The increasing importance of decentralized energy production based on renewable resources requires gas turbine systems due to their low emissions and flexible energy conversion. Therefore, a suitable hybrid power plant demonstrator consisting of an SOFC (solid oxide fuel cell) coupled to an MGT (micro gas turbine) is being set up at the German Aerospace Center (DLR). This facility requires a burner concept for low calorific gases capable of combusting the exhaust products of the fuel cell system anode side, here referred to as SOFC off-gas. The combustor behavior for the demonstrator system is investigated using an atmospheric combustor test rig at DLR. The main aspect of this work is the combustor operation inside the power plant system with varying power demands and also varying methane contents, representing biogas operation. This is leading to operating points with very low heating values (LHV) which require a flame stabilization strategy via direct addition of natural gas / biogas into the SOFC off-gas before entering the combustor. This is tested in view of expected impacts on electrical system efficiency and other critical system parameters. The combustion system is furthermore investigated in view of CO emissions in various significant operating points.
An atmospheric prototype burner is studied with numerical and experimental tools. The burner system is designed for operation in a hybrid power plant for decentralized energy conversion. In order to realize such a coupled system, a reliable combustion system has to be established. Numerical and experimental findings in the presented study demonstrate the capabilities of the novel burner system in suitable operation conditions. In this system, a solid oxide fuel cell (SOFC) is mounted upstream of the burner in the gas turbine system. The combination of both realizes a large operational flexibility with comparably high overall efficiency. Since the combustor is operated with SOFC off-gas, several challenges arise. Low calorific combustion needs careful burner design and numerical modeling, since the heat-loss mechanisms occur to be in the order of magnitude of thermal power output. Thus, different modeling strategies are discussed in the paper. The numerical studies are compared with experimental results and high-quality simulation results complement limited measured findings with easy-to-use low fidelity RANS models. A priori measurements are employed for the selection of investigation points. It is shown that the presented combustor system is able to cover low-calorific combustion over a large range of operation conditions, despite major heat-loss effects, which are characterized by means of numerical CFD (Computational Fluid Dynamics) modeling.
A hybrid power plant system concept based on a 30 kW el solid oxide fuel cell (SOFC) and a 3 kW el micro gas turbine (MGT) has been developed at the German Aerospace Center (DLR). At the Institute of Combustion Technology, a test rig with a MGT and emulated SOFC has been built, in order to investigate the effects of each system on the other, and to validate the concept of the hybrid power plant before coupling the systems. A control strategy for the emulator test rig has been elaborated and tested to allow for system operation at all expected load points. Different control loops based on PI and PID controllers have been parametrized and successfully tested. A turbine outlet temperature (TOT) controller with a feed-forward element was implemented and good performance during transient maneuvers has been shown. Start-up and shutdown routines, as well as an emergency stop maneuver, have been tested and integrated into the test rig state machine.
An increasing importance of decentralized energy production based on renewable resources in combination with rising power demands requires relatively small energy conversion systems in terms of power output. This situation is addressed at the German Aerospace Center with the development of a hybrid power plant demonstrator with a high temperature fuel cell. The power plant requires burner concepts for low calorific gases capable of combusting the exhaust products of the fuel cell anode side, here referred to as SOFC (solid oxide fuel cell) off-gas. The combustor behavior for the demonstrator plant is investigated in this work by means of numerical simulations and basic experiments. As a main focus, simulations are employed in order to characterize the combustion system in greater detail. Several full-and part-load conditions are treated with steady state reacting RANS (Reynolds Averaged Navier Stokes) simulations. The numerical investigations are validated with averaged flame images from OH*-chemiluminescence data. Based on this, a detailed insight into flow field and combustion characteristics is provided. It is shown that the employed CFD modeling strategy, accounting for turbulence, detailed combustion kinetics, and heat loss effects can be used as a reliable design and diagnostics tool, capturing flow field and combustion accurately with at the same time very low overall computational costs.
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