Right. My point is that, as with a battery, you'd need to store the product so you can reverse the reaction. Practically none of our current infrastructure for using CH4 is set up to store the output CO2 + H20; it just dumps it into the atmosphere.
If you want to reuse existing CH4 distribution pipelines, that's fine, but now you need to pipe back the CO2 to the plant that makes the CH4 in order to complete the cycle. That's much harder than pumping CH4 and would require a whole new parallel pipeline infrastructure that can transport highly pressurized CO2 back to the source plant. Alternatively, you could regenerate the CH4 on location, but then you need to pipe the energy in; in that case, you're not using CH4 to deliver energy at all.
You could make the CH4 from CO2 in coal or something, but then you're no longer carbon neutral (since that CO2 will end up in the atmosphere rather than the battery). You could also try carbon capture, but honestly thermodynamics suggests that that will never be a viable solution; far easier to order your coffee black than to unmix the cream from it.
My point is that, unless I'm misunderstanding something, existing distribution systems for natural gas are not useful for CH4-based battery storage.
> My point is that, as with a battery, you'd need to store the product so you can reverse the reaction.
No, you wouldn't, because CO2 is everywhere in the atmosphere. The methane making plant draws CO2 from the air around it; discharging batteries emit CO2 into the air around them. It's no different from plants taking in CO2 and animals breathing it out.
It's not necessary, just very advisable to do it.
The lower the concentration is, the harder it gets to extract something. Ideally CO_2 is captured where it is concentrated the most, i.e. right after generating it.
Most of the concentrated sources of CO2 are used to produce energy. The equation is like
something + O2 --> CO2 + energy
We want energy. The problem is that we want energy.
If you try to reverse it with
CO2 + more energy --> something + O2
the problem is that you need more energy to reverse the reaction. If you stick one 100% carbon capture magical device to a power plant, you will not produce any energy in the power plant. Moreover you will need to buy a lot of energy.
There are some process that emit a lot of CO2, but use it mostly for the chemical reactions and not to extract energy like in cement production [1] and somewhat in blast furnaces [2]. You can capture the CO2 produced by them to get guilty free cement or steel, but is much much much more efficient to use the energy to replace the energy of a power plant that burns coal / oil / whatever. (Call it carbon emission trading.)
Once we close all the power plants that burn coal / oil / whatever, it can be useful to add carbon capture.
If you want to reuse existing CH4 distribution pipelines, that's fine, but now you need to pipe back the CO2 to the plant that makes the CH4 in order to complete the cycle. That's much harder than pumping CH4 and would require a whole new parallel pipeline infrastructure that can transport highly pressurized CO2 back to the source plant. Alternatively, you could regenerate the CH4 on location, but then you need to pipe the energy in; in that case, you're not using CH4 to deliver energy at all.
You could make the CH4 from CO2 in coal or something, but then you're no longer carbon neutral (since that CO2 will end up in the atmosphere rather than the battery). You could also try carbon capture, but honestly thermodynamics suggests that that will never be a viable solution; far easier to order your coffee black than to unmix the cream from it.
My point is that, unless I'm misunderstanding something, existing distribution systems for natural gas are not useful for CH4-based battery storage.