Abstract

Combined Cycle power plants have recently become a serious alternative for standard coal and oil-fired power plants because of their high thermal efficiency, environmentally friendly operation, and short time to construct. The combined cycle plant is an integration of the gas turbine and the steam turbine, combining many of the advantages of the both thermodynamic cycles using a single fuel. The heat recovery steam generator forms the backbone of combined cycle plants, providing the link between the gas turbine and the steam turbine. The performance of the heat recovery steam generator (HRSG) strongly affects the overall performance of a combined cycle power plant. In the present paper performance simulation and exergy analysis of heat recovery steam generator is done to analyze the effects of various design and operating parameters on the performance of combined cycle power plant. For the exergy analysis, the selected objective is the minimization of thermal exergy losses, taking into account the irreversibility due to the temperature difference between the hot and cold streams and pressure loss in the hrsg. Exergy analysis of HRSG is necessary to find out the locations where exergy losses are more and how the irreversibilities will change with various hrsg parameters. The effect of various parameters like pinch point, approach point, steam pressure, steam temperature, mass flow rate of gas and gas temperature are studied on irreversibilities of different sections in HRSG, loss in exhaust gases, mass of steam generated and exergetic efficiency. It is found that selection of pinch point play an important role on the performance of HRSG. Exhaust gas temperature of the turbine and mass flow rate of gas increases the steam production. To reduce the exergy losses the approach point should be as low as possible and the temperature of feed water entering into the economizer should as high as possible. The steam pressure and temperature is selected such that having low latent heat and high quality to get more work.

Combined cycle power plants recently received worldwide interest not only because of their high energy conversion efficiency and low emission capability, but also due to their potential of using coal, the most widespread abundantly available fossil fuel. The design of combined cycle gas turbine power plant is inherently complex due to the existence of two different power cycles which are coupled through the heat recovery steam generator (HRSG). The HRSG is the nexus between the gas cycle and steam cycle, therefore any change in its design directly affects the cycle efficiency, power generation and many other variables in the cycle. The practical design of the HRSG is usually based on the concepts of pinch point and approach point that govern the gas and steam temperature profiles given by Linhoff and Hindmarsh [1].Pinch and approach points take into account both thermodynamic and economical points of view. Subrahmanyam.et.al [2] discussed various factors affecting the HRSG design for achieving highest combined cycle efficiency with cheaper, economical and competitive designs and with highest requirements to meet the shorter deliveries.P.K.Nag and D.Raha [3] made thermodynamic analysis of a coal based combined cycle power plant from the view points of both first law and second law.Horlock [4] presented detailed thermodynamic analysis of various schemes of combined power plants. A comprehensive computer simulation of a HRSG was developed by A.Ongiro.et.al [5] to predict the performance of heat recovery steam generator (HRSG).Nag and Mazumder, [6] reported the thermodynamic optimization of a waste heat recovery boiler with an economizer and evaporator and producing saturated Steam. They have reported the effect of different operating parameters on entropy generation for the waste heat recovery boiler. P.K. Nag and S. De [7] applied thermodynamic analysis to the optimal design and operation of a heat recovery steam generator (HRSG) generating saturated steam for a combined gas and steam power cycle. Manual Valdes and Jose.L.Rapun [8] presents a method for the optimisation of the HRSG based on the application of influence coefficients. C. Casarosa .et.al [9] have done thermoeconomic optimization based on the minimization of the total HRSG cost, after the reduction to a common monetary base of the costs of exergy losses and of installation. C. Casarosa and A. Franco [10] done thermodynamic optimization of the operative parameters for the heat recovery in combined power plants. They analysed various HRSG configurations from the simpler a single evaporater to the common configuration of two-pressure steam generator with five different sections

Fig.1.Schematic of simple combined cycle plant

Figure 1 shows the schematic of combined cycle power plant. In a combined cycle power plant (CCP) the HRSG represents the interface element between the gas turbine and the steam cycle. Here, the gas turbine exhaust gas is cooled down and the recuperated heat is used to generate steam and the steam thus produced is used to generate electric power. In order to provide better heat recovery in the HRSG more than one pressure level is used. With a single-pressure HRSG about 30% of the total plant output is generated in the steam turbine. A dual-pressure arrangement can increase the power output of the steam cycle by up to 10%, and an additional 3% can result with a triple-pressure cycle [11]. In the literature, different solutions for the increase of the heat recovery efficiency have been proposed. Among these is the well known Kalina cycle, where a mixed working fluid of variable composition is used to provide a better match between the temperatures of the hot and cold streams. An alternative, the use of a fluid with a high critical temperature, such as mercury or potassium, could be interesting for the "bottoming" cycle too [12].

IMPORTANCE OF PINCH AND APPROACH POINTS

Selection of pinch and approach points affects the size of economizer, evaporater and superheater.HRSGs for gas turbine exhaust are usually designed in unfired conditions and the performance evaluated at other unfired or fired conditions for several reasons as mentioned by Ganapathy [13].The reason for this is that two of the important variables which affect the gas and steam temperature profiles, namely the pinch and approach points, cannot be arbitrarily selected in the fired mode. In extracting heat from the flue gas, the ideal is that the flue gas temperature should drop at a constant rate all the way down the duct. This would be simple to arrange if one was trying to heat another gas, or a liquid which did not evaporate as it temperature increased. It would be then easy to keep a constant temperature difference between the flue gas and the fluid being heated at every point. This is not difficult in the economizer and super heater. In economizer water stays as liquid, in superheater it stays as steam. it is impossible to maintain a constant temp difference in the evaporator here as the water passes through the tubing , the water turns to steam, with the temperature staying the same all the way through. Sketch of temperature profile of single pressure heat recovery steam generator is shown in Figure.2.

Fig.2. Temperature profile of single pressure HRSG

The pinch point is the difference between the gas temperature leaving the evaporator and the temperature of saturated steam. Approach point is the difference between the temperature of saturated steam and the temperature of water entering the evaporator. In a heat recovery steam generator the steam production is affected by the conditions of gas turbine inlet temperature and the condition of exhaust gas such as flow rate, temperature and the gas composition entering to heat recovery steam generator. Ambient temperature can significantly affect the approach temperature, as a direct function. As the ambient temperature decreases, the approach temperature will also decrease, indicating that the economizer outlet temperature is getting closer to saturation and the risk of steaming with in the economizer is growing. In general, the pinch will react inversely with ambient temperature. When the inlet conditions to compressor is controlled by some means the ambient temperature will not affect the pinch and approach point

ANALYSIS

A single pressure heat recovery steam generator with a superheater, evaporater, and economiser is considered for the present analysis. The heat recovery steam generator is interface between gas turbine plant and steam turbine plant. The gas and steam temperature profiles are determined by assuming pinch and approach points. The gas flow rate into the heat recovery steam generator, gas inlet temperature to the hrsg, feed water temperature, temperature of steam leaving the superheater, steam pressure, and heat loss in heat recovery steam generator is 2%.To determine gas and steam temperature profiles and steam generation we first assume the pinch and approach points. The values known are gas flow rate (mg), gas temperature at inlet of HRSG (T4), temperature of steam leaving the superheater (Ta), and steam pressure (Ps).Once steam pressure is known, the saturated steam temperature (tsat) at the evaporater is obtained from steam tables. Once pinch point is selected, one can know the temperature of gas leaving the evaporater (T6) and the approach point gives the temperature of water leaving the economiser (Te), since the saturation temperature is known.

In an energy analysis, based on the first law of thermodynamics, all forms of energy are considered to be equivalent. The loss of quality of energy is not taken into account. For example, the change of the quality of thermal energy as it is transformed from a higher to a lower temperature can not be demonstrated in an energy analysis. It shows the energy flow is to be continuous. An exergy analysis, based on the first and second law of thermodynamics, shows the thermodynamic imperfection of a process, including all quality losses of materials and energy. For pinpointing and quantifying the irreversibilities an exergy analysis has to be performed. While doing exergy analysis the following assumptions are made M =650kg/sec, HRSG inlet g temperature=6000C, Pinch Point=200C, Approach Point=50C, temperature of steam = 5500C, pressure loss in H R S G = 0 . 0 5 b a r , h e a t l o s s i n H R S G = 2 % , C =1.147kJ/kgK,R =0.287,T =298K.

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[2]

Energy balance across evaporater and superheater gives

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Superheater energy balance gives

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The economizer energy balance gives

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[11]

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RESULTS AND DISCUSSION

Fig.3 Variation of mass flow rate of steam with pinch point for various flow rates of gas

Figure 3 shows the effect of pinch point on mass of steam produced for different gas flow rates. The mass of steam produced is high for low pinch point, because of more heat absorption from the flue gas. For the same pinch point, as the exhaust gas flow rate increases, the mass of steam produced also increases, which results more power output from the steam turbine. Figure 4 shows the steam produced with exhaust gas flow rate for various pinch points with and without heat loss in HRSG. The mass of steam generated increased with gas flow rate and also with decrease in pinch point.

Fig.4.Variation of steam flow rate with gas flow rate for various pinch points

Fig.5.Variation of mass flow rate of steam with approach point

Figure 5 represents the variation of mass flow rate of steam with approach point. When approach point increases the mass flow rate of steam and steam turbine work output decreases, because more amount of heat is required to generate steam and thus decreases the total mass of steam and further it decreases the turbine output. The variations of stack temperature (T7) with pinch and approach points are shown in Figure 6.With increase in pinch point temperatures the stack temperature also increases, because of large temperature difference between gas and steam temperature profiles.

Fig.6.Variation of stack temperature with pinch point

Fig.7.Variation of specific work output with pinch point for various max pressures

Figures 7 represent the specific output change with pinch point for different maximum steam pressures. The mass of steam and turbine work output decreases with increase in pinch point, but for same pinch point, with increase in maximum pressure the mass of steam and turbine work increases. At higher pinch points even the pressure is high the specific work output is decreased.

Fig.8.Variation of specific work output with approach point for different max pressures

Figures 8 demonstrate the change in specific work output with approach point for different maximum pressures. With increase in approach point there is decrease in turbine work output. For same approach point with increase in maximum pressure the mass of steam and turbine output increases but after certain approach point at high pressure with increase in approach point the mass of steam and turbine output is decreases compared at low pressure. It is because of less steam generation at high approach temperature.

Fig.9.Effect of mass flow rate of gas on Irreversibilities in HRSG components

Figure 9 shows the variation of irreversibilities in different sections (economiser, evaporater, and superheater) of Heat recovery steam generator (HRSG) with mass flow rate of gas. As the gas mass flow rate increases the irreversibilities also increases because of increase in availability. When gas flow rate is changed from 450kg/sec to 700 kg/sec the mass of steam generated increased from 64.5kg/sec to 100.5 kg/sec.

Fig.10. Effect of HRSG inlet temperature on Irreversibilities in HRSG components

Figure 10 shows the change in irreversibilities with HRSG gas inlet temperature. As the gas temperature increases the availability also increases. HRSG inlet temperature depends on the conditions of gas turbine cycle. The gas turbine inlet temperature is limited because of turbine material considerations .With increase in temperature the irreversibilities in economiser, evaporater increases because of increase in temperature difference between the two fluids. The irreversibility in evaporater follows the same path of superheater, but the total irreversibility decreases because of decrease in irreversibility of exhaust gas.

Fig.11. Effect of pinch point on irreversibilities

Fig.12.Effect of approach point on irreversibilities

Figure11 discussed the change in irreversibilities with pinch point. When the pinch point increases the temperature difference between steam and gas also increased. This causes increase in irreversibility, but in superheater the irreversibility decreases with increase in pinch point. Figure 12 shows the variation of irreversibilities with approach point. As the approach point increases the irreversibilities also increases because of large temperature difference in evaporater.Low approach point results less irreversibilities but increased heat transfer surface area.

Fig.13.Effect of steam pressure on irreversibilities

Fig.14.Effect of steam temperature on irreversibilities

Figure 13 discussed the change in irreversibility with steam pressure. The irreversibilities decrease with increase in steam pressure because of increase in saturation temperature of steam and decrease in temperature difference between steam and gas in the heat recovery steam generator. Figure 14 shows the change in irreversibility with steam temperature. As the steam temperature increases the irreversibility in the evaporater decreases because of increase in evaporater gas inlet temperature. With increase in steam temperature the total irreversibility increased because of increase in exhaust gas irreversibility.

Fig.15.Effect of pinch point on exergetic efficiency

Figure 15 shows the mass of steam generated and exergetic efficiency with pinch point. When the pinch point changed from 50C to 300C, the mass of steam generated decreased from 99.42 kg/sec to 89.1 kg/sec and exergetic efficiency changed from 81.28% to 72.9%. Lower pinch point results high mass flow rate of steam and exergetic efficiency but it requires high heat transfer surface area.

Fig.16.Effect of HRSG inlet temperature on Mass of steam and Exergetic efficiency

Figure 16 shows the variation of mass of steam generated and exergetic efficiency with HRSG gas inlet temperature. When HRSG gas inlet temperature is changed from 773K to 873 K, the mass of steam generated increased from 52.3 kg/sec to 93.2 kg/sec and exergetic efficiency changed from 56.8% to 76.2%.As the flue gas temperature increases the availability also increases which results high work output and causes high exergetic efficiency.

Fig.17.Effect of steam pressure on Exergetic efficiency

Figure 17 shows the change in mass flow rate of steam and exergetic efficiency with steam pressure. As the steam pressure increases from 120bar to 200 bar the mass of steam generated increased from 82.3kg/sec to 101.36 kg/sec, and exergetic efficiency increased from 66.52% to 81.69%.When the steam pressure increases the saturation temperature also increases and latent heat required for vaporization is decreased which results high mass flow rate of steam and increase in work output and exergetic efficiency.

Fig.18.effect of steam temperature on Exergetic efficiency

Figure 18 shows the change in mass of steam generated and exergetic efficiency with steam temperature. The mass of steam generated decreases because large amount of heat is required to get the desired superheat temperature. The exergetic efficiency decreases with steam temperature because of increase in total irreversibility of heat recovery steam generator

Conclusion

Combined cycle power plants play an important role to meet the present energy demand. The heat recovery steam generator (HRSG) forms the backbone of combined cycle power plant. In the present paper performance simulation and exergy analysis of heat recovery steam generator is done. The heat recovery steam generator performance is directly depends on the operating conditions. Pinch point, approach point, gas inlet temperature, and gas flow rate effects the mass of steam produced and power output of steam turbine. With increase in pinch and approach points the stack temperature also increases. Low pinch point results high steam production because of more heat utilization. As the steam pressure increases the efficiency also increases but depending on the material considerations we limit the steam pressure.

In exergy analysis the effect of various parameters like repinch point, approach point, steam pressure, steam temperature, mass flow rate of gas and gas temperature are studied on irreversibilities of different sections in HRSG, loss in exhaust gases, mass of steam generated and exergetic efficiency and came to the following conclusions. (1)Pinch point selection play an important role in hrsg performance, very low pinch point requires large heat transfer surface area and selection of high pinch point reduces the heat transfer. (2)Approach point is selected such that the water enters into the evaporater at less than saturation temperature, if the approach temperature increases the total irreversibility in the HRSG also increased. (3)Selection of steam temperature is such that it should be able to produce more work. At low steam temperature the turbine work output and cycle efficiency is less because of low enthalpy of steam. (4)Steam pressure is selected such that more mass of steam generated. At higher pressures because of low latent heat more steam is generated. (5)Mass flow rate of gas affects the HRSG performance such that, with increase in flow rate of exhaust gas the mass of steam generated and total irreversibility also increases. For each kg of steam generated 5.5 to 8 kgs of exhaust gas is required. (6)HRSG gas inlet temperatures also play an important role in HRSG performance. When the required steam temperature and mass of steam generated is more the gas temperature must be high. If the gas temperature is low we require large flow rate of gas and heat transfer area to get the required steam flow conditions.

NOMENCLATURE

AP = Approach Point,0C

C =Specific heat, kJ/kg K

hl = heat loss in HRSG

h = enthalpy, kJ/kg

I =Irreversibility (kW)

M = mass of gas, kg/s

M = mass of steam, kg/s

PP = Pinch Point, C

P = pressure (bar)

Q = heat absorbed by water and steam, kW

S =entropy (kJ/kgK)

T = Temperature ( C)

SUBSCRIPTS

a =hrsg steam outlet

b = condensor inlet

c =condensor outlet

d =economizer inlet

e =evaporater inlet

g =evaporater exit

net =net output

p =pump

s =steam cycle

st =steam turbin

4, 5, 6, 7…=Temperature of gas at different sections in HRSG

References

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[ 2 ] Subrahmanyam. N .V. R .S.S, Rajaram. S, Kamalnathan.N, "HRSGs for combined cycle power plants", Heat Recovery Systems & CHP, 1995, Vol. 15, No. 2, pp. 155-161.

[3] P.K.Nag and D.Raha, "Thermodynamic analysis of a coal-based Combined cycle power plant", Heat Recovery Systems &CHP, 1995, Vol. 15, No. 2, pp. 115- 129.

[4] Horlock, J.H., "Combined Power Plants", Pergamon Press, Oxford, 1992.

[5] A. Ong'iro, V. I. Ugursal, A. M. A1 Taweel, and J.D. Walker, "Modeling of heat recovery steam generator performance", Applied Thermal Engineering, 1997, Vol. 17, No. 5, pp. 427-446.

[6] P.K. Nag, S. Mazumder, "Thermodynamic optimization of a waste heat boiler", in: Proceedings of the 11th National Heat and Mass Transfer Conference, Madras, India, 1991

[7] P. K. Nag and S. De, "Design and operation of a heat recovery steam generator with minimum irreversibility", Applied Thermal Engineering, 1997, Vol. 17, No. 4, pp. 385-391.

[8] M.Valdes, J.Rapun, "Optimization of heat recovery steam generators for combined cycle gas turbine power plants", Applied Thermal Engineering, 2001, Vol 21, pp 1149-1159.

[ 9 ] C.Casarosa, F. Donatini, A . Franco , "Thermoeconomic optimization of heat recovery steam generators operating parameters for combined plants", Energy 29, 2004, p389414.

[10] C.Casarosa and A. Franco, "Thermodynamic Optimization of the Operative Parameters for the Heat Recovery in Combined Power Plants" International Journal of Applied Thermodynamics, Vol.4, (No.1), pp.43-52, March-2001.

[11] Deschamps P.J. "Advanced combined cycle gas alternatives with the latest gas turbines"ASME Journal of Engineering for Gas Turbines and Power, 1998, 120, p350357.

[12] Korobitsyn M.A. "New and advanced energy conversion technologies. Analysis of cogeneration combined and integrated cycles". PhD. Thesis, University of Twente, Enschede, The Netherlands, 1998.

[13] Ganapathy, Heat recovery steam generators: understanding the basics, Chemical Engineering Progress, pp32-45.

[14] T.J.Kotas, 'The Exergy method of Thermal Plant Analysis", Butterworth's, 1985

[15] Nag. P.K., "Power plant engineering" 2nd ed , Tata McGraw-Hill's, 2001.

[16] Ganesan.V, "Gas turbines" 1st edition, Tata McGraw Hill publishing company Ltd, New Delhi.

Performance Simulation And Exergy Analysis Of Hrsg In Combined Cycle Power Plant