A combustion turbine, also known as a gas turbine, is a type of internal combustion engine that converts fuel into mechanical energy through the combustion of air and fuel mixtures. The efficiency of a combustion turbine is influenced by several factors, including pressure ratio, ambient air temperature, turbine inlet air temperature, compressor and turbine efficiency, turbine blade cooling requirements, and performance enhancements such as recuperation, intercooling, inlet air cooling, reheat, steam injection, or combined cycle.
Understanding the Factors Influencing Combustion Turbine Performance
Pressure Ratio
The pressure ratio, which is the ratio of the compressor discharge pressure to the compressor inlet pressure, is a critical factor in determining the efficiency of a combustion turbine. Higher pressure ratios generally result in higher thermal efficiencies, but they also increase the complexity and cost of the turbine design. Typical pressure ratios for modern combustion turbines range from 15:1 to 30:1, with the higher end of the range being more common in larger, more efficient machines.
Ambient Air Temperature
The ambient air temperature has a significant impact on the performance of a combustion turbine. As the air temperature increases, the air density decreases, which reduces the mass flow through the turbine and decreases its power output. To mitigate this effect, many combustion turbines are equipped with inlet air cooling systems, such as evaporative coolers or chillers, which can improve the turbine’s efficiency by up to 20% on hot days.
Turbine Inlet Air Temperature
The turbine inlet air temperature, also known as the firing temperature, is another critical parameter that affects the efficiency of a combustion turbine. Higher turbine inlet temperatures generally result in higher thermal efficiencies, but they also increase the thermal stresses on the turbine blades and require more advanced cooling techniques. Modern combustion turbines can operate with turbine inlet temperatures ranging from 1,100°C to 1,500°C, with the higher end of the range being more common in the latest, most efficient designs.
Compressor and Turbine Efficiency
The efficiency of the compressor and turbine components within a combustion turbine also plays a significant role in the overall system efficiency. Improvements in aerodynamic design, materials, and manufacturing processes have led to steady increases in compressor and turbine efficiencies over time. Typical compressor and turbine efficiencies for modern combustion turbines range from 85% to 90%.
Turbine Blade Cooling Requirements
The high turbine inlet temperatures required for efficient combustion turbine operation can place significant thermal stresses on the turbine blades. To mitigate this, advanced cooling techniques, such as internal air cooling and film cooling, are employed to keep the blade temperatures within acceptable limits. The cooling requirements can have a significant impact on the overall system efficiency, as the cooling air is extracted from the compressor, reducing the available power output.
Performance Enhancements
Combustion turbines can be further optimized through the use of various performance enhancement technologies, such as:
- Recuperation: The use of a heat exchanger to recover exhaust heat and preheat the compressed air entering the combustor, improving overall efficiency.
- Intercooling: The use of a heat exchanger to cool the air between the compressor stages, reducing the work required by the compressor and improving efficiency.
- Inlet Air Cooling: The use of evaporative coolers or chillers to reduce the temperature of the inlet air, increasing the air density and improving power output.
- Reheat: The reheating of the exhaust gases before they enter the turbine, which can improve the overall efficiency of the cycle.
- Steam Injection: The injection of steam into the combustor, which can increase the mass flow through the turbine and improve power output.
- Combined Cycle: The integration of a combustion turbine with a steam turbine, where the exhaust heat from the combustion turbine is used to generate steam for the steam turbine, resulting in significantly higher overall system efficiencies.
Emissions and Environmental Considerations
Combustion turbines have very low emissions compared to other fossil-powered generation technologies. With catalytic exhaust cleanup or lean pre-mixed combustion, some large gas turbines can achieve emissions of less than 9 ppm NOx and 1 ppm CO. However, the specific emissions levels can vary depending on the type of combustion system used. For instance, the XONON system, which utilizes a flameless combustion system where fuel and air reacts on a catalyst surface, can achieve NOx levels below 3 ppm and CO and unburned hydrocarbons levels below 10 ppm.
Efficiency and Heat Rates
The efficiency of the Brayton cycle, which is the thermodynamic cycle used in combustion turbines, is a function of pressure ratio, ambient air temperature, turbine inlet air temperature, compressor and turbine efficiency, turbine blade cooling requirements, and any other performance enhancements. Newer machines are usually more efficient than older ones of the same size and general type due to improvements in these parameters over time.
Heat rates, which are a measure of the efficiency of a power plant, are also an important consideration in combustion turbine performance. Heat rates shown in Table 3-2 are from manufacturers’ specifications and are net of losses due to inlet and outlet pressure drop and parasitic power. Available thermal energy (steam output) is calculated from information provided by the vendors or published turbine data on turbine exhaust temperatures and flows.
Conclusion
In summary, the performance characteristics of combustion turbines are influenced by a variety of factors, including pressure ratio, ambient air temperature, turbine inlet air temperature, compressor and turbine efficiency, turbine blade cooling requirements, and performance enhancements. Newer machines are usually more efficient than older ones of the same size and general type, and emissions levels and heat rates are also important considerations in combustion turbine performance.
References:
– Enhanced turbine monitoring using emissions measurements and diagnostics, ScienceDirect, https://www.sciencedirect.com/science/article/am/pii/S0306261916304871
– A Holistic Methodology to Quantify Product Competitiveness and Define Innovation Requirements for Micro Gas Turbine Systems in Hydrogen-Based Energy Storage, ASME Digital Collection, https://asmedigitalcollection.asme.org/gasturbinespower/article/146/8/081001/1170270
– AP-42, Vol. I, 3.1: Stationary Gas Turbines, EPA, https://www3.epa.gov/ttnchie1/ap42/ch03/final/c03s01.pdf
– Combustion Turbines – EPA, https://www.epa.gov/sites/default/files/2015-07/documents/catalog_of_chp_technologies_section_3._technology_characterization_-_combustion_turbines.pdf
– Improving Combustion Turbine Performance with Process Gas Chromatographs, Emerson, https://www.emerson.com/documents/automation/application-note-improving-combustion-turbine-performance-process-gas-chromatographs-rosemount-en-71784.pdf
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