Advances in Gas Turbine Technology Part 9 doc

Adiabatic gas turbine: Data: see page 6 Power of the gas turbine: PGT Thermal power supplied by combustion chamber: Qcc The fuel flow is neglected... Advances in Gas Turbine Technolog

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Figures 13 and 14 give the modeling results Figure 13 shows the evolution of the heat exchange surface versus the inlet radius The greater the volute, the smaller the surface to volume ratio Small turbomachines therefore have a higher surface to volume ratio

The necessity of taking into account heat transfer in small turbomachines is largely confirmed by Figure 14: the heat losses in the volute are relatively greater

In this study, when the inlet radius is halved, the surface to volume ratio doubles and the heat losses are multiplied by about 2.5

Fig 13 Ratio of heat exchange surface (S) and the volume (V) of the volute versus the inlet radius

Influence of Heat Transfer on Gas Turbine Performance 231

Fig 14 Heat transfer in the volute versus the size of the machine

4 Conclusion

Internal and external heat transfer induces a drop in the performance of gas turbines This study shows that the performance of small turbomachines evaluated with the assumption of adiabaticity is not accurate

For a given operating point, the mass flow and the compression ratio recorded on the maps and the calculated performance do not correspond to the actual characteristics when the machine operates with heat transfer

The assumption that heat losses represent 15% of the work of adiabatic turbines, of which 60% is received by the compressor (non-insulated), leads to overestimating the power by 35% and the energy efficiency by 23%

Insulation of the turbine, although it seems to be a solution to maintain the operating characteristics of adiabatic turbines, leads in fact to increasing the drop in performance For the insulated version, the net power is overestimated by 51% and efficiency by 26.6% In the absence of an adiabatic gas turbine (ideal machine), which provides the best performance, we must avoid insulating the turbine in order not to decrease performance still further

To maintain the level of performance, and in particular the net power produced by the gas turbines, despite heat transfer, adjustments are needed They consist mainly in increasing the fuel flow, resulting in an increase in the turbine inlet temperature In the case of our study, the fuel flow increase is 3.5% in the non-insulated version and 8.5% in the insulated version The turbine inlet temperature increase is 6.4% in the insulated version and 11.8% in the non-insulated version

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Finally, this study confirms that the assumption of adiabaticity is not valid in turbochargers, micro and ultra-micro gas turbines Compared to the available thermal energy at the turbine inlet, heat losses increase with the surface to volume ratio which decreases in small-sized machines The quality of operation of small turbomachinery cannot be characterized with isentropic efficiency which has no physical meaning because of the relative importance of heat transfer

The proposal of a new performance indicator and the development of new maps available for any type of thermal turbomachines will therefore be the subject of our forthcoming investigations

5 Acknowledgment

The authors would like to acknowledge the French Cooperation EGIDE for funding this study

6 Appendix: Energy balance calculations

1 Adiabatic gas turbine:

Data: (see page 6)

Power of the gas turbine: PGT

Thermal power supplied by combustion chamber: Qcc

The fuel flow is neglected

Influence of Heat Transfer on Gas Turbine Performance 233 Thermal power lost in the exhaust gas: Qexh

=Q -P -P =9430.7-66-1526.2=7838.5kW

Q

2 Non insulated gas turbine:

Data: c = 7.17 (Figure 6); qm = 19.8 kg.s-1 (From the adiabatic compressor map) Ti2 = 604.20 K; Q12 = 625.2 kW (thermal power received by the compressor)

Power of the gas turbine: PGT

Thermal power received by the compressor: Q12

GT

P =1526.2 kW

P =P +P +P =1526.4+5663.7+66=7256.1kW

Search for new turbine inlet temperature

The variation in the expansion ratio of the turbine versus the reduced mass flow (Figure 3) shows that when the expansion ratio is greater than two (2), the reduced mass flow remains constant (Pluviose M., 2005) This reduced flow constant calculated in adiabatic conditions enables the new turbine inlet temperature (Ti3) corresponding to the new pressure (pi3) to be determined by the following equations

Advances in Gas Turbine Technology

The fuel flow is neglected

Pthl : power of thermal losses

3 Insulated gas turbine:

Data: c = 7.22 (Figure 6); qm = 19.5 kg.s-1 (from the adiabatic compressor map) Ti2 = 622.68

K, Q12 = 1042 kW (thermal power received by the compressor)

Power of the gas turbine: PGT

Thermal power received by the compressor: Q12

P =P =1526.2 kW

P =P +P +P =1526.4+5513.6+66=7105.8kWSearch for new turbine inlet temperature

Influence of Heat Transfer on Gas Turbine Performance 235

108819.5

The fuel flow is neglected

Berger, M., Gostiaux, B., 1992, Géométrie différentielle: variétés, courbes et surfaces France

Presses universitaires de Paris, ISBN : 2-13-044708-2

Cormerais, M 2007, Caractérisation expérimentale et modélisation des transferts thermiques

au sein d'un turbocompresseur d’automobile, Thèse de doctorat de l’école centrale de NANTES, pp 1-243

Diango, A., 2010, Influence des pertes thermiques sur les performances des turbomachines

Thèse de doctorat du Conservatoire national des arts et métiers, Paris, pp 1-244

Kreith, F., 1967, Principles of heat transfer, Masson, [trad.] Kodja Badr-El-Dine, Université

d'ALEP (Syrie), Colorado, International textbook Company Scranton, Pennsylvania, 1967 pp 1-654

Moreno, N., 2006, Modélisation des échanges thermiques dans une turbine radiale, Thèse de

doctorat de l'École nationale supérieure d'arts et métiers, pp 158

Padet, J., 2005, Convection thermique et massique, Techniques de l'ingénieur, BE 8206

Pluviose, M., 2005 Conversion d'énergie par turbomachines, Ellipses, pp 1.277.ISBN

2-7298-2320-4

Pluviose, M., 2002, Machines à fluides, Ellipses, pp 1-276, ISBN 2-7298-1175-3

Pluviose, M., 2005, Similitude des turbomachines à fluide compressible, Techniques de

Rautenberg & Al., 1981, Influence of heat transfer between turbine and compressor on the

performance of small turbocharger, International Gas Turbine Congress, Tokyo, Asme

paper, 1981

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Ribaud, Y., 2004, Overall Thermodynamics Model of an Ultra Micro turbine, Journal of

Thermal Science 2004, Vol 13, 4, pp 297-301

Sacadura, J F., 1993, Initiation aux transferts thermiques, Lavoisier Tec & Doc, Vol 4ème tirage

1993, pp 1-439, ISBN 2-85206-618-1

Verstraete, T & al., 2007, Numerical Study of the Heat Transfer in Micro Gas Turbines,

Journal of Turbomachinery ASME, Octobre 2007, Vol.129, DOI: 10.1115/1.2720874,

pp 835-841

Part 4 Combustion

In particular, air-blown gasified fuels provide low calorific fuel of 4 MJ/m3 and it is necessary to stabilize combustion In contrast, the flame temperature of oxygen-blown gasified fuel of medium calorie between approximately 9–13 MJ/m3 is much higher, so control of thermal-NOx emissions is necessary Moreover, to improve the thermal efficiency

of IGCC, hot/dry type synthetic gas clean-up is needed However, ammonia in the fuel is not removed and is supplied into the gas turbine where fuel-NOx is formed in the combustor For these reasons, suitable combustion technology for each gasified fuel is important In this paper, I will review our developments of the gas turbine combustors for the three type gasified fuels produced from the following gasification methods through experiments using a small diffusion burner and the designed combustors’ tests of the simulated gasified fuels

 Air-blown gasifier + Hot/Dry type synthetic gas clean-up method

 Oxygen-blown gasifier + Wet type synthetic gas clean-up method

 Oxygen-blown gasifier + Hot/Dry type synthetic gas clean-up method

Figure 1 provides an outline of a typical oxygen-blown IGCC system In this system, raw materials such as coal and crude are fed into the gasifier by slurry feed or dry feed with nitrogen The synthetic gas is cleaned through a dust removing and desulfurizing process The cleaned synthetic gas is then fed into the high-efficiency gas turbine topping cycle, and the steam cycle is equipped to recover heat from the gas turbine exhaust This IGCC system

is similar to LNG fired gas turbine combined cycle generation, except for the gasification and the synthetic gas cleanup process, primarily IGCC requires slightly more station service power than an LNG gas turbine power generation

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Gen-Gas turbine

Heat recovery steam generator

Stack

former

Trans- erator

Gen-GAS TURBINE HOT GAS CLEANUP

COAL GASIFIER

Fig 1 Schematic diagram of typical IGCC system

1.1 Background of IGCC development in the world

The development of the gas turbine combustor for IGCC power generation received considerable attention in the 1970s Brown (1982), summarized the overall progress of IGCC technology worldwide up until 1980 The history and application of gasification was also mentioned by Littlewood (1977) Concerning fixed-bed type gasification processes, Hobbs et

al (1993) extensively reviewed the technical and scientific aspects of the various systems Other developments concerning the IGCC system and gas turbine combustor using oxygen-blown gasified coal fuel include: The Cool Water Coal Gasification Project (Savelli & Touchton, 1985), the flagship demonstration plant of gasification and gasified fueled gas turbine generation; the Shell process (Bush et al., 1991) in Buggenum, the first commercial plant, which started test operation in 1994 and commercial operation in 1998; the Wabash River Coal Gasification Repowering Plant (Roll, 1995) in the United States, in operation since 1995; the Texaco process at the Tampa power station (Jenkins, 1995), in commercial operation since 1996; and an integrated coal gasification fuel cell combined cycle pilot plant, consisting of a gasifier, fuel cell generating unit and gas turbine, in test operation since 2002

by Electric Power Development Co Ltd in Japan Every plant adopted the oxygen-blown gasification method With regard to fossil-based gasification technology as described above, commercially-based power plants have been developed, and new development challenges toward global carbon capture storage (Isles, 2007; Beer, 2007) are being addressed

Meanwhile, from 1986 to 1996, the Japanese government and electric power companies undertook an experimental research project for the air-blown gasification combined cycle system using a 200-ton-daily pilot plant Recently, the government and electric power companies have also been promoting a demonstration IGCC project with a capacity of 1700 tons per day (Nagano, 2009) For the future commercializing stage, the transmission-end thermal efficiency of air-blown IGCC, adopting the 1773 K (1500°C)-class (average combustor exhaust gas temperature at about 1773 K) gas turbine, is expected to exceed 48%(on HHV basis), while the thermal efficiency of the demonstration plant using a 1473 K (1200°C)-class gas turbine is only 40.5% IGCC technologies would improve thermal efficiency by five points or higher compared to the latest pulverized coal-firing, steam power generation The Central Research Institute of Electric Power Industry (CRIEPI), developed an air-blown two-stage entrained-flow coal gasifier (Kurimura et al., 1995), a hot/dry synthetic gas cleanup system (Nakayama et al., 1990), and 150MW, 1773K(1500°C)-class gas turbine combustor technologies for low-Btu fuel (Hasegawa et al., 1998a) In order

Developments of Gas Turbine Combustors for Air-Blown and Oxygen-Blown IGCC 241

to accept the various IGCC systems, 1773K-class gas turbine combustors of medium-Btu fuels by wet-type or hot/dry-type synthetic gas cleanup methods have undergone study (Hasegawa et al., 2003, 2007)

The energy resources and geographical conditions of each country, along with the diversification of fuels used for the electric power industry (such as biomass, poor quality coal and residual oil), are most significant issues for IGCC gas turbine development, as has been previously described: The development of biomass-fueled gasification received considerable attention in the United States and northern Europe in the early 1980s (Kelleher, 1985), and the prospects for commercialization technology (Consonni, 1997) appear considerably improved at present Paisley and Anson (1997) performed a comprehensive economical evaluation of the Battele biomass gasification process, which utilizes a hot-gas conditioning catalyst for dry synthetic gas cleanup In northern Europe, fixed-bed gasification heating plants built in the 1980s had been in commercial operation; the available technical and economical operation data convinced small district heating companies that biomass or peat-fueled gasification heating plants in the size class of 5 MW were the most profitable (Haavisto, 1996) However, during the period of stable global economy and oil prices, non-fossil-fueled gasification received little interest Then, in the early 2000s when the Third Conference of Parties to the United Nations Framework Convention on Climate Change (COP3) invoked mandatory carbon dioxide emissions reductions on countries, biomass-fueled gasification technology began to receive considerable attention as one alternative With the exception of Japanese national research and development project, almost all of the systems using the oxygen-blown gasification are in their final stages for commencing commercial operations overseas

1.2 Progress in gas turbine combustion technologies for IGCCs

The plant thermal efficiency has been improved by enhancing the turbine inlet temperature,

or combustor exhaust temperature The thermal-NOx emissions from the gas turbines increase, however, along with a rise in exhaust temperature In addition, gasified fuel containing NH3 emits fuel-NOx when hot/dry gas cleanup equipment is employed It is therefore viewed as necessary to adopt a suitable combustion technology for each IGCC in the development of a gas turbine for each gasification method

Dixon-Lewis and Williams (1969), expounded on the oxidation characteristics of hydrogen and carbon monoxide in 1969 The body of research into the basic combustion characteristics

of gasified fuel includes studies on the flammability limits of mixed gas, consisting of CH4

or H2 diluted with N2, Ar or He (Ishizuka & Tsuji, 1980); a review of the flammability and explosion limits of H2 and H2/CO fuels (Cohen, 1992); the impact of N2 on burning velocity (Morgan & Kane, 1952); the effect of N2 and CO2 on flammability limits (Coward & Jones, 1952; Ishibasi et al, 1978); and the combustion characteristics of low calorific fuel (Folsom, 1980; Drake, 1984); studies by Merryman et al (1997), on NOx formation in CO flame; studies by Miller et al (1984), on the conversion characteristics of HCN in H2-O2-HCN-Ar flames; studies by Song et al (1980), on the effects of fuel-rich combustion on the conversion

of the fixed nitrogen to N2; studies by White et al (1983), on a rich-lean combustor for Btu and medium-Btu gaseous fuels; and research of the CRIEPI into fuel-NOx emission characteristics of low-calorific fuel, including NH3 through experiments using a small diffusion burner and analyses based on reaction kinetics (Hasegawa et al, 2001) It is widely

low-Advances in Gas Turbine Technology

et al (1994), on low NOx combustion technology (which quickly mixed fuel with air using the double cone burner from Alstom Power, called an EV burner); Cook et al (1994), on effective methods for returning nitrogen to the cycle, where nitrogen is injected from the head end of the combustor for NOx control; and Zanello and Tasselli (1996), on the effects of steam content in medium-Btu gaseous fuel on combustion characteristics In almost all systems, surplus nitrogen was produced from the oxygen production unit and premixed with a gasified medium-Btu fuel (Becker & Shetter, 1992), for recovering power used in oxygen production and suppressing NOx emissions Since the power to premix the surplus nitrogen with the medium-Btu fuel is great, Hasegawa et al studied low-NOx combustion technologies using surplus nitrogen injected from the burner (Hasegawa et al, 1998b) and with the lean combustion of instantaneous mixing (Hasegawa et al, 2003) Furthermore, Hasegawa and Tamaru(2007) developed a low-NOx combustion technology for reducing both fuel-NOx and thermal-NOx emissions, in the case of employing hot/dry synthetic gas cleanup with an oxygen-blown IGCC

1.3 Subjects of gas turbine combustors for IGCCs

The typical compositions of gasified fuels produced in air-blown or oxygen-blown gasifiers, and in blast furnaces, are shown in Tables 1 Each type of gaseous mixture fuel consists of

CO and H2 as the main combustible components, and small percentages of CH4 Fuel calorific values vary widely (2–13 MJ/m3), from about 1/20 to 1/3 those of natural gas, depending upon the raw materials of feedstock, the gasification agent and the gasifier type Figure 2 shows the theoretical adiabatic flame temperature of fuels which were: (1) gasified fuels with fuel calorific values (HHV) of 12.7, 10.5, 8.4, 6.3, 4.2 MJ/m3; and (2) fuels in which methane is the main component of natural gas Flame temperatures were calculated using a

CO and H2 mixture fuel (CO/H2 molar ratio of 2.33:1), which contained no CH4 under any conditions, and the fuel calorific value was adjusted with nitrogen In the case of gasified fuel,

as the fuel calorific value increased, the theoretical adiabatic flame temperature also increased Fuel calorific values of 4.2 MJ/m3 and 12.7 MJ/m3 produced maximum flame temperatures of

2050 K and 2530 K, respectively At fuel calorific values of 8.4 MJ/m3 or higher, the maximum flame temperature of the gasified fuel exceeded that of methane, while the fuel calorific value was as low as one-fifth of methane Furthermore, each quantity of CO and H2 constituent in the gasified fuels differed, chiefly according to the gasification methods of gasifying agents, raw materials of feedstock, and water-gas-shift reaction as an optional extra for carbon capture system However, it could be said that the theoretical adiabatic flame temperature was only a little bit affected by the CO/H2 molar ratio in the case of each fuel shown in Tables 1 That is to say, in air-blown gasified fuels, fuel calorific values are so low that flame stabilization is a problem confronting development of the combustor

Developments of Gas Turbine Combustors for Air-Blown and Oxygen-Blown IGCC 243

BFG:Blast furnace gas, COG:Coke-oven gas, RDF:Refuse derived fuel, Waste:Municipal solid waste, (a):No description, (b):Dry base

Table 1 Various gasified fuels

Fig 2 Relationship between equivalence ratio and adiabatic flame temperature for gasified fuels and CH4

On the other hand, in the case of oxygen-blown gasified fuels, flame temperature is so high that thermal-NOx emissions must be reduced Therefore, in oxygen-blown IGCC, N2

produced by the air separation unit is used to recover power to increase the thermal efficiency of the plant, and to reduce NOx emissions from the gas turbine combustor by reducing the flame temperature Furthermore, when hot/dry synthetic gas cleanup is employed, ammonia contained in the gasified fuels is not removed, but converted into fuel-NOx in the combustor It is therefore necessary to reduce the fuel-NOx emissions in each case of air-blown or oxygen-blown gasifiers

Because fuel conditions vary depending on the gasification method, many subjects arose in the development of the gasified fueled combustor Table 2 summarizes the main subjects of combustor development for each IGCC method

Equivalence ratio