Hydrogen has been touted as a solution to the climate crisis as it has the energy density required to power heavy goods vehicles and other forms of transport, as well as applications in heavy industry. But storing it has proved problematic—coal could be the unlikely answer.
“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.”
Geological formations could store large amounts of hydrogen and help to balance the grid. “Coal is well-studied, and we have been commercially producing gas from coal for almost a half century,” Liu said. “We understand it. We have the infrastructure. I think coal would be the logical place to do geological hydrogen storage.”
Liu was part of a team that analysed eight different types of coal from across the United States to understand their 'sorption' properties, an indicator of how much hydrogen they could hold.
All eight coals showed considerable sorption properties, with low-volatile bituminous coal from eastern Virgina and anthracite coal from eastern Pennsylvania performing the best in tests, the scientists reported in the journal Applied Energy.
“I think it’s highly possible that coal could be the very top selection for geological storage from a scientific perspective,” said Liu. “We find that coal outperforms other formations because it can hold more, it has existing infrastructure and is widely available across the country and near populated areas.”
Potentially promising sites include former methane reservoirs which have now been depleted, where methane once stuck to the surface of the coal in a process called adsorption. Hydrogen could be injected in the same way, with a layer of shale or mudstone on top acting as a seal. “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.”
Special equipment was required to run the experiments, as traditional equipment wouldn't have worked due to coal's weaker affinity with hydrogen. “We did a very novel and very challenging design,” Liu said. “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.”
This approach could also help revitalise former coal mining communities. “In the energy transition, it’s really coal 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.”
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