Repurposing mining residue as a CO2 adsorbent in ethanol steam reforming for hydrogen production

Abstract

An innovative and sustainable pathway for hydrogen production from sorption-enhanced steam reforming (SESR) of ethanol was evaluated. For the first time, a high-limestone-content mining residue – previously unexploited in SESR – is evaluated as a functional CO₂ adsorbent used in physical mixture with Ni-based catalytic systems. This dual-purpose application supports circular economy principles by transforming an abundant industrial by-product into a valuable material for renewable energy processes. Thermodynamic modelling guided the selection of optimal process parameters (600 °C, steam/ethanol molar ratio of 6, CaO/C ratio of 1), while Ni-based catalysts (10 wt% Ni-Al₂O₃ and 10 wt% Ni-MgAl₂O₄) were physical mixed with mining waste (MW) (1:1 w/w) and were tested over ten SESR reaction–regeneration cycles, with regeneration consisting solely of adsorbent decarbonation at 800 °C under N₂. Both systems achieved ethanol conversions exceeding 99 %, with peak dry-basis H₂ molar fractions of 82.2 % (Ni-Al₂O₃/MW) and 75.8 % (Ni-MgAl₂O₄/MW). CO molar fractions of 20 % (Ni-Al₂O₃/MW) and 15 % (Ni-MgAl₂O₄/MW) indicated incomplete water-gas shift (WGS) progression, which limited H₂ selectivity. Nonetheless, CO₂ capture was highly effective, with outlet molar fractions below 2 % for Ni-Al₂O₃/MW and below 5 % for Ni-MgAl₂O₄/MW. For Ni-Al₂O₃/MW, the pre-breakthrough period lasted approximately 56 min in the first cycle and 35 min in the tenth cycle. For Ni- MgAl₂O₄MW, the pre-breakthrough period was shorter (∼28 min) and also shows a slight reduction over the cycles. These findings represent a promising first assessment of this mining residue in SESR applications, demonstrating its stability and regeneration potential as a low-cost adsorbent for CO₂ removal in renewable hydrogen processes.