Enhanced thermochemical valorization of coconut husk through carbon dioxide integration: A sustainable approach to agricultural residue utilization

In this study, we aimed to develop a sustainable valorization platform for agricultural residues, mainly focusing on harnessing these residues as a renewable energy source. We conducted a case study involving the pyrolysis of coconut husk to create a sustainable pathway for its valorization. To enhance environmental compatibility, carbon dioxide (CO₂) was employed as the reaction medium. CO₂ acted as a partial oxidant, leading to the oxidation of carbons in the bio-oil and the formation of carbon monoxide (CO). In detail, this study experimentally proved the mechanistic contribution of CO₂ to shifting carbon in pyrogenic oil into gases, particularly CO. To enhance the reactivity of CO₂, the catalytic pyrolysis of coconut husk was conducted using a nickel-based egg-shell-type (Ni-ES) catalyst. Catalyst introduction resulted in the increased concentration of syngas compared with conditions without a catalyst in the presence of CO₂. In particular, syngas production from catalytic pyrolysis under CO₂ conditions was enhanced by >6.5-fold compared with that from single-stage pyrolysis under N₂ conditions. Simultaneously, the evolution of pyrogenic oil was significantly reduced under CO₂ condition in the presence of the Ni-ES catalyst. This confirms the catalytic potential in further optimizing the valorization of agricultural residues for sustainable energy production.