Influence of Turbine Inlet Temperature on the Efficiency of Externally Fired Gas Turbines

Many researchers have considered externally fired gas turbines (EFGT) as an option for the implementation of biomass-fueled power plants. The EFGT cycle with regeneration or the gas-vapor combined cycle using one EFGT, also known as externally fired combined cycle (EFCC), could lead to significant efficiency improvements if compared to current technology used for power generation from biomass. This work presents one improved numerical model used for the simulation of EFGT cycle. The results were obtained with a numerical model for the EFGT cycle coupled with a model for the high temperature heat exchanger (HTHE) that is necessary for the cycle implementation. The model of the heat exchanger is based in correlations for the Colburn and friction factors, obtained with CFD simulations. In previous work, the model included only laminar regime for the heat exchanger. The present work extends the correlations that describe the behavior of the heat exchanger to turbulent and transitional regimes. The updated model of the EFGT cycle is used to investigate the influence of the turbine inlet temperature over the cycle efficiency. The results obtained confirm that the pressure drop caused by the heat exchanger is one important parameter that influences the cycle efficiency. The feasibility of the EFGT cycle is discussed taking into consideration that the highest temperature in EFGT cycle is not in the turbine inlet, but in the high temperature heat exchanger.