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COGAS Propulsion for LNG Ships. September 11, 2007 Edwin G. Wiggins Rosenblatt Professor of Marine Engineering. LNG Ship Propulsion. Steam turbine without reheat until very recently Steam turbine with reheat has been proposed. LNG Ship Propulsion. Medium speed diesel

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## COGAS Propulsion for LNG Ships

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**COGAS Propulsion for LNG Ships**September 11, 2007 Edwin G. Wiggins Rosenblatt Professor of Marine Engineering**LNG Ship Propulsion**• Steam turbine without reheat until very recently • Steam turbine with reheat has been proposed**LNG Ship Propulsion**• Medium speed diesel • Burn gas with low pressure injection • Burn liquid fuel & reliquify the gas • Slow speed diesel • Burn gas with high pressure injection • Burn liquid fuel & reliquify the gas**Reliquifaction**• Consumes about 4 MW of power • Backup system required • 2 reliquifaction plants • combustor**LNG Ship Propulsion**• Combined Gas and Steam (COGAS) • Gas turbine produces most of the power • Gas turbine exhaust makes steam • Steam turbine produces additional power**LM 2500 Gas Turbine**• 16 compressor stages • 2 stage gas generator turbine (65% of power) • 6 stage power turbine (35% of power)**LM 2500 Gas Turbine**• It’s the size of a 40 foot container • It produces more than 30,000 brake horsepower**Blade Cooling to Increase Inlet Temperature**• Air cooling • Steam cooling • Allows higher gas inlet temperature**A Simple Schematic**• A simple gas cycle • Compressor • Heater • Gas turbine • 3 heat exchangers • Superheater • Boiler • Economizer**A Simple Schematic**• A simple steam cycle • Heat exchangers • Steam turbine • Condenser • Pump**Add a DFT**• Saturated steam to DFT • Two pumps**A 2 Pressure System**• 2 economizers • 2 boilers • 1 superheater • 2 steam turbines**Design Considerations**• Gas turbine pressure ratio • Gas turbine inlet temperature • Steam pressure • Heat exchanger pinch point ΔT**Representative Results**Air Standard Analysis Pressure Losses Ignored Single Boiler Steam System DFT at 50 psia**Effect of Gas Turbine Inlet Temp**• Gas power goes down as gas inlet temperature goes up. • Because steam power goes up • And gas mass flow goes down**Typical Boil-off Rates**• Loaded 7800 lb/hr • Ballast about half that**Effect of Gas Turbine Inlet Temp**• Fuel consumption goes down as • Gas inlet temperature goes up • Boiler pressure goes up**Factors Affecting Efficiency**• When boiler pressure increases by 700 psi • η increases 1% or 2% • When gas turbine inlet temp increases 250°F • η increases 2%**Cycle Efficiencies**• COGAS ~ 55% • Gas turbine alone ~ 34%**Economics of COGAS**• Steam plant increases first cost • (<$10 million) • Steam plant increases maintenance cost • (<$100 thousand per year) • But it decreases fuel cost • Calculate present worth of fuel savings**Natural Gas and Pollution**• Essentially no SOx • Much less CO2 • Hydrogen to carbon ratio is 4:1 • Liquid fuels are about 2:1 • NOx (influenced by temperature and pressure) • Less than diesel • More than steam**Compare IFO 380 to Natural Gas**• If IFO380 costs $350 per tonne • And the heating value is 18,500 BTU/lb • That’s $8.60 per MMBTU • Natural gas is cheaper**Compare MDO to Natural Gas**• If MDO costs $600 per tonne • And the heating value is 18,500 BTU/lb • That’s $14.74 per MMBTU • Natural gas is much cheaper**Economic Analysis - Assumptions**• Interest rate – 10% • Analysis period – 10 years • Gas turbine inlet temperature – 2510 F • Natural Gas Price - $7.00 per MMBTU • Boiler Pressure – 1200 psia**Economic Analysis - Results**• Annual Fuel Savings – $6.7 Million • Present Worth 10 years fuel savings at 10% • $41 Million**Weight Considerations**• Based on US Navy RACER design (extrapolated) • Steam power 17,500 hp • Steam plant weight 1500 tonne**Conclusion**• COGAS propulsion is technically feasible • It is also economically attractive

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