An Investigation of the Heat Recovery System in Externally Fired Evaporative Gas Turbines

Externally fired gas turbines have the features of using solid fuel and requiring no particulate cleaning up to protect the gas turbine path. The solid fuel can be, for example, coal or biomass. Evaporative gas turbines (e.g., HAT cycle) have the potential to enhance the power output and increase the efficiency without including a bottoming steam turbine. The integration of the two systems, so called externally fired evaporative gas turbine, can offer the features from both of the systems. In the present paper, the modified externally fired evaporative cycle with intercooling and recuperation is proposed and analyzed. The externally fired gas turbine system is divided into three main subsystems: gas turbine subsystem, solid fuel combustion subsystem and heat recovery subsystem. This paper presents an in-depth investigation of the heat recovery subsystem and its impacts on the total system. The effects of intercooler and aftercooler on the whole system have been addressed and discussed. The optimization strategies for multiple interaction variables, such as air temperature after the recuperation, water-to-air ratio and combustion air temperature for the externally fired combustor, have been provided. The optimization results show that the behavior of the heat recovery subsystem greatly affects the cycle efficiency and power output. Using exhaust heat to heat humid air in a recuperator and to preheat combustion air to the biomass combustor are important for improving the externally fired evaporative gas turbine system. With the electrical efficiency as the objective function of the optimization, there exists an optimum water-to-air ratio located at 0.17 to 0.20 for the system studied in this paper.