Flue Gas Recirculation in a Gas Turbine: Impact on Performance and Operational Behavior

The rigorous reduction of greenhouse gas emissions in the upcoming decades is only achievable with contribution from the following strategies: production efficiency, demand reduction of energy and carbon dioxide (CO₂) capture from fossil fueled power plants. Since fossil fueled power plants contribute largely to the overall global greenhouse gas emissions (> 25% [1]), it is worthwhile to capture and store the produced CO₂ from those power generation processes. For natural-gas-fired power plants, post-combustion CO₂ capture is the most mature technology for low emissions power plants. The capture of CO₂ is achieved by chemical absorption of CO₂ from the exhaust gas of the power plant. Compared to coal fired power plants, an advantage of applying CO₂ capture to a natural-gas-fired combined cycle power plant (CCPP) is that the reference cycle (without CO₂ capture) achieves a high net efficiency. This far outweighs the drawback of the lower CO₂ concentration in the exhaust. Flue Gas Recirculation (FGR) means that flue gas after the HRSG is partially cooled down and then fed back to the GT intake. In this context FGR is beneficial because the concentration of CO₂ can be significantly increased, the volumetric flow to the CO₂ capture unit will be reduced, and the overall performance of the CCPP with CO₂ capture is increased. In this work the impact of FGR on both the Gas Turbine (GT) and the Combined Cycle Power Plant (CCPP) is investigated and analyzed. In addition, the impact of FGR for a CCPP with and without CO₂ capture is investigated. The fraction of flue gas that is recirculated back to the GT, need further to be cooled, before it is mixed with ambient air. Sensitivity studies on flue gas recirculation ratio and temperature are conducted. Both parameters affect the GT with respect to change in composition of working fluid, the relative humidity at the compressor inlet, and the impact on overall performance on both GT and CCPP. The conditions at the inlet of the compressor also determine how the GT and water/steam cycle are impacted separately due to FGR. For the combustion system the air/fuel-ratio (AFR) is an important parameter to show the impact of FGR on the combustion process. The AFR indicates how close the combustion process operates to stoichiometric (or technical) limit for complete combustion. The lower the AFR, the closer operates the combustion process to the stoichiometric limit. Furthermore, the impact on existing operational limitations and the operational behavior in general are investigated and discussed in context of an operation concept for a GT with FGR.