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Combustion of hydrocarbons reveals hydrogen, but not sure at what efficiency. Is there a reaction to get hydrogen from hydrocarbons?

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    $\begingroup$ Combustion of hydrocarbons generally does not produce hydrogen because hydrogen itself will combust. $\endgroup$ – bon Jul 20 '16 at 8:55
  • $\begingroup$ You could considering cracking. It is an industrial process of getting hydrogen gas. However, the main purpose of cracking is usually to get some light alkene molecules such as ethene from higher petroleum fractions $\endgroup$ – Tan Yong Boon Jun 29 '17 at 8:52
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Hydrocarbons are in fact the main source of hydrogen gas. According to Wikipedia, 95% of all hydrogen is produced by the steam reforming reaction:

$$\ce{CH4(g) + H2O(g) -> 3H2(g) + CO(g)}$$

This is a very important industrial process which has been studied in depth. It is done at very high temperatures (~1000 °C) and under pressure (~20 atm), and can be "up to 75% efficient" (though efficiency here may not be defined as solely reaction yield).

As an aside, the fact that so much hydrogen is obtained from methane (itself generally sourced from natural gas, a fossil fuel) is one reason why hydrogen fuel cells currently have limited environmental benefit, though there is a huge amount of research into more efficient techniques for obtaining hydrogen from water splitting (especially using light) which could be carbon-neutral.

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  • $\begingroup$ Downside: Release of a great deal of carbon Monoxide. $\endgroup$ – Shadur Jul 20 '16 at 13:39
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    $\begingroup$ @Shadur Not really. Any plant producing hydrogen is either going to use the CO in some synthesis reaction, or use it to perform the water gas shift reaction to produce more hydrogen. $\endgroup$ – R.M. Jul 20 '16 at 14:49
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    $\begingroup$ is there any chance to get C and CO2 instead of CO? $\endgroup$ – njzk2 Jul 20 '16 at 17:03
  • $\begingroup$ @shadur the reaction mix is cooled, then passed through another reactor where the shift reaction CO + H2O -> CO2 + H2 occurs. After excess H2O has been condensed, the mixture goes to a pressure swing absorbtion (PSA) unit, where everything except H2 is absorbed onto molecular sieves. Once the sieves are fully charged the pressure is released and all the byproducts are flushed out, and sent to the furnace for burning. They therefore are not wasted but supply part of the heat needed to make the steam reforming reaction go. The PSA normally has 10 or more vessels to enable continuous operation. $\endgroup$ – Level River St Jul 20 '16 at 20:07

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