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Phillips Cascade ProcessPhillips Cascade ProcessPhillips Cascade Process
Simple to design and operateSimple cycle Frame 5 gas turbines mechanical driveNo helper turbine or large motor needed for start-upIncreased size with two gas turbine trains for each refrigerant processParallel compressor trains avoids capacity limitsIncreased CAPEX due to more (six) trains offset by increased availability 95-96% with parallel train operationLoss of one train does not cause plant shut downProduction carries on with reduced capacity Refrigerant and exchangers temperature not affected by one train trip enabling quick restart
Propane pre-coolingCentrifugal compressor (to 15 – 25 bar)Side-streams at 3 pressure levelsTypically requires a ~40 MW Gas Turbine (e.g. Frame 6) plus Helper Motor or Steam TurbineCompressor sizes reaching maximum capacity limitsAdded aerodynamic constraint; high blade Mach numbers due to high mole weight of propane (44)Prevents utilisation of full power from larger gas turbines (Frame 7)
Mixed refrigerant liquefaction and sub-coolingAxial LP for Shell Advised PlantCentrifugal HP compressor (45 – 48 bar)Typically requires ~70 MW Gas Turbine (e.g. Frame 7) plus Helper Motor or Steam Turbine
Two casing arrangements (LP and an HP)Axial LP / centrifugal HP compressor (45 – 48 bar)Typically requires ~70 MW Gas Turbine (e.g. Frame 7) plus Helper Motor or Steam TurbineLP and HP compressor speeds compromisedLP axial compressor (higher efficiency)HP centrifugal compressor
Mixed refrigerants for pre-cooling, liquefaction and sub-cooling dutiesLiquefin development studies presently oriented towards increasing capacity to 6 MTPA with:
2 x Frame 7 Gas Turbines for main compression2 x Frame 5 Gas Turbines for power generation
Higher capacities possible using:Frame 9 GTsElectric motorsSteam turbines etc.
APCI process uses larger and larger gas turbines to reduce CAPEX in a single train configuration; bigger gas turbine have lower $/kWFrame 7EA used for Mixed RefrigerantFrame 6 being replaced by Frame 7 for Propane for larger plantsThe plants are “single train” i.e. each machine is designed for 100% capacity and arranged in series
Aero-Derivative Gas Turbines for LNG Plant – Potential AeroAero--Derivative Gas Turbines Derivative Gas Turbines for LNG Plant for LNG Plant –– Potential Potential
Several established VendorsSize; may be built to exact process specificationMechanical drive up to 130 MW not a problemConstant speed power generation 600–1100 MWHigh reliability; 30 years life is achievableHigh availability; compressors & steam turbines may both achieve 3 years non-stop operation, no need for inspectionSteam is often required elsewhere in processMixed fuel; boilers can utilise varying fuel mix whereas gas turbines require fuel specification to be maintainedHigher thermodynamic efficiency than simple cycle GT (but lower efficiency than GT-steam combined cycle)Power output relatively unaffected by ambient conditions
Industrial Gas Turbines - ConsIndustrial Gas Turbines Industrial Gas Turbines -- ConsCons
Paucity of Vendors!Low thermal efficiency, high CO2 emissionsMaintenance is intensive, involving prolonged on-site work which reduces plant availabilityFixed sizes and fixed optimal speedsProcess and compressors must be designed around the GT (unlike steam turbines)Process may not make full use of the GT powerPower output highly sensitive to ambient conditions e.g. typical large GT:
Aero-Derivative Gas Turbines - ProsAeroAero--Derivative Gas Turbines Derivative Gas Turbines -- ProsProsHigher thermal efficiency than Industrial GT; 38-42% compared to 28-32% for similar size Industrial GTs in simple cycle
Smaller footprint area than Industrial GT because of aero design
Shorter maintenance period; modular design allows gas engine and power turbine sections to be swapped out
Off-site maintenance (in factory)
Thus, higher plant availability
Most engines have free power turbines for variable speed operation (within a range)
Large helper motors or steam turbines may not be needed for start-up
Aero-Derivative Gas Turbines - ConsAeroAero--Derivative Gas Turbines Derivative Gas Turbines -- ConsConsPaucity of Vendors (essentially only 2)!Higher NOX than Industrial GTsEngines need more care and maintenance due to higher operating pressures and temperatures and design complexityFixed sizes and fixed optimal speedsProcess and compressors must be designed around the GT (unlike steam turbines)Process may not make full use of the GT powerPower output highly sensitive to ambient conditionsFuel quality is critical – even more than in Industrials!Limited operating experience for LNG, although extensive for offshore mechanical drive and power generationPowers greater than 60 MW not available in simple cycleDry Low Emissions (NOX) technology adds complexityHigher risk technology than Industrial GTs
Variable Speed Electric Motors - ProsVariable Speed Electric Motors Variable Speed Electric Motors -- ProsProsCan be made to suit, allowing optimisation of process and compressorsHigher availability of LNG plant than if using GTs or Steam TurbinesReduced manning levelsMay avoid gearboxes for 3000-3600 rpm compressor speeds (large flow capacity compressors)Power generation may be off-siteLower CAPEX if power is bought from the gridSimple layout, reduced civil works
Variable Speed Electric Motors - ConsVariable Speed Electric Motors Variable Speed Electric Motors -- ConsConsMost LNG plant are in remote locations; off-site power generation of 400-500 MW not available!
Very high CAPEX if power generation is built alongside LNG
High OPEX (although savings may be possible)
Limited experience with high power VSDs; 45-55 MW is achievable, 65 MW is the maximum
Electrical issues at compressor start-up; grid peak current and fault levels
Power generation using GTs must happen somewhere; CO2, NOX and sensitivity to ambient conditions is similar to a GT (unless power generation is using a combined cycle)
Conclusions and ObservationsConclusions and ObservationsConclusions and ObservationsLNG drivers are predominately Industrial Heavy Duty Gas Turbines e.g. GE Frames 5, 6, 7 … even 9!Frame 5s generally used on older LNG plant, although ALNG in Trinidad was recently fitted with Frame 5Ds; these are demonstrating high overall availability at low CAPEX… 3.3 MTPA with 6 x Fr 5Fr 6 / Fr 7 combinations replaced Steam Turbines at MLNGNow Fr 6 / Fr 7 commonly used at NLNG, Oman LNG, Qatar LNG… 3.3 – 3.5 MTPAFr 7 / Fr 7 combinations used at Qatar LNG, but with poor use ofGT power because of non-optimal process, process had to be redesigned… ~4 MTPALarger and larger trains are pushing the limits of compressor technology i.e. Axials for Mixed Refrigerant and largest centrifugals for Propane
Conclusions and ObservationsConclusions and ObservationsConclusions and Observations
When parallel trains are used (instead of series) e.g. ALNG:
Smaller driver sizes can be used e.g. Frame 5sCompressor capacities are halved, so centrifugals may be used instead of axialsPlant availability is enhancedImproved operability, re-starting after a train failure is simpler and quickerPlant costs are surprisingly lower