Microsoft Word - Gerdsmeyer_GASTECH PAPER-final version.docKlaus Gerdsmeyer & Harry Isalski
Development in LNG Shipping
Ship BOG re-liquefaction patents in 1970s
Economics now look favourable
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tonnes/day Location
Peak Shaving Units 50 – 300 Over 100 units, since 1970s.
Mid-sized units 1000 – 4000 Less than 10
Base Load Plants 7000 – 10000 Over 100 units worldwide.
Newer Base Load Plants 11000 – 15000 several built, worldwide,
a few planned even larger
at >20000 tonnes/day.
Nitrogen Cycle Experience
200 TPD LNG Peak Shaver UK, 1970s, Closed Loop Nitrogen Cycle.
Many plants using nitrogen and air cycles for N2 and O2 liquefaction in very highly integrated processes.
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Complex facility, included pre-treatment for H2O & CO2 removal and included storage.
Nitrogen cycle used with similar configuration to that proposed for BOG re-liquefaction (Bryton).
Tractebel have operational experience of this unit since the 1970s.
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240 TPD LNG Peak Shaver using
Nitrogen Cycle
LNG liquefaction experience in Poland
Tractebel Project to increase cold production in a nitrogen rejection plant. Result is 60 TPD LNG.
Novel process scheme. A first in the world!!
More difficult than BOG re-liquefaction.
Expansion turbine with flashing flow (made by Ebara).
In commercial operation since October 2004.
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Novel.
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1200 TPD LNG Plant – Urumqi, China (together with Linde)
Plant in operation since Q4/2004.
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TGE Cascade Cycle experience on Ships.
In the market for ethylene ships with cascade refrigeration cycles, TGE has a market share of more than 70% during the last 10 years (32 of 42 ships).
A closed refrigeration system with screw compressor is used to re-condense cargo (ethylene) against evaporating refrigerant at approx. -38°C.
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Gas contains more nitrogen than in Base load plants.
Ship crew needs a simple process.
Process must work during sea voyage – motion.
Space is small on board ship.
Rapid start & stop is required.
Power, cooling & air available on board.
N2 make-up must be generated on board.
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Cascade – low power, complex for LNG application (several machines), moderately difficult to operate.
Combinations of above & proprietary processes are not appropriate for a small BOG unit.
Gas cycles – methane open cycle, nitrogen closed cycle – high power, low cost, easy to operate.
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Limited Space on ship.
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Large refrigerant circulation means high exergy losses through
pressure drops in equipment.
Optimise expander pressure ratio.
maximum; Compressor stages have about 3.5 pressure ratio.
Optimise BOG pressure to minimise power for liquefier
(compression energy costs too!).
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condensing and subcooling.
Composite Temperature vs. Enthalpy Curve
Nitrogen Cycle BOG Recondenser
Discharge pressure
0
0.5
1
1.5
2
2.5
3
3.5
•Previous experience used as a starting point.
•Cycle power of 0.75kWh/kg LNG obtained. Good for low pressure liquefaction & Nitrogen cycle with single expander.
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Single centrifugal, integrally geared compressor assembly, driven by electric motor and assisted by expander.
One multi-stream plate-fin heat exchanger.
BOG collection system.
LNG return system.
BOG machines:
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Can easily turn down to 60 % from 100%.
Will have power saving to about 70% - 75%.
Easy & rapid start-up and shut-down.
Can handle various BOG compositions even which high nitrogen content.
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BOG Re-liquefaction Economics.
•Assumptions: •LNG cost is at least 5$/MMBtU •HFO about 150 – 160 $/tonne. •BOG rate for liquefaction 6000 – 6500 kg/h •Machinery configuration as above.
•BOG re-liquefaction yields over 2 million $/year. •Payback in the region of 3 – 5 years. •Larger cargo delivery per voyage, up to 2 – 3%, even more on long voyages.
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Conclusions.
Economics is driving LNG ships to alternative fuels.
HFO is currently cheaper that BOG, and is likely to be so for some time.
BOG re-liquefaction is a sound option, based on mature technology, to enhance the economics of LNG transportation.