Applied Thermal Engineering 20 (2000) 179±197 www.elsevier.com/locate/apthermeng Gas turbine cogeneration systems: a review of some novel cycles Yousef S.H. Najjar* Mechanical Engineering Department, PO Box 9027, King Abdulaziz University, Jeddah, 21413, Saudi Arabia Received 24 October 1998; accepted 7 February 1999 Abstract The gas turbine engine is known to have a number of attractive features, principally: low capital cost, compact size, short delivery, high ¯exibility and reliability, fast starting and loading, lower manpower operating needs and better environmental performance, in relation to other prime movers, especially the steam turbine plant, with which it competes. However, it su€ers from limited eciency, especially at part load. Cogeneration, on the other hand, is a simultaneous production of power and thermal energy when the otherwise wasted energy in the exhaust gases is utilised. Hence, cogeneration with gas turbines utilises the engine's relative merits and boosts its thermal eciency. Thereby, the worldwide concern about the cost and ecient use of energy is going to provide continuing opportunities, for gas turbine cogeneration systems, in power and industry. In this work, ten research investigations carried out by the author and associates during the last ten years in the ®eld of gas turbine cogeneration in power and industry are reviewed brie¯y. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Gas turbines; Cogeneration; Novel cycles; Review 1. Introduction Cogeneration is the simultaneous production of various forms of energy from one power source (normally associated with heat and power-mechanical or electrical). The engine produces primary electrical power whereas thermal energy in the exhaust gases is converted in the heat recovery boiler HRB into steam. This process steam could be used in heating, * Tel.: +966-640-2000 ext. 4263; fax: +966-695-2182. 1359-4311/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 9 ) 0 0 0 1 9 - 8 180 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 absorption air conditioning, desalination and drying processes, and in the production of secondary electrical power in a combined cycle. Thus, cogeneration is an economically sound method for the conservation of resources [1], environmental improvement and ®nancial attractiveness [2]. Cogeneration with gas turbines is the most widely used, because the simple cycle gas turbine engine is known to feature: relatively low capital cost, high ¯exibility, high reliability without complexity [3], short delivery, early commissioning and commercial operation, and fast starting and loading. It is a compact engine, with lower manpower operating needs and ready availability [4]. The gas turbine is further recognised for its better environmental performance manifested in curbing of air pollution and reducing the greenhouse e€ect [5]. With all the advantages of the simple cycle gas turbine engine, utilities have to cope with limited eciencyÐespecially at part loadÐand the resulting dominance of fuel on generation cost [5]. Improving eciency could be implemented by increasing the turbine inlet temperature up to the metallurgical limit set by the material of the turbine blade; or/and by utilising the otherwise wasted heat in the exhaust gases, and thereby cogenerating power and heat. Exhaust energy of the gas turbine can be used for steam generation, drying, process ¯uid heating and preheating of combustion air for process heaters and boilers. The potential users of cogeneration may be chemical, petrochemical, textile, metals, paper and board, and agricultural industries [6] as well as space heating and air conditioning [7]. It is possible to use cogeneration in many circumstances, such as the development of new industrial facilities, major expansions to existing installations, replacement of ageing steam generation equipment, signi®cant changes in energy costs (fuel and electricity) and power sales opportunities [8]. 2. Factors a€ecting the design 2.1. Prime movers More recently, worldwide concern about the cost and ecient use of energy is providing continuing opportunities for gas turbine cogeneration systems. In principle, the simplest modi®cation to be introduced to the simple gas turbine cycle was to recover partially the exhaust energy in the heat exchanger of a recuperative cycle [9±11]. Exhaust energy can be recovered more eciently, however, in a hot water or heat recovery steam generator (HRSG). Such a cogeneration system may have a power-to-heat ratio (W/Q ) of 4±5 times that of steam turbines and 0.4±0.5 that of diesel engines. Hence the selection of the prime mover for cogeneration can be made only on a system-by-system basis, to match the energy demand with the system operating characteristics [12]. Each system has its own requirements for thermal and electrical energy. The type of fuel and space available would also a€ect the decision. It is clear that gas turbine engines enjoy better merits relative to steam turbines and diesel engines yielding higher rate of return [13], better ¯exibility, higher eciency especially when using aero-derived gas turbines that have good part-load eciencies [14], low downtime, and a removable gas generator that relates to most of the critical maintenance [15]. Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 181 2.2. Energy mapping and cascading In order to determine the feasibility of waste heat recovery, an energy mapping study needs to be carried out. The energy mapping and cascading concept can be utilised to optimise the energy use of a plant, where energy sources and energy users are important factors to be mapped and cascaded [16]. Five processes or cycles could be interconnected, each using high temperature source and rejecting low temperature energy. Each process uses a fraction of the energy available to it. By employing the energy cascading concept, the utilisation of energy can be maximised. For example, the rejected energy from the steel mill furnace can be recuperated to preheat combustion air for the gas turbine, which in turn can be bottomed with a HRSG to generate steam to drive the steam turbine and to produce process steam; reject energy from this cascade is sucient for driving an organic Rankine cycle which then rejects low quality energy to a cooling medium. Thus several di€erent industrial processes become interdependent. This may not be desirable in many industries, but the potential bene®t of the concept should warrant thorough consideration of all the pros and cons [17]. 2.3. Economics Fig. 1 shows, as an example, the anticipated annual savings and payback period of a cogeneration system installed in a city hotel [18]. Cogeneration ratio is the ratio of the declared cogeneration power to the contract electricity of the conventional system. It can be seen from the diagram, that the optimum capacity of the cogeneration system is 60±80% of the maximum simultaneous requirements of electric power. The ratio of total energy demands to total energy supply in the hotel is 81%. Utility thermal eciency is assumed to be 35%. The overall utilisation eciency in conversion of primary energy as fuel into useful heat and power is 76%. Total energy saving in energy supply with Fig. 1. Savings by CGS size in a hotel. Index for evaluation of savings for introduction of CGS into hotel [18]. 182 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 cogeneration vs conventional system is 25%. Thereby, the ®nal decision for the design is thermoeconomic where both performance and cost are optimised. 3. Analysis and discussion Several research teams investigated the cogeneration principle as related to gas turbine power plants and generated di€erent solutions and designs [19,20]. There are many gas turbine cogeneration projects around the world [21±24]. This paper is not intended to review the vast literature on gas turbine cogeneration, but to summarise the several research investigations carried out by the author and associates, during the last ten years, in the ®eld of gas turbine cogeneration in power and industry. 3.1. Fundamental studies 3.1.1. Waste energy utilisation in heat-exchange gas turbine cycles [25] One of the ways that can be used to increase the eciency of a shaft gas turbine engine is by installing a heat exchanger in one of the following two con®gurations: 1. After the low pressure turbine (usual case); Fig. 2. 2. After the high pressure turbine (suggested); Fig. 3. Analysis of ideal and real cycles for both con®gurations is done by using a computer program, where the following parameters were studied: heat exchanger e€ectiveness, turbine eciency, compressor eciency, ratio of turbine inlet pressure to compressor delivery pressure (P3/P2), and maximum temperature ratio (T3/T1). From the sensitivity analysis for both con®gurations, the usual con®guration is inferior to the suggested one in terms of the relative e€ects of compressor and turbine eciencies on the overall eciency, and turbine inlet pressure on overall thermal eciency and power output. However, the usual method is superior with respect to the relative e€ects of the compressor and turbine eciencies on power output. Fig. 2. Line diagram of the ideal usual case. Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 183 Fig. 3. Line diagram of the ideal proposed case. 3.1.2. Relative e€ect of pressure losses and ineciencies of turbomachines on the performance of the heat-exchange gas turbine cycle [26] The gas turbine engine is known to be relatively more sensitive to pressure losses and ineciencies of components than any other engine (Figs. 4 and 5). Therefore, a quantitative evaluation of the resulting performance is necessary. In this paper, a heat-exchange cycle, which is widely used due to its higher eciency, is considered. Performance, including work, thermal eciency and air mass ¯ow, is evaluated over a wide range of operating conditions, namely, compressor pressure ratio and eciency plus turbine eciency. A specially designed computer program was used. The analysis resulted in a set of curves which help the designer to easily estimate the percentage change in performance and engine cost relative to the ideal cycle over a wide range of operating conditions (Table 1). 3.1.3. Comparison of performance for cogeneration systems using single- or twin-shaft gas turbine engines [27] Currently, gas turbine systems account for over 50% of the new capacity installed in the United States. Moreover, options chosen for further development seem to aim, very clearly, at the immediate higher eciency mid-range aero-derivative market. Hence, twin-shaft engines are expected to be more widely used. Cogeneration with advanced aeroderivative engines has the prospect of attaining thermal eciency around 60% in the near future. In this work, performance of cogeneration systems associated with twin-shaft engines (Fig. 6) is compared with those related to single-shaft engines over a wide range of loading conditions. A computer program was devised to perform the calculations. The results showed the superior performance of the twin-shaft cogeneration systems at part Fig. 4. Single-shaft open cycle gas turbine with heat exchanger. 184 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 Fig. 5. Pressure losses. load. At a compressor pressure ratio rc of 8, the percentage of performance parameters relative to their values at design rc=10 are as follows: recoverable heat=13.8 and 77.4%; and overall eciency=45 and 93% for single- and twin-shaft cogeneration systems, respectively, as shown in Figs. 7±9. 3.2. Regenerative cycles using steam 3.2.1. Enhancing gas turbine engine performance by means of the evaporative regenerative cycle [28] The gas-turbine engine has relatively poor performance at part-load, and power output deteriorates during the hot season. Therefore the use of water injection in a regenerative cycle, utilising the exhaust waste energy in the recuperator and the water-heater, is expected to recover the de-rated power and improve the fuel economy at lower cost and higher reliability than in the combined cycle. In this research the performance of the evaporative regenerative Table 1 Performance table for ideal and actual cases plus performance ratio relative to the ideal case I Zc=1, Zt=1, E=1 II Zc=1, Zt=1, E=0.8 III Zt=1, Zc=0.87, E=1 IV Zt=0.87, Zc=1, E=1 rc/rcd rc W m Z W m Z W m Z W m Z 0.6 0.8 1.0 1.2 80.84 68.9 62.6 58.9 51.9 53 53 52.86 0.805 0.814 0.809 0.804 1.248 1.228 1.235 1.246 0.68 0.734 0.765 0.776 0.932 0.928 0.922 0.91 1.079 1.078 1.084 1.099 0.931 0.921 0.921 0.91 0.81 0.805 0.797 0.792 1.242 1.242 1.254 1.264 0.886 0.887 0.883 0.878 3 4 5 6 247.413 290.27 319.64 339.415 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 185 Fig. 6. Schematic drawing for a twin-shaft cogenerative system. cycle ERC (Fig. 10) is analysed parametrically, taking as the main variables the compressor pressure-ratio Rc, turbine inlet temperature T03, and water:air ratio w. A specially designed computer program was tailored for the analysis, where the main variables are changed over wide ranges. The relative e€ect of water injection is demonstrated by comparison of ERC with an equivalent regenerative cycle RC. The results show that ERC outperforms RC by about 57% in power and 13% in eciency, and that water injection would compensate for the power de-rating of engines during the hot season. Figs. 11 and 12 and Table 2 show the comparative performance for the two cycles. 3.2.2. Intercooled low-pressure turbo steam-injection gas turbine with cogeneration [29] Combined power plants seem an almost ideal solution for coping with demands for electrical power and heat; they give unique ¯exibility over wide ranges of loading. The gas turbine, because of its ability to burn a variety of fuels, may become the work-horse of all expanding Fig. 7. Variation of relative power drop with cogen vs compressor pressure ratio. 186 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 Fig. 8. Variation of overall eciency with compressor pressure ratio. Fig. 9. Variation of total heat recovered in the boiler with compressor pressure ratio. electricity-supply systems. In this paper a system of intercooled low-pressure steam-injected gas turbine (turbo-STIG) with cogeneration is compared with an intercooled cogenerated one (Fig. 13). Design point thermo-economic evaluation shows that the ®rst out-performs the second by about 21% in power output and 16% in overall eciency, with a payback period of 1.5 years (Tables 3 and 4). O€-design performance and useful energy in the bled steam were evaluated over wide ranges of the operating variables, namely compressor pressure ratio, turbine inlet temperature and injected steam:air ratio. A comparison between the two systems demonstrates Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 187 Fig. 10. (4) The regenerative cycle; (Q) the evaporative cycle. Fig. 11. Variation of work output with Rc. the superiority of the ®rst system. Sensitivity analysis shows the remarkable e€ect of turbine inlet temperature on the energy in the bled steam. 3.3. Gas turbines using hydrogen 3.3.1. Hydrogen fueled and cooled gas turbine engine [30] Hydrogen has many attractive features that give it a promising future as an alternative to hydrocarbon fuels in both steady and unsteady combustion processes. On the other hand, the gas turbine engine is gaining ®rm ground in di€erent ®elds of application. In this work, the performance of hydrogen fueled and turbine-blade cooled heat exchange gas turbine cycle is compared with a similar cycle using diesel fuel with the conventional method of compressor air 188 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 Fig. 12. Variation of thermal eciency with Rc. Table 2 (A) The performance parameters at design point, namely Rc=12, T03=1400 K and w = 12.88%; (B) The percentage relative improvement of the ERC over RC (A) Wn, kJ kgÿ1 Z, % SFC, kg k Whÿ1 RC ERC 316.496 497.365 0.41 0.462 0.207 0.183 (B) Wn Z SFC 57.15 12.683 11.6 Table 3 Comparison of performance of both systems at design conditions of Rc=16, T06=1400 K and SAR=0.08 Parameter System A System B Di€erence % Net work, kJ kgÿ1 air Total power, MW Energy in process steam QBL, MW Mass of steam, kg sÿ1 Speci®c fuel consumption, kg kWÿ1 hÿ1 Fuel energy, MW Corrected overall eciency, % 575.25 60.36 12.312 5.156 0.1541 108.88 59.55 476.49 50.0 34.163 11.704 0.2111 123.54 51.36 20.7 20.7 ÿ64 ÿ56 ÿ27 ÿ12 15.95 189 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 Fig. 13. Diagram of system A. Table 4 Economic analysis of the ILPT-STIG `system A', and the evaporative cogeneration cycle `system B' Economic item System A $ millions System B $ millions Capital cost investment Increment in investment Fuel saving/year Electric power saving/year Steam saving/year Relative saving/year Pay-back period, years 27.162 7.162 2.059 3.626 ± 4.903 1.49 20.00 ± ± ± 0.882 ± ± Fig. 14. Cycle con®guration of a heat-exchange gas turbine engine using hydrogen as fuel and for cooling. 190 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 for cooling (Fig. 14). The analysis covered obtaining power, speci®c fuel consumption and engine thermal eciency over a wide range of the operating variables namely compressor pressure ratio Rc and the cycle maximum temperature T03. A computer program was specially designed to carry out the whole computations. Results show that the hydrogen cycle is superior to the air cycle by about 50% in power and 9% in thermal eciency. Table 5 shows the comparative performance. 3.3.2. Cryogenic gas turbine engine using hydrogen for waste heat recovery and regasi®cation of LNG [31] Hydrogen has certain characteristics which make it convenient to use as a working substance in a closed gas turbine cycle, that recovers waste heat and delivers power, and rejects the heat for evaporation of LNG to be ready for further use (Fig. 15). Important operating parameters are compressor pressure ratio rc and turbine inlet temperature T3. A computer program is designed to carry out parametric performance calculations on, power output, reject heat and overall eciency. Sensitivity analysis shows that T3 is relatively more e€ective than rc with respect to power and overall eciency, whereas rc takes over when rejected heat for evaporation is considered of main concern (Fig. 16). Economic analysis shows that this system would be a remarkably viable alternative to the currently used heating system, in such an energy intense industry (Table 6). 3.3.3. The over-expansion gas turbine cycle using hydrogen [32] An interesting con®guration of the gas turbine engine is the over-expansion cycle, where an inverted gas turbine is added to the gas turbine engine in addition to the waste heat recovery comprising water and liquid hydrogen heaters (Fig. 17). The analysis of this cycle involved compressor pressure ratio and turbine inlet temperature as the main variables. Pressure losses in the heat exchangers and variation of polytropic eciencies of turbomachines with di€erent loads were considered. The results of this study show that the over-expansion cycle gives about 17% less speci®c work and 20% higher eciency than the conventional cycle (Figs. 18 and 19). Thus, it could be used conveniently in industrial stationary applications. Table 5 Comparison of performance of di€erent cycles at the design point (Rc=7, T03=1300 K) Diesel fuel H2 fuel Performance parameters Basic cycle Bleeding air cycle % change Basic cycle H2 cooling cycle % change Wn, kJ kgÿ1 SFC, kg kWÿ1 hÿ1 Z, % 289.12 0.2012 42 391.33 0.1858 45.57 +35.35 ÿ7.65 +8.5 283.66 0.0716 41.92 589.88 0.0605 49.6 +108 ÿ15.47 +18.32 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 191 Fig. 15. Line diagram of a hydrogen gas turbine cycle with intercooling, reheat and heat exchange. 3.4. Gas turbines and the re®nery 3.4.1. Saving energy in re®neries by means of expanders in ¯uidised-bed catalytic cracking [33] In petroleum re®ning, ¯uidised-bed catalytic cracking is one of the most commonly used processes for converting heavy high-boiling point components of crude oil into gasoline and distillate components. As an energy-conserving measure for such a process, an axial-¯ow compressor supplies air for combustion in the regenerator, where the coke deposits are burned o€ the catalyst; it also impels the catalyst through the system. Flue gases from the regenerator are expanded in an expansion turbine that drives the compressor, whereas the excess energy is Fig. 16. Variation of net work wn, rejected heat Qt and overall eciency E0 with turbine inlet temperature T3. 192 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 Table 6 Economic analysis of the waste-heat hydrogen cycle in comparison with the currently used burner system System Economic item Burner cost $ millions Waste heat hydrogen cycle cost $ millions Capital cost investment Increment in investment Engineering & sundries, 10% Gross increment in investment Fuel saving in the boiler/year Electric power saving/year Total savings/year Payback period, years 1.305 ± ± ± ± ± ± ± 6.2148 4.9098 0.49098 5.40078 0.578588 1.6647 2.243288 2.4 Fig. 17. Schematic and T±S diagrams for a combined inverted and gas turbine cycle. Table 7 Comparison of performance of the expander and gas-turbine cases Parameter Case A: expander Case B: gas turbine Compressor work, kJ kgÿ1 Turbine work, kJ kgÿ1 Net work, kJ kgÿ1 Gas turbine eciency, % Exhaust recoverable energy, kJ kgÿ1 Energy input, kJ kgÿ1 Steam ratio Overall eciency, % 218.3 389.85 171.55 28.4 308.14 605.00 1.0 42.8 720.14 891.69 171.55 28.19 1391.00 1206.23 4.514 46.73 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 193 Fig. 18. Variation of power with r for both the conventional and over-expansion cycles. used to drive an electric generator. The exhaust gases are utilised further in a heat-recovery boiler, to generate process steam (Figs. 20 and 21). With the help of a specially written computer program, the performance of the proposed system was analysed in a parametric study involving variation of the air pressure and expander inlet temperature. The system was economically evaluated and compared with the conventional cracking system associated with a gas-turbine engine (Table 7). The proposed system o€ers more advantages over the conventional one with regard to less investment and reduced size of components. Fig. 19. Variation of eciency with r for both cycles. 194 Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 Fig. 20. Schematic diagram showing energy-conservation arrangement: expander, A. 3.4.2. Energy conservation in the re®nery by utilising reformed fuel gas and furnace ¯ue gases [34] Decrease of fuel supplies and cost increases make it vital for industries, especially energy intensive ones, to consider conserving available sources and convert losses into sources of energy. In this paper, a gas turbine-based cogeneration system is suggested to utilise a re®nery's reformer gas in the gas turbine, and furnaces ¯ue gases together with the engine exhaust gases in a heat recovery steam generator, HRSG. This is proposed as an alternative to the currently used system where the gas turbine and the steam generator are used separately (Figs. 22 and 23). Operating variables comprising compressor pressure ratio and turbine inlet temperature are varied widely to evaluate performance; namely power, SFC, overall eciency and annual fuel savings at design and o€-design loading conditions using specially-designed computer program. Fig. 21. Schematic diagram showing energy-conservation arrangement: gas turbine with bleed air, B. Y.S.H. Najjar / Applied Thermal Engineering 20 (2000) 179±197 195 Fig. 22. Schematic diagram of the currently used system `A'. Results show that the proposed system o€ers 100% higher overall eciency (Fig. 24) and $5.25 million annual fuel saving for a 12 MWe gas turbine engine. 4. Conclusions 1. 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