Storing hydrogen in coal can help create green energy
Increasing emissions of greenhouse gases into the atmosphere negatively affects global warming due to the burning of fossil fuels.
In June 2021, the US Department of Energy launched the Hydrogen Shot initiative to accelerate breakthroughs in more accessible, affordable and reliable clean energy solutions over a decade. Hydrogen is an environmentally friendly fuel that has the potential to revolutionize the energy supply chain and reduce fuel consumption. As a universal energy carrier and chemical feedstock, hydrogen has advantages that combine all national energy resources—renewable, nuclear, and fossil fuels—and enable innovations in energy production and end-use that can help decarbonize the three most energy-intensive sectors of the economy: transportation , power generation and production. For example, excess electricity generated from solar, wind, hydropower, biomass can be used to produce hydrogen fuel by electrolysis of water. Water and fuel cell systems are an ideal alternative to internal combustion engines and boilers. The biggest advantage of proton exchange membrane fuel cells (PEMFCs) over internal combustion engines in automotive vehicles is the fact that PEMFCs do not produce emissions when using hydrogen as fuel and air as oxidizer. The roadmap for the Hydrogen Shot initiative aims to reduce the cost of hydrogen to $1 per kg in 1 decade (1 1 1). The key technical challenges for hydrogen and related technologies are cost, durability, reliability and performance, as well as the lack of hydrogen infrastructure. A full-scale hydrogen economy requires a massive energy storage system to store excess energy as a buffer and continuously meet demand. However, hydrogen is particularly difficult to store due to its high diffusivity in many materials, high cost, and low storage efficiency. Large-scale geological storage in salt caverns, saline aquifers, depleted natural gas or oil reservoirs, depleted coalbed methane (CBM) reservoirs, and man-made hard rock reservoirs offers opportunities for long-term energy storage. Several underground hydrogen storage options are currently implemented around the world, including the salt dome in Clemence (USA), salt cavern storage projects in Teesside (UK) and Kiel (Germany), aquifers in Bain (France) and Lobod. ( Czech Republic); and a depleted gas reservoir in Diadem (Argentina).
Selecting a suitable reservoir candidate for underground hydrogen storage is a complex task due to influencing factors including cap rock condition, permeability heterogeneity, multiphase process during injection/production, fluid-rock interaction caused by gas-brine buffering, and geological uncertainties etc. Geological storage can be a game changer for hydrogen storage because of its large capacity. Hydrogen is an ultra-small molecule (~0.12 nm) that can potentially penetrate the geological formation faster and faster (which will contribute to gas loss). Therefore, prior to any engineering implantation of hydrogen storage in the field, it is necessary to quantify the interaction of hydrogen with the host geological environment to obtain baseline values for engineering design.
The quest to develop hydrogen as a clean energy source that could curb our dependence on fossil fuels may lead to an unexpected result: coal.
A team of Pennsylvania State scientists has found that coal could be a potential way to store hydrogen gas in the same way that batteries store energy for future use, removing a major hurdle in the clean energy supply chain.
“We discovered that coal can be this geological hydrogen battery. You can put and store hydrogen in it and store it there, and take it out accordingly when you need to use it.” said Shimin Liu, associate professor of energy and mining at Pennsylvania State University.
Of all geological formations, coal mines are the most interesting mainly due to the effect of coal walls binding underground hydrogen using the sorption mechanism.
As a result of the scientists’ tests, several potential sites for gas storage were tested. It was further found that, in general, effective hydrogen storage in coal formations requires a clear understanding of several important reservoir parameters that determine the holding capacity and transport behavior of the formations, including adsorption/desorption, diffusion and permeability. It has been widely studied that gas sorption and diffusion depend on coal grade, temperature, pore size distribution, pore availability, as well as gas pressure, etc. Iglauer et al. They experimentally measured the ability to adsorb hydrogen, which could reach 0.6 mol H2/kg coal at 14.3 MPa. They also found that the adsorption capacity first increased strongly with pressure (up to 4 MPa) and then stabilized, while temperature had very little effect. Further measurements of hydrogen adsorption on three different grades of coal: semi-bituminous, bituminous and anthracite at elevated temperatures were carried out.
The results showed that the excess adsorption capacity increases with increasing gas pressure, but decreases with increasing temperature. The hydrogen sorption capacity of coal quantitatively determines its potential to become a candidate for storage in wells from geological formations. In addition, the diffusivity/permeability of gas in coal seams controls the efficiency and economy of field development. It is well known that the injection of sorbent-type gases (CH4, СО2, etc.) into coal can lead to significant swelling with a sharp violation of permeability, while it is interesting that no changes in the permeability of coal occur. was observed with injected H2 gas. The scientists observed that hydrogen pressure increased the contact angles of the brine at 25 °C, but had no effect at 50 or 70 °C.
In addition, it is well known that natural coal has a strong relationship with CH4, and it has been proven that the sorption capacity of coal depends on coal quality, gas type, gas pressure, moisture content, mineralogical composition, pore availability, pore surface area. , pore volume, pore shape, surface functional groups, etc. However, the behavior of the hydrogen flow, which includes sorption and diffusion into the microporous coal matrix, remains completely unknown at this stage, and mainly efforts are needed to quantify these mechanisms and further create a systematic methodology for integrating these fluxes into an overall flux simulation and providing a reliable estimate of hydrogen stocks.
Hydrogen is an environmentally friendly fuel and is promising for use in the most energy-intensive sectors of our economy — transportation, power generation, and manufacturing. But there is still a lot of work to do to build the hydrogen infrastructure and make it an affordable and reliable source of energy, scientists say. This includes developing a way to store hydrogen, which is currently expensive and inefficient. Geological formations are an intriguing option because they can store large amounts of hydrogen to meet peaks and troughs in energy use as energy demand changes daily or seasonally, the scientists said.
“Coal is well studied, and we have been engaged in the industrial production of gas from coal for almost half a century. We fully understand this process. We have the infrastructure in place, and I think coal would be a logical place for geological hydrogen storage,” Liu said.
To test this, scientists analyzed eight types of coal from coal fields in the United States to better understand their sorption and diffusion potential, as well as how much hydrogen they can hold. All eight coals demonstrated significant sorption properties, with low-volatility bituminous coal from eastern Virginia and anthracite from eastern Pennsylvania performing best in the tests, the researchers reported in the journal Applied Energy.
“I think it’s quite possible that coal may be the best choice for geological storage from a scientific point of view,” Liu said. “We think coal is superior to other seams because it can contain more, has existing infrastructure and is widely available across the country and near populated areas.”
Depleted coalbed methane reservoirs may be the best candidates. These reservoirs contain unconventional natural gas, such as methane, and have become an important source of fossil fuel energy over the past few decades. The methane sticks to the coal in a process called adsorption. Similarly, the introduction of hydrogen into the coal will cause this gas to be absorbed and stick to the coal. These formations often have a layer of shale or mudstone on top that acts as a seal that keeps the methane, or in this case, hydrogen, sealed until it’s needed and can be pumped back out, the scientists said.
“Many people identify coal as a stone, but it is actually a polymer. It has a high carbon content and many small pores that can store much more gas. Thus, carbon is like a sponge that can hold many more hydrogen molecules compared to other non-carbon materials,” Liu said.
Scientists have developed special equipment for experiments. Coal has a weaker affinity for hydrogen than other sorbing gases such as methane and carbon dioxide, so traditional pressurized sorption detection equipment would not work.
“We made a very new and very complex technical process. It took years to figure out how to do it right. We had to properly develop a system of trial and error experiments based on our previous experience with coal and shale,” said Liu.
Based on their results, the scientists determined that anthracite and semi-anthracite coals are good candidates for hydrogen storage in depleted coal seams, and low-volatile bituminous coals are better candidates for gas-bearing coal seams.
The development of hydrogen storage in coal mines can open up new economic opportunities for these regions, as well as create the country’s hydrogen infrastructure.
“During the energy transition, coal communities suffered the most economically. This is definitely an opportunity to repurpose the coal region. They already have experience in energy and the necessary skills for this job. If we can build the infrastructure and change their economic opportunities, I think that’s something we have to look at as a chance to revive the industry.” Liu said.
According to the scientists, further work will focus on the dynamic diffusion and dynamic permeability of the coal, characteristics that determine how quickly hydrogen can be pumped in and pumped back out.
“I think the state of Pennsylvania is a good place to do all this research — we have coal reserves, we have natural gas, we have both engineering and economics at the university. It’s a logical place to do it,” Liu said.
Also contributing from Pennsylvania was Ang Liu, instructor in the John and Willie Leone Family Department of Energy and Field Development.
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