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 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 and then quickly cooled to produce carbon and carbon dioxide, also known as the Boudouard Reaction.:

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 530 and 730 ºC, 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. The methane could be decomposed to carbon and hydrogen (see previous section), or used for the production of other hydrocarbons.

The 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. The carbon monoxide could be reduced to carbon and carbon dioxide (see previous section), returning the carbon dioxide to the cell for further reduction.

A number of processes utilizing carbon monoxide as a reducing agent have been proposed for lunar use. These processes would consume carbon monoxide and produce carbon dioxide. A direct electrolysis system could be used in this case on the produced carbon dioxide, with the carbon monoxide recirculated back into the system rather than reduced further.

Biological Reduction

Carbon could be produced by heating organic material in the absence of oxygen to produce charcoal. This would require some method of removing the ash which would inevitably be present.

Growing plants specifically to produce carbon in this fashion would probably be more energy intensive than other methods. However, processing of organic waste products into carbon presents an attractive recycling mechanism, as it can be utilized on the non-edible parts of food plants and even human feces. This process would most likely be carried out in conjunction with other carbon production methods, as the human population would need to be quite high for it to supply all the carbon.