Alternative fuels



Hydrogen is a potential fuel of the future. It is an invisible, colourless gas. Rarely found in its pure form, hydrogen is produced from a variety of domestic sources including natural gas, nuclear power, biomass and renewable power such as solar and wind.

In the energy industry, colours are used to differentiate the way it is produced and how much carbon is involved in the process. These colour codes are not determined by any universal naming convention so they may differ by region and over time. But for now, the terms below cover the most up to date and frequently used colour defnitions of a colourless gas that offers enormous potential as an alternative fuel for a carbon-free future:
  • Green hydrogen - Hydrogen produced with zero-carbon dioxide emissions, using renewable electricity to electrolyse water. Electrolysers use an electrochemical reaction to split water into hydrogen and oxygen.
  • Blue hydrogen - Produced mainly from natural gas, using a process called steam reforming. Carbon dioxide is a by-product of this process, which is trapped and stored using carbon capture and storage (CCS).
  • Grey hydrogen - Currently the most common form of hydrogen production, created from natural gas or methane as above, but without capturing and storing the by-product GHG emissions.
  • Black or brown hydrogen - Produced using fossil fuels, usually coal or lignite, the most polluting form of hydrogen production.
  • Yellow hydrogen - Produced using solar energy to power the electrolysis process.
  • Pink hydrogen - Produced using nuclear energy to power the electrolysis process


Ammonia (NH3) is a colourless gas with a distinct odour composed of nitrogen and hydrogen. As an energy source, ammonia has nine times the energy of lithium-ion batteries and is almost twice more energy-dense than liquid hydrogen.

The ammonia industry has informally adopted a colour scheme to describe the carbon intensity of the different methods for making ammonia. There are three ways of producing of ammonia, all of which have differing levels of carbon emissions emitted throughout the process:
  • Brown or conventional ammonia - achieved through the Haber-Bosch process where the hydrogen used to react with the nitrogen often comes for the steam reformation of methane, a process that emits CO2.
  • Blue ammonia - Conventional ammonia for which the by-product, CO2 has been captured and stored, reducing the climate impact compared with brown ammonia.
  • Green ammonia - Made with hydrogen that comes from water electrolysis powered by alternative energy.