Coal Formation

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Coal is a solid fossil fuel formed from ancient plants—including trees, ferns, and mosses—that grew in swamps and bogs or along coastal shorelines. Generations of these plants died and were gradually buried under layers of sediment. As the sedimentary overburden increased, the organic material was subjected to increasing heat and pressure that caused the organic material to undergo a number of transitional states to form coal. The mounting pressure and temperature caused the original organic material, which was rich in carbon, hydrogen, and oxygen, to become increasingly carbon-rich and hydrogen- and oxygen-poor. The successive stages of coal formation are peat (partially carbonized plant matter), lignite (soft brownish-black coal with low carbon content), subbituminous coal (soft coal with intermediate carbon content), bituminous coal (soft coal with higher carbon and lower moisture content than subbituminous coal), and anthracite (hard coal with highest carbon content and lowest moisture content). Because anthracite is the most carbon-rich, moisture-deficient form of coal, it has the highest heating value.

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