Subbituminous coal is a significant global energy resource, playing a considerable role in power generation across various countries. Its widespread availability and specific characteristics make it a frequently utilized fossil fuel.
Defining Subbituminous Coal
Subbituminous coal is a sedimentary rock that forms from the alteration of plant matter over millions of years through geological processes. It is ranked as an intermediate type of coal, positioned between lignite (a lower rank, younger coal) and bituminous coal (a higher rank, older coal) in the coalification spectrum. This ranking is determined by the amount of pressure and heat the organic material experienced over time, which influences its carbon content and energy output.
This coal typically appears dark brown to black and exhibits greater compaction and brightness compared to lignite. On a dry, ash-free basis, subbituminous coal contains approximately 42% to 52% carbon. Its calorific value, or heat content, generally ranges from about 19 to 26 megajoules per kilogram (approximately 8,200 to 11,200 British thermal units per pound) on a dry, ash-free basis.
Subbituminous coal often has a higher moisture content and more volatile matter than bituminous coal. While its heat content is lower than bituminous coal, it commonly features a low sulfur content, sometimes less than 1%. This lower sulfur can make it a preferred option for some power plants due to environmental regulations.
Global Deposits and Key Applications
Major deposits of subbituminous coal are found in various regions worldwide. In the United States, for instance, subbituminous coal is a substantial component of total coal production, accounting for approximately 46% in 2022. Wyoming is a leading producer, responsible for about 89% of U.S. subbituminous coal production, with other significant contributions coming from Montana, New Mexico, Colorado, and Alaska.
The primary application of subbituminous coal is in electricity generation, particularly in thermal power plants. These power plants burn the coal to heat water, producing steam that drives turbines to generate electricity. While its energy content is lower than bituminous coal, its availability in large, thick beds near the surface often results in lower mining costs, making it an economically attractive fuel for power generation. Subbituminous coal can also be liquefied and converted into petroleum and gas, though this is a less common application.
Environmental Impact
The extraction and combustion of subbituminous coal carry several environmental considerations. Burning coal, including subbituminous types, releases significant amounts of carbon dioxide (CO2), a primary greenhouse gas contributing to climate change. Nearly all of the carbon in coal is converted to CO2 during combustion. In 2022, CO2 emissions from coal combustion for energy accounted for about 19% of total U.S. energy-related CO2 emissions.
Beyond CO2, combustion also releases sulfur dioxide (SO2) and nitrogen oxides (NOx), which contribute to acid rain, smog, and respiratory illnesses. While some subbituminous coals have lower sulfur content than certain bituminous coals, SO2 is still a concern. Particulate matter (PM), including ash and unburned carbon, is another emission from coal combustion that contributes to smog, haze, and respiratory problems. The disposal of fly ash and bottom ash, which are residues from burning coal, can also pose environmental concerns if not managed properly, potentially leading to groundwater contamination.