ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 8-2012 COMBUSTION TURBINE INLET COOLING.pdf

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1、8.1CHAPTER 8COMBUSTION TURBINE INLET COOLINGAdvantages 8.2Disadvantages . 8.3Definition and Theory . 8.3System Types . 8.3Calculation of Power Capacity Enhancement and Economics. 8.6OWER OUTPUT capacity of all combustion turbines (CTs)Pvaries with ambient air temperature and site elevation. Therated

2、 capacities of all CTs are based on standard ambient air at15C, 60% rh, 101.35 kPa at sea level, and zero inlet and exhaustpressure drops, as selected by the International Organization forStandardization (ISO). For all CTs, increased ambient air tempera-ture or site elevation decreases power output;

3、 increased ambient airtemperature also reduces fuel efficiency (i.e., increases the heat rate,defined as fuel energy required per unit of electric energy pro-duced). However, the extent of the effect of these changes on outputand efficiency varies with CT design. This chapter provides adetailed disc

4、ussion on combustion turbine inlet cooling (CTIC).Additional information on applying CTIC to combined heat andpower systems (cogeneration) is provided in Chapter 7.There are two types of CTs: aeroderivative and industrial/frame.Figures 1 and 2 show typical effects of ambient air temperature onpower

5、output and heat rate, respectively, for these types of turbines.The actual performance of a specific CT at different inlet air temper-atures depends on its design. Figures 1 and 2 show that aeroderiv-ative CTs are more sensitive to ambient air temperature than areindustrial/frame CTs. Figure 1 (Punw

6、ani and Hurlbert 2005) showsthat, for a typical aeroderivative CT, an increase in inlet air temper-ature from 15 to 38C on a hot summer day decreases power outputto about 81% of its rated capacity: a loss of 19% of the rated capacity.Figure 2 (Punwani 2003) shows that, for the same change in ambient

7、air temperature, the heat rate of a typical aeroderivative CT increases(i.e., fuel efficiency decreases) by about 4% of the rated heat rate atISO conditions. Increasingly, industrial/frame CTs are using aerode-rivative technology to improve performance; thus, their performancecurves are moving towar

8、d those of the classic aeroderivative CT.In cogeneration and combined-cycle systems that use thermalenergy in CT exhaust gases for steam generation, heating, cooling,or more power generation, increases in ambient air temperature alsoreduce the total thermal energy available for these applications, a

9、sshown in Figure 3 (Orlando 1996).CTs in simple- and combined-cycle systems are particularlyqualified to meet peak electricity demand because of their ability tostart and stop more quickly than steam-turbine-based thermal powergeneration systems using coal, oil or gas, and nuclear plants. Forfossil

10、fuel power generation, combined-cycle systems are the mostfuel efficient (lowest heat rate of typically 7400 kJkWh) and steam-turbine-based systems are the least efficient (highest heat rate rangebetween 12 660 and 21 100 kJkWh, depending on turbine age). AThe preparation of this chapter is assigned

11、 to TC 1.10, CogenerationSystems.Fig. 1 Effect of Ambient Temperature on CT Output(Punwani and Hurlbert 2005)Fig. 2 Effect of Ambient Temperature on CT Heat Rate(Punwani 2003)Fig. 3 Effects of Ambient Temperature on Thermal Energy, Mass Flow Rate and Temperature of CT Exhaust Gases(Orlando 1996)8.2

12、2012 ASHRAE HandbookHVAC Systems and Equipment (SI)typical heat rate for a simple-cycle system is about 10 550 kJkWh.Therefore, to minimize fuel cost for power generation, the preferredorder of dispatching power to meet market demand is to operatecombined-cycle systems first, simple-cycle systems ne

13、xt, and steamturbines as the last resort.Electric power demand is generally high when ambient temper-atures are high. An example of an hourly profile of ambient temper-ature, system load, and CT output is shown in Figure 4 (Punwaniand Hurlbert 2005).When high ambient temperatures drive up power dema

14、nd, theuse of less efficient (high-fuel-cost) generation plants is requiredand that drives up the market price of electric energy. Figure 5(Hilberg 2006) shows the hourly load profile in one U.S. region fora single day in the summer. Although the peak electricity demandincreases by 80%, the peak pow

15、er price increases by over 400%.Figures 4 and 5 show that power output capacity decreases justwhen it is most needed, and when power is also most valuable.The trends shown in Figures 4 and 5 are not unique to the UnitedStates. The Middle East is seeing much higher growth rates in powerdemand, and th

16、at demand is also directly linked to hot-weatherpower usage. In some countries in the Middle East, over 40% ofpower usage is linked to air conditioning.CTIC is used by thousands of CT-based power plants to overcomethe ill effects of increased ambient temperature on CT performance.It can provide econ

17、omic and environmental benefits for plant own-ers, ratepayers, and the general public.ADVANTAGESCTIC offers economic as well as environmental benefits.Economic BenefitsMaximizes power output when most needed and most valuableReduces capital cost ($kW) for incremental capacityIncreases CT fuel effici

18、ency (lowers heat rate)Minimizes use of less-efficient steam-turbine-based systems, thushelping to minimize increase in rates to electricity usersEnvironmental BenefitsAllows minimum use of inefficient and polluting power plants byallowing maximum use of efficient and cleaner CT plantsConserves natu

19、ral fuel resourcesReduces emissions of pollutants (SOx, NOx, particulates, andhydrocarbons)Reduces emissions of global warming/climate change gas(CO2)Minimizes/eliminates new power plant siting issuesEmissions reductions from CTIC result from its displacement ofthe very-high-heat-rate steam turbine

20、peaker power plants, as shownin an example in Table 1.CTIC helps reduce the carbon footprint of energy use in twoways: it (1) improves the CTs energy efficiency and (2) increasesthe generation capacity of higher-efficiency systems and thus elim-inates or minimizes use of less-efficient power generat

21、ion systems.There are four major types of thermal power generation systems:Combined cycle (CC)Simple cycle (SC)Combined heat and power (CHP) or cogenerationCondensing steam turbineElectric power generation efficiencies of these systems aretypically in the ranges of 48 to 55% (i.e., heat rates of 690

22、0 to7400 kJ/kWh) for combined cycle, 34 to 42% (i.e., heat rates of8500 to 10550 kJ/kWh) for simple cycle, and 23 to 28% (i.e., heatrates of 12 700 to 15 600 kJ/kWh) for steam turbine. Even thoughpower generation efficiencies of CHP systems are very similar tothose of the CC or SC system that is par

23、t of the CHP, the overallenergy utilization efficiency of a CHP could be highest for facilitiesthat need to meet electric as well as coincidental thermal loads. Theoverall energy utilization efficiency is the lowest for steam-turbinesystems. Examples of electric generation efficiencies, overallFig.

24、4 Typical Hourly Power Demand Profile(Punwani and Hurlbert 2005)Fig. 5 Example of Daily System Load and Electric Energy Pricing Profiles(Hilberg 2006)Combustion Turbine Inlet Cooling 8.3energy utilization efficiencies, and carbon dioxide emissions of allfour systems are shown in Table 1. In this exa

25、mple, the CHP systemuses the same CT as for the combined- and simple-cycle systemsand has the lowest carbon dioxide emissions, followed by the com-bined-cycle, simple-cycle, and steam-turbine systems. Therefore,to minimize the carbon footprint of energy use, it is desirable tomaximize the use of CHP

26、 systems and minimize the use of steam-turbine systems.Because CTIC helps maximize the capacities of all CT-basedsystems, regardless of mode, they help minimize use of the least-energy-efficient systems using steam turbines during hot weatherand thus help reduce the carbon footprint of energy use.DI

27、SADVANTAGESPermanently higher CT inlet pressure drop that results in a smalldrop in the CT output capacity even when CTIC is not being used(magnitude of pressure drop varies with CTIC technology)Additional maintenance cost for CTIC equipmentDEFINITION AND THEORYA schematic flow diagram of a combusti

28、on turbine (CT) systemis shown in Figure 6.Power output of a CT is directly proportional to and limited bythe mass flow rate of the compressed air available to it from the aircompressor that provides high-pressure air to the combustion cham-ber of the CT system. An air compressor has a fixed capacit

29、y forhandling a volumetric flow rate of air for a given rotational speed ofthe compressor. Even though the volumetric capacity of a compres-sor remains constant, the mass flow rate of air it delivers to the CTchanges with ambient air temperature. The mass flow rate of airdecreases as the air density

30、 decreases with increasing ambient tem-perature. CTIC reduces the inlet air temperature, resulting inincreases in air density mass flow rate and power output. For moredetails, consult Stewart (1999).SYSTEM TYPESMany technologies are commercially available for CTIC, but theoverall approaches can be d

31、ivided into three major groups:Evaporative systemsChiller systemsLiquefied natural gas (LNG) vaporization systemsEach approach has advantages and disadvantages; for moreinformation, see Stewart (1999).The psychrometric paths for CTIC technologies are shown inFigure 7.Evaporative SystemsEvaporative c

32、ooling systems rely on cooling produced by evap-oration of water added into the inlet air. An ideal evaporative cool-ing process occurs at a constant wet-bulb temperature and cools theair to a higher relative humidity (i.e., water vapor content increases).When the warm ambient inlet air comes in con

33、tact with the addedwater, it transfers some of its heat to the liquid water and evaporatessome of the water. The process of heat transfer from inlet air towater cools the inlet air. Water added in the evaporative systems alsoacts as an air washer by cleaning the inlet air stream of airborne par-ticu

34、lates and soluble gases, which could be a significant benefit todownstream filter elements. Studies show that evaporative coolingalso reduces NOxemissions because of the increase in moistureadded to the air. The psychrometry of CTIC using evaporative sys-tems has been described in detail by Stewart

35、(1999).Evaporative systems can cool the inlet air up to 98% of the dif-ference between the ambient dry-bulb and wet-bulb temperatures.Therefore, the most cooling can be achieved during hot and/or dryweather. Design and hourly wet-bulb temperatures for many loca-tions can be found in Chapter 14 of th

36、e 2009 ASHRAE HandbookFundamentals on the accompanying CD and on the Weather Yearfor Energy Calculations 2 (WYEC2) Data and Toolkit CD (ASH-RAE 1997). This information is useful in evaluating the coolingpotential for locations in many climates.Table 1 Typical Combined-Cycle, Simple-Cycle, and Steam

37、Turbine SystemsUnit TypeTurbine Inlet Cooling CandidatesExisting Older Plants Steam TurbineCHP/Cogeneration Combined Cycle CT Simple Cycle CTPrime mover Frame CT Frame CT-STG Frame CT Condensing STG4Fuel Gas Gas Gas No. 6 OilPlant age, yr 30Heat rate, kJ/kWh 11 340 7385 11 340 13 715Generation capac

38、ity, MW 100 100 100 100Hours of operation 1 1 1 1Thermal energy need, MW 136 136 136 136Fuel use, MW (HHV)1Power generation 315 205 315 380Thermal energy20 160 160 160Total 315 365 475 540Energy efficiency, %Electric power generation 32 49 32 26Overall 75 65 50 44Carbon Emissions,3Mg/h 15.4 18.0 23.

39、3 26.61HHV = higher heating value.2CHP system provides thermal energy from the CT exhaust gases without using additional fuel. Other systemsuse 85% energy efficiency boiler for providing thermal energy needs.3Assuming natural gas of HHV of 37 260 kJ/m3.4STG = steam turbine generator.Fig. 6 Schematic

40、 Flow Diagram of Typical Combustion Turbine System8.4 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)Evaporative systems have the lowest capital costs among CTICsystems, and are the most common type in use. Their primary dis-advantage is that the extent of cooling produced is limited by thewet-b

41、ulb temperature (and thus, cooling is weather dependent). Inarid climates, these systems consume large quantities of water (e.g.,0.3 Lmin to cool 5 m3/s of air by 10 K), which comprises the majorcomponent of the system operating cost.There are two types of evaporative systems: direct and indirect.Di

42、rect Evaporative Cooling. Direct evaporative system types incommercial use are wetted media and fogging.Wetted Media. As the first approach adopted for CTIC, these sys-tems have a long history of success in a wide range of operating cli-mates. Inlet air is exposed to a thin film of water on the exte

43、ndedsurface of honeycomb-like wetted media. Water used for wettingmay or may not require treatment, depending on its quality and themedium manufacturers specifications. Typical pressure drop across the wetted media is about 75 Pa. Thehigher the degree of saturation required, the more media required

44、andhigher the pressure drop. A wetted-media system requires propercontrol of the chemistry of recirculating water (e.g. contaminantabsorption) and monitoring of media degradation (Graef 2004). Themedia may need to be replaced every 5 to 10 years, depending onwater quality, air quality, and hours of

45、operation.Fogging. This approach adds moisture to the inlet air by sprayingvery fine droplets of water. High-pressure fogging systems can bedesigned to produce droplets of variable sizes, depending on thedesired evaporation time and ambient conditions. The water dropletsize is generally less than 40

46、 m, and averages about 20 m. Foggingsystems typically require higher-quality water than wetted-mediasystems do. Generally, reverse-osmosis or demineralized water isused to ensure cleanliness throughout the system. Typical pressuredrop across the fogging section is about 25 Pa. Fogging nozzles maynee

47、d to be replaced every 5 to 10 years because of erosion. The high-pressure pumps require servicing at least annually.A fogging system that sprays more water than could evaporatebefore the inlet air enters the compressor is known as a wet com-pression, overspray, high-fogging, or over-fogging system

48、(Jollyet al. 2005; Kraft 2004; Schwieger 2004). Water in excess of thatrequired by fogging does not further reduce inlet air temperature. Itis ingested into the compressor section of the CT, and provides athreefold effect for the system:It ensures inlet air achieves maximum evaporative cooling.The a

49、dditional water evaporates in the compression stages of thecompressor, allowing the compressor to be cooler and require lesswork for air compression.Fig. 7 Psychrometric Chart Showing Direct and Indirect Inlet Air Cooling Processes(Schlom and Bastianen 2009)Combustion Turbine Inlet Cooling 8.5The excess water increases the total mass flow rate of gases enter-ing the CT and helps increase power output beyond that possiblewith fogging alone.There is debate in the industry whether wet compression is trulya CTIC technology; accordingly, this chapter addresses only theinlet fogging c