Lime plays a key role in many air pollution control applications. Lime is used to remove acidic gases, particularly sulfur dioxide (SO2) and hydrogen chloride (HCl), from flue gases. Lime-based technology is also being evaluated for the removal of mercury. Lime is more reactive than limestone, and requires less capital equipment. SO2 removal efficiencies using lime scrubbers range from 95 to 99 percent at electric generating plants. HCl removal efficiencies using lime range from 95 to 99 percent at municipal waste-to-energy plants. There are two main methods for cleaning flue gases from coal combustion 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 SO2 and HCl. There are two major dry processes: “dry injection” systems inject dry hydrated lime into the flue gas duct and “spray dryers” inject an atomized lime slurry into a separate vessel. A spray dryer is typically shaped like a silo, with a cylindrical top and a cone bottom. Hot flue gas flows into the top. Lime slurry is sprayed through an atomizer (e.g., nozzles) into the cylinder near the top, where it absorbs SO2 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 duct. A portion of the dried, unreacted lime and its reaction products fall to the bottom of the cone and are removed. The flue gas then 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, collected in particulate control devices. At electric generating plants, dry scrubbing is used primarily for low-sulfur fuels. At municipal waste-to-energy plants, dry scrubbing is used for removal of SO2 and HCl. Dry scrubbing is used at other industrial facilities for HCl control. Dry scrubbing methods have improved significantly in recent years, resulting in excellent removal efficiencies.
Wet Lime Scrubbing
In wet lime scrubbing, lime is added to water and the resulting slurry is sprayed into a flue gas scrubber. In a typical system, 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. Wet scrubbing treats high-sulfur fuels and some low-sulfur fuels where high-efficiency sulfur dioxide removal is required. Wet scrubbing primarily uses magnesium-enhanced lime (containing 3-8% magnesium oxide) because it provides high alkalinity to increase SO2 removal capacity and reduce scaling potential.
Comparing Lime and Limestone SO2 Wet Scrubbing Processes
More than ninety percent of U.S. flue gas desulfurization (FGD) system capacity uses lime or limestone. This trend will likely continue into the next phase of federally mandated SO2 reduction from coal burning power plants. In 2003, the National Lime Association sponsored a study by Sargent and Lundy to compare the costs of leading lime and limestone-based FGD processes utilized by power generating plants in the United States. The study included developing conceptual designs with capital and O&M cost requirements 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 sulfur content, dispatch, capital cost, and by-product production (gypsum and SO3 aerosol mitigation chemicals). The reports were updated in a 2007 report comparing wet limestone and dry lime based systems.
Because lime also reacts readily with other acid gases such as HCl, lime scrubbing is used to control HCl at municipal and industrial facilities. For example, at municipal waste-to-energy plants, dry lime scrubbing is used to control emissions from about 70 percent of the total U.S. capacity (as of 1998). HCl removal efficiencies using lime range from 95 to 99 percent. At secondary aluminum plants, for example, EPA identifies lime scrubbing as a maximum achievable control technology for HCl. EPA tests demonstrate removal efficiencies greater than 99 percent.
Different methods for controlling mercury emissions are being evaluated in the United States. One control technology being evaluated combines hydrated lime with activated carbon. The reagent, a registered product, consists of 95-97 percent lime and 3-5 percent activated carbon. Other calcium-based sorbents are also being evaluated as cost-effective alternatives for combined SO2 and mercury removal.