An appraisal on various hydrogen productions and their CO₂ emissions – a comparison on the Grey, blue and green pathways

The transition to a net-zero economy requires revolutionary change to the fundamentals of economies around the world. The need to decarbonise processes affects almost every aspect of industry and society, with energy, transport, and manufacturing making up some of the most polluting sectors. Many complementary solutions must be realised to surmount such challenges, and one suggestion is the greater use of hydrogen as a fuel or feedstock. For this to occur, hydrogen production must be significantly scaled up without introducing so many emissions as to counteract improvements made downstream. Although hydrogen gas can be found in underground deposits, almost all hydrogen is produced industrially from fossil fuels, notably via steam reforming process (grey hydrogen), which is usually associated with a considerable carbon footprint. Alternative production pathways with a lower carbon footprint have been developed for large-scale production. This perspective examines the impact of large-scale grey, blue, and green hydrogen production methods and their viability as sources of a cleaner fuel for heat and electricity generation.