Coal Ash and Flue Gas Desulfurization By-products
Nontraditional Soil Amendments
COAL ASH AND FLUE GAS DESULFURIZATION BY-PRODUCTS
These materials are generated during the combustion of coal, primarily at electric power generating stations and industrial boilers. Coarse, gritty material removed from the boiler bed is known as bottom ash, while fine particulate material removed from the combustion gasses is known as fly ash. Characteristics of coal fly ash depend primarily on the type of coal burned and result in a Class C or Class F categorization. Class C ash results primarily from burning western U.S. coal and thus is rare in Pennsylvania. Class C ash is high in calcium, may have a CCE as high as 50 percent, and may be used as a soil liming material. Class F fly ash results from burning most eastern U.S. coals, and thus is common in Pennsylvania. It is low in calcium and has no acid-neutralizing capacity. Thus, most coal ash produced in Pennsylvania has little, if any, agronomic value.
Coal ash may contain elevated levels of certain trace elements, most commonly boron, arsenic, and selenium. Boron is an essential plant nutrient but becomes plant-toxic at high concentrations, while arsenic and selenium are of more general environmental concern. Most coal-fired boilers must remove sulfur dioxide from their emissions. This is usually done by reaction with lime and generates materials known as flue gas desulfurization by-products (FGD). There are numerous types of FGD by-products with very different characteristics. Most contain significant amounts of gypsum (calcium sulfate), some contain large amounts of fly ash, and many contain unreacted lime. Because of their gypsum content, FGD materials can provide a source of highly soluble Ca as well as sulfur for crop nutrition. FGD materials that contain unreacted lime may have CCEs ranging from 20 to 60 percent and have value as liming materials. FGD materials are usually very fine-textured and can range in moisture content from a dry powder to a paste-like material to a slurry. Thus, the ability to haul and spread the material must be considered when deciding whether or not to use it. The relative amounts of gypsum, lime, and fly ash in FGDs can vary depending on boiler operating conditions, and trace element contents can vary depending upon the coal being burned. Therefore, before using any FGD material, it is very important to obtain several chemical analyses that show its CCE as well as total concentrations of major and trace elements.