Control-relevant modelling and linear analysis of instabilities in oxy-fuel combustion

Semi-closed gas turbine cycles based on oxy-fuel combustion have been proposed as an alternative to conventional gas turbine cycles for achieving CO₂-capture for CO₂ sequestration purposes. While combustion instabilities are a problem in modern conventional gas turbines in general, recent experimental results indicate that such instabilites also occur in oxy-fuel combustion. For conventional combustion processes, model based controllers have been successful at suppressing thermoacoustic instabilities. As a first step towards achieving the same for oxy-fuel combustion, we develop a control relevant model of oxy-fuel combustion. In oxy-fuel combustion CO₂ is used as working medium instead of air. From a control point of view, this gives an extra degree of freedom (compared to conventional combustion control). Analysis on the developed model show that secondary injection of CO₂ close to the flame is probably a better manipulated variable than fuel-injection, which has been a popular choice for control in conventional combustion processes.