Gas turbines are engines in which the chemical energy of the fuel is converted into mechanical energy in terms of kinetic energy. The thermodynamic process employed in gas turbines is the Brayton-cycle. There are two significant performance parameters: the pressure ratio and the firing temperature, which will be discussed further. Gas turbines present one of the cleanest means for electric power generation with fairly low carbon dioxide (CO2) and oxides of nitrogen (NOx) emissions. The available gas turbines cover a wide range of capacities. Gas turbines are well suited for Combined Heat and Power (CHP) and Combined Cooling, Heating, and Power applications because high-pressure steam can be produced from their high-temperature exhaust using Heat Recovery Steam Generators (HRSGs). Gas turbines have two main applications in modern industries, including "turbo generators" and "turbo compressors". Gas turbines offer flexibility in using a range of liquid and gaseous fuels.
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There are two types of land-based gas turbines, namely: Heavy Frame Engines and Aeroderivative Engines. Heavy Frame engines are regularly slower in speed, relatively low-pressure ratio (usually below 20), more restricted in operating speed range, more massive, have higher airflow, more gradually in start-up, and need more time and superfluous parts for maintenance.
Aeroderivative engines, as the name implies, are originated from jet engines and work at relatively high compression ratios, which can go up to 30. Despite being classified as a gas turbine, the fuel source for the aeroderivative turbine is not really gas. They are actually designed so that fuel and air are mixed and then ignited to accomplish the desired output. The engines are quite small and compact, making them ideal for smaller power outputs in smaller plants. Turbines with larger frames produce more emissions, but they need to be designed to reduce the pollutants emitted, such as NOx.
In a power generation gas turbine, air from the environment enters the inlet nozzle, and after passing through the inlet, air enters a multi-stage compressor, where its total pressure continuously grows to reach the design pressure ratio at the outlet of the compressor. The pressure increment is performed by supplying mechanical energy through the turbine. The working medium air leaves the compressor outlet at a relatively high total temperature and total pressure based on the compression pressure ratio. It then enters the combustion chamber, where fuel is supplemented. Within the combustion chamber, an intensive combustion process occurs, where the fuel's chemical energy is converted into thermal energy. The energy conversion process continues within the exit diffuser, where the kinetic energy of the exiting gas is partially converted into potential energy. Due to the power required to drive the compressor, energy conversion efficiency for a simple cycle gas turbine power plant is typically around 30 percent. Even the most efficient designs are limited to 40 percent. Achieving higher efficiency required a substantial increase in Turbine Inlet Temperature (TIT), requiring extensive cooling of the front turbine stages.