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Acid Gas/SO2 Controls

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Acid Gas/SO2 Control Technologies

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 rang e 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 so rbent liquid and is dissolved or diffused (scrubbed) into the liquid. The liquid interface for ga s 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 byproduct 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 po llutants can achieve extremely high levels of multipollutant control, including acid gases, SO2, fine partic ulates and heavy metals (i.e., mercury) from utility and industrial coal-fired boilers, waste-to-energy syst ems, 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 corro sive 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 fl ue gas streams. Scrubber costs have continued to decrease, largely because of technical innovations. Scrubber energy requirements have also continued todecrease, helping to lower operating costs. FGD systems are an increasingly significant part of a multipollutant 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 hy drofluoric 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, berylliu m, 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) 

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Dry Scrubbers

In a dry scrubber or FGD process, particles of an alkalin e 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 scrubb ers 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 increasing ly 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 charac terized 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 li me or sodium based, is atom ized into the hot flue gas to absorb the pollutants. The resulting dry material, incl uding 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. Oxidat ion 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 ar e designed for SO2 removal efficiencies of 70-95%.

A circulating dry scrubber uses an entrained fluidized be d reactor for contacting the reagent, usually hydrated lime, with sulfur dioxide and particulate laden flue ga s. The intensive gas-solid mixing that occurs in the reactor promotes the reaction of sulfur oxides in th e flue gas with the dry lime particles. The mixture of reaction products (calcium sulfite/sulfate), unreacted lim e, 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 calciu m 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 mois ture content of the lime. Circulating dry scrubbers can provide removal efficiencies of more than 90%.

Dry injection systems involve the injection of a dry sorb ent (normally lime or limestone) into the flue gas in the upper reaches of the boiler, or in the ductwork foll owing 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 re quired, or on small plants where the capital cost for other scrubber types may not be justified. Dry injectio n systems typically have removal efficiencies ranging

from 50-70%. 

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