Gas Combined CycleGas Turbine DesignIn gas turbine design the firing temperature, compression ratio, mass flow, andcentrifugal stresses are the factors limiting both unit size and efficiency. Forexample, each 55°C (100°F) increase in firing temperature gives a 10 - 13percent output increase and a 2 - 4 percent efficiency increase. The most criticalareas in the gas turbine determining the engine efficiency and life are the hot gaspath, i.e., the combustion chambers and the turbine first stage stationary nozzlesand rotating buckets. The components in these areas represent only 2 percent ofthe total cost of the gas turbine, yet they are the controlling factor in limiting gasturbine output and efficiency. The development process takes time, however,because each change of material may require years of laboratory and field teststo ensure its suitability in terms of creep strength, yield limit, fatigue strength,oxidation resistance, corrosion resistance, thermal cycling effects, etc.Manufacturers use various combustor arrangements: General Electric hasseveral combustors mounted in a ring around the turbine; Asea Brown Boverisometimes has a single combustor above the turbine; Siemens has twocombustors, one on each side of the turbine. Gas turbines can be fueled withnatural gas, diesel oil (distillate), and even residual or crude oil if appropriatecustomized fuel treatment facilities are installed and properly operated.Turbine nozzles and buckets are cast from nickel super alloys and are coatedunder vacuum with special metals (platinum-chromium-aluminide) to resist thehot corrosion that occurs ! the high temperatures encountered in the first stage ofthe turbine, particularly if contaminants such as sodium, vanadium andpotassium are present. Only a few parts per million of these contaminants cancause hot corrosion of uncoated components at the high firing temperatureencountered. With proper coating of nozzles and buckets and treatment of fuels
to minimize the contaminants, manufacturers claim the hot-gas-path componentsshould last 30,000 to 40,000 hours of operation before replacement, particularlythe hot-gas-path parts, that give rise to the relatively high maintenance cost forgas turbines (typical O&M annual costs of 4 percent of the capital cost).The continuing improvements in firing temperatures and compression ratios haspermitted manufacturers to increase the operating performance on the samebasic gas turbine frame or housing. For example, General Electric introduced itsFrame 7 series in 1970 with a rating of 45 MW, a firing temperature of 900°C(1650 °F) and an air flow of 0.8 million kgs (1.8 million lbs) per hour. Throughmany changes and upgrades the latest Model F of the same Frame 7 series hasa rating of 147 MW, a firing temperature of 1260°C (2300°F) and an air flow of1.5 million kgs (3.3 million lbs) per hour. One of the major advances made was toair cool nozzles and buckets using bleed air from the compressor to increase thefiring temperature while limiting the metal temperatures of the nozzles andbuckets to withstand hot corrosion and creep. This limiting of the maximumtemperature through air cooling while simultaneously increasing the mass flowwith more air compressor capacity permits higher power output. To increase thefinal compressor pressure additional compressor stages are added on thecompressor rotor assembly to give higher compression ratio thus providingadditional turbine power output. Typical industrial gas turbine compression ratiosare 16:1 and aeroderivative ratios are 30:1 with roughly 50 percent of the totalturbine power of either type being required just to drive the compressor.Compressor blading is special stainless steel, possibly coated by electroplatingwith nickel and cadmium to resist pitting in salt and acid environments.Compressor designs have been quite effective, as evident by the 200,000-hourlife of some early compressors installed in the 1950s.The gas turbine has the inherent disadvantage that reduced air density with highambient temperature or high elevation causes a significant reduction in poweroutput and efficiency, because the mass flow through the gas turbine is reduced.
A 28°C (50°F) results in about a 25 percent output reduction and a 10 percenthigher heat rate. Similarly, at 1000 meter (3300 ft) elevation the gas turbineoutput would be 15 percent lower than at sea level. Steam plants and diesels arenot affected to the same degree by ambient air temperature and elevationchanges.Aeroderivitive Versus Industrial Gas TurbinesThe advanced gas turbine designs available today are largely due to the hugesums that have been spent over the last 50 years to develop effective jet enginesfor military applications, including their adaptation as gas turbine propulsionsystems for naval vessels. The commercial aviation, electric power and to alesser extent, the sea and land transportation industries, have benefitedaccordingly. Given the aircraft designers need for engine minimum weight,maximum thrust, high reliability, long life and compactness, it follows that thecutting-edge gas turbine developments in materials, metallurgy andthermodynamic designs have occurred in the aircraft engine designs, withsubsequent transfer to land and sea gas turbine applications. However, there areweight and size limitations to aircraft engine designs, whereas the stationarypower gas turbine designers are seeking ever larger unit sizes and higherefficiency.To emphasize this difference in approach, today the largest aeroderivative powergas turbine is probably General Electrics 40 MW LM6000 engine with a 40percent simple-cycle efficiency and a weight of only 6 tons. This engine isadapted from the CF6-80C2 engine that is used on the CF6 military transportaircraft. By comparison, General Electrics largest industrial gas turbine, theFrame 9 Model F has an output of about 200 MW, an open-cycle efficiency of 34percent, but is huge compared to the LM6000 and weights 400 tons. Theaeroderivative is a light weight, close clearance, high efficiency power gasturbine suited to smaller systems. The industrial or frame type gas turbine tends
to be a larger, more rugged, slightly less efficient power source, better suited tobase-load operation, particularly if arranged in a combined-cycle block on largesystems. There is no significant difference in availability of two types of gasturbines for power use, based on the August 1990 Generation Availability Reportof the North American Electric Reliability Council. For the period 1985-1989 theaverage availability of 347 jet engines (1587 unit years) was 92 percent and thatfor 575 industrial gas turbines (2658 unit years) was 91 percent.Combined Cycle Sizes/CostsGas turbines of about 150 MW size are already in operation manufactured by atleast four separate groups-General Electric and its licensees, Asea BrownBoveri, Siemens, and Westinghouse/Mitsubishi. These groups are alsodeveloping, testing and/or marketing gas turbine sizes of about 200 MW.Combined-cycle units are made up of one or more such gas turbines, each with awaste heat steam generator arranged to supply steam to a single steam turbine,thus formatting a combined-cycle unit or block. Typical combined-cycle blocksizes offered by three major manufacturers (Asea Brown Boveri, General Electricand Siemens) are roughly in the range of 50 MW to 500 MW and costs are about$600/kW.Combined Cycle EfficienciesCombined-cycle efficiencies are already over 50 percent and research aimed at1370°C (2500°F) turbine inlet temperature may make 60 percent efficiencypossible by the turn of the century, according to some gas turbine manufacturers.Low-Grade Fuel for TurbinesGas turbines burn mainly natural gas and light oil. Crude oil, residual, and somedistillates contain corrosive components and as such require fuel treatmentequipment. In addition, ash deposits from these fuels result in gas turbine
deratings of up to 15 percent They may still be economically attractive fuelshowever, particularly in combined-cycle plants.Sodium and potassium are removed from residual, crude and heavy distillates bya water washing procedure. A simpler and less expensive purification system willdo the same job for light crude and light distillates. A magnesium additive systemmay also be needed to reduce the corrosive effects if vanadium is present.Fuels requiring such treatment must have a separate fuel-treatment plant and asystem of accurate fuel monitoring to assure reliable, low-maintenance operationof gas turbines.Alternative Combined Cycle DesignsGas dampers are often provided so the gas turbine exhaust can bypass the heatrecovery boiler allowing the gas turbine to operate if the steam unit is down formaintenance. In earlier designs supplementary oil or gas firing was also includedto permit steam unit operation with the gas turbine down. This is not normallyprovided on recent combined-cycle designs, because it adds to the capital cost,complicates the control system, and reduced efficiency.Sometimes as many as four gas turbines with individual boilers may beassociated with a single steam turbine. The gas turbine, steam turbine, andgenerator may be arranged as a single-shaft design, or a multishaft arrangementmay be used with each gas turbine driving a generator and exhausting into itsheat recovery boiler with all boilers supplying a separate steam turbine andgenerator. Combined-Cycle Shaft Arrangements
Combined Cycle Modular InstallationsOne significant advantage of combined-cycle units is that the capacity can beinstalled in stages with short lead time gas turbines being installed initially (1 to 2years) followed later by heat recovery boilers with the steam turbines (3 yearstotal). In this way each combined-cycle unit (i.e. block) can be installed in three(or more) roughly equal capacity segments.The modular arrangement of combined-cycle units also facilitates generationdispatching because each gas turbine can be operated independently (with orwithout the steam turbine) if part of the combined-cycle unit is down formaintenance or if less than the combined-cycle unit total capacity is required.This may give a higher efficiency for small loading than if the total capacity wasoperated.Furthermore, since combined-cycle units are available in sizes of roughly 50 MWto almost 500 MW (and 600 MW are expected to be available soon with 200 MWgas turbines), there are many selection possibilities for most sizes of powersystem.
Another point favoring staging a combined-cycle unit is that the gas turbine (orcombined-cycle) per kilowatt cost does not seem to increase significantly forsmaller units, as is the case for steam units due partly to the high cost of thesubstantial civil works necessary for steam plants regardless of steam unit size.Finally, combined-cycle units can be installed in 3 years while a steam unittypically requires 5 years, and once committed there is no power output from asteam unit until the complete unit is available.Fuels for Combined CyclesUsing present technology the combined-cycle unit can be fueled with natural gas,distillate, and even crude or residual oil with appropriate fuel treatment. Fuelingwith crude or residual oil, however, definitely results in extra capital costs for fueltreatment equipment. Operations suffer due to additional operating costs foradditives to counteract contaminants such as vanadium, lower availability due toadditional maintenance and water cleaning shutdowns to remove blade deposits,and reduced life because there is a greater tendency for hot gas path corrosiondue to blade deposits and corrosion.The daily (or even more frequent) testing of the residual or crude oil forcontaminants with appropriate adjustments of fuel treatment is critical to preventdamage to the gas turbine. Even with good operation there will be a reduction inefficiency with crude or residual oil fueling to reduce firing temperatures, asrecommended by most manufacturers for this mode of operation, and due to theblade deposits which build up between water-washing intervals. The gas turbinehas to be shut down periodically for cleaning and allowed to cool before washingcan be done by injecting water while rotating the unit using the starting motor.Operational Considerations of Combined CyclesThis gas turbine is the main component that requires maintenance on combined-cycle units. All manufacturers recommend specific intervals for hot-gas-path
inspections and for major overhauls, which usually involve hot-gas-path partchanges. During overhauls the condition of aeroderivatives may require that thecomplete engine or at least major components be sent to overhaul centers, whilethe industrial gas turbines usually will require only part changes on site.The type of fuel and mode of operation are critical in determining both themaintenance intervals and the amount of maintenance work required. It isestimated by one manufacturer that burning residual or crude oil will increasemaintenance costs by a factor of 3, assuming a base of 1 for natural gas, and bya factor of 1.5 for distillate fueling. Similarly, maintenance costs will be threetimes higher for the same number of fired hours if the unit is started, i.e. cycled,once every fired hour, instead of starting once very 1000 .fired hours. Peaking at110 percent of rating will increase maintenance costs by a factor of 3 relative tobase-load operation at rated capacity, for the same number of fired hours.The control system on combined-cycle units is largely automatic so, after a startis initiated by an operator, the unit accelerates, synchronizes and loads withautomatic monitoring and adjustment of unit conditions in accordance withpresent programs. The number of operators required in a combined-cycle planttherefore is lower than in a steam plant.Developed Country Combined Cycle InstallationsThe following key topics provide examples of developed country combined-cycleinstallations. Electricity Supply Board of Ireland Oil-to-Gas Conversion The electricity Supply Board of Ireland converted two old oil-fired steam plants to gas-fired combined cycle units in the late 1970s. Originally, there units were used for baseloaded operation, but recently change to intermediate load.
Refer To: World Bank IEN Working Paper #35: "Prospects for Gas- Fueled Combined-Cycle Power Generation in the Developing Countries", May 1991. Midland Nuclear Plant Conversion, U.S.A. Twelve Asea Brown Boveri 85 MW gas turbines and heat recovery boilers were installed to supply two 350 MW steam units originally installed for the Midland nuclear plant. This combined-cycle cogeneration plant will supply 1380 MW to Consumer Power Co. and process steam plus 60 MW of power to Dow Chemical Co. Refer To: World Bank IEN Working Paper #35: "Prospects for Gas- Fueled Combined-Cycle Power Generation in the Developing Countries", May 1991. LNG-Fired Combined-Cycle by Tokyo Electric The worlds largest regasified LNG-fueled combined-cycle plant is in operation near Tokyo in Japan. Fourteen 165 MW single-shaft combined-cycle units serve as mixed base-load and mid-range generation on the 41,000 MW Tokyo Electric Power Co. system. The plant capacity is 2,310 MW at 15°C ambient decreasing to 2,000 MW at 32°C. A unique feature is the low NOx emission level of 10 ppm due to the use of selective catalytic reduction equipment. Refer To: World Bank, IEN Working Paper #35: "Prospects for Gas- Fueled Combined-Cycle Power Generation in the Developing Countries", May 1991.Developing Country Combined Cycle Installations
The following list provides examples of Combined Cycle projects in developingcountries. These examples are discussed in greater detail in the associated KeyTopics. 5 x 300 MW in India 3 x 300 MW Gas Turbines in Malaysia 2 x 300 MW in Pakistan 5 Combined-Cycle plants in Mexico 300 MW in Egypt 772 MW in Thailand Combined-Cycle in BangladeshThe dollar per kilowatt capacity costs vary from $592/kW for a new 1,080 MWcombined-cycle plant in Egypt to $875/kW for a steam addition to convert fourgas turbines at Multan in Pakistan to a combined-cycle plant. Although theoperating performance of combined-cycle units in North America is reported tobe satisfactory with availability factors of about 85 percent, the developingcountry experience is less favorable, and in some countries the performance hasbeen poor.Developing Country Combined CycleInstallatons5 x 300 MW Installation in IndiaAs of 1991, only limited operational performance data are available on the 5 x300 MW combined-cycle units in India which were commissioned in 1990. Duringthe commissioning runs some blades failed on one gas turbine reportedly due topoor alignment; the manufacturer (Mitsubishi) made repairs and no furthertrouble has been reported.
These five combined-cycle blocks are operating on gas and the National ThermalPower Corporation (NTPC) reportedly is satis&ied with their performance andplans installing additional combined-cycle units. However, full operation of thecombined-cycle plants has not been possible due to power contract disputes.The gas price has not been finalized, so the NTPC selling price to the StateElectricity Boards has not been resolved, giving some uncertainty concerning theload dispatching of the combined-cycle units.3 x 300 MW Gas Turbine Installation in MalaysiaThe Paka 900 MW (3 x 300 MW Alsthom-Hitachi blocks) gas-fueled combined-cycle plant in Malaysia has had serious problems due to gas bypass damperjamming and gas turbine start-up train torque converter bearing faults. Availabilityreportedly has been only 55 to 60 percent in the past, but corrections have beenmade and higher availability is expected.Some of the problems have been attributed to split contracts for the gas turbine,boiler and steam unit components with interface difficulties, particularly on controlsystems. The country is still planning to install a large amount of combined-cyclecapacity in the 1990s (3,840 MW by 1999) expecting that "teething troubles" withthe technology will be overcome. However, some gas-fueled steam capacity willalso be installed, so that future power supply is not dependent on only onetechnology.2 x 300 MW Installation in Pakistan2 x 300 MW blocks of gas-fired combined cycle gas turbines were installed atGuddu, Pakistan.According to USAIDs consultant, RCG/Hagler Bailley, Inc. the availability of theplant has been good in the 80-85 percent range but efficiency has been lowerthen expected. No clear explanation of the reasons for the shortfall is provided.
Refer To: Annex 8 of the World Bank, IEN Working Paper #35: "Prospects forGas-Fueled Combined-Cycle Power Generation in the Developing Countries",May 1991
Five Combined Cycle Plants in MexicoMexico has the largest amount of combined-cycle capacity in any developingcountry totaling almost 1,900 MW at five plants. The units were installed in1975-1986 and are typically dual-fueled, natural gas and distillate. The operatingperformance has not been good; unit availability factors range from 38 percent to83 percent; efficiencies vary from 20 percent for units installed in 1975 to 39percent for later units. Comision Federal de Electricidad does not plan to installany additional combined-cycle units, partly because of their poor operatingrecord and also because there is surplus residual oil which can be used in steamplants.300 MW Installation in EgyptA 100 MW steam addition to 8 x 25 MW General Electric gas turbines. This plantis a 300 MW Talkha gas/distillate-fueled combined-cycle in Egypt. USAIDsconsultant, RCG/Hagler, Bailley, Inc. has reviewed the operational performanceof this plant and concluded that the availability of the plants has been quitereasonable -- in the 80-85 percent range -- but efficiency has been lower thanexpected without a clear explanation of the reasons for the shortfall. Gas turbinecomponent failures are the cause of most of the forced outages on these twocombined-cycle plants.Refer To: Annex 8 of the World Bank, IEN Working Paper #35: "Prospects forGas-Fueled Combined-Cycle Power Generation in the Developing Countries",May 1991772 MW Installation in ThailandThe early operation of the gas-fueled, Bang Pakong 772 MW combined-cycleaddition reportedly was troubled by low gas turbine start-up reliability, difficulties
in fuel changeover, vibration, gas damper distortion, gas supply problems, andthe lower system inertia of the combined-cycle units. The plant availability isunderstood to be only about 70 percent at present. Nevertheless, the ElectricityGenerating Authority of Thailand is planning to install more gas-fueled combined-cycled units to almost triple this type of capacity on its system by the end of the1990s.Combined Cycle Installation in BangladeshThere are substantial gas reserved in the eastern half of Bangladesh, socombined-cycle technology should be a logical generation candidate for thiscountry. Unfortunately, the operating experience has been poor on a small, UK-fi.anced GEC combined-cycle unit installed at Ashuganj in the early 1980s.Based on this experience, the Bangladesh Power Development Board (BPDB)prefers to install gas-fired steam units. This situation is reflected in a recentarticle by a consultant on system planning activities in Bangladesh:"The BPDB have rather limited and somewhat unsatisfactory experience of theirone CC unit, and their FORs for the smaller gas turbines which they haveoperated for a much longer period of time seem to be higher than average by afactor of at least two, and possibly more than three.Faced with these facts a cautious approach to planning was adopted using base-load regime but with high FORs for CC plant. With this approach, selection of CCplant by the program is limited and gas-fired steam plant claims most of the gasallocation available."