Deactivation of a steam reformer catalyst in chemical looping hydrogen systems: experiments and modeling

The utilization of real producer gases such as raw biogas or gasified wood for chemical looping hydrogen production implies the introduction of harmful contaminants into the process. Hydrogen sulfide represents one of the most challenging trace gases in the reformer steam iron cycle. The aim of the present work was an in-depth investigation of steam reforming with pure methane and synthetic biogas contaminated with selective concentrations of 1, 5 and 10 ppm of hydrogen sulfide. To validate the experimental data, the fixed-bed reactor system was modeled as one-dimensional pseudo-homogeneous plug flow reactor by an adapted Maxted model. In a preliminary thermodynamic study, the dry equilibrium composition was determined within a deviation of 4% for steam methane reforming (SMR) and 2% for synthetic biogas reforming compared to the experimental results. The impact of hydrogen sulfide on the reactivity of the catalyst was characterized by the residual methane conversion. The deactivation rate and extent is directly proportional to the concentration of H2S, as higher hydrogen sulfide concentrations lead to a faster deactivation and lower residual methane conversion. A comparison of the methane conversion as a function of sulfur coverage between experimental and simulated data showed good agreement. The predicted results are within <10% deviation for SMR and synthetic biogas reforming, except for sulfur coverages between 0.6 and 0.8. The temperature in the catalyst bed was monitored throughout the deactivation process to gather additional information about the reaction behavior. It was possible to visualize the shift of the reforming reaction front towards the bottom of the reactor caused by catalyst deactivation. The impact of sulfur chemisorption on the morphology of the steam reformer catalyst was analyzed by scanning electron microscope (SEM/EDS) and Brunnauer-Emmet-Teller techniques. SEM patterns clearly indicated the presence of sulfur as a sort of dust on the surface of the catalyst, which was confirmed by EDS analysis with a sulfur concentration of 0.04 wt%.