Methane in the cage

When a stream of methane is passed through highly chilled water at elevated pressure, a glassy mass is formed, consisting of water ice crystals in which methane molecules are caged. Such substances are called clathrates, or cage compounds (from the Latin ciatrutus, meaning caged). Methane clathrates are the most common in nature, but methane is not the only one capable of such bonds – molecules of carbon dioxide, hydrogen sulfide, ethene, and many other compounds can also be incorporated into the structure of water ice. There is one condition: the molecules, candidates for confinement in the ice cells, must have the appropriate dimensions, matching the size of the cages. The bonds are stable only under certain temperature and pressure conditions; under other conditions, the ice melts, releasing significant amounts of gas.

Methane clathrate doesn't look any different from ordinary ice. However, upon handling a sample, it turns out it's not cold! This is due to its very low thermal conductivity; methane ice feels like Styrofoam. The fire test is impressive – the ice burns with a steady, red flame, leaving a puddle of water behind. It is a highly concentrated fuel – 1 m³ of hydrate decomposes to produce 164 m³ of methane. The energy content of the hydrate is significantly higher than that of hard coal and crude oil, and similar to LNG – liquefied natural gas, the most concentrated carbon-based fuel used in technology.