Case study - WESPs: Securing a cleaner future for power-generating facilities


Courtesy of Beltran Technologies, Inc.

This article explains why wet electrostatic precipitators remain a viable technology for dealing with today's increasingly stringent compliance issues.

Despite recent, recession-related volatility in both power generation and consumption, industry analysts expect a resumption of long-term growth trends as continued global demand for energy — in both developed and developing nations —will be fueled by sheer population growth, increasing urbanization, and rising incomes and living standards.

Electric power generation activities, especially coal-fired boilers, face some of the most complex and oner-ous air-pollution-control challenges and tightest environmental regulations among all industrial sectors. High concentration 7oi fine particulate matter (PM-,.), sulfur dioxide, condensed hydrocarbons, and sulfuric acid mists are attracting the most concern due to their serious impacts on human health and ecological systems. Other pollutants include particulate-borne toxic metals (lead, mercury, arsenic, cadmium), hydrogen chloride, hydrogen fluoride, dioxins, furans, and greenhouse gases.

To reduce emissions from electric generating units, power 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, thanks to modern technological en-hancements, plant operators are turning with renewed interest to a basic technology that is more than 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 with up to 99.9 percent efficiency. Although WESPs may share similar operating principles and basic structures, they can vary greatly in design, materials, gas flow rates, and durability, as well as collection efficiency. Thus, it is important for engineers to recognize the key differences among these various systems.

Today, some of the more advanced WESPs are designed around a multistage system of ionizing rods, with star-shaped discharge points enclosed within

square or hexagonal tubes lined with grounded collection surfaces. This unique electrode geometry generates a corona field four to five times stronger than that of ordinary wet or dry ESPs. As flue gas travels through the tubular array, the intense corona induces 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.

The cool, saturated environment in the WESP is 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 — all of which can impair performance. By eliminating the need for mechanical or acoustical rappers, the cleansing system also minimizes 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 percent — 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 fiber-reinforced plastics (FRP).

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