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Green hydrogen versus blue hydrogen (and turquoise and grey hydrogen) - an explainer

Published: 03:31 22 Sep 2021 EDT

Renewable Energy Generation - Green hydrogen versus blue hydrogen (and grey hydrogen) - an explainer

Hydrogen, one of the smallest and lightest molecules in the universe, has excellent characteristics as a carrier or store of energy, which make it one of the most interesting tools in the road to reducing greenhouse gases and stopping global warming. 

While often thought of as a form of renewable energy, hydrogen is not really a fuel, as it takes energy to isolate it in the first place.

Fuel cells can create energy from splitting hydrogen and oxygen, while the process of electrolysis uses electricity to split water into hydrogen and oxygen. 

Not all hydrogen is truly clean and green, though, as the gas created is only as emission-free as the method by which it was produced.

Hydrogen is therefore grouped under three broad categories:

  • ‘Grey’ hydrogen is the currently dominant form of hydrogen manufacture, but as it is made using fossil fuels such as from coal gasification or steam methane reforming, this is not really green fuel at all.
  • ‘Blue’ hydrogen is where the CO₂ emitted during the production of grey hydrogen is sequestered via carbon capture and storage (CCS), so producing energy with pretty low emissions, with around 90-95% of CO2 captured.
  • ‘Green’ hydrogen, made by electrolysis using renewable energy, is the only version of the fuel that is free of CO2 at the point of use and at the point of production.
  • 'Turquoise' hydrogen is the newest addition, supposedly coming between blue and green. Turquoise hydrogen, like grey and blue hydrogen, is made using methane, with a process called methane pyrolysis producing hydrogen and solid carbon rather than as CO2 gas.

Green hydrogen is seen as potentially very useful for balancing the electricity grid as it can be created by fast-response electrolysers at a time of high renewable energy generation and used as power in fuel cells when power is in shorter supply.

Blue hydrogen was recently shown to cause more pollution than burning coal, according to a new study published in August 2021, with scientists at Cornell and Stanford Universities finding that the process does not prevent methane from entering the atmosphere as well as using large amounts of energy to separate and store the CO2.

But, as one of the authors of the study acknowledged, the study’s findings were based on worst-case scenarios. A thread on Twitter afterwards argued that using less extreme assumptions the global warming potential (GWP) would be 10 times lower than the study concluded.

Blue hydrogen, understandably, has received fervent backing from many who see it as a way of decarbonising (or maybe just greenwashing) oil & gas companies and chemicals companies, but the impact of blue hydrogen seems to depend very much on the entire production process, and depend very much on how much carbon leaks and is not captured and stored.

Although turquoise hydrogen uses methane as a feedstock, the methane pyrolysis process is driven by heat produced with electricity rather than through the combustion of fossil fuels. 

So in the future, turquoise hydrogen may be seen as a low-emission hydrogen if the thermal process is powered with renewable energy and the carbon is permanently stored or used. As the carbon output is in solid form rather than CO2, there is no requirement for carbon capture and storage, with the carbon potential being used in other applications such as a soil improver or to make goods such as tyres.

Turquoise hydrogen therefore has the potential to be zero-carbon if the electricity driving the pyrolysis is renewable, and can even be carbon negative if the feedstock is biomethane rather than natural gas.

In 2020, global production of all forms of hydrogen was around 70mln tonnes per year from direct manufacturing processes, with 98% of that produced from fossil fuels, mostly natural gas, according to analysis by the International Energy Agency, with only around 2% from lower-carbon or zero-carbon processes.

Backed by government 'roadmaps' to net zero, investment in hydrogen is rising fast - but as investors who care will want to know whether it is green. 

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