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White paper – Influencing and measured variables for gas turbines

and their importance for optimizing efficiency and emissions

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Gas turbines in industrial applications

Thanks to their outstanding performance in the peak load range, gas turbines have proven themselves as crucial and reliable components in numerous industrial applications. Gas turbines are used especially frequently in the electrical and heat generation sector, as well as in the oil and gas industry. In industrial energy production, gas turbines ensure that consumption peaks are covered and produce heat for buildings. Frequent areas of use are gas turbine works, combined heat and power (CHP) plants and cogeneration plants. A particularly high level of energy efficiency can be achieved using a combination of gas and steam turbines. In the oil and gas industry, gas turbines are in use as mechanical drives for pumps, compressors and generators in the transportation and processing of raw materials. Gas turbines are operated with liquid and gaseous fuels such as natural gas, gasoline, diesel, heating oil or petroleum. Optimizing the fuel- and exhaust gas-intensive process to the highest level of efficiency involves a complex interaction between the exhaust gas parameters and the combustion process settings of the gas turbine – the basis for an optimum performance. For a service technician, it is important to understand the function of the combustion process and the influence of the individual measurement parameters on the performance and pollutant emission of gas turbines.

Functional principle of gas turbines

Gas turbines are combustion engines which consist of three components: a preliminary compressor the central combustion chamber and the actual turbine. The design, performance and size of gas turbines differ depending on the application and area of use. However, their working principle is always the same, and is based on the thermodynamic cycle process according to James Prescott Joule (“Joule process”). Air is compressed via the blading of one or more compressor steps, and then mixes with a gaseous or liquid fuel in the combustion chamber, ignites and combusts.

A hot gas is produced from this mixture of compressed air and combustion gas, which can reach temperatures of +1,000°C, and which escapes to the downstream turbine component, and expands. Thermal energy is converted into mechanical energy. Subsequently, in the expansion turbine, the energy-rich, hot exhaust gas expands almost to ambient pressure, losing its velocity. During the expansion process, the exhaust gas transfers power to the turbine. Approximately 2/3 of this power is needed to drive the compressor (air intake). A directly coupled generator converts the mechanical energy into electrical energy. Roughly ¹/₃ of the power output remains available on the low pressure side for a second drive, for example for driving a generator, rotor, compressor or pump, before the hot gas is diverted to a downstream heat recovery boiler for the purpose of heating buildings.

Analyzing exhaust gas concentrations correctly

The concentration of the released exhaust gases provides important information on the efficiency of the combustion and how it can be increased. CO and NOX values provide information on the current status of the system and the adherence to the emission limit values. The air input between rich and lean and the correlating combustion chamber temperature influence the emission behaviour of the gas turbine.

Challenges in commissioning, operation and servicing

In emission measurements in gas turbines, the challenge is measuring not only at very high, but also at very low gas concentrations. At the right operating point, optimally adjusted gas turbines emit only low levels of CO and NOX. However, high gas concentrations can occur, for example when the plant is started up for testing purposes. The reduction of NO2 emissions as well as the avoidance of pressure loss in the combustion chamber are also important factors for the efficient operation of the gas turbine.

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