Optimization and Investigation of SO2 Adsorption Process for Solid Oxide Fuel Cell Cathode Protection

Abstract

Gas-phase contaminant removal is a critical process for power supply systems (e.g. fuel cells and gas turbines). Because of high-energy efficiency, low emissions, and fuel flexibility, Solid Oxide Fuel Cells (SOFCs) are promising energy converting devices. However, various types of contaminants on the cathode side of SOFCs can cause performance degradation, such as sea salt particles and sulfur compounds. Recently, researchers have shown significant degradation caused by SO2, which reacts with cathode side materials and blocks the active sites available for oxygen reduction reaction. The major SO2 source is the emission from marine fuels such as JP-5 and JP-8 whose sulfur content can be up to 1200 ppmw. It has also been reported that 5-20 ppm SO2 can be generated after combustion by using high sulfur containing hydrocarbon fuels. Moreover, researchers revealed that cell exposed to as low as 100 ppbv of SO2 underwent a current loss. Therefore, it is necessary to remove SO2 from cathode side down to a sub ppm level. In this study, SO2 breakthrough performance was studied by using various transition metal supported on gamma alumina. All of the transition metal containing adsorbents were prepared using incipient wetness impregnation method. Among the candidates, manganese-containing adsorbents exhibited the best sulfur capacity in the preliminary screening test. Furthermore, the effect of manganese loading, types of supports, and calcination temperature were further investigated. Addition of 5 wt.% Mn demonstrated the highest breakthrough and saturation capacity in the presence of O2 at room temperature. XRD and H2-TPR technique revealed that Mn3O4 and Mn2O3 co-existed in well-dispersed forms. Furthermore, the Mn/Al2O3 adsorbent showed better performance than Mn/ZrO2 and Mn/TiO2. A possible explanation was that Mn/Al2O3 possessed larger population of terminal (isolated) hydroxyl groups on the sorbent surface, which was beneficial during the SO2 adsorption process. According to the XPS study, the hydroxyl groups on 5Mn/Al2O3 and 5Mn/ZrO2 were 18.8 % and 15.9 %, respectively. Therefore, the XPS study can explain why the capacity of using Al2O3 support was higher than that of using ZrO2. Additionally, the role of manganese oxidation state was studied by XPS and H2-TPR technique. It was also believed that the activity of low valence manganese oxide species was higher than that of Mn(IV). This conclusion was supported by the study of calcination temperature effect. In chapter III, the TPR results show the average oxidation state (AOS) decreased monotonously with increasing calcination temperature. The SO2 removal capacity of 5Mn/Al2O3 calcined at 650 °C (AOS =1.57) exhibited the highest performance in the presence of 10 ppm SO2 in air. Oxygen chemisorption was also used to characterize the dispersion of manganese on the surface; however, the effect of active metal dispersion was not clear in this work and needs further investigation. The textual properties such as BET surface area, pore volume were also measured. They were important factors in SO2 adsorption process. The regeneration condition and aging tests were discussed in this part as well. The kinetics of non-catalytic gas-solid reaction between MnOx/-Al2O3 and SO2 at room temperature (20 °C) was investigated in Chapter IV. A mathematical model coupled with axial dispersion effect, external and internal diffusion resistance, and depletion mechanism of sorbent can not only fit the breakthrough results but also predict the breakthrough curves of packed bed (PB) and microfibrous entrapped sorbents (MFES) with acceptable deviations. The model consists of two important variables: initial sorption rate constant (ko) and depletion rate constant (kd), which can be determined by fitting the model to experimental data. Due to uniform flow pattern and minimized bed channeling, MFES outperformed conventional packed bed. The regeneration performance of MFES surpassed that of PB significantly in model anode exhaust gas (AEG) at ca. 650 °C. Furthermore, MFES maintained its breakthrough capacity up to 5 regeneration/adsorption cycles. Thus, the drawbacks of the packed bed due to flow maldistribution can be addressed employing MFES. In Chapter V, based on the mathematical model derived in the previous chapter, we could estimate the parasitic power consumed by the fuel cell system under various filter designs. For the case study, a 60 kW fuel cell stack was investigated. Generally speaking, when long term protection is required, composite bed and packed bed is the best option due to their high capacity. However, when long term protection is not the primary concern and high removal efficiency is required, MFES consumes the least parasitic power. Hence, it is vital to choose the best filter design based on various environmental condition. In Chapter VI, the strategy of protecting the Proton Exchange Membrane Fuel Cells (PEMFC) was discussed. Volatile Organic Compound was selected as the model impurity. Various activated carbons were used as sorbents for the fuel cell filters. Meanwhile, Yoon and Nelson model was used to describe this adsorption process. In this part, we also applied a chemical activation method to enhance the adsorption performance. Some proposed future works are listed in Chapter VII, including the investigation of altering the population of surface hydroxyl groups on the sorbent surface, UV-assisted SO2 adsorption on Mn/TiO2, incorporation of other additives to enhance the SO2 removal performance.