The hydrogen rainbow

When discussing hydrogen, you will often see reference to different colours of hydrogen. The colours refer to the method by which the hydrogen has been produced:

• Black and brown hydrogen is produced from different types of coal by gasification. This is the dirtiest form of hydrogen production.
• Grey hydrogen is produced from natural gas using steam reformation. At present, this is the most common type of hydrogen production, but carbon dioxide is produced as a biproduct.
• Blue hydrogen is a variant of grey hydrogen, but importantly the process includes carbon capture systems (“CCS”) to catch and store any carbon dioxide produced.
• Green hydrogen is the holy grail of sustainable hydrogen production. It is produced by electrolysing water using clean sources of energy. No carbon dioxide is produced at any stage of the process.

Beyond these basic colours the kaleidoscope becomes more specific and includes pink (produced using nuclear power, which may be blue or green), turquoise (produced by a new technique called methane pyrolysis which produces solid carbon as a by-product), yellow (produced by electrolysis using solar power) and white (naturally occurring geological hydrogen).

While hydrogen is generally considered to be a green alternative to natural gas, as may be seen from the rainbow of colours, it is critical to know how that hydrogen is being produced to understand just how green the hydrogen is.