The Sabatier reaction or Sabatier process involves the reaction of hydrogen with carbon dioxide at elevated temperatures and pressures in the presence of a nickel catalyst to produce methane and water. Optionally ruthenium on alumina (aluminum oxide) makes a more efficient catalyst. It is described by the following reaction:
It was discovered by the French chemist Paul Sabatier.
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Currently, oxygen generators onboard the International Space Station produce oxygen from water using electrolysis and dump the hydrogen produced overboard. As astronauts consume oxygen, carbon dioxide is produced which must then be removed from the air and discarded as well. This approach requires copious amounts of water to be regularly transported to the space station for oxygen generation in addition to that used for human consumption, hygiene, and other uses—a luxury that will not be available to future long duration missions beyond low Earth orbit.
NASA is currently investigating the use of the Sabatier reaction to recover water from exhaled carbon dioxide, for use on the International Space Station and future missions. (In April 2010, Sabatier hardware was delivered to the International Space Station on the STS-131 shuttle mission.)[1] The other resulting chemical, methane, would most likely be dumped overboard. As half of the input hydrogen becomes wasted as methane, additional hydrogen would need to be supplied from Earth to make up the difference. However, this creates a nearly closed cycle between water, oxygen, and carbon dioxide which only requires a relatively modest amount of imported hydrogen to maintain.
Ignoring other results of respiration, this cycle would look like:
The loop could be completely closed if the waste methane was pyrolyzed into its component parts:
The released hydrogen would then be recycled back into the Sabatier reactor, leaving an easily removed deposit of pyrolytic graphite. The reactor would be little more than a steel pipe, and could be periodically serviced by an astronaut where the deposit is chiselled out.
The Bosch reaction is also being investigated for this purpose. Though the Bosch reaction would present a completely closed hydrogen and oxygen cycle which only produces atomic carbon as waste, difficulties maintaining its higher required temperature and properly handling carbon deposits mean significantly more research will be required before a Bosch reactor could become a reality. One problem is that the production of elemental carbon tends to foul the catalyst's surface, which is detrimental to the reaction's efficiency.
The Sabatier reaction has been proposed as a key step in reducing the cost of manned exploration of Mars (Mars Direct) through In-Situ Resource Utilization. Hydrogen is combined with CO2 from the atmosphere, with methane then becoming a storable fuel and the water side product yielding oxygen to be liquefied for the oxidizer and hydrogen to be recycled back into the reactor. The original amount of hydrogen could be transported from Earth or separated from martian sources of water.[2]
The stoichiometric ratio of oxidizer and fuel is 3.5:1, for an oxygen:methane engine, however one pass through the Sabatier reactor produces a ratio of only 2:1. More oxygen may be produced by running the water gas shift reaction in reverse. When the water is split, the extra oxygen needed is obtained.
Another option is to make more methane than needed and pyrolyze the excess of it into carbon and hydrogen (see above section) where the hydrogen is recycled back into the reactor to produce further methane and water. In an automated system, the carbon deposit may be removed by blasting with hot Martian CO2, oxidizing the carbon into carbon monoxide, which is vented.
A fourth solution to the stoichiometry problem would be to combine the Sabatier reaction with the reverse water gas-shift reaction in a single reactor as follows:
3CO2 + 6H2 → CH4 + 2CO + 4H2O
This reaction is slightly exothermic, and when the water is electrolyzed, an oxygen to methane ratio of 4:1 is obtained, resulting in a large backup supply of oxygen. With only the light hydrogen transported from Earth, and the heavy oxygen and carbon extracted locally, a mass leveraging of 18:1 is afforded with this scheme. This in-situ resource utilization would result in massive weight and cost savings to any proposed manned Mars or sample return missions.
It has been proposed in a renewable energy dominated energy system to use the excess electricity generated by wind, solar photovoltaic, hydro, marine current, etc to make methane (natural gas).[3],[4] The methane can be injected into the existing gas network which in many countries has one or two years of gas storage capacity. The methane can then be used on demand to generate electricity (and heat - combined heat and power) overcoming low points of renewable energy production. The process is hydrolysis of water by electricity to create hydrogen (which can partly be used directly in fuel cells) and the addition of carbon dioxide CO2 (Sabatier process) to create methane. CO2 + 4H2 → CH4 + 2H2O The CO2 can be extracted from the air or fossil fuel waste gases by the amine process amongst many others. It is a low CO2 system and has similar efficiencies of today's energy system.