Comparative Energy and Exergy Analysis of a Thermal Power Plant

Transcription

1 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 618 Comparative Energy and Exergy Analysis of a Thermal Power Plant G. Buchi Babu and Dr. K. Sridhar Abstract--- Energy and exergy concepts come from thermodynamics and are applicable to all fields of science and engineering. Therefore, this paper intends to provide background for better understanding of these concepts and their differences for energy conversion system. The first law of thermodynamics used for analysis and optimization of energy systems. The use of energy as a measure for understanding and improving the efficiencies of energy systems can be misleading and confusing. First law analysis leads to false impressions about the energy conversion processes because it does not consider energy degradations during the processes. The second law of thermodynamics describes the quality of energy and degradations in the quality i.e., it compliments and enhances an energy balance by enabling calculation of both the true thermodynamic value of an energy carriers, and the real thermodynamic inefficiencies and losses from the processes or systems. The concept of exergy is extremely useful for this purpose. Exergy analysis, or the second law analysis as it is called, is based on both the first and second laws, and exergy based methods, therefore, most be adopted while designing or rehabilitating energy systems. In this paper a 500 MW coal based thermal power plant of APGENCO, Warangal is chosen for case study purpose. A detailed energy and exergy analysis is carried out for the each component of the plant. And the results show that, according to energy analysis the condenser is main source of losses and according to exergy analysis the boiler is the major source of losses where there are more irreversibilies. The use of supercritical technology is strongly recommended to improve the performance of boiler, it not only improves the performance of boiler, but also the performance of the turbine due to the enhanced steam parameters and higher pressure ratio across the turbine. Keywords--- Energy, Exergy, Exergy Efficiency and Exergy Destruction Ratio T I. INTRODUCTION HE science of thermodynamics is built primarily on two fundamental natural laws, known as the first and the second laws. The first law of thermodynamics (FLT) is simply an expression of the conservation of energy principle. It asserts that energy is a thermodynamic property and that during an interaction, energy can change from one form to another but the total amount of energy remains constant. The first law places no restriction on the direction of a process, but satisfying the first law does not ensure that the process can actually occur. This inadequacy of the first law to identify whether a process can take place is remedied by introducing another general principle, the second law of thermodynamics. The second law of thermodynamics (SLT) asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. The hightemperature thermal energy is degraded as it is transferred to a lower temperature body. The attempts to quantify the quality or work potential of energy in the light of the second law of thermodynamics has resulted in the definition of the properties entropy and exergy. Efficiency is one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or process is accomplished. Efficiency is also one of the most frequently misused terms in thermodynamics and is often a source of misunderstanding. This is because efficiency is often used without being G. Buchi Babu, Assistant Professor of Department of Mechanical Engineering, Warangal Institute of Technology and Science (WITS), Guddeppad X Road, Oorugonda, Atmakur(M), Warangal (A.P), g_buchibabu@yahoo.com. Dr. K. Sridhar, Professor of Department of Mechanical Engineering, Kakatiya Institute of Technology and Science (KITS), Opposite Yerragatu hillock, Hasanparthi, Warangal (A.P), kandurisridhar@rediffmail.com. PAPER ID: MET15

2 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 619 properly defined first. Efficiency traditionally has been primarily defined based on the first law (i.e., energy). In recent decades, exergy analysis has found increasingly widespread acceptance as a useful tool in the design, assessment, optimization and improvement of energy systems. Determining exergy efficiencies for an overall system and/or the individual components making up the system constitutes a major part of exergy analysis. A comprehensive analysis of a thermodynamic system includes both energy and exergy analyses in order to obtain a more complete picture of system behavior. To assist in improving the efficiencies of power plants, their thermodynamic characteristics and performances are usually investigated. Power plants are normally examined using energy analysis but, as pointed out previously, a better understanding is attained when a more complete thermodynamic view is taken, which uses the second law of thermodynamics in conjunction with energy analysis via exergy methods. Although exergy analysis can be generally applied to energy and other systems, it appears to be a more powerful tool than energy analysis for power cycles because of the fact that it helps determine the true magnitudes of losses and their causes and locations, and improve the overall system and its components. The objective of this paper is to analyze the results of energy analysis and exergy analysis performed on a 500 MW coal based thermal power plant of APGENCO, Warangal, Andhra Pradesh. Components of energy loss and exergy destruction are identified. And also effect of varying parameters on energy analysis and exergy analysis are investigated. II. ENERGY VS EXERGY The traditional method of assessing the energy disposition of an operation involving the physical or chemical processing of materials and products with accompanying transfer and/or transformation of energy is by the completion of an energy balance. This balance is apparently based on the first law of thermodynamics. In this balance, information on the system is employed to attempt to reduce heat losses or enhance heat recovery. However, from such a balance no information is available on the degradation of energy, occurring in the process and to quantify the usefulness or quality of the heat content in various streams leaving the process as products, wastes, or coolants. The exergy method of analysis overcomes the limitations of the first law of thermodynamics. The concept of exergy is based on both first law of thermodynamics and second law of thermodynamics. Exergy analysis can clearly indicate the locations of energy degradation in a process that may lead to improved operation or technology. It can also quantify the quality of heat in a reject stream. So, the main aim of exergy analysis is to identify the causes and to calculate the true magnitudes of exergy losses. Table 1 presents a general comparison of both energy and exergy. ENERGY is dependent on the parameters of matter or energy flow only, and independent of the environment parameters. has the values different from zero. is governed by the FLT for all the processes. is limited by the SLT for all processes. is motion or ability to produce motion. is always conserved in a process, so can neither be destroyed or produced. is a measure of quantity only. Table 1: Comparison of Energy and Exergy EXERGY is dependent both on the parameters of matter or energy flow and on the environment parameters. is equal to zero (in dead state by equilibrium with the environment). is governed by the FLT for reversible processes only (in irreversible processes it is destroyed partly or completely). is not limited for reversible processes due to the SLT. is work or ability to produce work. is always conserved in a reversible process, but is always consumed in an irreversible process. is a measure of quantity and quality.

3 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 620 III. PLANT DESCRIPTION 3.1. Configuration of Power Plant The power plant of 500 MW chosen for case study was erected in 2010 as KTPP stage I of APGENCO. This power plant is coal based and works in subcritical conditions of steam. The important conditions of steam are given below: Main steam (MS): bar and C. Reheat steam: C. Feed water temperature at economizer inlet: C. Condenser pressure: 9 kpa. Feed pump pressure: bar. Superheated steam first expands in high pressure turbine (HP turbine). HP turbine exhaust as cold reheat (CRH) is reheated to MS temperature and exits the reheater as hot reheat (HRH). This HRH expands in an intermediate pressure turbine (IP turbine) followed by a low pressure turbine (LP turbine). The LP turbine exhaust is condensed in condenser and this condensate is heated in three LP heaters. Feed water from the exit of LP heaters is fed to the HP heater with the help of boiler feed pump and enters into the boiler circuit. The schematic diagram of the plant is shown in figure 1. Figure 1: Power Plant Scheme In the boiler circuit, pulverized coal is burnt with air, the resulting combustion gases are cooled to heat the working fluid. Gases exit the airpreheater (APH) to chimney. Reheater (RH) is arranged in between two stages of superheater (SH), viz., panal SH and platen SH, as per the flue gas path. Working fluid path in boiler circuit is as follows: Economizer, Evaporator, Low Temperature Super Heater (LTSH), Panel SH, and Platen SH. In total six bleeds are taken from the turbines. The bleed pressures and corresponding destinations are given: 0.29 bar LPH bar LPH bar LPH bar Deaerator (DA) bar HPH bar HPH6 The sixth bleed is extracted from HP turbine exhaust, the bleeds to LP heaters are extracted from LP turbine and the remaining bleeds are extracted from IP turbine.

4 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies Fuel The fuel used in the boiler is coal. The ultimate analysis is given in the table 2. Table 2: Coal Ultimate Analysis [10] Element % by weight Carbon Hydrogen 2.54 Oxygen 7.05 Nitrogen 0.68 Sulphur 0.45 Moisture 12 Ash 42 Lower heating value(kj/kg) Exergy (kj/kg) [4] IV. THERMODYNAMIC ANALYSIS A detailed energy analysis and exergy analysis is carried out for all major components of the power plant Assumptions The following assumptions have been made during the analysis is Kinetic and Potential energies are neglected, Various components operate at study state conditions, Isentropic efficiency of pump is 80%, Generator efficiency is 98%, Excess air is 20%, and The reference environment used is environment of Beahr [4]: 1 atm, 25 0 C Table 3: Reference Environment Composition Element Mole fraction (%) N O H 2 O 3.12 Ar 0.90 CO Calculations The energy flow is given by: where m is mass flow and H is the enthalpy. En = mδh The physical exergy of a material stream at a given state is given by: Ex ph = H- H 0 -T 0 (S-S 0 ) where H is the enthalpy and S is the entropy at the state; H 0 and S 0 are the values at the environment state. The chemical exergy of the component gases of the environment on the molar basis is given by: Ex ch =mrt ln (P 0 /P 00 ) where P 00 is the partial pressure of the component and R is universal gas constant. The chemical exergy of an ideal gas mixture on the molar basis is is given by: Ex ch,mix =m[ i X i e ch i + RT 0 i X i ln X i ] where Ex ch is the standard chemical exergy of the component with mole fraction X. Standard chemical

5 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 622 exergy is based on standard values of the environment temperature and pressure. The exergy transfer with work interaction is associated with work transfer rate or shaft power. Because exergy is defined as the maximum work potential, it is equivalent to exergy in every respect. The exergy transfer rate (Ex) connected with the heat transfer rate Q is given by: Ex = A ( 1-T 0 /T ) Q da Where A is the heat transfer area and T is the temperature at which the heat transfer occurs. Finally, The total exergy is given by: Ex = Ex ph + Ex ch The energy or first law efficiency η I of a system or system component is defined as the ratio of energy output to the energy input of system or system component, i.e. η I = (Desired output energy / Input energy supplied) The exergy or second law efficiency is defined as η II = (Desired output / Maximum possible output) i.e., η II = (Exergy of product / Exergy of fuel) The exergy destruction ratio is given by: y d = (Exergy destruction / Total exergy of fuel) The exergy loss ratio is given by: The energy loss ratio is given by: Boiler y l = (Exergy loss / Total exergy of fuel) En l = (Energy loss / Energy of fuel) Inlet air molar composition and combustion gas composition are given in table 4 and table 5. Table 4: Inlet Air Composition Table 5: Combustion Gas Composition

6 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 623 Table 6: Exergy and Energy Flows Across the Boiler Turbine Generator efficiency is assumed as 98%, with generator output as 500 MW. This makes the turbine output as MW = W. Table 7: Exergy and Energy Flows Across the Turbine Stream Pressure, Temperature, Mass flow rate., Exergy flow, Energy flow, Stream No. bar C kg/s MW MW 1 MS CRH HRH Exhaust LPH LPH LPH DA HPH HPH Condenser

7 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 624 Table 8: Exergy and Energy Flows Across the Condenser Stream No. Pressure, bar Temperature, C Mass flow rate., kg/s Exergy flow, MW Energy flow, MW Results and Discussions After a detailed analysis of the power plant, the Exergy balance sheet and Energy balance sheet are given in table 9 and table 10 respectively. Table 9: Exergy Balance Sheet Table 10: Energy Balance Sheet The plant exergy efficiency is 35.41% and energy efficiency is 38.14%. There is not much difference in exergy and energy efficiencies. According energy analysis, across the condenser kg/s water is circulated and its temperature is rising from C to C. Thus, the condenser is major source of energy loss. According exergy analysis, about one third the exergy entering is lost in the combustion process and half in the boiler. Thus, the boiler is major source of exergy loss. V. CONCLUSIONS The exergy techniques presented in this paper provide a powerful and systematic tool for identifying the location, magnitude and source of real thermodynamic losses in an energy system. It is always recommended to go for an exergy analysis after an energy analysis. More than half of the exergy entering the plant is lost in boiler. Turbine and condenser have relatively very low losses compared to the boiler. The reasons for the high destruction in the boiler are: Combustion process is highly irreversible. Steam is operated in subcritical conditions, therefore temperature difference between steam and the combustion gas is high, which increases the irreversibility due to heat transfer.

8 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 625 Effect of excess air and inlet air temperature on boiler exergy efficiency is shown in figure 2. Figure 2: Effect of Excess Air and Inlet Air Temperature on Boiler Exergy Efficiency Some suggestions regarding the boiler are: Excess air may be reduced to improve efficiency. Heat transfer is much dependent on materiel properties, therefore material improvements may be considered to improve heat transfer performance. Working fluid operating in supercritical conditions is recommended as it decreases the temperature difference between steam and the combustion gas, and enhances the heat transfer. Here are some conclusions for the overall plant: Use of supercritical technology is strongly recommended as it not only improves the boiler performance, but also the performance of the turbine due to the enhanced steam parameters and high pressure ratio across the turbine. It is very important to identify which irreversibilities can be avoided and which cannot be. e.g, although combustion process is the major source of loss, it cannot be avoided. Also, it is important to identify whether the components are sensitive to the changes wish to be made. Use of cogeneration is always recommended wherever feasible. REFERENCES [1] N Arai, H Taniguchi, K Mouri and T Nakahara., Exergy Analysis on Combustion and Eenergy Conversion Process, Energy, Vlo. 30, No. 2-4, pp , [2] Bejan A., Tsatsaronis G., Moran M., Thermal Design and Optimization, Wiley ( 1996 ). [3] Cornelissen R L., Thermodynamics and sustainable development: The use of exergy analysis and reduction of irreversibility, Ph.D Thesis, University of Twinte, The Netherlands, [4] Cycle- Tempo release 5.0, Delft University of Technology, [5] Dincer I and Cengel Y A., Energy, Entropy and Exergy Concepts and Their Roles in Thermal Engineering, Entropy 2001, 3, [6] Moran M J and Shapiro H N., Fundamentals of Engineering Thermodynamics, 4th Ed., Wiley (2004). [7] Ministry of Power, Government of India, [8] 8.M V J J Suresh, K S Reddy and Ajit Kumar Kolar, Energy and Exergy Analysis of Thermal Power Plants Based on Advanced Steam Parameters, Advances in Energy Research (AER 2006). [9] M V J J Suresh, K S Reddy and Ajit Kumar Kolar, Energy and Exergy Based Thermodynamic Analysis of a 62.5 MW Coal-Based Thermal Power Plant A Case Study,presented at International Conference on Energy and Environment, August 28-30, Malasiya, [10] KTPP, stage I, APGENCO oparating data, Private communication. [11] Tsatsaronis G., Thermoeconomic analysis and optimization of energy systems, Prog. Energy Combust. Sci., 19, 1993 pp

9 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 626 [12] Wall G (1997), Exergy A uesful Concept within Resource Accounting, Institute of Theoretical Physics, Goteborg, Report No [13] Wall G (1986), Exergy A uesful Concept, Ph.D. Thesis, Chalmers University of Technology, S-412 Goteborg, Sweden, [14] Wall G and Gong M., On Exergetics, Economics and Optimization of Technical Processes to meet Environmenalt Conditions, Presented at TAIES 97, International Conference on Thermodynamic Analysis and Improvement of Energy Systems, Beijing, China, June 10-13, [15] Wall G and Gong M., On exergy and sustainable development Part 1: Conditions and Concepts, Exergy Int. J. 1(3) (2001) [16] Kotas T J (1995). The Exergy Method of Tharmal Power Plant Analysis,Reprint Ed., Krieger, Malabar, Florida. [17] Kamate S C and Gangavati P B., Exergy Analysis of Cogeneration Power Plants in Sugar Industries, Applied Thermal Engineering, Vol.29 No. 5-6, 2009, pp [18] Luo Y X and Wang X Y., Exergy Analysis of Throttle Reduction Efficiency Based on Real Gas Equations, Energy, Vol. 35, No. 1, 2010, pp [19] Boelman E C., and Asada H., Exergy and sustainable building, Open House International, vol.28, no. 1, pp [20] Atta A, Ganguly R and Sarkar L., Energy and Exergy Analysis of an Externally Fired Gas Turbine Cycle Integrated with Biomass Gasifier for Distributed Power Generation, Energy, Vol. 35, No. 1, 2010, pp