The major source of hydrogen is methane. Steam reforming extracts hydrogen from methane in a high-temperature and pressure tube inside a reformer with a nickel catalyst. Other fossil fuel sources include coal, heavy fuel oil and naphtha.
Green hydrogen is produced without fossil fuels or carbon dioxide emissions from biomass, water electrolysis and thermochemical (solar or another heat source) water splitting. However, these hydrogen sources are not economically competitive with steam reforming.
Starting with a natural gas () feedstock, the steps are as follows;
* Remove sulfur compounds from the feedstock, because sulfur deactivates the catalysts used in subsequent steps. Sulfur removal requires catalytic hydrogenation to convert sulfur compounds in the feedstocks to gaseous hydrogen sulfide (hydrodesulfurization, hydrotreating):
* Hydrogen sulfide is adsorbed and removed by passing it through beds of zinc oxide where it is converted to solid zinc sulfide:
* Catalytic steam reforming of the sulfur-free feedstock forms hydrogen plus carbon monoxide:
* Catalytic shift conversion converts the carbon monoxide to carbon dioxide and more hydrogen:
* Carbon dioxide is removed either by absorption in aqueous ethanolamine solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary solid adsorption media.
* The final step in producing hydrogen is to use catalytic methanation to remove residual carbon monoxide or carbon dioxide: