coal terms and definitions

8/6/2019 Coal Terms and Definitions

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Terms and Definitions Coal Combustion Byproducts Boiler slag is a molten ash collected at the base of slag tap and cyclone boilers that isquenched with water and shatters into black, angular particles having a smooth glassyappearance. Bottom ash consists of agglomerated ash particles formed in pulverized coalboilers that are too large to be carried in the flue gases and impinge on theboiler walls or fall through open grates to an ash hopper at the bottom of theboiler. 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 usedfor 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 sulfuremissions from boiler stacks. These systems include wet scrubbers, spray dryscrubbers, sorbent injectors, and a combined sulfur oxide (SOx) and nitrogenoxide (NOx) 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 iscaptured 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 aparticular coal. Coal is formed by the decomposition of plant matter without free access to air and under the influence of moisture, pressure, andtemperature. Over the course of the geologic process that forms coalcoalificationthe chemical composition of the coal gradually changes to



compounds of lower hydrogen content and higher carbon content in aromaticring 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 ahigh 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 8300to 10,500 Btu/lb on a moist, mineral-matter-free basis. Subbituminous coalsare 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 bycentrifugal separation that is used in particle collecting and fine grinding

operations. Cyclone firing refers to slagging combustion of coarsely pulverized coal in acylindrical (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 ashrequires the application of an electrostatic charge to the fly ash, which then iscollected on grouped plates in a series of hoppers. Fly ash collected indifferent hoppers may have differing particle size and chemical composition,depending on the distance of the hopper from the combustor. The ESP ashmay also be collected as a composite.



Fluidized-bed combustion (FBC) accomplishes coal combustion by mixingthe coal with a sorbent such as limestone or other bed material. The fuel andbed material mixture is fluidized during the combustion process to allowcomplete 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 fluegases, typically using a high-calcium sorbent such as lime or limestone. Thethree 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 bestationary or moving.

coal terms and definitions

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