Influence of catalysts on hydrogen production from wastewater generated from the HTL of human feces via catalytic hydrothermal gasification

The aqueous phase from the hydrothermal liquefaction of human feces is rich in organic matter. This study explored the usage of different catalysts on the catalytic hydrothermal gasification (CHG) of wastewater resulting from the hydrothermal liquefaction of human feces. The catalyst screening study revealed that NaOH (46.9%) and Raney Ni (41.2%) resulted in the highest H₂ composition, while Ru/AC resulted in the highest reduction in the liquid COD (97.7%). A catalyst mixture was then studied combining two heterogeneous catalysts, Raney Ni and Ru/AC, at different ratios to increase the H₂ composition. A weight ratio of 90% Raney Ni and 10% Ru/AC yielded a H₂ composition of 56.3%, a gas yield of 350.0 g/kg dry feed, and a H₂ yield of 10.61 mol/kg dry feed, indicating that catalyst synergy may exist between these two catalyst which further enhances H₂ production. Incorporation of a reaction coordinate diagram allowed for the direct comparison of energy recovery, COD_r, and H₂/COD value using a mixed Raney Ni and Ru/AC catalyst and varying temperatures and reaction times. Results showed that the optimal condition to maximize the H₂/COD content (9.5 mg H₂/g COD), the COD_r (64.3%), and energy recovery (30.4%) occurred at a temperature of 400 °C, a retention time of 60 min, and a catalyst to feedstock ratio of 0.1.