CFD model for tubular SOFC cell fed directly by biomass – Complete biomass gasification system integrated with SOFCs stack

Nowadays, with the advancing climatic emergency and the urgent need to reduce more and more the use of fossil fuels, we are constantly looking for new sources of renewable energy and devices capable of converting various forms of energy with high efficiency. In this context, the DB-SOFC project was launched, which is based on the use of biomass as a primary energy source for the production of syngas and SOFCs as energy conversion devices. The biomass chosen for this project is the olive kernel which is widely available in the area of interest of the project, that is the Euro-Mediterranean area. The biomass is initially gasified in a special reactor with CO2 as a gasifying agent, and the syngas produced is used as a fuel in SOFCs. Solid oxide fuel cells were chosen specifically thanks to their ability to electrochemically oxidize both pure hydrogen and carbonaceous fuel. This thesis follows the simulation work done on a single SOFC which had extracted from it the polarization curve and the efficiency of a single cell. The aim of the thesis is to simulate, through the COMSOL Multiphysics® software, the entire process of gasification and use of the fuel by all the fuel cells, thus extracting the velocity, temperature and pressure profiles of the syngas produced and the relative molar fractions of the gases at the outlet of the integrated system, using 3D geometry in the simulation. First, we tried to give a background on the state of the art of biomass and fuel cell technologies, explaining, when necessary, the theory behind these technologies. After that, I tried to explain the main chemical-physical characteristics of biomass, trying to characterize it as much as possible. At this point the physical equations the software must solve have been defined and the physical assumptions and hypothesis have been made to fully characterize the computational analysis of the problem. Starting from the constant parameters of the system, the geometry of the system, the mass flow rate of the biomass and CO2 were varied, trying to construct curves which, depending on the CO2/Biomass ratio, give relevant information on which geometry optimizes the process. The geometries taken into account in the xy plane have a square and circular geometry, while the height of the reactor along the z axis remains constant. The main variable, used to decide the most suitable geometry, is the total efficiency of the system. Finally, I graphed, in a 2D cross section of the system, the molar fractions of the various chemical species involved in the device, the temperature and all the most relevant results. Finally, a grid independence was performed to validate the model and calculate the uncertainty of the results. Some final observations and conclusions were also made regarding the project in its entirety.