[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.)

[Arkcon]

You want incomplete combustion.  That happens all the time, just deprive the system of some of the oxygen it needs.  Often we try to avoid incomplete combustion.  The carbon produced is finely divided, and blows out as soot, which is undesirable, unless you pine for Dickensian London scenes.  If the soot isn't finely divided, how will you get the lumps of carbon out of the reaction chamber.  Sometimes we like to use incomplete combustion, to make carbon monoxide, which is a useful reducer for iron ore to iron.  But we don't want that where animals are around, or they'll die.   All this ignores of course, you pay the same amount for the fuel, you just get less energy out.  No one advertises an engine on the "costs the same to run, produces less energy" theme.

[Jorge Stolfi]

Thanks for the reply.  

Can one get soot from methane? (Found no refs to that, only to the syngas [edit: and methanol] outcomes)  If so, that may be an answer: separating the soot from the H₂O would not be hard and should not consume much energy.  Soot seems to be an ideal carbon sink since it is inert and lasts forever.

As for the fuel price, methane is often an undesired byproduct of oil extraction and must be pumped back into the ground. (Flaring it, as was commonly done, emits CO₂ without any benefit.) So getting even part of its chemical energy converted into electricity without CO₂ emission may be profitable.

[Arkcon]

Well, methane is a useful fuel source, but the engineering costs of piping it about around is what makes it uneconomical, and underused, or wasted by flaming it off.  One use for it is being fed to methane using bacteria to produce single-cell protein for animal feed.  That application doesn't address the carbon footprint at all, but it just goes to show ... we could use methane, but we don't, because it costs too much, to use, from an engineering standpoint.  I'm sure there's pilot plants that use methane, or even local uses that are very important to the local economy, but its not universal, because the engineering costs just reduce viability.

Note:  you haven't addressed a fundamental.  You've picked an underused fuel source, and decided to offset carbon dioxide emission by simply wasting some, to produce more inert material -- carbon, that you now have to dispose of.  Just because carbon is fairly nonreactive, doesn't mean that a municipality wants it lying around, or buried.  The rules are the same, garbage is garbage, and you have to pay.  I'd bet anything, if you produced 10 million metric tons of gem quality diamonds in a big heap, once every passerby grabbed a handful to make jewelry, some pencil pusher will show up and say, "You going to take care of this eye-sore?"

Furthermore, the more fuel you use, and find an application for, if its a profitable app, then the more demand there is.  And that results in more consumption, to release more greenhouse gasses.  I suppose, there's the benefit of reducing methane release (itself a greenhouse gas), and sparing other fossil fuel use.  But look at it this way.  You're putting a great deal of effort, for a moderate benefit.  You're adding local costs, to benefit the whole world.  Engineering is often about constraining the possible chemistry to real world economical limitations.  You seem to be ignoring that.

[Jorge Stolfi]

Sorry, I wasn't clear enough.

I was thinking of thermoelectric plants.  These can be located far from people and perhaps closer to the methane producers; so disposing of the solid carbon in a landfill would not be a problem.

Brazil imports natural gas from Bolivia via pipeline to fuel its backup thermoelectric plants.  The decision was more political than technical/economical but the cost of the generated electricity, while higher than the national hydro, is still reasonable.  For Brazil natural gas is a nuisance since oil is plentiful, sells for a higher price, and is much easier to transport. But for Bolivia the gas is quite valuable since it is pretty much the only fossil energy source they've got.  Surely that is the case for other countries as well.

The pressure to reduce CO₂ emissions while energy demand increases is going to be a tough problem for  all countries that depend on fossil fuels.  If partial CH₄ combustion were technically viable, it would be a way out. Since it would emit no greenhouse gases (just water), it could still provide as much energy as needed, whereas oil and coal will not.

Anyway, thanks again for the comments.

[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.

[Arkcon]

To make an absolutely correct tally for the second case, you should also calculate CO +1/2 O₂ CO₂, I don't know if that produces or consumes energy.  Also, why do you add the reaction: H₂ +1/2 O₂ H₂O?  That isn't a part of your first reaction, and is cheating, a bit.  The hydrogen water reaction is highly energetic, but producing syngas (a mixture of CO and H₂) consumes energy.

http://en.wikipedia.org/wiki/Syngas

[Jorge Stolfi]

If my numbers are correct, we can deduce
CO + 0.5 O₂    CO₂ + 285 kJ
Indeed CO is flammable (and the blue color in gas flames is said to be due
to the above reaction.) 

As for the hydrogen burning, I added it for comparison only.  Note that my proposed reaction is formally equivalent to splitting CH₄ into C + 2H₂ and then burning the 2H₂.

[Arkcon]

Good, so the second group wasn't a tally for comparison, but some others to compare individually, OK, got it now.  So, what's next?

[Jorge Stolfi]

Next? Well, perhaps inspire some chemical engineer to invent a suitable process. 

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

Have fun...

[vmelkon]

It is an interesting proposal but wouldn't the bottom of the boiler be covered in carbon black and some heavy hydrocarbons? They would act as a insulator and require scrubbing constantly.

Another approach would be if you could somehow bind the methane to itself and produce hydrogen.

CH4 + CH4 => C2H6 + H2
and furthermore
C2H6 + CH4 => C3H8 + H2
and you keep going until you produce a heavy oil that can be stored underground.

The hydrogen would be used as the fuel.
It is probably not doable in reality.

[fledarmus]

There was a lot of effort in that field in the late 80's and early 90's. Mostly they were trying to oxidatively couple methane to form ethylene. Although the reaction could be demonstrated and is an exothermic reaction at high temperatures, as far as I know, nobody has been able to develop the perfect catalyst that would make this a commercially feasible reaction. I haven't seen much about it recently.

[Enthalpy]

Indeed, the solution is not to burn partially your methane, as this produces monoxide and hydrogen rather than vapour and soot, but first decompose the methane, and burn the hydrogen separately.

Decomposition can be done just by elevated temperature and needs little energy as compared to the combustion. Methane converts to heavier molecules which can be coked up to heavy tar, containing very little hydrogen (up to graphite would be a costly exaggeration). I'd even stop the process when the carbon-rich thing can still be washed away.

Be aware that polyaromatic compounds will form, and these are unhealthy. Graphite would improve that.

You already cited the main drawback with the proper figures (a further refinement distinguishes whether water exits as vapour or condensates at the boiler, telling a "upper" and "lower" calorific power): the energy of CO₂ isn't tapped then. That's why people consider instead to sequester the CO₂ produced at power stations, like: liquefy if to inject it in an old oil well, or react it with a silicate (=sand) to get silica and a solid carbonate. That's a current research and experiment topic in Germany, Australia and more.

CO₂ sequestration looks perfectly reasonable and cheap as compared with wind/Solar/nuclear electricity but it adds a significant cost to the ultra-cheap coal and gas energy, so most companies make strong efforts not to implement it.

[Jorge Stolfi]

Thanks for the reply, it is quite intriguing!   Extracting the carbon as heavier hydrocarbons instead of soot adds a potential economic bonus (plastics, asphalt, &c).

(Sorry for taking so long to reply.  I have been away from this forum for "a few" years and only saw your post today...)