Lignite (from Latin lignum 'wood'), often called brown coal, is a soft, brown, combustible sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35% and is considered the lowest rank of coal due to its relatively low heat content. When removed from the ground, it contains a very high amount of moisture, which partially explains its low carbon content. Lignite is mined all around the world and is used almost exclusively as a fuel for steam-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. Depending on the source, various toxic heavy metals, including naturally occurring radioactive materials, may be present in lignite and left over in the coal fly ash produced from its combustion, further increasing health risks.
Characteristics
Lignite is brownish-black in color and has a carbon content of 60–70 percent on a dry ash-free basis. However, its inherent moisture content is sometimes as high as 75 percent and its ash content ranges from 6–19 percent, compared with 6–12 percent for bituminous coal. 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.
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. 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. 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 to spontaneous 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 the calorific value of brown coal to a black coal equivalent fuel, and significantly reduce the emissions profile of 'densified' brown coal to a level similar to or better than most black coals. However, removing the moisture increases the cost of the final lignite fuel. Lignite rapidly degrades when exposed to air, in a process called slacking (or slackening).
Uses
Most lignite is used to generate electricity. However, small amounts are used in agriculture, in industry, and even, as jet, 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.
As fuel
Lignite is often found in thick beds located near the surface, making it inexpensive to mine. However, because of its low energy 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. It is often burned in power stations near the mines, such as in Poland's Bełchatów plant and Turów plant, Australia's Latrobe Valley and Luminant's Monticello plant and Martin 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 per megawatt-hour generated than for comparable black-coal plants, with the world's highest-emitting plant being Australia's Hazelwood Power Station until its closure in March 2017. The operation of traditional brown-coal plants, particularly in combination with strip mining, is politically contentious due to environmental concerns. The German Democratic Republic relied extensively on lignite to become energy self-sufficient, and eventually obtained 70% of its energy requirements from lignite. Lignite was also an important chemical industry feedstock via Bergius process or Fischer-Tropsch synthesis in lieu of petroleum, which had to be imported for hard currency following a change in policy by the Soviet Union in the 1970s, which had previously delivered petroleum at below market rates. East German scientists even converted lignite into coke suitable for metallurgical uses (high temperature lignite coke) and much of the railway network was dependent on lignite either through steam trains or electrified lines mostly fed with lignite derived power. East Germany was the largest producer of lignite for much of its existence as an independent state. In 2014, about 12 percent of Germany's energy and, specifically, 27 percent of its electricity came from lignite power plants, while in 2014 in Greece, lignite provided about 50 percent of its power needs. Germany has announced plans to phase out lignite by 2038 at the latest. Greece has confirmed that the last coal plant will be shut in 2025 after receiving pressure from the European Union and plans to heavily invest in renewable energy.
Home heating Lignite was and is used as a replacement for or in combination with firewood for home heating. It is usually pressed into briquettes for that use. 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 in home improvement stores and supermarkets.
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