Compliance with the increasingly stringent air pollution regulations of recent years is not the only reason forwardthinking industry leaders are adopting more sophisticated gas cleaning and air pollution control strategies. The desire to achieve higher performance efficiencies, control costs in a competitive marketplace, and stay ahead of the regulatory compliance curve is another driver of innovation and investment in exhaust and process gas management systems. Finally, for many industries, a decisive factor driving these new investments is the heavy cost of maintaining and replacing downstream equipment impacted by gas streams containing highly corrosive chemical components.
This is particularly true of industries that generate SOX and sulfuric acid, including metallurgical smelters and refineries, petroleum refineries, natural gas processing facilities, electric generating units, spent acid regeneration plants or municipal waste incinerators. In many cases, a common and cost-effective solution for capturing and utilizing SOX and corrosive sulfuric acid emissions is the incorporation of downstream sulfuric acid manufacturing plants. Operators of these facilities can take advantage of the high industrial market value of purified sulfuric acid, a primary industrial chemical used in fertilizer manufacturing; mineral processing; petroleum refining; wastewater processing; the manufacture of paints, dyes, detergents, lead batteries and explosives; and the synthesis of other chemicals, as in the alkylation of gasoline additives.
An efficient sulfuric acid manufacturing process strictly requires the removal of contaminants from the input gas streams, especially fine particulates and acid mists such as those emitted from metal ore roasters and smelters, petroleum refineries and coalfired industrial boilers. 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 maintenance and operating costs for affected industries.
For removing fine particulates, acid mists and other contaminants from the gas stream, the one technology that is almost universally specified for this application is the Wet Electrostatic Precipitator (WESP).
Primarily targeted at capturing submicron- scale particulate matter, saturated sulfuric or other acid aerosols and condensable organic chemicals, a well-designed and correctly operated WESP unit is often incorporated after the gas scrubbers. WESPs can achieve collection efficiencies with these materials of greater than 99.9 percent – far superior to other equipment.
Although the basic principle and design of the electrostatic precipitator have been around since the early 1900s, recent innovations have produced dramatic advances in efficiency, cost effectiveness, ease of maintenance and wider applicability. WESPs in particular have demonstrated a level of performance that promises to keep them as the system with demonstrated ability to control harmful industrial emissions.
However, it is important for engineers to recognize that there are key differences in features and benefits offered by the various precipitator systems. Although they may share the similar operating principles and basic structures, WESPs can vary greatly in design, materials, gas flow rate, durability – and collection efficiency.
A basic WESP is comprised of an array of ionizing electrodes such that negatively charged discharge rods generate a strong electric field and corona. These are surrounded by, or interfaced with, positively charged or grounded collection surfaces, which attract and hold the charged particles. In operation, the source gas is passed through the electrode array, which induces a negative charge in even the minutest, submicron-size particles, propelling them toward the grounded collection surfaces, where they adhere as the cleaned gas is passed through. The captured particles are cleansed from the plates by recirculating water sprays; residues, including aqueous sulfuric acid, are extracted for further use or disposal. The cleaned gas is ducted to downstream equipment or to the stack, depending on the application.
WESPs 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 on 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 in waste-toenergy applications, etc.
A traditional problem has been with high-resistivity contaminants, such as low-sulfur coal ash. However, newer WESP configurations and designs are available that overcome this challenge, using multistage ionizing rods, starshaped discharge points and space-saving hexagonal tube designs. This unique geometry generates a corona field that is four to five times more intense than other ESPs, achieving superior collection efficiency on resistant materials. This feature also allows higher velocity gas streams, resulting in faster through-put. WESPs impose a significantly lower pressure drop compared to scrubbers and fabric filters, and thus also contributes to increased production speeds. Furthermore, these gains in efficiency enable the use of smaller-scale, less-expensive equipment for a given set of operating parameters.
Another challenge for traditional precipitator designs was the re-entrainment back into the gas stream of particles from the collection surfaces. Dry-operating ESPs, especially those using mechanical or acoustical vibrating rapper machinery, were particularly susceptible to this phenomenon. Precipitators based on wet operation, however, minimize reentrainment, as the aqueous flushing is continuously effective. The elimination of mechanical rapping also reduces the higher cost and energy drain imposed by that equipment.
Other critical features to look for in WESP equipment are the more advanced electronic controls, which can optimize operating parameters such as gas flow, saturation, temperature, corona intensity, etc., to achieve maximum efficiency. In biomass gasification-to-energy applications, WESPs filter pre-combustion syngas, which can be a fraction of the volume of post-combustion exhaust from gas turbines; this supports greater efficiency and lower cost for pollution control, and downstream carbon capture and sequestration applications.
Wide ranging applications
Because it operates at cooler temperatures – usually at the process gas saturation temperature between 100 and 170° F – the WESP is uniquely adept at capturing condensable organic materials and acid mists, making this technology an invaluable component for sulfuric acid production plants. Since sulfur is a common element found in the earth, sulfuric acid and other sulfur-based pollutants are most associated with industrial processes that deal with materials originating underground:
- Metals and Metallurgical plants
- Mining and Extraction
- Smelting and Refining
- Finishing, Metallurgical Processing (steelmaking, alloys, etc.)
- Fossil fuels (coal, oil, gas)
- Petroleum refineries/processing
- Fossil fuel-fired power generating units
WESPS have proven invaluable in these industries, as well as in others, such as pulp and paper, biomass gasification, waste-toenergy technologies, etc. In each case, it is becoming clear that ESP technology will have an important role to play.
In both the electric power and transportation fuel industries, various sources of alternative, high-efficiency energy continue to emerge and evolve. However, for the near to mid-term, it is evident that the conversion of fossil fuels will remain this country’s principal source of energy – and of ambient air pollution.
Much attention has been devoted in recent years to the relative advantages and drawbacks of various carbonaceous feedstocks for power generation and transportation fuels – coal versus oil versus natural gas. Less conspicuous is the competition among the actual conversion processes that may be applied to those fuels. Beyond the conventional path of direct combustion, newer thermal conversion technologies, including gasification (e.g., coal, biomass and wastes), pyrolysis and plasma arc conversion may become the carbon-based energy technologies of the future; and in most cases, they may benefit from the utilization of WESP technology.
Whether the primary goal is capturing fugitive emissions, conditioning process gases or cleaning stack gas emissions, one common factor shared by most industries is the need to remove, at some stage in their process, toxic, hazardous and/or destructive chemicals, particles, aerosols and gaseous impurities. Among a long and growing list of pollutants coming under scrutiny by state and federal regulators are: particulates (especially fine, PM 0.25), SO2 and SO3, acid gases and aerosols (sulfuric and hydrochloric), nitric acid, mercury, other heavy metals, and organic air pollutants. As affected industries address these challenges, the utilization of electrostatic precipitators as a primary or adjunct gas treatment option may remain a valuable tool.