So.. What is Carbide lime slurry?
Carbide lime is a high quality hydrated lime slurry produced as a by-product of the generation of acetylene gas. It is produced as a 10% solids slurry and is stored in tanks, where the excess water is decanted to increase the solids content to the 30-40% range for distribution in 5000 gallon tank trucks.
Carbide lime has been used for decades as an alternative source for hydrated lime in such applications as municipal and industrial wastewater treatment, flu-gas desulfurization, sludge conditioning, road stabilization and other applications where normal commercial lime is used. The calcium hydroxide content of the dry solids is in the 90% range. The solids content of carbide lime will vary (averages around 30-35% solids) with each truck load. However, this variability is accounted for in our weighing and billing procedures.
Lime slurry can be easily stored in mild steel tanks. Agitation can be accomplished with a slow speed mechanical agitator, pump recirculation, air, etc. A lime slurry with a density of 35% solids will weigh approximately 10.3 pounds per gallon and contain 3.6 pounds dry solids per gallon.
HYDRATED LIME SLURRY
A calcium hydroxide (hydrated lime) slurry produced as, and delivered, in a slurry form. Hydrated lime slurry is usually delivered in 5000-gallon bulk tanker loads at an average density of 35% solids. We can also produce it elsewhere if conditions warrant. The calcium hydroxide content will be in the 96% range. Deliveries are made on an as-needed basis. Gilmour & Company provides the service of refilling the slurry storage tank when required.
Lime slurry can be easily stored in mild steel or poly tanks. Tank sizing needs to take into consideration use rate, delivery truck sizing and order lead time. Agitation can be accomplished with a slow speed mechanical agitator, pump recirculation, air, etc. Gilmour & Company’s hydrated lime slurry is relatively easy to keep in suspension. It is also relatively easy to re-agitate if the material does settle.
A lime slurry with a density of 35% solids will weigh approximately 10.3 pounds per gallon and contain 3.6 pounds dry solids per gallon.
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.
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.
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.