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# Uses of Lime

Environmental Flue Gas Desulfurization

Dry Lime Scrubbing Wet Lime Scrubbing Comparing Lime and Limestone SO2 Wet Scrubbing Processes HCl Removal Mercury Removal Biosolids and Sludge Animal Wastes Wastewater Hazardous Wastes Metallurgy Construction Other Uses 

# Flue Gas Desulfurization

Lime plays a key role in many air pollution control applications. Lime is used to remove acidic gases, particularly sulfur dio xide (SO 2 ) and hydrogen chloride (HCl), from flue gases. Lime-based technolo gy is also being evaluated for the removal of mercury. Lime is more reactive than limestone, and requires less capital equipment. SO 2removal efficiencies using lime scrubbers range from 95 to 99 percent at electric generating plants. HCl removal efficien cies using lime range from 95 to 99 percent at municipal wast e-to-energy plants. There are two main methods for cleaning flue gases from coal co mbustion at electric generating stations: dry scrubbing and wet scrubbing. Lime is used in both systems. Dry scrubbing is also used at municipal waste-to-energy plants and other industrial facilities, primarily for HCl control. 

## Dry Lime Scrubbing In dry scrubbing, lime is injected directly into flue gas to remove SO 2 and HCl. There are two major dry processes: “dry injection” systems inject dry hydrated lime into the flue gas duct and “spraydryers” inject an atomized lime slurry into a separate vesse l. A spray dryer is typically shaped like a silo, with a cylindrical top and a cone bottom. Hot flue gas flows into the top. Lim e slurry is sprayed through an atomizer ( e.g ., nozzles) into the cylinder near the top, where it absorbs SO 2 and HCl. The water in the lime slurry is then evaporated by the hot gas. The scrubbed flue gas flows from the bottom of the cylindrical section through a horizontal du ct. A portion of the dried, unreacted lime and its reaction products fa ll to the bottom of the cone and are removed. The flue gas th en flows to a particulate control device ( e.g ., a baghouse) to remove the remainder of the lime and reaction products. Both dry injection and spray dryers yield a dry final product, co llected in particulate control devices. At electric generatin g plants, dry scrubbing is used primarily for low-sulfur fuels. At municipal waste-to-energy plants, dry scrubbing is used for removal of SO 2 and HCl. Dry scrubbing is used at other industrial facilities for HCl cont rol. Dry scrubbing methods have improved significantly in re cent years, resulting in excellent removal efficiencies. 

## Wet Lime Scrubbing In wet lime scrubbing, lime is added to wa ter and the resulting slurry is sprayed into a flue gas scrubber. In a typical syste m, the gas to be cleaned enters the bottom of a cylinder-like tower and flows upward through a shower of lime slurry. The sulfur dioxide is absorbed into the spray and then precipitated as wet calcium sulfite. The sulfite can be converted to gypsum, a salable by-product. We t scrubbing treats high-sulfur fuels and some low-sulfur fuels wher e high-efficiency sulfur dioxide removal is required. Wet scr ubbing primarily uses magnesium-enhanced lime (c ontaining 3-8% magnesium oxide) because itprovides high alkalinity to increase SO 2 removal capacity and reduce scaling potential. 

## Comparing Lime and Limestone SO 2 

## Wet Scrubbing Processes More than ninety percent of U.S. flue gas desulfurization (FGD ) system capacity uses lime or limestone. This trend will likelycontinue into the next phase of federally mandated SO 2 reduction from coal burning power plants. In 2003, the National Lime Association sponsored a study by Sargent and Lundy to compare the costs of l eading lime and limestone-based FGD processes utilized by power generating plants in the United States. The study included develop ing conceptual designs with capital and O&M cost requirement s using up-to-date performance criteria for the processes. The results of the study are summarized in two reports: Wet FGD Technology Evaluation and Dry FGD Technology Evaluation . The reports present the competitive position of wet and dry limestone and lime-based processes relative to reagent cost, auxiliary power cost, coal su lfur content, dispatch, capital cost, and by-product productio n (gypsum and SO3 aerosol mitigation chemicals). The reports were updated in a 2007 reportcomparing wet limestone and dry lime based systems. 

## HCl Removal Because lime also reacts readily with othe r acid gases such as HCl, lime scrubbing is used to control HCl at municipal and indu strial facilities. For example, at municipal waste-to-energy plants, dry lime scrubbing is used to control emissions from about 70 pe rcent of the total U.S. capacity (as of 1998). HCl removal efficiencies using lime range from 95 to 99 percent. At secondary aluminum plan ts, for example, EPA identifies lime scrubbing as a maximum achievable control technology for HCl. EPA tests demonstrate removal efficiencies greater than 99 percent. 

## Mercury Removal Different methods for controlling mercury emissions are being ev aluated in the United States. One control technology being eva luated combines hydrated lime with activated carbon. The reagent, a registered product, consists of 95-97 percent lime and 3-5 percen t activated carbon. Other calcium-based sorbents are also being evaluated as cost-effective alternatives for combined SO 2 and mercury removal.

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