Most hydrogen today is produced from fossil fuels – steam methane reforming of natural gas, partial oxidation of coal or oil residues – and entails large CO2 emissions, from 8.5 tons of CO2 per ton of H2 from natural gas in modern facilities up to 20 tCO2/tH2 from coal. This fossil hydrogen can be called “grey hydrogen”.
Or sometimes, brown. The same color scheme applies to the ammonia produced from it, so we have “grey ammonia.” Or brown ammonia, your call. The exact carbon footprint depends on the fuel used and the efficiency of the facility, so you could easily identify many shades of grey.
Green is the colour of hydrogen produced from electrolysis of water, provided the process is run on green electricity, which most understand as renewables-based, although some insist nuclear power also qualifies if green means (very) low carbon. Reforming biogas would be green too but, because a significant amount of hydrogen might be used to make sustainable hydrocarbon fuels for, say, aviation, it might in the future be more useful to add hydrogen to biomass than to subtract hydrogen from biomass.
Blue is the colour of hydrogen produced from fossil fuels, but with capture and storage of carbon dioxide (CCS). The International Energy Agency systematically uses the term carbon dioxide capture and use or storage, “CCUS”. But CO2 is both used and stored only in enhanced oil recovery operations. From hydrogen and ammonia production, process CO2 is used in integrated ammonia/urea plant for manufacturing urea from ammonia – and released into the air soon after urea is used in the crops. Most other uses of CO2 involve manufacturing of hydrocarbons, starting with synthetic methane. However, reforming methane into hydrogen and CO2 for then making synthetic methane may not be the most clever thing to do. Let us stick to CCS to qualify for blue hydrogen, though depending on the fuel use, the share of CO2 captured, and the upstream methane emissions from natural gas exploitation, the blue may range from pale to dark, or from, say, sky blue to navy blue.
There is, however, another option to deliver clean hydrogen – and now another colour: turquoise, or green-blue (or blue-green). This is the colour of hydrogen from methane pyrolysis, a process that directly splits methane into hydrogen and solid carbon. Instead of being a waste, like CO2, that must be disposed of safely, solid carbon is potentially a resource, with various industrial uses. To be fair with electrolysis, it has also a valuable by-product, oxygen, absent from methane splitting.
Turquoise hydrogen and ammonia offer an interesting pathway in the transition from fossil fuels to renewables. Addressing climate change requires decarbonising not only the power sector but all end-use sectors (buildings, industry, transport), through material and energy efficiency, direct use of renewable heat and, most importantly, electrification. Electrification can be direct or indirect via the production of hydrogen and hydrogen rich fuels and feedstocks. Greenhouse gas emissions should peak as soon as possible (or, could we say now, never return to pre-Covid-19 levels) and achieve net zero in the second half of the century. Achieving this will require a deployment of renewable energy capacities much faster than we currently see. Turquoise hydrogen can help speed up this process and provide us with very low carbon ammonia.
Read the complete article about turquoise hydrogen here