Operational Aspects for Direct Coupling of Gas Turbine and Solid Oxide Fuel Cells

Hybrid power plants consisting of a gas turbine and solid oxide fuel cells (SOFC) promise high electrical efficiencies if both components are directly coupled and the SOFC is operated at elevated pressure. This contribution discusses various aspects of the pressure influences on electrochemistry at the electrodes to operating strategies of a hybrid power plant. The influence of pressure on SOFC performance has been investigated theoretically and experimentally. Experiments are carried out using a test rig that allows for characterization of SOFC stacks at pressures up to 0.8 MPa. Performance curves and electrochemical impedance spectra are used for evaluations. In addition to experimental investigations an SOFC stack model is developed based on an existing electrochemistry modeling framework. The stack model is experimentally validated and used for a theoretical analysis of pressure. As expected, Nernst potential increases with increasing pressure causing a higher open circuit voltage. Furthermore, gas diffusion is enhanced with increasing pressure and the charge transfer reaction is facilitated due to higher adsorption rates of reactants at the electrode surfaces. At constant operating conditions and efficiency an increase in SOFC power density of up to 83% is measured. If power density is kept constant, electrochemical efficiency is improved by up to 14 %. Results generally show that pressure influence is stronger at low pressures up to 0.5 - 1 MPa and weakens towards higher pressures. The influence of pressure on formation of nickel oxide and solid carbon is investigated. An analytical evaluation of the nickel oxidation propensity shows thatnickel oxidation is more likely to occur at higher pressures because the equilibrium partial pressure of oxygen in the anode gas increases. Carbon deposition is another degradation mechanism that can decrease the performance of an SOFC system. It was investigated via thermodynamic simulations using the software package Cantera. Thermodynamic equilibrium of gas mixtures with different oxygen to carbon ratios is calculated showing that the aptitude for carbon deposition is highly pressure dependent. Carbon deposition should be avoidable if oxygen to carbon ratio is kept above 2 within conditions that are relevant for hybrid power plants. The developed stack model is integrated into an existing validated gas turbine model that is extended to include further SOFC system components. A system operating strategy is presen