Thermodynamic Modelling and Performance of Combined Solid Oxide Fuel Cell and Gas Turbine Systems

Sammanfattning: Solid oxide fuel cells (SOFCs) are a ceramic type of fuel cell operating at elevated temperatures. By utilising the thermal energy from the SOFC in a heat engine, a hybrid cycle with high performance can be achieved. One promising concept is a pressurised SOFC in combination with a recuperated gas turbine cycle. The fuel cell, being the more efficient power generator of the two, represents the major output of such a system. Most likely intended for small-scale power and heat generation these systems can employ a microturbine. This work was concerned with thermodynamic modelling of combined solid oxide fuel cell and gas turbine (SOFC/GT) systems, and the evaluation of their performance. A detailed fuel cell model was developed by Azra Selimovic, and adapted and implemented in a process simulation program by the author. The fuel cell model considered a planar SOFC design with internal reforming and heat conduction in the solid part. Electrolyte-supported cells were assumed, which are typical for a high-temperature SOFC. The modelling approach was unique, integrating a two-dimensional cell model into a flowsheet model. Operational analysis was flexible as changes in the system parameters, as well as cell parameters, could be assessed on the overall behaviour of the systems. Although no validation of the system model was possible due to a lack of data, the fuel cell model was compared with other models in the literature with good results. Both parametric and to a lesser extent, conceptual variations of a reference SOFC/GT system were made, resulting in, generally, good performance under different operating conditions. The investigation showed greater effect on the system performance of the cell parameters (e.g., maximum solid temperature, internal cell resistance, cell geometry) than of the system parameters (e.g., pressure, turbine inlet temperature, recuperator size). By introducing conceptual changes into the SOFC/GT system, such as networking of stacks and cathode gas recycling, significant improvement of the system performance was achieved. In the first case, air and fuel were ducted in series between stacks, which led to higher utilisation of the fuel in the stacks and allowed for lower air flow rates. In the second concept, the air flow required by the cell could be decoupled from the flow to the gas turbine, thereby increasing the fuel utilisation and power fraction of the fuel cell part of the system. In addition, environmental aspects of SOFC/GT systems were covered, such as SOFC systems with CO2 abatement, biomass-fuelled SOFC/GT systems and design analysis of a combustor applied to burn SOFC off-gases. In the first study the concept of multistage oxidation (networking of stacks) was elaborated to capture CO2 in the anode (reacted fuel) stream. In the second study a gasifier and gas treatment system for biomass were modelled and implemented in a SOFC/GT flowsheet model. The combustor design analysis resulted in a non-premixed burner type adapted to the SOFC/GT application. Promising results were obtained in all studies.