Microfibrous Entrapped ZnO-Support Sorbents for High Contacting Efficiency H2S Removal from Reformate Streams in PEMFC Applications

摘要

A sintered microfibrous carrier consisting of 2.0 - 3.0 vol% of 4 and 8 um (dia.) Ni fibers is utilized to entrap from 20 to 30 vol% of 150-250 um (dia.) carbon and SiO2 support particulates. Zinc oxide is then placed onto the supports by impregnation at loadings ranging from 15 to 20 wt%. Two different sorbent recipes have been developed. ZnO/Carbon entrapped material for low temperature use is envisioned to operate as a last line of defense at stack temperatures. ZnO/SiO2 entrapped material is employed for regenerable use in a continuous batch mode at ca. 400°C to scavenge bulk H2S. The nano-dispersed nature of ZnO combined with the use of small support particulates promotes high ZnO utilization, high contacting efficiency, and high accessibility of ZnO. At equivalent bed volumes, microfibrous entrapped sorbents provide 2- to 3-fold longer breakthrough time for H2S (with a 67% reduction in sorbent loading), compared to packed beds of commercial 1-2 mm extrudates. Five-log reductions in H2S concentration with up to 67% ZnO utilization at breakthrough are achieved. Hydrogen sulfide concentrations from 50 ppmv (up to 20,000 ppmv) can be reduced to as little as 0.1 ppmv (at R.T.) and 0.6 ppmv (at 400°C) in 30% H2O at face velocities of 1.2-1.7 cm/s for layers as thin as 1.0 mm. Regenerability in air at 500-600°C is also facilitated by the nano-dispersed nature of the ZnO and the use of small support particulates. The recovery percentage of ZnO utilization using microfibrous entrapped sorbents is up to 5-fold higher than that for packed beds of 1-2 mm commercial extrudates. Furthermore, composite beds consisting of packed beds of large extrudates (ca. 1-2 mm dia.) followed by the above noted microfibrous entrapped sorbents as.polishing layers has been demonstrated with a great extension in gas life. This approach synergistically combines the high volume loading of packed beds and the overall contacting efficiency of small particulates.