Impacts of microplastics in municipal sludge pyrolysis-gasification: Melt-induced microaggregation and increased deposition risk

The escalating contamination of microplastics (MPs) is progressively introducing new challenges to the management and reutilization of municipal sludge (MS). Pyrolysis-gasification is an important MS disposal process. However, the influence of MPs on the process above remains understudied now. This work aimed to evaluate the characteristics of MPs residue in carbon resulting from medium-low temperature MS pyrolysis and explored their impacts on the gasification reactivity and deposition tendencies of the pyrolytic carbon. The results showed that microplastic residues in MS pyrolytic carbon were tiny and highly resistant to degradation, thereby significantly increasing their biological risks. Base on the analyses on morphological characterization, compound composition, volatile matter release, and carbonaceous structure, residual MPs were demonstrated to form porous "C-organic matter-C″ microaggregates in carbon, and corresponding melt-induced formation mechanisms were clarified. In gasification reactions, carbon defect structures and reactive matters introduced by MPs enhanced the gasification activity at low temperatures. Nevertheless, large aggregated volatile organic components and metals increased slag volume and agglomeration, so resulting in deposition risks. Given the elevated contamination of MPs in pyrolytic carbon, gasification emerges as a reliable strategy for mitigating environmental risks, but may require upgraded technologies (e.g., additive, and new reactor design) to reduce the deposition problem. These findings provide scientific foundation for the management of secondary contamination of microplastics in MS, thereby facilitating an environmentally friendly disposal process.