Switching to hydrogen could prolong the methane problem

Hydrogen’s potential as a clean fuel could be limited by a chemical reaction in the lower atmosphere, according to research from Princeton University and the National Oceanic and Atmospheric Association.

This is because hydrogen gas in the atmosphere reacts readily with the same molecule primarily responsible for breaking down methane, a potent greenhouse gas. If hydrogen emissions exceed a certain threshold, this joint reaction will likely cause methane to accumulate in the atmosphere – with climate consequences for decades.

“Hydrogen is theoretically the fuel of the future,” said Matteo Bertagni, a postdoctoral researcher at High Meadows Environmental Institute who works on the Carbon Mitigation Initiative. “In practice, however, it raises many environmental and technological concerns that have yet to be addressed.”

Bertagni is the first author of a research article published in nature communication, in which researchers modeled the effect of hydrogen emissions on atmospheric methane. They found that above a certain threshold, a leaky hydrogen economy, even replacing fossil fuel use, could cause short-term environmental damage by increasing the amount of methane in the atmosphere. The risk of damage is compounded by hydrogen production methods that use methane as an input, emphasizing the critical need to manage and minimize emissions from hydrogen production.

“We still have a lot to learn about the consequences of using hydrogen, so switching to hydrogen, an apparently clean fuel, doesn’t create new environmental challenges,” said Amilcare Porporato, Thomas J. Wu ’94 Professor of Civil and Environmental Engineering and the High Meadows Environmental Institute. Porporato is principal investigator and member of the leadership team of the Carbon Mitigation Initiative and is also an associate faculty member at the Andlinger Center for Energy and the Environment.

The problem boils down to a small, difficult-to-measure molecule known as the hydroxyl radical (OH). Often referred to as “the cleanser of the troposphere,” OH plays a critical role in removing greenhouse gases such as methane and ozone from the atmosphere.

The hydroxyl radical also reacts with hydrogen gas in the atmosphere. And since a finite amount of OH is produced each day, any increase in hydrogen emissions means more OH would be used to break down hydrogen, leaving less OH available to break down methane. As a result, methane would stay in the atmosphere longer, prolonging its warming effect.

According to Bertagni, the effects of a hydrogen spike, which could occur if government incentives for hydrogen production increase, could have decades-long climate impacts on the planet.

“If you emit some hydrogen into the atmosphere now, it will lead to a progressive accumulation of methane in the years to come,” Bertagni said. “Even though hydrogen in the atmosphere only has a lifetime of about two years, you’ll still have the methane feedback from that hydrogen 30 years from now.”

In the study, researchers identified the tipping point where hydrogen emissions would lead to increases in atmospheric methane, thereby undermining some of the near-term benefits of hydrogen as a clean fuel. By identifying this threshold, the researchers set targets for managing hydrogen emissions.

“It is imperative that we be proactive in setting hydrogen emission thresholds so that they can be used to design and implement future hydrogen infrastructure,” Porporato said.

For hydrogen, referred to as green hydrogen, which is produced by splitting water into hydrogen and oxygen using electricity from renewable sources, the critical threshold for hydrogen emissions is around 9%, according to Bertagni. That means that if more than 9% of the green hydrogen produced escapes into the atmosphere — whether at the point of production, at some point during transportation, or anywhere else along the value chain — atmospheric methane would increase over the next few decades, offsetting some of the climate benefits the move away from fossil fuels.

And for blue hydrogen, which is hydrogen produced by methane reforming followed by carbon capture and storage, the emissions threshold is even lower. Since methane itself is the primary input to the methane reforming process, blue hydrogen producers must consider direct methane leakage in addition to hydrogen leakage. For example, the researchers found that even with a methane release rate as low as 0.5%, hydrogen releases would need to be kept below about 4.5% to avoid increasing atmospheric methane concentrations.

“Managing hydrogen and methane leak rates will be critical,” Bertagni said. “If you just bleed out a small amount of methane and a little bit of hydrogen, the blue hydrogen you produce might not be much better than using fossil fuels, at least for the next 20 to 30 years.”

The researchers stressed the importance of the time scale over which the effect of hydrogen on atmospheric methane is considered. Bertagni said that in the long term (say, over the course of a century), moving to a hydrogen economy would still bring net benefits to the climate, even if methane and hydrogen leakage are high enough to warrant near-term warming. Eventually, he said, atmospheric gas concentrations would reach a new equilibrium, and switching to a hydrogen economy would demonstrate its climate benefits. But before that happens, the potential short-term consequences of hydrogen emissions could result in irreparable environmental and socio-economic damage.

So if institutions hope to meet mid-century climate targets, Bertagni warned that hydrogen and methane leakage into the atmosphere will need to be kept under control as hydrogen infrastructure begins to roll out. And since hydrogen is a small molecule that’s notoriously difficult to control and measure, he explained that managing emissions will likely require researchers to develop better methods of tracking hydrogen losses throughout the value chain.

“If companies and governments are serious about investing money in developing hydrogen as a resource, they need to make sure they’re doing it right and efficiently,” Bertagni said. “Ultimately, the hydrogen economy needs to be built in a way that doesn’t counteract efforts in other sectors to mitigate carbon emissions.”