Powder River Basin_map: Map showing subsurface mineral ownership in the Powder River Basin, Wyoming and Montana. (Source: USGS)

The Wyodak-Anderson coal zone of the Powder River Basin of Wyoming and Montana is among the most prolific coal resources in the world, and it supports the world's largest coal mine, the Black Thunder mine operated by Arch Coal, Inc. in Campbell County, Wyoming.

The zone contains some of world's largest continuous beds of coal, and it is of very high quality as measured by low amounts of sulfur, ash, and trace elements of concern to environmental and human health.

How much coal is in the Wyodak-Anderson coal zone? To answer that question we must first distinguish between the terms "coal reserves" and "coal resources." The term "resource" is a general term that describes naturally occurring deposits of coal in such forms and amounts that economic extraction is currently or potentially feasible. Identified resources are specific bodies of coal whose location, rank, quality, and quantity are known from geologic evidence supported by engineering measurements. Undiscovered resources are unspecified bodies of coal surmised to exist on the basis of broad geologic knowledge and theory. "Reserve" is a much more restrictive term, referring to that portion of the resource base that can mined today under existing economic, technological, environmental, and legal conditions. Note that as one moves form the resource to reserve category, there is an increasing degree of both economic and geological certainty.

The USGS estimates that the total coal resource of the Wyodak-Anderson coal zone is about 550 billion tons of coal. By way of comparison, the USGS estimates that the identified coal resources of U.S. are about 1,700 billion tons. The total resource (identified plus undiscovered) base is estimated at 4,000 billion tons. By any measure, the Wyodak-Anderson coal zone is a remarkable energy resource.

In terms of recoverable reserves, the importance of the Wyodak-Anderson coal is revealed in the dominant role that the state of Wyoming pays in U.S. production. The reserves in the producing mines in Wyoming account for 42% of the national total. The 17 active surface coal mines in Wyoming, most of which extract from the Wyodak-Anderson zone, produce more than 400 million tons of coal, equivalent to 36% of the nation's coal output. Those 17 mines alone account for about 7% of the global production of coal.

Wyodak-Anderson coal is shipped to over 130 power plants in 27 states, and is exported to Canada and Spain. It is particularly valued for its low ash and sulfur content. Wyodak-Anderson coal often blended with high sulfur eastern coal to bring power plant emissions down to mandated levels.

The source of organic material for the coal originated in swamps, estuaries, and deltas associated with the regression (retreat) of a large inland seaway that occupied central North America during the Cretaceous Period, which spanned the years between about 144 to 65 million years ago. The rapid accumulation of organic source material was sustained by high rainfall in tropical to subtropical (mean annual temperature of 20° C; annual precipitation rate of more than 90.55 in.) climate conditions.

The Wyodak-Anderson coal zone contains net coal (total thickness of all coals greater than 2.5 ft thick) that is more than 200 ft thick. The entire zone is more than 600 ft thick (measured from the top of the uppermost coal to the base of the lowermost coal). It consists of as many as six coal beds. The coal beds average 25 ft in thickness, and are separated by sedimentary rocks ranging from a few feet to 150 ft in thickness. The Wyodak Anderson coal beds merge into a single coal bed as much as 202 ft thick in the west-central part of the basin and as much as 120 ft thick in the eastern part of the basin.

In addition to very low sulfur content, the Wyodak-Anderson coal zone also has low concentrations of environmentally important trace elements such as antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, selenium, and uranium. Wyodak-Anderson coal has the lowest contents of these elements when compared to other coal within the Northern Rocky Mountains and Great Plains region, and coal from other coal-producing regions in the U.S.

The dragline known as 'Ursa Major' at the Black Thunder coal mine in Wyoming. Credit: Mining-technology.com

The Black Thunder coal mining complex is the largest surface coal operation in the world, and it has produced more coal to date than any other mine in the world. Located in Campbell County, Wyoming, the mining complex produces about 93 million tons of coal per year, equivalent to about 8% of U.S. coal output. That is used in about 100 power plants to generate about 5% of the nation's electricity. The energy content of the annual energy output of the mine is about 1.6 x 10¹⁸ joules, equivalent to almost 2% of the nation's total primary energy use. In 2004, Black Thunder became the first coal mine in the world to ship a cumulative one billion tons of coal since its first shipment on December 14, 1977. The mine complex is located on approximately 24,300 acres and has six active pit areas. Its fleet of five draglines includes Ursa Major, a Bucyrus-Erie 2570WS model, one of largest draglines in the world. It weighs 14.7 million pounds, is 238 feet tall, and its 110m-long boom carries a 122m³ bucket. The machine can dig the equivalent of an Olympic-sized swimming pool in just 25 minutes.

The mine produces high quality, low-sulfur, sub-bituminous coal that is suitable for power station fuel without any preparation except crushing. Black Thunder coal has a heating value of 20.3MJ/kg, a sulfur content of 0.5-0.7 lbs. SO₂/mmBtu, an ash content of about 5%, and a moisture content of 25-30%.

Sources

• Ellis, M.S., G.L. Gunther, A.M. Ochs, S.B. Roberts, E.M. Wilde, J.H. Schuenemeyer, H.C. Power, G.D. Stricker, and Dorsey Blake, Coal resources, Powder River basin, In U.S. Geological Survey Professional Paper 1625-A.
• Arch Coal, Inc., Black Thunder Coal Mine, Accessed August 16 2007.
• Mining technology.com, Black Thunder Coal Mine, WY, USA, Accessed August 16 2007.
• U.S. Bureau of Land Management, Final Environmental Assessment of the West Black Thunder Coal Lease Application as applied for by Thunder Basin Coal Company, Accessed August 16 2007.