Liquefied natural gas (LNG), exported widely as fuel, contains significant embedded energy beyond its burnable chemical energy. The energy that was used to cool and compress it into liquid form, known as “cold energy,” is an untapped resource. Researchers have proposed using LNG’s cold energy to cool carbon dioxide (CO2) into dry ice as part of a carbon capture process. In doing so, they hope to lower the energy required for carbon capture; however, it is still unclear how much energy could be saved (ACS Sustainable Chem. Eng. 2021, DOI: 10.1021/acssuschemeng.1c05892).
Japan is one of the top importers of LNG, says Koyo Norinaga of Nagoya University. After the LNG arrives on tanker ships, it gets fed into the local gas pipeline network, and the coldness generated when it expands into a gas is used for refrigeration at the seaport. But because Japan imports so much LNG—around 80 million metric tons annually—a significant amount of its cooling power is wasted and released to cool seawater, Norinaga says.
Norinaga, Hiroshi Machida, and their colleagues thought this cold energy could assist with capturing CO2 emissions at power plants or from the air. CO2 is usually captured with a sorbent like monoethanolamine, and one of the most energy-intensive steps in that process is regenerating the sorbent by heating it to force out the absorbed CO2 gas. But if the CO2 is cooled into dry ice as it exits the sorbent, the resulting pressure drop could pull more CO2 out of the sorbent, instead of requiring heat, Norinaga says.
Norinaga and his colleagues identified a commercially available amine sorbent that, when combined with an organic solvent, absorbs CO2 from flue gas at high pressure, releases it at low pressure, and would not itself vaporize in a low-pressure environment. The researchers calculated that if the CO2 absorption occurs at standard atmospheric pressure (101.3 kPa) and the desorption and dry ice deposition happens at 1 kPa, the required energy to capture CO2 could be as low as 0.25 GJ/ton under ideal conditions—almost one-eighth of the energy needed compared with the usual heat-driven sorbent regeneration process.
But some experts think that in practice, much more energy would be required. “The best studies that I have seen so far are something of the order of slightly higher than 2 GJ/ton CO2,” says Paul M. Mathias, who models chemical processes, including carbon capture, at Fluor Corporation. “These guys are claiming something really low.”
Nevertheless, Mathias agrees that finding uses for the “free” cold energy of LNG is worthwhile and suggests the proposed process could have an unexpected edge. In a typical CO2 capture process, the CO2 gas released from the sorbent needs to be compressed into a liquid to make it cheaper to transport, Mathias says. However, compression consumes significant energy, so Mathias encourages the researchers to directly convert the captured dry ice into a liquid or a pressurized gas. “That could be a pretty big advantage.”
Norinaga says that their calculated energy savings do include those from avoiding the compression process. The team is working on further experiments and simulations to calculate energy losses under nonideal, real-world conditions. The researchers are currently working in collaboration with Toho Gas Company and with funding from the Japanese government to develop a small pilot facility in Nagoya that would capture 1 ton of CO2 per year using their method.