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Chapter 3. Coal > What exactly is coal?

What exactly is coal?

Coal is fossilized land plants that have been buried deep in the earth for millions of years. Although the intense heat and pressure at those depths have converted the vegetation into a hard, black material that is primarily pure carbon, you can still see the leaves and stems of plants in the fossils. Most coal was formed during the Carboniferous and Permian periods, 363–245 million years ago, and in the Cretaceous period, 146–45 million years ago, when wetlands were widespread.[6] Dead vegetation that lay buried in large wetlands could not decay, because the water prevented oxygen in the air from reaching it.

There are four kinds of coal, depending on how long the plant material has been underground, at how high a temperature, and under how much pressure. Anthracite (hard coal) is the hardest and best coal fuel because it is 86% to 98% carbon and has the fewest impurities. Bituminous (soft coal) is the most abundant but is softer than anthracite, has a lot of impurities, and is only 69% to 86% carbon. Subbituminous (medium-soft coal) is even softer and has more water and impurities. Lignite (brown coal) is the worst stuff, very soft, 70% water, and less than 30% carbon. Of the four, anthracite, the cleanest and with the least water, is the best for home heating. The others are used primarily to produce electricity, and bituminous coal is also used to make coke, which in turn is used with iron to make steel.[7]

It is estimated that the Earth contains approximately 1 trillion tons of coal. The energy stored in this is somewhere between 4.76 and 7.64 million billion kilowatt-hours, a number hard to imagine. Here’s a try: Hoover Dam, as mentioned earlier, generates 4 billion kilowatt-hours a year, so the world’s total coal reserve contains the amount of energy that would be generated by 1 to 2 million Hoover Dams running at full capacity for one year, or, if you prefer, 1 Hoover Dam running for 1 to 2 million years, or about 100 Hoover Dams running for 10,000 years—about the length of time that human civilization has existed on Earth. No matter how you look at it, there is a lot of energy stored in coal, thanks to woody plants that over hundreds of millions of years did not decay but just lay quietly, buried and stored.

Like oil and natural gas, coal is distributed unevenly around the world, but coal is found in more locations than the other fossil fuels—70 nations have recoverable coal, but 10 nations currently produce 95% of the world’s coal (Figure 3.2).

Figure 3.2. Although there is a lot of coal around the world, these ten nations mine most of it.

The United States has a lot of coal—it is the second-largest producer, behind only China in its mining. Coal in the United States is concentrated in the central and southern Appalachian Mountains of the East and in the western Great Plains, with some in the Midwest (Figure 3.3).

Figure 3.3. Where coal is mined in the United States. Percentages are changes in production from 2005 to 2006. U.S. total production is approximately 1,170 million short tons.
(Source: Energy Information Administration, Quarterly Coal Report, October-December 2008, DOE/EIA-0121)[8]

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