12.1 Gas Turbine Configurations and Components

Gas turbines are powerhouses in energy production, combining compression, combustion, and expansion to generate electricity. This section breaks down the key components and configurations, showing how these systems work together to turn fuel into usable power.

Understanding gas turbines is crucial for grasping modern power generation. We'll explore different setups, from simple cycles to combined heat and power plants, and see how each configuration suits specific needs in the energy landscape.

Gas Turbine Power Plant Components

Main Components

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• Gas turbine power plants consist of a compressor, combustion chamber, turbine, and generator
• The compressor and turbine are mounted on the same shaft, allowing the turbine to drive the compressor

Auxiliary Systems

• Fuel system supplies and regulates the flow of fuel to the combustion chamber
• Lubrication system provides lubrication and cooling for bearings and other moving parts
• Cooling system removes excess heat from the turbine components, ensuring safe and efficient operation
• Control system monitors and regulates the operation of the gas turbine plant, including startup, shutdown, and load control

Gas Turbine System Operation

Compressor

• The compressor draws in atmospheric air and compresses it to high pressure
• Typically uses an axial flow compressor with multiple stages to achieve the desired pressure ratio
• Compressor efficiency is crucial for overall gas turbine performance

Combustion Chamber

• The combustion chamber (or combustor) mixes the compressed air with fuel and ignites the mixture
• Generates high-temperature, high-pressure gases to drive the turbine
• Common combustion chamber configurations include annular (single chamber surrounding the turbine), can-annular (individual cans arranged around the turbine), and silo (external combustion chamber) types

Turbine

• The turbine extracts energy from the hot gases, converting it into mechanical work
• Drives the compressor and the generator, which are coupled to the turbine shaft
• Typically consists of multiple stages of stationary (stator) and rotating (rotor) blades to efficiently extract energy from the gases

Generator

• The generator, coupled to the gas turbine shaft, converts the mechanical energy into electrical energy
• Usually a synchronous generator that maintains a constant frequency and voltage output
• Generator efficiency and cooling are important factors in overall plant performance

Thermodynamic Processes in Gas Turbine Cycles

Ideal Brayton Cycle

• The ideal gas turbine cycle is the Brayton cycle, consisting of four processes: isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection
• Isentropic compression: air is compressed adiabatically (no heat transfer) and reversibly (no entropy change) in the compressor
• Isobaric heat addition: heat is added to the compressed air at constant pressure in the combustion chamber
• Isentropic expansion: hot gases expand adiabatically and reversibly in the turbine, generating mechanical work
• Isobaric heat rejection: exhaust gases are released into the atmosphere at constant pressure

Actual Gas Turbine Cycle

• Actual gas turbine cycles deviate from the ideal Brayton cycle due to irreversibilities
• Friction, turbulence, and heat losses cause non-isentropic processes in the compressor and turbine, reducing efficiency
• Pressure losses occur in the combustion chamber and exhaust ducts, affecting cycle performance
• Heat addition and rejection processes are not truly isobaric due to pressure drops
• Cycle efficiency is lower than the ideal Brayton cycle efficiency due to these factors

Gas Turbine Configurations vs Applications

Simple Cycle Gas Turbines

• Simple cycle gas turbines are used for peak load power generation (quick start-up and load-following), emergency power supply, and mechanical drive applications (pumps, compressors)
• Advantages include quick start-up, flexible operation, and lower capital costs
• Disadvantages include lower efficiency compared to combined cycle plants

Combined Cycle Gas Turbine (CCGT) Plants

• CCGT plants utilize waste heat from the gas turbine exhaust to generate steam for a secondary steam turbine
• Achieve higher overall efficiency (up to 60%) by combining gas and steam cycles
• Suitable for base load power generation due to high efficiency and relatively low fuel costs
• Require longer start-up times and have higher capital costs compared to simple cycle plants

Cogeneration or Combined Heat and Power (CHP) Plants

• Cogeneration plants generate electricity and useful heat simultaneously
• Utilize waste heat from the gas turbine for industrial processes (process steam, heating) or district heating
• Achieve high overall efficiency (up to 80%) by utilizing both electrical and thermal energy
• Suitable for industries with high heat demand, such as chemical plants, refineries, and paper mills

Aeroderivative Gas Turbines

• Aeroderivative gas turbines are derived from aircraft jet engines and adapted for stationary power generation
• Offer high power density, compact size, and rapid start-up times
• Suitable for mobile power generation, offshore platforms, and remote locations where size and weight are critical factors
• Higher efficiency than industrial gas turbines of similar size, but higher maintenance costs