by Humans have been burning coal for thousands of years. Since the Industrial Revolution, coal has become a major source of electricity and global warming. But where does carbon come from? By studying how coal is formed, scientists can learn both about the deep past and what to expect when different coals are burned.
Coal is formed when bog plants are buried, compressed, and heated to become sedimentary rock in a process called carbonization. “Very basically, coal is fossilized plants,” James Hower (opens in new tab), a petrologist at the University of Kentucky, told Live Science. The creation of these plants fossils it involves “a lot of geological accidents,” he said.
Carbon formation begins with living plants. “When the tree is still alive, it can be damaged by burning or it can be attacked by insects,” Hauer said. “All these things will show up in the carbon record.” Traces of pollen, leaves, roots and even bugs in carbon, Hower said, can be used to reconstruction of ancient ecosystems. Fire damage (opens in new tab)for example, it gives evidence of ancient climates.
Then the plants die. “If carbon is retained at all, that tells you something about the overall environment,” Hower said. Plants on mountain slopes or in deserts are unlikely to become carbon because these environments are not conducive to peat formation.
“Of all the charcoal we see out there, a very, very high percentage came from swamps,” Hauer said.
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This is because when plants die in wetlands, they are covered by water and protected from oxygen. As a result, they do not decompose as quickly as in dry soil. Instead, the plants pile up in layers of peat on the soggy bog floor. This peat, which is sometimes a precursor to carbon, has a long history of its own: it is home to insects, fungi, bacteria and even burrowing tree roots. help break (opens in new tab) plants in a process called peating. “Any layer we see in a coal could be the product of tens or hundreds or thousands of years,” Hauer said.
Minerals that enter the peat from water or that are formed through chemical reactions are also sequestered in carbon. Fire clay coal (opens in new tab) In eastern Kentucky, Hauer said, it contains elements of rare earths from a volcanic eruption millions of years ago. the US Department of Energy is now funding (opens in new tab) technologies to extract these elements from coal waste for use in solar panels, windmills and batteries.
But minerals in coal also cause problems. Peat exposed to seawater, for example, often contains (opens in new tab) more sulphur. Burning coal with sulfur has additional human costs. while coal mining and coal smoke breathing are both generally dangerous (opens in new tab)coals with a high sulfur content may be more likely to spontaneously combusting (opens in new tab) in the mines and may also be associated with (opens in new tab) heart disease.
Not all peat is converted to carbon. some corrode or dry out. For the carbonization process to begin, the peat must be covered by something inorganic, such as silt from a wide river delta. “The river that goes back and forth for millions of years ends up being your depositional system,” Hauer said, referring to layers of accumulated sediment.
Over geologic time, peat becomes even more buried. Mountains erode and fill river valleys. forests grow on top. Within millions of years, new mountains rise. During these millennia, peat breaks down and gradually turns into carbon thanks to two factors: pressure and heat (opens in new tab). Most coal is between 60 and 300 million years old.
The pressure makes the peat more compact. Heat reorganizes (opens in new tab) the recognizable molecules in plants — such as carbohydrates or cellulose — and publications (opens in new tab) oxygen and hydrogen, leaving carbon and other elements behind.
Coal buried very deep experiences higher temperatures because it is closer to the Earth’s core. But geothermal heat (opens in new tab) they can also come to the surface of the Earth through volcanoes, hot springs and geysers. The amount of pressure and heat generally determines the coal’s rank: a measure of how far the coal has progressed on its journey from soggy peat to solid rock.
Lignite is the lowest grade of coal. Lignites and subbituminous coals still contain recognizable plant parts. Bituminous and sub-bituminous coals have been compressed and heated until they become hard. Anthracite coal, the rarest and highest grade, is smooth and shiny. it has been heated to a liquid in a process called metamorphism. To get to the anthracite rank, Hauer said, all you need to do is heat up for a while — even an hour will do the trick.
Charcoal burns without producing soot. they were historically used by coal-powered ships trying to avoid detection (opens in new tab) in time of war. Lignite and bituminous coal are mainly used to generate electricity. Release of lignite and sub-bituminous coal a little bit more (opens in new tab) carbon dioxide from bituminous coal when burned.
These differences are small, however, when coal is compared to other sources of electricity that have less impact on global warming. In general, coal produces twice as much carbon dioxide per kilowatt-hour as natural gas and 90 times more than wind energy, US Department of Energy (opens in new tab).
“Emissions from coal and coal-related industrial processes have obviously not been good for the climate,” Hauer said. “This is the reality we live in.”
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