The quest to develop hydrogen as a clean energy source that could curb our dependence on fossil
fuels may lead to an unexpected place — coal.
A team of Penn State scientists found that coal may represent a potential way to store
hydrogen gas, much like batteries store energy for future use, addressing a major hurdle
in developing a clean energy supply chain.
Hydrogen is a clean burning fuel and shows promise for use in the most energy intensive
sectors of our economy — transportation, electricity generation and manufacturing.
But 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.
Geologic formations are an intriguing option, because they can store large amounts of hydrogen
to meet the peaks and valleys as energy demand changes daily or seasonally.
The researcher says that they thought coal would be the logical place to do geological
hydrogen storage.
To put this to the test, the scientists analyzed eight types of coals from coalfields across
the United States to better understand their sorption and diffusion potential, or how much
hydrogen they can 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.
The researcher says, they 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.
Depleted coalbed methane reservoirs may be the best candidates.
These seams contain unconventional natural gas like methane and have become an important
source of fossil fuel energy over the last several decades.
The methane sticks to the surface of the coal, in a process called adsorption.
Similarly, injecting hydrogen into coal would cause that gas to absorb or stick to the coal.
These formations often have a layer of shale or mudstone on top that act as a seal keeping
methane, or in this case hydrogen, sealed until it is needed and pumped back out.
A lot of people define coal as a rock, but it’s really a polymer.
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 scientists designed special equipment to conduct the experiments.
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.
The researchers did a very novel and very challenging design.
It took years to figure out how to do this properly.
They had to properly design an experiment system, trial and error based on their previous
experience with coals and shales.
Based on their results, the scientists determined anthracite and semi-anthracite coals are good
candidates for hydrogen storage in depleted coal seams, and low-volatile bituminous coal
are better candidates for gassy coal seams.
Developing hydrogen storage in coal mining communities could bring new economic opportunities
to these regions while also helping create the nation’s hydrogen infrastructure.
Future work will focus on the dynamic diffusivity and dynamic permeability of coal, features
which determine how quickly hydrogen can be injected and pumped back out.