Study Spotlights Coal as a Hydrogen Storage Mechanism

A study out of Pennsylvania State University (PSU) found that geological coal formations can be used to store hydrogen gas, similar to how batteries store energy for electricity. 

 

A researcher from Pennsylvania State University’s Energy and Mineral Engineering Department holds a piece of coal. Image used courtesy of PSU

 

The researchers spent years developing a novel experiment design to measure hydrogen sorption capacity and diffusion behaviors in eight coal fields across the United States. All candidates showed substantial sorption capabilities, thus bringing an opportunity to tap into existing coal reserves to store hydrogen in the ongoing transition from fossil fuels to renewables. It could also expand the nation’s now-nascent hydrogen infrastructure landscape. 

Depleted coalbed methane reservoirs were identified as the best resource for hydrogen storage, featuring seams holding natural gas such as methane, which attaches to the surface of the coal via adsorption. 

PSU notes that the researchers found hydrogen storage in depleted coal seams works best with anthracite and semi-anthracite coals. At the same time, low-volatile bituminous coal is ideal for gassy coal seams. 

 

Hydrogen Sorption and Diffusion

Renewables such as solar photovoltaics and wind turbine generation are naturally intermittent, relying on weather patterns, and less able to adequately respond to unexpected demand spikes and drops, such as those seen in heat waves. Geologic coal formations show promise for addressing this volatility, as they can store large volumes of hydrogen for later use, similar to how batteries work. 

Hydrogen produces little to no greenhouse gas emissions, making it a promising renewable energy source. But several factors set the market back from widespread commercialization for powering the country’s largest polluting industries, such as transportation, electric power, and manufacturing. Hydrogen’s challenges are costs, durability, reliability, performance, and the lack of hydrogen infrastructure. 

In the study, published in Applied Energy, the PSU researchers noted that a full-scale hydrogen economy would require a bulk storage system to store excess energy as a buffer and fill the demand continuously. But hydrogen is difficult to store due to its high diffusivity in various types of material.

However, large-scale geologic storage within salt caverns, saline aquifers, depleted oil or natural gas reservoirs, and coalbed methane reservoirs extend new possibilities for long-duration energy storage. Of all these, depleted coalbed methane reservoirs offer a fixation of hydrogen via sorption, with plenty of existing infrastructure at coal production facilities and several sites that have been trialed for carbon dioxide and flue gas storage. 

Coal already has a substantial infrastructure behind it, widely available nationwide and near urban centers. According to the U.S. Energy Information Administration (EIA), America’s demonstrated reserve base—meaning in-place coal that can be mined commercially—is about 471 billion short tons, with 69% being underground mineable coal. Another measure, estimated recoverable reserves (which consider accessibility limits and recovery factors), totals 251 billion short tons, of which 58% is underground mineable coal. Lastly, recoverable reserves as existing mines are estimated to be 12 billion short tons, with around half being surface mineable. 

 

Map of coal fields by age, rank, and province in the U.S. Image used courtesy of the U.S. Geological Survey

 

The PSU researchers studied the sorption and diffusion behaviors of eight coal samples from major coal fields nationwide, evaluating their ranks, fixed carbon content, and other factors. While all showed promise, the best performance was achieved by low-volatile bituminous coal from the eastern part of Virginia and anthracite coal from eastern Pennsylvania. 

For context, bituminous coal makes up almost half of all U.S. coal production as of 2021 (the latest available dataset), according to the EIA. Anthracite only claims a tiny fraction of the pie. 

 

Underground Hydrogen Storage Use Cases

Hydrogen is already stored underground in salt caverns at a handful of sites in Europe and the U.S., according to the International Energy Agency (IEA). Still, those sites largely haven’t been tested to determine if hydrogen can be injected and extracted rapidly to keep up with the characteristic volatility of wind and solar resources. In PSU’s press release, the researchers said their future work would explore this realm, studying coal’s dynamic diffusivity and permeability to gauge if hydrogen can be injected and pumped out quickly. 

Per the IEA, some notable underground hydrogen storage use cases include HyStock in the Netherlands, a site testing pure hydrogen storage in salt caverns; Sun Storage in Austria, with mixed and pure hydrogen storage in gas fields and underground methanization via combined hydrogen and carbon dioxide injection; and Argentina’s Hychio, using mixed hydrogen storage in gas fields with the same combined method as Sun Storage. 

The PSU study also mentions a few underground hydrogen storage cases implemented worldwide, such as ConocoPhillips’s underground salt dome in Texas that stores about 1 billion cubic feet of hydrogen and Sabic Petroleum’s Teesside project in the U.K., which has stored near-pure hydrogen in three salt caverns since the early-1970s. Another salt cavern site in Kiel, Germany, has reached a 60% hydrogen storage capacity. 

Others include France’s Beynes aquifer, which stored town gas containing up to 60% hydrogen from the 1950s to the 1970s. And in Lobodice, Czech Republic, town gas containing 54% hydrogen was stored in a sandstone reservoir around 1,640 feet deep.