Modeling of solid oxide fuel cell performance with coal gasification

Abstract

Growing concern over greenhouse gas emissions has driven research into clean coal power production alternatives. Novel coal power plant designs that lower CO2 emissions are imperative in the coming decades to mitigate global temperature rise. High-efficiency stationary power systems that integrate coal gasification with solid oxide fuel cells (SOFCs) have been championed by the Department of Energy for the past couple of decades. However, many fundamental questions about this system still need to be addressed by modeling the complex coupling between SOFC's and gasification. More specifically, work is needed to characterize SOFC performance with a range of syngas (H₂+CO) mixtures produced by coal gasification. This thesis used a multiscale modeling approach to analyze SOFC performance with coal syngas at both the systems level and at the surface reaction scale. The first investigation in this thesis couples an equilibrium gasifier model to a detailed ID SOFC model to study the theoretical performance of the coupled system run on steam or carbon dioxide. The results of this study indicate that the system performs substantially better with steam gasification than with CO₂ gasification as a result of the faster electro-oxidation kinetics of H₂ relative to CO. The coupled system is then shown to reach higher current densities and efficiencies when the heat released by the fuel cell is sent to the gasifier instead of a bottoming cycle. 55-60% efficiency is then predicted for the system with heat transfer and steam gasification, making this technology competitive with other advanced system designs and almost twice as efficient as conventional coal-fired power plants. The second study in this thesis investigates SOFC behavior with H₂ and CO (syngas) mixtures that come from coal gasification. SOFC models typically neglect CO electrochemistry in the presence of H₂ and H₂0, assuming that the water-gas-shift reaction proceeds faster than CO electrooxidation. The results of this study show, however, that CO electro-oxidation cannot be neglected in syngas mixtures, particularly at high current densities for high CO-content syigas. First the simulations demonstrate that incoming CO is not all shifted to form H₂ by reforming reactions before reaching the electrochemical reaction sites. Furthermore, the results of this 'study confirm that direct electro-oxidation of CO contributes non-negligible current relative to H₂ at high anode overpotentials. Together these results show that CO electro-oxidation plays an important role in SOFC performance not only via water-gas-shift reforming, but also via direct electro-oxidation when H₂ is also present. This work suggests that accurate models for both surface reforming and direct electro-oxidation of CO in SOFC anodes must be included in order to capture performance when using coal syngas mixtures. Finally, a multi-step mechanism for the simultaneous electro-oxidation of H₂ and CO in SOFCs is implemented and studied. This mechanism combines a couple of reaction pathways: hydrogen (H) spillover to the electrolyte, and oxygen (O) spillover to hydrogen and CO on the anode. This mechanism is successfully verified in the model against a wide range of experimental data for mixtures of CO/CO₂, H₂/N₂, H₂/H₂0, H₂/CO, and H₂/CO₂ . The simulations show that H spillover is the dominant source of current at low anode activation overpotentials, but also demonstrate that the currents produced by 0 spillover are non-negligible at high overpotentials. Furthermore, it is shown that the current produced by 0 spillover to CO is not limited by the rate of CO adsorption on nickel, which leads CO to contribute more to cell performance at high currents. Together these three modeling studies demonstrate how coal can be efficiently converted to electricity via gasification and the simultaneous electro-oxidation of H₂ and CO in a solid oxide fuel cell.