We could generate hydrogen from rocks while storing CO2 in them

We desperately need clean hydrogen for processes that can’t be powered by renewable electricity – and it might be possible to produce large quantities of it from deeply buried rocks while trapping carbon dioxide.

Researchers at the University of Texas at Austin have shown that this process works for a common type of rock in laboratory studies. They now want to work with companies on field trials.

“We hope to demonstrate that we will be able to produce hydrogen economically while sequestering CO2,” says team member Orsolya Gelencsér. It might even be possible to produce geothermal energy simultaneously, she says.

The combustion of hydrogen only produces water and therefore does not cause global warming. Hydrogen could therefore play a major role in achieving net zero, for example by helping to decarbonize industrial processes such as fertilizer production and steel manufacturing.

The problem is that almost all hydrogen is currently produced from fossil fuels, which means a lot of CO2 is emitted during its production. One way to avoid these emissions is to use wind or solar power to split water, producing hydrogen and oxygen.

This is starting to happen, but the hydrogen thus produced is currently more expensive. Its large-scale production would also require large amounts of renewable energy, meaning less of this green energy would be available for other purposes, such as replacing coal-fired power plants.

Hence the recent renewed interest in natural or geological hydrogen. Several processes can generate hydrogen in rocks, and under the right conditions the gas can accumulate and be extracted in the same way as natural gas. It could be clean and cheap, but no one knows yet how much natural hydrogen is available. While some researchers believe there could be large quantities of hydrogen to be exploited, others – including Gelencsér – believe that natural hydrogen resources could be limited.

The only place where almost pure natural hydrogen is extracted and exploited is in a village in Mali called Bourakébougou, and even then on a very small scale.

“I think it’s a very special case,” says Gelencsér. Hydrogen is typically produced at low rates, and because its molecules are tiny, it’s rare for overlying rocks to provide a good seal and allow it to accumulate, she says.

So many groups around the world are now working on ways to generate hydrogen from rocks, rather than waiting for it to happen naturally. This approach is known as stimulated hydrogen production and trials of different methods are already underway.

One way to do this is to simply pump water underground. Water reacts with certain types of rock to form hydrogen in a process called serpentinization, which is the source of a large amount of natural hydrogen. Pumping more water speeds up the process.

What Gelencsér and others realized is that any CO2 added to the water should react with rocks and be trapped as carbonates. A company called Carbfix already mineralizes CO2 in Iceland by adding it to water pumped underground at a geothermal power plant.

Gelencsér and his colleagues conducted laboratory tests with a type of iron-rich volcanic rock. They placed rock samples in a vessel pressurized to 1.2 to 1.7 megapascals and heated to 90°C to simulate conditions at depth, and added either water with CO2 or water with argon, an inert gas, as a control. Water rich in CO2 releases more hydrogen, probably because the CO2 forms carbonic acid which dissolves some of the rock and thus allows more water to react with the rock. There has been CO2 mineralization, as expected, and hydrogen production could be further increased by adding nickel chloride as a catalyst, Gelencsér said at a recent meeting of the European Geosciences Union in Vienna.

The researchers were able to release about 0.5 percent of the hydrogen that was theoretically possible by reacting with the rock. They believe they need to increase this amount to 1 percent for the process to be feasible. One way to do this would be to go deeper, where temperatures are higher, because this enhances serpentinization, says Gelencsér. This would cost more, but it might also be possible to harness the higher temperatures to produce geothermal energy.

Globally, there are huge volumes of such iron-rich rocks that, even at 1% efficiency, could potentially produce far more hydrogen than the 100 million tonnes currently produced worldwide.

“It’s good work,” says Barbara Sherwood Lollar of the University of Toronto.

“There is growing interest in approaches combining stimulated geological hydrogen production and CO2 mineralization,” says Aliaksei Patonia of the University of Oxford in the United Kingdom. “Many groups and start-ups are exploring variations of this concept. »

If companies could charge for CO2 storage in this way, as Carbfix does, the additional revenue would reduce risks and make projects more attractive to investors, Patonia says. But it remains to be seen whether either of these approaches will be viable.

Sherwood Lollar thinks we should harness the small amounts of natural hydrogen we know about and explore boosted hydrogen production. His team has just shown that a mine in Timmins, Ontario, for example, emits around 140 tonnes of hydrogen per year, which could be exploited locally.

“There is no silver bullet,” she says. “Each of these potential approaches can and should help – and we must act quickly to do so. »