I just want to refer to a TV presentation by dr. Marco Alverà, where he described a new process to synthetize methane from spare CO2 and hydrogen from water electrolysis.
The final methane product could be burned without producing other CO2 than the employed one, so no new CO2 would be produced.
My addition now is that there is no need of energy for water electrolysis, because hydrogen could be produced by methane pyrolysis.
The path would be:
-pyrolize methane ai 1000 C to hydrogen and carbon.
-Reject (or use elsewhere) carbon.
-Burn about 20% of hydrogen to produce pyrolysis heat
-Recover spare CO2 from atmosphere
-Combine CO2 and Hydrogen to methane by the Sabatier synthesys
-There would be no need of water electrolysys, no energy consumption, no need of power stations or alternative energy sources.
-Final methane product could be burned by producing the same CO2 used to produce it
-About half of the heating value of the consumed methane would be lost. (this is the price to pay)
-About 20 % of the hydrogen from cracking would be consumed for methane cracking
The Sabatier reaction needs 4 hydrogen molecules to give one molecule of methane:
CO2 + 4 H2 -> CH4 + 2 H2O
To get these four molecules of hydrogen by methane pyrolysis, we need to crack two methane molecules. Therefore, the process you propose needs more than twice the methane it produces. The CO2 recovered to produce the methane will be emitted again when the methane is burnt. Where is the advantage?
That could be interesting if the methane source was biogenic, such as biomethane produced out of biogas or landfill gas. In that case you wouldn't be using fossil fuels to produce the hydrogen you need for the sabatier reaction, and when you burned the synthetic natural gas it would have equivalent emissions of greenhouse gases of 0 because that CO2 is kept in the short carbon cycle (~100 years) of the atmosphere.
However this would limit the scale of your production - you just don't have that much methane in a water treatment station (~250 Nm3/hr) or landfill to make that much of a dent in global consumption of natural gas you're trying to substitute.
Also, you can't forget that you lose 10% of the CO2 during CO2 capture using conventional capture technologies (amine scrubbing), and that DAC packs a huge energy consumption and costs to operate (~200 $ / tonne CO2).
Finally, methane pyrolisis has limited maturity (TRL < 8) compared to electrolysis. So it seems more reasonable to me to just invest in the typical green H2 + CO2 combination to produce synthetic natural gas rather than producing methane out of methane.
Let us use stoichiometry.
As a first step, 2 moles of methane are pyrolized to carbon and 4 moles of hydrogen: 2CH4=2C(wasted)+4H2
In a second step, all the hydrogen is reacted to atmospheric CO2 (Sabatier): 4H2+CO2=CH4+2H2O
------------------------------------------------------------------------------ 2CH4(fossil)+CO2=2C(wasted)+CH4(fresh)+2H2O
As a whole, from 2 moles of fossil methane we get 2 moles of carbon and 1 mole of clean methane, whose combustion would produce the same CO2 used to produce it, without pollution with new CO2.
2 moles of methane to be used to produce one. It seems a waste of fuel..
But how much methane should we burn to produce the energy for electrolysing water to hydrogen ? (Some are hypotizing it: make hydrogen by electrolysis, then combine it with CO2 in the Sabatier reaction to make climatically neutral methane)
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