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Power generation with gas turbine systems and combined heat and power
Applied Thermal Engineering 20 (2000) 1421±1429
www.elsevier.com/locate/apthermeng
Power generation with gas turbine systems and combined heat and power P.A. Pilavachi European Commission, Research Directorate-General, 200, Rue de la Loi, 1049 Brussels, Belgium
Abstract This article gives an overview of power generation with gas turbine and combined heat and power (CHP) systems. It also presents the European Union strategy for developing gas turbines and CHP systems. Ways to improve the performance of the several types of gas turbine cycle will be a major objective in the coming years. The targets are combined cycle eciencies above 60% industrial gas turbine system eciencies of at least 50% and small gas turbines eciencies above 35% and designs for the use of fuels with less than 25% heating value of that of natural gas. The main CHP targets are the reduction of the overall costs and the development of above 40 kW biomass-®red systems. 7 2000 The Commission of the European Communities. Published by Elsevier Science Ltd. All rights reserved. Keywords: Power generation; Gas turbine systems; Combined heat and power
1. Introduction The power generation industry uses a large amount of the primary energy demand in the European Union and power generation from gas turbine and combined heat and power (CHP) systems is an important and growing part of it. Therefore, there is a continuing need for improved energy eciency, coupled with a pressing need to reduce toxic and noxious emissions. In view of the uncertainties related to global warming, there is also a de®nite requirement to monitor and reduce, where possible, CO2 emissions. In long term, the potential exists for a signi®cant impact on both energy eciency and environmental protection. Therefore, the aim here is to develop technologies to help minimise the environmental impact of the production and use of energy in Europe. A driving force will be the Kyoto objectives for the European Member States to decrease by 8% the greenhouse gas emissions in 1359-4311/00/$ - see front matter 7 2000 The Commission of the European Communities. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 1 6 - 8
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2010 compared to the 1990 level. The promotion of the use of CHP is also expressed in the European Union CHP strategy [1]; the aim is to increase the participation of CHP from 9% of 1994 to 18% in year 2010. The use of gas turbines for power generation has increased in recent years and is likely to continue to increase particularly for distributed power systems. Applications fall into three categories, i.e. peak lopping (<10% utilisation), base load (about 100% utilisation) and in between. Gas turbines enjoy certain merits relative to steam turbines and diesel engines. They have high grade waste heat, lower weight per unit power, dual fuel capability, low maintenance cost, low vibration levels, low capital cost, compact size, short delivery time, high ¯exibility and reliability, fast starting time, lower manpower, and have better environmental performance. A further advantage is that gas ®ring produces less CO2 per unit power than does a liquid or solid fuel. Ways to improve the performance of several types of gas turbine cycle will be a major objective in the coming years. The targets are combined cycle eciencies above 60%, industrial gas turbine system eciencies of at least 50% and small gas turbine eciencies above 35% and designs for the use of fuels with less than 25% heating value of that of natural gas. The proportion of power generation using combined heat and power (CHP) is also growing mainly due to eciency improvements and environmental bene®ts. Ways to further improve the share of CHP in power generation is also of interest. Gas turbine cycles and CHP systems for power generation are discussed below.
2. Gas turbine systems Gas turbine technology is a critical one for the future, providing a clean, ecient and cost eective way of generating power from distributed generation and co-production schemes for large utility-size combined cycle plant. There are dierent gas turbine systems such as stand alone, combined with a steam turbine (``combined cycle'') or, less commonly at present, with a fuel cell. The main markets are in the power generation industry, the process industry, mobile applications (land and military marine) and gas and oil transmission pipeline pumping stations. The main technical barriers to the implementation of gas turbine technology are that the stand alone gas turbine has a lower eciency in its basic con®guration than an equal power output reciprocating engine. Furthermore the eciency decreases at partial load and burning of low heating value fuels may not be feasible, depending on the type of the turbine. The main non-technical barriers to the implementation are that maintenance requires more skilled personnel than does the reciprocating engine and that small gas turbines are too expensive compared to engines. Natural gas use is expected to grow. However, this will not be accessible in all geographical areas. Therefore, in the industrial market, the fuels mix is expected to become more diverse including naphtha, kerosine, gas condensates, natural gas liquids, alcohols, re®nery residues, biomass, etc. This fuel ¯exibility will of course need to be matched to a low emissions capability. Dierent technologies have been used to improve the gas turbine cycle eciency and an
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overview was given by several authors [2±4]. Other authors discuss proposals for upgrading existing steam cycle plants by topping with a gas turbine or by using a partial oxidation reactor [5]. Here, some examples of developments/modi®cations to the gas turbine cycle are presented. 2.1. Increased turbine inlet temperature Advancement in gas turbine eciency is attributable to increase turbine inlet temperature (TIT) up to the metallurgical limit set by the material of the turbine blades and last stage turbine stress level. This TIT increase has been achieved by the development of better materials including ceramics or thermal barrier coatings (TBC), and by blade cooling techniques frequently based on bleed air or steam ¯owing through complex internal passages (for small turbines of say less than 100 kW, the turbine blade geometry makes cooling very dicult, and for these units it will be necessary to use ceramic components). A large combined cycle gas turbine with metallic turbine blades (steam cooled) will enter service soon with an eciency of 60%. For large utility-size machines this trend will increase with the next generation of engines having turbine inlet temperatures above 15008C. 2.2. Waste heat recuperation A straightforward modi®cation of the simple gas turbine cycle is to recover partially the exhaust energy in a heat exchanger of a recuperative cycle. A recuperator is a heat exchanger located in a gas turbine exhaust. It enables waste heat to be transferred from the exhaust to the combustor inlet air, hence partially replacing fuel. It will reduce speci®c fuel consumption compared to a conventional simple cycle gas turbine, while ensuring exhaust temperature is still suitable for CHP. The biggest challenge to the designer of heat exchangers is in the small engine class (micro-turbines); eciencies over 30% can then be achieved. Gas turbine cycles with heat recovery are examples of what are generally called advanced cycles. Heat recovery schemes (recuperators or regenerators) are one of the most important ways of increasing the eciency of the power generation process by more than 40%; they also result in lower levels of pollution for a given output of electricity. There are technical problems in the use of recuperators such as unit cost, reliability or life, and fouling from the carbon generated by the combustion. 2.3. Steam or water injection The injection of steam in gas turbine combustors is a normal practice to boost power in many applications and is variously known also as the STIG or the Cheng cycle. This is a natural development of steam±water injection in regenerative cycles, which consists of a gas turbine combined with a heat recovery steam generator [2]. The Cheng cycle has been patented with plants already world wide. The power increase is up to about 20% due to greater mass ¯ow across the turbine stages; steam injection also leads to an improvement in the thermal eciency and also reduces NOx emissions. For both steam and water (see below) injection,
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there is also the cost of treated water. Another problem is from corrosion in the regenerative heat exchanger. Water injection has also been used into the combustion chamber, primarily to reduce emissions and also to boost power but, in contrast to steam injection, water injection leads to a decrease in thermal eciency. Water injection also tends to have the undesirable consequence of an increase in CO and unburned hydrocarbon emissions. However, the practice of water injection is being replaced due to the ability to have increased temperatures for increasing power and the so-called `dry low emissions' technologies for reducing emissions. Currently, one advanced method for achieving low NOx emissions includes selective catalytic reduction (<10 ppm); one author at least [6] claims that catalytic combustion can now deliver low cost NOx emissions. Another method is `dry low NOx`, which involves burning the fuel in rich or lean overall stoichiometry or a combination of the two. 2.4. Reduction of inlet air temperature The performance of turbines is adversely aected by high ambient temperatures. Several means of reducing the turbine inlet temperature (o-peak water chiller and ice storage and absorption refrigeration systems) have been proposed as a means of increasing turbine output. The feasibility of increasing turbine output power by using its exhaust gases to power an ejector refrigeration system was demonstrated [7]. It was shown that the system can decrease the compressor inlet temperature from 296.2 to 277.6 K which increases the turbine output by 12% (i.e. from 21.6 to 24.2 MW) during the periods of high ambient temperature and improves yearly averaged power output by 5.5% in a temperate climate. The energy in the turbine exhaust has the potential of producing additional cooling beyond that required to reduce the inlet temperature. The excess cooling available from the system could be used to provide chilled water for air-conditioning adjacent buildings or for industrial processes. 2.5. The humid air turbine cycle The main innovation of the humid air turbine (HAT) cycle is that steam is produced along the air¯ow, thus eliminating the heat recovery boiler. It consists of an inter-cooled gas turbine cycle having an air±water mixing evaporator before the combustion chamber and an exhaust gas recovery system. The eciency and power output are increased while the NOx is reduced. The system has two cooling stages after the compression stages, the mixing evaporator, the surface recuperator between the mixture and the exhaust gases, and the economiser before the gas discharge [2]. 2.6. The partial oxidation gas turbine cycle A gas turbine is used to generate high temperature pressurised stream containing oxygen. This oxidising gas is mixed with fuel and some steam; it then reacts in the partial oxidation reactor in the presence of a solid catalyst. The reactor exhaust is a high temperature gas, rich in CO and H2. The stoichiometry of the reaction is such that molar ¯ow rate increases during reaction. The air/fuel ratio is adjusted to limit the temperature of the reaction product, which
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is expanded in a free turbine and produces useful power. Finally, the turbine exhaust gas is used as fuel in a conventional steam generator. The partial oxidation gas turbine cycle can give eciencies above 60% [5]. 2.7. The combined cycle The combined cycle is the well known arrangement of a gas turbine with a steam turbine bottoming cycle. A waste boiler is designed to recover the energy in the exhaust of the gas turbine. There are several reasons for giving preference to the combined cycle rather than to the conventional coal ®red plant [3]. Thermal eciencies in excess of 60% are possible with current designs; capital cost is relatively low; construction times are short; plants are available in a wide range of con®gurations and capacities; the scheme is compatible with a wide range of fuels; emissions are relatively low; and the total power output of large units is convenient for the current form of centralised power generation. The combined cycle is therefore likely to remain attractive as a new plant for utility scale applications. 2.8. The Kalina cycle In the Kalina cycle, heat recovery is enhanced. The Kalina cycle is a bottoming cycle whose working ¯uid is a mixture of water and ammonia with a dierent composition in the boiler and the condenser. The use of a non-azeotropic mixture of ¯uids with dierent boiling points decreases the exergy losses in the heat recovery boiler when the hot ¯ow is a source of sensible heat, i.e. a ¯uid whose temperature changes during heat transfer. One of the features of the Kalina cycle is the use of distillation±condensation mixing processes to create ¯uids of dierent thermodynamic properties in dierent parts of the cycle. The main dierence between this cycle and the conventional Rankine cycle, as a bottomer, is related to dierent composition in the water±ammonia mixture in the various plant components which optimise heat transfer and reduce exergy losses. It is claimed that the Kalina cycle results in a net plant eciency of 58.8%, which is more that 2% better than the best Rankine cycle system. The Kalina cycle must be regarded as a possible competitor to the combined cycle. However, it is largely untried and judgement must be reserved until there is some commercial operating experience.
3. Combined heat and power (``cogeneration'') In all the cycles discussed so far, the sole purpose was to convert a portion of the heat transferred to the working ¯uid to work and then to electricity, which is the most valuable form of energy. The remaining portion of the heat is rejected to the environment. However, many industries require energy input in the form of process heat. This process heat could be used in heating, absorption refrigeration, air conditioning or desalination. Some process industries also heavily rely on process heat. It is important to match the temperature of
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the exhaust heat with the application because of the irreversibilities and associated losses in exergy. The utilisation factor of the ideal CHP plant will strive after 100%. Actual CHP plants have utilisation factors as high as 85±90% (large systems provide 40% electrical and 50% thermal energy while small systems give 30% electrical and 60% thermal). Although a small part only of the heat will disappear as losses before the heat reaches the consumers, the total eciency is still about 80% because of the heat losses in the stack gases. Since this technology makes it possible to produce electricity and heat using less fuel than a conventional specialised plant in combination with a separate heat generation, the level of emissions is correspondingly very low. The economics are viable because electricity which is not used locally can be sold to the grid, although see below as well. The CHP technology is characterised by the prime mover; engines (operated with gas, bio, diesel or bio-diesel) for output powers less than about 10 MW, turbines (operated with gas, fuel, steam, combined gas and steam system) or fuel cells (operated with fuels obtained from natural gas) are used. Either a steam turbine (Rankine) cycle or a gas turbine (Brayton) cycle or even a combined cycle can be used as the power cycle in a CHP plant. The main markets are industry (chemical, petrochemical, food, paper, etc.), buildings (residential, commercial, hospitals, schools, etc.), swimming pools and others. The criteria for selection are the eciency, the heat-to-power ratio (a low ratio being desirable) and the grade of heat. The size of CHP plants vary from 15 kWe (micro-cogeneration) for the requirements of an individual house to 400 MW for the chemical industry or district heating systems. The investment for a CHP system is between 4000 FF/kWe (for a 40 MW gas turbine system) to 6500 FF/kWe (for a 1 MW gas engine system). For comparison, a gas combined cycle costs 3500 FF/kWe, a coal power station costs 8000 FF/kWe while a nuclear power station costs 12000 FF/kWe. The cost of electricity produced by a CHP system is 22 cts/kWh (for a 40 MW gas turbine system) to 36 cts/kWh (for a 1 MW gas engine system). For comparison, the cost of electricity produced from a nuclear power station is 21 cts, from a coal thermal power station 22 cts and from a gas combined cycle 23 cts. A CHP plant of 3±10 MW has a payback period of four to ®ve years in the industrial sector and six to eight years in the tertiary sector [8]. Studies evaluate the maximum technical possible production of electricity from CHP in the European Union to 900±1000 TWh/year. This represents four times more electricity production by CHP of 1994 and 40% of the total annual electricity production in the EU in 1994. The present situation is that CHP production varies from one Member State to another from 1% to 40% of total power production [1]. The potential for micro-cogeneration is vast [9]. In the UK alone, the domestic gas boiler market is 12 million units. If 25% of this were suitable for micro-cogeneration, then this could result in 10,000 MWe of new cogeneration being installed (25% of the UK's electricity demand). From a purely technical standpoint the total demand of low temperature heat could theoretically be covered by CHP, but only at very high costs and low utilisation rates making it uninteresting from the energy and economic stand points. The opportunities for providing public heating are nearly exhausted in some Member States. The required increase in CHP in these Member States would therefore to a large extent have to come from the industrial sector. Climate and population density, fuel prices and availability, competitiveness of the electricity
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structure and environmental considerations are quite dierent in each of the EU countries. Therefore, same cogeneration policy cannot be applied in all Member States. The main non-technical barriers to the implementation of CHP systems are that: . the investment payback period could be high (up to 6 years); this is primarily due to the high investment cost and, sometimes, due to high fuel price. Also the price of excess electricity sold to the grid is often low; . the cost of grid connection (imposed by the buying company) might be high; . access to a gas network is not always possible; . there are still administrative and institutional barriers to CHP in several countries; . CHP technology and its bene®ts are not widely known; . there is possibility of increasing local pollution; . there is the requirement of a close matching of electric and heat load.
4. The European strategy The European Union strategy for gas turbine systems consists in developing: . combined cycles, above 50 MWe, having an energy conversion eciency above 60% LHV (in the medium term) and above 65% (in the long term); . advanced gas turbine systems (e.g. with regenerative heating, steam injection, ceramic gas turbines, etc.), between 10 to 50 MWe, having an energy conversion eciency above 50% LHV; . small scale gas turbines, up to 10 MWe, having energy conversion targets above 35%; gas turbines of 200±300 kW should also be developed. In all the three cases above, low maintenance should be aimed for, with availability above 90% and 95% reliability (on an annual basis) for the short to medium term, and 97% reliability for the longer term. . gas turbines having the capacity to use fuels with a heating value below 25% of that of natural gas, and also having emission levels below 20 ppm (NOx) (in the medium to long term). The European strategy for CHP systems consists in: . reducing the speci®c investment and operating costs of CHP systems by more than 20%, in the short term; in the longer term, to achieve a mean CO2reduction of 0.6 kg/kWh produced by CHP as opposed to separate power and heat generation. This will include activities related to gas turbines, reciprocating engines, fuel cells, etc. . developing micro CHP units of up to 20 kWe and mini units of 20±500 kWe with at least 90% overall eciencies and emission levels at least as good as those of conventional systems. This will be based on prime mover development: very small gas engines, Stirling engines and fuel cells. The objective would be to reduce the investment and operating costs by more than 20%, in the short term;
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. developing integrated tri-generation systems in order to compete further with existing techniques to supply heating, cooling and electricity, thus increasing the environmental bene®ts and competitiveness of the European economy. The technologies that can be used include absorption cycle chilling (using heat to create cooling) or compression cycle chilling (cogeneration to drive refrigeration compressors); the chilling equipment can either be based centrally, with chilled water piped to users, or can be located on the premises of the user; . developing systems above 40 kW, in the short to medium term, based on conversion of biomass, with high reliability and availability, high electricity to heat ratio and high electric eciency (more than 20% for small to medium scale; more than 40% beyond 35 MWe); . integrating CHP in processes, plants or sites in the process industry to considerably reduce overall costs. In addition to electricity and heat, generation of chemicals should also be considered. By employing the energy cascading concept, the use of energy can be maximised. For example, rejected energy from the process industry can be recuperated to preheat combustion air for the combined gas and steam cycle; rejected energy can drive an organic Rankine cycle. The EC is supporting projects in these ®elds and further information can be obtained from the Work Programme [10]. Substantial and appropriate industrial ®nancial involvement would normally be required to ensure relevance to industrial needs, to con®rm their commitment to the project and to facilitate the widest exploitation. The involvement of an equipment manufacturer is desirable in the case of the development of units.
5. Conclusions The power generation industry uses a large amount of the primary energy demand in the European Union. Therefore, there is a continuing need for improved energy eciency coupled with a pressing need to reduce emissions. The use of gas turbines for power generation has increased in recent years and is likely to continue to increase particularly for distributed power systems. The proportion of power generation using CHP is also growing mainly due to eciency improvements and environmental bene®ts.
References [1] Une strateÂgie communautaire pour promouvoir la production combineÂe de chaleur et d'eÂlectricite (PCCE) et supprimer les obstacles aÁ son deÂveloppement. COM 514 ®nal, Commission des CommunauteÂs europeÂennes, 1997. [2] S.S. Stecco, Non-conventional thermodynamic cycles for the nineties: comparisons and trends, in: P.A. Pilavachi (Ed.), Energy Eciency in Process Technology, Elsevier Applied Science, New York, 1993, pp. 123± 139. [3] T. Heppenstall, Advanced gas turbine cycles for power generation: a critical review, Applied Thermal Engineering 18 (1998) 837±846. [4] C.F. McDonald, D.G. Wilson, The utilisation of recuperated and regenerated engine cycles for high-eciency gas turbines in the 21st century, Applied Thermal Engineering 16 (8/9) (1996) 635±653.
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[5] G. Heyen, B. Kalitventze, A comparison of advanced thermal cycles suitable for upgrading existing power plant, Applied Thermal Engineering 19 (1999) 227±237. [6] J.Ch. Solt, The ultimate NOx solution for gas turbines, in: International Gas Turbine & Aeroengine Congress & Exhibition, The American Society of Mechanical Engineers, New York, 1998, pp. 1±4. [7] G.J. Kowalski, Improving turbine performance by cooling inlet air using a waste heat powered ejector refrigerator, in: Proceedings of the ASME Advanced Energy Systems Division AES-36, 1996, pp. 501±508. [8] J.P. Tabet, La cogeÂneÂration: aspects techniques et eÂconomiques, Bulletin du droit de l'environnement industriel 5 (1997) 10±15. [9] COGEN Europe, An introduction to micro-cogeneration. COGEN Europe Brie®ng 8, 1999. [10] Work programme: energy, environment and sustainable development. Programme for Research, Technological Development and Demonstration under the Fifth Framework Programme, European Commission, 1999.
www.elsevier.com/locate/apthermeng
Power generation with gas turbine systems and combined heat and power P.A. Pilavachi European Commission, Research Directorate-General, 200, Rue de la Loi, 1049 Brussels, Belgium
Abstract This article gives an overview of power generation with gas turbine and combined heat and power (CHP) systems. It also presents the European Union strategy for developing gas turbines and CHP systems. Ways to improve the performance of the several types of gas turbine cycle will be a major objective in the coming years. The targets are combined cycle eciencies above 60% industrial gas turbine system eciencies of at least 50% and small gas turbines eciencies above 35% and designs for the use of fuels with less than 25% heating value of that of natural gas. The main CHP targets are the reduction of the overall costs and the development of above 40 kW biomass-®red systems. 7 2000 The Commission of the European Communities. Published by Elsevier Science Ltd. All rights reserved. Keywords: Power generation; Gas turbine systems; Combined heat and power
1. Introduction The power generation industry uses a large amount of the primary energy demand in the European Union and power generation from gas turbine and combined heat and power (CHP) systems is an important and growing part of it. Therefore, there is a continuing need for improved energy eciency, coupled with a pressing need to reduce toxic and noxious emissions. In view of the uncertainties related to global warming, there is also a de®nite requirement to monitor and reduce, where possible, CO2 emissions. In long term, the potential exists for a signi®cant impact on both energy eciency and environmental protection. Therefore, the aim here is to develop technologies to help minimise the environmental impact of the production and use of energy in Europe. A driving force will be the Kyoto objectives for the European Member States to decrease by 8% the greenhouse gas emissions in 1359-4311/00/$ - see front matter 7 2000 The Commission of the European Communities. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 1 6 - 8
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2010 compared to the 1990 level. The promotion of the use of CHP is also expressed in the European Union CHP strategy [1]; the aim is to increase the participation of CHP from 9% of 1994 to 18% in year 2010. The use of gas turbines for power generation has increased in recent years and is likely to continue to increase particularly for distributed power systems. Applications fall into three categories, i.e. peak lopping (<10% utilisation), base load (about 100% utilisation) and in between. Gas turbines enjoy certain merits relative to steam turbines and diesel engines. They have high grade waste heat, lower weight per unit power, dual fuel capability, low maintenance cost, low vibration levels, low capital cost, compact size, short delivery time, high ¯exibility and reliability, fast starting time, lower manpower, and have better environmental performance. A further advantage is that gas ®ring produces less CO2 per unit power than does a liquid or solid fuel. Ways to improve the performance of several types of gas turbine cycle will be a major objective in the coming years. The targets are combined cycle eciencies above 60%, industrial gas turbine system eciencies of at least 50% and small gas turbine eciencies above 35% and designs for the use of fuels with less than 25% heating value of that of natural gas. The proportion of power generation using combined heat and power (CHP) is also growing mainly due to eciency improvements and environmental bene®ts. Ways to further improve the share of CHP in power generation is also of interest. Gas turbine cycles and CHP systems for power generation are discussed below.
2. Gas turbine systems Gas turbine technology is a critical one for the future, providing a clean, ecient and cost eective way of generating power from distributed generation and co-production schemes for large utility-size combined cycle plant. There are dierent gas turbine systems such as stand alone, combined with a steam turbine (``combined cycle'') or, less commonly at present, with a fuel cell. The main markets are in the power generation industry, the process industry, mobile applications (land and military marine) and gas and oil transmission pipeline pumping stations. The main technical barriers to the implementation of gas turbine technology are that the stand alone gas turbine has a lower eciency in its basic con®guration than an equal power output reciprocating engine. Furthermore the eciency decreases at partial load and burning of low heating value fuels may not be feasible, depending on the type of the turbine. The main non-technical barriers to the implementation are that maintenance requires more skilled personnel than does the reciprocating engine and that small gas turbines are too expensive compared to engines. Natural gas use is expected to grow. However, this will not be accessible in all geographical areas. Therefore, in the industrial market, the fuels mix is expected to become more diverse including naphtha, kerosine, gas condensates, natural gas liquids, alcohols, re®nery residues, biomass, etc. This fuel ¯exibility will of course need to be matched to a low emissions capability. Dierent technologies have been used to improve the gas turbine cycle eciency and an
P.A. Pilavachi / Applied Thermal Engineering 20 (2000) 1421±1429
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overview was given by several authors [2±4]. Other authors discuss proposals for upgrading existing steam cycle plants by topping with a gas turbine or by using a partial oxidation reactor [5]. Here, some examples of developments/modi®cations to the gas turbine cycle are presented. 2.1. Increased turbine inlet temperature Advancement in gas turbine eciency is attributable to increase turbine inlet temperature (TIT) up to the metallurgical limit set by the material of the turbine blades and last stage turbine stress level. This TIT increase has been achieved by the development of better materials including ceramics or thermal barrier coatings (TBC), and by blade cooling techniques frequently based on bleed air or steam ¯owing through complex internal passages (for small turbines of say less than 100 kW, the turbine blade geometry makes cooling very dicult, and for these units it will be necessary to use ceramic components). A large combined cycle gas turbine with metallic turbine blades (steam cooled) will enter service soon with an eciency of 60%. For large utility-size machines this trend will increase with the next generation of engines having turbine inlet temperatures above 15008C. 2.2. Waste heat recuperation A straightforward modi®cation of the simple gas turbine cycle is to recover partially the exhaust energy in a heat exchanger of a recuperative cycle. A recuperator is a heat exchanger located in a gas turbine exhaust. It enables waste heat to be transferred from the exhaust to the combustor inlet air, hence partially replacing fuel. It will reduce speci®c fuel consumption compared to a conventional simple cycle gas turbine, while ensuring exhaust temperature is still suitable for CHP. The biggest challenge to the designer of heat exchangers is in the small engine class (micro-turbines); eciencies over 30% can then be achieved. Gas turbine cycles with heat recovery are examples of what are generally called advanced cycles. Heat recovery schemes (recuperators or regenerators) are one of the most important ways of increasing the eciency of the power generation process by more than 40%; they also result in lower levels of pollution for a given output of electricity. There are technical problems in the use of recuperators such as unit cost, reliability or life, and fouling from the carbon generated by the combustion. 2.3. Steam or water injection The injection of steam in gas turbine combustors is a normal practice to boost power in many applications and is variously known also as the STIG or the Cheng cycle. This is a natural development of steam±water injection in regenerative cycles, which consists of a gas turbine combined with a heat recovery steam generator [2]. The Cheng cycle has been patented with plants already world wide. The power increase is up to about 20% due to greater mass ¯ow across the turbine stages; steam injection also leads to an improvement in the thermal eciency and also reduces NOx emissions. For both steam and water (see below) injection,
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there is also the cost of treated water. Another problem is from corrosion in the regenerative heat exchanger. Water injection has also been used into the combustion chamber, primarily to reduce emissions and also to boost power but, in contrast to steam injection, water injection leads to a decrease in thermal eciency. Water injection also tends to have the undesirable consequence of an increase in CO and unburned hydrocarbon emissions. However, the practice of water injection is being replaced due to the ability to have increased temperatures for increasing power and the so-called `dry low emissions' technologies for reducing emissions. Currently, one advanced method for achieving low NOx emissions includes selective catalytic reduction (<10 ppm); one author at least [6] claims that catalytic combustion can now deliver low cost NOx emissions. Another method is `dry low NOx`, which involves burning the fuel in rich or lean overall stoichiometry or a combination of the two. 2.4. Reduction of inlet air temperature The performance of turbines is adversely aected by high ambient temperatures. Several means of reducing the turbine inlet temperature (o-peak water chiller and ice storage and absorption refrigeration systems) have been proposed as a means of increasing turbine output. The feasibility of increasing turbine output power by using its exhaust gases to power an ejector refrigeration system was demonstrated [7]. It was shown that the system can decrease the compressor inlet temperature from 296.2 to 277.6 K which increases the turbine output by 12% (i.e. from 21.6 to 24.2 MW) during the periods of high ambient temperature and improves yearly averaged power output by 5.5% in a temperate climate. The energy in the turbine exhaust has the potential of producing additional cooling beyond that required to reduce the inlet temperature. The excess cooling available from the system could be used to provide chilled water for air-conditioning adjacent buildings or for industrial processes. 2.5. The humid air turbine cycle The main innovation of the humid air turbine (HAT) cycle is that steam is produced along the air¯ow, thus eliminating the heat recovery boiler. It consists of an inter-cooled gas turbine cycle having an air±water mixing evaporator before the combustion chamber and an exhaust gas recovery system. The eciency and power output are increased while the NOx is reduced. The system has two cooling stages after the compression stages, the mixing evaporator, the surface recuperator between the mixture and the exhaust gases, and the economiser before the gas discharge [2]. 2.6. The partial oxidation gas turbine cycle A gas turbine is used to generate high temperature pressurised stream containing oxygen. This oxidising gas is mixed with fuel and some steam; it then reacts in the partial oxidation reactor in the presence of a solid catalyst. The reactor exhaust is a high temperature gas, rich in CO and H2. The stoichiometry of the reaction is such that molar ¯ow rate increases during reaction. The air/fuel ratio is adjusted to limit the temperature of the reaction product, which
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is expanded in a free turbine and produces useful power. Finally, the turbine exhaust gas is used as fuel in a conventional steam generator. The partial oxidation gas turbine cycle can give eciencies above 60% [5]. 2.7. The combined cycle The combined cycle is the well known arrangement of a gas turbine with a steam turbine bottoming cycle. A waste boiler is designed to recover the energy in the exhaust of the gas turbine. There are several reasons for giving preference to the combined cycle rather than to the conventional coal ®red plant [3]. Thermal eciencies in excess of 60% are possible with current designs; capital cost is relatively low; construction times are short; plants are available in a wide range of con®gurations and capacities; the scheme is compatible with a wide range of fuels; emissions are relatively low; and the total power output of large units is convenient for the current form of centralised power generation. The combined cycle is therefore likely to remain attractive as a new plant for utility scale applications. 2.8. The Kalina cycle In the Kalina cycle, heat recovery is enhanced. The Kalina cycle is a bottoming cycle whose working ¯uid is a mixture of water and ammonia with a dierent composition in the boiler and the condenser. The use of a non-azeotropic mixture of ¯uids with dierent boiling points decreases the exergy losses in the heat recovery boiler when the hot ¯ow is a source of sensible heat, i.e. a ¯uid whose temperature changes during heat transfer. One of the features of the Kalina cycle is the use of distillation±condensation mixing processes to create ¯uids of dierent thermodynamic properties in dierent parts of the cycle. The main dierence between this cycle and the conventional Rankine cycle, as a bottomer, is related to dierent composition in the water±ammonia mixture in the various plant components which optimise heat transfer and reduce exergy losses. It is claimed that the Kalina cycle results in a net plant eciency of 58.8%, which is more that 2% better than the best Rankine cycle system. The Kalina cycle must be regarded as a possible competitor to the combined cycle. However, it is largely untried and judgement must be reserved until there is some commercial operating experience.
3. Combined heat and power (``cogeneration'') In all the cycles discussed so far, the sole purpose was to convert a portion of the heat transferred to the working ¯uid to work and then to electricity, which is the most valuable form of energy. The remaining portion of the heat is rejected to the environment. However, many industries require energy input in the form of process heat. This process heat could be used in heating, absorption refrigeration, air conditioning or desalination. Some process industries also heavily rely on process heat. It is important to match the temperature of
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the exhaust heat with the application because of the irreversibilities and associated losses in exergy. The utilisation factor of the ideal CHP plant will strive after 100%. Actual CHP plants have utilisation factors as high as 85±90% (large systems provide 40% electrical and 50% thermal energy while small systems give 30% electrical and 60% thermal). Although a small part only of the heat will disappear as losses before the heat reaches the consumers, the total eciency is still about 80% because of the heat losses in the stack gases. Since this technology makes it possible to produce electricity and heat using less fuel than a conventional specialised plant in combination with a separate heat generation, the level of emissions is correspondingly very low. The economics are viable because electricity which is not used locally can be sold to the grid, although see below as well. The CHP technology is characterised by the prime mover; engines (operated with gas, bio, diesel or bio-diesel) for output powers less than about 10 MW, turbines (operated with gas, fuel, steam, combined gas and steam system) or fuel cells (operated with fuels obtained from natural gas) are used. Either a steam turbine (Rankine) cycle or a gas turbine (Brayton) cycle or even a combined cycle can be used as the power cycle in a CHP plant. The main markets are industry (chemical, petrochemical, food, paper, etc.), buildings (residential, commercial, hospitals, schools, etc.), swimming pools and others. The criteria for selection are the eciency, the heat-to-power ratio (a low ratio being desirable) and the grade of heat. The size of CHP plants vary from 15 kWe (micro-cogeneration) for the requirements of an individual house to 400 MW for the chemical industry or district heating systems. The investment for a CHP system is between 4000 FF/kWe (for a 40 MW gas turbine system) to 6500 FF/kWe (for a 1 MW gas engine system). For comparison, a gas combined cycle costs 3500 FF/kWe, a coal power station costs 8000 FF/kWe while a nuclear power station costs 12000 FF/kWe. The cost of electricity produced by a CHP system is 22 cts/kWh (for a 40 MW gas turbine system) to 36 cts/kWh (for a 1 MW gas engine system). For comparison, the cost of electricity produced from a nuclear power station is 21 cts, from a coal thermal power station 22 cts and from a gas combined cycle 23 cts. A CHP plant of 3±10 MW has a payback period of four to ®ve years in the industrial sector and six to eight years in the tertiary sector [8]. Studies evaluate the maximum technical possible production of electricity from CHP in the European Union to 900±1000 TWh/year. This represents four times more electricity production by CHP of 1994 and 40% of the total annual electricity production in the EU in 1994. The present situation is that CHP production varies from one Member State to another from 1% to 40% of total power production [1]. The potential for micro-cogeneration is vast [9]. In the UK alone, the domestic gas boiler market is 12 million units. If 25% of this were suitable for micro-cogeneration, then this could result in 10,000 MWe of new cogeneration being installed (25% of the UK's electricity demand). From a purely technical standpoint the total demand of low temperature heat could theoretically be covered by CHP, but only at very high costs and low utilisation rates making it uninteresting from the energy and economic stand points. The opportunities for providing public heating are nearly exhausted in some Member States. The required increase in CHP in these Member States would therefore to a large extent have to come from the industrial sector. Climate and population density, fuel prices and availability, competitiveness of the electricity
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structure and environmental considerations are quite dierent in each of the EU countries. Therefore, same cogeneration policy cannot be applied in all Member States. The main non-technical barriers to the implementation of CHP systems are that: . the investment payback period could be high (up to 6 years); this is primarily due to the high investment cost and, sometimes, due to high fuel price. Also the price of excess electricity sold to the grid is often low; . the cost of grid connection (imposed by the buying company) might be high; . access to a gas network is not always possible; . there are still administrative and institutional barriers to CHP in several countries; . CHP technology and its bene®ts are not widely known; . there is possibility of increasing local pollution; . there is the requirement of a close matching of electric and heat load.
4. The European strategy The European Union strategy for gas turbine systems consists in developing: . combined cycles, above 50 MWe, having an energy conversion eciency above 60% LHV (in the medium term) and above 65% (in the long term); . advanced gas turbine systems (e.g. with regenerative heating, steam injection, ceramic gas turbines, etc.), between 10 to 50 MWe, having an energy conversion eciency above 50% LHV; . small scale gas turbines, up to 10 MWe, having energy conversion targets above 35%; gas turbines of 200±300 kW should also be developed. In all the three cases above, low maintenance should be aimed for, with availability above 90% and 95% reliability (on an annual basis) for the short to medium term, and 97% reliability for the longer term. . gas turbines having the capacity to use fuels with a heating value below 25% of that of natural gas, and also having emission levels below 20 ppm (NOx) (in the medium to long term). The European strategy for CHP systems consists in: . reducing the speci®c investment and operating costs of CHP systems by more than 20%, in the short term; in the longer term, to achieve a mean CO2reduction of 0.6 kg/kWh produced by CHP as opposed to separate power and heat generation. This will include activities related to gas turbines, reciprocating engines, fuel cells, etc. . developing micro CHP units of up to 20 kWe and mini units of 20±500 kWe with at least 90% overall eciencies and emission levels at least as good as those of conventional systems. This will be based on prime mover development: very small gas engines, Stirling engines and fuel cells. The objective would be to reduce the investment and operating costs by more than 20%, in the short term;
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. developing integrated tri-generation systems in order to compete further with existing techniques to supply heating, cooling and electricity, thus increasing the environmental bene®ts and competitiveness of the European economy. The technologies that can be used include absorption cycle chilling (using heat to create cooling) or compression cycle chilling (cogeneration to drive refrigeration compressors); the chilling equipment can either be based centrally, with chilled water piped to users, or can be located on the premises of the user; . developing systems above 40 kW, in the short to medium term, based on conversion of biomass, with high reliability and availability, high electricity to heat ratio and high electric eciency (more than 20% for small to medium scale; more than 40% beyond 35 MWe); . integrating CHP in processes, plants or sites in the process industry to considerably reduce overall costs. In addition to electricity and heat, generation of chemicals should also be considered. By employing the energy cascading concept, the use of energy can be maximised. For example, rejected energy from the process industry can be recuperated to preheat combustion air for the combined gas and steam cycle; rejected energy can drive an organic Rankine cycle. The EC is supporting projects in these ®elds and further information can be obtained from the Work Programme [10]. Substantial and appropriate industrial ®nancial involvement would normally be required to ensure relevance to industrial needs, to con®rm their commitment to the project and to facilitate the widest exploitation. The involvement of an equipment manufacturer is desirable in the case of the development of units.
5. Conclusions The power generation industry uses a large amount of the primary energy demand in the European Union. Therefore, there is a continuing need for improved energy eciency coupled with a pressing need to reduce emissions. The use of gas turbines for power generation has increased in recent years and is likely to continue to increase particularly for distributed power systems. The proportion of power generation using CHP is also growing mainly due to eciency improvements and environmental bene®ts.
References [1] Une strateÂgie communautaire pour promouvoir la production combineÂe de chaleur et d'eÂlectricite (PCCE) et supprimer les obstacles aÁ son deÂveloppement. COM 514 ®nal, Commission des CommunauteÂs europeÂennes, 1997. [2] S.S. Stecco, Non-conventional thermodynamic cycles for the nineties: comparisons and trends, in: P.A. Pilavachi (Ed.), Energy Eciency in Process Technology, Elsevier Applied Science, New York, 1993, pp. 123± 139. [3] T. Heppenstall, Advanced gas turbine cycles for power generation: a critical review, Applied Thermal Engineering 18 (1998) 837±846. [4] C.F. McDonald, D.G. Wilson, The utilisation of recuperated and regenerated engine cycles for high-eciency gas turbines in the 21st century, Applied Thermal Engineering 16 (8/9) (1996) 635±653.
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[5] G. Heyen, B. Kalitventze, A comparison of advanced thermal cycles suitable for upgrading existing power plant, Applied Thermal Engineering 19 (1999) 227±237. [6] J.Ch. Solt, The ultimate NOx solution for gas turbines, in: International Gas Turbine & Aeroengine Congress & Exhibition, The American Society of Mechanical Engineers, New York, 1998, pp. 1±4. [7] G.J. Kowalski, Improving turbine performance by cooling inlet air using a waste heat powered ejector refrigerator, in: Proceedings of the ASME Advanced Energy Systems Division AES-36, 1996, pp. 501±508. [8] J.P. Tabet, La cogeÂneÂration: aspects techniques et eÂconomiques, Bulletin du droit de l'environnement industriel 5 (1997) 10±15. [9] COGEN Europe, An introduction to micro-cogeneration. COGEN Europe Brie®ng 8, 1999. [10] Work programme: energy, environment and sustainable development. Programme for Research, Technological Development and Demonstration under the Fifth Framework Programme, European Commission, 1999.