Valorization of sludge using microwave pyrolysis for green bio-energy: Combined effects of key parameters on the directional optimization of high-quality syngas

Abstract

• The key influencing factors of sludge microwave pyrolysis to produce biogas were screened. • The combined effect of the key parameters on the syngas quality optimization was explored. • The yield and the lower heating vale of syngas reached 34.31 mol/kg DS and 15.48 MJ/kg DS . • Improved syngas quality was achieved in the less time by the synergistic control of parameters. • The mechanism of high-quality syngas generated by sludge microwave pyrolysis was revealed. To directionally convert sludge into the high-quality syngas, the effects of various parameters on the biogas produced by the sludge microwave pyrolysis are explored, and the syngas generation mechanism under the synergistic effect of parameters is revealed. The results showed that the pyrolysis final temperature, reaction time, and the amount of the microwave absorber were the key factors that affected the biogas yield. A response surface analysis was used to explore the synergistic effect of each key parameter on the yield and the lower heating vale (LHV) of syngas. The pyrolysis temperature was the key to driving syngas generation. Properly prolonging the reaction time decreased the formation of tar by-products and increased the syngas yield. The addition of a microwave absorber increased the heating rate by enhancing the conversion efficiency of microwave energy, and this contributed to accelerating the dehydrogenation and the Boudouard reactions to promote the generation of H 2 and CO. Under high temperature conditions (900 °C), a higher syngas yield (10.25–34.31 mol/kg DS ) and LHV (4.85–14.53 MJ/kg DS ) were obtained in a shorter period time by synergistically optimizing the amount of microwave absorbers (8–10 g) and the reaction time (28–40 min). As the pyrolysis temperature increased, the H 2 content gradually rose to the peak, while the content of CO continued to increase. By analyzing the evolution of the specific pyrolysis products, the secondary pyrolysis of bio-oil at high temperatures and the Boudouard reaction were found to be the primary pathways that promoted syngas generation. This research provides strategic guidance for producing high-quality syngas using the microwave pyrolysis of waste biomass, thereby minimizing the exploration cost of basic experiments and providing directional control of the syngas quality.