| Abstract |
This study aims to reduce the environmental impact of impurity adsorption systems on hydrogen production. Biomass is a carbon-neutral hydrogen fuel with a low environmental impact. However, during the production of biomass-derived hydrogen (biohydrogen), the hydrogen sulfide contained in the gasified biomass reduces the performance of fuel cells when hydrogen is used. Generally, hydrogen sulfide (H2S) is removed via impurity adsorption. However, common adsorbents such as metal oxides have a significant environmental impact. Therefore, adsorbents with lower environmental impacts are required. Thus, to improve metal depletion from the circular economy perspective, this study proposes using neutralized sediment, a waste product from the mining wastewater treatment, as an H2S adsorbent. In this study, we discuss the environmental impact of H2S adsorption systems that use neutralized sediments as adsorbents. Although the use of waste materials has a small environmental impact, it is necessary to consider the environmental impact of the inputs and outputs related to waste material generation based on life cycle assessment (LCA). Because fossil fuel-generated electricity and chemical neutralizers are input in mine wastewater treatment, the use of neutralized sediment may have a large environmental impact than the use of metal oxides from the viewpoint of LCA. Therefore, we quantitatively evaluated the environmental impact of neutralized sediment using LCA and investigated whether the environmental impact of using neutralized sediment is less than that of using metal oxides. In addition, it is not appropriate to include the production stage in the system boundary to meet the LCA standards because mineral water treatment, which is the production stage of neutralized sediment, is conducted whether neutralized sediment is utilized or not as an H2S adsorbent. Therefore, an LCA that does not include the production stage was also conducted to indicate the environmental benefit of waste utilization according to the actual situation. The amount of neutralized sediment used, which is calculated by the sulfur capture capacity, is necessary for the LCA calculation. However, the sulfur capture capacity of neutralized sediment has never been investigated. Hence, dynamic adsorption tests were conducted during gasification at low (40-120 degrees C) and high (200-300 degrees C) temperatures to investigate the sulfur capture capacity of the neutralized sediment in each temperature range. The results showed that the sulfur capture capacity of the neutralized precipitates was 2.28 (at 40 degrees C) and 5.73 (at 300 degrees C) g S/100 g sorbent. The results of the LCA, with and without including the neutralized sediment production process, based on the sulfur capture capacity, demonstrated that the use of neutralized sediment as an adsorbent improved the global warming potential (GWP) by 75.1% and 98.9%, respectively, compared with the use of metal oxides. In other words, this study quantitatively indicated that the use of neutralized sediment as H2S adsorbents has a smaller environmental impact than existing adsorbents and shows the importance of considering the actual situation in LCA for waste utilization. |