The Great Gas Hydrate Escape
1/18/2012 - Pacific Northwest National Laboratory - RICHLAND, Wash. -- For some time, researchers have
explored flammable ice for low-carbon or alternative fuel or as a
place to store carbon dioxide.
Now, a computer analysis of the ice
and gas compound, known as a gas hydrate, reveals key details of its
The results show that hydrates can hold hydrogen at an
optimal capacity of 5 weight-percent, a value that meets the goal of
a Department of Energy standard and makes gas hydrates practical and
The analysis is the first time researchers have accurately
quantified the molecular-scale interactions between the gases --
either hydrogen or methane, aka natural gas -- and the water
molecules that form cages around them.
A team of researchers from
the Department of Energy's Pacific Northwest National Laboratory
published the results in Chemical Physics Letters online December
The results could also provide insight into the process of replacing
methane with carbon dioxide in the naturally abundant “water-based
reservoirs,” according to the lead author, PNNL chemist Sotiris
"Current thinking is that you need large amounts of energy to push
the methane out, which destroys the scaffold in the process," said
"But the computer modeling shows that there is an
alternative low energy pathway. All you need to do is break a single
hydrogen bond between water molecules forming the cage -- the
methane comes out, and then the hydrate reseals itself."
Gas hydrates -- especially methane hydrates, which store natural gas
-- look like ice but actually hold burnable fuel.
deep in the ocean, water and gas interweave in the hydrates, but
little is known about their chemical structure and processes
occurring at the molecular level.
They have been known to cause
problems for the petroleum industry because they tend to clog pipes
and can explode.
A methane hydrate produced the bubble of methane
gas that contributed to 2010's Gulf of Mexico oil spill.
In previous work, Xantheas and colleagues used computer algorithms
and models to examine the water-based, ice-like scaffold that holds
Water molecules form individual cages made with 20 or 24
Multiple cages join together in large lattices. But those
scaffolds were empty in the earlier analysis.
To find out how fuels can be accommodated inside the water cages,
Xantheas and PNNL colleague Soohaeng Yoo Willow built computer
models of the cages with either hydrogen gas -- in which two
hydrogen atoms are bound together -- or methane gas, a small
molecule made with one carbon and four hydrogen atoms.
In the hydrogen hydrates, which could potentially be used as
materials for hydrogen fuel storage, a small hollow cage made from
20 water molecules could hold up to a maximum of five hydrogen
molecules and a larger cage made from 24 water molecules could hold
up to seven.
The maximum storage capacity equates to about 10 weight-percent, or
the percentage of hydrogen by mass in the chunks of ice, although
packing hydrogen in that tight puts undue strain on the system.
Department of Energy's goal for hydrogen storage -- to make the fuel
practical -- is above 5.5 weight-percent.
Experimentally, hydrogen storage researchers typically measure much
less storage capacities.
The computer model showed them why: The
hydrogen molecules tended to leak out of the cages, reducing the
amount of hydrogen that could be stored.
The researchers found that adding a methane molecule to the larger
cages in the pure hydrogen hydrate, however, prevented the hydrogen
gas from leaking out.
The computer model showed the researchers that
they could store the hydrogen at high pressure and practical
temperatures, and release it by reducing the pressure, which melts
Understanding how the gas interacts and moves through the cages can
help chemists or engineers store gas and remove it at will.
and Xantheas’ computer simulations showed that hydrogen molecules
could migrate through the cages by passing between the figurative
bars of the water cages.
However, the cages also had gates:
Sometimes a low-energy bond between two water molecules broke,
causing a water molecule to swing open and let the hydrogen molecule
The “gate” closed right after the molecule passed through
to reform the lattice.
With methane hydrates, some fuel producers want to remove the gas
safely to use it.
Others see the emptied cages as potential storage
sites for carbon dioxide, which could theoretically keep it out of
the atmosphere and ocean, where it warms the earth and acidifies the
So, Willow and Xantheas tested how methane could migrate
through the cages.
The water cages were only big enough to comfortably hold one methane
molecule, so the chemists stuffed two methanes inside and watched
Quickly, one of the water molecules forming the cage
swung open like a gate, allowing one methane molecule to escape. The
gate then slammed shut as the remaining methane scooted into the
middle of the cage.
"This process is important because it can happen with natural gas.
It shows how methane can move in the natural world," said Xantheas.
"We hope this analysis will help with the technical issues that need
to be addressed with gas hydrate research and development."
Xantheas said performing computer simulations with carbon dioxide
instead of methane might help determine whether it's chemically
feasible to store carbon dioxide in hydrates.
This work was supported by the Department of Energy Office of
Science (BES). Computer resources used were at the National Energy
Research Scientific Computing Center at DOE's Lawrence Berkeley
National Laboratory in Berkeley, Calif.
Reference: Soohaeng Yoo Willow and Sotiris S. Xantheas, 2011/12.
Enhancement of Hydrogen Storage capacity in Hydrate Lattices, Chem.
Phys. Lett. Dec. 22, 2011, doi: 10.1016/j.cplett.2011.12.036.
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