Biomass-powered Solid Oxide Fuel Cells: Experimental and Modeling Studies for System Integrations

Biomass is a sustainable energy source which, through thermo-chemical processes of biomass gasification, is able to be converted from a solid biomass fuel into a gas mixture, known as syngas or biosyngas. A solid oxide fuel cell (SOFC) is a power generation device that directly converts the chemical energy of a fuel to electricity. Therefore, biomass-powered SOFCs could be highly efficient. Typically, in addition to carbon dioxide and water vapor, the major components of syngas produced from biomass gasification include hydrogen, carbon monoxide and methane which are potential fuels for SOFCs, which make integration possible between SOFCs and biomass gasifiers. However, the syngas is also comprised of trace species such as tars, H2S, HCl, and alkali compounds, among others, which could be detrimental to SOFCs if they are contained within the feeding syngas stream. Therefore, the syngas must be pretreated in order to reduce these trace species to a level that SOFCs are able to tolerate. With various gas treatments, the overall system performance would fluctuate, and therefore, the influence of the gas treatment methods on the system performance must be understood. The most prominent among the trace species is tar. The effect of tars on the performance of SOFCs has yet to be studied, however, it is known that, even though tar can possibly poison the fuel cell through carbon deposition, it may also become a fuel for SOFCs. Furthermore, SOFC systems are currently designed in general for employing natural gas. Due to the fact that SOFC systems are very sensitive to the fuel types, it is necessary to completely understand the system response when switching from natural gas to biosyngas to enable a better controllability for future experiments. The research scope of this thesis is limited to the aforementioned issues. The objective of this thesis is to provide a fundamental study to ensure a safe and efficient system integration. The study is limited to an existing downdraft fixed-bed gasifier and a 5 kWe SOFC CHP system due to these two units entering the commercial market. The approach utilized, however, could be further adopted for the large scale power plants based on biomass gasifiers and SOFCs. The research begins with the evaluation of technologies involved biomass-powered SOFCs in chapter 2. Technologies regarding biomass gasification, gas cleanup and fuel cells are discussed based on literature surveys. The review begins by briefly summarizing conventional gasifiers including fixed-bed and fluidized bed gasifiers, which are implented for biomass gasification. Following that, details are indicated for SOFC performance affected by the trace species such as particulates, H2S and available cleaning technologies. The combination of biomass gasifiers with fuel cells including proton exchange membrane fuel cells (PEMFC), molten carbonate fuel cells (MCFC), and SOFCs is then reviewed with an emphasis on the development of SOFC technology and the study of integration between biomass gaisifers and SOFCs. Chapter 3 presents a thermodynamic study of the influence of cleaning technology on the energetic and exergetic performance of the integrated gasifier–SOFC system with distinctive system configurations. Two gas cleaning systems, specifically, a combined high and low temperature gas cleaning system and a high temperature gas cleaning system are considered to connect the gasifier with the SOFC system. The influence of the steam addition for the suppression of carbon deposition and various heat sources for steam generation on the system performance is evaluated. The performance of the SOFC system operating with natural gas and biosyngas is also compared. The installed SOFC system, particularly the embedded pre-reformer and anode off-gas recirculation was initially designed for natural gas. This design is desirable as it effectively uses the steam in the anode off-gas and the heat generated in the stack. As SOFC performance is very sensitive to gas composition and operating conditions, both of which are affected by the anode recirculation, an evaluation of the recirculation behavior on safety issues regarding carbon deposition and nickel oxidation and system performance are presented in chapter 4. An important finding is that, by not implementing the recirculation, the biosyngas-fueled SOFC system effectuates a much higher net electrical efficiency, less initial investment and simpler system configuration in comparison to that when recirculation is implemented. Tolerance of SOFCs to the trace species from biomass gasification is not yet fully understood. The influence of biomass gasification tars on SOFC performance and mitigation of carbon deposition are experimentally evaluated in chapter 5&6. Well-controlled operational conditions assist in the suppression of carbon deposition. Chapter 5 presents the influence of operating conditions including steam levels, current density and time on stream on the performance of SOFCs with Ni–YSZ anodes fueled by tar-containing biosyngas at 800 °C. Changes in impedance spectra and polarization curves of SOFCs following tar exposure were analyzed to assess the cell performance. The biosyngas composition and the tar concentration employed in these measurements were identical to those measured from the commercial air-blown biomass gasifier that is to be connected to the studied SOFC system. Operating this type of SOFC with the tar concentrations could result in severe damage to the cell due to carbon formation on the anodes. Scanning Electron Microscopy (SEM) indicated carbon deposition which affected the performance of the SOFC, as is exhibited by the impedance spectra and anode polarization curves of the cells after exposure to tars. However, the risk of carbon deposition could be alleviated by increasing steam levels and current loads. Chapter 6 presents a similar study of the effects of tar on SOFC performance, but possesses a focus on Ni–GDC anodes and various operating temperatures levels (700, 800 and 900 °C) under both dry and wet conditions. Polarization behavior, electrochemical impedance spectroscopy, and cell voltage degradation were analyzed to evaluate the cell performance. It is most likely that the cells with Ni–GDC anodes did not suffer from carbon deposition under the wet conditions studied. Dry tar-containing syngas for SOFCs is unlikely to cause carbon formation under a mild current load; however, it may induce carbon formation at open circuit. The effect of carbon dioxide that is capable of suppressing carbon deposition was experimentally investigated, and an enhanced performance was observed under the conditions studied. Under carbon risk-free operating conditions, the cell voltage increases when raising the feeding tar concentration, indicating that tar performs as fuel for SOFCs. Numerical simulation is an efficient tool for the evaluation of SOFCs’ response when switching fuels. Chapter 7 presents such a numerical study with the focus on the evaluation of kinetic models for methane steam reforming for SOFCs operation with multiple fuels. Three frequently employed kinetic models were selected in order to examine their impacts on the performance of a tubular SOFC. The resulting thermo-electrochemical behaviors derived from these models were compared. It was discovered that all three kinetic models are reasonably accurate in terms of the polarization behavior, but they significantly affected the local thermo-electrochemical performance. A more rapid kinetic model was adopted based on the evaluation of these three kinetic models in order to evaluate the performance of the tubular SOFC in terms of local electrochemical performance, anode oxygen partial pressure and overall SOFC performance when performing with multiple fuels. Chapter 8 draws the conclusions regarding the work presented in this dissertation, and recommendations are suggested for future research activities