FORMATION OF COAL
Coal is a sedimentary rock that had its origin on the surface of the earth as an accumulation of inorganic and organic debris. Coal is predominantly made up of organic plant material, in particular ancient wood, leaves, stems, twigs, seeds, spores, pollen, and other parts of aquatic and land plants. When the debris first begins to pile up it is termed peat; the earths crust subsides, and more sediments are piled on top of the organic material, causing it to sink ever deeper into the sedimentary layer.
Layers of peat may be separated by clay and sand deposited during times of flood or other breaks in the accumulation cycle. As the peat accumulates, organic processes begin to break the plant debris down, both physically and chemically. Physically, small insects, worms, and fungi break the fragments into smaller pieces. As the peat solidifies, the small fragments formed are termed macerals, and can be identified microscopically as coming from plant products. At the same time, the peat is squeezed by overlying material, driving out its water content and compacting the plant debris into rock. Chemically, the plant material is slowly converted into simpler organic compounds ever richer in carbon. These combined processes are called sedimentation, After sedimentation, the peat is buried deeper while pressure and heat build up. It is the heat and pressure that slowly transforms the peat into coal through the process of maturation. To generate one foot of coal it took approximately five feet of raw organic material.
Coal are deposited over a narrow range of sedimentary environments, such as swamps or bogs. In all cases the fresh, organic plant material needs to be buried quickly and protected from oxidation. In order for the organic matter to be preserved, the plant debris must accumulate in a local area of restricted oxygen supply.
CLASSIFICATION OF COAL
There are two main recognized ways to classify coal by rank or by grade. Coal rank is a measure of the degree of coalification or heat content and coal grade is a measure of the coal purity. For the purposes of the Primer, Rank will be used to describe coal and its relationship to methane production.
Rank
The degree of coalification or metamorphosis undergone by a coal, as it matures from peat to anthracite, has a significant bearing on its physical and chemical characteristics, and is referred to as the 'rank' of the coal. The major ranks of coal from lowest to highest are lignite, sub-bituminous, bituminous, semianthracite and anthracite. The higher the coal rank the higher the temperature and pressure of coal formation. The higher coal ranks have a greater percent of carbon. As moisture and volatiles are driven off during coal maturation carbon is left behind. With an increase in carbon content there is also an increase in the heat content of the coal.
The earths crust exhibits an average geothermal gradient of about 1.5° F for every 100 feet of burial depth. As coal seams are depressed ever-deeper into the earth under accumulating sediments, much of the water and volatile matter are driven away, leaving behind the fixed carbon as well as residual amounts of ash, sulfur, and tiny amounts of a few assorted trace elements. The extent of this de-volatilization varies according to the deepest depth of ultimate burial, resulting in a continuous series of coal grades according to the relative percentages of fixed carbon they contain.
Lignite is the lowest rank of coal and is characterized as browner and softer. Lignites have a high oxygen content (up to 30 percent), a relatively low fixed carbon content (20-35 percent), and a high moisture content (30-70 percent) (WCI).. Lignite is not particularly efficient in producing energy per mass of fuel. These coals are also susceptible to spontaneous combustion
Sub-bituminous coals usually appear dull black and waxy. Sub-bituminous coals have a fixed carbon content between 35 to 45 percent and a moisture content of up to 10 percent. These coals are frequently used for electrical generation.
Bituminous coals are dense black solids, frequently containing bands with brilliant colors. The carbon content of these coals ranges from 45 to 80 percent and the water content from 1.5 to 7 percent (WCI). The coals are well suited for the production of metallurgical coke, power generation, cement making, and to provide heat and steam in industry. Because of their higher fixed carbon content and lower moisture content, bituminous coals contain more energy per pound than sub-bituminous coals, which in turn contain more energy than lignite coal.
Layers of peat may be separated by clay and sand deposited during times of flood or other breaks in the accumulation cycle. As the peat accumulates, organic processes begin to break the plant debris down, both physically and chemically. Physically, small insects, worms, and fungi break the fragments into smaller pieces. As the peat solidifies, the small fragments formed are termed macerals, and can be identified microscopically as coming from plant products. At the same time, the peat is squeezed by overlying material, driving out its water content and compacting the plant debris into rock. Chemically, the plant material is slowly converted into simpler organic compounds ever richer in carbon. These combined processes are called sedimentation, After sedimentation, the peat is buried deeper while pressure and heat build up. It is the heat and pressure that slowly transforms the peat into coal through the process of maturation. To generate one foot of coal it took approximately five feet of raw organic material.
CLASSIFICATION OF COAL
There are two main recognized ways to classify coal by rank or by grade. Coal rank is a measure of the degree of coalification or heat content and coal grade is a measure of the coal purity. For the purposes of the Primer, Rank will be used to describe coal and its relationship to methane production.
Rank
The degree of coalification or metamorphosis undergone by a coal, as it matures from peat to anthracite, has a significant bearing on its physical and chemical characteristics, and is referred to as the 'rank' of the coal. The major ranks of coal from lowest to highest are lignite, sub-bituminous, bituminous, semianthracite and anthracite. The higher the coal rank the higher the temperature and pressure of coal formation. The higher coal ranks have a greater percent of carbon. As moisture and volatiles are driven off during coal maturation carbon is left behind. With an increase in carbon content there is also an increase in the heat content of the coal.
The earths crust exhibits an average geothermal gradient of about 1.5° F for every 100 feet of burial depth. As coal seams are depressed ever-deeper into the earth under accumulating sediments, much of the water and volatile matter are driven away, leaving behind the fixed carbon as well as residual amounts of ash, sulfur, and tiny amounts of a few assorted trace elements. The extent of this de-volatilization varies according to the deepest depth of ultimate burial, resulting in a continuous series of coal grades according to the relative percentages of fixed carbon they contain.
Lignite is the lowest rank of coal and is characterized as browner and softer. Lignites have a high oxygen content (up to 30 percent), a relatively low fixed carbon content (20-35 percent), and a high moisture content (30-70 percent) (WCI).. Lignite is not particularly efficient in producing energy per mass of fuel. These coals are also susceptible to spontaneous combustion
Sub-bituminous coals usually appear dull black and waxy. Sub-bituminous coals have a fixed carbon content between 35 to 45 percent and a moisture content of up to 10 percent. These coals are frequently used for electrical generation.
Bituminous coals are dense black solids, frequently containing bands with brilliant colors. The carbon content of these coals ranges from 45 to 80 percent and the water content from 1.5 to 7 percent (WCI). The coals are well suited for the production of metallurgical coke, power generation, cement making, and to provide heat and steam in industry. Because of their higher fixed carbon content and lower moisture content, bituminous coals contain more energy per pound than sub-bituminous coals, which in turn contain more energy than lignite coal.