H2S removal with ZnO during fuel processing for PEM fuel cell applications

The efficacy of using ZnO as an absorber for the removal of H₂S from small fuel processor steam reformate streams was examined. At temperatures below 300 °C, H₂S can in principle be reduced to below 100 ppbv, required for safe operation of PEM fuel cells. The ZnO adsorbent performed predictably based on ZnO-H₂S-ZnS-H₂O equilibria with steam, hydrogen, and CO₂ in the feed. However, addition of CO even at levels as low as 1 vol% drastically lowered the sulfur removal capability of the ZnO. This is consistent with the formation of COS by the reaction H₂S + CO = H₂ + COS. ZnO is not an efficient absorber for COS. Indirect evidence is also provided for the formation of COS via the reaction CO₂ + H₂S = COS + H₂O, which can occur in special cases when CO₂ is present but neither H₂O nor CO is present in the feed. The potential for COS formation compromises the ability of ZnO to deliver very low sulfur concentration reformate.