Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad, is a soft brown fuel with characteristics that put it somewhere between coal and peat. It is considered the lowest rank of coal; it is mined in Germany, Russia, the United States, India, Australia and many European countries, and it is used almost exclusively as a fuel for steam-electric power generation. Up to 50% of Greece's electricity and 24.6% of Germany's comes from lignite power plants.
Lignite is brownish-black in color and has a carbon content of around 25-35%, a high inherent moisture content sometimes as high as 66%, and an ash content ranging from 6% to 19% compared with 6% to 12% for bituminous coal.
The energy content of lignite ranges from 10 to 20 MJ/kg (9 to 17 million BTU per short ton) on a moist, mineral-matter-free basis. The energy content of lignite consumed in the United States averages 13 million Btu/ton (15 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter). When reacted with quaternary amine, amine treated lignite (ATL) forms. ATL is used in drilling mud to reduce fluid loss.
Lignite has a high content of volatile matter which makes it easier to convert into gas and liquid petroleum products than higher ranking coals. However, its high moisture content and susceptibility to spontaneous combustion can cause problems in transportation and storage.
Because of its low energy density, 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 constructed very close to any mines, such as in Australia's Latrobe Valley and Luminant's Monticello plant in Texas. Carbon dioxide emissions from brown coal fired plants are generally much higher than for comparable black coal plants, with the world's highest emitting being Hazelwood Power Station, Victoria. The operation of brown coal plants, particularly in combination with strip mining, can be politically contentious due to environmental concerns.
Lignite is geologically younger than higher-grade coals, originating mainly in the Tertiary period.
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.
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