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Chemistry LibreTexts

15.6: Coal - The Carbon Rock of Ages

  • Page ID
    152235
  • Learning Objectives

    • Describe coal and its processing.
    • List the products of coal burning that promotes pollution.

    Coal is a complex solid material derived primarily from plants that died and were buried hundreds of millions of years ago and were subsequently subjected to high temperatures and pressures. It is a combustible black or brownish-black sedimentary rock with a high amount of carbon and hydrocarbons. Coal is classified into four main types, or ranks depending on the types and amounts of carbon present and on the amount of heat energy the coal can produce, including anthracite, bituminous, subbituminous, and lignite (highest to lowest ranked, pictured in Figure \(\PageIndex{1}\).

    Screenshot (41).png
    Figure \(\PageIndex{1}\) Different types of coal. Images obtained from Wikimedia Commons.

    There are four distinct classes of coal (Table \(\PageIndex{1}\)); their hydrogen and oxygen contents depend on the length of time the coal has been buried and the pressures and temperatures to which it has been subjected. The most abundant form in the United States is bituminous coal, which has a high sulfur content becauseof the presence of small particles of pyrite (FeS2). The combustion of coal releases the sulfur in FeS2 as SO2, which is a major contributor to acid rain.

    Table \(\PageIndex{1}\) Properties of Different Types of Coal.
    Type % Carbon Hydrogen:Carbon Mole Ratio % Oxygen % Sulfur US Deposits
    anthracite 92 0.5 3 1 Pennsylvania, New York
    bituminous 80 0.6 8 5 Appalachia, Midwest, Utah
    subbituminous 77 0.9 16 1 Rocky Mountains
    lignite 71 1.0 23 1 Montana

    Peat, a precursor to coal, is the partially decayed remains of plants that grow in swampy areas. It is removed from the ground in the form of soggy bricks of mud that will not burn until they have been dried. If a peat bog were buried under many layers of sediment for a few million years, the peat could eventually be compressed and heated enough to become lignite, the lowest grade of coal; given enough time and heat, lignite would eventually become anthracite, a much better fuel.

    For us to use the potential energy stored in coal, it first must be mined from the ground. This process in itself uses a great deal of resources and has its own environmental impacts. Coal then typically undergoes processing to make it suitable for use in coal-fire power plants. Finally, the processed coal is burned in these power plants, and the kinetic energy released from its combustion is harnessed for electricity generation or other purposes. Figure \(\PageIndex{2}\) below is a schematic diagram showing a typical layout of a coal-fire power plant. You can also watch a short video of a virtual tour of a coal power plant at the URL provided below.

    Power Plant Virtual Tour

    Video \(\PageIndex{1}\) Coal-Fueled Power Plant Virtual Tour Video

    Screenshot (42).png
    Figure \(\PageIndex{2}\) Diagram of a typical steam-cycle coal power plant (proceeding from left to right). Image by US Tennessee Valley Authority – Public domain. www.tva.com

    The Global Carbon Cycle

    Figure \(\PageIndex{3}\) illustrates the global carbon cycle, the distribution and flow of carbon on Earth. Normally, the fate of atmospheric CO2 is to either (1) dissolve in the oceans and eventually precipitate as carbonate rocks or (2) be taken up by plants. The rate of uptake of CO2 by the ocean is limited by its surface area and the rate at which gases dissolve, which are approximately constant. The rate of uptake of CO2 by plants, representing about 60 billion metric tons of carbon per year, partly depends on how much of Earth’s surface is covered by vegetation. Unfortunately, the rapid deforestation for agriculture is reducing the overall amount of vegetation, and about 60 billion metric tons of carbon are released annually as CO2 from animal respiration and plant decay. The amount of carbon released as CO2 every year by fossil fuel combustion is estimated to be about 5.5 billion metric tons. The net result is a system that is slightly out of balance, experiencing a slow but steady increase in atmospheric CO2 levels. As a result, average CO2 levels have increased by about 30% since 1850.

    Figure \(\PageIndex{3}\) The global carbon cycle.

    Most of Earth’s carbon is found in the crust, where it is stored as calcium and magnesium carbonate in sedimentary rocks. The oceans also contain a large reservoir of carbon, primarily as the bicarbonate ion (HCO3). Green plants consume about 60 billion metric tons of carbon per year as CO2 during photosynthesis, and about the same amount of carbon is released as CO2 annually from animal and plant respiration and decay. The combustion of fossil fuels releases about 5.5 billion metric tons of carbon per year as CO2.

    Pollution from Coal Burning

    In the United States and most of the world, most of the coal consumed is used as a fuel to generate electricity. Burning coal produces emissions such as sulfur dioxide (SO2) and nitrogen oxides (NOx) that are associated with acid rain (more on this in chapter 6). Carbon dioxide (CO2), another emission resulting from burning coal, is a major greenhouse gas that is associated with global warming (see Chapter 13 ). Burning of coal is classified as incomplete combustion, since the reaction of carbon (in coal) with oxygen produces carbon monoxide and/or carbon (soot) in addition to carbon dioxide.

    Burning coal produces emissions that also impact human health. Emissions such as sulfur dioxide (SO2), nitrogen oxides (NOx) and particulates contribute to respiratory illnesses. Particulates also contribute to a condition among coal miners and other coal workers known as coal workers' pneumoconiosis (CWP) or black lung disease, which results from long exposure to coal dust. Inhaled coal dust progressively builds up in the lungs and is unable to be removed by the body; this leads to inflammation, fibrosis, and in worse cases, tissue death (necrosis).

    Coal is the largest source of mercury and also a source of other heavy metals, many of which have been linked to both neurological and developmental problems in humans and other animals. Mercury concentrations in the air usually are low and of little direct concern. However, when mercury enters water, either directly or through deposition from the air, biological processes transform it into methylmercury, a highly toxic chemical that accumulates in fish and the animals (including humans) that eat fish.

    Reducing the Environmental Impacts of Coal Use

    Regulations such as the Clean Air Act and the Clean Water Act require industries to reduce pollutants released into the air and water. Below are some actions that have been taken to reduce the negative impacts of coal on human and environmental health:

    Clean coal technology: Industry has found several ways to reduce sulfur, NOx, and other impurities from coal before burning.

    • Coal consumers have shifted toward greater use of low sulfur coal.

    • Power plants use scrubbers, to clean SO2, NOx, particulate matter, and mercury from the smoke before it leaves their smokestacks. In addition, industry and the U.S. government have cooperated to develop technologies that make coal more energy-efficient so less needs to be burned.

    • Research is underway to address emissions of carbon dioxide from coal combustion. Carbon capture & sequestration separates CO2 from emissions sources and recovers it in a concentrated stream. The CO2 can then be sequestered, which puts CO2 into storage, possibly underground, where it will remain permanently.

    • Reuse and recycling can also reduce coal’s environmental impact. Land that was previously used for coal mining can be reclaimed and used for airports, landfills, and golf courses. Waste products captured by scrubbers can be used to produce products like cement and synthetic gypsum for wallboard.

    Summary

    • Coal is a complex solid material derived primarily from plants that died and were buried hundreds of millions of years ago and were subsequently subjected to high temperatures and pressures.
    • Coal is classified into four main types, depending on the types and amounts of carbon present and on the amount of heat energy the coal can produce, including anthracite, bituminous, subbituminous, and lignite.
    • The processing of coal is described as well as the environmental and health impacts of its use.
    • Research is underway and various approaches are implemented (clean coal technology, use of low sulfur coal, use of scrubbers etc.) to reduce the environmental impacts of coal use.

    Contributors and Attributions

    Libretext: Introduction to Environmental Science (Zendher et al.)

    Libretext: General Chemistry (Petrucci et al.)

    Anne Marie Helmenstine, Ph.D. “What Is a Combustion Reaction?” ThoughtCo, www.thoughtco.com/combustion-reactions-604030.

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