Mechanism of gas production under microwave/conventional pyrolysis of sewage sludge: Mechanism of microwave energy action on oxygen-containing functional groups

Microwave pyrolysis (MWP) of sewage sludge (SS) has obvious advantages in the quality improvement of syngas. However, it is unclear the mechanism of syngas production and microwave energy on oxygen-containing functional groups during MWP of SS. In this paper, the effect of pyrolysis methods on syngas generation is investigated through macroscopic experimental quantitative analysis combined with microscopic DFT simulations. Compared with conventional pyrolysis (CP), MWP showed higher gas yields. Further quantitative analysis of the gas composition revealed that microwaves excited the gasification reaction of CO2 and promoted the production of H2 and CO, and increased by 39.89%. Kinetic calculations revealed that microwaves decreased the activation energy of H2 and CO by 50.84% and 29.59%, respectively. Tar analysis revealed that microwaves promoted a large number of oxygen-containing functional groups transferred to tar at low temperatures. As temperature increases, the oxygen-containing functional groups in the tar undergo a secondary reaction in response to microwave, resulting in numerous H radicals and O radicals being transferred to the gas phase, promoting the yield of H2 and CO. Combined with DFT to explore the decomposition path of all typical oxygen-containing functional groups in tar, it found that the decomposition of these functional groups into H2 and CO requires high energy to complete, and microwave was expected to achieve the process. Finally, the mechanism of microwave energy in the process of SS pyrolysis was proposed, which provides a feasible idea for the subsequent directional regulation of microwave pyrolysis of SS.