Unit 10: Energy Challenges // Section 3: Fossil Fuels: Coal
Oil, natural gas, and coal, the traditional fossil fuels that have powered modern societies since the Industrial Revolution, remain the dominant world energy sources today. These fuels account for roughly 80 percent of world energy use and 86 percent of that of the United States, where the remainder comes from nuclear power and renewable fuels (Fig. 5).
Figure 5. World energy use by fuel source
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Source: Key World Energy Statistics, p.6. © Office for Economic Co-operation and Development/International Energy Agency (2006).
Fossil fuels hold energy stored in plant tissues by photosynthesis millions of years ago. When these ancient plants and the animals that fed on them died, they were buried in sediments, where Earth's heat and compression from the weight of overlying rock eventually turned the deposits into coal, oil, and natural gas. Exploring for and extracting these fossil fuels requires an intimate knowledge of the Earth's structure and history, and employs many of today's geoscientists.
Coal, the first fossil fuel exploited by humans for energy on a large scale, is a carbonaceous rock formed from buried plants in ancient forests or swamps. These plant materials are initially converted to peat—a loose, brown, organically rich soil that itself is an important energy resource in some areas. As more rock layers press down on the buried deposits, geothermal energy heats the peat and reduces its oxygen and hydrogen content, converting it to coal (Fig. 6). As materials go through this process, known as thermal maturation, their energy content by weight increases.
Figure 6. Coal formation
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Source: © Steve Greb, Kentucky Geological Survey.
Coal comes in several grades that reflect its thermal maturity and energy content:
- Brown coal (lignite), the first type of coal to form when plant matter is compacted, has an energy value of 9 to 17 million British thermal units (Btu) per ton. Because it has a low energy content, larger volumes are needed relative to higher-grade coals in order to generate the same amount of power.
- Sub-bituminous coal (16 to 24 million Btu/ton) and bituminous coal (19 to 30 million Btu/ton) are characteristically dark black and represent the most important coal grade for energy production (both direct heating and electricity generation) throughout the world.
- Anthracite coals are metallic gray and have a very high energy content, typically 22 to 28 million Btu per ton. Most readily accessible anthracite reserves in the eastern United States have been exhausted, and the remaining deposits are generally reserved for use in processing metals because of anthracite's high energy output and low volatile content.
Coal is extracted in both subsurface and surface strip mining operations. These processes have significant but different environmental impacts. Underground mining has relatively low immediate impact at the surface, but can cause ground subsidence when mineshafts collapse. Coal dust and methane gas (which is commonly found along with coal) raise significant risks of explosions. Worldwide, several thousand miners on average die each year in coal mining-related accidents.
In contrast, the impacts of strip mining—removing soils and overburden to extract shallow coal deposits—are highly visible at the surface. Strip mining operations generally leave permanent scars on the landscape. In its most extreme form, mountaintop removal, land is clear-cut and leveled with explosives to expose coal seams, with most of the removed overburden dumped into neighboring valleys (Fig. 7).
Figure 7. Mountaintop removal site, Kayford Mountain, West Virginia (2005)
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Source: © Vivian Stockman/www.ohvec.org. Flyover courtesy SouthWings.
Coal often contains a significant amount of sulfur, in either organic or metallic compounds, such as the mineral pyrite. When rain or groundwater comes in contact with coal, it produces sulfuric acid. Acid drainage from coal mines can pollute surrounding areas long after the mines are shut down. Many underground mines are dug to levels below the water table, so they flood easily after they are abandoned. When this happens, contaminated water flows out of mines, lowering the pH of lakes, rivers, and streams and leaching toxic heavy metals from the ground. Runoff from abandoned mines is a major source of water pollution in states with large coal industries like West Virginia and Pennsylvania.
Beyond the mine, coal produces significant amounts of atmospheric pollution and greenhouse gas emissions when it is burned. Coal combustion generates sulfate and nitrogen emissions that contribute to acid deposition, regional haze, and smog. It also produces mercury, which accumulates in the fatty tissues of animals and fish and can harm humans who consume certain species. (For more on these issues, see Unit 11, "Atmospheric Pollution.") And coal is the most carbon-intensive of all fossil fuels, so it produces a disproportionate share of total greenhouse gas emissions from energy use. On average, coal contains roughly 30 percent more carbon per unit of energy than crude oil and 75 percent more than natural gas (footnote 2). (For more discussion of GHG emissions from energy consumption, see Unit 12, "Earth's Changing Climate.")
A variety of technologies exist to make coal cleaner to burn, including filtration systems that reduce particulate emissions, scrubbing systems to reduce hazardous sulfur and nitrogen emissions, and methods for removing mercury. Moreover, coal can be turned into a form of syngas (synthetic natural gas), which can be burned with smaller environmental impacts. Many of these technologies are proven and some are in use at modern power generation facilities. It is generally very costly to retrofit older power plants with these capabilities, however. Technologies that could capture a large part of the carbon dioxide from coal-burning power plants, for subsequent storage away from the atmosphere, are under intensive development but are certain to be expensive.