CO₂-assisted catalytic municipal sludge for carbonaceous biofuel via sub- and supercritical water gasification

This study uses CO₂ as a reaction gas (99.999%) during the sub- (360 °C) and supercritical water gasification (380 °C–440 °C) of municipal sludge to realize the synergistic regeneration of CO₂ and sludge into biofuels. The results showed that the reaction efficiency of sludge under CO₂ atmosphere could be enhanced by adjusting the reaction parameters to increase the yield of biogas and bio-oil. Temperature was the primary driving force to promote the gasification reaction between CO₂ and sludge. With increasing temperature, CO₂ catalyzed the decomposition of fatty acids to form the biogas. An increase in the gasification temperature enhanced the reformation reaction between CO₂ and the biogas, thus facilitating the generation of carbonaceous gases (CO, CH₄, and C₂–C₄). Besides, CO₂-assisted catalysis primarily facilitated the decomposition of organic matter by increasing the H⁺ concentration of reaction system. According to the elements distribution, more carbon was transferred to the biogas, and the oxygen content in the bio-oil was reduced in the presence of CO₂. Correspondingly, the lower heating value of biogas and bio-oil increased to 30.18 MJ/kg and 6.32 MJ/Nm³. Using CO₂ as a reaction gas has the potential to optimize the quality of sludge-derived biofuels and convert CO₂ into carbonaceous fuels.