PARTIAL oxidation of methane as energy source

[Jorge Stolfi]

Methane from natural gas could be a great source of energy (eg. in thermoelectric plants), but the need to curb CO₂ emissions stands in the way.

I wonder whether one could get useful energy from some partial combustion reaction of CH₄, that yields H₂O plus some carbon-containing solid that could be safely buried or stored. While that would fail to recover the full energy content of methane, it might be better than nothing.

The question has two parts: (1) find a reaction that is theoretically possible, exothermic, and carbon-trapping; and (2) figure out whether it could be achieved in practice.

The simplest answer to (1) may be

CH₄ + O₂ → C + 2H₂O​

I suppose that it would be exothermic, but not very much. Correct?

The solid C-containing product could also be any of a large number of C_xH_yO_z compounds, such as oxalic acid C₂H₂O₄, mellitic anhydride C₁₂O₉, ... With these partly oxidized end-products one may perhaps recover a large fraction of the heat yielded by full combustion.

Part (2) of the question seems harder, as the carbon seems easier to oxidize than the hydrogens. It seems that attempt at partial combustion with 1:1 methane oxigen ratio usually gives syngas:

CH₄ + O₂ → CO + H₂O + H₂​

However it seems that at high pressure the reaction is shifted to

CH₄ + 0.5O₂ → H₃COH​

This reaction seems to be a commercially interesting synthesis route for methanol, but could it be also a useful source of heat? (Note that additional processing would be needed to convert the methanol into a safely disposable solid.)

 

[uby]

Kinetics plays the pivotal role in such schemes. You'd need to find a selective catalyst to promote the desired reaction under conditions that suppress other reactions. As you note, carbon affinity for oxygen is much higher than hydrogen - so it will be difficult to suppress CO/CO2 formation completely. While higher pressures might be useful in shifting thermodynamic equilibrium, you'll still need to fight against kinetics.

As for whether it can be a useful source of heat: doubtful. As you said, these reactions are not particularly exothermic.

 

[Jorge Stolfi]

Thanks for the comment. The kinetics is a concern for part (2); as for part (1), what is your guess about the fraction of energy that could be obtained? (My guess for C(s) + 2H₂O is around 30% of CO₂ + 2H₂O; unfortunately I do not know enough chemistry to compute it.)

What about, say, 2CH₄ + 3.5O₂ → 3H₂O + C₂H₂O₄ (oxalic acid)? Note that stoichometrically this is almost a complete burn...

 

[Jorge Stolfi]

If I computed correctly (please check, I am not a chemist),

CH₄ + O₂ → C + 2H₂O + 434 kJ​

Compare to

CH₄ + 2O₂ → CO₂ + 2H₂O + 871 kJ​

C + 0.5 O₂ → CO + 110 kJ​

C + O₂ → CO₂ + 395 kJ​

2H2 + O₂ → 2H₂O + 572 kJ​

All energy values are per mol of reaction, at standard conditions. Thus the hypothetical semi-combution of methane would give about half as much energy as the full combustion.

 

[Jorge Stolfi]

It seems that methane will partly decompose to C + 2H₂ at 1500-2500K; but other byproducts are generated as well, and it may not be easy to recover the energy used to heat it up to that temperature.

Methane can be cleanly decomposed to carbon black and hydrogen by radiofrequency heating. The process is probably too inefficient energywise, although it is commercially used for the production of high-quality carbon black:

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01420624

 

[uby]

It seems that methane will partly decompose to C + 2H₂ at 1500-2500K; but other byproducts are generated as well, and it may not be easy to recover the energy used to heat it up to that temperature.

Methane can be cleanly decomposed to carbon black and hydrogen by radiofrequency heating. The process is probably too inefficient energywise, although it is commercially used for the production of high-quality carbon black:

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01420624

I don't have any numbers on hand, but in my experience you can crack methane with a decent yield at less than 1000K in an inert atmosphere without catalysts. This would place an upper temperature limit for your process that might limit the efficiency of any cycle designed to recover usable heat from the process.