Researchers have devised a novel method to boost the natural-gas storage capacity of porous adsorbent materials, which can then be kept and transported at low pressure (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.6b00919). The trick is to seal high-pressure gas inside porous beads using hydrocarbon plugs that can be slowly removed to release the gas, effectively turning the beads into tiny gas tanks. The advance could lead to a compact, lightweight, and low-cost technology for storing natural gas, which has been a critical hurdle for its widespread adoption as a vehicle fuel.
Natural gas, which is mostly methane, is the cleanest burning of all fossil fuels. But it hasn’t taken hold as a vehicle fuel—it’s mostly used in buses—because of its low energy density by volume. To overcome that, it is stored in liquefied form using expensive cryogenically cooled systems, or compressed and stored at high pressure (20–30 megapascals) in large, bulky tanks.
A promising alternative is to store natural gas in porous, high-surface-area adsorbent materials, which should allow lower storage pressures and hence less costly and bulky tanks. The Advanced Research Projects Agency-Energy (ARPA-E) set a challenge to pack 315 L of natural gas per liter of sorbent (or 12.5 MJ of energy per liter) at a low pressure of 3.5 MPa. Scientists have tried microporous adsorbents such as zeolites, metal-organic frameworks (MOFs), and carbon nanotubes for this, but so far none have met ARPA-E’s requirements.
Moises A. Carreon of Colorado School of Mines, Shiguang Li of the Gas Technology Institute, Miao Yu of the University of South Carolina, and their colleagues came up with the idea of loading natural gas into known adsorbents and then sealing it in with a coating that acts like a pressure valve. “So there is high pressure gas inside but outside is a low pressure,” Li says.
To demonstrate the concept, they first coated commercially available zeolite beads with amorphous silica and an alumina layer, which reduces the material’s pore sizes to 1.34 to 1.42 nm. Shrinking the pores reduced leakage of adsorbed gas, the team found.
Then the researchers injected methane at 5 MPa into a tank containing the coated beads. The gas enters the beads through the coating’s pores and sticks. Once the beads were fully loaded, the researchers exposed them to 2,2-dimethylbutane vapor. The hydrocarbon molecules enter the coating’s pores and seal them. The researchers then decreased the pressure of the tank and stored the beads at a pressure of just 0.1 MPa.
Tests showed that the zeolite beads remained sealed at room temperature for up to 24 hours. To release the methane, the researchers heated the beads slowly up to 150–200 °C in order to gradually drive off the 2,2-dimethylbutane sealant from the pores so that the pressurized gas isn’t released all at once.
The coated beads stored twice the amount of methane as uncoated beads at 0.1 MPa. And, at that same pressure, they could store five to seven times more gas per liter than advanced adsorbents like MOFs. By using the sealing technique with MOFs, which currently have a capacity of less than 8 MJ/L at 3.5 MPa, the researchers say they could make adsorbents that exceed the ARPA-E requirements. Carreon says that they are now tweaking their coating and sealing chemistry to apply it to MOFs.
Yu says coated adsorbent pellets loaded with natural gas could be stored in lightweight cartridges. “At a gas station, you’d dump out cartridges with empty sorbent and swap it with a new cartridge.”
“This is a clever and elegant approach to improve methane storage in porous structures at low pressures,” says Haiqing Lin of the University of Buffalo. Used with advanced porous materials, he says the approach shows the promise of meeting ARPA-E targets and making natural-gas vehicles more appealing.