Simulation of a Multistage Compressor at Low Load Operation with Additional Bleed Air Extraction for Minimum Environmental Load Reduction

The need for more flexible operation of gas-fired power plants has led manufacturers to exploit possibilities to retrofit existing systems and increase turndown capabilities, lowering the minimum environmental load (MEL) - the lowest output at which the unit can operate and still meet environmental emissions limits. A possible measure for the compressor to enable a reduced MEL is to extract significant mass flow rates through the bleed ports in operation with a closed IGV to lower the mass flow entering the combustor, enabling a further load reduction while maintaining emissions. For this measure to be implemented, a stable operation of the compressor has to be ensured at the reduced MEL conditions. It is well known that bleed-air offtake at full speed shifts the loading towards the rear stages of the compressor. At MEL, the rear stages already operate at an increased loading compared to base-load. Therefore, to confirm the viability of bleed offtake as a turndown strategy, the effects on performance and stability have to be quantified for operation at MEL. Of particular interest is how an increase in air extraction through the bleed ports influences the stability of the compressor at MEL, especially when the offtake is from low pressure. Information on the degradation of the stability margin due to the additional bleed-air extraction at MEL is gained through numerical simulations. Full-compressor CFD simulations of an F-class gas turbine at reduced MEL conditions are performed. The influence of several geometrical and numerical modeling details is studied, and the model is validated against a comprehensive set of experimental data. The numerical results show good agreement with the experimental data, even with increased bleed air extraction. It can be concluded that bleed-air extraction is a capable method of reducing the compressor discharge mass flow rate significantly. The extraction rate is limited by the stability of the last compressor stages. To verify the integrity of the whole GT at operation under such conditions, e.g., the thermal state of hot parts, further analysis should be performed.