Wet electrostatic precipitators an effective technology
In an effort to reduce emissions produced by various industries, plant designers have called upon an arsenal of gas cleaning equipment and techniques, including wet and dry flue-gas scrubbers, venturi scrubbers, cyclones, and fabric filters. These systems can be cost-effective at controlling large particulates, oxides of sulfur and nitrogen, and other hazardous air pollution. However, they usually are inefficient or ineffective on such problematic contaminants as fine particulates, acid mists, oily residues, or fine, condensed organic compounds.
Therefore, plant operators are turning with renewed interest to modern wet electrostatic precipitators (WESPs). Thanks to innovative technological enhancements, WESPs can clean complex gaseous emissions of particulates and mists down to submicron scale (PM 2.5) with up to 99.9% efficiency.
Today, some of the more advanced WESPs are designed around a multistage system of ionizing rods with star-shaped discharge points. These are encased within square or hexagonal tubes which are lined with grounded collection surfaces. The unique electrode geometry generates a corona field four to five times stronger than that of conventional wet or dry ESPs.
As flue gas travels through the tubular array, the intense corona induces a powerful negative charge, propelling even submicron-size particulates and sulfuric acid droplets toward the collection surfaces, where they adhere as cleaned gas is passed through. The surfaces are intermittently cleansed of residues by recirculating water sprays.
The cool, saturated environment in the WESP makes the system highly effective on condensable or oily compounds, while the continuous aqueous flushing process prevents re-entrainment of particles, sticky residue build-ups, and particle resistivity which can impair performance. By eliminating the need for mechanical or acoustical rappers, the cleansing process also minimizes energy and maintenance costs.
With virtually no mechanical obstruction of flue gas emissions, there is very little pressure drop through the WESP, and gas velocities can be extremely high. This enables plant engineers to use smaller-scale, less costly equipment for specific gas volumes and still achieve collection efficiencies of 99.9%.
Due to their versatility and efficiency on a wide variety of problematic emission components, and under diverse operating conditions, wet electrostatic precipitators are playing an essential gas cleaning role in numerous industrial sectors and applications. Even in those plants which rely primarily on various scrubber technologies to control pollutants in the gaseous state, WESPs are often used as critical, downstream adjunct equipment to capture contaminants the others miss, especially when flue gases are cooled to the condensation levels of various undesirable chemical compounds.
Metallurgical industries benefit
Among the world’s primary heavy industries, the mining and metallurgical sector, from exploration to extraction, faces some of the most complex and onerous air pollution-control challenges, and some of the tightest environmental regulations. Ore concentrators, smelters, roasters, converters, and other refining stages can release a profusion of pollutants into the atmosphere.
High concentrations of fine particulate matter (PM 2.5) and sulfur dioxide, especially from roasters and smelters, are attracting the most concern due to their serious impacts on human health and ecological systems. Particulate matter itself can become an airborne vehicle for toxic metal oxides or sulfides. Other potential pollutants include sulfuric acid mists and heavy metals, hydrogen chloride, hydrogen fluoride, dioxins, furans, and greenhouse gases.
These environmental concerns are taking on increased importance as multinational metals and mining companies venture into ever more remote corners of the globe, driven by the increasing scarcity of higher grade ore deposits in traditional, more easily accessible mining regions. Consequently, much of the world’s new exploration and development is occurring within the borders of developing nations, focusing on aggressive exploitation of proven mineral reserves or on promising new explorations.
At the same time, these nations have begun to demand more from foreign mining and processing companies—with particular emphasis on stringent pollution control, social responsibility, and sustainable development. The new risk/reward equation in emerging markets means these companies must devote greater resources to emissions control, including conscientious investments in new capital equipment, or risk serious disruption, delay, and even blockage of major projects.
For metallurgical operations, the competitive drive to achieve regulatory compliance, improve performance, and control costs has stimulated major innovations in exhaust and process gas cleaning technologies.
One common but crucial strategy is the construction of downstream sulfuric acid plants to convert SO3 and SO2 into purified sulfuric acid, the world’s most widely used and readily marketable industrial chemical. In fact, most sulfuric acid on the market today is a by-product of ferrous and nonferrous smelting operations.
To remain competitive in price and quality, a sulfuric acid manufacturing plant requires the maximum possible removal from input gas streams of fine particulates, acid mists, and other impurities. This removal step helps protect catalyst beds and other components from corrosion, fouling, or plugging, and prevents the formation of “black” or contaminated sulfuric acid. It also ensures that fewer H2SO4 and other contaminants are present in the final stack gases.
Success in Zimbabwe
An excellent example of the successful application of WESPs in mining and metallurgical operations is Mopani Copper Mines, Plc, located in the town of Mufulira, in Zambia’s mineral-rich Copperbelt region. In 2010, copper exports accounted for 78% of Zambia’s merchandise exports, making it one of the largest producers in the world. The Mopani facility utilizes pyrometallurgical smelting processes, which produce high levels of sulfur dioxide, sulfuric acid mists, particulates, and other emissions from the metallic concentrate.
In 2007, under pressure from Zambian regulators, Glencore International AG, the mine’s owner, engaged a former engineering subsidiary of Monsanto, Inc., to enlarge and modernize its sulfuric acid plant. Mopani had been using reverse jet scrubbers to clean the smelting off-gases, but these alone were not effective on acid mists and fumes. The acid plant expansion included a pair of WESP units designed and built by Beltran Technologies of Brooklyn, N.Y. The new gas cleaning equipment helped reduce acid mists by 94% and particulates by 99.5%, while handling 11 kg/hr of sulfuric acid mists and producing 244 metric tons per day of clean sulfuric acid product.
The corrosive nature of the Mopani smelter emissions required that special attention be given to the materials used in construction of the precipitators. Therefore, the Beltran WESPs were custom fabricated using fiberglass reinforced plastic (FRP) and high nickel-chromium alloys. Although precipitators used in acid mist applications historically have been made with lead, engineers have found FRP components to be less expensive, easier to construct and maintain, and extremely corrosion resistant. The electrically conductive sections of the WESPs are protected with a specially engineered, conductive FRP material. The high-voltage insulators are kept continuously clean using a purge-air system, further reducing maintenance costs.
Smelting operations at the Mopani Copper Mine were expanded again in early 2014 to meet the steadily growing worldwide demand for copper. To handle the significantly increased gas volumes at Mopani’s newly added sulfuric acid plants, Beltran installed six more WESP units. Today, Mopani Copper Mines Plc is again meeting emission control requirements set by the Zambian government.
The Mopani Copper facility provides one example of how global industrial companies, no matter where they conduct operations, can rely on newer generations of WESPs to help meet the most complex, exacting emission control requirements while ensuring superior performance, productivity, and profit.
Ronald F. Vitale is a freelance technical writer based in Suffern, N.Y.