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Anthracite

Anthracite

Gist

Anthracite: The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter. Bituminous: Bituminous coal is a middle rank coal between subbituminous and anthracite.

Summary

anthracite, the most highly metamorphosed form of coal. It contains more fixed carbon (86 percent or greater on a dry, ash-free basis) than any other form of coal and the least amount of volatile matter (14 percent or less on a dry, ash-free basis), and it has calorific values near 35 megajoules per kilogram (approximately 15,000 British thermal units per pound), not much different from the calorific values for most bituminous coal. Anthracite is the least plentiful form of coal. In the United States it is found mostly in northeastern Pennsylvania and makes up less than 2 percent of all coal reserves in the country. Smaller amounts of anthracite occur in South Africa, Australia, eastern Ukraine, western Canada, China, and other countries.

Anthracites are black to steel gray and have a brilliant, almost metallic lustre. They can be polished and used for decorative purposes. Hard and brittle, anthracites break with conchoidal fracture into sharp fragments. Unlike many bituminous coals, they are clean to the touch. Although anthracites are difficult to ignite, they burn with a pale blue flame and require little attention to sustain combustion. In the past they were used for domestic heating because they produce little dust upon handling, burn slowly, and emit relatively little smoke. Anthracite is rarely used for this purpose today because of its limited abundance and relatively high cost and the ready availability of other sources of energy (e.g., natural gas and electricity) for heating purposes.

Although anthracites usually occur in geologically deformed areas, such as in the intensely folded sedimentary rocks of the anthracite region of Pennsylvania, their origin is due to higher than normal heating caused by the presence of nearby igneous intrusions or high geothermal gradients. Both of these phenomena produce temperatures much higher than those reached at depth in most sedimentary basins. For instance, in Antarctica, large igneous sills intruded the coal measures and converted some of the existing bituminous coal to anthracite. Temperatures ranging from 170 to 250 °C (about 340 to 480 °F) are thought to be necessary for the formation of anthracite.

Details

Anthracite, also known as hard coal and black coal, is a hard, compact variety of coal that has a submetallic lustre. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the highest ranking of coals.

The Coal Region of Northeastern Pennsylvania in the United States has the largest known deposits of anthracite coal in the world with an estimated reserve of seven billion short tons. China accounts for the majority of global production; other producers include Russia, Ukraine, North Korea, South Africa, Vietnam, Australia, Canada, and the United States. Total production in 2020 was 615 million tons.

Anthracite is the most metamorphosed type of coal, but still represents low-grade metamorphism, in which the carbon content is between 86% and 97%. The term is applied to those varieties of coal which do not give off tarry or other hydrocarbon vapours when heated below their point of ignition. Anthracite is difficult to ignite, and burns with a short, blue, and smokeless flame.

Anthracite is categorized into several grades. Standard grade is used predominantly in power generation, and high grade (HG) and ultra high grade (UHG), are used predominantly in the metallurgy sector. Anthracite accounts for about 1% of global coal reserves, and is mined in only a few countries around the world.

Names

Anthracite derives from the Greek, literally "coal-like". Other terms which refer to anthracite are black coal, hard coal, stone coal, dark coal, coffee coal, blind coal (in Scotland), Kilkenny coal (in Ireland), crow coal or craw coal, and black diamond. "Blue Coal" is the term for a once-popular and trademarked brand of anthracite, mined by the Glen Alden Coal Company in Pennsylvania, and sprayed with a blue dye at the mine before shipping to its Northeastern U.S. markets to distinguish it from its competitors.

Culm has different meanings in British and American English. In British English, culm is the imperfect anthracite, located predominantly north Devon and Cornwall, which was used as a pigment. The term is also used to refer to some carboniferous rock strata found in both Britain and in the Rhenish hill countries, also known as the Culm Measures. In Britain, it may also refer to coal exported from Britain during the 19th century. In American English, "culm" refers to the waste or slack from anthracite mining, mostly dust and small pieces not suitable for use in home furnaces.

Properties

Anthracite is similar in appearance to the mineraloid jet and is sometimes used as a jet imitation.

Anthracite differs from ordinary bituminous coal by its greater hardness (2.75–3 on the Mohs scale), its higher relative density of 1.3–1.4, and luster, which is often semi-metallic with a mildly green reflection. It contains a high percentage of fixed carbon and a low percentage of volatile matter. It is also free from included soft or fibrous notches and does not soil the fingers when rubbed. Anthracitization is the transformation of bituminous coal into anthracite.

The moisture content of fresh-mined anthracite generally is less than 15 percent. The heat content of anthracite ranges from 26 to 33 MJ/kg (22 to 28 million Btu/short ton) on a moist, mineral-matter-free basis. The heat content of anthracite coal consumed in the United States averages 29 MJ/kg (25 million Btu/ton), on the as-received basis, containing both inherent moisture and mineral matter.

Since the 1980s, anthracite refuse or mine waste has been used for coal power generation in a form of recycling. The practice known as reclamation is being applied to culm piles antedating laws requiring mine owners to restore lands to their approximate original condition.

Chemically, anthracite may be considered as a transition stage between ordinary bituminous coal and graphite, produced by the more or less complete elimination of the volatile constituents of the former, and it is found most abundantly in areas that have been subjected to considerable stresses and pressures, such as the flanks of great mountain ranges. Anthracite is associated with strongly deformed sedimentary rocks that were subjected to higher pressures and temperatures (but short of metamorphic conditions) just as bituminous coal is generally associated with less deformed or flat-lying sedimentary rocks. The compressed layers of anthracite that are deep mined in the folded Ridge and Valley Province of the Appalachian Mountains of the Coal Region of East-central Pennsylvania are extensions of the same layers of bituminous coal that are mined on the generally flat lying and undeformed sedimentary rocks further west on the Allegheny Plateau of Kentucky and West Virginia, Eastern Ohio, and Western Pennsylvania.

In the same way the anthracite region of South Wales is confined to the contorted portion west of Swansea and Llanelli, the central and eastern portions producing steam coal, coking coal and domestic house coals.

Anthracite shows some alteration by the development of secondary divisional planes and fissures so that the original stratification lines are not always easily seen. The thermal conductivity is also higher; a lump of anthracite feels perceptibly colder when held in the warm hand than a similar lump of bituminous coal at the same temperature.

Anthracite has a history of use in blast furnaces for iron smelting; however, it lacked the pore space of metallurgical coke, which eventually replaced anthracite.

History of mining and use

In southwest Wales, anthracite has been burned as a domestic fuel since at least medieval times, when it was mined near Saundersfoot. More recently, large-scale mining of anthracite took place across the western part of the South Wales Coalfield until the late 20th century.

In the United States, anthracite coal history began in 1790 in Pottsville, Pennsylvania, with the discovery of coal made by the hunter Necho Allen in what is now known as the Coal Region. Legend has it that Allen fell asleep at the base of Broad Mountain and woke to the sight of a large fire because his campfire had ignited an outcrop of anthracite coal. By 1795, an anthracite-fired iron furnace had been built on the Schuylkill River.

Anthracite was first experimentally burned as a residential heating fuel in the US on 11 February 1808, by Judge Jesse Fell in Wilkes-Barre, Pennsylvania, on an open grate in a fireplace. Anthracite differs from wood in that it needs a draft from the bottom, and Judge Fell proved with his grate design that it was a viable heating fuel.[citation needed] In spring 1808, John and Abijah Smith shipped the first commercially mined load of anthracite down the Susquehanna River from Plymouth, Pennsylvania, marking the birth of commercial anthracite mining in the United States. From that first mine, production rose to an all-time high of over 100 million tons in 1917.

The difficulty of igniting anthracite inhibited its early use, especially in blast furnaces for smelting iron. With the development of the hot blast in 1828, which used waste heat to preheat combustion air, anthracite became a preferred fuel, accounting for 45% of US pig iron production within 15 years. Anthracite iron smelting was later displaced by coke.

From the late 19th century until the 1950s, anthracite was the most popular fuel for heating homes and other buildings in the northern US, until it was supplanted by oil-burning systems, and more recently natural gas systems. Many large public buildings, such as schools, were heated with anthracite-burning furnaces through the 1980s.

During the American Civil War, Confederate blockade runners used anthracite as a smokeless fuel for their boilers to avoid revealing their position to the blockaders.

The invention of the Wootten firebox enabled locomotives to directly burn anthracite efficiently, particularly waste culm. In the early 20th century US, the Delaware, Lackawanna and Western Railroad started using only the more expensive anthracite coal in its passenger locomotives, dubbed themselves "The Road of Anthracite", and advertised widely that travelers on their line could make railway journeys without getting their clothing stained with soot. The advertisements featured a white-clad woman named Phoebe Snow and poems containing lines like "My gown stays white / From morn till night / Upon the road of Anthracite". Similarly, the Great Western Railway in the UK was able to use its access to anthracite (it dominated the anthracite region) to earn a reputation for efficiency and cleanliness unmatched by other UK companies.

Internal combustion motors driven by the so-called "mixed", "poor", "semi-water" or "Dowson gas" produced by the gasification of anthracite with air (and a small proportion of steam) were at one time the most economical method of obtaining power, requiring only 1 pound per horsepower-hour (0.6 kg/kWh), or less. Large quantities of anthracite for power purposes were formerly exported from South Wales to France, Switzerland and parts of Germany. As of April 2013, widespread commercial anthracite mining in Wales has now ceased, although a few large open cast sites remain, along with some relatively small drift mining operations.

Anthracite today

Anthracite generally costs two to six times as much as regular coal. In June 2008, the wholesale cost of anthracite was US$150/short ton, falling to $107/ton in 2021; it makes up 1% of U.S. coal production.

The principal use of anthracite today is for a domestic fuel in either hand-fired stoves or automatic stoker furnaces. It delivers high energy per its weight and burns cleanly with little soot, making it ideal for this purpose. Its high value makes it prohibitively expensive for power plant use. Other uses include the fine particles used as filter media, and as an ingredient in charcoal briquettes. Anthracite was an authorised fuel in terms of the United Kingdom's Clean Air Act 1993, meaning that it could be used within a designated Smoke Control Area such as the central London boroughs.

Additional Information

Hard and very brittle, anthracite is dense, shiny black, and homogeneous with no marks of layers. Unlike the lower rank coals, it has a high percentage of fixed carbon and a low percentage of volatile matter. Anthracites include a variety of slow-burning fuels merging into graphite at one end and bituminous coal at the other. They are the hardest coals on the market, consisting almost entirely of fixed carbon, with the little volatile matter present in them chiefly as methane, CH4. Anthracite is usually graded into small sizes before being burned on stokers: the ‘meta-anthracites’ burn so slowly as to require mixing with other coals, while the ‘semi-anthracites’, which have more volatile matter, are burned with relative ease if proper fired. Most anthracites have a lower heating value than the highest-grade bituminous coals. Anthracite is used principally for heating homes and in gas production.

Some semi-anthracites are dense, but softer than anthracite. This grade is shiny gray and somewhat granular in structure. The grains have a tendency to break off in handling and produce coarse, sand-like slack.

Other semi-anthracites are dark gray and distinctly granular. The grains break off easily in handling and produce a coarse slack. The granular structure has been produced by small vertical cracks in horizontal layers of comparatively pure coal separated by very thin partings. The cracks are the result of heavy downward pressure and shrinkage of the pure coal because of a drop in temperature.

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