Effects of a Real Syngas On PdCu-Alloy Hydrogen Separation Membranes

Gasification is generally recognized as a viable near-term approach for addressing national fuel security and economic sustainability concerns, while simultaneously addressing the control of greenhouse gas emissions. The gasification process can utilize a variety of carbon-based feed stocks such as coal and biomass to produce synthesis gas (syngas). Integrating a water-gas shift reactor with simultaneous hydrogen separation (a WGS membrane reactor) can enhance CO-conversion beyond that attainable in conventional reactors decreasing the necessity for multiple reactors, added catalysts, and large quantities of steam, while maximizing H₂ yield and producing a highly concentrated, high pressure stream of carbon dioxide ready for co-sequestration. Dense metal membranes are a promising technology for the separation of H₂ from these other product gases due to their high permeability and high selectivity for H₂. However, impurities typically contained in coal derived syngas can have adverse effects on metal-based separation membranes. Sulfur compounds, particularly hydrogen sulfide, are probably the most detrimental to metal membrane technologies. H₂S is well known to severely impact the performance of dense metal membranes. These effects include catalytic poisoning and corrosion which can result in severe performance degradation and mechanical failure. In laboratory studies, the effects of minor contaminants such as As and Se compounds are not often considered.  Therefore, developing a thorough understanding of the effects of syngas constituents, especially H₂S but including minor components, on metal membrane-based H₂ separation membranes is imperative to facilitate the acceptance and deployment of membranes in gasification technologies.

NETL’s Office of Research and Development is currently utilizing a variety of experimental and computational techniques to evaluate and develop potential sulfur tolerant membrane alloys. A variety of binary and ternary Pd-based alloys (Pd combined with Cu, Ag, Au, Pt, etc.) are being studied for application in the severe conditions and environments associated with the gasification process. Previously, the performance of these Pd-based alloy membranes were evaluated under laboratory conditions in the absence and presence of 1000 ppm H₂S in H₂ at conditions consistent with syngas processing. The observed H₂ flux trends on exposure to 1000 ppm H₂S in H₂ were correlated with results obtained from post test XRD and SEM/EDS membrane characterization. Additionally, coupon samples of these alloys were exposed to real syngas at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama to evaluate their tolerance to actual gasifier conditions. The coupons were exposed to syngas containing about 250 ppm H₂S for approximately five weeks at about 400°C for most of the exposure period. The post-exposure coupons were characterized by methods including XRD and SEM/EDS to evaluate morphological changes and corrosion deposit formation. These results are compared to the performance data collected on the same alloys at NETL under similar controlled laboratory conditions. The NCCC results demonstrate that the B2-structured palladium alloys tend to be more corrosion resistant than the fcc-structured alloys, which is in agreement with laboratory studies. In the B2 alloys, corrosion products formed a very thin surface coating contain high levels of arsenic and selenium in addition to sulfur. The membrane alloys are effective in scavenging some impurities, especially As and Se, from the syngas and indicate that minor gas stream impurities are much more important than previously thought. These experiments represent a rare opportunity to study alloy performance under actual gasifier conditions and long exposure time.