Steam methane reforming technique is used to obtain
Grey Hydrogen
Steam methane reforming yields grey hydrogen — option 2.
The reaction. Methane from natural gas is reacted with steam over a nickel catalyst at about 800 °C :
\(\text{CH}_{4}+\text{H}_{2}\text{O}\rightarrow\text{CO}+3\text{H}_{2}\)
followed by the water-gas shift reaction, which converts the carbon monoxide and yields more hydrogen :
\(\text{CO}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{H}_{2}\)
The carbon dioxide is released to the atmosphere, and it is that release which fixes the colour label.
The colour code of hydrogen describes not the gas — which is always the same — but the method and emissions of its production :
| Colour | How it is made | Carbon released |
|---|---|---|
| Grey | Steam methane reforming of natural gas | Yes, vented to the atmosphere |
| Blue | Same as grey, but with carbon capture and storage | Mostly captured |
| Brown / black | Gasification of lignite (brown) or of bituminous coal (black) | Yes, and the most of all |
| Green | Electrolysis of water using renewable electricity | None |
| Pink | Electrolysis using nuclear electricity | None |
Why grey dominates. Steam methane reforming is by far the cheapest route and supplies the great majority of the world’s hydrogen, which goes mainly into ammonia for fertiliser and into petroleum refining. Its drawback is that roughly nine to twelve tonnes of carbon dioxide are released for each tonne of hydrogen produced.
The Indian context. The National Green Hydrogen Mission, approved in January 2023, targets five million tonnes a year of green hydrogen by 2030, precisely to displace the grey hydrogen now used in fertiliser and refinery plants.
Hence, the answer is Grey Hydrogen.
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