Coal is a form of fossil fuel. Coal is generally believed to form when plant matter is buried under sediment, under anaerobic conditions, and then subjected to geologically long periods of heat and pressure.
Impurities in coal make it a more polluting fossil fuel than natural gas.
- "Gas works, coke works, and other coal carbonization plants". Department of Energy (UK). 1995. p. 7. ISBN 1 85112 232 X. http://publications.environment-agency.gov.uk/PDF/SCHO0195BJKP-E-E.pdf. Retrieved 2012-06-24.