Scientists in Australia claim to have made a "eureka moment" breakthrough in gas separation and storage that could drastically reduce energy consumption in the petrochemical industry while making hydrogen much easier and safer to store and transport in powder form.
|Researchers at Deakin University have described a novel mechanochemical process that can safely store gases in powders while using very little energy in a repeatable process.|
Researchers at Deakin University's Institute for Frontier Materials claim to have discovered a super-efficient method of mechanochemically trapping and holding gases in powders, with potentially enormous and far-reaching industrial implications.
Mechanochemistry is a relatively new term that refers to chemical reactions triggered by mechanical forces rather than heat, light, or electric potential differences. Ball milling provides the mechanical force in this case, which is a low-energy grinding process in which a cylinder containing steel balls is rotated so that the balls roll up the side and then drop back down, crashing and rolling over the material inside.
The team has demonstrated that grinding specific amounts of specific powders with precise pressure levels of specific gases can trigger a mechanochemical reaction that absorbs the gas into the powder and stores it there, resulting in what is essentially a solid-state storage medium that can safely hold the gases at room temperature until they are needed. The gases can be released as needed by heating the powder to a certain temperature.
The process is repeatable, and Professor Ian Chen, the co-author of the new study published in the journal Materials Today, tells us over the phone that the boron nitride powder used in the initial experiments loses only "a couple of percent" of its absorption capability with each storage and release cycle. ""Graphene and boron nitride are both very stable," he says. We're considering a restoration treatment that will clean the powders and restore their absorption levels, but we need to prove it works first."
A radical transformation of the massive gas separation industry
In terms of numbers, the results are absolutely astounding. This process, for example, could separate hydrocarbon gases from crude oil while using less than 10% of the energy required today. "At the moment, the petrol industry uses a cryogenic process," Chen explains. "When several gases combine, they cool everything down to a liquid state at very low temperatures and then heat it all together to purify and separate them. Different gases evaporate at different temperatures, which is how they are separated."
Of course, cryogenics is an energy-intensive process, and the Deakin team discovered that its ball milling process could be tuned to separate out gases just as effectively while using far less energy. They discovered that different gases are absorbed at different milling intensities, gas pressures, and time periods. Once the first gas has been absorbed into the powder, the process can be restarted with a different set of parameters to trap and store the next gas. Similarly, some gases are released from the powders at higher temperatures than others, providing a second method of separating gases if they are stored together.
|Gas separation by mechanochemical means using ball milling University of Deakin|
Using boron nitride powder, the team was able to separate a mixture of alkyne, olefin, and paraffin gases in their experiments. The process takes time; some gases were completely absorbed after two hours, while others were only partially absorbed after 20 hours. However, Chen believes that this is simply a matter of fine-tuning: "We're still working on different gases with different materials. We hope to publish another paper soon, and we also hope to collaborate with the industry on some real-world applications."
Even if it takes some time, the cost savings - as well as energy and emissions savings - make a compelling case for widespread adoption. "A 20-hour milling process consumes US$0.32 in energy," according to the paper. "It is estimated that the ball-milling gas adsorption process consumes 76.8 KJ/s to separate 1,000 liters (220 gals) of olefin/paraffin mixture, which is two orders of magnitude less than the cryogenic distillation process."
Even when the energy required to heat the powder to several hundred degrees and release the gas is considered, the process is extremely efficient. And cryogenic distillation is a critical but energy-intensive process; according to a 2016 study published in Nature, cryo-distillation separation of just the olefins propene and ethene, which are required for plastics, consumes roughly as much energy globally as all of Singapore - 0.3 percent of total global energy consumption. Distillation as a whole account for 10-15% of global energy consumption. As a result, this technology has the potential to make a significant global contribution.
Another area with enormous potential is solid-state hydrogen storage.
The gas separation use case would be a significant advancement in and of itself, but the team believes that by securely storing gas in powders, it has also unlocked a compelling way to store and transport hydrogen, which could play a key role in the coming clean energy transition.
Pure hydrogen is currently stored as a gas or as a cryogenic liquid. The gaseous form must be stored at around 700 times the normal atmospheric pressure at sea level, or more than 10,100 psi, which requires a significant amount of energy to compress and storage tanks capable of safely handling high-pressure loads. The liquid form must be cooled to 20.28 K (252.87 °C, 423.17 °F) below the boiling point of hydrogen at atmospheric pressure, and it must be kept cold and sometimes pressurized for as long as it is stored. This requires even more effort.
"For at least half a century, the scientific community has been trying to find a suitable sponge-type material that can store large amounts of hydrogen," Chen says. "We recently reported a technique for storing paraffin, but we can store much more hydrogen. It doesn't take much energy and it's safe; under normal conditions, it's quite stable, and the hydrogen won't be released unless it's heated to several hundred degrees. So there's a real chance that this will become a viable solid-state storage technology - not just for hydrogen, but also for ammonia and other fuel gases."
While heating the powder to hundreds of degrees may appear to be an energy-intensive process, Chen claims that the round trip from gas to powder and back to gas uses far less energy than even compressed gas."It's difficult to give exact figures because we're currently only conducting small-scale experiments in comparison to the gas separation study," he says. However, we believe it uses one-third, if not one-quarter, of the energy required to compress hydrogen gas. This can be improved on a larger scale or by optimizing the grinding conditions and materials. We're working to reduce the amount of energy needed to release the gas, and the more gas you store, the less energy is needed to release it. However, there is still a lot of work to be done."
With hydrogen safely stored in powder, it can be moved and warehoused extremely easily and safely - this could be a very compelling way to move bulk quantities of hydrogen for export or distribution, because it's both cheaper and easier to handle than gas or liquid, and the equipment required to release the gas for use at the other end will be fairly simple.
Chen believes the powder could also be used as a direct fuel for cars and trucks. "It may also have benefits in mobile applications," he adds, "which is currently the most difficult issue in the hydrogen energy community." However, if you want to do this in a vehicle, we'll need to consider a suitable tank or container, how to release it at a controlled rate and speed, what the fueling process will look like... and so on."
How does it compare in terms of volume and weight density? Chen claims that the powder has a hydrogen weight percentage of about 6.5 percent. "Every gram of material will store approximately 0.065 grams of hydrogen," he claims. "That is already higher than the US Department of Energy's target of 5%. In terms of volume, we want to store around 50 liters (13.2 gal) of hydrogen for every gram of powder."
However, comparing these weight and volume densities to gaseous or liquid hydrogen is complicated because many factors are involved. Fifty liters (11 gals) per gram may seem like a lot, but at atmospheric pressures, hydrogen is 467 times less dense than when compressed to 700 bar in a tank. So, each gram of powder stores roughly the same amount of hydrogen as 0.11 liters (3.62 fl. oz.) of compressed H2 gas.
Similarly, 6.5 percent appears to be a very small weight fraction - for every kilogram of hydrogen, you must also carry 14.4 kilograms of boron nitride. That has to be a deal breaker for any weight-sensitive use case, right? Not quite - as ZeroAvia's Val Miftakhov once told us, current compressed hydrogen tanks are much heavier than the fuel they carry, so you're still carrying at least 9 kg of tank for every 1 kg of hydrogen contained within. So, while the powder would still require its own container and heat-release system, it might not be that far off the mark.
It doesn't appear to be a solution for aviation, especially given the ultra-lightweight GTL cryogenic liquid tanks we looked at in April, which are claimed to increase the mass fraction of hydrogen by more than 50% even with all ancillary equipment, allowing hydrogen-powered planes to fly four times as far as current jet-fuel planes for half the fuel cost.
However, aviation is a particularly weight-sensitive mode of transportation. Powder-sequestered hydrogen could be so inexpensive, convenient, and simple to use that it becomes a no-brainer in long-haul trucking, for example. ""We really want to work with some truck companies," Chen says, "because our storage is far superior to the current best results." We want to collaborate with them to identify any obstacles to making this technology useful in vehicles. We need industry help on this."
Boron nitride is readily available in industrial quantities and is relatively inexpensive, but Chen believes the technique should also work with other materials. ""We're not limited to boron nitride; we're just using it as an example," he says. Another example would be graphene, and we are continuing to investigate other materials."
Clearly, this advancement has potentially enormous implications, as it could significantly reduce energy use, emissions, the green energy transition, and even fuel and chemical prices. The team has filed provisional patent applications, and we are excited to see what is possible as the method is refined and tailored to useful applications.
The research is published in the journal Materials Today.