Detailed exergetic evaluation of heavy-duty gas turbine systems running on natural gas and syngas

Abstract

Gas turbine systems are widely used for the production of electricity in a simple or combined-cycle mode today. Based on their ability to allow a fast load change, gas turbine systems will become even more important in the future since the volatile production of renewable energies will increase. In this study, a state-of-the-art gas turbine running on natural gas, having an overall net efficiency of approximately 40%, is modeled using Aspen Plus® and characteristic parameters are identified. Based on these parameters, a gas turbine running on syngas was simulated. The emphasis here is on a very detailed evaluation of the inefficiencies. The models consider cooling and sealing flows. The syngas considered in this study is typically used in IGCC processes with carbon capture resulting in a high concentration of hydrogen.For both systems, twelve types of inefficiencies were identified and rated. A comparison of the inefficiencies within each system and between both systems represented by their exergy destruction ratios is presented.In case of the gas turbine running on natural gas, the most important results show that the stoichiometric combustion, followed by the addition of excess air represent the largest inefficiencies. When just applying an isentropic efficiency, the exergy destructions associated with expansion and mixing at different temperatures and pressures of a gas turbine stage cannot be further sub-divided. Hence, this grouping of inefficiencies results to the third position. The effect of mixing at different compositions and the compression follows.In the second case considered here (use of syngas instead of natural gas), the effects of mixing and adding excess air become more significant due to a higher specific heat capacity of the combustion gas. In both cases, the exergy destruction associated with mixing at different compositions can be neglected except the one at the inlet of the pre-mixed combustor, which strongly depends on the particular conditions of the fuel gas. Inefficiencies such as convective cooling of the vanes and blades, heat loss, losses associated with the shaft and generator were found to represent a very small part of the overall exergy destruction. The resulting exergy destructions and losses are shown in an exergy flow diagram.