Is hydrogen a green fuel? Why are there full-page adverts in newspapers from oil companies about their investment in blue hydrogen? But surely hydrogen is colourless? Are you confused about the colour of hydrogen, read on to find out about the hydrogen colour code.
There’s been a lot of talk about hydrogen as one of the components of the UK government’s net-zero strategy. This includes major investment in the Northwest and a lobby grouping of local industrial and commercial potential users, the Northwest Hydrogen Alliance. However, the terminology used to describe the hydrogen in many news stories can cause confusion.
So why do we need to understand the colour of hydrogen? It is very important to understand what the colour means because some hydrogen production uses fossil fuels and it’s questionable whether these are carbon neutral.
The colourless colour of hydrogen
If you look up hydrogen in any reference source, such as Wikipedia you will find it described as colourless and odourless. So, the colour of hydrogen is not a sensory experience, but a shorthand used to describe the way it has been made.
This brings up another important aspect of hydrogen’s role in renewable energy technology. It is not intrinsically a renewable energy source, but it can be used to convert electrical energy produced from renewable sources, such as wind, hydropower or solar radiation, into a more easily storable or transportable form.
This sort of hydrogen is generally described as green hydrogen. Green hydrogen is produced by the electrolysis of water and when the resulting hydrogen is used, it is turned back into water while releasing energy. Every time we convert one form of energy into another, we lose efficiency, so even green hydrogen is less efficient than using the solar or wind generated electricity immediately, but it provides a way of storing renewable energy for when there is no wind or solar energy – such as at night.
Good and carbon neutral colours
Green hydrogen is generally seen as a component of any future carbon neutral economy and it would almost certainly play a role in some transportation methods, such as aviation and maritime – where battery power is impractical due to the excess weight.
There is another method that has been suggested for hydrogen production by electrolysis of water. This uses nuclear power to produce pink hydrogen confusingly this is sometimes called purple hydrogen.
Clearly pink hydrogen is carbon neutral but there is less logic for its use in a net-zero economy. Nuclear power is certainly likely to be used in the medium term as we close down coal production, but its role is to contribute towards base load, the power that is required 24/7 to run our energy dependent civil and industrial structure. Green hydrogen complements renewable energy sources; pink hydrogen does not.
Finally, we need to look at another carbon neutral hydrogen – white hydrogen. White hydrogen is the term used to describe naturally occurring hydrogen in the earth’s crust. This has been known for some time as a geological curiosity but has attracted more attention recently as we attempt to reduce the world’s carbon consumption. This would be another fossil fuel and may well be extracted by problematic geoengineering methods such as fracking – but it would be carbon free.
Not so good colours
The hydrogen projects that are currently being discussed for construction in the Northwest will use another colour of hydrogen, blue hydrogen. Blue hydrogen is produced from methane, natural gas, by a process called steam reforming where steam and gas are mixed at high temperatures to produce hydrogen and carbon dioxide (CO2), the CO2 is separated and either used in industrial processes or stored.
This is already one of the methods used to produce hydrogen for use in ammonia manufacture and explains why the gas price crisis in 2022 led to a shortage of CO2 once fertiliser production was suspended in the UK due to a shortage of affordable methane. However, if we ramp up steam reforming to produce hydrogen from CO2, we have the issue of storing all the unwanted CO2 to prevent it being released to the atmosphere.
In some ways this is an easier problem than capturing CO2 from fossil fuel power generators but there is no commercial scale (in terms of both large volumes and low cost) solutions to this issue available at present (although there is substantial research in place in the UK and elsewhere). Without a fool-proof and economically viable solution to this issue, blue hydrogen is not a long-term route to a zero-carbon energy strategy.
Black and blue
Older readers may dimly remember the production of gas in gasworks present in towns and cities in the UK. This also used steam reforming but with coal as the feedstock, to produce both hydrogen and carbon monoxide (CO) as a process gas that was both highly flammable and highly poisonous. This is fundamentally the same chemical process as produces blue hydrogen but with the CO not converted to CO2 before use as a gas. Hydrogen produced this way is termed black hydrogen, sometimes called brown hydrogen when used mixed with CO and grey hydrogen when the CO is converted to CO2 and released to the atmosphere.
Why is the UK government (and other governments worldwide) so keen on blue hydrogen? There are two possibilities. First it is relatively easy to upscale blue hydrogen production because it is already used in the nitrogen fertiliser industry and there is over 150 years of expertise in related grey and black hydrogen manufacture. This would allow a transition to a ‘hydrogen economy’ while green hydrogen routes are being developed and refined.
The second possibility is that this allows the continued dominance of the energy industry by the oil and gas majors because we would still be using a fossil fuel – methane – as our source of energy but would be burying the CO2 (probably literally) to prevent it entering the atmosphere. However, this does depend on developing viable carbon capture and storage on a tremendous scale and still leaves us reliant on a finite fossil fuel gas resource.
Completing the hydrogen spectrum
There are a few more colours that are sometimes mentioned in articles concerning a future hydrogen economy. These all cover new ways of making hydrogen either from water or from methane. Of these the most promising is the direct splitting of water into hydrogen and oxygen using sunlight. This is attempting to mimic the photosynthesis process in plants that uses sunlight to make sugars and oxygen. This is still at very early stages of development; however, the product has already been termed yellow hydrogen (after the colour of the sun).
Finally, it is possible to use very high temperatures – pyrolysis – to directly split methane into hydrogen and solid carbon. This is attractive as it is easier to store solid carbon and prevent it entering the atmosphere as CO2 than it is to store CO2 gas. If these very high temperatures are achieved by burning some of the methane gas as fuel, this is termed turquoise hydrogen and if the high temperatures are achieved using a nuclear reactor, we call it red hydrogen.
Is the future of hydrogen green?
Green hydrogen and possibly, in the future, yellow hydrogen both have a role to play in a future net-zero energy economy. Blue hydrogen may have a short-term use in the transition to a hydrogen economy but beware of its use as a greenwashing tool by fossil fuel interests.
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