Analysis of Effects of Evaporative Inlet Cooling on Gas Turbines

Transcription

1 Analysis of Effects of Evaporative Inlet Cooling on Gas Turbines Ranu Rajoria 1, Arif khan 2* 1 Assistant Professor, AIRT,Kaiload Kartal Bapass Road, Indore, MP, India 2 Research Scholar, Department of Mechanical Engineering AIRT, Indore, MP, India ABSTRACT This paper deals with the analytical study of a gas turbine power plant incorporated with an evaporative inlet air cooler. The study is performed on four different configurations of the, each one with the evaporative inlet air cooler. The gas turbine is best suited for the peak hour power generation and is less expensive to implement, efficient, cleaner than most of other alternatives and the potential to be integrated as a combined power plant further, are some of the features which clearly makes it one of the better choices for power generation. Variation of ambient air temperature on the performance of gas turbine (efficiency, specific output) are investigated along with the mass flow rate of water in the pump of the evaporative inlet cooler, the pump work for it is also investigated in the present analysis. The programming of the variation of specific heat capacities of air in the compressor stage wise and of the flue gas in the turbine stage wise is developed by the MATLAB software Keywords: - Reheat, Regenerative, MATLAB software INTRODUCTION: Power generation using gas turbines provides flexibility to augment the power output during periods when high tariff rates is paramount importance, in the backdrop of continuous increase in fuel prices and the perpetual growth in energy demands.(11) A gas turbine is less expensive to implement, efficient, cleaner than most of other alternatives and the potential to be integrated as a combined power plant further, are some of the features which clearly makes it one of the better choices for power generation. It is only in the recent past many research and developments have been done in the designing of efficient gas turbine. The gas turbine proves to be more advantageous power plant in the league of various other means of producing mechanical power. The reasons behind it are: No reciprocating and rubbing members, hence reduced vibration and balancing problems, Highly reliable, High power - to - weight ratio. In gas turbine power plant, Brayton Cycle is the practical working The components of a simple gas turbine are, compressor, combustion chamber and the turbine to which generator is coupled. The effect of the inlet air temperature on the is well known, the temperature and humidity ratio at the inlet of the compressor affects the net efficiency and the plant heat rate. Configurations of gas turbine : Simple gas turbine Intercooled Reheat Regenerative and Various other configurations including above all or some of them according to the need History of gas turbine : Man has learned the use of power from mechanical means quite a long time ago. Before that the primary source of power, or prime mover was chiefly man s muscles. After that animals were trained to help with. They also learned to harness the wind and running streams, but still were mainly depended on muscle power. The world got revolutionized when man learned to convert the heat of chemical reactions onto mechanical energy. Hence heat engines came into existence. Low compressor efficiency, and Temperature limitations of the materials available at that time. Objectives: The main objectives of the research are as following: To perform the analysis of evaporative inlet cooler and evaluate the pump work,(7) To perform the thermodynamic analysis (First law) of four configurations of gas turbine viz. simple, intercooled, reheat and regenerative. ISSN: Page 57

2 RESEARCH METHODOLOGY: This study deals with the thermodynamic analysis of gas turbine with an evaporative inlet air cooling. The present parametric study of the gas turbine has been carried out by modeling its various elements and deriving governing equations. In this chapter the modeling of various elements of the gas turbine is considered. 1. Gas turbine : The gas turbine in many respect is the most satisfactory means of producing mechanical power among the others. Further the economics of power generation by a gas turbine is now quite captivating due to its low capital cost, it is compact as compared to water and steam turbines and its high reliability and flexibility in operation. 2. Gas turbine modeling: A Brayton with four different configurations has been considered for this present analysis.(12) An evaporative inlet air cooler is incorporated with the compressor for each configuration to cool the air at the inlet of the compressor and a difference of 10 C between the ambient temperature and the temperature at the compressor inlet, i.e. in each case it is considered that the inlet evaporative cooler cools the ambient air to 10 C. Real gas effect is considered i.e. specific heat of gas vary with respect to the temperature and specific gas constant R = kj/kg-k. The gas turbines s taken here are open type ideal s and all the processes in the are steady flow. Compression and expansion processes are adiabatic and changes in K.E and P.E. are negligible. Fuel taken was natural gas with kj/kg-k as calorific value. The ambient pressure was taken as 1 bar = 10 5 Pa. Heat capacity ratio of ambient air (γ) is taken as 1.4. The pressure ratio is taken as 10 and for staging in the compressor, stage pressure ratio ( has been taken as 1.2. Pressure loss is considered in the combustion chamber and in the ducts comprising a total of 4%. 3. Gas turbine modeling: The gas turbine includes modeling of following components Axial flow compressor, Combustion chamber, Axial flow gas turbine 4. Compressor modeling: In the present analysis axial flow compressor is considered. In an axial flow compressor the flow is in axial direction of the machine, as the name suggests. The compressor s prime motive is to increase the pressure of the fluid passing through it. An axial flow compressor consists of an alternating sequence of fixed and moving blades known as stators and rotors respectively. The stator are fixed blades and are spaced around stationary casing, whereas the rotors are set of moving blades and are fixed to the rotating spindle. A row of moving blades with succeeding set of stationary is known as a stage of the compressor. Whenever air flows through this stage the energy of air is increased by the action of rotor blade which exerts torque on the fluid. This torque is provided by an external source. 5. Combustion chamber modeling: burned. The combustor burns large quantity of fuel in the presence of compressed air provided by the compressor. It produces in turn hot gases which then is supplied to gas turbine. Chemical energy is converted into heat energy in the combustor, which is later Combustion chamber or simply combustor in a gas turbine is the part where fuel is converted into work by the turbine. The process involves breaking down of heavy hydrocarbon molecules into lighter fraction molecules. Generally air-fuel ratio of open type gas turbines varies from 50:1 to 200:1 to get efficient combustion and keep the turbine inlet temperature down to permissible limit. Combustors are carefully designed first to mix and ignite the air and fuel and then mix in more air to complete the combustion process. Now a days three main configurations of the combustor exists: can, annular and can-annular. Natural gas (methane) is used in the combustion chamber as the fuel which results in less carbon emission. It is burnt in the presence of compressed air from the compressor and hereafter the working fluid of the changes from air to gas. The composition of natural gas by mass are considered as 0.94CH C 2 H N 2 6. Gas turbine modeling: In the modeling an axial flow gas turbine is considered. Expansion of the gas from high pressure to low pressure produces the work. Mostly turbines possess more than one stages. The stage consists of a ring of fixed nozzle blades followed by rotor blade ring. The turbine stages are classified normally as an impulse stage and a reaction stage. In the present ISSN: Page 58

3 model reaction stage is considered in which expansion of the gas takes place both in the stator and in the rotor. Two functions occurred in the rotor, i.e. it converts the kinetic energy of the gas into work and contributes a reaction force on the rotor blades. For the evaluation of the gas turbine work output, the assumptions and expressions used are: The gas turbine used is of axial flow type, The thermodynamic losses in an axial flow gas turbine are incorporated in the model by bringing in the concept of polytrophic efficiency. 7. Gas modeling: The composition of air by mass is considered as 77.44% N % O % Ar % H 2 O % CO 2 The specific heat of air varies with temperature and also with pressure in actual conditions, if the pressure is very high. In the present configuration however, it is assume that specific heat of air varies with temperature only. The specific heat of air at constant pressure has been adopted from The temperature ranges for the specific heat of air is considered is: For the temperature range of 250 to 599K and valid up to 100 bar, C P,a = T T T 3 For the temperature range of 600 to 1500 K and valid up to 100 bar, C P,a = T T T 3 In the combustion chamber fuel is burnt in the presence of air and hot gases are produced. In this study the specific heat of gas varies with the variation in the temperature. The specific heat of gas at constant pressure has been adopted from [25]; For temperature range of K and valid up to 100 bar, A= B= 8. Inlet evaporative cooler modeling: The present analysis deals with the evaporative type inlet air cooler. (1)Using sprays to sprinkle fine mist of water through which air is passed, as a result air drops temperature to wet bulb temperature. Both the factors, ambient temperature and relative humidity dictates the temperature of the air coming out of the cooler. Thus the evaporative inlet air cooler works as an adiabatic saturator that delivers air to the compressor with 100% relative humidity at lowered temperatures. RESULTS AND DISCUSSION Table - Simple gas, Intercooled, Reheat,. Regenerative, find the without Inlet Air Cooling Ambient temp. ( C) Simple gas Efficienc y Intercoole d. Reheat Efficienc y Regenerati ve B= C P,g = For the temperature range K and valid up to 100 bar, A= Variation of efficiency without evaporative inlet cooling shows the variation of the efficiency with respect to the ambient temperature without evaporative inlet cooling. The efficiency of the regenerative is more than the other three efficiencies. With rise in temperature the efficiencies of simple, intercooled ISSN: Page 59

4 and the reheat s remains almost the same whereas regenerative shows slight decrement and reaches almost equal to the simple. As the temperature at the inlet to the compressor increases the density of air decreases accordingly and to maintain the constant turbine inlet temperature the required input to the combustion chamber increases. Table - Simple gas, Intercooled, Reheat,. Regenerative, find the with Inlet Air Cooling Ambi ent temp. ( C) Inlet air temp.( C) Simple gas Efficie ncy Intercoole d. Reheat Efficie ncy Regenerati ve evaporative inlet cooling. The following conclusions have been drawn from the work: Performance curves shows that the pump work for the evaporative inlet cooler is in accordance with the mass flow rate of water, Performance curves shows that the specific output of the s slightly increases at evaporative inlet cooling. ACKNOWLEDGEMENT: The author is highly grateful to the Principal of Astral Institute Of Technology, & Research Indore for providing this opportunity to carry out the present work. The author is thankful to Prof. Ms.Ranu Rajoria (Head of Department of Mechanical Engineering, AITR Indore) who has been of great help in the conclusion of present work. The author would like to express a deep sense to gratitude and thanks to Ms.Ranu Rajoria, Assistant professor, Department of Mechanical Engineering, AITR, who as the supervisor provided wise counsel and able guidance. The author has to acknowledge with thanks all other staff members of the Mechanical Engineering Department for extending the helping hand to complete this work. I would thank all people not listed above, who have contributed in this research work. REFERENCES: Variation of efficiency with evaporative inlet cooling shows the variation of the efficiency with respect to temperature at the compressor inlet with evaporative inlet cooling. Here also with rise in temperature the efficiencies of simple, intercooled and the reheat s shows slight increment as compared to the previous i.e. without inlet evaporative whereas regenerative shows slight decrement but not as much in the previous one. As the temperature at the inlet to the compressor increases the density of air decreases accordingly and to maintain the constant turbine inlet temperature the required input to the combustion chamber increases. CONCLUSION: The present model involves the analysis of four configurations of the gas turbine with [1] Alhazmy Majed M., Jassim Rahim K., Zaki Galal M., Performance Enhancement of Gas Turbines by Inlet Air-Cooling in Hot and Humid climates, International Journal of Energy Research, Vol.30,pp ,2006. [2] Johnson Rony. Chand Susheel, Turbine Inlet Air Cooling A Powerful Means to Enhance Performance of Power Plants, International O&M Conference on Indian Power Stations, [3] Bhargava R., Bianchi M., Melino F., Peretto A., Parametric Analysis of Combined Cycles Equipped With Inlet Fogging, ASME, Vol.128,pp ,2006. [4] Ameri M., Hejazi S.H., The Study of Capacity Enhancement of the Chabahar Gas Turbine Installation Using an Absorption Chiller, Applied Thermal Engineering, Vol.24, pp.59-68, [5] Kakaras E., Doukelis A., Karellas S., Compressor Intake Air Cooling In Gas Turbine Plants, Energy, Vol.29,pp , [6] Ibrahim K.T., Rahman M.M., Abdalla A.N., Optimum Gas Turbine Configuration for Improving the Performance of Combined Cycle Power Plant, Procedia Engineering, Vol.15, pp , [7] Santos A.P., Analysis of Gas Turbine Performance With Inlet Air Cooling Techniques Applied to Brazilian Sites, J. Aerospace. Technol.Manag., Vol.4(3), pp , [8] Boonnasa S., Namprakai P., Muangnapoh T., Performance Improvement of Combined Cycle Power Plant by Intake Air Cooler Using Absorption Chiller, Energy, Vol.31, pp , ISSN: Page 60

5 [9] Mohanty, B., Palso G., Enhancing gas turbine performance by intake air cooling using an absorption chiller, Heat recovery systems and HPC, Vol.15, pp.41-45, [10] Gareta, R., Romeo, L.H., Gil, A., Methodology for economic evaluation of gas turbine air cooling systems in combined power plant, Energy, Vol.29, pp , [11] Dawoud, B., Zurigat, Y.H., Bortmany, J., Thermodynamic assessment of power requirement and impact of different gas turbine inlet air cooling techniques at two different locations in Oman, Applied Thermal Engineering, Vol.25, pp , [12] Al-Amiri, A.M., Zamzam, M.M., systematic assessment combustion turbine of inlet air cooling techniques, Journal of Engineering for Gas Turbines and Power, Vol.127, pp , ISSN: Page 61