After grey, blue, and green: turquoise hydrogen the horse to bet on?

It’s getting colorful but complicated. We already know grey hydrogen, the least environmentally friendly form, as it still generates CO2 when produced from gas, and blue hydrogen, where the CO2 is captured and stored. There is ‘green’ hydrogen, generated mainly by electrolysis using renewable electricity from solar or wind energy.

But why not consider ‘turquoise’ hydrogen (H2), also extracted from natural gas, but with the carbon (C) left as a solid byproduct that can be reused? Pyrolysis is a promising technology, a new report by Hydrogen Europe and German DVGW proofs. And as this process consumes the least energy, it might be a horse to bet on.

Technical neutral approach

Hydrogen Europe is the Brussels-based European association representing the interest of the hydrogen industry and its roughly +400 stakeholders. German DVGW is a recognized standardization body for the gas and water industry and a center for technical and scientific know-how.

The study they presented in Brussels last week includes life cycle emissions calculations showing that hydrogen from pyrolysis is clean and can even be carbon-negative.

Representing the industry that today still depends up to 95% on ‘grey’ hydrogen made from natural gas, they argue for “a technology-neutral approach to hydrogen. That will provide airtime to lesser-known production methods such as pyrolysis to enable its development and contribution to the energy transition.”

Pyrolysis process

So what is this ‘turquoise hydrogen’ all about? Like ‘grey’ hydrogen, it is produced from fossil sources. Grey hydrogen is obtained from fossil fuels such as crude oil or natural gas via steam reforming. In that process, the CO2 is released unhindered into the air, making it the environmentally least preferred method but currently the cheapest.

By capturing that CO2 in the process and storing it like in underground former gas wells, you get a ‘CO2-neutral’ form of ‘blue’ hydrogen. But especially the Carbon Capture and Storage (CCS) of the C02 in gaseous form is technically complex and expensive.

“In the pyrolysis process for making turquoise hydrogen, methane (CH₄) is separated directly into hydrogen, and solid carbon at very high temperatures in the absence of oxygen,” the authors of the report, Prof. Dr. Gerald Linke, Chairman of the Board of DVGW and Jorgo Chatzimarkakis, CEO of Hydrogen Europe say.

“It is much easier to handle than the gaseous CO₂ produced in steam reforming, and the carbon can be used in its solid form in various production processes or safely deposited,” they add. Per kilogram of H₂, about 3 kg of solid carbon (C) is produced.

Depending on the allotrope (graphite, graphene, soot, carbon tubes, turbostratic carbon, etc.) and the purity of the carbon produced, it may be used for various applications,” Robert Obenaus-Emler, a researcher at the University of Leoben, says.

Solid carbon as a byproduct

This can be high-tech applications like carbon nanotubes, high-performance materials like carbon fibers used in today’s racing cars or aerospace industry, supercapacitors, and micro-porous carbon tanks.

But it can also be used as a raw material for rubber and plastic products and in the asphalt and ceramics industry, as an additive for lubricants, casting powders, electrode materials for the metallurgical industry, and as a feedstock for batteries, he adds.

Other good news, the authors say, is that pyrolysis requires less energy than other forms of hydrogen production despite the high temperatures (above 1 000°C) needed. Methane (CH₄) requires seven times less energy than a water molecule (H₂O) with electrolysis to produce the same amount of hydrogen.

“Pyrolysis is considerably more efficient than other processes, with a specific energy demand of 37.8 kJ/mol H2 compared with 63.3 kJ/mol H₂ for steam reforming or 285.9 kJ/ mol H₂ for hydrogen production by electrolysis,” it’s argued with technical figures.

And there are – like always – disadvantages too. “Challenges include the energy demand of the high-temperature reaction in combination with the uncontrolled deposit of solid carbon on the reactor walls and catalysts, as well as the possible formation of undesirable gaseous byproducts,” the German DBI (Gastechnologisches Institut) warns.

But to avoid these disadvantages, DBI is developing an innovative process with the steps of “hydrogen production” and “carbon deposition” separated, they claim.

Two decades to replace fossil

So is turquoise hydrogen the Holy Grail? Probably not, as there are several other methods to produce ‘green’ hydrogen maturing nowadays. Like biomass fermentation, algae and
thermochemical cycles, or the hydrogen-solar panels researchers at the Belgian KU-Leuven university are developing? The latter uses only solar energy to filter the hydrogen out of the ambient air directly. How much cheaper can you produce?

We should bear one thing in mind: the majority of final energy consumption in Europe is currently covered by molecules – i.e., natural gas, mineral oil or coal – and only one-fifth by electricity, even though half of this is now renewable,” the authors of the report say. “This means fossil fuels must be rapidly replaced in the next two decades.” And every little extra helps.









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