Activation and functionalization of biochar-based catalysts obtained from sewage sludge and cotton stalks to improve hydrogen-rich gas production

Biomass treatment and efficient energy recovery pose formidable challenges in solid waste management. The activation and functionalization of biochar-based catalysts (BC) derived from the co-pyrolysis of municipal sewage sludge (MSS) and cotton stalks (CS) were systematically investigated. These catalysts were then reintroduced into the same co-pyrolysis system of MSS and CS to evaluate their role in enhancing hydrogen-rich gas production efficiency through in-situ catalytic upgrading. The response surface methodology (RSM) was used to identify the best co-pyrolyzed biochar preparation conditions that would obtain the maximum specific surface area (SSA). BC with the maximum SSA was prepared at a pyrolysis temperature of 900°C, a CS-to-MSS mass ratio of 6:4, and a residence time of 66   min. Potassium hydroxide (KOH), calcium nitrate anhydrous (Ca(NO 3 ) 2 ), and nickel nitrate (Ni(NO 3 ) 2 ) were used for the activation and functionalization of BC. Ni/Ca@KWBC with 30% active components (15% Ni and 15% Ca) loading was the best catalyst with the highest H 2 yield (5.03   mmol/g) and production rate (20.30%). After reuse of the catalyst for five cycles, no serious reduction of H 2 yield is found. Ni primarily promoted reforming reactions that directly increased the H 2 yield, such as the water–gas shift reaction and the steam–methane reforming reaction. Ca adsorbed CO 2 and reduced the activation energy of the reaction, assisting in shifting the equilibrium of the water–gas shift reaction toward H 2 generation. This closed-loop system uniquely repurposes biochar—a traditional low-value byproduct—as a catalyst, offering a novel pathway for integrated waste management and energy production.