The Hydrogen Rainbow — Time for a globally recognised system?
Hydrogen is the lightest chemical element in the periodic table. It is the most abundant chemical in the universe. It is odourless. It is also colourless yet this hasn’t stopped it being referred to by a huge array of colours. So why the rainbow of different types and what do they all mean?
Well, before we start, it’s probably worthwhile stating that the varied colours do not actually describe different types of Hydrogen. All of them refer to the same diatomic molecule i.e. H2 which as we started with is colourless. The different colours are actually in relation to how the Hydrogen is produced — more specifically:
The type of energy source used in the production process and;
The environmental impact in terms of Carbon Dioxide (CO2) emissions of this production process.
So let’s start with Green. Green is a colour traditionally associated with ‘environmentally friendly’ products and as such, it would be safe to assume that this is the type of Hydrogen produces the lowest CO2 emissions. In fact, it’s produced without any CO2 as a by-product.
With this process, the ‘input material’ is simply water (H2O) and this water undergoes electrolysis which is the process of using electricity to split water into Hydrogen and Oxygen (O2). This reaction takes place in a unit called an electrolyzer. The Hydrogen produced can be collected and stored for further use and the Oxygen (O2) can be safely vented to atmosphere.
However, in order to be considered green, the electricity used in the production process needs to come from a source where CO2 is not produced as a by-product. As such, it must come from renewable sources such as wind or solar or a combination of such renewable sources.
Actually, Hydrogen produced via electrolysis of water using just solar power can be referred to as yellow but it’s still green! Considering all of the positive associations with the word green as described above and the fact that yellow is quite a recent term to be used then perhaps such production processes are better off sticking with green!
That brings us on to nuclear. Nuclear power plants produce no greenhouse gas emissions during operation, and over their life-cycle, they produce approximately the same amount of CO2e (CO2e or CO2 equivalent is the Global Warming Potential/GWP of a range gases in relation to CO2 which is 1) emissions per unit of electricity as wind, and about 1/3 of the emissions per unit of electricity when compared with solar. Arguably therefore, based on the criteria outlined at the start of this post, Hydrogen generated through electrolysis powered by nuclear energy could be described as green. Nuclear power plants though have not traditionally been associated with the term green for a whole host of reasons which won’t be expanded upon here. As such, this form of H2 generation has been referred to as purple, red and now more commonly pink. Considering many people associate red with danger and pink with joy and happiness then it’s clear to see why pink now predominates the phraseology used for this type of Hydrogen.
So what other colours are there and are there any other colours needed to describe novel, emerging production technologies? Well, let’s start with the existing colours.
With Blue Hydrogen, the input material is natural gas (Methane — CH4) rather than water.
The natural gas is split into Hydrogen and CO2 either by Steam Methane Reforming (SMR) or Auto Thermal Reforming (ATR). SMR is a process in which methane is heated, with steam, usually with a catalyst, to produce a mixture of Carbon monoxide (CO) and Hydrogen. ATR is a process which produces a mixture of Hydrogen and Carbon monoxide, by partially oxidizing a hydrocarbon feed (in this case methane) with Oxygen and steam with subsequent catalytic reforming.
As can be seen, this process produces CO2 and so to mitigate the associated environmental impact, the CO2 needs to be captured and then stored which is done through a process called Carbon Capture Usage and Storage (CCUS).
Because this type of H2 production process doesn’t actually avoid the creation of greenhouse gases then it is sometimes described as ‘low-carbon hydrogen’.
If the Hydrogen is produced in the same manner as Blue but WITHOUT using CCUS then it is known as Grey Hydrogen and this is the most common form of hydrogen production. Grey is not the most positive of colours and so it seems quite obvious why this was chosen here.
Black and Brown too do not ordinarily evoked positive emotions and one would be correct in assuming that black and brown hydrogen are the most environmentally damaging. Any Hydrogen made from fossil fuels through the process of gasification is sometimes called black or brown Hydrogen interchangeably. However, typically black refers to that produced from the gasification of black coal and brown to that produced from the gasification of lignite (brown coal) and these are the oldest forms of producing Hydrogen. Gasification involves taking these input materials and heating them (without combustion) using a controlled amount of Oxygen (and/or steam) to produce a synthetic gas (syngas) comprising of predominantly CO, H2 and CO2. The H2 content can be increased through the water shift reaction (a reaction between the CO and H2O). The H2 can then be separated from the syngas. Both the CO and the CO2 produced are released to atmosphere hence the high environmental impact.
We now come to Turquoise . This is produced by breaking methane down into Hydrogen and solid carbon using a process called pyrolysis. Pyrolysis is a similar thermal conversion process to gasification, as described previously, but is in the absence of Oxygen and whilst producing the Hydrogen that can be collected and stored, it also leaves a ‘char’ i.e. the solid carbon. This carbon can either be utilised in other industrial processes or simply buried. Turquoise Hydrogen therefore avoids this Carbon being incorporated in to gaseous CO2 that would have been released to the atmosphere.
Finally, we come to white hydrogen. White represents purity and innocence and as such is probably the most appropriate when considering naturally-occurring geological hydrogen found in underground deposits. This could be created through fracking but viable strategies to utilise these deposits do not currently exist which is why there are the different processes as described here to generate Hydrogen artificially exist.
Now let’s look to the novel, emerging Hydrogen production technologies — what colours would be most appropriate to these?
For example, the gasification of waste biomass that would ordinarily be destined for landfill to produce Hydrogen. If the decomposition of this waste biomass in the landfill would have produced more CO2e than that produced by utilising it instead as the input material in the Hydrogen production process (the landfill resulting in both CO2 and Methane which has a GWP 21 times that of CO2) then it could result in net negative emissions. What colour would this be or would it be super green? Coupled with CCUS — super, super green? What about using plastics within mixed wastes as the input material where ordinarily the biogenic component of these mixed wastes too would have created significant landfill gases?
Unfortunately, it is the case that there is no fixed, global system for applying these colour definitions to Hydrogen at present. Considering the evolution in production technologies, the importance of Hydrogen as a key solution in the global clean energy transition, the potential for these definitions to possibly change over time, between countries and even production companies themselves plus the clearly very important Psychological associations of colours then perhaps now is the time that there should be international agreement on colour definitions.
