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PostPosted: May 19, 2011 2:46 am 
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Joined: Jan 10, 2007 5:09 pm
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Location: Los Altos, California
LNG is apparently 13% of natural gas market today, with Spain and France taking 24 and 13 billion m^3/year. If I were using LNG to spin a turbine, I'd look into using the expanded fuel as a heat sink to cool the incoming air. Seems like I might be able to get some more compression ratio without melting my turbine blades.

That got me thinking: the output of a gas turbine is really hot, because of the limit on compression ratio. Instead of transferring the heat to steam for a GTCC, perhaps you could directly expand the turbine exhaust to something well below atmospheric pressure (but still well above ambient temperature), then dump heat to ambient, and finally recompress the exhaust to ambient. Among other things, you might be able to use an expanded LNG input to condense the ~15% of the output which is H2O, which could then be recompressed without a lot of extra work. Better still, the other 85% of the output would be cooled in a crossflow heat exchanger rather than the GTCC steam plant's condenser, and the CHX is more thermodynamically efficient.

-Iain


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PostPosted: Jun 11, 2011 10:15 pm 
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Joined: May 24, 2009 4:42 am
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Location: Calgary, Alberta
I have looked at similar ideas off and on for years and haven't found anything 'magic' yet. For example one idea was to partially expand the hot gasses in a CCGT and then put them through the heat recovery steam generator at pressure, then expand the relative cool gas so that the leaving temp is lower than ambient, stealing thermal energy from the environment.

Sounds nice, but it doesn't work out. The problem is by cooling the gases down before the final expansion step dramatically reduces the power produced in that last expansion step.

Your first idea has a lot of promise, by pumping LNG to high pressure, heating and expanding, one can recovery 100% of the thermal energy added as mechanical work, which is very unusual for any thermal cycle. The only challenge is that most of the high pressure pipelines are already operating a high pressure already (~90 bar) leaving little room for an expansion step exhausting at 90 bar. However if that gas is being burned in a CCGT, there would be a few more options with a lower gas delivery pressure ~30 bar.

Regarding your second idea, I suspect that would not work out so well, as the energy required to recompress up to ambient is quite significant and the efficiencies also work against you, the compressor might be 85% and the turbine 92%.

One thing that is very effective, using the cooling effect of regasification to cool the compressor inlet air of the CCGT. This is very effective and commonly done, especially in hot countries, where the performance impact can be very significant.


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PostPosted: Feb 15, 2013 12:55 pm 
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Joined: Sep 21, 2011 9:10 pm
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You might find some magic in MHD generators. This technology relies on very hot gases (such as those exiting a gas turbine) to generate electricity with no moving parts. (no loss to inertia - or downtime for repairs) The downside, is that it's far more expensive than other combined cycle technologies.

http://en.wikipedia.org/wiki/MHD_generator

Quote:
MHD generators are different from traditional electric generators in that they operate at high temperatures without moving parts. MHD was developed because the hot exhaust gas of an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency. MHD was developed as a topping cycle to increase the efficiency of electric generation, especially when burning coal or natural gas. MHD dynamos are the complement of MHD propulsors, which have been applied to pump liquid metals and in several experimental ship engines.


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PostPosted: Feb 15, 2013 8:23 pm 
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Joined: Jun 05, 2011 6:59 pm
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Location: NoOPWA
Send the atmosphere thru a pre-turbine into a chilling chamber which contains a HEX that boils the LNG. Then compress the chilled atmosphere, intro the NG, burn them, and then thru the power turbine. The pre-turbine can drive the compressor and the power turbine will drive the generator without the loss to the compressor. If the boiled LNG is at a significantly higher pressure than the compressed air, it can go thru a turbine of its own, adding to the output, tho it may be needed to help drive the compressor.

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DRJ : Engineer - NAVSEA : (Retired)


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