Depletion of these reserves will lead to intense pressure to develop similar deposits of coal at greater depths and at greater cost. For the past 25 years, production of this coal has increased from about 25 to about 383 million short tons, a more than fifteenfold increase. This high-quality coal has been increasingly utilized by electric power generating plants in States in the western, midwestern, southern, and southeastern U.S. and foreign countries. The high production of this coal has reduced the amount of remaining low-cost, clean, and compliant coal. Such coal serves as the main combustible fuel, or as a blend with non-compliant coal, in many of these power plants.
Most Tertiary coal is considered a clean, low-contaminant, compliant coal resource.
The quantity of trace elements of environmental concern named in the 1990 Clean Air Act Amendments is of increasing importance in meeting compliance standards. These trace elements include antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, selenium, and uranium. The Northern Rocky Mountains and Great Plains region of Wyoming, Montana, and North Dakota contains a vast expanse of coal-bearing rocks of Tertiary age. The apparent rank of Tertiary coal ranges from lignite to subbituminous in the shallower strata and from subbituminous to bituminous in the deeper strata. In 1998 these coal zones, which range in thickness from 25 to 140 ft, produced more than 340 million short tons from 25 mines and made up more than 34 percent of the total U.S. coal production in that year.