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Acid Gas/SO2 Controls

Wet Scrubbers

Scrubber is a general term that describes air pollution control devices or systems that use absorption, both physical and chemical, to remove pollutants from the process gas stream. Scrubber systems rely on a chemical reaction with a sorbent to remove a wide range of pollutants, including sulfur dioxide (SO2), acid gases, and air toxics, from flue gases. When used to remove or "scrub" SO2 from the flue gas, these devices are commonly called flue gas desulfurization (FGD) or scrubber systems. FGD or scrubber systems are generally classified as either "wet" or "dry". Wet scrubbers are increasingly recognized as an important part of a multi-pollutant control program.

In a wet scrubber, a liquid sorbent is sprayed into the flue gas in an absorber vessel. The gas phase or particulate pollutant comes into direct contact with a sorbent liquid and is dissolved or diffused (scrubbed) into the liquid. The liquid interface for gas and particle absorption include liquid sheets, wetted walls, bubbles and droplets. In the wet processes, a wet slurry waste or by-product is produced. Most wet FGD systems use alkaline slurries of limestone or slaked lime as sorbents. Sulfur oxides react with the sorbent to form calcium sulfite and calcium sulfate. Uptake of the pollutant by the sorbent results in the formation of a wet solid by-product that may require additional treatment, or when oxidized, results in a gypsum by-product that can be sold.

Scrubber technologies for wet scrubbing of gaseous pollutants can achieve extremely high levels of multi-pollutant control, including acid gases, SO2, fine particulates and heavy metals (i.e., mercury) from utility and industrial coal-fired boilers, waste-to-energy systems, and other industrial processes. Wet scrubber technology can be applied to difficult processes such as gas absorption and particle collection, treating combustible particles, and removal of wet, sticky or corrosive particles. Wet scrubbers are used in industrial process mercury removal and to remove ionic forms of mercury from the gas stream of coal-fired power generation facilities.

Wet scrubbers generally have relatively small space requirements, low capital cost, and are able to process high temperature, high acidity, and high humidity flue gas streams. Scrubber costs have continued to decrease, largely because of technical innovations. Scrubber energy requirements have also continued to decrease, helping to lower operating costs. FGD systems are an increasingly significant part of a multi-pollutant control approach, even as the energy requirements of these systems are decreasing to where these systems now consume only about 1% of total boiler output. Where low-cost high-sulfur fuels are available, or where the required reductions are very high, scrubbing is often a viable control option.

New wet scrubbers routinely achieve SO2 removal efficiencies of 95%, with some scrubbers achieving removal efficiencies of up to 99%. Scrubbers have been used in the EPA Acid Rain Program on coal-fired boilers, which are significant sources of hydrochloric acid (HCl) and hydrofluoric acid (HF). According to the EPA and others, both wet and dry scrubbers have been shown to reduce HCl emissions by 95% and more, and wet scrubbers have been shown to reduce HF emissions by more than one-third. Others have reported ranges of 87-94% removal of chlorine and 43-97% removal of fluorine by both wet and dry scrubbers. In addition, wet scrubbers also provide significant removal of arsenic, beryllium, cadmium, chromium, lead, manganese, and mercury from flue gas.

Wet scrubbers can be generally grouped by geometric designs and method for gas-liquid contact. Several groupings include packed-bed, counter-flow, cross-flow, bubble-plate, open spray (single and double loop) tower, dual-flow tray, cyclonic, and venturi designs. However, there are many proprietary systems designed around specific industry needs. Design and operating parameters include scrubber geometrical shape, liquid spray or injection locations, gas residence time, gas velocities, gas and liquid temperatures, gas and liquid pressure drop, and, liquid/gas flow rate ratio.

Dry Scrubbers

In a dry scrubber or FGD process, particles of an alkaline sorbent are injected into a flue gas, producing a dry solid by-product. In dry FGD scrubbing, the flue gas leaving the absorber is not saturated (the major distinction between wet and dry scrubbers). Dry scrubbers generally involve simple designs and low capital and maintenance costs. Scrubber costs have continued to decrease, largely because of technical innovations. However, dry scrubbers are increasingly being recognized as an important part of a multi-pollutant control program. Dry scrubber energy requirements, while less than wet FGD processes, continue to decrease which helps to lower operating costs.

Generally, the fate of absorbent in dry FGD is characterized as either once-through or regenerable. Dry scrubbers systems can be grouped into three categories: spray dryers, circulating spray dryers, and dry injection systems. All these systems offer multi-pollutant control opportunities by combining acid gas, SO2, particulate control, and air toxics, including mercury.

In a spray dryer, a slurry of alkaline reagent, typically lime or sodium based, is atomized into the hot flue gas to absorb the pollutants. The resulting dry material, including fly ash, is collected in a downstream particulate control device, typically an electrostatic precipitator or fabric filter. In some cases, a portion of the dry material is recycled into the lime slurry mixture. Oxidation of the mixture results in a gypsum by-product that can be sold. A lime spray dryer can typically be installed on industrial and utility boilers, and on municipal and hazardous waste incinerators. Spray dryers commonly are designed for SO2 removal efficiencies of 70-95%.

A circulating dry scrubber uses an entrained fluidized bed reactor for contacting the reagent, usually hydrated lime, with sulfur dioxide and particulate laden flue gas. The intensive gas-solid mixing that occurs in the reactor promotes the reaction of sulfur oxides in the flue gas with the dry lime particles. The mixture of reaction products (calcium sulfite/sulfate), unreacted lime, and fly ash is carried to a downstream particulate collector that is separated from the gas stream. Part of the dry waste product is removed for disposal, but most of the waste product is mixed with fresh calcium hydroxide for use in the reactor. Water spray is introduced into the fluidized bed separately to enhance performance (for maximum SO2 capture with minimum lime utilization) by optimizing the surface moisture content of the lime. Circulating dry scrubbers can provide removal efficiencies of more than 90%.

Dry injection systems involve the injection of a dry sorbent (normally lime or limestone) into the flue gas in the upper reaches of the boiler, or in the ductwork following the boiler. Sulfur oxides react directly with the dry sorbent, which are collected in a downstream particulate control device. Because a separate scrubber vessel is not needed, capital costs are minimized. Low capital costs are partially offset by lower reagent utilization, which result in higher operating costs for equivalent SO2 removal rates. Dry injection systems are generally applied when lower removal efficiencies are required, or on small plants where the capital cost for other scrubber types may not be justified. Dry injection systems typically have removal efficiencies ranging from 50-70%.

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