Hydrogen: a spectrum of opportunities

As the world attempts to extract itself from the grip of fossil fuels, scientists are rapidly searching for viable alternatives that can carry us into a greener future. One of these alternative fuels is hydrogen. 

Hydrogen is one of the building blocks of organic life alongside carbon, however pure elemental hydrogen is hard to come by—it is only present in the atmosphere at around 0.6 parts per million. Industrial-scale hydrogen manufacturing is therefore necessary for it to be a practical option.

So, what are the different sources and methods of hydrogen production? Read on to find out.

Colours of hydrogen

Once isolated, hydrogen is very promising as a fuel source for combustion engines, fuel cells, and as an alternative to natural gas heating.

While hydrogen is a colourless gas, discussion around diatomic hydrogen has been colour-coded to easily distinguish between different sources and production processes. They aren’t formally defined but they are a useful nomenclature when it comes to understanding sustainability in the industry.

Grey

Grey hydrogen is the most common method of production at present, accounting for almost 50% of the total manufactured hydrogen. This process takes natural gas—namely methane—and reacts it with water in a process called steam methane reforming. This reaction produces hydrogen gas and carbon monoxide, the latter of which is toxic, so it is further reacted with water to produce hydrogen and carbon dioxide through the water-gas shift reaction.

This process is very energy-intensive to maintain a high-heat environment, plus it also produces significant amounts of carbon dioxide (5-6 tons for every ton of hydrogen gas produced).

Brown & black

Brown and black hydrogen refers to that which is extracted from brown and black coal. The coal is heated with steam and oxygen in a process called coal gasification, and partially oxidises to produce carbon dioxide, carbon monoxide, water, and hydrogen gas. The carbon monoxide can be further reacted to reduce toxic by-products.

Blue

This method of hydrogen production is exactly the same as grey, brown, and black, referring to any hydrogen made from fossil fuel feedstocks. However, the carbon dioxide produced in the reaction is captured to avoid its release into the atmosphere and reduce impact to climate change effects. It can be either stored deep underground (in the hopes that it will again become fossil fuels) or repurposed for other industrial processes. 

Green

Green hydrogen most often refers to the process of manufacturing hydrogen via electrolytic splitting of water, where the electricity is sourced from strictly renewable energy. This process uses electricity to decompose water into hydrogen and oxygen gases, using oxidation and reduction processes. 

Green hydrogen can also more broadly refer to any renewable form of hydrogen production, including gasification or steam reformation of biomass and biofuels, as opposed to fossil fuels.

As it currently stands, only about 4% of industrially produced hydrogen comes from electrolysis, with an even smaller percentage produced through purely renewable electricity sources. This is primarily because water splitting is not an inexpensive process, but as it becomes more commonplace, this cost is set to become comparable to gas and petroleum. 

Yellow 

Yellow hydrogen is a less specific form of green hydrogen, in that it is used to describe water splitting via electrolysis using any source of electricity. It is sometimes used to specify solar-powered electrolysis, but can also include other sources of electrical power such as wind, hydroelectricity, or the burning of fossil fuels.

Purple

Also known as pink or red hydrogen, this is again a hydrolytic process to create hydrogen from the splitting of water. However, this refers specifically to electricity which is produced via nuclear fission in nuclear power plants. While it doesn’t emit greenhouse gases like carbon dioxide, there are other environmental impacts associated with nuclear waste. 

Turquoise

Like grey hydrogen, this method uses natural gas as its feedstock. However, rather than steam reformation, the methane is heated so it thermally decomposes into hydrogen gas and solid carbon (as opposed to carbon dioxide). This is environmentally friendly with respect to greenhouse gas emissions, however the amount of energy required to heat methane sufficiently that it will decompose (just over 1000°C) is not insignificant.

White

This is one of the lesser-known colours of hydrogen, referring to the rare naturally occurring geological hydrogen. This is usually extracted as a by-product of fossil fuel refining.

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Sources:

• https://nacfe.org/emerging-technology/electric-trucks-2/making-sense-of-heavy-duty-hydrogen-fuel-cell-tractors/ 
• https://blog.csiro.au/green-blue-brown-hydrogen-explained/ 
• https://en.wikipedia.org/wiki/Hydrogen_production 
• https://pubs.acs.org/doi/pdf/10.1021/ef0101964
• https://pubs.acs.org/doi/10.1021/acs.iecr.1c01679 
• https://www.nature.com/articles/s41560-022-01114-6#Sec1