What does “green” hydrogen mean, and what can we use it for?

Hydrogen often comes up in conversations about clean energy, and a range of colors is sometimes used to describe a completely colorless clean-burning chemical that can be used as a substitute for coal, oil or gas in a wide variety of applications. But for its use to have net environmental benefits, hydrogen must be produced from clean sources, rather than from fossil fuels, which is the current standard method.

• So-called “brown hydrogen” is usually obtained from coal and has been produced to supply cities with light since the early 19th century: through water and heat, the coal undergoes gasification, and in that process, the chemicals in the coal react to produce what is known as town gas, now known as synthesis gas, which contains a mixture of carbon dioxide, carbon monoxide, hydrogen, methane and ethylene, along with small amounts of other gases. The first two are of no value for power generation, making the process highly polluting compared to other methods, although chemical companies can distill hydrogen from this mixture relatively easily. In addition, waste-to-energy incinerators often use similar processes to generate brown hydrogen from biomass and petrochemicals. The vast majority of synthetic gas still comes from coal.
• So-called “grey hydrogen” makes up the vast majority of the hydrogen produced today…