July 18. Coal

Coal is known from most geologic periods, even the Precambrian, when it probably derived from algal accumulations. But the Carboniferous, especially the later part of the period called Pennsylvanian in the United States, was probably the time when more coal formed than any other period. That was pretty much directly proportional to the widespread environments that encouraged the growth of plants in settings that led to their burial and alteration to coal.  

Depending on how much the original plant matter is compressed and heated up, you get various grades of coal. The least compacted type with the most impurities is called lignite, which is essentially compressed peat, a loose accumulation of plant matter. Lignite has only about 30% carbon content and high moisture. The next grade is called bituminous, around 70% carbon and fewer impurities and lower moisture than lignite. The word bituminous means “containing bitumen,” and bitumen was the name of the tarry substance used as pitch or glue in Roman times and earlier. Lignite comes from a Latin word meaning wood.

The highest grade of coal, anthracite, can have carbon content as high as 95% or more. It is essentially a low-grade metamorphic rock, low in moisture and volatiles and other impurities. The grades of coal are proportional to their heat content when burned, and consequently the price of coal follows its grade, with anthracite most expensive. Anthracite’s name comes from the Greek word for coal.

The biggest deposits of anthracite in the world are in the Pennsylvanian rocks of Pennsylvania, but today more anthracite is mined in China than any other country. China is also the world’s largest consumer of coal. Anthracite accounts for only about 1% of all the coal reserves known, but its value means about 9% of total coal production is anthracite.

Bituminous coal is the bulk of coal mined and burned for fuel and energy generation. About a third of the world’s energy comes from coal, with 40% of electricity generated by burning coal. And about 70% of all the steel made relies on coal to fuel the blast furnaces and refineries where steel is fabricated.

Today the leading producers of coal, in order, are China, with almost half of the total world production, followed by the US, India, Indonesia, and Australia. Close to eight billon tons of coal is mined worldwide every year.

While burning coal is the primary end use, there are lots of products that come from coal and the coal tars derived from coal. Heat-resistant black plastics, aspirin pills, dyes, mothballs, and more were ultimately made from chemicals distilled from coal. There’s a fair amount about these topics in my other book, What Things Are Made Of.

—Richard I. Gibson