How Coal Works

Many people think coal represents a bygone way of life and that America has moved on to safer and cleaner energy sources. On the contrary, coal provides roughly half the nation’s electricity—far more than any other source of power—and our coal use has nearly tripled since 1960. Our coal use will continue to expand if the power industry succeeds in building the many power plants it has proposed for construction.

Coal’s proponents claim coal power is cheap. While the direct price of electricity from the nation’s aging fleet of coal plants may be low, it doesn’t reflect the staggering and lasting costs of coal-related air and water pollution, mining accidents, permanently altered landscapes, and, most importantly, climate change. Even the newest proposed plants – which would cost far more than existing plants—would have major impacts on air and water quality, and almost the same mining and climate impacts as existing plants. 

Technology is evolving that has the potential to substantially reduce coal’s contribution to global warming by capturing carbon emissions before they are emitted.  This technology could become an important part of the battle against global warming, but it remains to be seen whether it will work at a commercial scale and at what cost. 

Meanwhile, a 2009 UCS study found that we can dramatically reduce our coal use—and all the environmental and social costs associated with it—while saving energy consumers money with policies that aggressively promote energy efficiency and renewable power. 

How Coal Was Formed

Coal was formed when dead plant matter submerged in swamp environments was subjected to geological forces of heat and pressure over hundreds of millions of years.  As time went by, the plant matter evolved from moist and low-carbon peat, to coal, which is much higher in energy and carbon content. Coal itself has a wide variation in properties, so it is categorized into 4 ranks—lignite, sub-bituminous, bituminous, and anthracite—in order of increasing carbon and energy content.  Most of the coal burned in U.S. power plants is of the bituminous or subbituminous variety.

Figure 1: The Process of Coal Formation (Source: KGS)

Coal of all types can vary widely in the amount of sulfur contained.  These differences are determined by the conditions under which the coal formed.  Low-sulfur coal deposits formed in a freshwater environment, while those containing higher proportions of sulfur developed in brackish swamps or marine-influenced environments.[1] In the United States, the sulfur content of coal resources varies along geographic lines, with most of the eastern coal containing high levels of sulfur, and the younger western coal containing much less.

How Coal is Mined

In 2007, the Nation burned roughly 1.1 billion short tons of coal, enough to fill a railroad car every 3 seconds. The electricity sector accounted for over 90 percent of all coal used, with the rest being burned mostly in industrial and commercial settings.[2] Getting such vast quantities of coal to power plants for burning relies on a vast network of mines, where companies excel at extracting it from the Earth at prolific rates.

Removing coal is simple in principle: just expose it, break it up, and cart it off to be burned. In practice, coal mining is an energy intensive, labor intensive and money intensive undertaking. Underground mining remains one of the most hazardous of occupations, killing and injuring many in accidents, and causing chronic health problems. Most underground mining occurs in the East, while surface mining dominates in the West.

Due to a variety of influences, the coal mining industry has undergone dramatic change over the past few decades. More productive, but also more environmentally damaging surface mining methods have overtaken underground mining as the predominant way of extracting coal. The worst manifestation of this trend has been the rise in prevalence of mountaintop removal mining, in which tops of whole mountains are blasted away to get to the coal seams underneath. The amount of coal produced per worker has also risen dramatically, resulting in overall employment in mining being roughly cut in half between the 1980s and the 2000s. Even the portion of coal still mined underground is extracted with the help of extremely power, efficient machines, and many fewer humans. Overall, coal mining in the United States today looks much different than it did even a few decades ago.

Figure 2: U.S. Coal Mining Employment (Source: National Mining Association, 2007)

Underground Mining

Two dominant methods together account for the vast majority of underground coal mining: “room and pillar” and “longwall” mining. 
In room and pillar mining, seams of coal are mined partially, leaving large pillars of coal intact to support the overlying layers of rock.  Mining by this method creates a network of alternating open spaces and large pillars of coal.  When mining in one part of the coal seam has been finished, miners practice “retreat” mining, extracting as much as possible of the coal in the remaining pillars on the way out, while the roof of the seam begins to collapse behind them.

Figure 3: Room-and-pillar mining (Source: U.S. EIA)

Longwall mining involves cutting long tunnels into a coal seam and removing the extracted coal by conveyor belt.  As the miners and the machinery move along the seam, a hydraulic support system temporarily holds the ceiling of overlying rock in place.

Longwall mining (Source: Direct Industry)

Both room and pillar and longwall mining are mostly now performed with very powerful mining machines, which rapidly cut coal from the face of coal seams.  These machines are phasing out the use of less productive and more dangerous traditional methods, which require intermittently drilling the coal seam and blasting it with explosives.

Surface Mining

Strip mining

Surface mining is employed when the coal seam is located much closer to the surface.  It requires many fewer workers per unit of coal produced and uses very large machinery.  Extracting the coal first requires clearing the vegetation and soil from the immediate surface.  Then the large intermediate layer of sediment and rock, referred to by the industry as “overburden”, must be blasted and removed, either by a process called “draglining” or in large truckloads.  With the underlying coal seam finally exposed, it is removed in strips and moved by conveyor belt or truck to its final destination.  After mining of the coalbed is completed, the law requires that the contours of the land be restored and replanted with vegetation, though to date the acres of surface mines that have been successfully reclaimed is relatively low compared with acres of newly mined land.[3]

Mountaintop Removal Mining

While coal mining has long caused environmental damage, the most destructive mining method by far is a relatively new type of surface mining called mountaintop removal (MTR).  Currently practiced in southern West Virginia and eastern Kentucky, this method requires stripping all trees from the mountaintop and then blasting away the top several hundred feet with explosives.  The resulting debris is dumped into an adjacent valley, burying the streams and destroying everything that once grew there.  The practice leaves behind a flattened area with soils so poor they can only support exotic grasses, a profound change from a once diverse and heavily forested ecosystem. The damages from MTR include great loss and fragmentation of ecosystems, higher risk of local flooding and runoff, contamination of local groundwater resources, and occasional deadly accidents.

Mountaintop Removal Mining (Source: Ohio Valley Environmental Coalition)

Where Our Coal Comes From

While coal is currently mined in 26 states, the sources of U.S. coal are highly concentrated.  In 2007, 63 percent of all the coal produced in the United States came from just three states—Kentucky, West Virginia, and Wyoming—with each contributing 10, 13, and 40 percent of U.S. coal production, respectively.[4]

Figure 4: U.S. Coal Resources (Source: DOE EERE)

Not long ago, underground mines in the Appalachian region provided most of America's coal. In 1950, underground mines produced 421 million short tons, 75 percent of the coal produced. However, surface mining has overtaken underground mining as the dominant method, accounting for 70 percent of production in 2007.  As such, Wyoming was the largest single source of U.S. coal in 2007, most of it coming from the enormous Powder River Basin.

Although lower production costs have been an important factor in this transition, another has been the sulfur content of the coal. The Clean Air Act, especially after the 1990 amendments, drove a shift toward increasing use of low-sulfur coal as one of the main methods of reducing sulfur dioxide emissions from coal plants.  Since the predominantly subbituminous coal west of the Mississippi contains much less sulfur per unit energy released in combustion, this trend favored western surface mines as a dominant source of newer coal production.  

Figure 5: U.S. Coal Productions (Source: U.S. EIA)

How Coal is Transported and Processed

Coal predominantly gets around the nation by train, with almost 70 percent of the coal transported around the United States in 2007 arriving to its destination via railroad. Barges and trucks each accounted for about 11 percent of coal transportation, with the rest delivered mainly by tramway, conveyor belt, or slurry pipeline. In some cases coal is transported great distances. For instance, Powder River Basin coal is shipped by rail to power plants as far away as Georgia. Coal can go a long way, traveling as far as the distance from Wyoming to Georgia.[5]

Source: PictureQuest

A common way of avoiding the transportation costs associated with coal is by burning it in “mine mouth” coal plants.  As soon as it is mined the coal is immediately put on a conveyor belt that runs directly into the power plant. The growing presence of mine mouth plants has been facilitated by expanding use of long distance electricity transmission.

Before coal is shipped long distances, it undergoes a process of preparation to lower shipping costs and facilitate use in power plants.  Preparation generally includes crushing the coal and removing heavy, extraneous non-coal materials. If coal is high in sulfur or other impurities, it is washed with a water or chemical bath, removing up to 40 percent of inorganic sulfur in the coal.

Not all coal is prepared using the same process. Eastern coal commonly undergoes washing to meet environmental regulations, while low-sulfur western coal typically is crushed and resized without being washed. Unfortunately, the contaminants and non-coal material removed during washing have to go somewhere, and normally are deposited in large waste reservoirs. The volume of waste created is clearly demonstrated by the fact that, in 2002, about 25 percent of the raw coal going into washing plants was left behind as waste.[6]

How Coal is Burned

Almost all coal plants operating today use “pulverized coal” (PC) technology, which involves grinding the coal, burning it to make steam, and running the steam through a turbine to generate electricity.  A newer technology known as integrated gasification combined cycle (IGCC) converts coal into a gas, runs the gas through a combustion turbine to generate electricity, and uses the excess heat from that process to generate additional electricity via a steam turbine (hence the term “combined cycle”).

Figure 6: Inside an IGCC Power Plant (Source: U.S. EPA, 2006)

When pulverized coal burns, it emits enormous quantities of carbon dioxide  along with other pollutants like sulfur dioxide, nitrogen oxide, mercury, and microscopic particulate matter. A portion of existing plants and all new ones have pollution control technology to reduce emissions of some of these pollutants, especially sulfur dioxide and particulates. Common methods of pollution control include the use of scrubbers and filters.  Scrubbers use a wet limestone slurry to absorb sulfur as it passes through. Filters are collections of large cloth bags that catch particulates as they go through the cloth. Still, smaller particulates are less likely to be absorbed, and can pass out the smokestack into the air.

The newer IGCC technology is currently more expensive than pulverized coal technology, but it has certain environmental advantages.  While modern pollution controls for nitrogen oxides, sulfur dioxide, and particulate matter can dramatically reduce emissions from pulverized coal plants (by 90 to 99 percent), IGCC plants are capable of even greater reductions.  It is also easier and less costly to capture and dispose of mercury from an IGCC plant than from a pulverized coal plant, which will be increasingly important as mercury restrictions come into effect in the years ahead.

At the moment, there is no commercially available control technology that can be added to coal plants in order to reduce their CO2 emissions in the same way that scrubbers and filters can be installed to capture sulfur dioxide and particulate emissions, respectively.  However, carbon capture and storage (CCS) is an emerging technology that could allow plant operators to capture CO2, transport it to a “geologic sequestration” site, and pump it into the ground, where it would ideally remain safely stored over the long term. 

Figure 7: Geologic Sequestration of CO2 (Source: Alberta Geological Survey)

With respect to capturing carbon, IGCC has an advantage over pulverized coal technology. Since its gasification process allows for the separation and capture of CO2 before combustion, the gas is still in a relatively concentrated and pressurized form.  Pulverized coal plants can only capture CO2 after combustion, when it is far more diluted and harder to separate, increasing the projected costs.  The IGCC and pulverized coal varieties of CCS fall under what are called “pre-combustion” and “post-combustion” technology, respectively.

Pre- and post-combustion technologies are both expected to capture between 85 and 95 percent of a plant’s CO2. However, capturing and compressing CO2 is a very energy-intensive process, causing large reductions in the amount of net energy the plant is able to produce for the grid. In fact, the state-of-the-art method for capturing CO2 from pulverized coal plants is expected to reduce the plant’s energy output by a quarter or more (assuming CCS is built into the original plant design and not added as a retrofit, in which case it would reduce power output even more).  Although this output loss for IGCC plants would be smaller, reductions in output of greater than 15 percent are still expected.  When factoring in the likely additional fuel used just to power the CO2 removal process, the actual amount of CO2 avoided per unit of electricity would fall to the 80 or 90 percent range.

UCS presents a more detailed analysis of carbon capture and storage technology in its publication Coal Power in a Warming World: A Sensible Transition to Cleaner Energy Options.

Environmental and Public Health Impacts of Coal

Coal and Global Warming

Of the many environmental and public health risks associated with coal, the most serious in terms of its universal and potentially irreversible consequences is global warming. The scientific community has reached an overwhelming consensus that Earth’s climate is warming—with potentially devastating future impacts— and that human activities such as the burning of fossil fuels and deforestation are largely to blame. Coal-fired power plants are the largest single source of CO2 emissions in the United States, emitting as much as all modes of transportation combined in 2007. 

Figure 8: The Sources of U.S. Heat-Trapping Emissions in 2005 (Source U.S. EIA, 2008)

The United States is not alone in this regard.  China and India both are rapidly developing economically and plan on using their own very large reserves of coal to accelerate this process.  While China is taking important steps to improve its energy efficiency and to build renewable sources of power, it has already passed the United States in annual CO2 emissions, and is still set to expand its coal production much further.  These trends make it even more important that the United States continue to invest in low-cost alternatives to coal and pass these technologies on to the developing world.    

Click here for more information about coal and global warming.

Other Environmental Impacts

Beyond global warming, coal is responsible for countless environmental damages across the entire fuel cycle, from mining, to burning, to the disposal of waste products.

As discussed above, mining can lead to serious environmental impacts, including alteration of landscapes and resulting ecosystem destruction, water contamination, and human health and safety hazards. MTR mining has proven to be extremely destructive in this respect. Between 1985 and 2001, more than 7 percent of Appalachian forests were cut down and more than 1,200 miles of streams buried or polluted mining coal this way. In addition, black lung disease continues to kill about 1,000 former coal miners annually in the United States.

Impurities that are removed from coal before combustion are commonly stored in slurry reservoirs, where they pose great risks for nearby humans and the environment. There are over 700 such impoundments in Appalachia, and they may hold hundreds of millions of gallons of mine waste. Contaminants from these reservoirs can easily leach into surface and groundwater supplies. In extreme cases, the dams holding these reservoirs can fail, flooding local waterways and putting both wildlife and downstream communities at risk.

The nation's worst ever black water spill happened on October 11, 2000 near Inez, Kentucky. Just after midnight 306 million gallons of coal sludge, laced with coal cleaning chemicals and the heavy metals present in coal, leaked from a coal slurry impoundment at a Martin County Coal Company mountaintop removal site. The sludge leaked into an underground mine then burst out two portals into the Coldwater and Wolf Creeks. The Martin County slurry released 30 times more liquid than the Exxon Valdez.

Simply moving coal from one place to another has a significant environmental impact, with coal transportation accounting for about half of U.S. freight train traffic.  These trains, as well as trucks and barges that transport coal, run on diesel—a major source of nitrogen oxide and soot.

The burning of coal creates some of the most damaging impacts.  In addition to contributing to global warming through substantial emissions of carbon dioxide, coal plants give off the following pollutants:

  • Sulfur dioxide, which produces acid rain.  Coal combustion is the leading source of U.S. sulfur dioxide emissions.
  • Nitrogen oxides, key contributors to ground-level ozone (smog) and respiratory illnesses.
  • Particulate matter (soot), which produces haze and can cause chronic bronchitis, aggravated asthma, and premature death.  (Both sulfur dioxide and nitrogen oxides transform into particulates in the atmosphere).
  • Mercury, a neurotoxin that can contaminate waterways, make fish unsafe to eat, and cause birth defects.  As with sulfur dioxide, coal burning is the leading source of mercury emissions in the U.S.
  • Hydrocarbons, carbon monoxide, volatile organic compounds (VOCs), arsenic, lead, cadmium, and other toxic heavy metals.

After combustion, the remaining coal ash and sludge is often disposed of in unlined and unmonitored landfills and reservoirs.  Heavy metals and toxic substances contained in this waste can contaminate drinking water supplies and harm local ecosystems.  Even worse, failed reservoirs can flood coal waste into surrounding areas.  In 2008, a Tennessee Valley Authority coal ash pond spilled into the nearby environment, requiring over $1 billion in estimated clean-up costs.

The Future of Coal

Coal is abundant in America, though not as abundant as previously thought. In 2007 the National Academy of Sciences reported that the country likely has at least a 100-year supply at today’s consumption levels, but it could not confirm the often-quoted assertion that the nation has a 250-year supply of coal. Since then, the U.S. Geologic Survey substantially reduced its estimate of the amount of coal that is economically recoverable in the Powder River Basin, the nation’s most important coal field. However, even if the U.S. coal reserve remains ample, other factors may limit coal use. With expected future limits on carbon dioxide emissions, production of coal will very likely have to decrease over time, in the absence of the success of carbon capture and sequestration on a large scale. 

We are beginning to see signs already of the effect of expected future carbon dioxide regulations, and other policy and market changes, on the expansion of coal burning electricity.  Power companies are starting to integrate the future price of carbon into their cost estimates for new plants, and this is seriously impacting their decisions over whether to build new coal plants.  While there are still many coal plant proposals in the pipeline, over 100 coal plant proposals were cancelled or rejected by regulators in the last few years, in many cases in recognition of the financial risks posed by likely future carbon regulations.

Given the urgent need to dramatically reduce greenhouse gas emissions, and given coal power’s enormous contribution to those emissions, options for reducing the CO2 from coal plants are attracting increasing attention.  As noted above, carbon capture and storage technology could play a significant role in reducing plant emissions in the future.

However, there are many currently available alternatives to coal power that would allow us to meet our energy needs and reduce our emissions of greenhouse gases and other pollutants.  A 2009 UCS analysis (Climate 2030: A National Blueprint for a Clean Energy Economy) found that policies that promote more aggressive investments in energy efficiency and renewable energy would allow the United States to dramatically reduce its dependence on coal power—by about 85 percent relative by 2030 to what would be used under business-as-usual projections, while reducing power plant carbon emissions by 84 percent and saving consumers and businesses money.

Figure 9: Sources of Electricity, Blueprint Case vs Reference Case (Source: UCS, 2009)


[1] Kennedy, Bruce. Surface Mining. SME 1990, pg. 68
[2] EIA 2007 U.S. Coal Consumption by End-use Sector
[3] Ward, Ken. “Twice as many acres mined as reclaimed, report says”. West Virginia Gazette. March 14, 1999.
[4] EIA 2007 Coal production by state data
[5] EIA 2007 Coal distribution data
[6] Bonskowski, R. et al. 2006. Coal production in the United States: an historical overview. Energy Information Administration.  

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