Download - Gerdsmeyer GASTECH PAPER-final version

ON-BOARD RELIQUEFACTION FOR

LNG SHIPS

Gastech, Bilbao, March 16, 2005

Klaus Gerdsmeyer & Harry Isalski

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Presentation Overview

�The history of LNG liquefaction

�Development in LNG Shipping

�Experience in liquefaction

�Tractebel design approach

�Plant description

�Plant economics

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History of LNG Re-liquefaction

�Liquefaction of LNG began in the 1940s

�First base load Plant in the 1960s

�LNG Peak Shaving plants in the 1970s

�Ship BOG re-liquefaction patents in 1970s

�Economics now look favourable

�Technology is mature

�Machinery well developed with high efficiencies

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Modern LNG Ship Characteristics.

� Capacity 145,000 to 230,000

m3 LNG.

� BOG 4000 – 6000 kg/hr.

� New propulsion systems

� High LNG costs.

� Low fuel oil costs.

� BOG RE-LIQUEFACTION!!

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LNG Liquefaction Capacities.

Type of LNG Liquefier Capacity Plants &

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.

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Nitrogen Cycle Experience

� 200 TPD LNG Peak Shaver UK, 1970s, Closed Loop Nitrogen Cycle.

� Many plants using nitrogen and air cyclesfor N2 and O2 liquefaction in very highlyintegrated processes.

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Tractebel Experience in Belgium.

� 240 TPD liquefier for peak shaving applications.

� Complex facility, included pre-treatment for H2O & CO2 removal and included storage.

� Nitrogen cycle used with similar configuration to thatproposed for BOG re-liquefaction (Bryton).

� Tractebel have operational experience of this unitsince the 1970s.

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Peak Shaving LNG Plants in 1970s - Belgium

240 TPD LNG Peak Shaver using

Nitrogen Cycle

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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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LNG Production using turbo-expander.

Polish client – KRIO.

Cryogenic section is small.

Demonstrates TractebelExperience in cryogenicplant design.

Novel.

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Tractebel project - Mid-sized LNG Plant, China

� 1200 TPD LNG Plant – Urumqi, China (togetherwith 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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Cascade Re-liquefaction System.

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Characteristics for LNG BOG Re-liquefaction on Ship.

� Gas is dry and CO2 & H2S free.

� 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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Choice of Cycle Arguments.

� Pre-cooled mixed refrigerant – low power, complex, difficult to operate.

� Single stage mixed refrigerant – low power, difficult to operate.

� 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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Nitrogen Cycle Selection Criteria.

� Previous experience in industry.

� Flexibility of process.

� Low inventory of hydrocarbons.

� Ability to accept pitch and roll during voyage.

� Limited Space on ship.

� Simple to operate and easy to install on board.

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Process Optimisation Heuristics.

� High delta T means high exergy losses.

� Large refrigerant circulation means high exergy losses through

pressure drops in equipment.

� Larger capacity machines means better efficiency.

� Optimise expander pressure ratio.

� Compressor wheel coupled to expander has 1.4 pressure ratio

maximum; Compressor stages have about 3.5 pressure ratio.

� Optimise BOG pressure to minimise power for liquefier

(compression energy costs too!).

� Reduce flash gas into tank as far as practical

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Composite Cooling Curves for BOG unit.

Wide delta Ts for cold end.

Large exergy losses in cold end.

Red is N2 cooling, BOG cooling,

condensing and subcooling.

Blue is N2 cycle reheat curve.

Composite Temperature vs. Enthalpy Curve

Nitrogen Cycle BOG Recondenser

Temperature

En

tha

lpy

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Cycle Pressure Optimisation.

� Has significant effect on

reduction of overall power.

� Suction Pressure important

so that make-up is easy.

� Discharge pressure

important to minimise

exergy losses.

� Need to monitor how

machine efficiency changes

with pressure ratio and

number of stages.

Pressure Parameter vs. Specific Power

0

0.5

1

1.5

2

2.5

3

3.5

Pressure Parameter

Sp

ecif

ic P

ow

er

(kW

/kg

)

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Cycle Pressure Optimisation.

•BOG compression optimised.

•Considerable work done to optimise pressure ratio in cycle.

•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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Process Flowscheme.

LIQUID DROPLETS RETURN

BOIL-OFF GAS

COMPRESSOR

MAIN EXCHANGER

START-UP VALVE

MAIN LIQUID CONTROL VALVE

INTEGRATEDEXPANDER-

COMPRESSOR

AFTERCOOLER

CYCLECOMPRESSOR

1ST INTERCOOLER

2ND INTERCOOLER

NITROGEN MAKE-UP

LIQUID TO TANK

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BOG Re-liquefaction Plant Characteristics.

� Two BOG gas compressors, each 100% duty.

� 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.

� Ancillaries.

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Cycle Compressor.

Cycle Compressor:

Approx. 45 tonne

package.

7.1m x 4m x 3m

BOG machines:

Approx. 25 tonnes

package.4.9m x 3.5m x 2.8m

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Plate-fin Heat Exchanger.

Approx. 16 tonne

package with box

and insulation.1.8m x 2.2m x 4.8m

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Arrangement of Liquefaction System.

SAFE ZONEHYDROCARBON ZONE

BULKHEAD

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Plant Flexibility.

� Able to handle “ballast” & “cargo” voyages.

� 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.

� Tractebel has an economic solution and broad technical experience to implement BOG re-liquefaction on board LNG ships.