Lignite 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 used almost exclusively as a fuel for steam-electric power generation.
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 - 20 MJ/kg (9-17 million BTU per 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/ton), on the as-received basis (i.e., containing both inherent moisture and mineral matter). The energy content of lignite consumed in Victoria, Australia averages 8.4 MJ/kg (6.5 million BTU/ton). 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. However, it is now known that efficient processes that remove latent moisture locked within the structure of brown coal will relegate the risk of spontaneous combustion to the same level as black coal, will transform the calorific value of brown coal to a black coal equivalent fuel while significantly reducing the emissions profile of 'densified' brown coal to a level similar to or better than most black coals.