Combustion instabilities in combined cycle gas turbine

Abstract

With increasing discussion and legislation around emissions, gas turbines have evolved to limit their emissions, especially NOx emissions. The way of operating the gas turbine, with lean premixed combustion reduces the flame temperature in the combustion chamber which results in low NOx emissions. The downside of this way of operating is the increased sensitivity of the combustion system to thermo-acoustic instabilities. These instabilities arise due to an interaction between unsteady heat release at the flame and the acoustics of the combustion system. At lean premixed conditions, a fluctuation in equivalence ratio results in a large fluctuation in flame speed which is related to the heat release. This makes the lean premixed combustion very susceptible to acoustic pressure waves travelling through and reflecting inside the combustion system, as these pressure waves can influence the mass flow of gas and air in the premixing duct and thus changing the composition of the fuel mixture that is convected towards the flame. An additional effect to the unsteady heat release are fluctuations of the total mass flow at the burner outlet, which are convected towards the flame front, an increase in combusted fuel will result in an increase in heat release.

To analyze the sensitivities of the combustion system, a thermo-acoustic model will be constructed. The model will be a one-dimensional and linearised system, where conservation equations across the (thermo-)acoustic elements of the combustion system are rewritten into transfer matrix form. Validation will be done by analyzing operational data from the HW09 power plant and comparing this with the model response. The combustion dynamics are analysed for frequencies in the range 0 - 300 Hz.

It was found that the HW09 is susceptible to combustion instabilities when the pressure drop across the premix gas nozzles is low, which results in a decrease in fuel supply impedance. This increases the pressure amplitude in the 90 Hz range, this behaviour corresponds to the modelled results when changes to the gas preheating temperature or fuel nozzle compositions are made in the thermo-acoustic model, which influence the pressure drop over the gas nozzles. This is accompanied with a decreased amplification at the critical 120 Hz range. This is a known side-effect, as the 90 Hz needs to remain dominant for a stable combustion system. This means the pressure drop over the gas nozzles balances on a fine line, too low will result in high 90 Hz pressure waves and too high will result in a more unstable 120 Hz frequency. Several of the hybrid burners in the annular combustion chamber are fitted with a cylindrical burner outlet. This changes the flow field out of the burner and is used to allow a higher maximum load for the gas turbine. The inclusion of this burner ring makes the combustion system upstream of the burner outlet more resistant to acoustic pressure fluctuations in the frequency range up to 120 Hz. The burner system does become more sensitive to acoustic pressure fluctuations for frequencies higher than 120 Hz. Fuel composition influences the combustion behaviour of a fuel mixture. Low-calorific fuel shows to be inherently more unstable in this gas turbine configuration as opposed to high-calorific fuel. If hydrogen is added to high-calorific fuel, the flame speed is increased as is the influence of both equivalence ratio perturbations and velocity perturbations on unsteady heat release.The increase in flame speed results in a reduction of flame length, which changes the convective time delay and shifts the instability of the combustion system from the 120 Hz area towards the 180 Hz area.

The thermo-acoustic model responds to changes of operational parameters in an equal manner as was found from data analysis. The results are comparable up to 200 Hz, after which the flame can no longer be assumed to be compact, which means compressibility, 2D and 3D effects have to be taken into account. Fuel composition analysis shows the importance of flame length and convective time delay on combustion stability. For hydrogen addition it can shift the critical frequency towards the second harmonic frequency of the combustion chamber, this will result in rapidly increasing pressure waves and high accelerations. With increased knowledge of the effects of hydrogen addition or low-calorific fuel on the flame dynamics, the model can be a predictive tool to investigate the combustion behaviour when the change to this fuel mixture will be made. The ability to predict the influence of different operational parameters or changes to burner geometry on the stability of the combustion system can be very beneficial for analysis of future changes to the gas turbine and its operation.