Short answers, pulled from the story.
Coal is a combustible black or brownish-black sedimentary rock made mostly of carbon, with variable amounts of hydrogen, sulfur, oxygen and nitrogen. Bituminous coal, on a dry, ash-free basis, is about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen and 1.8% sulfur by weight. Around 10% of coal is ash.
Coal forms when dead plant matter is protected from oxidation and turns into peat, which deep burial then converts to coal over millions of years through heat and pressure. About 90% of all coal beds were deposited during the Carboniferous and Permian periods. Temperature matters more than pressure or time, with lignite forming under mild conditions and anthracite under higher heat.
The ranks of coal, from lowest to highest, are lignite or brown coal, sub-bituminous coal, bituminous coal and anthracite, with peat as a precursor and graphite as a near-pure-carbon extreme. Anthracite has the highest carbon content and fuel value. The key practical split is between thermal coal, used to generate electricity, and metallurgical or coking coal, used to make steel.
Coal is used primarily as fuel, with 68% of global coal use going to electricity generation in 2022. Metallurgical coal is baked into coke to smelt iron ore and make steel, and coal can also be gasified into syngas to produce chemicals like methanol, ammonia and urea, or liquefied into synthetic gasoline and diesel.
Coal is estimated to cause 800,000 premature deaths globally every year, mostly in India and China, through emissions of sulfur dioxide, nitrogen oxides, particulates and heavy metals. Breathing coal dust causes black lung disease, and in the US alone an estimated 1,500 former coal workers die each year from it. Coal is also the largest single source of carbon dioxide contributing to climate change.
Burning coal emitted over fifteen billion tonnes of carbon dioxide in 2024, more than a quarter of total global greenhouse gas emissions. Each megawatt-hour of coal-fired electricity emits around a tonne of carbon dioxide, double the roughly 500 kilograms from a natural gas plant. World gross carbon dioxide emissions from coal reached 14.5 gigatonnes in 2016.
China mines almost half the world's coal and is the largest consumer and importer, followed by India with about a tenth. In 2022 China used 4,520 megatonnes, more than half the global total, while Indonesia was the largest exporter by volume at 471 megatonnes, followed by Australia and Russia.