To all the colours of hydrogen I’ve loved before

Tim Baxter
10 min readFeb 25, 2023

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One of my more perverse hobbies is to collect “colours” of hydrogen. I admit that it’s weird, and it is just shy of a being the kind of hobby you shouldn’t admit to in polite company, but I shan’t apologise. Molecular hydrogen (i.e H₂) is naturally a colourless gas, but energy nerds have developed a growing but imperfect shorthand system to refer to the multitudinous ways in which the simplest molecule in the universe might be made.

From a climate perspective, ultimately what matters is the emissions intensities of the processes used to manufacture hydrogen — and not really their colour. Nonetheless, I dutifully collect the colours I see in the wild like they are some kind of deranged climate and energy Pokemon. I’m sharing my collection here. Let me know if I’ve missed any that you’ve heard of.

Upfront: When I refer to whether the process is relatively low or high greenhouse gas emissions, I am not considering the indirect warming impact of fugitive hydrogen emissions, which will differ across methods because the rates of leakage will differ. To my knowledge, this hasn’t been studied in a systematic way for any method of hydrogen production, and given that many of the methods below are just barely past the point where you might successfully describe them as “hypothetical” without furiously giggling in a fit of audacity, I suspect some won’t ever be studied.

(While it is not a greenhouse gas in its own right, through a complex set of interactions operational and inadvertent releases of molecular hydrogen from hydrogen production will have a warming impact on the atmosphere. This mostly occurs because fugitive hydrogen emissions subtly increase the atmospheric lifetime of methane, a live-fast-die-young greenhouse gas that is responsible for around one-third of the shift in the planet’s temperature that we have seen so far.)

Let’s begin!

Source: Wikimedia commons (Oren neu dag)

Grey hydrogen: Unmitigated steam methane reforming. Splits methane (CH₄) into molecular hydrogen and carbon dioxide, with the methane almost universally being derived from fossil (aka “natural”) gas. Today, grey hydrogen is an essential pillar of most fertiliser production worldwide and is central to ensuring that the planet is capable of supporting billions of people. It is essential that we find alternatives to this process because it is very emissions intensive: for every tonne of hydrogen that is produced, more than ten tonnes of carbon dioxide are released to the atmosphere. If the methane is derived from fossil gas, then there is also a separate additional atmospheric burden as a result of emissions intensive upstream processes. Most hydrogen manufactured today is derived from fossil methane using steam methane reforming.

Brown hydrogen: Coal gasification using brown coal. This process combines the carbon in coal with water vapour and oxygen to create carbon monoxide — or in more recent iterations of the process, carbon dioxide — and hydrogen. This process was historically used to create town gas, which was used in industry and households around the world before today’s “natural gas” (methane, which was literally “naturally occurring gas”, as opposed to manufactured gas) began to be exploited at industrial scale. Worldwide, coal gasification is the second most common source of hydrogen today, though it is much less common than steam methane reforming. Coal gasification is roughly twice as emissions intensive as steam methane reforming, with around twenty tonnes of carbon dioxide produced for every tonne of hydrogen. Again, there is a significant additional burden from upstream greenhouse gas emissions. Recently, the world’s first shipment of liquefied hydrogen, which was produced from Victorian brown coal, was shipped to Japan in a pilot program.

Black hydrogen: As above, but using geologically older — and so more pure — black coal instead of brown. Brown coal is generally preferred in hydrogen manufacture over black.

Blue hydrogen: Almost universally, steam methane reforming linked to carbon capture and storage, though very rarely it is used to describe coal gasification with carbon capture and storage. Despite significant hyperventilation and hand waving, to date not so much as one single molecule of blue hydrogen has ever been created. In theory, blue hydrogen — particularly SMR plus CCS — can be a low emissions process and so it has passionate advocates, particularly among the fossil fuel industries and their defenders. However, the theoretical potential for blue hydrogen to be a low emissions source of hydrogen only holds if you ignore those processes upstream in the fossil fuel supply chain that create substantial greenhouse gas emissions and presume that carbon capture and storage functions at a very high capture rate. Independent assessments have shown that the combination of upstream emissions and process inefficiencies mean that in most instances the unabated combustion of fossil gas will have a lower greenhouse gas emissions intensity per unit of energy than blue hydrogen from gas. That is, from a climate perspective we would be better to just burn gas — a dangerous fossil fuel — than to go to the effort of converting it to hydrogen and then attempting to sequester the produced carbon dioxide underground using CCS, a technology that fails whenever it is important. That said, the fact that we would be better off burning the gas in the first place than making blue hydrogen from it holds even if you don’t just presume that CCS fails — which it will — but instead presume that it will be more successful than it ever has been. Some dickhead once made up the term ‘clean hydrogen’ in an attempt to pretend that steam methane reforming with CCS is indistinguishable from green hydrogen. You should forever entirely ignore anyone who tries that kind of bullshit.

Turquoise hydrogen: Methane pyrolysis. This process sees methane (again, CH₄) split into pure carbon and hydrogen. This process is theoretically capable of producing low emissions hydrogen because the pure carbon can be stored, or even used commercially. There should be no — or at least absurdly minimal — carbon dioxide created from the process. However, its low emissions status is conditional on whether the large amount of thermal energy required by the pyrolysis process comes from zero — or at least low — emissions sources. Also, as with blue hydrogen, if the methane used in the process is derived from fossil fuels, the reality of emissions intensive processes upstream poses a considerable challenge to its status a low emissions source of hydrogen. Methane pyrolysis has not been proven at scale and is not created in commercial quantities.

Natural hydrogen (sometimes alternatively known as white or gold hydrogen): Molecular hydrogen that occurs naturally in the earth’s crust. Until recently, it was commonly understood that there was very little naturally occurring pure molecular hydrogen on earth. Very recently, there has been considerable excitement — though to be frank, comparatively little proof — that there are a number of large, commercially-viable pockets of pure hydrogen at exploitable depths beneath the earth’s crust. One target location is in South Australia and an exploration permit has been granted over the Yorke Peninsula and surrounds. The village of Bourakébougou in the Koulikoro region of western Mali is currently being powered by a small natural hydrogen powered generator after an accidental discovery in the 1980s. Exploiting natural hydrogen would produce no direct greenhouse gases other than from the energy used in extraction (e.g. diesel burned by machinery that is used to drill down to the naturally-occurring hydrogen). There are a number of natural processes that might lead to pure molecular hydrogen accumulating in the earth’s crust — fifteen or so I think, though to be honest I couldn’t explain them all to you. After confessing to that significant personal failing, I can say that most are dependent on the presence of either iron- or uranium-rich rock formations.

Gold or white hydrogen — definition 2: Hydrogen created through supercritical carbon dioxide fracturing (aka “fracking”). This as-yet-mostly-theoretical process would see the carbon dioxide from steam methane reforming — see ‘grey hydrogen’ above — or other processes used to frack hydrocarbon rich rocks. Ordinarily, fracking (more often in the form of ‘hydraulic fracturing’) involves the injection of water — along with sand and an alphabet soup of chemicals — underground instead. Supercritical carbon dioxide fracturing would result in the carbon from the injected carbon dioxide, as well as the carbon contained in the methane that might have been produced if water was used instead of SCCO2, remaining underground while pure molecular hydrogen is extracted. In theory, this would produce very low emissions hydrogen, with all the usual caveats relating to the potential for emissions intensive upstream processes and the source of process energy.

Gold hydrogen — definition 3: Hydrogen created with genetically engineered microbes. (No, I’m not making this shit up!) At least one company is looking into creating hydrogen by inserting genetically engineered oil-eating microbes into depleted oil fields. The hope is that while mopping up the remaining hydrocarbons, these microbes will create commercially-viable quantities of pure hydrogen. This will necessarily lead to considerable quantities of carbon dioxide being extracted along with the hydrogen, though the second —perhaps greater — hope is that this CO₂ could be reinserted back into the field after it is separated from the hydrogen. It would be a polite understatement to describe such plans as ‘ambitious’. Surprisingly, the risks with this planned process are somehow even more obvious than they are numerous, even while there are rather a lot of risks. I wanted to be a neutral arbiter of fact in this piece, but even as someone who is generally fine with genetic engineering, I would prefer it if we didn’t do this one.

Orange hydrogen: Engineered natural hydrogen production. This process pumps carbon dioxide rich water deep underground into iron-rich rocks. It is intended to engineer some of the same process that would create natural hydrogen. Similar — though not identical — to the second definition of gold hydrogen given above, this would result in carbon dioxide sequestration. While this would be relatively low emissions, the total emissions would depend on the source of carbon dioxide — particularly if it is derived from fossil fuels and there are significant upstream emissions — and the source of process energy.

Purple hydrogen (sometimes alternatively known as pink hydrogen): Thermochemical hydrogen production where the heat is provided from waste heat in advanced nuclear reactors. This as-yet-unused process would see water split into its component parts — hydrogen and oxygen — at temperatures in excess of 500°C. To my knowledge… thermochemical hydrogen production is not possible using the current generation of nuclear reactors and is conditional on Gen-IV nuclear reactors. Despite some progress, Gen-IV reactors have been a promising technology that has been just ten more years away for twenty years. It is currently just ten more years away. As such, there is currently no hydrogen being produced using this method.

Yellow hydrogen: Thermochemical hydrogen produced from solar energy. This is the same as purple hydrogen, but rather than using waste nuclear heat, concentrating solar thermal arrays are used to focus solar energy and reach the temperatures necessary to split water. Over three hundred different methods have been proposed for the creation of thermochemical hydrogen with solar energy, but to date none are are producing commercially-viable quantities of hydrogen.

Dark hydrogen: Extra-planetary hydrogen. One for the futurists, dark hydrogen refers to hydrogen that is extracted from other planets and moons in the universe. Physicists usually have a particular state of hydrogen in mind when they refer to dark hydrogen. That is, a slightly magnetic phase of hydrogen that does not transmit or reflect visible wavelengths of light and that occurs under pressure just beneath the surface of gas giants like Jupiter and Saturn. In my experience, forward-thinking energy nerds tend to be a little less constrained to that definition and use the term dark hydrogen to refer any hydrogen derived from somewhere other than earth. Who cares if it releases greenhouse gases?! We’re in space motherfu — !! (It doesn’t though.)

Green hydrogen: Renewable energy powered electrolysis. There are several different processes that can produce hydrogen with electrolysis, but generally these cause water to be split into hydrogen and oxygen by driving an electric current through it. The various methods each have their pros and cons, but polymer electrolyte membrane (or PEM) electrolysis is generally considered the most promising for renewable energy, particularly for wind and solar where the energy input will vary. While it is rapidly improving, electrolysis is an energy hungry and inefficient process and, by definition, it will always be so. This means that it will be valuable in future for soaking up peaks in the supply of electricity that are likely in an electric grid with a high penetration of wind and solar. The first target for this hydrogen should be the decarbonisation of the existing hydrogen supply chain, so we can ensure the planet continues to be able to support the current population while annoying the malthusians. Where processes can be directly powered by grid electricity — or where electricity can reliably be stored — it will almost always be preferable to do that rather than to use renewably-derived hydrogen. While in a strict sense, the term green hydrogen should arguably be limited to just those instances where the required electricity is entirely — or at least mostly — directly supplied by renewable sources, it is very often used to describe any forms of hydrogen production using electrolysis. This can occur even when grid-connected electrolysers are being primarily supplied with electricity derived from coal and gas. In some instances, the claim to a green status for these projects occurs because some kind of renewable energy certificate or other form of offset has been purchased and surrendered, but occasionally projects will claim to be “green” simply because they use electrolysis. In these cases, the emissions impact of hydrogen production will depend on the integrity of the offset method and emissions intensity of the electricity supplied. Even in South Australia — with a high penetration of renewable energy and no coal — unabated hydrogen production using grid-connected electricity has a comparable greenhouse gas intensity to unabated steam methane reforming (grey hydrogen). In Australia’s most emissions intensive grids — Victoria, New South Wales and Queensland — it has a worse emissions intensity than even coal gasification.

Purple hydrogen (or pink) — definition 2: As above, but where the electricity supplied is primarily, or exclusively, derived from nuclear power stations.

That’s nearly all of them. I remember once seeing another kind of blue hydrogen, but I can’t remember its definition. My annoyance at the fact that I know I have forgotten this was the inspiration for writing down all the rest.

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Tim Baxter

Climate and energy researcher for my day job, but these opinions are written on my own time.