Reverse Water-Gas Shift Reaction
Reverse Water-Gas Shift Reaction
Description
Process
Uses
Advantages
Disadvantages
References
Description
The RWGS reaction was discovered in the 19th century as a method of producing water from carbon dioxide and hydrogen, with carbon monoxide as a side product. In the context of human missions to Mars, it has been proposed as a complement to the Sabatier/water electrolysis (SE) process to produce methane and oxygen from hydrogen and carbon dioxide on the surface. Alternatively, it can be used with water electrolysis to generate carbon monoxide and oxygen. The oxygen is used for breathing or as oxidizer, while the carbon monoxide can be used as a moderate specific-impulse fuel (with oxygen as the oxidizer) or as a feedstock to generate higher hydrocarbons (see Fischer-Tropsch reactions)
Process
CO2 + H2 → CO + H2O
The reactor itself is very similar to a Sabatier unit; a simple steel pipe filled with catalyst. According to experiments done by Pioneer Astronautics in Lakewood, Colorado, the best catalyst for this reaction is
Uses
The RWGS reaction’s chief attribute is that it, when used alongside water electrolysis, can generate any amount of oxygen from the equivalent amount of carbon dioxide with only a tiny amount of hydrogen. The hydrogen is recovered from the water via electrolysis and recycled back into the reactor’s feed end. When used with the Sabatier and water electrolysis reactions, the RWGS can provide an oxidizer/fuel (O/F) ratio of 3.5:1 (3.5 units of oxygen to 1 unit of methane) compared with 2:1 for the SE process alone.
Advantages
For the purposes of CO2 separation, the RWGS is far more efficient and requires a fraction of the power, compared to solid-oxide or molten carbonate electrolysis.
Disadvantages
This reaction has an equilibrium constant of 0.1 even at temperatures of 400 C or above, so it must be fed with either a hydrogen-rich or a carbon dioxide-rich mixture to ensure satisfactory results. Excess hydrogen (or excess carbon dioxide) is captured from the exhaust with a filtering membrane and fed back into the reactor.
References
R. Zubrin, The Case for Mars, pp. 153
