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Cogeneration with gas turbine engines
Heat Recovery Systems & CHP Vol. 13, No. 5, pp. 471-480, 1993 Printed in Great Britain
0890-4332/93 $6.00 + .00 © 1993 Pergamon Press Ltd
REVIEW P A P E R
COGENERATION WITH GAS TURBINE ENGINES Y. S. H.
NAJJAR,M. AKYURT,O. M. AL-RABGHIand T. ALP
College of Engineering, King Abdulaziz University, Jeddah 21413, Saudi Arabia (Received 26 November 1992) Abstract-42ogeneration, simply, is the generation of energy for one process from the excess energy supplied to another process. Cogeneration, then, is nothing more than an economically sound method for the conservation of resources [1]. Thus, the benefits from the use of cogeneration may be cited as energy conservation, environmental improvement and financial attractiveness to investors. In the near future energy etficiency must no longer be a choice, but a c o m m i t m e n t - - h e n c e the timely importance of this review.
1.
INTRODUCTION
Cogeneration, when associated with the gas turbine, is a relatively old technology which dates back to pre-World War II years. The concept of cogeneration involves the generation of steam and electricity in one operation. The "topping cycle" generates electricity when the turbine is directly coupled to a generator. The waste heat from the exhaust of the turbine is then recovered in the form of steam or hot water. In the "bottoming cycle" a gas turbine may be used as a combustor only [2]. The exhaust from the turbine may be fed to a steam generator (HRSG), and the resulting steam may be used as process steam or in a steam Rankine cycle. A secondary cycle with an organic fluid may be used at the condenser end to drive a turbine for generating byproduct electricity. During the decade of the 1960s, industrial users recognized the gas turbine as a reliable prime mover for base load process applications. Accordingly, gas turbine cogeneration systems were installed in various industries. More recently, worldwide concern about the cost and efficient use of energy is providing continuing opportunities for gas turbine cogeneration systems. In this connection, Allen and Kovacik [3] reviewed cogeneration principles applicable to the development of gas turbine energy supply systems. Negri et al. [4], as well as several other research teams [5-11] conducted essentially theoretical studies on the cogeneration idea and its several variations. Najjar and others carried out several performance studies related to different gas turbine cycle combinations for the purpose of evaluating energy conservation and utilization, including heat exchange, organic Rankine cycle and heat pumps [12-18]. In principle, the simplest modification to be introduced to the simple gas turbine engine cycle was to recover part of the exhaust energy in the heat exchanger of a recuperative cycle [12, 13, 19]. Exhaust energy can be recovered more efficiently, however, in a hot water or heat recovery steam generator (HRSG). Such a cogeneration system may have a power-to-heat ratio of 4 to 5 times that of steam turbines, in addition to the greater potential for generating power in excess of on-site needs [20]. The steam may be used in heating, cooling and many other industrial processes. In order to determine the feasibility of waste energy recovery, an energy mapping study needs to be carried out. The energy mapping and cascading concept can be utilized to optimize the energy use of a plant, where energy sources and energy users are important factors to be mapped and cascaded as shown in Figs 1 and 2 [2]. The organic Rankine cycle is used in energy cascading as a low bottoming cycle [16, 21]. The exhaust energy of the gas turbine can be used for steam generation, drying, or process fluid heating, as well as preheating combustion air for process heaters and boilers [3]. The potential users of cogeneration systems, also called combined heat and power (CHP), may be chemical, petrochemical, textile, metals, paper and board, and agricultural industries [22]. 471
472
Y. S, H. NAJJAR el a/.
2250 "--]
furnace
2ooo_j @
high pressure steam boiler waste heat range 1
1750 --I
[ heat user range
~-~ 1500
heat source range
--
I
gas turbine 125o
~
1000 - -
~
750 - -
steam boiler
500 - -
bottoming cycle
250 - 0
--
Fig. I. E n e r g y m a p o f a p r o c e s s p l a n t (after K i a n g [2]).
It is thus possible to use congeneration in many circumstances, such as the development of new industrial facilities, major expansions to existing installations, replacement of ageing steam generation equipment, significant changes in energy costs (fuel and power) and power sales opportunities [23]. Thus, it is no wonder that public utilities enter into joint ventures with private CHP operators [24]. Cogeneration projects may be small in size based on internal combustion engines [25-28]. However, large scale cogeneration, especially with gas turbines, has become a permanent part of the industry [29-32]. In general, planning and technical studies need to take into account such factors as capital and operating costs, reliability and demand profiles, all of which affect the decision of selecting the small scale CHP [33], and need to consider such factors as coal prices relative to gas prices when deciding on large systems [34]. Several research teams investigated the congeneration principle as related to gas turbine power plants and generated different solutions and designs. Pasha [35] studied the various factors affecting the design, cost and performance of a heat recovery steam generator, considering both natural and forced circulations. Kesseli and Wells [36] described a cost-competitive 30 kW gas turbine cogeneration system for solar electric applications. 2. TYPES OF COGENERATION WITH APPLICATIONS The gas turbine is the center of many exciting new power generating technologies such as the integrated coal-gasification combined-cycle (IGCC), pressurized fluidized bed combustion (PFBC) and compressed air energy storage (CAES) [37]. Moreover, gas turbine CHP plants are highly 2250--
furnace heat input
2000--
1750--
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1500--
I
1250-1000--
gas turbine and high pressure s t e a m boiler heat input
750-500
--
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--
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--
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Fig. 2. Energy cascading (after Kiang [2]).
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competitive with steam turbines and internal combustion engines as prime movers, yielding a higher rate of return [38], better flexibility, higher efficiency, especially when using aero-derived gas turbines that have good part-load efficiencies [39] and low downtime, with the aero-derivative type having a removable gas generator that relates to most of the critical maintenance [40]. Cogeneration with gas turbines encompasses heating air for fired heaters [41], production of steam for cooling gas turbine blades [42], using heat rejected from closed-cycle gas turbines [43, 44], producing chilled water or cold air from an absorption system [10, 45], converting heat for space heating by the use of heat pumps [17, 18], using liquefied hydrogen as fuel and coolant [46], producing air for compressed air storage plants [47] and producing power from an absorption cycle to drive reverse osmosis desalination [48]. Other cogeneration systems include steam power plants [49-51], diesel power plants [52, 53], fuel cells [54] and nuclear power plants [55, 56]. There are many gas turbine cogeneration projects around the world, some producing hot water and steam for district heating [57], plus combined-cycle [58, 59] for power, heating and air conditioning [60] and other applications [15]. 3. I N D U S T R I A L COGENERATION Within the last few years, gas turbines have been integrated into several industries. The paper industry, a long-standing supporter of combined heat and power, has emerged in the last decade as a leading industrial market for gas turbines [61-68]. In each case, a two-or three-stage steam boiler is used for heat recovery from the exhaust of the gas turbine. Instead of using gas-fired driers, the tail end may be used directly for pulp-drying. It is reported that the gas turbine CHP is replacing ageing diesel cogeneration systems in some leather works [69]. Combined-cycle CHP plants are further reported to be supplying chemical complexes with all of their steam and electricity requirements [70]. The said complexes may include salt refineries, electrolytic plants for chlorine production, caustic soda plants, natural gas-based methanol plants and a range of derivative products. Cooke and Parizot [71] reported the use of a gas turbine exhaust in preheating combustion oxygen for cracking furnaces used in ethylene production. Large gas turbine cogeneration plants are now employed in enhanced oil recovery from heavy oil deposits which require the injection of large amounts of steam to make oil production feasible at economic flow rates [72, 73]. It is further reported [74] that the dairy industry makes extensive use of waste heat boilers in conjunction with gas turbines. It has been suggested that gas turbines fired by blast furnace gas can be installed in combined-cycle in steel works, where a multi-cannular type of combustor may be used to burn the low BTU fuel. The gas turbine, generator, steam turbine and the fuel gas compressor can be coupled to make a single-shaft combination [75]. One step in producing kaolin requires large spray driers, the heat for which can be supplied directly from the exhaust gas of the gas turbine engine [76]. Energy costs may be reduced in the brick and tile industry by utilizing gas turbine cogeneration [77]. Incineration plants boost their efficiency of power generation when steam is superheated in a gas turbine-waste heat boiler combination rather than in the incinerator boiler [78]. Heat recovery may be achieved more efficiently without heat transfer surfaces if the fluid to be cooled is very viscous or highly contaminated, such as the aqueous stream leaving an alcohol distillation column that contains grape peels or grains and organic fragments and solids, which if allowed to pass through a heat exchanger, would cause serious fouling problems [79]. Novel cogeneration gas turbine cycles for saving energy in energy-intensive industries, such as refineries and related industries, have been recently studied and analyzed [45, 80, 81]. The power plant coupled with sea-water desalination is another proposed area of application for waste heat recovery from gas turbines. Aly [48, 82, 83] suggested various methods for waste heat recovery with respect to different desalination plant designs. Jabboury and Darwish [84] presented a method to predict the performance of steam generators with heat recovery/steambottoming cycle combined with a sea-water desalination plant at various steam and gas conditions. In another study, Jabboury and Darwish [85] were concerned with the prediction of gas turbine cogeneration (power/desalting) plant performance under different environmental and loading conditions.
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DESIGN
CONSIDERATIONS
Gas turbines are steadily gaining popularity as prime movers in industrial applications, particularly those suitable for cogeneration. Analysis of the most efficient arrangement has been considered for conserving energy in the industrial environment [86, 87]. Boilers for recovering exhaust heat from gas turbines are very different from conventional boilers [88]. Significant improvements in performance are obtained with design modifications such as a counter-current configuration [89], fire or water-tube, tube size, and fin configurations. Boiler performance must be evaluated by considering all of the possible modes of operation with regard to steaming gas velocity, metal temperatures, heat flux, corrosion, erosion, tube bundle vibration and noise [90]. In general, the exchange of heat from the gas to the fluid should occur at the highest temperature difference possible. This can best be accomplished by making the gas and the fluid temperature gradients nearly parallel to each other, utilizing a multi-pressure level unit. When a multi-level pressure boiler cannot be justified, selecting the lowest possible usable pressure level for steam generation will achieve the highest steam production and the greatest heat recovery [91]. There are some design techniques which aid enhancement of heat recovery. These include selection of materials which do not corrode at flue gas condensation [92], and heat transfer enhancement methods whether passive or active, such as extended, treated, roughened surfaces, swirl flow devices, porous structures, additives and aids such as surface and fluid vibration, electrostatic fields and jet impingement [93]. 5. P E R F O R M A N C E
AND
ECONOMICS
Some methods have been proposed [94] that combine energy and exergy analyses with economic considerations in order to identify the relative portions, for heat and electricity, of the total cost of a cogeneration system. Leibowitz and Tabb [7] developed an integrated system with enhanced performance through the implementation of several engineering development tasks; namely, control integration, mechanical equipment integration, rotary screw gas compressor application, and application of steam injection. Rice [95] undertook an analysis of seven commercially available gas turbines, showing the effect of pressure ratio, exhaust temperature, intercooling, regeneration, and turbine inlet temperature with regard to power output, heat recovery, and overall efficiency. He then added some complexities such as reheating, steam injection, and steam cooling to examine their effects on overall heat balance [96]. Other researchers [97-99] developed computer programs to analyze heat balance in cogeneration systems at different loads. Ito et al. [100] investigated the relation of fuel cost to the operation policy on the energy demand of a cogeneration plant used for district heating and cooling. Mitchell [101] specified a method to quantify five distinct attributes of surplus power from a cogeneration facility. 6.
IMPACT
OF
COOLING
It is known that the efficiency of the gas turbine engine is relatively low at design point, and it deteriorates further at part-load and at off-design, when the ambient air temperature increases. The present trend in design is towards improving efficiency and power output by increasing the engine pressure ratio and turbine inlet temperature. This, however, results in the increase of NO, as the undesired pollutant. The compressor air is usually used for cooling the combustor and the blades of the turbine, thus enabling the utilization of higher combustion and turbine inlet temperatures [102, 103]. Extensive research has been carried out on turbine blade film cooling [104-109]. It was shown that film cooling effectiveness is mainly governed by the momentum flux ratio, injected and mainstream, on both suction and pressure surfaces, independent of density ratio. Further studies were carried out on convective heat transfer in a rotating coolant channel [110] and a turbine vane shroud [111]. Steam cooled gas turbine blades were investigated, and the results were compared with those using air [112].
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Stecco and Facchini [113] developed a computer model for cooled expansion in gas turbines and found that using water or steam increases efficiency and specific power more than other coolants. Cooling effects differ according to cycle. In the simple cycle, the best performance is achieved with mixing of the coolant in the main stream. The authors concluded that in combined-cycles, however, performance is improved when there is no mixing. Bettocchi et al. [114] investigated cooling with multiple-expansion reheating combined gas-steam cycle and found that a higher power ratio between the gas turbine and the steam section can be achieved with cooling. The highest efficiency was obtained when the cooling air was not at the maximum compressor pressure. It was further proposed to utilize exhaust waste heat to produce steam for injection into the air stream at the inlet to the compressor, predicting a 4% rise in thermal efficiency [115]. One of the side benefits of injecting steam before or after the compressor is the reduction of NOx emissions [116]. Water spray has been used for precooling high pressure turbine cooling air to stand higher temperatures [117]. It is anticipated that variations of atmospheric conditions such as temperature, humidity and pressure will be important factors in gas turbine performance. Thermodynamic analyses have revealed that thermal efficiency and specific output decrease with the increase of humidity and ambient temperature [118]. An extensive study was undertaken on the effect of humidity on engine performance, and correlations were formulated [119]. Recently, EI-Hadik [120] carried out a parametric study on the effects of ambient temperature, pressure, humidity and turbine inlet temperature on power and thermal efficiency. He concluded that the ambient temperature has the greatest effect on gas turbine performance, which increases with the turbine inlet temperature and pressure ratio. As mentioned previously, the gas turbine engine can be used in a cogeneration system, utilizing the heat from the exhaust gases to drive an absorption refrigeration system to produce cool air or chilled water, both of which can be utilized in different applications [45, 121-123]. An effective method of overcoming the problem of NOx and improvement of performance is the precooling of the inlet air. The increased density of the cooled air increases the mass flow through the engine, resulting in a significant increase in gas turbine power output with a slight improvement in efficiency. The NOx emission is reduced. A further consequence of precooling the inlet air is that maintenance costs are reduced, as these depend heavily on the temperature of the hot section [124]. Improvements in power and efficiency due to inlet air precooling are enhanced further as the ambient temperature increases [10]. The increase in air mass flow also increases the flow rates of fuel and combustion gases, and hence the amount of recoverable waste heat is increased. Inlet air precooling also reduces the impact of surge in the front stages of the compressor when running at part-load. Furthermore, the variations of specific power and efficiency with ambient temperature are reduced as a result of inlet air precooling. The optimum pressure ratios for maximum power and efficiency become higher than those of the conventional regenerative cycle [125]. It is possible, therefore, to utilize air precooling in conjunction with larger engines. It was found for aircraft engines that the positive effect of precooling increases with increasing compressor pressure ratio [126]. In gas turbine cogeneration plants, air precooling reverses the drop in power-to-heat ratio usually seen with increasing ambient temperatures [127]. Air precooling may be implemented by evaporative cooling [128] or by using a cycle configuration adopting the bleed-air system [129]. A further possibility is the utilization of an absorption refrigeration machine that is driven by the recovered heat from the exhaust gases of the engine [125]. The latter heat recovery method can be modified to have a waste heat boiler in the exhaust duct of the steam power plant. The tail-end gases coming from the waste heat boiler can be used to power the generator of the absorption machine [124]. Due to the reduction in compressor work, the cooling of inlet air has been considered as a means of improving performance. Air precooling, utilizing a cold energy recovery system, has been considered in connection with the use of liquid hydrogen [126, 130, 131] and LNG [44, 132]. Various suction/air configurations have been studied [129] for overcoming the main drawbacks of the gas turbine engine, that is, fairly poor thermal efficiency, and the significant variation of
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thermal efficiency and specific power with variations in ambient temperature. The most practical method used to date for inlet air cooling is evaporative cooling with its subsequent increase in mass flow through the turbine and the heat recovery boiler, thus boosting power and steam production [128]. The system presently in use is the conventional wetted rigid media system, although a direct mixing evaporative cooling system has also been suggested [133]. However, evaporative systems are excellent in regions of high temperature and low humidity [134]. There is growing interest in using the absorption system of refrigeration for inlet air precooling [125]. In cogeneration systems, inlet cooling was found to reverse the drop in power-heat ratio with increasing ambient temperature, and increase capacity at the cost of efficiency [8]. An absorption refrigeration system was proposed [ 124] that utilized the exhaust waste heat, whereby 200 kW MW of refrigeration needed by a mechanical chiller was saved. Using a two-stage lithium-bromide absorption chiller that runs on turbine exhaust, Hufford [135] reported that the available chilled water (tons), as converted to SI units, from the hot gas stream may be calculated from the formula: chilled water (tons) = 1.12 (0.97) (M) (T~ - T2) (COP)/3.5, where 1.12 is the average specific heat (kJ kg ~K ~), 0.97 is the transmission effÉciency in the ductwork, M is the gas mass flow rate (kg s ~), T~ is the inlet gas temperature (°C), T2 is 190°C, and COP is equal to 1.14. The highest and lowest allowable exhaust gas temperatures were 870 and 260°C, respectively. However, there is considerable concern about the possibility of using such high temperatures (260°C and above) in a lithium-bromide system without crystallization taking place. A recent study [136] on inlet air cooling showed that the absorption system offers less cost per kW and a shorter payback period than mechanical systems. The authors further stated that the installation of cooling coils in the engine inlet airstream is not complicated; coils using 7°C chilled water can be designed with an overall pressure drop of 25 mm Hg at rated inlet airflow. The cooling coils with drift eliminators must be downstream of the inlet air filters. Nasser and El-Kalay [137] proposed a heat recovery cooling system to conserve energy in gas turbine power stations located in the Arabian Gulf. The authors reasoned that in a gas turbine system, the output is inversely proportional to the ambient temperature, which varies by more than 30°C from summer to winter in the Arabian Gulf region. This causes a sizable drop in power during the summer time. Accordingly, the authors suggested using a lithium-bromide/water absorption cooling system to cool the intake air to the compressor. The cooling system was to be powered from the waste heat of the exhaust gases. The authors claimed that the useful power output may thus be increased by more than 20% during the summer without consuming more fuel. Again there is doubt about the validity of using exhaust gas directly into a lithium-bromide absorption system. Precooling of the inlet air by waste heat absorption refrigeration (WHAR), however, has substantial potential due to the following factors: (a) Since there is no need to use vapor compression refrigeration in WHAR, power will be saved at the rate of 200 kW/MW of refrigeration when compared with the bleed-air system [124]. (b) There is no need to utilize chloro-fluoro-carbons with WHAR, which are currently recognized as heavily implicated in the erosion of the ozone layer, and the atmospheric greenhouse effect [123]. (c) W H A R permits the use of combined gas turbine-steam turbine configuration, supplying back pressure steam to a single-effect absorption chiller. (d) In the case when the cooling load increases, the steam turbine may be omitted, and the high pressure steam is fed directly to a double-effect chiller [123]. Recently, it has been reported that the primary application of chillers are base-load gas turbine plants, but for a peaking station an ice-storage system was chosen where low-cost off-peak electric power was used during high-cost peaking situations [134]. As such, it is similar to compressed air storage. In an ammonia-based cycle, approximately 24 k W h of electrical energy is required to generate one ton of ice, with each ton having the ability to cool approximately 4000 kg of air from 40°C down to T'C.
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7. C O N C L U S I O N S
1. Making use of the waste energy in the form of cogeneration offers efficient use of resources to prevent further degradation of the environment, and to achieve the goal of greater economic competitiveness. 2. Beside district heating and air-conditioning, cogeneration has great potential in different industries such as: paper; dairy; brick; refinery operations such as ethylene production and enhanced oil recovery; and chemical complexes including salt refineries, chlorine production, caustic soda plants and natural gas based methanol plants. 3. Certain design considerations should be taken to enhance heat recovery from the exhaust energy with emphasis on economics. 4. Cooling with gas turbine engines enhances the performance of the engine by: (a) using air or steam for cooling the combustor and turbine blades; (b) cooling the intake air by using water evaporation; mechanical and absorption refrigeration for base-load applications; and ice-storage for peaking plants. Exhaust energy is utilized in the last two applications. Acknowledgement--This work was sponsored by King Abdulaziz City for Science and Technology, Riyadh, through grant No. AR-12-39.
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Jeffs, Expanded CHP system will make Italian paper mill energy independent. Cogeneration September/October 38-40 (1989). 65. R. Farmer, Tampella Espanola commissions 9 MW Mars powered CHP system. Gas Turbine Worm April 9--11 (1989). 66. R. Farmer, Combined cycle repowering more than doubles paper mill CHP power output. Gas Turbine Worm September/October 15 18 (1989). 67. R. Farmer, 42 MW cogeneration plant saving Proctor & Gamble $8 million a year. Cogeneration May/June 32 35 (1990). 68. E. Dunckel, Paper industry leads European industrial CHP drive. Cogeneration September/October 36-40 (1990). 69. E. Jeffs, Gas-fired micro-CHP system replaces diesel cogen sets at leather works. Cogeneration March/April 56 57 (1988). 70. R. Farmer, Dutch company in joint venture with utility on CHP plant. Gas Turbine World August 28-29 (1988). 71. D. H. Cooke and W. D. Parizot, Cogenerative direct exhaust integration of gas turbine in ethylene production. ASME Paper No. 90-GT-139 (1990). 72. R. Farmer, 300 MW enhanced oil recovery cogen plant is saving 7000bpd of crude. Gas Turbine Worm November/December 20-21, 24-25 (1985). 73. I. Stambler, Sycamore and Midway EOR plants rate high in cogen efficiency. Gas Turbine WorM January 8-13 (1988). 74. E. Nieuwlaar and B. DeVries, Energy conservation and cogeneration in the dairy industry. J. Heat Recovery Systems 4, 341-345 (1984).
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75. H. Takano, Y. Kitauchi and H. Hiura, Design for the 145 MW blast furnace gas-firing gas turbine combined cycle plant. J. Engng. Gas Turbines Power Ill, 218-224 (1989). 76. P. Jane and P. E. Hill, Direct heat cogeneration applications for kaolin producers. ASME Paper No. 90-GT-179 (1990). 77. L. Barthel, Energy cost reduction in the brick and tile industry--private electricity generation with waste-heat recovery unit power plant. Kawasaki Heavy Industries Leaflet, 1/84 (1984). 78. R. T. Wickmeijer, Improvements in incinerators by means of gas turbine-based cogensystems. ASME Paper No. 90-GT-180 (1990). 79. E. Finkelstein, Heat recovery without transfer surfaces. J. Heat Recovery Systems 3, 177-182 (1983). 80. Y. S. H. Najjar, Saving energy in refineries by means of expanders in fluidized bed catalytic cracking. J. Inst. Energy 66, 31-38 (1991). 81. Y. S. H. Najjar and M. B. Habeebullah, Energy conservation in the refinery by utilizing reformed fuel gas and furnace flue gases. Heat Recovery Systems & CHP II, 138-152 (1991). 82. S. E. Aly, Gas turbine waste-heat recovery distillation system. Heat Recovery Systems & CHP 7, 375-382 (1987). 83. S. E. Aly, Vapor compression distillation using waste-heat absorption systems. Desalination 68, 57-68 (1988). 84. B. G. Jabboury and M. A. Darwish, Effects of the operating parameters of heat recovery steam generators on combined cycle sea-water desalination plant performance. Heat Recovery Systems & CHP 10, 255-267 (1990). 85. B. G. Jabboury and M. A. Darwish, Performance of gas turbine co-generation power desalting plants under varying operating conditions in Kuwait. Heat Recovery Systems & CHP 10, 243-253 (1990). 86. A. Bosio and G. Manfrida, Computer-aided choice of optimal solutions for gas turbine cogeneration systems. A S M E Cogen Turbo, pp. 245-261 (1988). 87. S. Horii, K. Ito and P. S. Pak, Optimal planning of gas turbine cogeneration plants based on mixed-integer linear programming. Int. J. Energy Res. 11, 507-518 (1987). 88. R. W. Foster-Pegg, Capital cost of gas turbine heat recovery boilers. Chem. Engn. 93 (14), 73-78 (1986). 89. B. Brian, Wash water heat recovery. J. Imaging Tech. 11, 67-73 (1985). 90. V. Ganapathy, Heat recovery boiler design for cogeneration process applications covers many parameters. Oil Gas J. 83 (48), 123-125 (1985). 91. R. W. Precious and A. Pasha, The design of gas turbine heat recovery systems. Int. Power Gen. September 38-41 (1984). 92. L. M. Hossfeld and D. J. Hawkins, A cost-effective technology for flue gas condensation heat recovery. ASME Paper No. 85-JPGC-Fu-I (1985). 93. D. A. Reay, Heat transfer enhancement--a review of techniques and their possible impact on energy efficiency in the UK. Heat Recovery Systems & CHP 11, 1-40 (1991). 94. C. A. Frangopoulos, Costing of heat and electricity from a cogeneration system. A S M E Cogen-Turbo, pp. 349-356 (1988). 95. 1. G. Rice, Thermodynamic evaluation of gas turbine cogeneration cycles: Part l--heat balance method analysis. ASME Paper No. 86-GT-6 (1986). 96. I. G. Rice, Thermodynamic evaluation of gas turbine cogeneration cycles: Part 2---complex cycles analysis. ASME Paper No. 86-GT-7 (1986). 97. J. C. Stewart and C. F. Hsun, A computer program to analyze cogeneration plant heat balances and equipment design. ASME Paper No. 87-GT-27 (1987). 98. J. W. Baughn, A. A. McKillob and K. Trelwen, An analysis of the performance of a gas turbine cogeneration plant. J. Engng Power 109, 816-820 (1983). 99. F. Gang, C. Ruxian and L. Rumou, Off-design performance of fully fired combined cycles cogenerating. ASME Paper No. 90-GT-370 (1990). 100. K. Ito, R. Yokoyama, S. Akagi and Y. Mutsomuto, Influence of fuel cost on the operation of a gas turbine--waste heat boiler cogeneration system. J. Engng Gas Turbine Power 112, 112-123 (1990). 101. E. D. Mitchell, Cogeneration design considering optimum use of surplus power. A S M E Cogen-Turbo, pp. 393-396 (1989). 102. A. K. Sinha and D. G. Bogard, Gas turbine film cooling--flow field due to second row of holes. ASME Paper No. 90-GT-44 (1990). 103. G. E. Andrews, A. A. Asere, M. L. Gupta, Full coverage discrete hole film cooling--the influence of the number of holes and pressure loss. ASME Paper No. 90-GT-61 (1990). 104. R. E. McClelland, An investigation of convection cooling of small gas turbine blades using intermittent cooling air. D. Engng thesis, Univ. of Detroit (1988). 105. A. J. Teekaram, C. J. Forth and T. V. Jones, Use of foreign gas to stimulate the effects of density ratios in film cooling. J. Turbomach. 3, 57-62 (1989). 106. A. J. Teekaram, C. J. Forth and T. V. Jones, Film cooling in presence of mainstream pressure gradients. ASME Paper No. 90-GT-334 (1989). 107. S. Kikawa and K. Ozaki, Effect of density ratio of injected and main stream gases on film cooling effectiveness on a gas turbine blade. Heat Trans. Jpn. Res. 17 (6), 21-30 (1988). 108. S. Z. Koplev, M. N. Galkin and A. N. Boiko, Investigation of heat transfer during development of a cooling system for gas turbine blades. Power Engng (NY) 27 (3), 91-98 (1989). 109. Z. B. El-Qun and J. M. Owen, Preswirl blade cooling effectiveness in an adiabatic rotor-stator system. J. Turbomach. 3, 522-529 (1989). 110. J. O. Medwell, W. D. Morris, Y. J. Xia and C. Taylor, Investigation of convective heat transfer in a rotating coolant channel. ASME Paper No. 90-GT-329 (1990). I I I. D. Granser and T. Schulenberg, Prediction and measurement of film cooling effectiveness for a first-stage turbine vane shroud. ASME Paper No. 90-GT-95 (1990). 112. M. Obata, J. Yamaga and H. Taniguchi, Heat transfer characteristics of a return-flow steam-cooled gas turbine blade. Exper. Therm. Fluid Sci. 323-332 (1989). l l3. S. S. Stecco and B. Facchini, A computer model for cooled expansion in gas turbines. A S M E Cogen-Turbo, pp. 201-209 (1989).
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114. R. Bettocchi, G. Cantore and G. Negri di Montenegro, The coding air influence on the performance of the gas-steam combined cycle power plants with multiple expansion reheating gas turbine. A S M E Cogen-Turbo, pp. 201 209 (1989). 115. A. I. Khandwawala and S. A. Asif, Prediction of the effects of steam addition on performance of gas turbines with fixed-bed regenerators. J. Heat Recovery Systems 4, 1-8 (1984). 116. M. Perkavec, NO x suppression in heavy duty gas turbines. Diesel Gas Turbine Worldwide October 10 12 (11991). 117. A. Masashi, A. Tetsu, T. Kiyomi and K. Akinori, Research and development on the HPT of the AGTJ-100B. ASME Paper No. 87-GT-263 (1987). 118. Y. Tsujikawa and T. Sawada, Characteristics of hydrogen-fueled gas turbine cycle with intercooler, hydrogen turbine and hydrogen heater. Int. J. Hydrogen Energy 10, 677 683 (1985). 119. J. Bird and W. Grabe, Humidity effects on gas turbine performance. ASME Paper No. 91-GT-329 (1991). 120. A. A. EI-Hadik, The impact of atmospheric conditions on gas turbine performance. J. Engng. Gas Turbines Power 112, 590-596 (1990). 121. O. Yang and G. T. Sato, A study on suction cooling gas turbine cycle with turbo-refrigerating machine using the bleed air. Bull. J S M E 14, 493 (1971). 122. 1. Stambler, Sunlaw's cogen refrigeration plants rake up big profits in first three years. Gas Turbine Worm July/August 24-27 (1989). 123. P. Chester, Gas as a fuel source in CHP systems. Energy Worm August 12-15 (1989). 124. W. F. Malewski and G. M. Holldorf, Power increase of gas turbines by inlet air precooling with absorption refrigeration utilizing exhaust waste heat. ASME Paper No. 86-GT-67, Int. Gas Turbine Conf. Dusseldorf, 8-12 June (1986). 125. O. Yang, M. Tsujita and G. T. Sato, Thermodynamic study on the suction cooling gas turbine cycle combined with absorption-type refrigerating machine using wasted heat. Bull. J S M E 13, 1111-1122 (1970). 126. Y. Tsujikawa, T. Sawada and M. Hirano, Effects of precooling of suction air on the performance of liquid hydrogen-fueled supersonic aircraft engine. In Proc. 6th Worm Hydrogen Energy Conf. Hydrogen Energy Progress Vl, Vienna, Austria, July, pp. 1174-1184. Pergamon Press, New York (1986). 127. J. W. Boughn and R. A. Kerwin, A comparison of the predicted and measured thermodynamic performance of a gas turbine cogeneration system. ASME Paper No. 86-GT-162 (1986). 128. D. J. Moel|er and D. A. Kolp, Simpson paper company: first 35 MW IM5000 in cogeneration plant. ASME Paper No. 84-GT-55 (1984). 129. O. Yang and G. T. Sato, The suction cooling gas turbine cycles. Tokyo Joint Int. Gas Turbine Conf. Tokyo, Japan, 4-7 October, Paper No. JSME-2 (1971). 130. Y. Tsujikawa and T. Sawada, Analysis of a gas turbine and a steam turbine combined cycle with liquefied hydrogen as fuel. Int. J. Hydrogen Energy 7, 499-502 (1982). 131. Y. S. H. Najjar, A cryogenic gas turbine engine using hydrogen for waste heat recovery and regasification of LNG. Int. J. Hydrogen Energy 16, 129-134 (1991). 132. M. Arai, M. Okai and N. Okamoto, Practical application of an LNG cold energy recovery power plant. Tech. Rev. Mitsubishi Heavy lndustrv 21, 263-270 (1984). 133. J. P. Nolan and V. J. Twombly, Gas turbine performance improvement direct mixing evaporative cooling. Int. Gas Turbine Aeroengine Congr. Expo. ASME Publ., GT 368 (1990). 134. M. Piolenc, LES "iced" inlet nets utility another 14 MW of peaking at zero fuel cost. Gas Turbine Worm January 20 25 (1992). 135. P. E. Hufford, Absorption chillers improve cogeneration energy efficiency. ASHRAE J. March 46-53 (1992). 136. I. S. Ondryas, D. A. Wilson, M. Kawamoto and G. L. Haub, Options in gas turbine power augmentation using inlet-air chilling. ASME Paper No. 90-GT-250 (1990). 137. A. E. Nasser and M. A. E1-Kalay, Heat recovery cooling system to conserve energy in gas-turbine power stations in the Arabian Gulf. Appl. Energy 38, 133-142 (1991).
0890-4332/93 $6.00 + .00 © 1993 Pergamon Press Ltd
REVIEW P A P E R
COGENERATION WITH GAS TURBINE ENGINES Y. S. H.
NAJJAR,M. AKYURT,O. M. AL-RABGHIand T. ALP
College of Engineering, King Abdulaziz University, Jeddah 21413, Saudi Arabia (Received 26 November 1992) Abstract-42ogeneration, simply, is the generation of energy for one process from the excess energy supplied to another process. Cogeneration, then, is nothing more than an economically sound method for the conservation of resources [1]. Thus, the benefits from the use of cogeneration may be cited as energy conservation, environmental improvement and financial attractiveness to investors. In the near future energy etficiency must no longer be a choice, but a c o m m i t m e n t - - h e n c e the timely importance of this review.
1.
INTRODUCTION
Cogeneration, when associated with the gas turbine, is a relatively old technology which dates back to pre-World War II years. The concept of cogeneration involves the generation of steam and electricity in one operation. The "topping cycle" generates electricity when the turbine is directly coupled to a generator. The waste heat from the exhaust of the turbine is then recovered in the form of steam or hot water. In the "bottoming cycle" a gas turbine may be used as a combustor only [2]. The exhaust from the turbine may be fed to a steam generator (HRSG), and the resulting steam may be used as process steam or in a steam Rankine cycle. A secondary cycle with an organic fluid may be used at the condenser end to drive a turbine for generating byproduct electricity. During the decade of the 1960s, industrial users recognized the gas turbine as a reliable prime mover for base load process applications. Accordingly, gas turbine cogeneration systems were installed in various industries. More recently, worldwide concern about the cost and efficient use of energy is providing continuing opportunities for gas turbine cogeneration systems. In this connection, Allen and Kovacik [3] reviewed cogeneration principles applicable to the development of gas turbine energy supply systems. Negri et al. [4], as well as several other research teams [5-11] conducted essentially theoretical studies on the cogeneration idea and its several variations. Najjar and others carried out several performance studies related to different gas turbine cycle combinations for the purpose of evaluating energy conservation and utilization, including heat exchange, organic Rankine cycle and heat pumps [12-18]. In principle, the simplest modification to be introduced to the simple gas turbine engine cycle was to recover part of the exhaust energy in the heat exchanger of a recuperative cycle [12, 13, 19]. Exhaust energy can be recovered more efficiently, however, in a hot water or heat recovery steam generator (HRSG). Such a cogeneration system may have a power-to-heat ratio of 4 to 5 times that of steam turbines, in addition to the greater potential for generating power in excess of on-site needs [20]. The steam may be used in heating, cooling and many other industrial processes. In order to determine the feasibility of waste energy recovery, an energy mapping study needs to be carried out. The energy mapping and cascading concept can be utilized to optimize the energy use of a plant, where energy sources and energy users are important factors to be mapped and cascaded as shown in Figs 1 and 2 [2]. The organic Rankine cycle is used in energy cascading as a low bottoming cycle [16, 21]. The exhaust energy of the gas turbine can be used for steam generation, drying, or process fluid heating, as well as preheating combustion air for process heaters and boilers [3]. The potential users of cogeneration systems, also called combined heat and power (CHP), may be chemical, petrochemical, textile, metals, paper and board, and agricultural industries [22]. 471
472
Y. S, H. NAJJAR el a/.
2250 "--]
furnace
2ooo_j @
high pressure steam boiler waste heat range 1
1750 --I
[ heat user range
~-~ 1500
heat source range
--
I
gas turbine 125o
~
1000 - -
~
750 - -
steam boiler
500 - -
bottoming cycle
250 - 0
--
Fig. I. E n e r g y m a p o f a p r o c e s s p l a n t (after K i a n g [2]).
It is thus possible to use congeneration in many circumstances, such as the development of new industrial facilities, major expansions to existing installations, replacement of ageing steam generation equipment, significant changes in energy costs (fuel and power) and power sales opportunities [23]. Thus, it is no wonder that public utilities enter into joint ventures with private CHP operators [24]. Cogeneration projects may be small in size based on internal combustion engines [25-28]. However, large scale cogeneration, especially with gas turbines, has become a permanent part of the industry [29-32]. In general, planning and technical studies need to take into account such factors as capital and operating costs, reliability and demand profiles, all of which affect the decision of selecting the small scale CHP [33], and need to consider such factors as coal prices relative to gas prices when deciding on large systems [34]. Several research teams investigated the congeneration principle as related to gas turbine power plants and generated different solutions and designs. Pasha [35] studied the various factors affecting the design, cost and performance of a heat recovery steam generator, considering both natural and forced circulations. Kesseli and Wells [36] described a cost-competitive 30 kW gas turbine cogeneration system for solar electric applications. 2. TYPES OF COGENERATION WITH APPLICATIONS The gas turbine is the center of many exciting new power generating technologies such as the integrated coal-gasification combined-cycle (IGCC), pressurized fluidized bed combustion (PFBC) and compressed air energy storage (CAES) [37]. Moreover, gas turbine CHP plants are highly 2250--
furnace heat input
2000--
1750--
t..)
1500--
I
1250-1000--
gas turbine and high pressure s t e a m boiler heat input
750-500
--
250
--
0
--
low pressure steam boiler heal input bottoming cycle heat Input waste heat
Fig. 2. Energy cascading (after Kiang [2]).
Cogeneration with gas turbine engines
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competitive with steam turbines and internal combustion engines as prime movers, yielding a higher rate of return [38], better flexibility, higher efficiency, especially when using aero-derived gas turbines that have good part-load efficiencies [39] and low downtime, with the aero-derivative type having a removable gas generator that relates to most of the critical maintenance [40]. Cogeneration with gas turbines encompasses heating air for fired heaters [41], production of steam for cooling gas turbine blades [42], using heat rejected from closed-cycle gas turbines [43, 44], producing chilled water or cold air from an absorption system [10, 45], converting heat for space heating by the use of heat pumps [17, 18], using liquefied hydrogen as fuel and coolant [46], producing air for compressed air storage plants [47] and producing power from an absorption cycle to drive reverse osmosis desalination [48]. Other cogeneration systems include steam power plants [49-51], diesel power plants [52, 53], fuel cells [54] and nuclear power plants [55, 56]. There are many gas turbine cogeneration projects around the world, some producing hot water and steam for district heating [57], plus combined-cycle [58, 59] for power, heating and air conditioning [60] and other applications [15]. 3. I N D U S T R I A L COGENERATION Within the last few years, gas turbines have been integrated into several industries. The paper industry, a long-standing supporter of combined heat and power, has emerged in the last decade as a leading industrial market for gas turbines [61-68]. In each case, a two-or three-stage steam boiler is used for heat recovery from the exhaust of the gas turbine. Instead of using gas-fired driers, the tail end may be used directly for pulp-drying. It is reported that the gas turbine CHP is replacing ageing diesel cogeneration systems in some leather works [69]. Combined-cycle CHP plants are further reported to be supplying chemical complexes with all of their steam and electricity requirements [70]. The said complexes may include salt refineries, electrolytic plants for chlorine production, caustic soda plants, natural gas-based methanol plants and a range of derivative products. Cooke and Parizot [71] reported the use of a gas turbine exhaust in preheating combustion oxygen for cracking furnaces used in ethylene production. Large gas turbine cogeneration plants are now employed in enhanced oil recovery from heavy oil deposits which require the injection of large amounts of steam to make oil production feasible at economic flow rates [72, 73]. It is further reported [74] that the dairy industry makes extensive use of waste heat boilers in conjunction with gas turbines. It has been suggested that gas turbines fired by blast furnace gas can be installed in combined-cycle in steel works, where a multi-cannular type of combustor may be used to burn the low BTU fuel. The gas turbine, generator, steam turbine and the fuel gas compressor can be coupled to make a single-shaft combination [75]. One step in producing kaolin requires large spray driers, the heat for which can be supplied directly from the exhaust gas of the gas turbine engine [76]. Energy costs may be reduced in the brick and tile industry by utilizing gas turbine cogeneration [77]. Incineration plants boost their efficiency of power generation when steam is superheated in a gas turbine-waste heat boiler combination rather than in the incinerator boiler [78]. Heat recovery may be achieved more efficiently without heat transfer surfaces if the fluid to be cooled is very viscous or highly contaminated, such as the aqueous stream leaving an alcohol distillation column that contains grape peels or grains and organic fragments and solids, which if allowed to pass through a heat exchanger, would cause serious fouling problems [79]. Novel cogeneration gas turbine cycles for saving energy in energy-intensive industries, such as refineries and related industries, have been recently studied and analyzed [45, 80, 81]. The power plant coupled with sea-water desalination is another proposed area of application for waste heat recovery from gas turbines. Aly [48, 82, 83] suggested various methods for waste heat recovery with respect to different desalination plant designs. Jabboury and Darwish [84] presented a method to predict the performance of steam generators with heat recovery/steambottoming cycle combined with a sea-water desalination plant at various steam and gas conditions. In another study, Jabboury and Darwish [85] were concerned with the prediction of gas turbine cogeneration (power/desalting) plant performance under different environmental and loading conditions.
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Y.S.H. NAJJAR et al. 4.
DESIGN
CONSIDERATIONS
Gas turbines are steadily gaining popularity as prime movers in industrial applications, particularly those suitable for cogeneration. Analysis of the most efficient arrangement has been considered for conserving energy in the industrial environment [86, 87]. Boilers for recovering exhaust heat from gas turbines are very different from conventional boilers [88]. Significant improvements in performance are obtained with design modifications such as a counter-current configuration [89], fire or water-tube, tube size, and fin configurations. Boiler performance must be evaluated by considering all of the possible modes of operation with regard to steaming gas velocity, metal temperatures, heat flux, corrosion, erosion, tube bundle vibration and noise [90]. In general, the exchange of heat from the gas to the fluid should occur at the highest temperature difference possible. This can best be accomplished by making the gas and the fluid temperature gradients nearly parallel to each other, utilizing a multi-pressure level unit. When a multi-level pressure boiler cannot be justified, selecting the lowest possible usable pressure level for steam generation will achieve the highest steam production and the greatest heat recovery [91]. There are some design techniques which aid enhancement of heat recovery. These include selection of materials which do not corrode at flue gas condensation [92], and heat transfer enhancement methods whether passive or active, such as extended, treated, roughened surfaces, swirl flow devices, porous structures, additives and aids such as surface and fluid vibration, electrostatic fields and jet impingement [93]. 5. P E R F O R M A N C E
AND
ECONOMICS
Some methods have been proposed [94] that combine energy and exergy analyses with economic considerations in order to identify the relative portions, for heat and electricity, of the total cost of a cogeneration system. Leibowitz and Tabb [7] developed an integrated system with enhanced performance through the implementation of several engineering development tasks; namely, control integration, mechanical equipment integration, rotary screw gas compressor application, and application of steam injection. Rice [95] undertook an analysis of seven commercially available gas turbines, showing the effect of pressure ratio, exhaust temperature, intercooling, regeneration, and turbine inlet temperature with regard to power output, heat recovery, and overall efficiency. He then added some complexities such as reheating, steam injection, and steam cooling to examine their effects on overall heat balance [96]. Other researchers [97-99] developed computer programs to analyze heat balance in cogeneration systems at different loads. Ito et al. [100] investigated the relation of fuel cost to the operation policy on the energy demand of a cogeneration plant used for district heating and cooling. Mitchell [101] specified a method to quantify five distinct attributes of surplus power from a cogeneration facility. 6.
IMPACT
OF
COOLING
It is known that the efficiency of the gas turbine engine is relatively low at design point, and it deteriorates further at part-load and at off-design, when the ambient air temperature increases. The present trend in design is towards improving efficiency and power output by increasing the engine pressure ratio and turbine inlet temperature. This, however, results in the increase of NO, as the undesired pollutant. The compressor air is usually used for cooling the combustor and the blades of the turbine, thus enabling the utilization of higher combustion and turbine inlet temperatures [102, 103]. Extensive research has been carried out on turbine blade film cooling [104-109]. It was shown that film cooling effectiveness is mainly governed by the momentum flux ratio, injected and mainstream, on both suction and pressure surfaces, independent of density ratio. Further studies were carried out on convective heat transfer in a rotating coolant channel [110] and a turbine vane shroud [111]. Steam cooled gas turbine blades were investigated, and the results were compared with those using air [112].
Cogeneration with gas turbine engines
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Stecco and Facchini [113] developed a computer model for cooled expansion in gas turbines and found that using water or steam increases efficiency and specific power more than other coolants. Cooling effects differ according to cycle. In the simple cycle, the best performance is achieved with mixing of the coolant in the main stream. The authors concluded that in combined-cycles, however, performance is improved when there is no mixing. Bettocchi et al. [114] investigated cooling with multiple-expansion reheating combined gas-steam cycle and found that a higher power ratio between the gas turbine and the steam section can be achieved with cooling. The highest efficiency was obtained when the cooling air was not at the maximum compressor pressure. It was further proposed to utilize exhaust waste heat to produce steam for injection into the air stream at the inlet to the compressor, predicting a 4% rise in thermal efficiency [115]. One of the side benefits of injecting steam before or after the compressor is the reduction of NOx emissions [116]. Water spray has been used for precooling high pressure turbine cooling air to stand higher temperatures [117]. It is anticipated that variations of atmospheric conditions such as temperature, humidity and pressure will be important factors in gas turbine performance. Thermodynamic analyses have revealed that thermal efficiency and specific output decrease with the increase of humidity and ambient temperature [118]. An extensive study was undertaken on the effect of humidity on engine performance, and correlations were formulated [119]. Recently, EI-Hadik [120] carried out a parametric study on the effects of ambient temperature, pressure, humidity and turbine inlet temperature on power and thermal efficiency. He concluded that the ambient temperature has the greatest effect on gas turbine performance, which increases with the turbine inlet temperature and pressure ratio. As mentioned previously, the gas turbine engine can be used in a cogeneration system, utilizing the heat from the exhaust gases to drive an absorption refrigeration system to produce cool air or chilled water, both of which can be utilized in different applications [45, 121-123]. An effective method of overcoming the problem of NOx and improvement of performance is the precooling of the inlet air. The increased density of the cooled air increases the mass flow through the engine, resulting in a significant increase in gas turbine power output with a slight improvement in efficiency. The NOx emission is reduced. A further consequence of precooling the inlet air is that maintenance costs are reduced, as these depend heavily on the temperature of the hot section [124]. Improvements in power and efficiency due to inlet air precooling are enhanced further as the ambient temperature increases [10]. The increase in air mass flow also increases the flow rates of fuel and combustion gases, and hence the amount of recoverable waste heat is increased. Inlet air precooling also reduces the impact of surge in the front stages of the compressor when running at part-load. Furthermore, the variations of specific power and efficiency with ambient temperature are reduced as a result of inlet air precooling. The optimum pressure ratios for maximum power and efficiency become higher than those of the conventional regenerative cycle [125]. It is possible, therefore, to utilize air precooling in conjunction with larger engines. It was found for aircraft engines that the positive effect of precooling increases with increasing compressor pressure ratio [126]. In gas turbine cogeneration plants, air precooling reverses the drop in power-to-heat ratio usually seen with increasing ambient temperatures [127]. Air precooling may be implemented by evaporative cooling [128] or by using a cycle configuration adopting the bleed-air system [129]. A further possibility is the utilization of an absorption refrigeration machine that is driven by the recovered heat from the exhaust gases of the engine [125]. The latter heat recovery method can be modified to have a waste heat boiler in the exhaust duct of the steam power plant. The tail-end gases coming from the waste heat boiler can be used to power the generator of the absorption machine [124]. Due to the reduction in compressor work, the cooling of inlet air has been considered as a means of improving performance. Air precooling, utilizing a cold energy recovery system, has been considered in connection with the use of liquid hydrogen [126, 130, 131] and LNG [44, 132]. Various suction/air configurations have been studied [129] for overcoming the main drawbacks of the gas turbine engine, that is, fairly poor thermal efficiency, and the significant variation of
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thermal efficiency and specific power with variations in ambient temperature. The most practical method used to date for inlet air cooling is evaporative cooling with its subsequent increase in mass flow through the turbine and the heat recovery boiler, thus boosting power and steam production [128]. The system presently in use is the conventional wetted rigid media system, although a direct mixing evaporative cooling system has also been suggested [133]. However, evaporative systems are excellent in regions of high temperature and low humidity [134]. There is growing interest in using the absorption system of refrigeration for inlet air precooling [125]. In cogeneration systems, inlet cooling was found to reverse the drop in power-heat ratio with increasing ambient temperature, and increase capacity at the cost of efficiency [8]. An absorption refrigeration system was proposed [ 124] that utilized the exhaust waste heat, whereby 200 kW MW of refrigeration needed by a mechanical chiller was saved. Using a two-stage lithium-bromide absorption chiller that runs on turbine exhaust, Hufford [135] reported that the available chilled water (tons), as converted to SI units, from the hot gas stream may be calculated from the formula: chilled water (tons) = 1.12 (0.97) (M) (T~ - T2) (COP)/3.5, where 1.12 is the average specific heat (kJ kg ~K ~), 0.97 is the transmission effÉciency in the ductwork, M is the gas mass flow rate (kg s ~), T~ is the inlet gas temperature (°C), T2 is 190°C, and COP is equal to 1.14. The highest and lowest allowable exhaust gas temperatures were 870 and 260°C, respectively. However, there is considerable concern about the possibility of using such high temperatures (260°C and above) in a lithium-bromide system without crystallization taking place. A recent study [136] on inlet air cooling showed that the absorption system offers less cost per kW and a shorter payback period than mechanical systems. The authors further stated that the installation of cooling coils in the engine inlet airstream is not complicated; coils using 7°C chilled water can be designed with an overall pressure drop of 25 mm Hg at rated inlet airflow. The cooling coils with drift eliminators must be downstream of the inlet air filters. Nasser and El-Kalay [137] proposed a heat recovery cooling system to conserve energy in gas turbine power stations located in the Arabian Gulf. The authors reasoned that in a gas turbine system, the output is inversely proportional to the ambient temperature, which varies by more than 30°C from summer to winter in the Arabian Gulf region. This causes a sizable drop in power during the summer time. Accordingly, the authors suggested using a lithium-bromide/water absorption cooling system to cool the intake air to the compressor. The cooling system was to be powered from the waste heat of the exhaust gases. The authors claimed that the useful power output may thus be increased by more than 20% during the summer without consuming more fuel. Again there is doubt about the validity of using exhaust gas directly into a lithium-bromide absorption system. Precooling of the inlet air by waste heat absorption refrigeration (WHAR), however, has substantial potential due to the following factors: (a) Since there is no need to use vapor compression refrigeration in WHAR, power will be saved at the rate of 200 kW/MW of refrigeration when compared with the bleed-air system [124]. (b) There is no need to utilize chloro-fluoro-carbons with WHAR, which are currently recognized as heavily implicated in the erosion of the ozone layer, and the atmospheric greenhouse effect [123]. (c) W H A R permits the use of combined gas turbine-steam turbine configuration, supplying back pressure steam to a single-effect absorption chiller. (d) In the case when the cooling load increases, the steam turbine may be omitted, and the high pressure steam is fed directly to a double-effect chiller [123]. Recently, it has been reported that the primary application of chillers are base-load gas turbine plants, but for a peaking station an ice-storage system was chosen where low-cost off-peak electric power was used during high-cost peaking situations [134]. As such, it is similar to compressed air storage. In an ammonia-based cycle, approximately 24 k W h of electrical energy is required to generate one ton of ice, with each ton having the ability to cool approximately 4000 kg of air from 40°C down to T'C.
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7. C O N C L U S I O N S
1. Making use of the waste energy in the form of cogeneration offers efficient use of resources to prevent further degradation of the environment, and to achieve the goal of greater economic competitiveness. 2. Beside district heating and air-conditioning, cogeneration has great potential in different industries such as: paper; dairy; brick; refinery operations such as ethylene production and enhanced oil recovery; and chemical complexes including salt refineries, chlorine production, caustic soda plants and natural gas based methanol plants. 3. Certain design considerations should be taken to enhance heat recovery from the exhaust energy with emphasis on economics. 4. Cooling with gas turbine engines enhances the performance of the engine by: (a) using air or steam for cooling the combustor and turbine blades; (b) cooling the intake air by using water evaporation; mechanical and absorption refrigeration for base-load applications; and ice-storage for peaking plants. Exhaust energy is utilized in the last two applications. Acknowledgement--This work was sponsored by King Abdulaziz City for Science and Technology, Riyadh, through grant No. AR-12-39.
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