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FAQ's - FGT

Our customer service department answers many questions regarding our lime products for all the industries we serve. The questions range from lime applications and use to storage, shipping and handling. As a resource to our customers and website viewers, the listing below represents the most frequently asked questions.

 

Are dry FGD systems only suited for smaller industrial applications?

Dry lime-based FGD systems are suitable for both small industrial applications and large applications including coal and oil-fired power generating systems.

 

What is the difference between wet and dry lime-based FGD systems?

Wet and dry systems are distinguished mainly by the type of by-product produced immediately after capture of sulfur dioxide. In dry lime FGD, the product of reaction of lime with sulfur dioxide is a dry powder. In wet lime FGD, the reaction product is a liquid. In dry FGD, milk of lime is sprayed directly into hot flue gas containing sulfur dioxide. Sulfur dioxide reacts directly with lime, and the heat of the flue gas evaporates water in the milk of lime, leaving a dry by-product. The by-product is removed from the gas using either fabric filters in a so-called baghouse or in an electrostatic precipitator. In wet lime FGD, sulfur dioxide is removed from flue gas using a two-step chemical process, compared with dry FGD which is a one-step process flue gas containing sulfur dioxide flows into the side of and up through a cylindrical vessel called an absorber (see figure). A liquid mixture of water, products of previous reaction of sulfur dioxide and lime, including an alkaline salt (magnesium sulfite), is sprayed into the gas from above. Sulfur dioxide is grabbed by the alkaline salt and is absorbed into the liquid mixture (step 1). The liquid mixture containing sulfur dioxide falls into the bottom of the absorber, where a pool of the liquid mixture is held. The sulfur dioxide, which is acidic, lowers the pH of the liquid pool. Milk of lime is then added to the pool to raise pH to about 6. Calcium hydroxide in the lime reacts with most of the sulfur dioxide to form a solid by-product, and magnesium hydroxide reacts with the remainder of the sulfur dioxide to replenish the alkaline salt (step 2). The replenished liquid mixture is then available to be sprayed again into the top of the absorber tower to remove sulfur dioxide from additional flue gas.

 

I require FGD, but I have no space on my site for disposal of FGD by-products. Are there lime-based FGD systems that produce by-products which I can sell or provide for beneficial uses?

Yes. Lime-based systems can produce by-products both for sale and for a number of beneficial uses. Wet lime systems produce gypsum (calcium sulfate), a beachsand-like product which is purchased by building products manufacturers for production of plasterboard. Gypsum is also used by cement manufacturers as a set retarder. In some areas of the US, agricultural soils are deficient in sulfur or high in alkali, and gypsum is used as a soil amendment to increase crop yields. Dry lime FGD systems produce dry by-products rich in alkali which have also been used for soil amendment. Additional beneficial uses for lime FGD by-products include structural fill, mining mortars, and production of lightweight aggregate.

 

What references are available that describe lime-based and other options for flue gas desulfurization (FGD) and compare costs of leading systems?

A good general reference produced by US EPA, Controlling SO2 Emissions: A Review of Technologies, is available through the National Technical Information Service ([EPA/600/R-00/093, Order No. PB2001-101224; Telephone: 703-605-6000; 800-553-6847 U.S. only]. Leading FGD systems for large industrial or power generating plants are wet lime FGD, dry lime FGD, and wet limestone FGD. Lime-based systems have lower equipment costs than limestone-based systems. Wet lime systems are suited for flue gases with all ranges of sulfur dioxide content up to 10000 parts per million by volume of sulfur dioxide and where up to 99% sulfur removal is required. Dry lime systems have the lowest equipment costs and are suited for flue gases with less than 1500 parts per million by volume of sulfur dioxide and where up to 94% sulfur removal is required.

 

I have other pollutants in my flue gas that I need to remove in addition to sulfur dioxide. Will a lime-based FGD system remove other pollutants?

Lime will remove additional acid gases such as hydrogen chloride (HCl), hydrogen fluoride (HF) and sulfur trioxide (SO3). Wet lime and dry lime systems achieve over 95% removal of HCl and HF, and dry lime systems also achieve over 95% removal of SO3. Sulfur trioxide in combination with moisture produces acid mist, which produces a dark brown or bluish plume and greatly increases visual opacity of flue gas. Mercury is also efficiently removed by lime-based systems. Dry lime FGD in combination with activated carbon is a proven technology for removal of more than 70% of mercury from flue gases produced by incineration and other industrial facilities. Wet lime scrubbers efficiently remove water-soluble forms of mercury.

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