Fast Prediction of Combustor Stability Limits in Humidified Micro Gas Turbines Using Hybrid 0D/1D Model

Abstract

Abstract Cycle humidification is considered as an interesting route to enhance the micro Gas Turbine (mGT) operational flexibility, ensuring a place for mGTs in future small-scale decentralized power production. Indeed, humidifying the mGT cycle when there is no heat demand increases the electrical efficiency of the cycle, and thus the operational flexibility of the unit. However, the rather small operating range of the mGT combustor under these diluted conditions remains the main limitation, restricting the humidification rate, and thus the potential cycle improvement. Therefore, a complete characterization of the combustor performances for a range of operating conditions in combination with an increasing water-to-air ratio allows quantifying the operability limit of the engine under these unconventional diluted conditions. To improve the mGT cycle further, the limits to reach stable, complete, and low emissions combustion are assessed using a hybrid 0D/1D model of the combustor of typical mGT, namely the Turbec T100 mGT, for different humidification pathways (steam or water injection, saturation tower, ...). This model combines 0D Chemical Reactor Networks with 1D unstretched laminar flame calculations. The results of this computationally fast and flexible model are experimentally validated for both dry and wet cycle operation. For moderate water fractions, the simulation results show that complete combustion with low CO emissions, but at reduced temperature, could be achieved at a constant equivalence ratio compared to the dry reference case. At increased water fraction, stable and complete combustion at similar CO emission levels could still be achieved by increasing the equivalence ratio, compensating for the reduced temperature in the combustion chamber. Finally, at high water fractions, the results show unacceptably high CO levels, as a result of incomplete combustion due to the too-low temperature or too-low flame speed. With these simulations, we were able to assess the influence of humidified conditions on the combustion stability as well as to determine the operating limit helping so future cycle improvements.