Innovative wet electrostatic precipitators assure competitive edge in acid marketplace
The unsteady global economic recovery continues to subject many core industries to persistent volatility in profits, prices, markets and capital investments. However, some analysts remain optimistic about the global international trade in sulfur in all forms, particularly sulfuric acid.
This versatile mineral acid is both the world’s most widely used chemical, and the one with the highest production volume. Its use as both a primary and intermediate raw material spans hundreds of industrial processes, especially agricultural fertilizer manufacturing, which consumes 70 percent of H2SO4 production. This application alone may continue to fuel a vigorous global trade in this chemical. Sulfur provides both a direct nutritive value for plants, as well as an indirect value as a soil amendment. It also facilitates a plant’s use of the three other major nutrients: nitrogen, phosphorus and potassium.
Growth in the fertilizer industry, and consequently in the global sulfuric acid trade, is expected to be driven by the following factors:
—The increasing demand for foodstuffs by burgeoning global populations.
—A decrease in accessible arable land due to rapid industrialization and urbanization, especially in developing countries.
—Concern over sulfur deficiency in soils resulting from intensive farming techniques; higher-yielding crop varieties; higher-volume irrigation systems; less use of traditional manures and other sulfur-rich fertilizers; and—ironically—a significant reduction in the amount of atmospheric sulfur available for soil deposition due to more stringent emission control regulations.
As global trade in sulfuric acid accelerates, acid plant operators will be under increasingly intense competition to supply the purest, highest quality product. At the same time, operators must remain pricecompetitive by achieving maximum costefficiency as they maintain these high levels of purity.
Most sulfuric acid on the market today is sourced as a non-discretionary by-product captured from ferrous and non-ferrous smelting. This industry faces some of the most complex and onerous air pollution-control challenges, and some of the tightest environmental regulations, among all industrial sectors. Ore concentrators, smelters, roasters, converters and other refining equipment can release an array of toxic pollutants into the atmosphere. High concentrations of fine particulate matter and sulfur dioxide are attracting the most concern due to their serious impacts on human health and ecological systems.
In an effort to reduce these emissions, plant designers have deployed an arsenal of gas cleaning equipment and techniques, including wet and dry flue-gas scrubbers, venturis, cyclones and fabric filters. In addition, thanks to innovative technological enhancements, plant operators are turning with renewed interest to modern wet electrostatic precipitators (WESPs), which can clean flue gases of acid mists, condensed organics or fine particulates down to submicron scale with up to 99.9 percent efficiency.
To remain competitive in price and quality, 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 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.
WESPs can vary greatly in design, materials, gas flow rates and durability, as well as collection efficiency. It is thus important for engineers to recognize the key differences among these various systems.
Today’s advanced WESPs are designed around a multistage system of ionizing 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 avoids corona quenching due to high particle densities, and 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 submicronsize 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. The continuous aqueous flushing process prevents reentrainment of particles, sticky residue buildups and particle resistivity. By eliminating the need for mechanical or acoustical rappers, the cleansing system also minimizes energy costs.
With very little pressure drop through the WESP, gas velocities can be extremely high, boosting efficiency. Plant engineers can use smaller-scale, less costly equipment and still achieve superior collection efficiencies.
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.
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 this context, the role of wet electrostatic precipitators should continue to grow as an essential primary or adjunct gas treatment option.