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The Sabatier reaction converts CO2 to methane. Can a vehicle that burns methane recover it (at least some part) from the exhaust gases via the Sabatier reaction? I am interested to know how this would be implemented, and whether such a vehicle has ever been built or prototyped.

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    $\begingroup$ Draw a pfd of the supposed engine, and then ask your question yourself. $\endgroup$ – mart Nov 9 '15 at 7:24
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According to Wikipedia:

It [Sabatier reaction ] involves the reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400 °C) and pressures in the presence of a nickel catalyst to produce methane and water. $$ CO_2 + 4H_2 \rightarrow CH_4 + H_2O + \text{Energy} $$

as you can see from the previous chemical reaction, to obtain 1 KMole of $CH_4$ you should have 4 Kmole of Hydrogen as reactants and a reaction medium of 300–400 °C (with god knows how much of catalyst you need).

Comparing to a regular vehicle internal combustion engine running on Methane:

  • Hydrogen is not available in ambient air to the degree that could allow a sustainable Sabatier reaction (it's not Oxygen), meaning you need a separate Hydrogen tank along side with your engine (not to mention the explosive nature of Hydrogen).
  • Preheating the reactants to 300–400 °C would require an external heating system if done using hydrocarbon fuel, it would require an additional combustion chamber and a heat exchanger.
  • Higher heating value of Methane = 889 kJ/Mole and HHV of Hydrogen = 286 kJ/Mole (Wikipedia), so theoretically speaking, the Sabatier reaction would provide you with 1 Mole of CH4 having heating value of 889 kJ by supplying the reactor 4 Moles of H2 that had a heating value of 4 * 286 = 1144 kJ !!
  • The resulting Methane and water have a ratio of 1:1, introducing this mixture with this amount of water to the combustion chamber will give you a VERY low efficiency combustion process, So naturally you need a separation mechanism to lower water content as possible before entering combustion chamber.

So has anyone researched/built a vehicle that burns methane and it recovers it (at least some part) from the exhaust gases via Sabatier reaction?

I really have no idea but from the previous points I don't see it as feasible at all as a conventional internal combustion engine running on Methane.

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  • $\begingroup$ 1) hydrogen won't be sourced from air, but from the combustion of hydrocarbons (huge amount of H2 in them) and/or electrolysis of H2O (if necessary). 2) heating of 300-400C is very easily achievable, since the exhaust gases are usually 1000C. 3) I don't understand what you mean there, there is no scientific term "heating value" that I know of. 4) of course methane will be isolated from water (not very hard process) $\endgroup$ – sloupioc Jul 29 '15 at 11:56
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    $\begingroup$ @sloupioc (1) Hydrogen from combustion?? (2) Do you have any idea about the size of that "easily achievable" heater? (3)Google it? (4) Everything in your engine is not "hard" and doable - the question is "is it worth it?". $\endgroup$ – Algo Jul 29 '15 at 12:16
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    $\begingroup$ @sloupioc Regarding point 3, heating value is another term for heat of combustion. $\endgroup$ – Chris Mueller Jul 29 '15 at 12:29
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    $\begingroup$ @sloupioc (1) That's a whole another reactor now (And it's not combustion), and again why extract hydrogen from natural gas rather than using it as a fuel already :-))? (2) Exhaust gases are hot but hydrogen tank isn't that's why you need a heater (4) Sure, it seems like a fun project to design, anyway I don't see it as an efficient system at all :)) $\endgroup$ – Algo Jul 29 '15 at 12:56
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    $\begingroup$ @sloupioc Semin et al 2008 give an exhaust temperature for a CNG ICE of 750K. You're fighting against the laws of thermodynamics. $\endgroup$ – EnergyNumbers Jul 29 '15 at 15:04
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Algo has given you a good answer about the detail. There is a much broader, more widely-applicable, kind of answer.

Converting any form of energy to heat introduces huge inefficiencies. It is the lowest-quality form of energy. So if you want to do any work with that energy at all, it is best to do it before it gets turned into heat, whenever possible.

Once it's in the form of heat, then the quality of the energy (what we call its exergy, that is it's ability to do work) depends on the difference between its temperature, and the temperature of the cold reservoir you're using as your heat sink. That's typically, but not always, ambient temperature. If you want to do any work with that heat, then you do it when the difference in temperatures is highest, and when the cold-reservoir temperature is lowest.

So you wouldn't go from heat to chemical energy to kinetic energy.

Whenever you do an energy-conversion, you lose some ability to do work - i.e. you lose some exergy. When you do non-heat to heat conversion, you lose a lot of exergy. So round-trip processes, where you go from, say, chemical energy (methane) to heat and then back again, are really inefficient, and you'll get much less out than you put back in.

So no, you wouldn't use the Sabatier process to power a vehicle, because that would involve:

chemical energy -[combustion]-> heat -[Sabatier]->
    chemical energy -[combustion]-> heat -[pistons]-> kinetic energy

And it really does not matter which conversion process is in that box that's currently marked "Sabatier": this convoluted series of conversions would not make sense, whatever that process is.

Instead, you'd just go straight from chemical energy to heat to kinetic energy, which is what combustion engines do.

Going from methane to heat to methane is going to be very inefficient. You'd only do it in really exceptional circumstances. I can't think of any right now, but I'm sure someone could contrive a corner-case where it made sense; e.g. some peculiar circumstance where you were able to move high-grade heat but could not move methane.

So it would be pointless to put a Sabatier (or similar) process in a methane-powered car. If you needed higher efficiency, the thing to do is to invest in vehicle efficiency: lower speeds, cleaner-burn higher-temperature engines, lighter vehicle weight, more aerodynamic profile, higher-efficiency transmission, kinetic-energy recovery systems, and so on. What you wouldn't do is add weight through adding inefficient unnecessary kit just to turn your heat back into fuel: just burn less fuel in the first place.

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