coke and tire-derived fuel. These fuels are often much cheaper than higher-grade fuels.

Where can we go?

Because wet ESP technology is flexible, the array of applications in which it can be applied is indeed very large. Here is a brief list of some novel areas where the technology may be a good fit:

Regenerative catalytic oxidizers (RCOs). RCOs consume less energy than RTOs. A limiting factor in their use, however, is gas cleanliness, i.e., too much particulate can foul or poison the catalytic media. In situations where a wet ESP is required to protect a downstream RTO, there may still be too much particulate to allow for catalytic operation. In these cases, if the performance of the wet ESP could be enhanced, then the RTO could be changed into a catalytic unit with significant energy savings. As fuel prices continue to rise, this approach could become more attractive.

Mist elimination – Wet ESPs are excellent mist eliminators. In situations where very high mist removal efficiency is required, a wet ESP may be an appropriate approach. Also, because they are so efficient on relatively large water droplets, wet ESPs could be adapted for the recovery of drift from cooling towers.

Gas separation. Research on gas separation membrane technology continues at a fast pace in laboratories all over the world. Application of these membranes shows a great deal of promise. Keeping the membranes clean could be an important role for wet precipitation technology particularly because wet ESPs are efficient and non-plugging, unlike filters.

Nuclear waste processing. Vitrification is the most likely fate of much of the high- and low-grade nuclear waste generated in nuclear power plants. The challenge of the vitrication process is cleaning the gas stream after it exits the glass furnace. Because the particulate matter will be radioactive, an extremely high removal efficiency will be required. However, because the amount of particulate matter will be substantial, filters, which can plug, can be impractical. The open-flow, high-efficiency wet ESP fits the bill.

As the first industrial wet ESP approaches its 100th anniversary, the industry is seeing more and more examples of wet precipitation as a leading technology for particulate removal. With society demanding more from technology to preserve the environment while providing abundant and affordable energy Dr. Cottrell's simple and effective wet ESP may be just beginning to come into its own. PE