Gas Phase Desulfurization Using Regenerable Microfibrous Entrapped Metal Oxide Based Sorbents for Logistic PEM Fuel Cell Applications

Abstract

This dissertation presents results of R&D efforts to develop a thin, low pressure drop, high efficiency zinc oxide based sorbent using glass fibrous media as carrier to remove gaseous sulfur compounds from reformates for logistic PEM fuel cell power systems. The glass fibrous entrapped sorbents (GFES) contain 3 vol.% glass fibrous media, 22 vol.% particles (100~200 um) and 75 vol.% voidage. Therefore, GFES yielded much lower pressure drops than packed beds at the same test conditions. In thin bed tests, GFES demonstrated exceptional desulfurization and regeneration performance, compared with the packed beds of ZnO extrudates (1 mm) and particles of similar size (80~100 mesh) at equivalent reactor volume. Fundamental kinetic studies were conducted to investigate the improvements observed using GFES. The experimental results at 400 °C indicated that the desulfurization process using ZnO/SiO2 and GFES sorbents was controlled by the external mass transfer rate at a face velocity less than 11 cm/s, while the process using ZnO extrudates suffered from severe intra-particle mass transfer resistance. A modified Amundson model was applied to describe the relationship between the apparent rate constant (ka) and the sharpness (lumped K) of a breakthrough curve. Based on this model, the influences of microfibrous media and high voidage were discussed. The sorbent was also evaluated for sulfur removal from realistic reformates. The effects of CO, CO2 and water on the desulfurization performance were examined for ZnO based sorbents at 400 °C. Water and CO contents determine the H2S and COS breakthrough respectively, therefore total sulfur breakthrough. The homogeneous and heterogeneous COS formation pathways were revealed experimentally. Moreover, the low temperature performance of ZnO/SiO2 and GFES was also studied. It was found that the addition of copper dopant to ZnO/SiO2 could significantly improve the sulfur capacity and regenerability for the desulfurization applications at stack temperatures. Due to the high sulfur removal efficiency and low ZnO density, the GFES can be employed as desulfurizer for H2S removal at extremely low concentrations or as polishing layers in composite beds (packed beds followed by polishing layers downstream) to improve the overall breakthrough capacity