Dynamic LCA of H₂S adsorption from bio-syngas using mine residues

Hydrogen production from biomass and fuel cells is effective with respect to its small eco-burden and high energy efficiency. However, it is established that hydrogen sulfide (H2S) in gasified biomass (bio-syngas) can reduce the efficiency of fuel cells. To maintain high efficiency, H2S is removed by impurity adsorption. Although metal oxides are generally used as adsorbents, these can adsorb H2S only a single time, and the large amounts used represent an eco-burden. To overcome this problem, the use of mine waste (neutralized sediment) containing highly reactive iron was examined in a previous study, with H2S as an adsorbent to maintain a small eco-burden. It was found that its eco-burden is smaller than that of using metal oxides at 300°C. However, the authors treated the inlet H2S concentration, which affects sulfur capture capacity, as a constant, whereas the H2S concentration of biogas typically fluctuates dynamically between 50 and 1,200 ppmv. In addition, it is necessary to consider real situations from the perspective of a dynamic life cycle assessment. Consequently, it is necessary to determine the amounts of adsorbent used by considering changes in inlet H2S concentration. In this study, we investigated the relationship between changes in the inlet H2S concentration and sulfur capture capacity of neutralized sediment. The amounts used and eco-burden of neutralized sediment were calculated based on our experimental results, and on the basis of our findings, we established that the global warming potential of the proposed system at 300°0 was 70.7% smaller than that obtained using a metal oxide.