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Grex > Science > #84: Found while looking for something else: methane hydrates | |
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russ
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Found while looking for something else: methane hydrates
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Oct 31 01:45 UTC 2002 |
While searching for the density of frozen methane (which I
didn't find in the first 6 pages of Google results), I came
across a description of methane hydrate. It was interesting
enough that I thought I'd share the details here, and some
of my speculations.
The average composition of methane clathrate is 5.75 molecules
of water to 1 molecule of methane, or 1 molecule methane per
approximately 119.5 AMU. The heat of combustion of methane is
23875 BTU/lbm, or about 212 kcal/mol if I've done my numbers
right.
Here's where I get speculative. I've read that methane hydrates
form easily in water which is sufficiently pressurized (deep)
and cold. If the formation created a lot of heat, you'd expect
that the process would be rate-limited by the ability to conduct
heat away (else the temperature would rise too high to keep
forming more hydrate). I've read nothing to indicate that this
happens. So I'll assume for the sake of speculation that the
heat of formation of methane hydrate is -10 cal/gram, or about
1/8 the heat of formation of water-ice.
A quantity of methane hydrate containing a mole of methane masses
119.5 grams. My speculative number indicates that this hydrate
would require at most 1195 calories of heat to dissociate it into
free methane and liquid water.
This is less than 1% of the heat of combustion of the methane.
The lesson seems obvious: methane hydrate deposits on the ocean
floor can potentially be liberated by warming them gently using
the combustion of a small fraction of the methane, and the rest
harvested for whatever use is contemplated. Who needs foreign oil?
(And if anyone can tell me the density of frozen methane, and the
volume fraction of crystals in liquid you can get before the "slush"
stops flowing, please tell me. I'm intensely curious about the
possibilities for rocketry.)
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| 9 responses total. |
rcurl
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response 1 of 9:
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Oct 31 03:34 UTC 2002 |
There has been speculation that global warming may release the methane
hydrates in the seabed, causing an enormous acceleration in global
warming.
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gull
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response 2 of 9:
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Oct 31 20:13 UTC 2002 |
We better harvest them now, then. ;)
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russ
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response 3 of 9:
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Nov 1 01:12 UTC 2002 |
Indeed. And the combustion of fossil fuels is a major contributor
to this possibility.
Which is why using the sea-floor methane is so important. This would:
1.) Eliminate at least part of the threat of its release. (The
quantities are so enormous that it's impossible to get rid of
it all.)
2.) Displace oil and coal, reducing greenhouse gas emissions.
3.) If the methane is reformed into hydrogen on the spot, the
carbon dioxide could be reburied in the same sediments.
(I understand that CO2 forms hydrates similarly to CH4.)
This would make the entire process near-zero emissions.
There would be substantial geopolitical fallout from the displacement
of oil by domestic gas, but that's not a scientific issue.
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rcurl
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response 4 of 9:
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Nov 1 07:28 UTC 2002 |
3) would not work as the ocean is buffered so that the CO2 would be
converted to bicarbonate. It is an acid. Currently, it is believed
that the current higher CO2 levels are causing some disruption of
ocean buffering leading to coral death. The ocean does not need
more CO2. (It's deposit as the solid hydrate, even if possible, would
not make it chemically inert.)
1) and 2) are not actually benefits in regard to global warming. You
would just be burning a different fossil fuel. It would extend the
life of oil deposits, but contribute nothing to the reduction of
release of CO2 except for the small difference in CO2 per unit
heat of combustion.
I suspect that the mining of the methane hydrates will cause an enormous
concomitant release of additional methane hydrates to the environment
as methane, because of the disturbance of the deposits, exposing them
to water circulation, or just dispersing the solid into the ocean, where
it would float and decompose (the s.g. of the hydrate is about 0.9).
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russ
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response 5 of 9:
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Nov 2 07:04 UTC 2002 |
Re #4: The ocean waters are buffered, but the sediments are not
necessarily so. It's been proposed to take CO2 from generating
plants, *freeze* it into multi-ton torpedoes, and drop it from
ships deep into ocean sediments. Before it could escape to the
water it would have to pass through the sediments again, where it
would have opportunity to be bound chemically as carbonates (maybe).
I don't see any great difference between doing this and replacing
methane clathrates with CO2 clathrates in the same sediments. Of
course, I'm not in a position to analyze the concept fully; I'm
just tossing it out as something which might have possibilities.
Other possibilities include dumping CO2 down old gas and oil wells.
Even if you dump all combustion products to the atmosphere, methane
is still a huge win over coal for global warming and a smaller win
over oil. Putting the carbon back in the seabed would be a further
decrease in the ratio of atmospheric CO2/energy. If we can obtain
a 70% reduction over current rates, we'll arrest the trend.
An off-site correspondent tells me he recalls that methane ice has
a density of something like 0.4, but doesn't have his references
ready to hand to confirm it.
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russ
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response 6 of 9:
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Nov 2 16:06 UTC 2002 |
(Re #4, cont'd:) Dispersal of the solid is a problem, which is why
I did not anticipate open mining. Any methane released as gas would
require a solid cover to catch it (an umbrella in reverse, so to
speak); the same cover would tend to catch any solid which floats up.
(If climate conditions are making the clathrates unstable, losing
some from the mining process is still better than losing them all.)
This suggests a possibility for mining: anchor a cover over an area
of sediment, add hot water and stir. If the sediment is made fluid,
its components will separate by density. If any of the methane is
liberated as gas, it will help to stir the mix.
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russ
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response 7 of 9:
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Nov 3 03:34 UTC 2002 |
Here's part of what I found when I went looking for information
on oceanic methane clathrates today:
Physical chemistry of methane clathrates:
http://www.mbari.org/ghgases/peerart/practpchem.pdf
http://www.wikipedia.org/wiki/Methane_hydrate
This is a wiki and thus not reliable, but interesting. Quote:
It is thought that as much as 20 times the current known
reserves of natural gas may be contained within ocean-floor
clathrate deposits, representing a potentially important future
source of fossil fuel. Methane clathrates remain stable at
temperatures up to 18 0C.
Interesting... it decomposes at 18 C. Other sources implicate methane
clathrates in tsunamis; sediments cannot consolidate while embedded in
a clathrate matrix and temperature increases with depth, yielding a
breakdown in the clathrate and an abrupt reduction in the shear
strength of the sediments possibly leading to catastrophic failure.
The conclusion I draw is that it's not required to raise the temperature
of the sediment to more than about 18 C, which sets a ceiling on the
amount of heat required. If the ocean surface waters are that warm or
warmer, the heat could even be "free for the pumping".
http://ethomas.web.wesleyan.edu/ees123/clathrate.htm
Much more authoritative source, great-looking intro to the topic. Quote:
The extent of worldwide gas hydrate occurrences in the oceans
has been evaluated using seismic exploration, because gas
hydrates are characterized by the occurrence of an anomalous
reflector parallel to the sea floor (Bottom Simulating Reflector
or BSR), and cross-cutting ordinary sedimentary structures,
because of the mismatch between the overlying high-velocity
clathrates and underlying, low-velocity gas-bearing sediments
(e.g., Kvenvolden 1988, MacDonald, 1990). The permafrost
reservoir has been estimated at about 400 GtC in the Arctic
(MacDonald, 1990), but no estimates have been made of possible
Antarctic reservoirs. The oceanic reservoir has been estimated to
be about 10,000 to 11,000 GtC (e.g., MacDonald, 1990; Kvenvolden,
1998). This oceanic clathrate reservoir is thus enormous (at
almost a third of the size of the deep ocean reservoir; Fig. 2),
and only small changes in its extent can have major effects on
the atmospheric reservoir. Even the permafrost reservoir is on
the order of hundreds of gigatons, not much smaller than the
total amount of carbon in the terrestrial biosphere.
Just FYI, that 10,000 GtC represents about 600 million quads (quadrillion
BTU) of energy. If even 10% of that can be harvested, it would be an
enormous resource.
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russ
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response 8 of 9:
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Nov 4 00:32 UTC 2002 |
I checked sources today, and total US energy consumption is under
100 quad per year. If the USA uses 25% of world fuel consumption,
that would make the world consumption about 400 quad per year.
600 million quads represents 1.5 million years of fuel at that rate.
(Far freakin' out, man.)
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tsty
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response 9 of 9:
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Nov 8 22:38 UTC 2002 |
even with inflation, .5 million years makes it worth trying.
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