A lignite stockpile (above) and a lignitebriquette
Lignite (derived fromLatinlignum meaning 'wood'), often referred to asbrown coal,[1] is a soft, brown,combustiblesedimentary rock formed from naturally compressedpeat. It has acarbon content around 25–35%[1][2] and is considered thelowest rank ofcoal due to its relatively lowheat content. When removed from the ground, it contains a very high amount ofmoisture, which partially explains its low carbon content. Lignite is mined all around the world and is used almost exclusively as a fuel forsteam-electric power generation.
Lignite combustion produces less heat for the amount of carbon dioxide and sulfur released than other ranks of coal. As a result, lignite is the most harmful coal to human health.[3] Depending on the source, varioustoxic heavy metals, includingnaturally occurring radioactive materials, may be present in lignite and left over in thecoal fly ash produced from its combustion, further increasing health risks.[4]
Lignite is brownish-black in color and has a carbon content of 60–70 percent on a dry ash-free basis. However, itsinherent moisture content is sometimes as high as 75 percent[1] and itsash content ranges from 6–19 percent, compared with 6–12 percent forbituminous coal.[5] As a result, its carbon content on the as-received basis (i.e., containing both inherent moisture and mineral matter) is typically just 25-35 percent.[2]
The energy content of lignite ranges from 10 to 20 MJ/kg (9 to 17 million BTU/short ton) on a moist, mineral-matter-free basis. The energy content of lignite consumed in the United States averages 15 MJ/kg (13 million BTU/short ton), on the as-received basis.[6] The energy content of lignite consumed in Victoria, Australia, averages 8.6 MJ/kg (7.4 million BTU/short ton) on a net wet basis.[7]
Lignite has a high content of volatile matter which makes it easier to convert into gas and liquid petroleum products than higher-ranking coals. Its high moisture content and susceptibility tospontaneous combustion can cause problems in transportation and storage. Processes which remove water from brown coal reduce the risk of spontaneous combustion to the same level as black coal, increase thecalorific value of brown coal to ablack coal equivalent fuel, and significantly reduce the emissions profile of 'densified' brown coal to a level similar to or better than most black coals.[8][9] However, removing the moisture increases the cost of the final lignite fuel.
Lignite rapidly degrades when exposed to air, in a process calledslacking (orslackening).[10]
Lignite mine in the background ofLützerath, Germany
Most lignite is used to generate electricity.[2] However, small amounts are usedin agriculture,in industry, and even, asjet,in jewelry. Its historical use as fuel for home heating has continuously declined and is now of lower importance than its use to generate electricity.
Lignite is often found in thick beds located near the surface, making it inexpensive to mine. However, because of its lowenergy density, tendency to crumble, and typically high moisture content, brown coal is inefficient to transport and is not traded extensively on the world market compared with higher coal grades.[1][7] It is often burned in power stations near the mines, such as in Poland'sBełchatów plant andTurów plant, Australia'sLatrobe Valley andLuminant'sMonticello plant andMartin Lake plant in Texas. Primarily because of latent high moisture content and low energy density of brown coal,carbon dioxide emissions from traditional brown-coal-fired plants are generally much higher permegawatt-hour generated than for comparable black-coal plants, with the world's highest-emitting plant being Australia'sHazelwood Power Station[11] until its closure in March 2017.[12] The operation of traditional brown-coal plants, particularly in combination withstrip mining, is politically contentious due to environmental concerns.[13][14]
TheGerman Democratic Republic relied extensively on lignite to become energyself-sufficient, and eventually obtained 70% of its energy requirements from lignite.[15] Lignite was also an important chemical industry feedstock viaBergius process orFischer-Tropsch synthesis in lieu of petroleum,[16] which had to be imported forhard currency following a change in policy by theSoviet Union in the 1970s, which had previously delivered petroleum at below market rates.[17] East German scientists even converted lignite intocoke suitable for metallurgical uses (high temperature lignite coke) and much of therailway network was dependent on lignite either throughsteam trains orelectrified lines mostly fed with lignite derived power.[17] As per the table below, East Germany was the largest producer of lignite for much of its existence as an independent state.
In 2014, about 12 percent ofGermany's energy and, specifically, 27 percent of Germany's electricity came from lignite power plants,[18] while in 2014 inGreece, lignite provided about 50 percent of its power needs. Germany has announced plans tophase out lignite by 2038 at the latest.[19][20][21][22] Greece has confirmed that the last coal plant will be shut in 2025 after receiving pressure from theEuropean Union[23] and plans to heavily invest inrenewable energy.[24]
Lignite was and is used as a replacement for or in combination withfirewood for home heating. It is usually pressed intobriquettes for that use.[25][26] Due to the smell it gives off when burned, lignite was often seen as a fuel for poor people compared to higher value hard coals. In Germany, briquettes are still readily available to end consumers inhome improvement stores and supermarkets.[27][28][29][30]
An environmentally beneficial use of lignite is in agriculture. Lignite may have value as an environmentally benignsoil amendment, improving cation exchange and phosphorus availability in soils while reducing availability of heavy metals,[31][32] and may be superior to commercial K humates.[33] Lignite fly ash produced by combustion of lignite in power plants may also be valuable as a soil amendment and fertilizer.[34] However, rigorous studies of the long-term benefits of lignite products in agriculture are lacking.[35]
Lignite may also be used for the cultivation and distribution ofbiological control microbes that suppress plant pests. The carbon increases theorganic matter in the soil while the biological control microbes provide an alternative to chemical pesticides.[36]
Leonardite is a soil conditioner rich inhumic acids that is formed by natural oxidation when lignite comes in contact with air.[37] The process can be replicated artificially on a large scale.[38] The less matured xyloid (wood-shaped) lignite also contains high amounts of humic acid.[39]
Reaction withquaternary amine forms a product called amine-treated lignite (ATL), which is used indrilling mud to reduce fluid loss during drilling.[40]
Lignite may have potential uses as an industrialadsorbent. Experiments show that its adsorption ofmethylene blue falls within the range ofactivated carbons currently used by industry.[41]
Jet is a form of lignite that has been used as a gemstone.[42] The earliest jet artifacts date to 10,000 BCE[43] and jet was used extensively in necklaces and other ornamentation in Britain from theNeolithic until the end ofRoman Britain.[44] Jet experienced a brief revival inVictorian Britain.[45]
Okefenokee Swamp, a modern peat-forming swampPartial molecular structure of a lignin-derived organic molecule in lignite
Lignite begins as partially decayed plant material, or peat. Peat tends to accumulate in areas with high moisture, slow landsubsidence, and no disturbance by rivers or oceans – under these conditions, the area remains saturated with water, which covers dead vegetation and protects it from atmospheric oxygen. Otherwise, peat swamps are found in a variety of climates and geographical settings. Anaerobic bacteria may contribute to the degradation of peat, but this process takes a long time, particularly in acidic water. Burial by other sediments further slows biological degradation, and subsequent transformations are a result of increased temperatures and pressures underground.[46]
Lignite forms from peat that has not been subjected to deep burial and heating. It forms at temperatures below 100 °C (212 °F),[1] primarily by biochemical degradation. This includes the process of humification, in which microorganisms extract hydrocarbons from peat and form humic acids, which decrease the rate of bacterial decay. In lignite, humification is partial, coming to completion only when the coal reaches sub-bituminous rank.[47] The most characteristic chemical change in the organic material during formation of lignite is the sharp reduction in the number of C=O and C-O-R functional groups.[48]
Lignite deposits are typically younger than higher-ranked coals, with the majority of them having formed during theTertiary period.[1]
Lignite is often found in thick beds located near the surface.[1][7] These are inexpensive to extract using various forms ofsurface mining, though this can result in serious environmental damage.[49] Regulations in the United States and other countries require that land that is surface mined must be restored to its original productivity once mining is complete.[50]
Strip mining of lignite in the United States begins withdrilling to establish the extent of the subsurface beds.Topsoil andsubsoil must be properly removed and either used to reclaim previously mined-out areas or stored for future reclamation.Excavator and truckoverburden removal prepares the area fordragline overburden removal to expose the lignite beds. These are broken up using specially equipped tractors (coal ripping) and then loaded into bottom dump trucks usingfront loaders.[51]
Once the lignite is removed, restoration involves grading the mine spoil to as close an approximation as practical of the original ground surface (Approximate Original Contour or AOC). Subsoil and topsoil are restored and the land reseeded with various grasses. InNorth Dakota, aperformance bond is held against the mining company for at least ten years after the end of mining operations to guarantee that the land has been restored to full productivity.[50] A bond (not necessary in this form) formine reclamation is required in the US by theSurface Mining Control and Reclamation Act of 1977.[52]
TheLatrobe Valley inVictoria,Australia, contains estimated reserves of some 65 billion tonnes of brown coal.[53] The deposit is equivalent to 25 percent of known world reserves. The coal seams are up to 98 m (322 ft) thick, with multiple coal seams often giving virtually continuous brown coal thickness of up to 230 m (755 ft). Seams are covered by very littleoverburden (10 to 20 m (33 to 66 ft)).[53]
A partnership led byKawasaki Heavy Industries and backed by the governments of Japan and Australia has begun extracting hydrogen from brown coal. The liquefied hydrogen will be shipped via the transporterSuiso Frontier to Japan.[54]
The largest lignite deposits in North America are theGulf Coast lignites and the Fort Union lignite field. The Gulf Coast lignites are located in a band running fromTexas toAlabama roughly parallel to the Gulf Coast. The Fort Union lignite field stretches fromNorth Dakota toSaskatchewan. Both are important commercial sources of lignite.[10]
Lignite can be separated into two types: xyloid lignite orfossil wood, and compact lignite or perfect lignite.
Although xyloid lignite may sometimes have the tenacity and the appearance of ordinary wood, it can be seen that the combustible woody tissue has experienced a great modification. It is reducible to a fine powder bytrituration, and if submitted to the action of a weak solution ofpotash, it yields a considerable quantity ofhumic acid.[39]Leonardite is an oxidized form of lignite, which also contains high levels of humic acid.[55]
Jet is a hardened,gem-like form of lignite used in various types of jewelry.[42]
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Subcoal – Coal substitute made from recycled waste
^Kim Thi Tran, Cuc; Rose, Michael T.; Cavagnaro, Timothy R.; Patti, Antonio F. (November 2015). "Lignite amendment has limited impacts on soil microbial communities and mineral nitrogen availability".Applied Soil Ecology.95:140–150.Bibcode:2015AppSE..95..140K.doi:10.1016/j.apsoil.2015.06.020.
^Jones, Richard; Petit, R; Taber, R (1984). "Lignite and stillage:carrier and substrate for application of fungal biocontrol agents to soil".Phytopathology.74 (10):1167–1170.doi:10.1094/Phyto-74-1167.
^abNeuendorf, K. K. E. Jr.; Mehl, J. P.; Jackson, J. A., eds. (2005).Glossary of Geology (5th ed.). Alexandria, Virginia: American Geological Institute. p. 344.
^Ibarra, JoséV.; Muñoz, Edgar; Moliner, Rafael (June 1996). "FTIR study of the evolution of coal structure during the coalification process".Organic Geochemistry.24 (6–7):725–735.Bibcode:1996OrGeo..24..725I.doi:10.1016/0146-6380(96)00063-0.
^Turgeon, Andrew; Morse, Elizabeth (22 December 2012)."Coal". National Geographic.Archived from the original on 25 September 2021. Retrieved25 September 2021.
^ab"Reclamation Process".Mining Lignite Coal for our Energy Future. BNI Coal.Archived from the original on 25 September 2021. Retrieved25 September 2021.
^"Mining Process".Mining Lignite Coal for our Energy Future. BNI Coal. Retrieved25 September 2021.
^"Reclamation Bonds".Office of Surface Mining Reclamation and Enforcement.Archived from the original on 2 March 2022. Retrieved18 March 2022.
^abDepartment of Primary Industries, Victorian Government, Australia, ‘Victoria Australia: A Principle Brown Coal Province’ (Fact Sheet, Department of Primary Industries, July 2010).
^Appunn, Kerstine (7 August 2018)."Germany's three lignite mining regions".The Clean Energy Wire. Archived fromthe original on 26 November 2018. Retrieved5 July 2019.Germany has been the largest lignite producer in the world since the beginning of industrial lignite mining. It still is, followed by China, Russia, and the United States. The softer and moister lignite (also called brown or soft coal) has a lower calorific value than hard coal and can only be mined in opencast operations. When burned, it is more CO2 intensive than hard coal.
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