Terms and Definitions

Coal Combustion Byproducts

Boiler slag is a molten ash collected at the base of slag tap and cyclone boilers that is quenched with water and shatters into black, angular particles having a smooth glassy appearance.

Bottom ash consists of agglomerated ash particles formed in pulverized coal boilers that are too large to be carried in the flue gases and impinge on the boiler walls or fall through open grates to an ash hopper at the bottom of the boiler. Bottom ash is typically gray to black in color, is quite angular, and has a porous surface structure.

FBC materials consist of unburned coal, ash, and spent bed material used for sulfur control. The spent bed material (removed as bottom ash) contains reaction products from the absorption of gaseous sulfur oxides (SO₂ and SO₃).

FGD materials are derived from a variety of processes used to control sulfur emissions from boiler stacks. These systems include wet scrubbers, spray dry scrubbers, sorbent injectors, and a combined sulfur oxide (SO_x) and nitrogen oxide (NO_x) process. Sorbents include lime, limestone, sodium-based compounds, and high-calcium coal fly ash.

Fly ash is the coal ash that exits a combustion chamber in the flue gas and is captured by air pollution control equipment such as electrostatic precipitators, baghouses, and wet scrubbers.

Coal Types

Bituminous coal is soft coal that may vary from low to high volatile content, with calorific values ranging from 10,500 to 14,000 Btu/lb on a moist, mineral-matter-free basis. Bituminous coal is prevalent in the eastern United States.

Coal rank indicates the degree of coalification that has occurred for a particular coal. Coal is formed by the decomposition of plant matter without free access to air and under the influence of moisture, pressure, and temperature. Over the course of the geologic process that forms coal—coalification—the chemical composition of the coal gradually changes to compounds of lower hydrogen content and higher carbon content in aromatic ring structures. As the degree of coalification increases, the percentage of volatile matter decreases and the calorific value increases. The common ranks of coal are anthracite, bituminous, subbituminous, and brown coal/lignite.

Excluded minerals are minerals (inorganic compounds) that may be mined with the coal but are not an intrinsic part of the coal.

Included minerals are minerals (inorganic compounds) that are part of the coal particle and matrix.

Lignite, or brown coal, is the lowest-rank solid coal. Lignite typically has a high moisture content and calorific values of less than 8300 Btu/lb on a moist, mineral-matter-free basis. U.S. lignites are found in North Dakota, Montana, Texas, and other Gulf Coast states.

Subbituminous coal is a black coal with calorific values ranging from 8300 to 10,500 Btu/lb on a moist, mineral-matter-free basis. Subbituminous coals are found in the western United States, primarily in Montana, Wyoming, and Alaska, with significant additional deposits in New Mexico and Colorado.

Coal Combustion

Baghouse is a facility that removes fly ash from the flue gas by the use of fabric filter bags.

Cyclone is the cone-shaped air-cleaning apparatus which operates by centrifugal separation that is used in particle collecting and fine grinding operations.

Cyclone firing refers to slagging combustion of coarsely pulverized coal in a cylindrical (cyclone) burner. Some wet-bottom boilers are not cyclone-fired. The primary byproduct is a glassy slag referred to as boiler slag.

Electrostatic precipitator (ESP) collection of coal combustion fly ash requires the application of an electrostatic charge to the fly ash, which then is collected on grouped plates in a series of hoppers. Fly ash collected in different hoppers may have differing particle size and chemical composition, depending on the distance of the hopper from the combustor. The ESP ash may also be collected as a composite.

Fluidized-bed combustion (FBC) accomplishes coal combustion by mixing the coal with a sorbent such as limestone or other bed material. The fuel and bed material mixture is fluidized during the combustion process to allow complete combustion and removal of sulfur gases. Atmospheric FBC (AFBC) systems may be bubbling (BFBC) or circulating (CFBC). Pressurized FBC (PFBC) is an emerging coal combustion technology.

Flue gas desulfurization (FGD) is removal of the sulfur gases from the flue gases, typically using a high-calcium sorbent such as lime or limestone. The three primary types of FGD processes commonly used by utilities are wet scrubbers, dry scrubbers, and sorbent injection.

Gasification is the conversion of coal to a combustible gas, volatiles, char, and ash/slag. Byproducts from gasification systems vary widely.

pc (pulverized coal) combustion refers to any combustion process that uses very finely ground (pulverized) coal in the process.

Slag is the nonmetallic product resulting from the interaction of flux and impurities in the smelting and refining of metals.

Slag cyclone is the primary combustion chamber for a cyclone-fired boiler. Ash from the coal melts in the cyclone and is removed as a slag.

Stoker firing refers to the combustion of coal on a grate, which may be stationary or moving.

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