Keio Research Highlights

Unconventional sources of natural gas, such as so-called 'flammable ice' reserves located in the Sea of Japan, may soon find wider use thanks to carbon capture technology developed by Keio researchers¹.

Generating electricity from natural gas instead of coal or oil is a promising way to reduce greenhouse gas emissions. This energy, composed primarily of methane (CH₄) and other light hydrocarbons, burns cleaner than other fossil fuels because it is relatively free of impurities like carbon dioxide (CO₂) and hydrogen sulfide. However, rising demand means that lower-quality natural gas supplies, with trickier purification requirements, are being targeted for extraction.

Chemical absorption and membrane separation are typical techniques used to remove CO₂ impurities from natural gas. But these treatments suffer from disadvantages such as the use of corrosive liquids and the need to constantly clean clogged filters. Ryo Ohmura and colleagues from Keio University's Department of Mechanical Engineering investigated a way to avoid such complications by using simple water molecules with surprising gas-trapping abilities.

Clathrate hydrates are ice-like complexes that form naturally on ocean seabeds, where temperatures plummet and pressures are enormous. These conditions cause water molecules to hydrogen-bond into cages and trap gases such as CH₄ or CO₂ at densities tens to hundreds of times greater than possible through typical compression techniques (see image).

The hydrates are also selective, explains Ohmura, and only stabilize when the gas compound fits suitably inside the aqueous cage: molecules too big or too small cause the complex to collapse. Since CO₂ molecules are larger than CH₄ molecules, the hydrates can be used to purify natural gas by selectively encapsulating and removing unwanted CO₂.

The team tested the carbon-capturing performance of hydrates under conditions that model the continuous operations of natural gas refineries. Using a small-scale reactor, they passed a CO₂-CH₄ mixture through a hydrate slurry and analyzed compositions for over 35 hours. Eventually, steady-state conditions ― with significantly reduced CO₂ concentrations ― were reached.

The researchers then compared the experimental data to computational algorithms designed to uncover the thermodynamic and kinetic factors behind hydrate purification. "With more accurate models, we can identify ways to improve the hydrate formation, and understand long-term CO₂ capture more precisely," says Ohmura.

This technology has other, intriguing applications that could even change what we eat for dessert. Ohmura and his team are developing hydrates embedded with CO₂ that can carbonate frozen food ― a potential way to inject champagne-like effervescence into ice creams and jellies.

1. Tomita, S., Akatsu, S. & Ohmura, R. Experiments and thermodynamic simulations for continuous separation of CO₂ from CH₄ + CO₂ gas mixture utilizing hydrate formation. Applied Energy 146, 104-110 (2015). | article