Methane Fuel Production Figure 5 shows a basic Sabatier reactor plant, utilizing a terrestrial hydrogen source to produce methane (e.g. as discussed by Baker and Zubrin [5]). Once this source is completely converted to methane, no further oxygen can be produced. The stoichiometric O/F mass ratio produced is 2:1. An advantage is that all the hydrogen can be quickly converted to methane and water (for later electrolysis), recycling the hydrogen to extinction at a later time, thus eliminating some storage problems. If the liquid hydrogen is well insulated and used in a steady state process, it has adequate cooling capacity to liquefy the product O2 and CH4. While the expanding H2 gas has the capacity to do a modest amount of work in steady state, it could be a significant source of power in a plant operated by quickly converting all H2 to CH4 and H2O, with later electrolysis of the initial product water to extinction. This mode of operation could have significant benefits for a manned mission if properly integrated. It is appropriate to note that because the Sabatier reaction is irreversible, it could be carried out at ambient pressure, using the work available in the hydrogen to run an inductor. However, compression is necessary before the water can be removed from the product stream, and the most convenient place to compress the gas is at the CO2 inlet. The plant of Figure 6 is the same as in Figure 5, except a CO2 electrolysis cell with conversion efficiency of 1 has been included. Hence, we can obtain the desired O/F ratio of 3.5:1 by putting CO to waste. There is no reason to process the CO further, since this only leads to a lower thermodynamic efficiency than a unit based on water electrolysis only. Here, once the hydrogen runs out, we can continue to produce oxygen from the atmosphere in the ceramic membrane cell. The ceramic membrane cell in a Sabatier reaction based plant should be operated to produce oxygen at storage pressure regardless of inlet conditions. The electrical power requirements for this plant are slightly less than for that of Figure 5. Figure 7 shows the process of Figure 6 with a conversion efficiency of 0.25, and CO disproportionation included to regenerate CO2. The plant in Figure 8 is the same as in Figure 5, except here a higher pressure is maintained in the plant. This has the effect of eliminating several pumps for compression of finished products, and reducing the overall size of the plant, since less gas volume is required. In a Sabatier reaction based process it is reasonable to compress the incoming Martian atmosphere stream to the CH4 storage pressure. Higher pressure favors products, and also increases the operating temperature of the condenser, substantially reducing radiator requirements. Methane would pass through the condenser directly to liquefaction and storage. Oxygen is produced in a liquid water electrolyzer, where it can be produced at the desired oxygen storage pressure without regards to the Sabatier reactor pressure.
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