Thermal coal: How energy resides
Why a kilo of black coal beats two of lignite
What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Coal, lignite and peat hold ancient solar energy as chemical bonds in carbon-rich plant remains, concentrated by burial heat and pressure over millions of years.
Form Chemical energy in carbon-hydrogen and carbon-carbon bonds of fossilised plant matter Origin Fossil solar (ancient photosynthesis)How it got there
Fusion in the Sun Sunlight Photosynthesis in swamp plants Waterlogged plant debris accumulates as peat Burial, heat and pressure drive off water and gases Lignite, then sub-bituminous and bituminous coal, then anthraciteHow concentrated it is
| Measure | Value | Note |
|---|---|---|
| Bituminous (steam) coal, net calorific value | 25.8 (range 19.9 to 30.5) MJ/kg1 | IPCC default for 'other bituminous coal'. |
| Sub-bituminous coal | 18.9 (11.5 to 26.0) MJ/kg1 | Lower rank, more moisture. |
| Lignite (brown coal) | 11.9 (5.5 to 21.6) MJ/kg1 | Wet and bulky, so usually burned next to the mine. |
| Peat | 9.76 (7.8 to 12.5) MJ/kg1 | The youngest, least altered stage. |
| Anthracite | 26.7 (21.6 to 32.2) MJ/kg1 | Highest rank, nearly pure carbon. |
Coal is the compressed remains of plants that captured sunlight. In waterlogged swamps the debris could not rot completely and built up as peat over thousands of years. Burial then squeezed out water and volatile compounds, raising the carbon content and density with each step in 'rank' from lignite to anthracite. The whole process takes on the order of a few million years.
The energy is released by burning, which recombines the carbon with oxygen. Rank matters a great deal: a kilogram of good bituminous coal holds about 26 MJ, more than twice the energy of a typical kilogram of lignite (12 MJ) or peat (10 MJ). Low-rank fuels carry so much water and ash per unit of energy that shipping them far rarely pays, so lignite power stations sit beside their mines, while bituminous steam coal moves across oceans.