Lignite is brownish-black in color and has a carbon content from as low as 20-25 percent up to 60–70 percent, a high inherent moisture content sometimes as high as 75 percent, and an ash content ranging from 6–19 percent, compared with 6–12 percent for bituminous coal. lignite at Tagebau Garzweiler in Germany The energy content of lignite ranges from 10 to 20 MJ/kg on a moist, mineral-matter-free basis. The energy content of lignite consumed in the United States averages , on the as-received basis. The energy content of lignite consumed in Victoria, Australia, averages . Lignite has a high content of volatile matter which makes it easier to convert into gas and liquid petroleum products than higher-ranking coals. Unfortunately, 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.
Uses
Because of its low energy density 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 Australia's Latrobe Valley and Luminant's Monticello 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 generated than for comparable black-coal plants, with the world's highest-emitting plant being 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. In 2014, about 12 percent of Germany's energy and, specifically, 27 percent of Germany's electricity came from lignite power plants, while in 2014 in Greece, lignite provided about 50 percent of its power needs. An environmentally beneficial use of lignite can be found in its use in cultivation and distribution of biological control microbes that suppress plant disease causing microbes. The carbon enriches the organic matter in the soil while the biological control microbes provide an alternative to chemical pesticides. Reaction with quaternary amine forms a product called amine-treated lignite, which is used in drilling mud to reduce fluid loss during drilling.
Geology
Lignite begins as an accumulation of partially decayed plant material, or peat. Burial by other sediments results in increasing temperature, depending on the local geothermal gradient and tectonic setting, and increasing pressure. This causes compaction of the material and loss of some of the water and volatile matter. This process, called coalification, concentrates the carbon content, and thus the heat content, of the material. Deeper burial and the passage of time result in further expulsion of moisture and volatile matter, eventually transforming the material into higher-rank coals such as bituminous and anthracite coal. Lignite deposits are typically younger than higher-ranked coals, with the majority of them having formed during the Tertiary period.
The Latrobe Valley in Victoria, Australia, contains estimated reserves of some 65 billion tonnes of brown coal. The deposit is equivalent to 25 percent of known world reserves. The coal seams are up to 100 metres thick, with multiple coal seams often giving virtually continuous brown coal thickness of up to 230 metres. Seams are covered by very little overburden.
Types
Lignite can be separated into two types. The first is xyloid lignite or fossil wood and the second form is the 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 by trituration, and if submitted to the action of a weak solution of potash, it yields a considerable quantity of humic acid. Leonardite is an oxidized form of lignite, which also contains high levels of humic acid. Jet is a hardened, gem-like form of lignite used in various types of jewelry.
