Beltran Technologies, Inc.

Advanced WESP designs assure superior gas cleaning performance

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Courtesy of Beltran Technologies, Inc.

Nowadays, many industries are facing formidable environmental challenges and there is a growing need for effective, economical gas cleaning and air pollution control equipment. Michael Beltran of Beltran Technologies, Inc. reports on a renewed interest in high-efficiency wet electrostatic precipitators (WESPs) and discusses their importance for controlling sulphuric acid mists and other pollutants in a wide range of industries including mining, metallurgical and power generation.

The global economic recovery continues to be sporadic among most of the world’s economies, with many industries experiencing persistent volatility in profits, prices, markets and capital investments. Yet, some financial analysts are predicting an eventual resumption of long-term growth trends in such basic industries as mining and metallurgy, petroleum refining, and fossil-fuelled (primarily coal) electric power generation. Experts anticipate that global demand for mined resources, transportation fuels and energy, in both developed and developing nations, will be driven by relentless population growth, increasing urbanisation, rising incomes, and expectations of higher living standards among populations exposed to an array of digital communications technologies and social media.

While by no means a boom, these trends, coupled with a growing worldwide concern over environmental issues, portend a continuing and perhaps sharper focus on the formidable environmental challenges facing these and other industries, and the growing need for effective, economical gas cleaning and air pollution control equipment.

Many of these basic industries face some of the most complex and onerous air pollution-control challenges, and some of the tightest environmental regulations, of all industrial sectors. In metallurgical operations, ore concentrators, smelters, roasters, converters and other refining stages can release an array of toxic pollutants into the atmosphere. High concentrations of fine particulate matter (PM 0.25) and sulphur dioxide, especially from roasters and smelters, are attracting the most concern due to their serious impacts on human health and ecological systems.

Extractive and fossil-based power industries normally deal with materials originating within the earth’s crust, including sulphur, one of the most common elements, and one of the most reactive--and thus one of the most pervasive sources of hazardous compounds such as sulphur trioxide and sulphuric acid mists. Other constituents found in these basic industrial emissions include, heavy metals (lead, mercury, arsenic, cadmium), hydrogen chloride, hydrogen fluoride, dioxins, furans and greenhouse gases. Particulate matter itself can be a vehicle for oxides or sulphides of lead, zinc, iron, antimony, mercury and other toxics. Fugitive particulates can be released during the handling and transport of ores, concentrates and tailings. Acidic vapours can be produced during pyrometallurgy and other secondary processing.

In an effort to reduce concentrations of these solid, liquid and gaseous contaminants, plant designers and operators have deployed an arsenal of gas cleaning and emission control equipment and techniques, including wet and dry flue-gas scrubbers, venturis, cyclones and fabric filters. In addition, largely due to the advent of modern technological enhancements, plant operators are turning with renewed interest to a basic technology that is over a century old, yet incomparably efficient: wet electrostatic precipitators, or (WESPs). A typical advanced WESP can clean the flue gas of acid mists, condensed organics or fine particulates down to submicron scale (PM 0.25) with up to 99.9% efficiency.

When concentrations of sulphur compounds in flue gases exceed 5-7% of gas volumes, a common and cost-effective solution for cleaning the gas while capturing and utilising the these compounds is the incorporation of downstream sulphuric acid manufacturing plants. Operators of these facilities seek to capitalise on the market value of purified sulphuric acid, a primary industrial chemical with hundreds of applications such as fertilizer, paints, lead batteries and petroleum refining.

However, an efficient sulphuric acid manufacturing process requires the maximum possible removal from input gas streams of fine particulates, acid mists, condensable organic compounds and other contaminants. This is necessary for protecting downstream components such as catalyst beds from corrosion, fouling and plugging, as well as for preventing the formation of a “black” or contaminated acid end-product. Proper gas cleaning also results in lower costs for maintenance, operations and equipment replacement. For removing these and other contaminants from flue gases, the WESP is the one technology that is becoming almost universally specified.

Although they may share similar operating principles and basic structures, WESPs can vary greatly in design, materials, gas flow rates and durability, as well as collection efficiency. It is thus important for engineers to recognise the key differences among these various systems.

Today, some of the more advanced WESPs are designed around a multistage system of ionising rods with star-shaped discharge points, enclosed within square or hexagonal tubes which are lined with grounded collection surfaces. The unique electrode geometry generates a corona field 4-5 times stronger than that of ordinary wet or dry ESPs. The multistage charging configuration also assures maximum corona field strength with a minimum of energy load.

As flue gas travels through the tubular array, these intense corona fields induce a negative charge, propelling even submicron- size particulates and acid mists toward the collection surfaces, where they adhere as cleaned gas is passed through. The surfaces are cleansed of residues by recirculating water sprays. A heated purge-air stream should be used to keep the high-voltage insulators dry, reducing maintenance costs. Since fine particles have little significant mass, they generally pass through scrubbers, venturis and other devices, but are captured with remarkable efficiency by advanced WESP equipment.

The cool, saturated environment in the WESP is highly effective on condensable or oily compounds, which can elude conventional equipment that relies on temperatures as high as 700-800°F (371-427°C). The continuous aqueous flushing process prevents re-entrainment of particles, sticky residue build-ups and particle resistivity which can impair the performance of dry ESPs. By eliminating the need for mechanical or acoustical rappers, the cleansing system also minimises energy costs.

With virtually no mechanical obstruction, 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 and still achieve collection efficiencies of 99.9% – far superior to wet or dry scrubbers, cyclones, fabric filters and other equipment.

Other critical features to look for in WESP equipment are sophisticated electronic controls linked to a close-coupled gas flow management system; these components can squeeze even more efficiency out of the system by optimizing such operating parameters as gas velocity, saturation, temperature, corona intensity, etc. Also, to prevent premature deterioration, critical WESP surfaces should be constructed with modern, corrosion-resistant materials such as high nickel-chromium alloys or fibre-reinforced plastics (FRP).

Most WESP designs allow for a wide range of adaptability within existing industrial installations, and are often positioned downstream from wet or dry flue-gas desulphurisation or other pollution control equipment. A new mission for WESPs is emerging as a result of the increasing reliance by industry on selective catalytic reduction (SCR) systems to control nitrogen oxide emissions (NOx) resulting from fuel combustion in the presence of air. The SCR increases the conversion of SO2 to SO3, which reacts with water vapour to form sulphuric acid mists. These mists in turn overload or escape the scrubbers, and emerge as an acidic, visible plume. Fortunately, the WESP is ideally suited to prevent this occurrence.

Forward-thinking industrial plant operators around the world constantly seek out and deploy more advanced gas cleaning technologies throughout their enterprises, not only to stay ahead of the regulatory compliance curve, but also to achieve superior operating performance and to control maintenance and other costs in a competitive marketplace. In the United States, installations representing the top 12% for gas-cleaning efficiency are used as the official benchmark for determining the recently established “Most Achievable Control Technology,” or MACT, performance levels for industrial boilers. Other proposals in the U.S. and throughout the world indicate continued stringency in the regulatory environment. These and other future air pollution control regulations will impact the development of more advanced remedial technologies. Yet, as an enabling technology for the mining, metallurgical, power generation and other critical industries, the role of wet electrostatic precipitators should continue to grow as an essential primary or adjunct gas treatment option.

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