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# Acid Gas/SO2 Controls

Wet Scrubbers Scrubber is a general term that describes air pollu tion control devices or systems that use absorption , both physical and chemical, to remove pollutants from th e process gas stream. Scrubber systems rely on a chemical reaction with a sorbent to remove a wide r ange of pollutants, including sulfur dioxide (SO2), acid gases, and air toxics, from flue gases. When used t o remove or "scrub" SO2 from the flue gas, these devices are commonly called flue gas desulfurizatio n (FGD) or scrubber systems. FGD or scrubber system s are generally classified as either "wet" or "dry". Wet scrubbers are increasingly recognized as an imp ortant 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 wit h a sorbent liquid and is dissolved or diffused (sc rubbed) into the liquid. The liquid interface for gas and p article absorption include liquid sheets, wetted wa lls, bubbles and droplets. In the wet processes, a wet s lurry waste or by-product is produced. Most wet FGD systems use alkaline slurries of limestone or slake d lime as sorbents. Sulfur oxides react with the so rbent to form calcium sulfite and calcium sulfate. Uptake ofthe pollutant by the sorbent results in the format ion of a wet solid by-product that may require additional tr eatment, or when oxidized, results in a gypsum by-p roduct that can be sold. Scrubber technologies for wet scrubbing of gaseous pollutants can achieve extremely high levels of mul tipollutant 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 scrubb er technology can be applied to difficult processes su ch as gas absorption and particle collection, treat ing combustible particles, and removal of wet, sticky o r corrosive particles. Wet scrubbers are used in in dustrial process mercury removal and to remove ionic forms o f 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 pr ocess high temperature, high acidity, and high humidity f lue gas streams. Scrubber costs have continued to decrease, largely because of technical innovations. Scrubber energy requirements have also continued t o decrease, helping to lower operating costs. FGD sys tems are an increasingly significant part of a mult ipollutant control approach, even as the energy requ irements of these systems are decreasing to where these systems now consume only about 1% of total bo iler output. Where low-cost high-sulfur fuels are available, or where the required reductions are ver y high, scrubbing is often a viable control option. New wet scrubbers routinely achieve SO2 removal eff iciencies of 95%, with some scrubbers achieving removal efficiencies of up to 99%. Scrubbers have b een used in the EPA Acid Rain Program on coal-fired boilers, which are significant sources of hydrochlo ric acid (HCl) and hydrofluoric acid (HF). Accordin g to the EPA and others, both wet and dry scrubbers have bee n shown to reduce HCl emissions by 95% and more, and wet scrubbers have been shown to reduce HF emis sions by more than one-third. Others have reported ranges of 87-94% removal of chlorine and 43-97% rem oval of fluorine by both wet and dry scrubbers. In addition, wet scrubbers also provide significant re moval of arsenic, beryllium, cadmium, chromium, lea d, 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-f low, bubble-plate, open spray (single and double lo op) tower, dual-flow tray, cyclonic, and venturi design s. However, there are many proprietary systems desi gned around specific industry needs. Design and operatin g 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 rat io. 

## Dry Scrubbers In a dry scrubber or FGD process, particles of an a lkaline sorbent are injected into a flue gas, produ cing a dry solid by-product. In dry FGD scrubbing, the flu e gas leaving the absorber is not saturated (the major distinction between wet and dry scrubbers). Dry scr ubbers generally involve simple designs and low cap ital and maintenance costs. Scrubber costs have continue d to decrease, largely because of technical innovations. However, dry scrubbers are increasingl y being recognized as an important part of a multipollutant control program. Dry scrubber energy requ irements, while less than wet FGD processes, contin ue to decrease which helps to lower operating costs. Generally, the fate of absorbent in dry FGD is char acterized as either once-through or regenerable. Dr y scrubbers systems can be grouped into three categor ies: spray dryers, circulating spray dryers, and dr y injection systems. All these systems offer multi-po llutant control opportunities by combining acid gas , SO2, particulate control, and air toxics, including merc ury. In a spray dryer, a slurry of alkaline reagent, typ ically lime or sodium based, is atomized into the h ot flue gas to absorb the pollutants. The resulting dry materia l, including fly ash, is collected in a downstream particulate control device, typically an electrosta tic precipitator or fabric filter. In some cases, aportion of the dry material is recycled into the lime slurry mixtu re. 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 d ryers commonly are designed for SO2 removal efficiencies of 70-95%. A circulating dry scrubber uses an entrained fluidi zed bed reactor for contacting the reagent, usually hydrated lime, with sulfur dioxide and particulate laden flue gas. The intensive gas-solid mixing thatoccurs in the reactor promotes the reaction of sulfur oxid es in the flue gas with the dry lime particles. The mixture of reaction products (calcium sulfite/sulfate), unreac ted lime, and fly ash is carried to a downstream pa rticulate collector that is separated from the gas stream. Pa rt of the dry waste product is removed for disposal , but most of the waste product is mixed with fresh calci um hydroxide for use in the reactor. Water spray is introduced into the fluidized bed separately to enh ance performance (for maximum SO2 capture with minimum lime utilization) by optimizing the surface moisture content of the lime. Circulating dry scru bbers can provide removal efficiencies of more than 90%. Dry injection systems involve the injection of a dr y sorbent (normally lime or limestone) into the flu e gas in the upper reaches of the boiler, or in the ductworkfollowing the boiler. Sulfur oxides react directlywith the dry sorbent, which are collected in a downstream pa rticulate control device. Because a separate scrubb er vessel is not needed, capital costs are minimized. Low capital costs are partially offset by lower rea gent utilization, which result in higher operating costs for equivalent SO2 removal rates. Dry injection sy stems are generally applied when lower removal efficiencies a re required, or on small plants where the capital c ost for 

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# Page 1 of 2 Acid Gas/SO2 Controls

- Institute of Clean Air Companies

1/14/2013 http://www.icac.com/?Acid_Gas_SO2_ControlInstitute of Clean Air Companies 2025 M Street NW · Suite 800 Washington, DC 20036 T: 202-367-1114 · F: 202-367-2114 E: icacinfo@icac.com Anti-Trust & Privacy other scrubber types may not be justified. Dry inje ction systems typically have removal efficiencies r anging from 50-70%.

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# Page 2of 2 Acid Gas/SO2 Controls

- Institute of Clean Air Companies

1/14/2013 http://www.icac.com/?Acid_Gas_SO2_Control