Hydrogen is widely touted as an essential feature of the energy transition and the road to net-zero as governments and businesses look to shift away from traditional fossil fuels and scramble to cut carbon emissions.
Abstract Big Bang - hydrogen rainbow. Credit: Mikhail Doroshenko / Shutterstock
Hydrogen is the most abundant element in the universe and was theorised to have been formed during the Big Bang. The following image is an abstract concept of the Big Bang. Credit: Mikhail Doroshenko / Shutterstock
However, not all fuels are made equal. Despite the gas itself being colourless, a number of types of hydrogen, colour-coded by industry over the years, could lead to confusion for those not in the know.
Read more: Hydrogen "not cheap" & "needs support" says World Energy Council
This guide will help readers distinguish between the various forms of hydrogen, their uses, and their role in the road to reducing emissions.
In its raw form, hydrogen is sometimes referred to in the industry as "white hydrogen". This is hydrogen as it naturally occurs.
Hydrogen, alongside helium, is currently one of the most readily available elements in the universe. The gas makes up 75% of the mass of the known universe and is currently theorised to have been created during the Big Bang.
However, no real methods currently exist to utilise white hydrogen, so the gas must be sequestered or otherwise generated through different means.
The primary form of hydrogen in use today is called "grey hydrogen", which is currently the most common form used in industry. Grey hydrogen refers to hydrogen that is generated with the use of natural gas, methane, or through a process known as "steam reforming" and involves no carbon capture.
More than 70 million metric tonnes of grey hydrogen are produced globally every year, which offers room for a switch to more renewable generation and is a burgeoning market ripe for innovation.
In the processes for these types of generation, the greenhouse gases used in the production of grey hydrogen production are not captured and leak into the atmosphere, which only adds to current greenhouse gas emissions. There are currently a number of initiatives to slowly phase out the use of grey hydrogen and replace it with its more environmentally friendly forms.
However, despite its various downsides, grey hydrogen still generates fewer emissions than those produced by "black hydrogen" or "brown hydrogen", which, as their names suggest, is hydrogen generated through the burning of fossil fuels, mainly lignite (brown) or coal (black).
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Both black and brown hydrogen are highly polluting and result in not only high carbon dioxide emissions, but can also release carbon monoxide - an odourless, tasteless, yet toxic greenhouse gas that can kill in high dosages.
Should the greenhouse gases - primarily carbon - used in the production of grey hydrogen be captured or otherwise sequestered, then the resulting product is known as "blue hydrogen". Blue hydrogen is mainly generated through the process of steam reforming mentioned earlier.
Blue hydrogen. Credit: Alexander Limbach / Shutterstock
Blue hydrogen is often referred to as "low-carbon hydrogen" - a reference to the fact it produces far fewer emissions than the other types of hydrogen generated through fossil fuels.
However, there have been warnings in academia that the use of blue hydrogen may be more polluting than burning coal or natural gas, with other concerns being raised that 10-20% of emissions from blue hydrogen cannot be sequestered.
The gases released by blue hydrogen generation are captured using carbon capture and storage (CCS) technology and usually stored deep underground.
When people refer to hydrogen as the "fuel of the future", it is likely they are referring to "green hydrogen". Simply put, this refers to hydrogen generated through renewable energy - primarily electrolysis powered by wind energy or solar power - where no greenhouse gases are emitted.
Green hydrogen is currently considered essential in the energy transition owing to its extreme versatility. There are also rumours it may be able to use existing fossil fuel infrastructure, which could help governments cut costs in the switch.
However, green hydrogen is currently three times more expensive than natural gas in the US, with similar concerns raised in other parts of the world.
More astonishingly, the only by-product of green hydrogen generated through electrolysis is water vapour, which means there is little environmental risk associated with its generation.
Green hydrogen has as many applications as grey hydrogen for industrial use, from steelmaking, to use in hydrogen fuel cells for the automotive sector or to helping decarbonise cement.
Read more: Steeling the future: Using hydrogen to make green steel
While the term "green hydrogen" is quite broad and generally refers to any zero-emission hydrogen generated through electrolysis, when hydrogen is primarily generated through solar energy, it may be referred to as "yellow hydrogen" - though this is a far less common term.
However, electrolysis does not need to rely on just wind or solar energy to generate hydrogen. A newer term, known as "pink hydrogen" refers to any hydrogen generated through electrolysis powered by nuclear energy.
France's EDF Energy is currently planning to produce pink hydrogen at the controversial Sizewell C plant in Suffolk, England, which has seen backlash from environmental activists.
In addition to this, the high temperatures used in the generation of pink hydrogen could be used in steam reforming, leading to slightly lower emissions in the production of blue and grey hydrogen.
A more novel form of hydrogen production that sits between blue and green is known as "turquoise hydrogen". A newer term, it is often being touted as a lower-carbon form of blue hydrogen while mitigating the higher costs associated with green hydrogen production.
It is generated through a process known as "methane pyrolysis". Like blue hydrogen, it still uses methane as feedstock, but the heat used in the process is generated through electricity - usually renewably sourced electricity - rather than from the burning of fossil fuels.
Like most of the methods referenced, a byproduct of turquoise hydrogen is carbon. However, the carbon generated through this method often comes out in a solid form, though storage technology is sometimes utilised.
As a result of this, carbon capture technology is not necessarily required to stop greenhouse gas emissions, and solid carbon can be used in other applications, such as in the manufacturing of car tyres or batteries.
Hydrogen can also be generated through biomass, which may be a good way to get rid of excess waste, like with those seen in schemes to convert plastic waste into hydrogen.
Read more: Plastic waste-to-hydrogen technology greenlit for trial
It is likely that, as new methods for energy generation develop, new ways of turning that energy into hydrogen will develop, which could open new doors to efficiency and applications for the most abundant element in the universe.
The push for net-zero will likely ensure these methods become more and more sustainable.
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