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Is geological hydrogen a green solution to our energy needs?

A growing number of companies are interested in extracting hydrogen gas from under the ground, with at least one claiming it is “zero carbon”. Now, the first assessment of this kind of geologic hydrogen production has found that while it would still lead to increased global warming, this would be comparable to or lower than hydrogen production from renewable sources. However, at least one expert thinks the study underestimates the warming effect.

Burning hydrogen produces only water, so it is widely seen as a green alternative to natural gas. However, at present, most hydrogen is produced from natural gas, which results in high greenhouse gas emissions.

A few companies are producing so-called blue hydrogen, which involves capturing some of these emissions. Then there is green hydrogen, made by splitting water using power from wind or solar, thus keeping emissions down.

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Now, there is a new contender, known as geologic hydrogen, which is produced deep underground by processes such as rocks reacting with water. Largely ignored until recently, it is now attracting a lot of commercial interest. Earlier this month, for instance, a start-up called Koloma revealed that it has raised $90 million to explore geologic hydrogen production.

But huge uncertainties remain about how much hydrogen could be extracted. “As it is so early in development, we don’t have much information about how abundant and cheap it will be to produce geologic hydrogen, so it remains to be seen whether or not it can be a major energy source,” says Adam Brandt at Stanford University in California.

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It is also unclear what geologic hydrogen’s environmental impact would be, so Brandt has made the first estimate of its likely greenhouse emissions in a study funded by Smart Gas Sciences, a company interested in geologic hydrogen.

One issue is that while hydrogen isn’t a greenhouse gas, it alters atmospheric chemistry in a way that increase the lifespan of greenhouse gases, such as methane. This means any hydrogen leakage causes global warming.

Geological hydrogen also contains varying amounts of methane and nitrogen, so the gas must be purified and then compressed before transport and use, which requires energy and so can lead to further emissions

As a base case, Brandt assumes a source that is 85 per cent hydrogen, 12 per cent nitrogen and 1.5 per cent methane, where waste gas is re-injected underground and the process is powered using some of the hydrogen. In this case, he estimates that the production of 1 kilogram of pressured hydrogen would result in the emission of the equivalent of 0.4 kilograms of carbon dioxide.

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However, emissions would be several times higher in various scenarios. If the waste gas was burnt off, the carbon intensity would rise to 1.2 kg of CO2 equivalent per kg of hydrogen. If the source gas contained less hydrogen, it would be 1.5, while an unproductive well could be as high as 16.1.

Comparing this with other sources is tricky, as different methods and assumptions result in widely varying estimates. But one recent study concluded that hydrogen from natural gas has a carbon intensity of 71 to 82 kg CO2 equivalent per kg of hydrogen. For blue hydrogen it was 15 to 53, and for green hydrogen it was 3 to 13.

Another study by Kiane de Kleijne at Radboud University in the Netherlands estimated that the carbon intensity of green hydrogen is between 0.4 and 17 kg CO2 equivalent per kg of hydrogen.

So if Brandt’s estimates are correct, geologic hydrogen could be the greenest source of all. “I think it is good news,” he says.

But Robert Howarth at Cornell University in New York isn’t convinced that geologic hydrogen will result in lower greenhouse gas emissions than green hydrogen. He thinks the study underestimates the warming effect for several reasons – for instance, a paper published last month concluded that the warming effect of hydrogen is much higher than previously thought.

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Assumptions about leakage are also critical, says Howarth. “Hydrogen is the smallest molecule in the universe, and so we would expect it to be very, very leaky.  The more the gas is handled, stored and transported, the greater the emissions will be and the worse the greenhouse gas consequences.”

“My take-home is that the greenhouse gas footprint of geological hydrogen remains poorly known because we do not know what to expect for hydrogen emissions,” he says.

One big advantage geologic hydrogen has over green hydrogen is that it doesn’t require renewable energy, meaning this energy remains available for other things. However, we need to slash emissions by 2050 and it isn’t clear if any significant quantity of geologic hydrogen can be produced by then, says de Kleijne.

“I don’t think it should be considered a replacement of other ways of producing hydrogen with lower emissions that are in much further stages of technological development,” she says.

Journal reference:

Joule DOI: 10.1016/j.joule.2023.07.001

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