Methanation is the conversion of carbon monoxide and carbon dioxide (CO
x) to methane (CH
4) through
hydrogenation
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or Saturated ...
. The methanation reactions of CO
x were first discovered by
Sabatier and
Senderens in 1902.
CO
''x'' methanation has many practical applications. It is a means of carbon oxide removal from process gases and is also being discussed as an alternative to
PROX
PROX is an acronym for PReferential OXidation, that refers to the preferential oxidation of carbon monoxide in a gas mixture by a catalyst. It is intended to remove trace amounts of CO from H2/CO/CO2 mixtures produced by steam reforming and water- ...
in fuel processors for mobile
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
applications.
Methanation as a means of producing
synthetic natural gas has been considered since the 1970s.
More recently it has been considered as a way to store energy produced from solar or wind power using
power-to-gas systems in conjunction with existing
natural gas storage.
Chemical reactions
The following reactions describe the methanation of carbon monoxide and carbon dioxide respectively:
:
CO + 3H2 -> CH4 + H2O -206 kJ/mol
:
CO2 + 4H2 -> CH4 + 2 H2O -164 kJ/mol
The methanation reactions are classified as
exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
and their energy of formations are listed.
There is disagreement on whether the CO
2 methanation occurs by first associatively adsorbing an
adatom
An adatom is an atom that lies on a crystal surface, and can be thought of as the opposite of a surface vacancy. This term is used in surface chemistry and epitaxy, when describing single atoms lying on surfaces and surface roughness. The word ...
hydrogen and forming oxygen intermediates before hydrogenation or dissociating and forming a carbonyl before being hydrogenated.
CO is believed to be methanated through a dissociative mechanism where the carbon-oxygen bond is broken before hydrogenation with an associative mechanism only being observed at high H
2 concentrations.
Methanation reaction over different carried metal
catalysts
Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
including Ni, Ru and Rh has been widely investigated for the production of CH
4 from
syngas
Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as ...
and other power to gas initiatives.
Nickel is the most widely used catalyst due to its high selectivity and low cost.
Industrial applications
Creation of synthetic natural gas
Methanation is an important step in the creation of synthetic or
substitute natural gas (SNG). Coal or wood undergo gasification which creates a producer gas that must undergo methanation in order to produce a usable gas that just needs to undergo a final purification step.
The first commercial synthetic gas plant opened in 1984 and is the
Great Plains Synfuel plant in Beulah, North Dakota.
It is still operational and produces 1500 MW worth of SNG using coal as the carbon source. In the years since its opening, other commercial facilities have been opened using other carbon sources such as wood chips.
In France, the AFUL Chantrerie, located in Nantes, started in November 2017 the demonstrator MINERVE. This methanation unit of 14 Nm3/day was carried out by Top Industrie, with the support of Leaf. This installation is used to feed a CNG station and to inject methane into the natural gas boiler.
Ammonia synthesis
In
ammonia production CO and CO
2 are considered
poisons
A poison is any chemical substance that is harmful or lethal to living organisms. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figurati ...
to most commonly used catalysts.
[{{Cite journal, last=Khorsand, first=Kayvan, year=2007, title=Modeling and simulation of methanation catalytic reactor in ammonia unit, url=https://www.researchgate.net/publication/26498529, journal=Petroleum & Coal, volume=49, pages=46–53] Methanation catalysts are added after several hydrogen producing steps to prevent
carbon oxide buildup in the ammonia synthesis loop as methane does not have similar adverse effects on ammonia synthesis rates.
See also
*
Biological methanation
*
Biomethane
Renewable natural gas (RNG), also known as biomethane, is a renewable fuel made from biogas that has been upgraded to a quality similar to fossil fuel, fossil natural gas and has a methane concentration of 90% or greater. By removing carbon diox ...
*
Hydrogen economy
The hydrogen economy is an umbrella term for the roles hydrogen can play alongside low-carbon electricity to reduce emissions of greenhouse gases. The aim is to reduce emissions where cheaper and more energy-efficient clean solutions are not ava ...
*
Renewable natural gas
*
Sabatier reaction
References
Organic reactions
Methane
Chemical processes
Hydrogenation