Numerical Analysis of Combustion of the Exhaust Gas of a Solid Oxide Fuel Cell
thesisposted on 28.10.2014 by Daniele Bongiovanni
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
As the energy demand and the pollution concerns continue to grow, new sources of energy and environmental friendly power generation systems are being developed. In this context, syngas is considered as an attractive fuel, as it can be produced from a broad range of fossil fuel and biomass sources, including agriculture and municipal waste, using gasification processes. It is mainly composed of H2 and CO, along with varying amounts of N2, CO2, and H2O. In the following work a comprehensive study about a low-energy-content fuel, presenting the characteristic components of syngas, is developed. This fuel differs from common syngas in the relative ratios between the species of the mixture; in the reported case the presence of H2 and CO is largely reduced. The present research deals with the combustion of the exhaust gas of a solid oxide fuel cell. The exhaust stream from the SOFC stack, fed with biogas, contains CO2, H2O and some trace amount of H2 and CO resulting from incomplete utilization of fuel in the SOFC stack. Consequently exhaust anodic fuel can be combusted with depleted air leaving the cathodic compartments of the stack, with the aim of exploiting its retained energy content and enhance the global efficiency of the system. This combustion process, however, generates several undesirable compounds, especially NOX . Modifying of the combustion atmosphere, a change from air to oxy-combustion, allows elimination of nitrogen oxides. Recently, oxygenenriched combustion is obtaining greater acceptance, in fact its potential value in terms of both improved heat transfer and reduction of NOX can overcome the cost penalty of burning a fuel in a oxidizer other than air. For these reasons, both pure oxygen and air solutions are covered. An extensive investigation of the considered mixture is provided, beginning with equilibrium calculation as a preliminary analysis, continuing with opposed flow flame simulations, that allow a better understanding about the suitable combustion temperatures, the production of pollutant compounds, the influence of strain rate on the diffusion flame and the flammability limits. In the second part of the thesis, the main focus shifts towards the development of a detailed study about the extinction behavior of syngas fuels. For this matter, a common syngas mixture (50% H2- 50% CO) is employed as fuel for counterflow flames. Multidimensional simulations are carried out using an advance software for unsteady reacting flows. Studies about extinction limits, blow off extinction and effects related to scalar dissipation rate have been extensively covered in the present thesis as principal objective of this part. Differences and analogies between one- and two-dimensional simulations outputs are reported and discussed, introducing an interesting field of research.