Coal is a black or brown rock formed from ancient plant matter. Hundreds of millions of years ago, swamp forests died and were buried under sediment. Heat and pressure compacted the remains into carbon-rich seams. Today miners extract it from underground shafts or open pits.

Not all coal is the same. Lignite is soft and low in energy. Sub-bituminous and bituminous coals are harder and burn hotter. Anthracite is the hardest, with the highest carbon content. Power plants burn most coal to make electricity. Steelmakers use a special type, coking coal, to smelt iron ore.

Coal powered the Industrial Revolution and still generates a large share of global electricity. It is also the dirtiest fossil fuel. Burning it releases more CO2 per unit of energy than oil or gas. It emits sulfur dioxide, mercury, and particulate matter that harm health. Many countries are phasing it out. Others are building new plants. The transition is uneven.

Types of coal

• Lignite
• Sub-bituminous
• Bituminous
• Anthracite
• Coking coal for steel