design of gas turbine power plant for university of lagos

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Design of Gas Turbine Power Plant for University of Lagos Saka Oluwadamilola 110404085 Mechanical Engineering University Of Lagos

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Design of Gas Turbine Power Plant for University of Lagos to replace the EKEDP power supply

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MEG401_Project_110404085

Design of Gas Turbine Power Plant for University of Lagos

Saka Oluwadamilola110404085

Mechanical EngineeringUniversity Of Lagos

A Class Project in:MEG401: TURBOMACHINERYEngr. Francis OnorohTable of ContentsDesign of Gas Turbine Power Plant for University of Lagos1I.ABSTRACT3II.NOTATION4III.INTRODUCTION5IV.THEORY7A.THEORY OF OPERATION7B.BRAYTON CYCLE8C.COMPONENTS OF A GAS TURBINE PLANT81.Compressor82.Combustion Chamber133.Gas Turbines154.Vortex Blading15D.GAS TURBINE FUELS16E.ADVANTAGES OF DISADVANTAGES OF GAS TURBINE POWER PLANT161.Advantages162.Disadvantages16F.TURBINE PERFORMANCE171.Turbine Power Output172.System Efficiency17V.UNILAG POWER CONSUMPTION REPORT18VI.POWER PLANT DESIGN19A.TURBINE DESIGN DATA19B.DESIGN ANALYSIS193.Velocity Diagram204.Velocities205.Work done216.Efficiency227.Mass flow rate23VII.POWER PLANT ECONOMICS24VIII.DISCUSSION OF RESULTS26IX.RECOMMENDATIONS27X.CONCLUSION28XI.REFERENCES29

ABSTRACTThe gas turbine engine is gaining acceptance throughout the world as a reliable, flexible and efficient power generation plant. The more advanced the technology, the higher the generated power and efficiency. This paper aims to design a gas turbine power plant to generate power for the University of Lagos community. This design is concerned with finding the particulars of the geometry, gas angles, flow velocities, work done and mass flow rate. The overall performance and stage efficiency are evaluated. The financial analysis of the plant is done, and corresponding costs are calculated. The cost per kWh is calculated and compared with the Nigeria Federal Government distribution cost per kWh. Finally, several recommendations are made for optimum performance of the gas turbine power plant.

NOTATIONCabsolute velocity, m/sCaaxial velocity, m/sCpspecific heat at constant pressure, kJ/kgKCvspecific heat at constant volume, kJ/kgKh enthalpy, kJ/kgl distance along blade span measured from root, mm mass flow rate, kg/sn number of bladesN rotational speed of blade row, rpmr radius, mU blade speed, m/sW specific power output, kW ratio of specific heatss stage efficiency, % degree of reaction density, kg/m3

INTRODUCTIONOf all the forms of energy, electricity is the easiest to produce, to transport, to use and to control. So, it is mostly the terminal form of energy for transmission and distribution. Electricity in bulk quantities is produced in power plants, which can be of the following types: (a) Thermal, (b) Nuclear, (c) Hydraulic, (d) Geothermal and (e) Gas turbine. Thermal, nuclear and geothermal power plants work with steam as the working fluid and have many similarities in their cycle and structure. A Gas Turbine is an engine that employs gas flow as the working medium by which heat energy is transformed into mechanical energy. The use of gas turbines for generating electricity dates back to 1939. Today, gas turbines are one of the most widely-used power generating technologies. Gas turbines are a type of internal combustion (IC) engine in which burning of an air-fuel mixture produces hot gases that spin a turbine to produce power. It is the production of hot gas during fuel combustion, not the fuel itself that the gives gas turbines the name. Gas turbines can utilize a variety of fuels, including natural gas, fuel oils, and synthetic fuels. Combustion occurs continuously in gas turbines, as opposed to reciprocating IC engines, in which combustion occurs intermittently. Gas turbines are comprised of three primary sections mounted on the same shaft: the compressor, the combustion chamber (or combustor) and the turbine. The compressor can be either axial flow or centrifugal flow. Axial flow compressors are more common in power generation because they have higher flow rates and efficiencies. Axial flow compressors are comprised of multiple stages of rotating and stationary blades (or stators) through which air is drawn in parallel to the axis of rotation and incrementally compressed as it passes through each stage. The acceleration of the air through the rotating blades and diffusion by the stators increases the pressure and reduces the volume of the air. Although no heat is added, the compression of the air also causes the temperature to increase. The compressed air is mixed with fuel injected through nozzles. The fuel and compressed air can be pre-mixed or the compressed air can be introduced directly into the combustor. The fuel-air mixture ignites under constant pressure conditions and the hot combustion products (gases) are directed through the turbine where it expands rapidly and imparts rotation to the shaft. The turbine is also comprised of stages, each with a row of stationary blades (or nozzles) to direct the expanding gases followed by a row of moving blades. The rotation of the shaft drives the compressor to draw in and compress more air to sustain continuous combustion. The remaining shaft power is used to drive a generator which produces electricity. Approximately 55 to 65 percent of the power produced by the turbine is used to drive the compressor. To optimize the transfer of kinetic energy from the combustion gases to shaft rotation, gas turbines can have multiple compressor and turbine stages. Because the compressor must reach a certain speed before the combustion process is continuous or self-sustaining initial momentum is imparted to the turbine rotor from an external motor, static frequency converter, or the generator itself. The compressor must be smoothly accelerated and reach firing speed before fuel can be introduced and ignition can occur. Turbine speeds vary widely by manufacturer and design, ranging from 2,000 revolutions per minute (rpm) to 10,000 rpm. Initial ignition occurs from one or more spark plugs (depending on combustor design). Once the turbine reaches self-sustaining speed above 50% of full speed the power output is enough to drive the compressor, combustion is continuous, and the starter system can be disengaged. Because of the power required to drive the compressor, energy conversion efficiency for a simple cycle gas turbine power plant is typically about 30 percent, with even the most efficient designs limited to 40 percent. A large amount of heat remains in the exhaust gas, which is around 600C as it leaves the turbine. By recovering that waste heat to produce more useful work in a combined cycle configuration, gas turbine power plant efficiency can reach 55 to 60 percent. However, there are operational limitations associated with operating gas turbines in combined cycle mode, including longer startup time, purge requirements to prevent fires or explosions, and ramp rate to full load.

THEORY

A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between. The basic operation of the gas turbine is similar to that of the steam power plant except that air is used instead of water. Fresh atmospheric air flows through a compressor that brings it to higher pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature flow. This high-temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure, producing a shaft work output in the process. The turbine shaft work is used to drive the compressor and other devices such as an electric generator that may be coupled to the shaft. The energy that is not used for shaft work comes out in the exhaust gases, so these have either a high temperature or a high velocity. The purpose of the gas turbine determines the design so that the most desirable energy form is maximized. Gas turbines are used to power aircraft, trains, ships, electrical generators, or even tanks. Examples of gas turbine configurations: (1) turbojet, (2) turboprop, (3) turboshaft (electric generator), (4) high-bypass turbofan, (5) low-bypass afterburning turbofan.THEORY OF OPERATION

In an ideal gas turbine, gases undergo three thermodynamic processes: an isentropic compression, isobaric (constant pressure) combustion and an isentropic expansion. Together, these make up the Brayton cycle. In a practical gas turbine, mechanical energy is irreversibly transformed into heat when gases are compressed (in either a centrifugal or axial compressor), due to internal friction and turbulence. Passage through the combustion chamber, where heat is added and the specific volume of the gases increases, is accompanied by a slight loss in pressure. During expansion amidst the stator and rotor blades of the turbine, irreversible energy transformation once again occurs. If the device has been designed to power a shaft as with an industrial generator or a turboprop, the exit pressure will be as close to the entry pressure as possible. In practice it is necessary that some pressure remains at the outlet in order to fully expel the exhaust gases. In the case of a jet engine only enough pressure and energy is extracted from the flow to drive the compressor and other components. The remaining high pressure gases are accelerated to provide a jet that can, for example, be used to propel an aircraft.

BRAYTON CYCLEAs with all cyclic heat engines, higher combustion temperatures can allow for greater efficiencies. However, temperatures are limited by ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand high temperatures and stresses. To combat this many turbines feature complex blade cooling systems. As a general rule, the smaller the engine, the higher the rotation rate of the shaft(s) must be to maintain tip speed. Blade-tip speed determines the maximum pressure ratios that can be obtained by the turbine and the compressor. This, in turn, limits the maximum power and efficiency that can be obtained by the engine. In order for tip speed to remain constant, if the diameter of a rotor is reduced by half, the rotational speed must double. For example, large jet engines operate around 10,000 rpm, while micro turbines spin as fast as 500,000 rpm. Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternative-rotor assembly (see image above), not counting the fuel system. However, the required precision manufacturing for components and temperature resistant alloys necessary for high efficiency often makes the construction of a simple turbine more complicated than piston engines. More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers. Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. These bearings are being surpassed by foil bearings, which have been successfully used in micro turbines and auxiliary power units.COMPONENTS OF A GAS TURBINE PLANTThe construction and operation of the components of a gas turbine plant are necessary for proper understanding and design.CompressorThe high flow rates of air through the turbine and the relatively moderate pressure ratios necessitate the use of rotary compressors. The types of compressor commonly used are the following. Centrifugal compressors Axial flow compressorsA centrifugal compressor consists of an impeller with a series of curved radial vanes as shown in Figure 1. Figure 1

Air is sucked in near the hub, called the impeller eye and is whirled round at high speed by the vanes on the impeller rotating at high rpm. The static pressure of air increases from the eye to the tip of the impeller. Air leaving the impeller tip flows through diffuser passages (scroll) which convert the kinetic energy to pressure energy (Figure 2). Figure 2

The compressors may have single inlet or double inlet. In a double inlet impeller having an eye on either side, air is drawn in on both sides (Figure 3). Figure 3

The impeller is subjected to approximately equal forces in the axial direction. About half the pressure rise occurs in the impeller vanes, and half occurs in the diffuser passages. If the air flow into the impeller eye is in the axial direction (Figure 1), the blade velocity diagram at inlet is shown in Figure 4(a). By using fixed guide blades, the inlet velocity to the impeller eye is inclined at an angle, known as pre-whirl (Figure 4(b)).Figure 4

At exit from the impeller the flow is in the radial direction and the blade velocity is larger, since the radius of the impeller is larger at outlet. The blade velocity diagram is shown in Figure 5(a) being the case of radially inclined blades and (b) being that of blades inclined backwards at an angle 2. Figure 5

The inertia of the air trapped between the impeller blades, however, causes the actual whirl velocity to be less than the ideal whirl velocity. It is known as slip.For low pressure ratios (less than 4/1) the centrifugal compressor is lighter and is able to operate effectively over a wide range of mass flows at any speed. Using titanium alloys pressure ratios above eight have now been achieved. For larger units with higher pressure ratios the axial-flow compressor is more efficient and is usually preferred. For industrial and large marine gas turbine plants axial compressors are normally used, although some units may employ two or more centrifugal compressors with intercooling between stages. Centrifugal compressors are cheaper to produce, more robust and have a wider operating range than the axial-flow type. An axial-flow compressor is similar to an axial-flow turbine with a succession of moving blades on the rotor shaft and fixed blades arranged around the stator (casing). Air flows axially through the moving and fixed blades, with diffuser passages throughout which continuously increases the pressure and decreases the velocity. Stationary guide vanes are provided at entry to the first row of moving blades (Figure 6). Figure 6

The work input to the rotor shaft is transferred by the moving blades to the air, thus accelerating it. The spaces between the blades as well as the stator blades from diffusing passages decreasing velocity and increasing pressure. There can be a large number of stages (5 to 14) with a constant work input per stage. An equal temperature rise in the moving and fixed blades is usually maintained. The axial velocity of air is also kept constant throughout the compressor. A diffusing flow is less stable than a converging flow as in a turbine and for this reason the blade shape and profile are more important for a compressor than for a reaction turbine. Typical blade sections of an axial-flow compressor are shown in Figure 7(a) and the corresponding velocity diagrams in Figure 7(b).Figure 7

Blades are usually of twisted section designed according to free vortex theory. Due to nonuniformity of the velocity profile in the blade passages the work that can be put into a given blade passage is less than that given by the ideal diagram.Combustion ChamberIn an open cycle gas turbine plant combustion may be arranged to take place in one or two large cylindrical can-type combustion chambers with ducting to convey the hot gases to the turbine. Combustion is initiated by an electric spark and once the fuel starts burning, the flame is required to be stabilized. A pilot or recirculated zone is created in the main flow to establish a stable flame which helps to sustain combustion continuously. The common methods of flame stabilization are by swirl flow and by bluff body. Figure 8

Figure 8 shows a can-type combustor with swirl flow flame stabilization. About 20 per cent of the total air from the compressor is directly fed through a swirler to the burner as primary air, to provide a rich fuel-air mixture in the primary zone, which continuously burns, producing high temperature gases. Air flowing through the swirler produces a vortex motion creating a low pressure zone along the axis of the combustion chamber to cause reversal of flow. About 30 per cent of total air is supplied through dilution holes in the secondary zone through the annulus round the flame tube to complete the combustion. The secondary air must be admitted at right points in the combustion chamber, otherwise the cold injected air may chill the flame locally thereby reducing the rate of reaction. The secondary air not only helps to complete the combustion process but also helps to cool the flame tube. The remaining 50 per cent of air is mixed with burnt gases in the tertiary zone to cool the gases down to the temperature suited to the turbine blade materials. Figure 8

Figure 9 shows a can-type combustor with a bluff body stabilizing the flame. The fuel is injected upstream into the air flow and a sheet metal cone and perforated baffle plate ensure the necessary mixing of fuel and air. The low pressure zone created downstream side causes the reversal of flow along the axis of the combustion chamber to stabilize the flame. Sufficient turbulence is produced in all three zones of the combustion chamber for uniform mixing and good combustion. The air-fuel ratio in a gas turbine plant varies from 60/1 to 120/1 and the air velocity at entry to the combustion chamber is usually not more than 75m/s. There is a rich and a weak limit of flame stability and the limit is usually taken at flame blowout. Instability of the flame results in rough running with consequent effect on the life of the combustion chamber. Because of the high air-fuel ratio used, the gases entering the turbine contain a high percentage of oxygen and therefore if reheating is performed, the additional fuel can be burned satisfactorily in turbine exhaust, without needing further air for oxygen.The term "combustion efficiency" is often used in this regard, which is defined as the ratio of "theoretical fuel-air ratio for actual temperature" to the "actual fuel-air ratio for actual temperature rise". Theoretical temperature rise depends on the calorific value of the fuel used, the fuel-air ratio and the initial temperature of air. To evaluate the combustion efficiency, the inlet and outlet temperatures and the fuel and air mass flow rates are measured. The fuel used in aircraft gas turbine is a light petroleum distillate or kerosene of gross calorific value of 46.4MJ/kg. For gas turbines used in power production or in cogeneration plants, the fuel used can be natural gas.Figure 9

Gas TurbinesLike steam turbines, gas turbines are also of the axial-flow type (Figure 10). The basic requirements of the turbines are light weight, high efficiency, reliability in operation and long working life. Large work output can be obtained per stage with high blade speeds when the blades are designed to sustain higher stresses. More stages are always preferred in gas turbine power plants, because it helps to reduce the stresses in the blades and increases the overall life of the turbine. The cooling of gas turbine blades is essential for long life as it is continuously subjected to high temperature gases. Blade angles of gas turbines follow the axial-flow compressor blading (Figure 7(a)), where the degree of reaction is not 50 per cent. It is usually assumed for any stage that the absolute velocity at inlet to each stage is equal to the absolute velocity at exit from the moving blades and that the same flow velocity is constant throughout the turbine. The degree of reaction, R, as defined for a steam turbine, is valid for gas turbines also. It is the ratio of the enthalpy drop in the moving blades to the enthalpy drop in the stageVortex BladingThis is the same name given to the twisted blades which are designed by using three dimensional flow equations with a view to decrease fluid flow losses. A radial equilibrium equation can be derived and it can be shown that one set of conditions which satisfy this equation is as follows:-Constant axial velocity-Constant specific work over the annulus-Free constant vortex at entry to the moving blades-Duct Work: The duct work consists of ducts between the compressor and the combustion chamber, combustion chamber to the turbine, and the exhaust dust. The ducts must be sized to minimize the pressure losses, as the loss in pressure directly reduces the capacity of the plant. Ducts should be supported from the floor to reduce vibration. Expansion joints must be provided to allow for dimensional changes due to temperature variation.GAS TURBINE FUELSGas turbines are basically designed to operate on petroleum-based fuels like natural gas, kerosene, aviation fuel and residual fuel oil. Other fuels like powdered coal, sewage gas, etc. are also being actively considered. The main requirements of a gas turbine fuel are low vanadium content and low ash content. An additive is placed in gas turbine fuel to reduce corrosion of the blades and to prevent the deposit of carbon and ash.ADVANTAGES OF DISADVANTAGES OF GAS TURBINE POWER PLANT1. Advantagesi. Very high power-to-weight ratio, compared to reciprocating engines;ii. Smaller than most reciprocating engines of the same power rating.iii. Moves in one direction only, with far less vibration than a reciprocating engine.iv. Fewer moving parts than reciprocating engines.v. Greater reliability, particularly in applications where sustained high power output is requiredvi. Waste heat is dissipated almost entirely in the exhaust and is very usable for boiling water in a combined cycle, or for cogeneration.vii. Low operating pressuresviii. Low cost of lubrication and maintenanceix. High operation speeds.x. Can run on a wide variety of fuels.xi. Very low toxic emissions of CO and HC due to excess air, complete combustion and no "quench" of the flame on cold surfacesDisadvantagesi. Cost is very highii. Less efficient than reciprocating engines at idle speediii. Longer startup than reciprocating enginesiv. Less responsive to changes in power demand compared with reciprocating enginesTURBINE PERFORMANCE1. Turbine Power OutputTo minimize the size and weight of the turbine for a given output power, the output per pound of airflow should be maximized. This is obtained by maximizing the air flow through the turbine which in turn depends on maximizing the pressure ratio between the air inlet and exhaust outlet. The main factor governing this is the pressure ratio across the compressor which can be as high as 40:1 in modern gas turbines. In simple cycle applications, pressure ratio increases translate into efficiency gains at a given firing temperature, but there is a limit since increasing the pressure ratio means that more energy will be consumed by the compressor.System EfficiencyThermal efficiency is important because it directly affects the fuel consumption and operating costs.Simple Cycle TurbinesA gas turbine consumes considerable amounts of power just to drive its compressor. As with all cyclic heat engines, a higher maximum working temperature in the machine means greater efficiency (Carnot's Law), but in a turbine it also means that more energy is lost as waste heat through the hot exhaust gases whose temperatures are typically well over 1,000C . Consequently simple cycle turbine efficiencies are quite low. For heavy plant, design efficiencies range between 30% and 40%. (The efficiencies of aero engines are in the range 38% and 42% while low power micro turbines (