Production of hydrogen via microalgae air gasification over acid mine sludge waste coated on monolith

[Display omitted] •The microalgae air gasification was performed to produce H2-rich gas.•Ni-loaded acid mine sludge showed a high catalytic effect toward H2 generation.•Gas yield and H2 selectivity were maximized using monolith.•Increasing temperature and sludge/microalgae ratio enhanced the H2 generation.•Raising ER increased the gas yield, but was not conductive for H2 production. In this study, air gasification of microalgae employing Nickel-loaded acid mine sludge waste (Ni/AMS) with the packed bed and monolith-type reactor designs was performed for the first time. The effects of various factors such as reaction temperature, equivalence ratio (ER), and AMS-to-biomass ratio were also evaluated. The gas yield and H2 selectivity greatly increased over the Ni/AMS catalyst compared to those produced using the AMS catalyst owing to the dual function of the Fe2O3 redox properties and metallic Ni. By applying monolith-type supports (M-AMS and M-Ni/AMS), the yield of gas and H2 selectivity significantly increased compared to those obtained using packed bed reactors. The gas yield and H2 selectivity reached 74.30 wt% and 42.32 vol%, respectively, employing monolith-type Ni/AMS (M-Ni/AMS). This can be ascribed to the augmented mass and heat transfer in the monolithic structure. Increasing the reaction temperature also increased gas yield and H2 selectivity. Furthermore, although the gas yield gradually increased with increasing ER, the selectivity of H2 increased with increasing ER till 0.3 and then decreased with further enhancement of the ER. However, decreasing the AMS-to-biomass ratio reduced the gas yield and selectivity of H2. As a result of microalgae water-washing pretreatment, the gas yield and H2 selectivity enhanced favorably. Overall, this study provides new prospects for the use of waste, such as AMS, and monolith frameworks for the air gasification of microalgae to increase H2 production.