International Space Station Life Support
Currently, oxygen generators on board 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.) 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:
- 2H2O → O2 + 2H2 → (respiration) → CO2 + 2H2 + 2H2 (added) → 2H2O + CH4 (discarded)
The loop could be completely closed if the waste methane was pyrolyzed into its component parts:
- CH4 + heat → C + 2H2
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.
Read more about this topic: Sabatier Reaction
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