Gas turbine for power generation

Gas turbines have been used to generate electricity since 1939. They are now one of the most common power generation technologies. Gas turbines are a type of internal combustion (IC) engine in which hot gases produced by the combustion of an air-fuel mixture spin a turbine to generate power. The name “gas turbine” refers to the production of hot gas during fuel combustion rather than the fuel itself.

What is the working principle of a gas turbine?

The compressor, the combustion chamber (or combustor), and the turbine are all mounted on the same shaft in a gas turbine. Axial flow or centrifugal flow compressors are available. Because of their higher flow rates and efficiencies, axial flow compressors are more common in power generation. Axial flow compressors are made up of several stages of rotating and stationary blades (or stators) through which air is drawn parallel to the axis of rotation and incrementally compressed. The air is accelerated through the rotating blades and diffused by the stators, increasing pressure and decreasing volume. Although no heat is added, the compression of the air raises the temperature.

As the compressor must reach a certain speed before the combustion process can be considered continuous – or self-sustaining – initial momentum is imparted to the turbine rotor by an external motor, static frequency converter, or the generator itself. Before fuel can be introduced and ignition can occur, the compressor must be smoothly accelerated and reach firing speed. Turbine speeds vary greatly depending on manufacturer and design, ranging from 2,000 to 10,000 revolutions per minute (rpm). One or more spark plugs initiate ignition (depending on combustor design).

Once the turbine reaches self-sustaining speed (above 50% of full speed), the power output is sufficient to drive the compressor, combustion is continuous, and the starter system can be turned off.

Manufacturers like General Electric and Woodward design and develop control systems for gas and steam turbines. General Electric control system component examples include IS200VAICH1D, IS220PTCCH1A, IS420ESWBH3A etc.

Gas turbine performance – Aeroderivative / Heavy duty gas turbines.

The Brayton cycle is the thermodynamic process used in gas turbines. The pressure ratio and firing temperature are two important performance parameters. The engine’s fuel-to-power efficiency is improved by increasing the difference (or ratio) between the compressor discharge pressure and the inlet air pressure. This compression ratio is design-dependent. Industrial (heavy frame) or aero-derivative gas turbines can be used to generate electricity. Industrial gas turbines are intended for stationary use and have lower pressure ratios, typically up to 18:1.

Aeroderivative gas turbines are lighter-weight compact engines adapted from aircraft jet engine design that operate at higher compression ratios – up to 30:1 – than conventional gas turbines. They are more fuel efficient and emit less pollution, but they are smaller and have higher initial (capital) costs. The compressor inlet temperature is more sensitive to aero-derivative gas turbines.

The temperature at which the turbine operates has an effect on efficiency as well; with higher temperatures resulting in greater efficiency. However, the thermal conditions that the turbine blade metal alloy can tolerate limit the turbine inlet temperature. Gas temperatures at the turbine inlet can range from 1200 oC to 1400 oC, but some manufacturers have increased inlet temperatures to 1600 oC by designing blade coatings and cooling systems to protect metallurgical components from thermal damage.

Gas turbine Working efficiency

The energy conversion efficiency for a simple cycle gas turbine power plant is typically around 30% due to the power required to drive the compressor, with even the most efficient designs around 40%. The exhaust gas, which is around 600 oC as it exits the turbine, retains a significant amount of heat. Gas turbine power plants can achieve 55 to 60 percent efficiency by recovering waste heat and using it to produce more useful work in a combined cycle configuration. However, there are operational limitations to running gas turbines in combined cycle mode, such as longer startup time, purge requirements to prevent fires or explosions, and ramp rate to full load.

Compressed air is mixed with fuel, which is injected through nozzles. Pre-mixing of the fuel and compressed air is an option, or the compressed air can be introduced directly into the combustor. Under constant pressure conditions, the fuel-air mixture ignites, and the hot combustion products (gases) are directed through the turbine, where they expand rapidly and impart rotation to the shaft. The turbine is also divided into stages; with each stage having a row of stationary blades (or nozzles) to direct the expanding gases, followed by a row of moving blades. The shaft’s rotation causes the compressor to draw in and compress more air in order to maintain continuous combustion.

The remainder of the shaft power is used to power a generator, which generates electricity. The compressor uses about 55 to 65 percent of the power generated by the turbine. Gas turbines can have multiple compressors and turbine stages to optimize the transfer of kinetic energy from combustion gases to shaft rotation.