EDUCATIONAL

GAS TURBINE

  In 1831, scientist Michael Faraday discovered that when a magnet is moved inside a coil of wire, an electric current flows in the wire. An electricity generator is a device that converts a form of energy into electricity. Generators operate because of the relationship between magnetism and electricity. Generators that convert kinetic (mechanical) energy into electrical energy produce nearly all of the electricity that consumers use.  

  The use of gas turbines for generating electricity. 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. 

How Do Gas Turbines Work?

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 compressors 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. 

Gas Turbine Performance

The thermodynamic process used in gas turbines is the. Two significant performance parameters are the pressure ratio and the firing temperature. The fuel-to-power efficiency of the engine is optimized by increasing the difference (or ratio) between the compressor discharge pressure and inlet air pressure. This compression ratio is dependent on the design. Gas turbines for power generation can be either industrial (heavy frame) or aero-derivative designs. Industrial gas turbines are designed for stationary applications and have lower pressure ratios – typically up to 18:1. Aeroderivative gas turbines are lighter weight compact engines adapted from aircraft jet engine design which operates at higher compression ratios – up to 30:1. They offer and lower emissions, but are smaller and have higher initial (capital) costs. Aeroderivative gas turbines are more sensitive to the compressor inlet temperature.