[co-author: Mostafa Al Khonaizi ]
Hydrogen is not found freely in the atmosphere, and the pathways of producing hydrogen differ significantly in their carbon intensity. These pathways are typically referred to by colors grey, blue, and green based on the various inputs required to produce the hydrogen, with conventional means of producing hydrogen emitting most significant carbon dioxide into the atmosphere.
Grey Hydrogen
Grey hydrogen is produced using fossil fuel hydrocarbon feedstocks, most typically, natural gas. As such, it is the most carbon-intensive pathway of generating hydrogen. The conventional methods use either steam methane reforming or autothermal reforming processes to separate hydrogen from carbon and make up about 75% of total hydrogen production.