Lunar Carbon Production

Introduction

Lunar carbon is found in trace amounts in the lunar regolith, where it can be extracted by heating (see Volatiles). This process results in a number of carbon compounds, chiefly carbon monoxide (CO), carbon dioxide (CO₂), and methane (CH₄). It is desirable to produce elemental carbon from these feedstocks for production of lunar steel, as well as various other uses. In addition, processes to reduce these substances to elemental carbon would be necessary in order to recycle carbon consumed in various industrial processes. A number of methods have been proposed for this.

Carbon Monoxide Reduction

Carbon monoxide can be subjected to temperatures of around 700°C to produce carbon and carbon dioxide, a reaction that occurs in sooty chimneys.:

2 CO ==> C + CO₂

This would recover half the carbon present in the gas. Further reduction of the carbon dioxide would be required to obtain the rest.

Methane Reduction

Methane may be useful on its own as a feedstock for producing various hydrocarbons. If elemental carbon is desired, it can be subjected to thermal decomposition at high temperatures, producing hydrogen as a byproduct:
CH₄ ==> C + 2 H₂

Production of carbon and hydrogen in this manner has been tested with various catalysts. All had issues with carbon deposition fouling the catalyst surface. Uncatalyzed production seems to require temperatures significantly greater than 900°C .

Uncatalyzed production has the advantage that any vessel capable of holding and heating the methane could be used as a reactor, even a simple pipe, which could be periodically subjected to an auger to remove deposited carbon.

Carbon Dioxide Reduction

Bosch Reaction

In the Bosch Reaction, carbon dioxide is reacted with hydrogen in the presence of an iron catalyst at temperatures between 530º and 730º C, producing carbon and water in a slightly exothermic process. The water is split, recovering the hydrogen and producing oxygen.

CO₂ + 2 H₂ ==> C + 2 H₂O (Bosch Reaction)
2 H₂O ==> O₂ + 2 H₂ (Water Splitting)
Net Reaction: CO₂ ==> C + O₂

This possesses the same disadvantage as low temperature methane decomposition, namely that the produced carbon builds up on the catalyst surface, reducing the efficiency. A combination of continuous mechanical scraping and large catalyst surfaces could make the reaction useable.

The bosch reaction is a subject of current research for space based carbon dioxide reduction

Sabatier Reaction

Another way to produce carbon from carbon dioxide is by use of the Sabatier reaction, which again involves reacting carbon dioxide with hydrogen, this time in the presence of a nickel catalyst. This process produces water and methane as reaction products:

CO₂ + 4 H₂ ==> CH₄ + 2 H₂O

The water is split to recover hydrogen and oxygen, as in application of the Bosch reaction, and the methane is decomposed to carbon and hydrogen (see previous section).

This Sabatier Reaction is currently utilized on board the International Space Station, except that the methane produced is dumped overboard.

Direct Co2 Electrolysis

Another option is to directly electrolyze carbon dioxide, resulting in oxygen and carbon monoxide.
2 CO₂ ==> 2 CO + O₂

An appropriate membrane could be utilized to separate the oxygen, and the carbon monoxide could be reduced to carbon and carbon dioxide (see previous section). The carbon dioxide would be returned to the cell to be electrolyzed again.

Biological Reduction

Carbon could be produced from carbon dioxide by plant growth, followed by heating in the absence of oxygen to produce charcoal. This method is especially attractive if food plants are used, utilizing the non-edible portions of said plants for carbon production. This process would result in a fair amount of ash present in the resulting carbon, and would require more input energy than the other processes mentioned, though possibly at a lower technological and maintenance level. The demand for food products would probably need to be quite high compared to the demand for elemental carbon for this approach to be used as a primary carbon source.