Lignite or Brown coal is a brown fossil fuel and the lowest rank of coal. It is a softer coal with a high moisture content and contains the greatest amount of compounds other than carbon - such as sulfur and mercury. The carbon content of lignite is only about 70%. Also, lignite represents the youngest rank of fuel, with approximate ages of around 60 000 000 years.
Lignite is the first "stage" of coal that forms after peat is heated and compressed after sediment piles on top of layers of peat. Since lignite has a lower carbon content and has not been buried very long, it does not have as high an energy density as harder black coals. The energy density of lignite is only 3500 kcal/kg, significantly lower than the contents of higher quality coals such as sub-bituminous, bituminous, and anthracite coal.
Since lignite has a relatively low energy density, the coal is burned close to the mines to get the most output from the coal without expending too much energy transporting it (these are called mine mouth operations). Lignite's high moisture content and lower carbon content results in more carbon dioxide emissions than harder black coals. In these power plants the coal is burned and used in industrial boilers. 79% of all lignite coal is used in these boilers to generate electricity, and 13.5% is used to generate synthetic natural gas. A small 7.5% is used to produce various fertilizer products. Virtually none is used as a home heating fuel due to its poor burn quality.