Increased emissions of greenhouse gasses into the atmosphere have adversely been contributing to global warming as a result of burning fossil fuels. Hydrogen is a clean fuel that can potentially revolutionize the energy supply chain and decarbonize fuel consumption.
However, much work remains to build a hydrogen infrastructure and make it an affordable and reliable energy source. This includes developing a way to store hydrogen, which is currently expensive and inefficient. Geological storage can be a game changer for hydrogen storage because of its high capacity.
Now, researchers at Penn State say that coal, which is not generally thought of as a clean fuel source, could be a great material to store hydrogen gas, much like batteries store energy for future use.
“We found that coal can be this geological hydrogen battery,” said Shimin Liu, associate professor of energy and mineral engineering at Penn State. “You could inject and store the hydrogen energy and have it there when you need to use it.”
To put this theory to the test, researchers analyzed eight different types of coals from coalfields across the United States to better understand their sorption and diffusion potential or how much hydrogen they can hold. The best of the bunch was low-volatile bituminous coal from eastern Virginia and anthracite coal from eastern Pennsylvania.
“A lot of people define coal as a rock, but it’s really a polymer,” Liu said. “It has high carbon content with a lot of small pores that can store much more gas. So, coal is like a sponge that can hold many more hydrogen molecules compared to other non-carbon materials.”
The research team built special pressure-producing equipment to force the hydrogen into the coal. Coal has a weaker affinity with hydrogen compared to other sorbing gases like methane and carbon dioxide, so traditional pressurized equipment for determining sorption would not have worked.
“We did a very novel and very challenging design,” said Liu. “It took years to figure out how to do this properly. We had to properly design an experiment system, trial, and error based on our previous experience with coals and shales.”
Based on their results, the team determined anthracite and semi-anthracite coals are good candidates for hydrogen storage in depleted coal seams, and low-volatile bituminous coal are better candidate for gassy coal seams.
Future work will focus on coal’s dynamic diffusivity and dynamic permeability, features that determine how quickly hydrogen can be injected and pumped back out. The research could offer hope to communities that have been hit hard by the shift away from coal to fight climate change.
“In the energy transition, it’s really cool communities that have been the most impacted economically,” Liu said. “This is certainly an opportunity to repurpose the coal region. They already have the expertise – the energy engineer and skills. If we can build an infrastructure and change their economic opportunities – I think that’s something we should consider.”
- Ang Liu, Shimin Liu. Hydrogen sorption and diffusion in coals: Implications for hydrogen geo-storage. Applied Energy, 2023; DOI: 10.1016/j.apenergy.2023.120746