Wet electrostatic precipitators are proven for emission control and gas cleaning

Industrial plant engineers have a wide variety of pollution control and gas cleaning systems from which to choose. Regardless of the manufacturing process, from automotive production to zinc smelter operations, wet electrostatic precipitators (WESPs) have proven particularly valuable in industries that emit sulfur oxides and sulfuric acid, such as metallurgical smelters and refineries; petroleum refineries; fossil-fueled power plants and industrial boilers; and municipal waste incinerators.

WESPs are also valuable in the manufacturing of electronic components such as semiconductors, printed circuit boards and microchips. Potential air emissions from these industries include doping agents; hazardous gases; organic solvent vapors; particulates; and sulfuric, hydrochloric, and other acids. For controlling particulate emissions, and condensed organic compounds, WESPs remain the technology of choice.

Another forward-looking technology, biomass gasification, requires high-efficiency cleaning of syngas produced from the thermochemical conversion of carbonaceous wastes. Then, for maximum energy production (via synfuel engines, gas turbines or liquid fuel combustion), the gas must be purified to extremely high standards. All these plants are impacted by emissions control and gas cleaning problems.

What’s available

Systems and equipment available to plant engineers include wet and dry flue-gas scrubbers, cyclones, fabric filters, thermal oxidizers and dry electrostatic precipitators. These types of systems can be cost-effective in controlling large-scale particulates, oxides of sulfur and nitrogen and other hazardous air pollutants. However, they are usually inefficient or ineffective on such problematic industrial contaminants as fine particulates, acid mists, heavy metals or condensed organic compounds. In these cases, engineers continue to rely on modern versions of a technology that has been used for years to reduce dust and fumes from industrial exhaust and process gases: the wet electrostatic precipitator.

The basic WESP design makes use of an array of negative discharge electrodes surrounded by grounded collection surfaces. Source gas is passed through the array, which induces a negative charge in even the most minute, submicron-size particles, impelling them toward the collection surfaces. There they adhere as the cleaned gas is passed through. The captured particle residues are purged from the plates by recirculating water sprays.

The simple elegance of the basic WESP design concept makes it versatile over a broad range of industries, applications, operating conditions, locations and gas chemistries. Still, it is important for engineers to recognize that there are key differences in features and benefits among the various precipitator systems. WESPs can vary greatly in design, materials, gas flow rates and durability, as well as collection efficiency.

Subtle differences

Today’s advanced WESPs are designed around a multistage system of ionizing rods bristling with star-shaped discharge points, enclosed within various collector tube shapes, such as round, space-saving square or hexagonal tubes. This unique electrode geometry generates a corona field 4-5 times more intense than that of other electrostatic precipitator designs, resulting in greater particle migration velocity and collection efficiency. Fine particulates and aerosols, which have little significant mass and easily escape through venturi and other scrubbers, are captured at up to 99.9% efficiency with a well-designed WESP.

Wet electrostatic precipitators can process a wide range of gas streams. They are often used downstream from wet or dry flue gas desulfurization units, which cannot capture fine particulates and acid aerosols. They are also superior when applied to high ash content and sticky residues (which may also contain mercury and heavy metals), oily residues/tars, mercury (as condensed oxide) and emissions from municipal solid waste (MSW) in waste-to-energy plant applications, as well as others.

Compared to WESPs, the challenge for traditional dry precipitator designs is the possible re-entrainment back into the gas stream of particles from the collection surfaces. Dry-operating ESPs, especially those using mechanical, acoustical, or vibrating rapper methods, are particularly susceptible to this phenomenon. Precipitators based on wet operation, however, minimize re-entrainment, as the aqueous flushing is operating continuously. The elimination of these rapping methods also reduces the higher cost and energy requirement imposed by that equipment.

Because the WESP processes gases in a cooler, saturated environment — usually between 100° – 170° F. — it is uniquely adept at capturing condensable organic materials and acid mists, such as found in sulfuric acid plants like Mopani Copper.

Mopani Copper case history

Mopani Copper Mines Plc, a unit of Glencore Xstrata based in Switzerland, operates sulfuric acid production facilities at their copper smelter plants in Mufulira and Kitwe in Zambia. The sulfuric acid plants currently have nine wet electrostatic precipitators designed and engineered by Beltran Technologies for sulfuric acid gas cleaning.

Industrial-grade sulfuric acid, still the most widely used industrial chemical in the world, continues to be sourced primarily as a nondiscretionary byproduct from the roasting, smelting and refining of nonferrous metals (70%), and from natural gas processing, electric power generation and spent acid regeneration. These industries are usually heavy emitters of particulates, sulfur and nitrogen oxide gases, and sulfuric acid mists, among other pollutants. They are also subject to increasingly strict environmental regulations.

When concentrations of sulfur dioxide from these operations exceed 5-7% of exhaust-gas volumes, a common and cost-effective solution is the incorporation of a downstream sulfuric acid manufacturing plant. Owners of these facilities can capitalize on the high industrial market value of purified sulfuric acid, while achieving greater operating efficiencies and easier regulatory compliance.

An efficient sulfuric acid manufacturing process requires the maximum possible removal from input gas streams of fine particulates, acid mists, condensable organic compounds and other contaminants. This high level of gas-cleaning efficiency is necessary to prevent poisoning of the catalysts and fouling or plugging of the catalyst beds. An optically pure input gas is essential for avoiding the formation of a “black” or contaminated acid end-product.

Hyundai Steel case history

In the recent past, Hyundai Steel, Seoul, South Korea purchased Hanbo Steel Co. Hanbo Steel had operated the steel plant for some time using a Japanese horizontal wet electrostatic precipitator that was inefficiently removing particulate matter. After Hyundai bought the steel plant, it revamped most of the air pollution control equipment to comply with the country’s regulations. The Hyundai Group has the reputation of being a leading company in Korea. And they wanted to eliminate emission problems.

Hyundai manufactures steel for cars, buses, trucks and high-speed trains. They use a scarfing process, i.e., a thermochemical exothermic reaction of oxygen and fuel which reacts with and removes surface defects from cast steel.

The reaction of oxygen with the steel results in a high concentration emission of submicron iron oxide particulate difficult to remove by ordinary methods. The scarfing process produces a peak emission of submicron particulate in a concentration of 2500 mg/Sm³. The Korean Environmental Agency requires an outlet concentration not to exceed 5 mg/Nm³. This requires the WESP to have a high-performance efficiency of 99.8% and the process has a high of volume 4,000 Am³/min. The scarfing process produces a great deal of particulate and visible emission. Hyundai Steel quenches the exhaust gas using water spray in a 100-meter length tunnel. Gases then go to the wet electrostatic precipitators to remove the submicron particulate and visible emission.

For an exhaust gas volume of 4,000 Am³/min, Beltran Technologies designed two WESPs operating concurrently. Beltran guaranteed 99.8% at an inlet condition of 2500 mg/Sm3 and an outlet concentration of 5 mg/Sm³. However, Beltran achieved more than 99.8% in the WESP operation. Beltran attained under 1 mg/Sm3 during the WESP operation which is a more than 99.96% submicron particulate reduction.

For steel mills, sulfuric acid plant production, metallurgical/mining operations and so many other industrial applications, the WESP has a long history of achievement reducing fine particulate aerosols and visible emissions.

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