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Parametric analysis of a dual fuel parallel coupled combined cycle
Energy 26 (2001) 1063–1074 www.elsevier.com/locate/energy
Parametric analysis of a dual fuel parallel coupled combined cycle T.J. Chmielniak *, J. Kotowicz, J. Lyczko Institute of Machines and Power Generation Systems, Silesian Technical University, 18 Konarskiego Street, Gliwice 44-100, Poland
Abstract In this paper an optimal interconnection of a coal steam plant with a gas combined cycle is analyzed. In the first part the model of such a connection is introduced in order to calculate the integral and marginal cycle efficiency. A parametric analysis is then performed on the hypothetical plant structure. The main goal of the analysis was to estimate the influence of some parameters on the considered cycle efficiency. 2001 Published by Elsevier Science Ltd.
1. Introduction The Polish fuel market situation [4] and the rapid growth of implementation of the gas and gas/steam combined cycles all over the world encouraged the research on possibility of improving the efficiency of existing steam power plants by the addition of gas turbine in order to form combined cycle (‘repowering’). Four possible types of dual fuel repowering can be considered: (a) Systems arranged in series with topping gas turbine (the flue gases of the gas turbine are channeled into the combustion chamber of the coal-fired steam boiler (SB), ‘hot windbox’ or ‘topping’ repowering), Fig. 1. (b) Parallel coupled systems (interconnected at the water–steam system, the coupling consisting of partial by-pass of regenerating heat exchangers and/or SB, by additional gas/water heat exchangers or a heat recovery boiler fed by the gas turbine exhaust gases), Fig. 2. (c) Mixed systems, combining (a) and (b) (there may be several possible variants), one of them is shown in Fig. 3. (d) Dual fuel systems with steam supercharging of the combustion chamber. * Corresponding author.
0360-5442/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 6 9 - X
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Nomenclature A AR BWd Nel q Q QLPR QHPR
theoretical ratio of power rate of gas and steam part (defined in Eq. (3)) real (actual) ratio of power rate of gas and steam part (defined in Eq. (19)) chemical energy heat flow in fuel [kW] electric power [kW] specific energy consumption [kJ/kWe s] (defined in Eq. (2)); heat flow [kW] flow of heat passed to condensate in low-pressure regeneration part [kW] regeneration part [kW]
Greek symbols a b g d hel hel0 hmarg
coefficient of low-pressure part (condensate) coupling (defined in Eq. (9)) coefficient of high-pressure part (feedwater) coupling (defined in Eq. (11)) coefficient of the steam generation part coupling (defined in Eq. (13)) coefficient of coupling gas and steam cycles (defined in Eq. (16)) electric efficiency electric efficiency of autonomous cycle marginal efficiency
Superscripts I II B DCC CC G WB S ST SB
base fuel (i.e. coal) additional fuel (i.e. gas) boilers: steam and waste heat together dual fuel combined cycle combined cycle gaseous part waste heat boiler, HRSG steam part steam turbine; steam turbogenerator steam boiler
Diagrams GTU dSI HPR LPR Qflue QFW
gas turbine unit (set) desulfurization installation high-pressure regeneration heat exchangers low-pressure regeneration heat exchangers flue gases heat flux feedwater heat flux
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Qloss Qs/w SB STU WB
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loss of heat flux steam/water heat flux coal-fired steam boiler steam turbine unit (turbine, generator, condenser, etc.) waste heat recovery boiler
Fig. 1. Scheme of serial dual fuel combined cycle.
Fig. 2. Scheme of parallel coupled dual fuel combined cycle.
Fig. 3. Sample scheme of mixed structure of dual fuel combined cycle.
A considerable interest in dual fuel combined cycles [1–3,5] has been observed, in recent years particularly in the countries, where a large portion of the electrical power plants is based on solid fuel (hard coal and lignite). This paper focuses on parametric calculations of the dual fuel parallel coupled combined cycle performances for different coupling structures. The theoretical ratio of the power rate of the gas and steam part was chosen as the main independent parameter in the performed calculations for thermal and marginal efficiency. Because the number of possible structures of the considered cycles is very high, in the first step of the analysis thermal efficiency was concidered as a selected criterion. Thermo-economic analysis of the chosen coupled structure will be the subject of further investigation.
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2. A model of a parallel coupled dual fuel combined cycle For general consideration it has been assumed that the gas and the steam parts of the combined cycle may be coupled by a recovery boiler in three different ways, viz. (a) by partial or total low-pressure regeneration (LPR) of the feedwater preheating train; (b) by partial or total preheating of the feedwater in the recovery boiler, replacement of the high-pressure part of the feedwater heater train; (c) by the generation of steam in a heat recovery steam generator. 2.1. Efficiency of a dual fuel parallel coupled combined cycle power station The thermal efficiency of a dual fuel combined cycle may be assessed as a generated electric and supplied with fuel power ratio ⫽ hDCC el
NSel+NGel (BWd)I+(BWd)II
(1)
Gas turbine efficiency (hGel) and specific energy consumption (qG) definitions will be applied then. For a specific gas turbine supplied by selected fuel the following parameters are defined: (BWd)II 1 ⫽ G⫽qG NGel hel
(2)
Let A be defined as a theoretical ratio of the electric power generated in separated gas and steam portions (autonomous cycles before coupling) NGel A⫽ S Nel0
(3)
Let efficiency (hSel0) of the autonomous steam part be expressed by the formula NSel0 hSel0⫽ (BWd)I
(4)
Substituting Eqs. (2)–(4) into Eq. (1) we get the following formula: NSel +A NSel0 DCC hel ⫽ 1 +AqG S hel0
(5)
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If it is assumed, that the changes of energy supplied to SB (BWd)I are negligibly small, as a result of coupling steam and gas portion (SB operates with semiconstant load and efficiency) and using heat balance equations, Eq. (5) can be expressed as a function of coupling steam and gas systems. To determine steam turbogenerator electric power ratio of coupled (NSel) and autonomous (NSel0) system, following balance equations (cf. Fig. 4) are needed. For autonomous system 앫 앫 앫 앫
steam turbogenerator unit (STU) balance equation; SB balance equation; high-pressure and low-pressure regeneration (HPR/LPR) systems balance equations; useful heat Eq. (6) and SB thermal efficiency Eq. (7) definitions SB ⌬QSB⫽QSB⫺QFW
(6)
⌬QSB hSB⫽ (BWd)I
(7)
For coupled system 앫 앫 앫 앫 앫
STU balance equation; SB balance equation; waste heat recovery boiler (WB) balance equation; HPR/LPR systems balance equations; recovery boiler useful heat flux definitions (HRSG section)
Fig. 4. Scheme of dual fuel coal-gas parallel coupled combined cycle — general model.
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⌬QWB⫽QWB⫺QWB FW
(8)
If the following coefficients are defined a parametric form of Eq. (1) can be formulated a — waste heat boiler heat flux portion and the total heat flux in low-pressure feedwater preheating train ratio QWB LPR a⫽ QLPR
(9)
where ST QLPR⫽QWB LPR⫹QLPR
(10)
b — waste heat boiler heat flux portion and total heat flux ratio exchanged in high-pressure feedwater preheating train QWB HPR b⫽ QHPR
(11)
where ST QHPR⫽QWB HPR⫹QHPR
(12)
g — ratio of heat flux passed into steam cycle by HRSG (⌬QWB) and total (by HRSG and by SB) heat flux (⌬QB) ⌬QWB g⫽ ⌬QB
(13)
where ⌬QB⫽⌬QWB⫹⌬QSB
(14)
Value of each coefficient may be between (0,1). Its value represents the coupling level of the cycle. By applying these balance equations and defining Eq. (7)–Eq. (14), we get the following formula describing the efficiency of a dual fuel parallel coupled combined cycle: 1 A+ d hDCC ⫽ el 1 +AqG hSel0
(15)
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where (1−g)⌬QS d⫽ S ⌬Q +bQHPR+aQLPR
(16)
In Ref. [6] we often encounter the idea of a marginal efficiency hDCC marg. This efficiency has been defined as the ratio of electric power obtained by introducing the gas unit and the amount of energy supplied in the gaseous fuel (Eq. (17)). hDCC marg⫽
NGel+⌬NSel (BWd)II+⌬(BWd)I
(17)
Transforming the above equation we get a formula enabling dual fuel combined cycle marginal efficiency calculation 1 A+ −1 d hDCC marg⫽ AqG
(18)
The marginal efficiency (hDCC marg) may be useful when we would like to compare the dual fuel cycle based on existing steam power plant with a combined cycle power plant which is based on the same gas turbine unit. 2.2. Actual and theoretical power ratio of gas and steam parts The coupling of two autonomous (steam and gas) systems into a dual fuel combined cycle increases the conversion’s efficiency of the fuels chemical energy into electricity. A higher efficiency results from the utilization of the waste heat flue gas (from the gas turbine) in the steam part. The effect is an increase of power rate of the steam turbogenerator (⌬NSel). One of the characteristic parameters of combined cycles is the theoretical electric power ratio of the gas and steam parts (A). In order to compare parallel coupled combined cycles we apply this dimensionless index. This index is more important while considering the dual fuel combined cycles than those one fuel ones. In the parallel coupled dual fuel combined cycles, both subcycles (i.e. the gas and the steam cycle) can operate either in coupled, or autonomous mode. The overall power rate of the power plant as well as the power ratio of both parts then change considerably. While carrying out multivariant analyses it is much easier to apply theoretical power ratio (A) defined by Eq. (3), which does not depend on the way of coupling on both portions, in contradiction to the actual (real) power ratio (AR). As it may be easily proved, the relation between the theoretical and the actual power ratio of gas and steam part can be expressed by the formula AR⫽Ad
(19)
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where d is determined by Eq. (16). Considering already existing systems it would probably be more convenient to use only the actual power ratio (AR), however in the case of such specific systems, as parallel combined cycles, the application of both indices gives more detailed information about the potential possibilities of this type of power plant. 3. Selected results of numerical calculations 3.1. Assumptions We have chosen parallel combined cycle power plant unit with electric efficiency of autonomous steam part to hSel0=0.35. The system was connected parallel with a gas turbine (hGel equal to: 32, 36 and 40%) and recovery boiler (HRSG and water preheaters). It has been assumed that the efficiency of the units realizing the cycle does not depend on the load. 3.2. Investigations aiming at an optimization of the process The aim of these investigations was to optimize the configuration of the dual fuel combined cycle both quantitavely and qualitatively. For various kinds of couplings of gas part with the steam part we tried to arrive at an optimal solution, which would ensure an efficient flow of working medium in the respective installations. The results gathered in this way present a set of data for further analysis, with the purpose to optimize the installation with respect to other criteria taking into account the economical and ecological aspects of the investigated cycle. Selected results of these investigations have been presented in the form of diagrams; the axis of ordinates presents the theoretical power ratio of gas and steam parts (A). Coefficient A has been chosen by changing power rate of gas part, with the efficiency of SB remaining constant (constant flux of live steam). 3.3. Results of calculations Figs. 5–10 illustrate selected results of the calculations in the form of diagrams. The diagrams 5–7 concern a cycle, in which the flue gases leaving the gas turbine heat feedwater (b) and condensate (a). It is observed that for each gas turbine (hGel) there is a definite value A, for which the efficiency of the cycle is the highest, due to the amount of heat which the exhaust gas can transfer to the feedwater and condensate. An increase of the heat flux in the combustion gas passed into the recovery water heater exceeding the optimal value does not bring about any positive thermodynamic effects, but it raises only the temperature of the combustion gas leaving the recovery boiler, which again causes higher energy losses QWB loss . The diagrams 8–10 present the results of investigations concerning a cycle in which the recovery boiler generates the steam (g-HRSG) for IP part of steam turbine while the remainig heat is transfered to the main condensate (a). In both these cases, the value of the coefficient a tends to amax⬇75% together with the power
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Fig. 5. Efficiency hDCC of dual fuel combined cycle (a+b) as A function. el
Fig. 6. a and b coefficients as A function.
ratio A. The maximum value of the coefficient a results from the structure of the coupling of gas and steam part: only three out of four condensate preheaters are bypassed. In the case of systems coupled with steam generation (g) in HRSG, the efficiency of the coupled combined cycle does not display such a distinct extremum. Observing, however, the marginal efficiency, we see a distinct deviation of the approximating curves, which are also in this case strictly connected with the amount of heat to be recovered in HRSG. The similar results can be observed among others, in [6]. The flat shape of the characteristics of the marginal efficiency is of essential importance, it can again indicate a flat course of the characteristics of dual fuel power plant at a partial load (when the steam capacity of a boiler is reduced).
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Fig. 7. Marginal efficiency hDCC marg of the cycle (a+b) as A function.
Fig. 8.
Efficiency hDCC of dual fuel combined cycle (a+g) as A function. el
The efficiency of parallel dual fuel combined cycles is most affected by the power ratio of gas and steam part A and the way in which both these parts have been connected, although the influence of their autonomous efficiency must not be neglected. The temperature of combustion gases leaving the gas turbine is of less importance, though still essential, particularly in the case of a system with steam generation (g) in HRSG. The best index of the assessment of the repowering structure is a marginal effect. Marginal efficiency may be compared with the efficiency of a classical combined cycle. The marginal effect results from the introduction of gas part or the connection of both parts into dual fuel combined cycle, and as such it should be taken into account when assessed and compared with other cycles.
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Fig. 9. a and g coefficients as A function.
Fig. 10. Marginal efficiency hDCC marg for coupling (a+g) as A function.
4. Conclusions Based on the investigations of parallel coupled systems, one can conclude that adding gas turbine to the existing conventional part of the plant allows a simultaneous increase of both the power rate and the efficiency. Even the application of only small gas turbines makes it possible to increase the power rate (and also its availability) considerably, and at the same time also the efficiency of the whole dual fuel power plant. The developed methods of analyzing dual fuel combined cycles permit the evaluation of various
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coupling structures (two of which have been dealt with in this paper), and also become a universal tool in the investigations of dual fuel combined cycle effectiveness. We can calculate emissions of dual fuel parallel coupled combined cycle if thermal efficiency, structure of the cycle and fuels characteristics are known. Approximate techno-economic analysis of the dual fuel parallel coupled combined cycles is difficult to be generalized — too many independent arguments for objective functions present a large number of results and reliability of them is low. Therefore the techno-economic analysis should be performed only for selected results of earlier investigations. Introduction of real characteristics of plant components into calculation model allows the assessment of performance of such systems at both full and partial loads. References [1] Arakelyan EK, Timoshenko NI, Tsanyew SW, Klyevcov AW, Gorbachinsky SI. Investigation of the indices of steam and gas plant with a semi-dependent scheme. Teploenergetika 1994;1(January):39–42 [in Russian]. [2] Braam ALH, Borisov NA, van Aart FJJM. Gas turbine topping of a 800 MW gas fired power station in Ukraine. In: Power-Gen Europe ’97, 1997, Madrid (Spain), 17–19 June. [3] Haas B, Necker P, Scha¨ fer K, Bu¨ th R, Cossmann R. Parallel combined-cycle cogeneration plant Altbach/Deizisau Neckarwerke AG. In: Power-Gen Europe ’97, 1997, Madrid (Spain), 17–19 June. [4] Janiczek R. The aim of the changes in the fuel structure of the national power industry. In: IX Conference: Electric and thermal-electric power stations modernization and fine coal processing plant building, 1995, Zakopane, 9–11 October [in Polish]. [5] Melli R, Naso V, Sciubba E. Modular repowering of power plants with normal ratings lower than 180 MW: A rational design approach and its application to the Italian Utility System. J Energy Res Technol 1994;8(September):201–10. [6] Pfost H, Rukes B. Gas turbines increase power and efficiency of steam power plants. In: Power-Gen Europe ’98, 1998, Milan (Italy), 9–11 June.
Parametric analysis of a dual fuel parallel coupled combined cycle T.J. Chmielniak *, J. Kotowicz, J. Lyczko Institute of Machines and Power Generation Systems, Silesian Technical University, 18 Konarskiego Street, Gliwice 44-100, Poland
Abstract In this paper an optimal interconnection of a coal steam plant with a gas combined cycle is analyzed. In the first part the model of such a connection is introduced in order to calculate the integral and marginal cycle efficiency. A parametric analysis is then performed on the hypothetical plant structure. The main goal of the analysis was to estimate the influence of some parameters on the considered cycle efficiency. 2001 Published by Elsevier Science Ltd.
1. Introduction The Polish fuel market situation [4] and the rapid growth of implementation of the gas and gas/steam combined cycles all over the world encouraged the research on possibility of improving the efficiency of existing steam power plants by the addition of gas turbine in order to form combined cycle (‘repowering’). Four possible types of dual fuel repowering can be considered: (a) Systems arranged in series with topping gas turbine (the flue gases of the gas turbine are channeled into the combustion chamber of the coal-fired steam boiler (SB), ‘hot windbox’ or ‘topping’ repowering), Fig. 1. (b) Parallel coupled systems (interconnected at the water–steam system, the coupling consisting of partial by-pass of regenerating heat exchangers and/or SB, by additional gas/water heat exchangers or a heat recovery boiler fed by the gas turbine exhaust gases), Fig. 2. (c) Mixed systems, combining (a) and (b) (there may be several possible variants), one of them is shown in Fig. 3. (d) Dual fuel systems with steam supercharging of the combustion chamber. * Corresponding author.
0360-5442/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 6 9 - X
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Nomenclature A AR BWd Nel q Q QLPR QHPR
theoretical ratio of power rate of gas and steam part (defined in Eq. (3)) real (actual) ratio of power rate of gas and steam part (defined in Eq. (19)) chemical energy heat flow in fuel [kW] electric power [kW] specific energy consumption [kJ/kWe s] (defined in Eq. (2)); heat flow [kW] flow of heat passed to condensate in low-pressure regeneration part [kW] regeneration part [kW]
Greek symbols a b g d hel hel0 hmarg
coefficient of low-pressure part (condensate) coupling (defined in Eq. (9)) coefficient of high-pressure part (feedwater) coupling (defined in Eq. (11)) coefficient of the steam generation part coupling (defined in Eq. (13)) coefficient of coupling gas and steam cycles (defined in Eq. (16)) electric efficiency electric efficiency of autonomous cycle marginal efficiency
Superscripts I II B DCC CC G WB S ST SB
base fuel (i.e. coal) additional fuel (i.e. gas) boilers: steam and waste heat together dual fuel combined cycle combined cycle gaseous part waste heat boiler, HRSG steam part steam turbine; steam turbogenerator steam boiler
Diagrams GTU dSI HPR LPR Qflue QFW
gas turbine unit (set) desulfurization installation high-pressure regeneration heat exchangers low-pressure regeneration heat exchangers flue gases heat flux feedwater heat flux
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Qloss Qs/w SB STU WB
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loss of heat flux steam/water heat flux coal-fired steam boiler steam turbine unit (turbine, generator, condenser, etc.) waste heat recovery boiler
Fig. 1. Scheme of serial dual fuel combined cycle.
Fig. 2. Scheme of parallel coupled dual fuel combined cycle.
Fig. 3. Sample scheme of mixed structure of dual fuel combined cycle.
A considerable interest in dual fuel combined cycles [1–3,5] has been observed, in recent years particularly in the countries, where a large portion of the electrical power plants is based on solid fuel (hard coal and lignite). This paper focuses on parametric calculations of the dual fuel parallel coupled combined cycle performances for different coupling structures. The theoretical ratio of the power rate of the gas and steam part was chosen as the main independent parameter in the performed calculations for thermal and marginal efficiency. Because the number of possible structures of the considered cycles is very high, in the first step of the analysis thermal efficiency was concidered as a selected criterion. Thermo-economic analysis of the chosen coupled structure will be the subject of further investigation.
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2. A model of a parallel coupled dual fuel combined cycle For general consideration it has been assumed that the gas and the steam parts of the combined cycle may be coupled by a recovery boiler in three different ways, viz. (a) by partial or total low-pressure regeneration (LPR) of the feedwater preheating train; (b) by partial or total preheating of the feedwater in the recovery boiler, replacement of the high-pressure part of the feedwater heater train; (c) by the generation of steam in a heat recovery steam generator. 2.1. Efficiency of a dual fuel parallel coupled combined cycle power station The thermal efficiency of a dual fuel combined cycle may be assessed as a generated electric and supplied with fuel power ratio ⫽ hDCC el
NSel+NGel (BWd)I+(BWd)II
(1)
Gas turbine efficiency (hGel) and specific energy consumption (qG) definitions will be applied then. For a specific gas turbine supplied by selected fuel the following parameters are defined: (BWd)II 1 ⫽ G⫽qG NGel hel
(2)
Let A be defined as a theoretical ratio of the electric power generated in separated gas and steam portions (autonomous cycles before coupling) NGel A⫽ S Nel0
(3)
Let efficiency (hSel0) of the autonomous steam part be expressed by the formula NSel0 hSel0⫽ (BWd)I
(4)
Substituting Eqs. (2)–(4) into Eq. (1) we get the following formula: NSel +A NSel0 DCC hel ⫽ 1 +AqG S hel0
(5)
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If it is assumed, that the changes of energy supplied to SB (BWd)I are negligibly small, as a result of coupling steam and gas portion (SB operates with semiconstant load and efficiency) and using heat balance equations, Eq. (5) can be expressed as a function of coupling steam and gas systems. To determine steam turbogenerator electric power ratio of coupled (NSel) and autonomous (NSel0) system, following balance equations (cf. Fig. 4) are needed. For autonomous system 앫 앫 앫 앫
steam turbogenerator unit (STU) balance equation; SB balance equation; high-pressure and low-pressure regeneration (HPR/LPR) systems balance equations; useful heat Eq. (6) and SB thermal efficiency Eq. (7) definitions SB ⌬QSB⫽QSB⫺QFW
(6)
⌬QSB hSB⫽ (BWd)I
(7)
For coupled system 앫 앫 앫 앫 앫
STU balance equation; SB balance equation; waste heat recovery boiler (WB) balance equation; HPR/LPR systems balance equations; recovery boiler useful heat flux definitions (HRSG section)
Fig. 4. Scheme of dual fuel coal-gas parallel coupled combined cycle — general model.
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⌬QWB⫽QWB⫺QWB FW
(8)
If the following coefficients are defined a parametric form of Eq. (1) can be formulated a — waste heat boiler heat flux portion and the total heat flux in low-pressure feedwater preheating train ratio QWB LPR a⫽ QLPR
(9)
where ST QLPR⫽QWB LPR⫹QLPR
(10)
b — waste heat boiler heat flux portion and total heat flux ratio exchanged in high-pressure feedwater preheating train QWB HPR b⫽ QHPR
(11)
where ST QHPR⫽QWB HPR⫹QHPR
(12)
g — ratio of heat flux passed into steam cycle by HRSG (⌬QWB) and total (by HRSG and by SB) heat flux (⌬QB) ⌬QWB g⫽ ⌬QB
(13)
where ⌬QB⫽⌬QWB⫹⌬QSB
(14)
Value of each coefficient may be between (0,1). Its value represents the coupling level of the cycle. By applying these balance equations and defining Eq. (7)–Eq. (14), we get the following formula describing the efficiency of a dual fuel parallel coupled combined cycle: 1 A+ d hDCC ⫽ el 1 +AqG hSel0
(15)
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where (1−g)⌬QS d⫽ S ⌬Q +bQHPR+aQLPR
(16)
In Ref. [6] we often encounter the idea of a marginal efficiency hDCC marg. This efficiency has been defined as the ratio of electric power obtained by introducing the gas unit and the amount of energy supplied in the gaseous fuel (Eq. (17)). hDCC marg⫽
NGel+⌬NSel (BWd)II+⌬(BWd)I
(17)
Transforming the above equation we get a formula enabling dual fuel combined cycle marginal efficiency calculation 1 A+ −1 d hDCC marg⫽ AqG
(18)
The marginal efficiency (hDCC marg) may be useful when we would like to compare the dual fuel cycle based on existing steam power plant with a combined cycle power plant which is based on the same gas turbine unit. 2.2. Actual and theoretical power ratio of gas and steam parts The coupling of two autonomous (steam and gas) systems into a dual fuel combined cycle increases the conversion’s efficiency of the fuels chemical energy into electricity. A higher efficiency results from the utilization of the waste heat flue gas (from the gas turbine) in the steam part. The effect is an increase of power rate of the steam turbogenerator (⌬NSel). One of the characteristic parameters of combined cycles is the theoretical electric power ratio of the gas and steam parts (A). In order to compare parallel coupled combined cycles we apply this dimensionless index. This index is more important while considering the dual fuel combined cycles than those one fuel ones. In the parallel coupled dual fuel combined cycles, both subcycles (i.e. the gas and the steam cycle) can operate either in coupled, or autonomous mode. The overall power rate of the power plant as well as the power ratio of both parts then change considerably. While carrying out multivariant analyses it is much easier to apply theoretical power ratio (A) defined by Eq. (3), which does not depend on the way of coupling on both portions, in contradiction to the actual (real) power ratio (AR). As it may be easily proved, the relation between the theoretical and the actual power ratio of gas and steam part can be expressed by the formula AR⫽Ad
(19)
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where d is determined by Eq. (16). Considering already existing systems it would probably be more convenient to use only the actual power ratio (AR), however in the case of such specific systems, as parallel combined cycles, the application of both indices gives more detailed information about the potential possibilities of this type of power plant. 3. Selected results of numerical calculations 3.1. Assumptions We have chosen parallel combined cycle power plant unit with electric efficiency of autonomous steam part to hSel0=0.35. The system was connected parallel with a gas turbine (hGel equal to: 32, 36 and 40%) and recovery boiler (HRSG and water preheaters). It has been assumed that the efficiency of the units realizing the cycle does not depend on the load. 3.2. Investigations aiming at an optimization of the process The aim of these investigations was to optimize the configuration of the dual fuel combined cycle both quantitavely and qualitatively. For various kinds of couplings of gas part with the steam part we tried to arrive at an optimal solution, which would ensure an efficient flow of working medium in the respective installations. The results gathered in this way present a set of data for further analysis, with the purpose to optimize the installation with respect to other criteria taking into account the economical and ecological aspects of the investigated cycle. Selected results of these investigations have been presented in the form of diagrams; the axis of ordinates presents the theoretical power ratio of gas and steam parts (A). Coefficient A has been chosen by changing power rate of gas part, with the efficiency of SB remaining constant (constant flux of live steam). 3.3. Results of calculations Figs. 5–10 illustrate selected results of the calculations in the form of diagrams. The diagrams 5–7 concern a cycle, in which the flue gases leaving the gas turbine heat feedwater (b) and condensate (a). It is observed that for each gas turbine (hGel) there is a definite value A, for which the efficiency of the cycle is the highest, due to the amount of heat which the exhaust gas can transfer to the feedwater and condensate. An increase of the heat flux in the combustion gas passed into the recovery water heater exceeding the optimal value does not bring about any positive thermodynamic effects, but it raises only the temperature of the combustion gas leaving the recovery boiler, which again causes higher energy losses QWB loss . The diagrams 8–10 present the results of investigations concerning a cycle in which the recovery boiler generates the steam (g-HRSG) for IP part of steam turbine while the remainig heat is transfered to the main condensate (a). In both these cases, the value of the coefficient a tends to amax⬇75% together with the power
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Fig. 5. Efficiency hDCC of dual fuel combined cycle (a+b) as A function. el
Fig. 6. a and b coefficients as A function.
ratio A. The maximum value of the coefficient a results from the structure of the coupling of gas and steam part: only three out of four condensate preheaters are bypassed. In the case of systems coupled with steam generation (g) in HRSG, the efficiency of the coupled combined cycle does not display such a distinct extremum. Observing, however, the marginal efficiency, we see a distinct deviation of the approximating curves, which are also in this case strictly connected with the amount of heat to be recovered in HRSG. The similar results can be observed among others, in [6]. The flat shape of the characteristics of the marginal efficiency is of essential importance, it can again indicate a flat course of the characteristics of dual fuel power plant at a partial load (when the steam capacity of a boiler is reduced).
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Fig. 7. Marginal efficiency hDCC marg of the cycle (a+b) as A function.
Fig. 8.
Efficiency hDCC of dual fuel combined cycle (a+g) as A function. el
The efficiency of parallel dual fuel combined cycles is most affected by the power ratio of gas and steam part A and the way in which both these parts have been connected, although the influence of their autonomous efficiency must not be neglected. The temperature of combustion gases leaving the gas turbine is of less importance, though still essential, particularly in the case of a system with steam generation (g) in HRSG. The best index of the assessment of the repowering structure is a marginal effect. Marginal efficiency may be compared with the efficiency of a classical combined cycle. The marginal effect results from the introduction of gas part or the connection of both parts into dual fuel combined cycle, and as such it should be taken into account when assessed and compared with other cycles.
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Fig. 9. a and g coefficients as A function.
Fig. 10. Marginal efficiency hDCC marg for coupling (a+g) as A function.
4. Conclusions Based on the investigations of parallel coupled systems, one can conclude that adding gas turbine to the existing conventional part of the plant allows a simultaneous increase of both the power rate and the efficiency. Even the application of only small gas turbines makes it possible to increase the power rate (and also its availability) considerably, and at the same time also the efficiency of the whole dual fuel power plant. The developed methods of analyzing dual fuel combined cycles permit the evaluation of various
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coupling structures (two of which have been dealt with in this paper), and also become a universal tool in the investigations of dual fuel combined cycle effectiveness. We can calculate emissions of dual fuel parallel coupled combined cycle if thermal efficiency, structure of the cycle and fuels characteristics are known. Approximate techno-economic analysis of the dual fuel parallel coupled combined cycles is difficult to be generalized — too many independent arguments for objective functions present a large number of results and reliability of them is low. Therefore the techno-economic analysis should be performed only for selected results of earlier investigations. Introduction of real characteristics of plant components into calculation model allows the assessment of performance of such systems at both full and partial loads. References [1] Arakelyan EK, Timoshenko NI, Tsanyew SW, Klyevcov AW, Gorbachinsky SI. Investigation of the indices of steam and gas plant with a semi-dependent scheme. Teploenergetika 1994;1(January):39–42 [in Russian]. [2] Braam ALH, Borisov NA, van Aart FJJM. Gas turbine topping of a 800 MW gas fired power station in Ukraine. In: Power-Gen Europe ’97, 1997, Madrid (Spain), 17–19 June. [3] Haas B, Necker P, Scha¨ fer K, Bu¨ th R, Cossmann R. Parallel combined-cycle cogeneration plant Altbach/Deizisau Neckarwerke AG. In: Power-Gen Europe ’97, 1997, Madrid (Spain), 17–19 June. [4] Janiczek R. The aim of the changes in the fuel structure of the national power industry. In: IX Conference: Electric and thermal-electric power stations modernization and fine coal processing plant building, 1995, Zakopane, 9–11 October [in Polish]. [5] Melli R, Naso V, Sciubba E. Modular repowering of power plants with normal ratings lower than 180 MW: A rational design approach and its application to the Italian Utility System. J Energy Res Technol 1994;8(September):201–10. [6] Pfost H, Rukes B. Gas turbines increase power and efficiency of steam power plants. In: Power-Gen Europe ’98, 1998, Milan (Italy), 9–11 June.