Methane

Methane is a simple hydrocarbon, resulting from decomposition and fermentation of organic substances, in the form of natural gas, swamp gas, and biogas.

Methane is used as fuel for heating devices, engines, fuel cells, and gas turbines that generate electrical energy, as well as in many industrial chemical processes. It has also been regarded both as a fossil fuel and as a renewable energy source. Although it shows high potential to become an environmentally safe motor fuel, given its cleaner exhaust and abundant occurrence, it is also a potent greenhouse gas (GHG) 25 times more powerful than carbon dioxide in effecting climate change.

At room temperature, methane (CH₄) is a colorless, odorless, highly flammable, and nontoxic gas. Liquid methane does not burn unless subjected to high pressure (normally 4 to 5 atmospheres). Under certain circumstances, methane reacts explosively with oxygen or chlorine. Containers of methane should be stored in well-ventilated places, kept away from ignition sources, and protected against electrostatic charge. Usually, methane is stored liquefied in gas containers at minus 160 degrees Celsius and under high pressure of 150 bars. These conditions increase its density enormously. Upon contact with methane in this form, frostbites can occur. Methane is both the simplest alkane and the simplest hydrocarbon, soluble in benzene, diethyl ether, and ethanol but only slightly soluble in acetone and water. At the molecular level, its carbon and hydrogen bonds form a tetrahedron.

History

Etymologically, the Greek methy denotes wine and hyle means wood and refers to the distillation of methyl alcohol from wood. Medieval alchemists knew methane as a component of fermentation gas, also called swamp air. In 1667, Thomas Shirley developed it first, and Joseph Priestley observed and described the formation of methane during decomposition in 1772. A few years later, between 1776 and 1778, it was isolated by Alessandro Volta. He noticed ascending gas bubbles when studying marshes around Lake Maggiore and started experimenting, inventing the prototype of a gas lighter, called a Volta pistol, and gas lamps. In 1856, Marcellin Berthelot first synthesized methane using carbon disulfide and hydrogen sulfide. Pyrolysis, in the form of wood gasification, is another method used to produce methane. During World War II, wood gas was used to fuel automobiles.

Chemistry

Combustion of methane releases much energy as heat. An optimal energy yield results only when sufficient oxygen is available. Insufficient oxygenation causes incomplete combustion, undesirable by-products such as carbon monoxide (CO), and carbon (carbon black), also yielding less effective energy. First methane forms HCHO (or H₂CO). This formaldehyde gives a formyl radical (HCO), also forming CO. The process is called oxidative pyrolysis:

CH₄ + O₂ g CO + H₂ + H₂O

Following oxidative pyrolysis, the H₂ oxidizes, forming H₂O and releasing heat:

2H₂ + O₂ g 2H₂O

Finally, the CO oxidizes, forming CO₂ and releasing more heat:

2CO + O₂ g 2CO₂

In summary, when methane burns, the products are carbon dioxide and water:

CH₄ + 2O₂ g CO₂ + 2H₂O + 891 kJ/mol (at standard conditions) gH_R = -802 kJ/mol

where kJ denotes kilojoules (J denotes Joules) and mol denotes moles.

Methane melts at minus 182.6 degrees Celsius and boils at minus 161.7 degrees Celsius. The standard molar entropy S⁰ is 188 J/mol·K (kelvin), and the heat capacity C is 35.69 J/mol·K. At 90.67 K and 0.117 bar, methane has a triple point of 190.56 K and a critical pressure of 45.96 bar. The heating value H_i is about 35.89 MJ·m^-3.

In addition to oxygenation, methane is very reactive with all halogens, such as fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), in a procedure called free-radical halogenation. The universal formula for the potential reactions is:

CH₄ + X₂ g CH₃X + HX

where X is a halogen. For example, with chlorine, methane forms chloromethane, dichloromethane, chloroform, and carbon tetrachloride. In step 1, radical generation, the required energy comes from ultraviolet (UV) radiation, or heating. In step 2, radical exchange, the following reactions occur:

CH4 + Cl· g CH3· + HCl + 14 kJ

CH3· + C12 g CH3Cl + Cl· + 100 kJ

In step 3, radical extermination, these reactions occur:

2 Cl· g C12 + 239 kJ

CH3· + Cl· g CH3Cl + 339 kJ

2 CH3· g CH3CH3 + 347 kJ

In addition to its widespread occurrence in nature, methane can be synthesized from carbon and hydrogen or by means of catalytic reaction from carbon monoxide and hydrogen, as follows:

CO + 3H2 g H2O + CH4

Occurrence

The major source of methane is geological deposits of natural gas known as gas fields. Gas at shallow levels (that is, at low pressure) is formed primarily by anaerobic decay of organic matter, called methanogenesis, and secondarily from reworked methane from deep beneath the Earth’s surface. In general, sediments buried deeper, and therefore under higher pressures and at higher temperatures, than those that form oil generate natural gas, which often is released during volcanic eruptions. Natural gas is a mixture of gases, including 75 to 98 percent methane, depending on its source, evolving from hermetically sealed, dead biomass under high pressure. Other components of natural gas can include ethane, propane, butane, pentane, and inert gases.

Although a part of natural gas, methane is not only a fossil fuel. In fact, it is continously produced during all organic fermentation and degradation processes, which occur in swamplands, paddy fields, landfills, and industrial livestock farms. Methane is produced by the gases of ruminants (such as cows), but also by the enteric fermentation of wild animals, during their decomposition of glucose into carbon dioxide and methane, as follows:

C₆H₁₂O₆ g 3 CH₄ + 3 CO₂

Sources differ in estimating methane expulsion, ranging between 60 and 100 kilograms of methane per year per animal for cattle. Globally, livestock are increasing (corresponding to a growing human population) at a rate that has made methane production from ruminants a factor relevant to the climate.

The transformation of the polymer substances takes place in a multilevel process, including hydrolysis (breaking down complex organic compounds into simple sugars, amino acids, and fatty acids, similar to the beginning of human digestion), acidogenesis (resulting in carbon dioxide, hydrogen, alcohol, fatty acids, and acetic acid), acetogenesis (producing more carbon dioxide and acetic acid), and finally methanogenesis.

This last stage is performed by bacteria that produce methane and water from carbon dioxide and hydrogen (CO₂ + 4 H₂ g CH₄ + H₂O) or transform acetic acid into methane and carbon dioxide:

(CH₃COOH g CH₄ + CO₂)

Biogas, as produced in biodigesters, consists of 50 to 75 percent methane, 20 to 45 percent carbon dioxide, and 5 percent hydrogen, nitrogen, sulfide, and helium. It is counted among the renewable energy sources, derived in a multilevel anaerobic bacterial decomposition of organic material, such as dung, slurry, sewage sludge, or municipal waste, at temperatures of 20 to 55 degrees Celsius.

Methane hydrates, which are icelike combinations of methane and water on the seabed formed at high pressure and low temperature, are considered a promising future source of methane. It is estimated that globally there are between 500 and 3,000 gigatons of carbon stored in methane hydrate. In comparison, the carbon content of the known coal reserves is about 900 gigatons. However, regardless of methane’s high potential, drilling for methane hydrate is very risky, because the continental slopes, consisting in large part of methane clathrates, could become unstable.

Also, global warming and a simultaneous warming of the oceans may cause melting and vaporization with a concomitant release of methane that could fortify and accelerate the greenhouse effect drastically. Fossil methane deposits are part of the natural methane cycle, and therefore it is necessary to keep the Earth at temperatures that allow the existence of life.

Uses

Methane is used as fuel for gas turbines or steam boilers to generate electrical power. Its heat of combustion is lower than that of other hydrocarbons, but it produces more heat per mass unit than more complex hydrocarbons, as follows:

heat of combustion = 891 kJ/mol molecular mass = 16.0 g/mol heat per mass = 55.7 kJ/g

As part of natural gas, methane is used in private homes for domestic heating and cooking, containing energy of 39 megajoules per cubic meter, or 1,000 British thermal units (Btu) per standard cubic foot. In the chemical industry, methane is used for the production of hydrogen, methanol, acetic acid, hydrocyanic acid, carbon disulfide, and many other organic compounds.

Greenhouse Gas

In addition to carbon dioxide, methane is one of the six major greenhouse gases (GHGs), those gases responsible for increasing the greenhouse effect relevant to climate change. However, methane has an even greater potential than carbon dioxide to effect climate changes: Its density is less than that of air, which is why it rises into higher atmospheric layers. The Earth’s atmospheric methane concentration has increased by about 150 percent since 1750, and it accounts for 20 percent of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases. In 2021, global atmospheric methane levels were measured at over 1,890 parts per billion, the highest level in the past 400,000 years. Historically, methane concentrations in the world’s atmosphere have ranged between 300 and 400 parts per billion during the glacial periods, commonly known as ice ages, and between 600 and 700 parts per billion during the warm interglacial periods.

Worldwide emissions of methane amount to about 600 million tons per year, of which about 60 percent are caused by human activities. In 2021, livestock farming caused about 32 percent of global methane emissions. One cow emits between 150 and 250 liters of methane per day, caused by microbial decay of cellulose in the digestive organs. Rice cultivation accounted for an 8 percent share of global methane emissions. Landfills and wastewater accounted for about 20 percent of methane emissions. More methane—nearly 25 percent of all emissions—is emitted into the atmosphere through the extraction and distribution of oil and gas. Arctic methane release from thawing permafrost regions is expected to increase in response to global warming. A leaking borehole at the bottom of the North Sea releases an estimated 11 million liters of methane every hour, of which about one-third gets into the atmosphere. Forest fires release chloromethane (CH₃Cl). Additional methane occurs as a by-product of oil refinement, as well as during coking and sulfurization of coal and during steel production from blast-furnace gases.

Researchers say about 30 percent of the increase in global temperatures since the Industrial Revolution is due to methane. Although it has a much shorter atmospheric lifetime than carbon dioxide--about twelve years vs. centuries--methane absorbs more energy in the atmosphere. It also has a negative impact on air quality because it can result in ground-level ozone.

Compared with carbon dioxide, methane’s global warming potential is 25 times higher, according to a study by the Intergovernmental Panel on Climate Change. This means that a methane emission will have 25 times the impact on global temperature that a carbon dioxide emission of the same mass has over the following 100 years. Photochemically produced hydroxyl radicals (OH) oxidize methane and lower its concentration in the atmosphere, as follows:

OH + CH₄ g CH₃- + H₂0

Between 2006 and 2009, concerns arose that plants could be huge emitters of methane into the atmosphere, accounting for as much as 30 percent. By now, however, it has been shown that trees compensate for GHG emissions; their methane emissions are small in comparison with their ability to store carbon in leaves, wood, and bark, thus acting as a carbon sink.

Scientists have also measured a large amount of methane in the air above the oceans and have considered cyanobacteria as the source of these emissions. Usually cyanobacteria eat phosphate molecules, but in times when nutrients are scarce they substitute phosphonates such as methylphosphonate, and release methane. On the other hand, in the Black Sea, scientists have found sulfate-reducing bacteria that transform methane in reaction with sulfate:

CH₄ + SO₄^2- g H₂S + CO₂

In waters and soils containing oxygen, methane is oxidized by Pseudomonia methanica bacteria to carbon dioxide and water. Research suggests aquatic ecosystems may emit up to half of global methane. Impacted aquatic ecosystems and freshwater ecosystems may emit more methane than natural and coastal ecosystems.

Health

Methane is an asphyxiant, causing temporary symptoms such as hyperventilation, increased heart rate, low blood pressure, numbness in the limbs, drowsiness, mental disorder, and memory loss—all resulting from the oxygen deficiency upon inhalation of methane in an enclosed space.

Methane-air mixtures can therefore become hazardous. When methane reaches 5 to 12 percent by volume, due to an unperceived escape of gas, explosions can result. Miners are familiar with this dangerous phenomenon in the form of fire damps; mine gas released through coal mining amounts to about 6 percent of anthropogenic methane emissions. Efforts are under way to capture mine gas and use it to generate electrical power.

In November 2021, President Joe Biden announced plans to impose new regulations on methane at the United Nations climate change summit. The regulations included placing new limits on the amount of methane emitted from US oil and gas rigs and restoring rules that had been ended during the Donald Trump administration that prevent methane leaks from oil and gas wells. Experts noted that the regulations would reduce methane emissions by 41 million tons between 2023 and 2035.

Bibliography

Demirbas, Ayhan. Methane Gas Hydrate. New York: Springer, 2010.

Friedman, Lisa. "Biden Administration Moves to Limit Methane, a Potent Greenhouse Gas." The New York Times, 5 Nov. 2021, www.nytimes.com/2021/11/02/climate/biden-methane-climate.html. Accessed 8 Aug. 2024.

"Global Methane Assessment Released." World Meteorological Organization, 7 May 2021, wmo.int/media/news/global-methane-assessment-released. Accessed 8 Aug. 2024.

"Methane and Climate Change." International Energy Agency, 2022, www.iea.org/reports/global-methane-tracker-2022/methane-and-climate-change. Accessed 2 Aug 2024.

"Methane Emissions Continue to Rise." NASA Earth Observatory, earthobservatory.nasa.gov/images/146978/methane-emissions-continue-to-rise. Accessed 8 Aug. 2024.

Rosentreter, Judith A., et al. "Half of Global Methane Emissions Come from Highly Variable Aquatic Ecosystem Sources." Nature Geoscience, vol. 14, 2021, pp. 225-239, doi.org/10.1038/s41561-021-00715-2. Accessed 2 Aug. 2024.