Fire and Ice: The Story of Gas Hydrates
A gas hydrate crystal structure
This material consists of small cage-like structures of ice that contain within them methane (natural gas) molecules. A clump of gas hydrates looks much like a snowball -- but it is flammable! You could set it on fire and watch it burn with a clean blue flame.
Why haven't you ever seen gas hydrates? Why aren't we using them as an energy source?
Unlocking the Energy Cage
Immense quantities of natural gas can be found in icy gas-hydrate deposits, but getting it out is a great challenge.
Ice to Burn
How big is one trillion cubic feet (tcf)? Even though this is an immense volume, it is fairly easy to imagine its size. Here is one way. Go to a local soccer field or football field and stand at one end, near the goal. Look down the field at the other end, and imagine how long a line of about 30 such fields would be, if laid end to end. (This distance is about 3 km (about 1.9 mi), or about 3500 walking steps.) Now turn 90 degreed to your right, and imagine the same distance in that direction. Finally, look straight up, and imagine a vertical line into the sky extending that same distance. You have just looked along the three edges of a cube that would enclose about one trillion cubic feet of space! On average, the people of the world consume about seven tcf of natural gas each month!
The people of the world use natural gas (methane, CH4), one of the fossil fuels, to provide 25% of the everyday energy they use. At present, this amounts to burning about 2.4 trillion cubic meters (85 trillion cubic feet) of natural gas each year. Unfortunately, we have found enough natural gas reserves in the world to last only another 60 years at this rate of consumption. This means that the grandchildren of today's high school students might see the end of natural gas usage, as we know it.
There is some good news to this bleak outlook. There happens to be another world resource of natural gas sufficient to provide 100% of our current energy needs for as much as 2000 years into the future. That takes care of the grandchildren! Unfortunately, we have not yet found a way to extract this natural gas economically. But we are working on it.
These particular natural gas reserves, called gas hydrates, consist of small cage-like structures of ice that contain within them methane (natural gas) molecules. The basic hydrate unit is a hollow crystal of water molecules with a single molecule of natural gas trapped inside. The crystals fit together in a tight latticework. The few times gas hydrates have been seen intact, they looked like ice. But they don't act like ice: when lit with a match, they burn!
This is a particularly "hot" topic for the world of the 21st century. The vast gas hydrates estimates worldwide have led several countries to start research and exploration programs to understand hydrate behavior, identify accumulations, and develop possible recovery methods. Japan, India, USA, Canada, Norway, and Russia are among the countries with ongoing gas hydrate investigations.
|A gas hydrate crystal structure. Each unit cell of this gas hydrate consists of 46 water molecules that form two small dodecahedral voids and six large tetradecahedral voids. Gas hydrates can only hold small gas molecules such as methane and ethane. At conditions of standard temperature and pressure (STP), one volume of saturated methane hydrate will contain as much as 189 volumes of methane gas. This large gas-storage capacity of gas hydrates may represent an important source of natural gas.|