Gas Goes Solid
Nearly 95 percent of the known gas fields in the world are too small to justify the costs required pipe the gas to a plant, turn it into a liquid, and then transport it on specially equipped tankers.
But a handful of researchers have an idea that could make these fields worth mining: rather than figure out cheaper ways to transport this cleaner-burning energy source from point A to point B as a liquid, why not change natural gas into a solid substance that’s easier and cheaper to transport?
Japanese researchers Hajime Kanda at Mitsui Engineering and Shipbuilding in Tokyo and Yasuhara Nakajima of Japan’s National Maritime Research Instititute think they’ve found a solution with the aid of hydrates, solid crystals in which natural gas-composed chiefly of methane-is caged inside of water molecules.
For decades, researchers have been looking for ways to gather these crystals from their deep-ocean deposits and reap what they expect could be a natural gas harvest. Kanda and Nakajima are taking an opposite approach. Rather than extracting methane from hydrates, they want to turn methane into hydrates-essentially, transforming the colorless and odorless gas into small pellets that can be easily stored, transported, and eventually turned back into natural gas. A few months ago Mitsui, in partnership with Osaka University, opened a demonstration plant near Tokyo to promote the concept and show that it works. If the Mitsui’s process proves feasible and economical, many untapped natural gas deposits could become vital energy sources.
Changing natural gas into a hydrate form for cheaper transport gained attention in the early 1990s. Norwegian petroleum engineers first proposed the idea after comparing the transport economics of liquid natural gas to natural gas hydrates, knowing that hydrates could store large amounts of natural gas in a small space. “More than 180 standard cubic feet of gas can be stored in one cubic foot of hydrate,” says Rudy Rogers, professor of chemical engineering at Mississippi State University, and an authority on industrial use of gas hydrates.
Another major advantage: “transporting natural gas as hydrates can be done at higher temperature and lower pressure than liquid natural gas, and the risk of ignition in transport is much lower,” explains Hugh Guthrie, who studies natural gas at the U.S. Department of Energy’s National Energy Technology Laboratory in Morgantown, WV. Much of the high cost of liquid natural gas comes from temperature and pressure demands on piping, shipping, and storage facilities.
Producing the hydrates requires mixing natural gas with water in a continuously stirred tank reactor. When gas is piped into the water from the bottom, hydrates form on the surface of the gas bubbles. Removing the residual water leaves behind a residue of hydrate powder. Kanda and Nakajima envision a hydrate-pellet production plant close to gas fields in Southeast Asia. From there, a pellet carrier would transport the hydrate load to plants where the pellets would be turned back into gas and piped to market.
The company’s demonstration plant produces as much as 600 kilograms of hydrates per day, moving the methane through all the necessary phases: hydrate formation, storage, pelletizing, and “controlled dissociation,” or separation of the gas and water. Whereas a liquid natural gas facility requires temperatures of -162 C, Mitsui’s plant operates at -10 C, which means huge savings in cooling costs. Kanda says the project, which is co-sponsored by the government’s New Energy and Industrial Technology Development Organization, demonstrates that hydrates can be a successful vector for gas transport.
Mitsui’s only significant competition in gas hydrate technology comes from another Japanese company, Mitsubishi. Mitsubishi is pursuing its own gas-to-solid technology based on a hydrate-oil slurry, a process whose main drawback is that it produces thousands of tons of excess water that need to be removed.
Because hydrates are still a mysterious substance, there are many scientific and engineering obstacles that could make the process cost prohibitive. But Kanda and his colleagues are optimistic. Japan, the world’s number two energy consumer, is investing heavily in hydrate research, especially as public opinion turns increasingly against nuclear energy as a crude-oil alternative. Mitsui researchers hope their tiny white pellets will be just what their country needs.
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