Energy Analysis of Omotosho Phase 1 Gas Thermal Power Plant

Transcription

1 International Journal of Engineering & Technology Sciences (IJETS) 1(4): , ISSN Academic Research Online Publisher Research Article Energy Analysis of Omotosho Phase 1 Gas Thermal Power Plant Egware H.O a and Obanor A.I b* a,b, Department of Mechanical Engineering, University of Benin, P.M.B 1154, Benin City, Nigeria *Corresponding author henryegware@yahoo.com,aiobanor@yahoo.com. A b s t r a c t Keywords: Energy analysis, Efficiency, Power output, Gas Turbine, Ambient temperature, Performance. Accepted:15 July Many managers of electrical generating utilities are striving to improve the efficiency, heat rate and performance of their existing thermal power stations. The thermal performance of the plant has to be investigated to achieve this aim. This paper describes the use of energy analysis in evaluating the performance of Omotosho Phase I Gas Thermal Power Plant. The data used were obtained from data record book and discussions with personnel during visit to the plant. The efficiency of plant was calculated at various load levels and ambient temperature. The average overall efficiency of the units was found to be 30.28% at ambient temperature range of C and power output of MW. This shows that power output and overall efficiency decreases as ambient temperature increases and 66.98% of fuel input is heat lost through the flue gases to the environment was the largest source of energy loss in the turbine units. Based on the results obtained compressor air intake cooling and combined cycle systems are recommend to optimize thermal performance of the plant. Academic Research Online Publisher. All rights reserved. 1. Introduction Gas turbines can be started and stopped easily which make them every useful at peak period in energy demand. Gas turbines are widely used for electricity generation in Nigeria because of the availability and low prices of natural gas compared to distillate fuels in the country [1]. For these reasons they are used are based load units. Energy analysis need to be carried out to know sources of losses during conversion of energy process in the plant and factors affecting the performance of the plant for efficient used of gas turbine power plant[2]. Energy analysis is the traditional method of assessing the way energy is used in an operation involving the physical or chemical processing of materials and the transfer and/or conversion of energy [3, 4]. This usually entails performing energy balances, which are based on the First Law of

2 Thermodynamics (FLT) and evaluating energy efficiencies. This energy balance is employed to determine and sometimes to enhance waste and heat recovery. Energy analysis of heat and power cycles are today the most common way of evaluating thermal systems regarding, for instance, fuel utilization and electrical efficiency. Throughout the whole 20th century, energy analysis matured and is today considered a well-established tool to evaluate thermal systems [5, 6]. Energy analysis is founded on the first law of thermodynamics, and together with the continuity equation over the system and its components, this type of analysis becomes a powerful method [7, 8]. The major aim of an energy analysis is to optimize the thermal efficiency of a system. The thermodynamic cycle upon which all gas turbines operate is called the Brayton cycle [9].The major components of a gas turbine are compressor, combustors, turbine and generator. Some researchers have carried this analysis in Nigeria such as [10, 11] in their works. Both authors did not consider the work ratio and effect of ambient temperature as it affect power out and efficiency of the plants used for their analyses, which are include in this work. Research has shown that turbine Power out and efficiency decreases with increase in compressor inlet temperature [12, 13, 14, and 15].This paper studies energy analysis Omotosho Phase I Thermal Power plant. 2. Methodology 2.1. Data Collection Data used for this study were collected from the power station s log book and Omotosho Power Station Phase I Final Report[16,17], such as average daily power generated, mass flow rate, pressure and temperature. Relevant plant and working fluid parameters were obtained from appropriate thermodynamics tables [18, 19] 2.2.Omotosho Phase I Thermal Power Station The Omotosho power station is located in Omotosho town, Ondo State in south west region of Nigeria. Phase I was completed in November 2008 and currently operated by Power Holding Company of Nigeria (PHCN). Phase I has an installed generating capacity of 335 MW (8 X41.875MW). It utilizes natural gas as its combustion fuel which it gets directly through piping networks from Nigeria Gas Company (NGC); in conjunction with Shell Petroleum Development Company (SPDC) and Chevron Nigeria Limited. 207 P age

3 The phase I power station consists of eight (8) gas turbine units each having an installed generating capacity of 42.1 MW. The units GT1, GT2, GT3, GT4, GT5, GT6, GT7 and GT8 are so named in the order of their arrangement in the power plant. Each gas turbine unit is fired with natural gas. The eight turbines are arranged parallel in a single turbine hall, each gas turbine drives a generator with an output voltage of 10.5 kv which is stepped up to a terminal voltage of 330 kv, and is fed into the National Grid. The gas turbine shaft speed of 5163 revolution per minute (rpm) is reduced to 3000 rpm of the generator by a speed reducer. 2.2.Thermodynamic Operational Principle of the Plant It operates on open Brayton cycle. Schematic and temperature entropy (T s) diagrams are shown in Figures 1 and 2 respectively. When the plant is started for the first time, an electric motor is used to run the axial flow compressor until it reaches 60% of the turbine shaft speed. The compressor uses 60% of the turbine shaft work to run itself, so the maximum thermal efficiency is 40%. When the turbine starting system is actuated, ambient air is drawn through the air intake plenum assembly, filtered at point 1and is compressed in the 17 stage - axial flow compressor. For pulsation protection during start up, the 11 th stage extraction valve is opened and the variable inlet guide vanes are in the closed position. At 95% speed, the extraction bleed valves closes automatically and the compressed air from the compressor flows into the annular space surrounding the ten combustion chambers at point 2 through which it flows into the combustion liners. It enters the combustion zone through metering holes in each of the combustion liners for proper fuel combustion. Fuel is provided to 10 equal flow lines, each terminating at a fuel nozzle. The nozzles introduce the fuel into the combustion chamber at a rate consistent with the speed and load requirements of the gas turbine. The introduced fuel mixes with the compressed air and is ignited using one or both of the spark plugs. At the instant when the fuel mixture is ignited in one combustion chamber; flame is propagated through cross-connecting fire tubes to all other combustion chambers. When the turbine rotor approximates operating speed, combustion pressure causes the spark plugs to retract thus removing their electrode from the hot flame zone. It is designed for proper dilution and cooling. The hot gas from the combustion chamber expand into 10 separate transition pieces attached to aft end of the chamber liners and flow from there into the 3 stage turbine of the plant at point 3. In each nozzle of the turbine the kinetic energy of the hot gases is increased, with associated pressure drop in each following row of moving buckets. A portion of the kinetic energy of the hot gas jet is converted to useful work on the turbine rotor. 208 P age

4 After passing through the third stage buckets, the gases are directed into the exhaust hood and diffusers, which contains a series of turning vanes to turn the gases from axial direction to radial direction with minimum exhaust hood losses. At point 4 the gases are passed into the exhaust plenum and are introduced to the atmosphere through exhaust stack. Some of the work developed by the turbine is used to drive the compressor as stated before, and the remainder is available for useful work at the output flange of the gas turbine, where its speed is reduced by a speed reducer to 3000 rpm of the generator. The generator converts the mechanical energy to electrical energy. Fig.1: A Schematic Diagram of Omotosho Phase I Thermal Power Plant T 3 2ʹ 2 4ʹ 4 1 Fig.2: The T- s Diagram of Omotosho Phase I Thermal Power Cycle s 209 P age

5 2.4. Gas Turbine Plant Parameters The design data from the main plant and auxiliaries are presented in Tables 1 and 2. They are manufacturer specification for the plant. Table 1: The Manufacturer Specification and Ratings Data for the Gas Turbine Units. Type of fuel Natural gas Lower Heating Value (LHV) kj/kg Compressor pressure ratio, p 2 /p :1 Generator out put power, P electrical 42.1 MW Specific heat capacity of water, c pw 4.2 kj/kgk Specific heat capacity of air c pa kj/kgk Specific heat capacity of exhaust gas, kj/kgk c pexh Table 2: Power Consumption by Auxiliaries Seal oil pump (kw) 7 Lube oil cooling fan (kw) 20 Load compartment vent fan (kw) 7 Turbine compartment vent fan (kw) 7 Torque Adjuster (kw) 1 Cooling tower water fan (kw) Cooling water pump motor (kw) 40 Close cooling water pump motor 27.5 (kw) 2.5. Energy Analysis Equations The Equations (1-16) using the energy analysis are obtained from [2, 6, 9, 10, 20, 21] based on Figure Thermal power Work done by compressor is given by Wc = ṁ a c pa (T 2 T 1 ) (1) Work done by turbine is given by W T = ṁ exh c pexh (T 3 T 4 ) (2) Thermal Power, P thermal P thermal = W T Wc (3) 210 P age

6 Mechanical Losses The friction losses at the bearings are transformed to heat which is removed by the lubricating oil and dissipated in the oil coolers. Mass flow rate of lube oil, ṁ o = lube oil volume flow rate x Density of water Heat removed from the bearing at the oil cooler, Q o Q o = ṁ o c po (T o2 T o1 ) (4) Generator Losses The generator losses are translated to heat that is removed initially by air, the heat is then passed to the cooling water, which is then dissipated in water coolers. Mass flow rate of cooling water, ṁ w = cooling water volume flow rate specific volume (5) Heat removed from the generator at the water cooler, Q w Q w = ṁ w c pw (T w2 T w1 ) (6) Flue Gas Losses Flue gas losses, Q exh = ṁ exh c pexh (T 4 T 1 ) (7) Power consumed by Auxiliaries, Paux P aux = Total of the power consumed by the auxiliary devices (8) Electrical Power Generated, P electrical P electrical = P thermal P mech losses P gen losses P aux (9) Mechanical Efficiency, ɳ m ɳ m = P thermal P mech losses P thermal (10) Generator Efficiency, ɳ g ɳ g = P thermal P mech losses Pgen losses P thermal P mech losses (11) Thermal Efficiency, ɳ thermal 211 P age

7 ɳ thermal = P thermal ṁ f X LHV (12) Overall Efficiency, ɳ o ɳ o = P electrical ṁ f X LHV (13) Specific Fuel Consumption (SFC) SFC = ṁ f X 3600 P electrical (14) Heat Rate (HR) HR = SFC X LHV (15) Work Ratio (WR) WR = P thermal W T (16) 3. Results and Discussion 3.1. Performance Data of Gas Turbine Units The data used for this analysis were recorded values from the station log book from January 2008 to December 2011[17]. They are recorded at various loads, ambient temperature and other parameters during when each of the gas turbine units operate at normal conditions (When the plant operates without any identifiable mechanical fault). A summary of the operating conditions and parameters for this period is shown in Table 3. Table 3:Actual Performance Data for Gas Turbine Power T 1 T 2 P 2 (Bar) ṁ a (kg/s) ṁ f (kg/s) ṁ exh (kg/s) T 3 T 4 T w1 T w2 T o1 T o The performance of the gas turbine is calculated using Equations (1 16) from the values in Tables 1-3.The results of these calculations are summarized in Tables 4.The power out and overall efficiency 212 P age

8 was plotted against ambient temperature of various GT as shown in Figure 3. The energy flow diagram is represented as Sankey diagram in Figure 4. Table 4: Performance of the Gas Turbine T 1 Power Energy Input Thermal Power Turbine WorK mech. Losses Gen. Losses Aux. Losses Flue Gas Losses Mech,ɳ m (%) Gen,ɳ G (%) Thermal, ɳ Thermal (%) Overall,ɳ o (%) SFC (kg/kwh) Heat Rate (kj/kwh) Work Ratio Power output and Overall efficiency Ambient teperature Power Overall,ɳo (%) Fig.3: Change in Power out and overall efficiency with ambient temperature for the Gas Turbine 213 P age

9 Fuel in MW Compressed air MW Air Combustor MW Turbine Mech losses MW Flue gas, stack and Other losses MW Gen losses MW Aux. losses MW Electrical Power to Grid MW Fig.4: Energy flow diagram for Omotosho Phase I 3.2 Discussion Data for the actual performance of are available for power outputs at MW and temperature range of 21 0 C 33 0 C. Data for the design performance was available only for the rated output of 42.1 MW at International Standard Organization (ISO) condition. 214 P age

10 The overall efficiency and power output was plotted against the ambient temperature for each of the GT as shown in Figures 3. The average overall efficiency value for the Omotosho Phase I is 30.28%. From Figure 3, it was observed that as the compressor inlet temperature increases, as the power output and overall efficiency decreases, which agrees with the statement stated in [12-15]. The highest overall efficiency was 32.40% observed at a temperature of 21 0 C and a corresponding power output of MW, which is also the highest power output recorded during this period. The lowest overall efficiency of 28.73% was observed at a temperature of 33 0 C and a corresponding power output of 31.00MW. The performance curves for the GT as shown figure reveal that Overall efficiency and net power output decreases with increase compressor inlet temperature. The design power output of 42.1MW was not achieved because the plant was designed at ISO condition temperature of 15 0 C, an ambient temperature that does not usually occur in the local environment without inlet air cooling. From the performance values in Tables 4, it can be seen that as the compressor inlet temperature increases the specific fuel consumption increases and heat rate increases while work ratio decrease. Increase in SFC and HR confirm what was stated in [13]. The decrease in work ratio reduces the effectiveness of the plant because a high work ratio is less susceptible to irreversibility than low a work ratio. The mechanical losses were relatively constant and were not dependent on power output; this can be attributed to the fact that the turbine speeds are constant irrespective of power output since frictional losses are proportional to turbine speed as obtained in [10,11 ]. The generator losses depend on the power factor at which the output is being generated. The least generator loss will occur when the power factor is unity. The power lost to the auxiliaries is constant since a good number of the required auxiliaries are operational irrespective of the power output level. The 66.98% of fuel input is largest heat loss from exhaust of flue gas in the plant to environment as shown in Figure Conclusion and Recommendation The efficiencies of Omotosho Phase I gas turbine units studied in this analysis were seen to decrease with increase in ambient temperature. At load levels in the range of MW and corresponding ambient temperature of C, the power output decreases as the ambient temperature into the compressor increases. The average overall efficiency was found to be 30.28% for all units. 215 P age

11 The heat lost through the flue gases at a maximum exhaust temperature of C to the environment was found to be the largest source of energy loss in the turbine units. The results achieved from calculations also reveal that the specific fuel consumption, heat rate increases and power out and efficiency decreases with increase in ambient temperature. The work ratio decreases with increase in ambient temperature, which shows that the plant is susceptible to high irreversibility at high compressor inlet temperature. Based on the results obtained compressor air intake cooling and combined cycle systems are recommend to optimize thermal performance of the plant. Gas turbine plants should be designed using the local weather data of the country where they are to be installed. Acknowledgments The authors wish to acknowledge the contributions of the members of staff of Omotosho Phase I Thermal Power Station for their invaluable assistance when it mattered. References [1] Abam FI,Ugot IU, Igbong DI. Performance Analysis and Components Irreversibilities of a (25 MW) Gas Turbine Power Plant Modelled with a Spray Cooler, American Journal of Engineering and Applied Sciences 2012; 5(1): [2] Egware HO. Exergy Analysis of Omotosho Phase I Gas Thermal Power Station (Unpublished M.Eng Project), University of Benin, Nigeria,. [3] Dincer J, Rosen M A. Exergy, Energy, Environment and Sustainable Development, Exergy Hand Book 1 st edition, Pdf file ;( 3) 2007 [4] Moran MJ, Shapiro HN. Fundamentals of Engineering Thermodynamics, 3 rd edition, John Wiley & Sons,; [5] Coplan CO. Exergy Analysis of Combined Cycle Cogeneration Systems (M.Sc Thesis), Department of Mechanical Engineering, Middle East Technical University; [6] Cengel YA, Boles MA. Thermodynamics An Engineering Approach,5 th edition, McGraw Hill, Boston,; 2006 [7]Jassim RK, Zaki GM, Alhazmy MM. Energy and Exergy Analysis of Reverse Brayton Refrigerator for Gas Turbine Power Boosting, International Journal of Exergy 2009; 6 (2): [8]Abam DSP, Moses NN. Computer Simulation of Gas Turbine Performance, Global Journal of Researches in Engineering 2011; 11, (1),Version 1.0. ISSN: [9] Saravanamuttoo H, Roger GFC, Cohen H, Straznicky PV. Gas Turbine Theory, 6 th edition, Pearson Education Ltd, England; 2009 [10] Ighodaro OO. Performance Appraisal of Delta IV Power Station Ughelli (M.Eng Thesis), University of Benin, Nigeria; P age

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