IUBAT- International University of Business Agriculture and Technology Founded 1991 by Md. Alimullah Miyan
COLLEGE OF ENGINEERING AND TECHNOLOGY(CEAT) LECTURE SLIDE - 2
Course Title: Power Plant Engineering Course Code : MEC 403
Course Instructor: Engr. Md. Irteza Hossain
Gas Turbine Power Plant
GAS TURBINES • Invented in 1930 by Frank Whittle • Patented in 1934 • First used for aircraft propulsion in 1942 on Me262 by Germans during second world war • Currently most of the aircrafts and ships use GT engines • Used for power generation • Manufacturers: General Electric, Pratt &Whitney, SNECMA, Rolls Royce, Honeywell, Siemens – Westinghouse, Alstom • Indian take: Kaveri Engine by GTRE (DRDO)
Gas turbine power plant Gas turbine: Working principle : Air is compressed(squeezed) to high pressure by a fan-like device called the compressor. Then fuel and compressed air are mixed in a combustion chamber and ignited. Hot gases are given off, which spin the turbine wheels. Most of the turbine’s power runs the compressor. Part of it drives the generator/machinery.
Gas turbine power plant… Gas turbine: Description:
Gas turbines burn fuels such as oil,
i) ii)
nature gas and pulverised(powdered) coal. Instead of using the heat to produce steam, as in steam turbines, gas turbines use the hot gases directly to turn the turbine blades. Gas turbines have three main parts: Air compressor Combustion chamber
iii) Turbine
Gas turbine power plant… Gas turbine: Air compressor: The air compressor and turbine are mounted at either end on a common horizontal axle(shaft), with the combustion chamber between them. Gas turbines are not self starting. A starting motor initially drives the compressor till the first combustion of fuel takes place, later, part of the turbine’s power runs the compressor. The air compressor sucks in air and compresses it, thereby increasing its pressure.
Gas turbine power plant… Gas turbine: Combustion chamber: In the combustion chamber, the compressed air combines with fuel and the resulting mixture is burnt. The greater the pressure of air, the better the fuel air mixture burns. Modern gas turbines usually use liquid fuel, but they may also use gaseous fuel, natural gas or gas produced artificially by gasification of a solid fuel. Note : The combination of air compressor and combustion chamber is called as gas generator.
Gas turbine power plant… Gas turbine: Turbine: o The burning gases expand rapidly and rush into the turbine, where they cause the turbine wheels to rotate. o Hot gases move through a multistage gas turbine. o Like in steam turbine, the gas turbine also has fixed(stationary) and moving(rotor) blades. o The stationary blades guide the moving gases to the rotor blades and adjust its velocity. o The shaft of the turbine is coupled to a generator or machinery to drive it.
Gas turbine power plant… Applications of gas turbine: Gas turbines are used to drive pumps, compressors and high speed cars. Used in aircraft and ships for their propulsion. They are not suitable for automobiles because of their very high speeds. Power generation(used for peak load and as stand-by unit). Note : Gas turbines run at even higher temperatures than steam turbines, the temperature may be as high as 1100 – 12600C. The thermal efficiency of gas turbine made of metal components do not exceed 36%. Research is underway to use ceramic components at turbine inlet temperature of 13500C or more, and reach thermal efficiencies over 40% in a 300 kW unit.
Layout of a gas turbine power plant
Layout of gas turbine power plant… Starting motor: Gas turbines are not self starting. They require a starting motor to first bring the turbine to the minimum speed called coming –in speed, for this purpose a starting motor is required. Low pressure compressor(LPC): The purpose of the compressor is to compress the air. Air from the atmosphere is drawn into the LPC and is compressed.
Intercooler: The air after compression in the LPC is hot. It is cooled by the intercooler. The intercooler is circulated with cooling water.
Layout of gas turbine power plant… High pressure compressor(HPC): The air from the intercooler enters the HPC where it is further compressed to a high pressure. The compressed air passes through a regenerator.
Regenerator(Heat exchanger): The air entering the combustion chamber(CC) for combustion must be hot. The heat from the exhaust gases is picked up by the compressed air entering the combustion chamber.
Combustion chamber: The fuel(natural gas, pulverized coal, kerosene or gasoline) is injected into the combustion chamber. The fuel gets ignited because of the compressed air. The fuel along with the compressed air is ignited sometimes with a spark plug.
Layout of gas turbine power plant… High pressure compressor(HPC): The air from the intercooler enters the HPC where it is further compressed to a high pressure. The compressed air passes through a regenerator.
Regenerator(Heat exchanger): The air entering the combustion chamber(CC) for combustion must be hot. The heat from the exhaust gases is picked up by the compressed air entering the combustion chamber.
Combustion chamber: The fuel(natural gas, pulverized coal, kerosene or gasoline) is injected into the combustion chamber. The fuel gets ignited because of the compressed air. The fuel along with the compressed air is ignited sometimes with a spark plug.
Layout of gas turbine power plant… High pressure turbine (HPT): In the beginning the starting motor runs the compressor shaft. The hot gases(products of combustion) expands through the high pressure turbine. It is important to note that when the HPT shaft rotates it infact drives the compressor shaft which is coupled to it. Now the HPT runs the compressor and the starting motor is stopped. Note : About 66% of the power developed by the gas turbine power plant is used to run the compressor.
Only 34% of the power developed by the plant is used to generate electric power.
Layout of gas turbine power plant… Low pressure turbine (LPT): The purpose of the LPT is to produce electric power. The shaft of the LPT is directly coupled with the generator for producing electricity. The hot gases(products of combustion) after leaving the HPT is again sent to a combustion chamber where it further undergoes combustion. The exhaust gases after leaving the LPT passes through the regenerator before being exhausted through the chimney into the atmosphere.
The heat from the hot gases is used to preheat the air entering the combustion chamber. This preheating of the air improves the efficiency of the combustion chamber.
Gas turbine power plant… Advantages of gas turbine power plant : Storage of fuel requires less area and handling is easy. The cost of maintenance is less. It is simple in construction. There is no need for boiler, condenser and other accessories as in the case of steam power plants. Cheaper fuel such as kerosene , paraffin, benzene and powdered coal can be used which are cheaper than petrol and diesel. Gas turbine plants can be used in water scarcity areas. Less pollution and less water is required. Disadvantages of gas turbine power plant : 66% of the power developed is used to drive the compressor. Therefore the gas turbine unit has a low thermal efficiency. The running speed of gas turbine is in the range of (40,000 to 100,000 rpm) and the operating temperature is as high as 1100 – 12600C. For this reason special metals and alloys have to be used for the various parts of the turbine. High frequency noise from the compressor is objectionable.
Gas Turbine Power Plants – Advantages Compared to Steam-Turbine, Gas Turbine offers : 1. Greater Power for a given size and weight, 2. High Reliability, 3. Long Life, 4. More Convenient Operation. 5. Engine Start-up Time reduced from 4 hrs to less than 2 min…!!
Thermodynamic Cycles Applications of Thermodynamics
Power Generation Power Cycles
Engines Devices / Systems used produce Net Power Output.
Refrigeration Refrigeration Cycles
to
External Heat is supplied to the Working Fluid from an external source such as a Furnace / Geothermal Well / Nuclear Reactor, etc.
Refrigerators / Heat Pumps / A.C. Devices / Systems used to produce Refrigeration Effect. Internal Heat is supplied to the Working Fluid by burning the Fuel within the System Boundaries.
Introduction Thermodynamics Cycles
Gas Cycles
Vapour Cycles
Working Fluid remains in Gaseous Phase throughout the Cycle.
Working Fluid exists in Vapor Phase during part of the Cycle, and in liquid phase during remaining part.
Introduction Thermodynamics Cycles
Closed Cycles
Open Cycles
Working Fluid returns to Initial State at the end of the cycle, and is Recirculated.
Working Fluid is Renewed at the end of each cycle, and thus us NonRecirculated.
Brayton Closed Cycle – Analysis
Made up of Four Internally Reversible processes: 1-2
Isentropic Compression (in a Compressor)
2-3
Constant-Pressure Heat Addition
3-4
Isentropic Expansion (in a Turbine)
4-1
Constant-Pressure Hat Rejection
Brayton Closed Cycle – Analysis Neglecting changes in Kinetic and Potential energies, the Energy Balance
for a Steady-Flow Process, on a Unit–Mass Basis :
q in q out
win
wout
q in
h3 h 2
CP T3 T 2
q out
h 4 h1
CP T 4 T1
Thermal Efficiency of Ideal Brayton Cycle
:
hexit
hinlet
Brayton Closed Cycle – Analysis Processes 1-2 and 3-4 are
Isentropic, P2 = P3 and P4 = P1.
Substituting and simplifying the equation : th , Brayton
1
1
1
rp
where;
rp
P2 P1
Brayton Closed Cycle – Analysis th , Brayton
1
1
1
rp
Thermal Efficiency of an Ideal Brayton
Cycle depends on the Pressure Ratio of the gas turbine and the Specific Heat
th , Brayton
f rp ,
Ratio of the working fluid.
for γ = 1.4
Brayton Closed Cycle – Analysis Highest Temperature occurs at the end of the Combustion process (state 3), and
it is limited by the maximum temperature that the turbine blades can withstand. This limits the Pressure Ratios that can be used in the cycle. For a fixed Turbine Inlet Temperature T3, the Net Work Output per Cycle increases with
the
Pressure
Ratio,
reaches
a
maximum, and then starts to decrease, Compromise between the Pressure Ratio (thus the Thermal Efficiency)
and the Net Work Output. Generally, the Pressure Ratio ranges from about 11 to 16.
Back Work Ratio Usually, more than one-half of the
Turbine Work Output is used to drive the Compressor.
Back Work Ratio
CompressorWork TurbineWork
1 2
In contrast to Steam Power Plants, where Back Work Ratio is only a few percent. ..!! This is due to : 1. Liquid is compressed in Steam Power Plants instead of a gas. 2. Steady-Flow Work is proportional to Sp. Volume of the working fluid.
Therefore, the turbines used in Gas-Turbine Power Plants are larger than those used in Steam Power Plants of the same net power output…!!
Brayton Closed Cycle – Analysis Functions of Air in Gas Turbines : 1. Supplies the Necessary Oxidant for the combustion of the fuel. 2. As a Coolant to keep the temp. of various components within safe limits. Drawing in more air than is needed for the complete combustion of the fuel. Air–Fuel Mass Ratio of 50 or above is common.
Treating the Combustion Gases as Air does not cause any appreciable error.
Gas Turbine Power Plants – Applications Two Major Application Areas :
1. Aircraft Propulsion 2. Electric Power Generation.
Aircraft Propulsion
Electric Power Generation
Actual Gas-Turbine Cycles • For actual gas turbines, compressor and turbine are not isentropic
Regenerative Brayton Cycle
For the Brayton cycle, the turbine exhaust temperature is greater than the compressor exit temperature. Therefore, a heat exchanger can be placed between the hot gases leaving the turbine and the cooler gases leaving the compressor. This heat exchanger is called a regenerator or recuperator..
Gas Turbine Cycle – Intercooling Net Work Output of Gas Turbine can be ↑ by ↓ the Compressor Work Input. Multistage + Intercooling…!!!
Gas Turbine Cycle – Intercooling Three Internally Reversible processes: 1-c Isentropic Compression, till Pr. is Pi c-d
Constant-Pressure Cooling,
↓ from Tc to Td d-2
Isentropic Compression, State 2.
Gas Turbine Cycle – Intercooling Work Input per unit Mass Flow on the P–V Diagram : 1–c–d–2–a–b–1.
Without Intercooling : Single Stage Isentropic Compression from State 1 to State 2’. Work Area ≡ 1–2’–a–b–1. Crosshatched Area ≡ Reduction in work due to Intercooling.
Gas Turbine Cycle – Intercooling When using multistage compression, cooling the working fluid between the stages will reduce the amount of compressor work required. The compressor work is reduced because cooling the working fluid reduces the average specific volume of the fluid and thus reduces the amount of work on the fluid to achieve the given pressure rise. To determine the intermediate pressure at which intercooling should take place to minimize the compressor work, we follow the approach shown in Chapter 7. For the adiabatic, steady-flow compression process, the work input to the 3 0 2 4 compressor per unit mass is 4
wcomp = v dP = v dP
v dP
1
1
2
3
v dP
Gas Turbine Cycle – Intercooling This yields
P2
P1 P4
or, the pressure ratios across the two compressors are equal.
P2 P1
P4 P2
P4 P3
Inter cooling is almost always used with regeneration. During inter cooling the compressor final exit temperature is reduced; therefore, more heat must be supplied in the heat addition process to achieve the maximum temperature of the cycle. Regeneration can make up part of the required heat transfer.
Brayton with Intercooling, Reheat, & Regeneration • For max performance
Gas Turbine Cycle – Reheat For Metallurgical Reasons, the Temperature of the Gaseous Combustion Products entering the turbine must be limited. This temperature can be controlled by providing Air in Excess of the Amount required to Burn the Fuel in the combustor. As a consequence, the gases exiting the combustor contain Sufficient Air to support the Combustion of Additional Fuel. Gas Turbine Power Plants take advantage of the Excess Air by means of a Multistage Turbine with a Reheat Combustor between the stages. With this arrangement the Net Work per Unit of Mass Flow can be increased.
NOTE : Reheat is used for ↑ in Output Power. It may not ↑ the Efficiency…!!
Gas Turbine Cycle – Reheat
After expansion from State 3 to State a in the first turbine, the gas is Reheated at Constant Pressure from State a to State b.
The expansion is then completed in the second turbine from State b to State 4.
Gas Turbine Cycle – Intercooling + Reheat + Regenerator
Example 1 Inlet conditions to a Brayton cycle are 1 bar and 300 K. The cycle pressure ratio is 8. The temperature at the inlet to the turbine is 1300 K. Calculate a. The gas temperature at the exit of the compressor and turbine b. the back work ratio c. the thermal efficiency
1300 K
rp =8
300 K
Example 2 In the plant of Example 1, let the compressor and the turbine have the isentropic efficiencies of 0.8 and .85 respectively each. Calculate the performance parameters of the cycle. a. the back work ratio b. the thermal efficiency c. the turbine exit temperature 1300 K
rp =8
300 K
Regeneration • Use heat exchanger called recuperator or regenerator • Counter flow
Regeneration • Effectiveness
• For cold-air assumptions
Example 3
Determine the thermal efficiency of the gas turbine described in the previous problem if a regenerator having an effectiveness of 80percent is installed
Brayton with Intercooling, Reheat, & Regeneration
Example 4
