USING LIME FOR FLUE GAS TREATMENT

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 the removal of acidic gases: dry scrubbing and wet scrubbing. Both methods are used for cleaning flue gases from the combustion of coal to produce electric power. Dry scrubbing is also used at municipal waste-to-energy plants and other industrial facilities, primarily for HCl control. Lime is used in both systems.

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 also 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 lime wet 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 is used primarily for high-sulfur fuels and some low-sulfur fuels where high-efficiency sulfur dioxide removal is required. Wet scrubbing is a primary use for magnesium-enhanced lime (containing 3-8% magnesium oxide), which provides high alkalinity that increases SO2 removal capacity and reduces scaling potential.

COMPARING LIME AND LIMESTONE SO2 WET SCRUBBING PROCESSES:
Over 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), as summarized in technical paper presented in May 2003.

HCl REMOVAL: Because lime also reacts readily with other acid gases such as HCl, lime scrubbing is used to control HCl at other types of municipal and industrial facilities:

• 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, the U.S. Environmental Protection Agency identifies lime scrubbing as a maximum achievable control technology for HCl. EPA tests demonstrate removal efficiencies greater than 99 percent.
  

MERCURY REMOVAL: Many different methods for controlling mercury emissions are being evaluated in the U.S. 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.

USING LIME TO TREAT BIOSOLIDS AND SLUDGES

Lime can be used for effective treatment of sewage biosolids, as well as industrial sludges and petroleum wastes.

Sewage Biosolids. Quicklime and calcium hydroxide (hydrated lime) have been used to treat biological organic wastes for more than 100 years. The treatment of human wastewater sludges (i.e., biosolids) by lime treatment is specifically prescribed in U.S. EPA regulations (40 C.F.R. 503). There are many examples of wastewater treatment systems using lime stabilization.

How Lime Treatment Works--Lime treatment controls the environment needed for the growth of pathogens in biosolids and converts sludge into a usable product. Lime stabilization is a cost-effective option that generally has lower capital costs than alternative treatment options. The mechanism of lime treatment of biological wastes is based on several chemical reactions:

• Calcium hydroxide is an alkaline compound that can create pH levels as high as 12.4. At pH levels greater than 12, the cell membranes of harmful pathogens are destroyed. The high pH also provides a vector attraction barrier, preventing flies and other insects from infecting the treated biological waste. Because lime has low solubility in water, lime molecules persist in biosolids. This helps to maintain the pH above 12 and prevent regrowth of pathogens.
• When quicklime (CaO) is used, an exothermic reaction with water occurs. This heat release can increase the temperature of the biological waste to 70ºC, which provides effective pasteurization.
• The high pH also will precipitate most metals that are present in the waste and reduces their solubility and mobility. Lime will also react with phosphorus compounds.
• The solubility of calcium hydroxide also provides free calcium ions, which react and form complexes with odorous sulfur species such as hydrogen sulfide and organic mercaptans. Thus the biological waste odors are not >covered over= but actually destroyed.
• The addition of lime also increases the solids content of the waste, making it easier to handle and store.

  Lime use can help meet EPA’s Part 503 Requirements--EPA has established federal requirements for the safe treatment, beneficial use, and disposal of biosolids (40 CFR Part 503). For biosolids that are to be beneficially used, lime stabilization is one of the technologies identified to meet the requirements to address pathogens.

  The Part 503 regulations establish two classes -- Class A and B -- that specify performance goals and the degree of treatment biosolids must receive before beneficial use or disposal:

• Class B biosolids contain higher pathogen concentrations than Class A, but have levels low enough for some beneficial uses, such as land application with restrictions. To meet Class B requirements using lime stabilization, the pH of the biosolids must be elevated to more than 12 for 2 hours and subsequently maintained at more than 11.5 for 22 hours.
• Class A biosolids contain extremely low pathogen concentrations and have few or no use restrictions. To meet Class A requirements using lime stabilization, the Class B elevated pH requirements are combined with elevated temperatures (70oC for 30 minutes) or other EPA-approved time/temperature processes.

In addition to regulating pathogen concentrations, the Part 503 regulations include requirements for reducing the tendency of biosolids to attract disease vectors such as rodents and insects. Lime treatment is one of the methods sanctioned in the regulations. To meet vector attraction reduction requirements using lime, the pH must be raised to 12 or higher for 2 hours and subsequently maintained above pH 11.5 for another 22 hours without further alkali addition. Most lime treatment facilities have the flexibility to produce either Class A or Class B biosolids, thus increasing disposal and recycling options.

Lime-treated biosolids can be re-used--Lime-treated biosolids are safe and promote recycling. As EPA notes, “properly prepared biosolids provide a rich source of the essential fertilizer elements needed by plants to produce food.” U.S. EPA, “Biosolids Recycling: Beneficial Technology for a Better Environment,” (June 1994). Reuse of lime-stabilized biosolids is not limited to use on farmland. Biosolids have also been used as a soil substitute for landfill cover, and in reclamation of mining-disabled land. Exceptional quality biosolids can also be sold for public use as a commercial fertilizer or soil conditioner.

Lime use is cost-effective--Lime stabilization is generally more cost-effective than alternative biosolids options. A series of studies comparing lime stabilization to composting, thermal drying, and digestion technologies found that lime stabilization has unit costs as much as 60 percent lower than alternatives. Reduced capital cost requirements of lime stabilization are even more dramatic – particularly important for municipalities with limited capital budgets. In general, lime stabilization is a non-proprietary process, although patented processes are available.

Industrial Sludges and Petroleum. Quicklime and hydrated lime can be used in the treatment of many industrial sludges by correcting pH for further treatment, neutralizing acidic wastes, and removing or immobilizing contaminants. Specific examples include sulfite/sulfate sludges and petroleum waste.

Calcium sulfite/sulfate waste--Calcium sulfite and sulfate wastes from desulfurizing stack gases, lime neutralization of acid waste effluent, and waste accumulated in the manufacture of superphosphate fertilizers, when untreated, are lacking in bearing strength and are prone to leach objectionable amounts of the sulfate ion into the ground water. However, this material, when mixed with 2-3% lime and 15-30% pozzolan--such as fly ash, volcanic ash, pulverized slag, etc.--develops considerable bearing strength, erosion resistance and is non-leaching. The stabilized material can be used in constructing embankments and earth dams. In addition, a synthetic gypsum can be crystallized from sulfite sludges from wet scrubbers. The gypsum produced from hydrated lime in this manner is very white and is a saleable product.

Petroleum wastes--Restoration of waste oil ponds to environmentally safe land for beneficial uses has been achieved using either commercial lime (mainly quicklime) or lime kiln dust. Either material is used to dewater the oily waste to the extent that the dried sludge can be compacted and the pond area converted to useful land.

USING LIME TO TREAT ANIMAL WASTES

The Animal Waste Problem--An emerging issue in the U.S. is the growing environmental threat caused by animal wastes. Current management practices have begun to create environmental problems because of the consolidation of the livestock industry into much larger facilities, and the resulting concentration of waste-producing activities. Concentrated animal feeding operations (“CAFOs”) for beef cattle, swine, and poultry can create numerous problems, including excess nutrient loading of agricultural land, eutrophication of surface waters, groundwater contamination, pathogen release, and offensive odors. There have been a number of incidents in which large numbers of people have been sickened by water or food contaminated by animal wastes. These problems will only get worse—the amount of animal manure produced annually is estimated to be 10 times the amount of municipal sewage—and much of that manure currently receives little or no treatment. In addition to solid animal manure, there are large amounts of other animal wastes, such as poultry bedding, urine, and carcasses which also are environments problems and are estimated to total up to 100 times the amount of human wastewater biosolids.

EPA’s CAFO rule—The Environmental Protection Agency is in the process of developing a new rule to regulate concentrated animal feeding operations. If the final rule resembles the proposed rule, many more of these 40,000 facilities will be required to institute effective treatment of animal wastes than presently do. When this happens, the need for cost-effective treatment methods will become acute.

Lime Treatment for Animal Wastes--Lime treatment is a multi-functional, cost-effective, politically acceptable option with respect to many of the challenges posed by animal wastes, just as it has played an important role in biosolids (sewage) treatment.

Lime Can Help Control Excess Nutrients—Animal wastes contain phosphorus and nitrogen, and these nutrients can be returned to the soil as fertilizer. However, the quantities of animal wastes produced means that there is an excess of these nutrients for the soil and crops to absorb, and runoff causes damaging eutrophication of surface waters. Lime will volatilize the nitrogen (and with the use of new technology, convert it into a usable concentrated fertilizer), and can precipitate the phosphorus to an insoluble form, reducing the excess nutrient problem. Lime can also be used to precipitate most metals that are present in the waste and reduce their mobility.

Lime Can Help Control Pathogens--Lime inhibits pathogens by controlling the environment required for bacterial growth. Calcium hydroxide (hydrated lime) is an alkaline compound that can create pH levels as high as 12.4. At pH levels greater than 12, the cell membranes of harmful pathogens are destroyed. The high pH also provides a vector attraction barrier (i.e., prevents flies and other insects from infecting the treated biological waste). Because lime has low solubility in water, lime molecules persist in biosolids. This helps to maintain the pH above12 and prevent regrowth of pathogens. In addition, when quicklime (calcium oxide, or CaO) is used, an exothermic reaction with water occurs. This heat release can increase the temperature of the biological waste to 70°C, which provides pasteurization and also helps dry out the solid waste.

Lime Can Help Control Odors--Lime treatment also reduces odors, particularly hydrogen sulfide, which is not only a nuance odor but also can be very dangerous if localized high concentrations build up. In addition to high pH, lime provides free calcium ions, which react and form complexes with odorous sulfur species such as hydrogen sulfide and organic mercaptans. Thus the biological waste odors are not ‘covered over’ but actually destroyed.

Lime Treatment is Cost-Effective--Lime treatment of animal wastes is economically attractive. For biosolids, lime treatment is often a least cost alternative—for example, unit treatment costs of lime stabilization of biosolids have been estimated to be less than half the costs of aerobic and anaerobic digestion. There are a number of innovative technologies that use lime or lime-derived materials to treat animal wastes and generate a usable agricultural product. Because of the versatility of lime it can be used for the treatment of most animal wastes, including hogs, cattle, dairy, and poultry.

USING LIME TO TREAT WASTEWATER

Lime is extensively used in the treatment of municipal wastewaters, as well as the treatment of industrial liquid wastes.

Municipal Wastewater Treatment. In advanced wastewater treatment plants, lime precipitation is employed in tertiary processes in which phosphorus is precipitated as complex calcium phosphates along with other suspended and dissolved solids. Due to the high pH of 10.5-11.0 maintained by lime, the stripping of nitrogen, another nutrient, is facilitated. Thus, the removal of phosphorus and nitrogen helps prevent eutrophication (algae build-up) in surface waters.

When alum and ferric chloride are employed for coagulation, lime is used to counteract the low pH induced by these acid salts and to provide the necessary alkalinity for efficient nitrogen removal.

In sewage plants where sewage sludge is removed by vacuum or pressure filtration, lime and ferric chloride are employed as filter aids in the conditioning of the sludge and for final clarification of the effluent.

Industrial Wastewater. Lime has numerous applications in treating industrial wastewaters, especially where neutralization of acidic wastes is required. In steel plants, sulfuric acid-based waste pickle liquors are neutralized with lime in which the iron salts are precipitated. Lime is also a neutralizer and precipitant of chrome, copper, and heavy metals in processes for treating discharges from plating plants.

Lime is used to neutralize sulfuric acid wastes from rayon plants and to neutralize and precipitate dissolved solids from wastes of cotton textile finishing plants (dye works).

Vegetable and fruit canning wastes can be clarified with lime alone or with supporting coagulants as an alternate to lagooning of the liquid waste. In citrus canning, lime assists in clarifying wastewaters and in the processing of citrus pulp by-products.

For a fact sheet on the use of lime to neutralize acidic wastewaters, see http://www.lime.org/ACIDNEUTfinal.pdf.

Acid Mine Drainage. Highly acidic drainage from active or abandoned mines is frequently neutralized with lime. Further clarification of the discharge is achieved by precipitation of iron contained in this pyritic leachate. Coal washing plants use lime to neutralize the acidic waste or process water to reduce corrosion on steel equipment and to recover the water for reuse.

USING LIME TO TREAT DRINKING WATER

In terms of annual tonnage, lime ranks first among chemicals used in the treatment of potable and industrial water supplies—in 2001, nearly a million metric tons. It is used by many municipalities to improve water quality, especially for water softening and arsenic removal. Indeed, the American Water Works Association (AWWA) has issued standards that provide for the use of lime in drinking water treatment.

Softening--In water softening hydrated lime is used to remove carbonate hardness (caused by bicarbonates and carbonates of calcium and magnesium) from the water. Hardness caused by other calcium and magnesium salts, called noncarbonate hardness, is generally treated by means of the lime-soda process, which entails the precipitation of magnesium by lime. The co-produced calcium salt reacts with the soda ash to form a calcium carbonate precipitate. Lime enhanced softening can also be used to remove arsenic from water. Recent changes to the national drinking water standard for arsenic have increased the need for this treatment. The U.S. EPA has issued new guidance on enhanced lime softening to remove arsenic, see http://www.epa.gov/safewater/mdbp/coaguide.pdf.

pH Adjustment/Coagulation--Hydrated lime is widely used to adjust the pH of water to prepare it for further treatment. Lime is also used to combat "red water" by neutralizing the acid water, thereby reducing corrosion of pipes and mains from acid waters. The corrosive waters contain excessive amounts of carbon dioxide (carbonic acid). Lime precipitates the CO2 to form calcium carbonate, which provides a protective coating on the inside of water mains.

Lime is used in conjunction with alum or iron salts for coagulating suspended solids incident to the removal of turbidity from "raw" water. It serves to maintain the proper pH for most satisfactory coagulation conditions. In some water treatment plants, alum sludge is treated with lime to facilitate sludge thickening on pressure filters.

Effect on Pathogen Growth--By raising the pH of water to 10.5-11 through the addition of lime and retaining the water in contact with lime for 24-72 hours, lime controls the environment required for the growth of bacteria and certain viruses. This application of lime is utilized where "phenolic water" exists, because chlorine treatment tends to produce an unpalatable water due to the phenol present. This process, called "excess alkalinity treatment," also removes most heavy metals.

Removal of Impurities---One of the most common methods of removing silica from water is the use of dolomitic lime. The magnesium component of this lime is the active constituent in silica removal. Lime is also used to remove manganese, fluoride, organic tannins and iron from water supplies.

USING LIME TO TREAT HAZARDOUS WASTES

Lime is widely used to treat hazardous wastes both currently generated process wastes and previously disposed or abandoned materials. Lime stabilizes most metals by converting them to more chemical stable forms that are less likely to leach. In addition, lime can react with soils to solidify materials, further reducing the leaching of hazardous wastes. Lime can also be used to neutralize acidic materials.

Under the U.S. EPA's land disposal restrictions regulations, currently generated hazardous wastes that are to be land disposed must be pretreated using the Abest demonstrated available technology. For hazardous wastes containing metals, metals stabilization or metals precipitation is frequently required, and lime is identified by EPA as suitable to treat these wastes (see 40 C.F.R. Part 268.42).

EPA also endorses lime stabilization as a key technology for hazardous waste site cleanups (see, e.g., Handbook for Stabilization/Solidification of Hazardous Wastes (EPA/540/2-86/001, June 1986). In 1997, for example, EPA announced a proposed cleanup plan as part of the Anaconda Regional Water, Waste, and Soils Project for 14,000 acres in Anaconda, Montana. A key element of the plan is to treat arsenic-containing soils with lime and organics. Copper mining created environmental contamination in the 300 square mile area and concern about potential human exposures. EPA recommended in-place lime treatment over the option of excavating and treating the tailings and contaminated groundwater. (Nearby, the Warm Springs Pond is already being used to capture and treat water contaminated with metals (copper, zinc, and arsenic) that threaten the Clark Fork River. The contaminated waters are treated with a lime solution.)