Coal is a combustible rock formed from prehistoric biomass. Like oil and natural gas, coal is considered a “fossil fuel” because it was formed from decaying plant material over hundreds of millions of years.
Coal is a combination of pure carbon and hydrocarbons with varying amounts of moisture, minerals and heavy metals. It was the first fossil fuel used extensively by humans, and is still vitally important today, generating 39 percent of the world’s electricity, 49 percent of U.S. electricity and 36.5 percent of Texas’ electricity in 2006.
Coal is found on every continent and in some 70 countries. The U.S., Russia, China and India have the world’s largest reserves. The World Coal Institute in London estimates proven world coal reserves at 984 billion metric tons (more than 1 trillion U.S. tons), enough to last for more than 190 years at current rates of consumption.
In Texas in 2006, coal mining provided 2,241 jobs, earning an estimated $167.6 million in wages. Other contributions of coal to the economy are indirect. Texas coal is mined at the surface, and the surface owner, usually large utilities, does not report the value of the coal nor does the owner owe state taxes on coal production, although federal taxes are owed.
Throughout recorded history, some degree of industrialization has accompanied the widespread use of coal. Some of the earliest archeological evidence of the human use of coal dates back to about 6,000 years ago in northeastern China. The Romans used coal they found in Britain both as jewelry and as fuel for their forts and blacksmiths’ foundries until their exit from the islands in the fifth century A.D. Their knowledge of coal’s fuel value was lost to their British subjects for almost seven centuries.
By the 11th century, the Chinese were using charcoal and coke, a material derived from coal, to make iron. Britain’s use of coal in the eighteenth century led to the widespread availability of cheap iron and helped spur the Industrial Revolution.
In early America, English settlers reported an abundance of coal in the new country. Coal outcrops were found throughout the Appalachian Mountains and, in 1758, a new settlement named Pittsburgh was founded in an area of particularly abundant coal supplies. Within a few short years, Pittsburgh coal helped America begin its own industrialization.
Today, the world consumes about 4.4 billion short tons annually, a 38 percent increase in 20 years. (A short ton is 2,000 pounds, the measure used in the U.S. and in this chapter. The metric “tonne” of 2,200 pounds is used by some sources cited in this chapter; these figures have been converted to short tons throughout.) The majority of this coal is used for electricity generation and steel production.
Coal is one of the world’s most widely used fuels. In the U.S., coal produces 22.5 percent of the British thermal units (Btu) consumed for all purposes from all sources – about the same as natural gas (22.4 percent), but less than petroleum (39.8 percent).
Coal began as peat, a soft deposit formed by plant and animal matter collecting in boggy areas some 360 to 290 million years ago. As the material aged, sank and became buried by sediments over eons – a process called coalification – ever-increasing overburden pressure and heat squeezed out moisture and impurities to create four “ranks,” or grades, of coal. These are, in descending order of hardness and heat content, anthracite, bituminous coal, subbituminous coal and lignite. Each type of coal has specialized uses.
Of the four grades of coal, the hardest and rarest is anthracite, which is also geologically the oldest and purest, with the lowest moisture and mineral content. As such, it burns hottest, producing about 25 million Btu per ton, and produces the lowest emissions of all coals. In the U.S., anthracite is found only in northeastern Pennsylvania, and is used almost exclusively for home heating.
Bituminous and subbituminous coals, the most abundant types in the U.S., are found in Appalachia, the Midwest, Wyoming and Montana. The Powder River Basin (PRB) in Wyoming and Montana is a major source of this coal in the U.S. In addition to having a higher moisture and mineral content than anthracite, these coals contain bitumen, a thick tar-like material used in steelmaking and road building.
In the U.S., bituminous coal is often used to generate electricity. Its heat content averages 24 million Btu per ton, only slightly lower than that of anthracite. Subbituminous coal ranks between bituminous and lignite in its hardness and moisture content, and has a higher mineral content than bituminous coal. Its heat value averages 17 to 18 million Btu per ton. Bituminous coal is found in the eastern and midwestern U.S., while subbituminous is mined only in the western U.S., most prominently in the Powder River Basin.
This lower-grade coal is most often used to fire boilers, either to generate electricity or to create heat for industrial processes such as smelting. It also can be transformed into coke, which has its own applications in industrial processes.
Coal production contributed 2,241 mining jobs to the Texas economy in 2006. Wages were estimated to be $167.6 million. Texas has 13 active lignite mines, most supporting a nearby coal-fired electricity generation plant or industrial facility (known generally as “mine mouth” operations). Five other Texas mines are in reclamation, meaning that they are no longer in operation and the mine sites are being reclaimed for other uses. One is not operating but is not yet in reclamation.
Rail is the overwhelming choice for coal transportation in the U.S., shipping some 71 percent of the nation’s coal by weight in 2006, according to EIA. Eleven percent was shipped by truck, 10 percent by river barges, 7 percent by short-distance means, such as tramways, conveyers and slurry pipelines (pipelines carrying a mixture of water and finely ground coal), and 1 percent was undocumented .
Of the 680 million tons of coal shipped by railroad in the U.S., electricity generation plants received 93.7 percent; industrial plants received 4.5 percent; 1.5 percent went to coking plants; and the remainder went to other residential and commercial uses. More than 95 percent of the 85 million tons of coal shipped by conveyors or slurry pipelines went to electricity generation plants; 4.7 percent went to industrial plants; and the remainder went to other residential and commercial uses.
Because tramways, conveyors and slurry pipelines are generally short-distance hauls, one can infer that the power plants they serve are mine mouth operations. Slurry pipelines carry either a paste made of equal parts pulverized coal and water, or a compressed “log” of coal using water for flotation. The slurry contains the same trace minerals of copper, lead and other metals as dry coal, so it must be dewatered and demineralized before it is suitable for burning.
Until recently, the nation’s longest slurry pipeline in operation was the Black Mesa pipeline, which ran for 273 miles from a mine in northeastern Arizona to an electric plant in southern Nevada. In early 2007, however, the pipeline and the power plant it served shut down.
Texas lignite generally is not transported for significant distances because most of its major consumers – electric utilities, aluminum smelters and other industrial users – are located within a short distance of active mines. Because it is not shipped, the fuel’s total cost usually is lower than that of other coals that must be transported.
To generate electricity, coal can be burned directly or gasified and then burned more cleanly. If burned directly, the coal is ground into a very fine powder and then blown into large combustion chambers. The resulting heat either drives turbines directly or boils water to drive steam turbines, which then drive generators to create electricity. If the turbines can do both, the process is called “combined cycle.”
Gasification is a different process that can use coal, biomass, petroleum coke, petroleum residues or other organic waste. Under high heat, high pressure and controlled amounts of pure oxygen, most of the feedstock does not burn but instead breaks into its component parts.
The resulting synthetic natural gas, called “syngas,” is primarily hydrogen and carbon monoxide. It can be burned to drive turbines, either directly or by boiling water or both. Mineral impurities can be removed before they combine with other elements to become regulated emissions such as NOX, SOX and H2S. The burned coal is reduced to ash and removed. The ash is either sold for use as an ingredient in concrete or as a roadbed material, or made into synthetic gypsum used in wallboard manufacturing. Occasionally, the ash is deposited in landfills.
As of November 2007, Texas had only one coal gasification plant in the planning stages. Eastman Chemical is proposing to build a gasification plant near Beaumont.