There is no consensus on the colour definitions to identify hydrogen. The industry has used a kaleidoscope of colours to identify the production method of hydrogen but has not yet reached a consensus on the colour of each production process. The colour definitions used today may change over time, and even between countries.
To produce hydrogen, it must be separated from other elements in the molecules where it occurs. Therefore, there are many different sources of hydrogen and ways of producing it for use as a fuel or an intermediate energy carrier.
Below is a list of colours that we have identified to date, but we expect that the list will grow when the industry comes up with alternative production methods.
White Hydrogen is sometimes referred to as geological hydrogen, generated in the earth’s crust and can be found (rarely) as geological underground deposits; while there is some exploration in this field, capturing this hydrogen is still a long way off.
White-Hydrogen can be produced from carbon-based matter through thermo-chemical steam conversion, a process that breaks the molecular bond between carbon and hydrogen atoms. The resulting hydrogen-rich synthesis gas from this process is then processed with pressure swing adsorption (PSA) or membrane separation to yield high-purity clean White Hydrogen and minor hydrocarbons as an off-gas from the PSA process. This off-gas is used to generate energy for this Thermochemical conversion process, making this fully self-sufficient and sustainable and carbon neutral without greenhouse gas emissions. The additional benefit of this process is the elimination or reduction of the global waste problem and producing hydrogen at a fraction of the cost compared to electrolitic hydrogen.
Green electrolytic hydrogen is produced in smaller and more expensive production systems using solar and wind power to generate the energy required for the electrolysers to spit hydrogen from water. However, besides the large area of land needed, solar and wind are not without limitations and are not available 24/7. Furthermore, a typical breakdown for green hydrogen production through electrolysis demands at least 55 kW of electricity and ≈60 litres of raw water ( 9 litres of purified water) per kg of hydrogen produced. Therefore, the primary disadvantage of green hydrogen is the cost and arguably the most expensive and unreliable way to green hydrogen.
The oldest way of producing hydrogen is by transforming coal into gas. Gasification processes convert organic or fossil-based carbonaceous materials into carbon monoxide, hydrogen, and carbon dioxide. Gasification is achieved at very high temperatures with a controlled amount of oxygen and/or steam. The carbon monoxide reacts with water to form carbon dioxide and more hydrogen via a water-gas shift reaction.
This hydrogen is known as black or brown depending on the type of coal used: brown (lignite) or black (bituminous) coal. It results from a highly polluting process since both CO2 and carbon monoxide cannot be reused and released into the atmosphere.
Most Grey hydrogen indicates it is from natural gas without capturing the CO2 emissions from the carbon separation via stream reforming, where the CO2 emission is not captured. As a result, grey hydrogen production emits approx. 9.3kg of CO2 per kg of hydrogen.
Blue hydrogen is produced from steam reforming natural gas and the emissions from the steam reforming process are captured and stored (CSS). Blue hydrogen is often considered carbon neutral even though “low carbon” would be more accurate since around 10-20% of the generated CO2 cannot be captured.
A new way of extracting hydrogen from natural gas, methane pyrolysis, is currently in the experimentation phase. The gas is decomposed at very high temperatures generating hydrogen and solid carbon. This hydrogen is then referred to as “turquoise” or low carbon-hydrogen.
Pink hydrogen is obtained from electrolysis through nuclear energy but can also be referred to as purple or red hydrogen.
The colour yellow sometimes indicates hydrogen produced via electrolysis, but often is also used to indicate that the electricity used for the electrolysis comes from mixed sources based on availability (from renewables to fossil fuels).
