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Unraveling Co-Pyrolysis Mechanisms for Municipal Sludge and Microplastics: Thermodynamic, Kinetic, and Product Insights
Summary
Wastewater treatment plants produce large quantities of sewage sludge, which is often contaminated with microplastics from household and industrial sources. This study tested whether co-pyrolyzing sludge with polyethylene (HDPE) or PET plastic waste at high temperatures could improve energy recovery while processing microplastics. Adding 30% HDPE maximized the overall pyrolysis efficiency and changed the chemical reaction pathways, while PET had stronger facilitating effects at mid-range temperatures. The research suggests that co-pyrolysis could serve the dual purpose of sludge disposal and microplastic destruction, though the altered reaction kinetics and product mixtures require careful management.
This study aimed to characterize the impacts of high density polyethylene (HDPE) and polyethylene terephthalate (PET) on the co-pyrolysis mechanisms and products of municipal sludge (MS) by using thermogravimetric analysis. Compared with PET, the addition of 30% HDPE maximized the comprehensive pyrolysis index of MS from 7.68 to 20.37 × 10−6 %3/(min2·°C3). Between 350 and 500 °C, the facilitatory effect of the MS-PET co-pyrolysis was stronger than that of HDPE-MS. Between 500 and 1000 °C, the addition of PET/HDPE exerted an inhibitory effect on the MS pyrolysis. Prior to adding either plastic, the two main pyrolysis stages of MS followed distinct reaction models: a first-order reaction between 162.6 and 431.5 °C and a sixth-order (F6) reaction between 431.5 and 735.8 °C. However, the addition of HDPE transformed the high-temperature stage kinetics from the F6 model to nucleation growth. Throughout the (co-)pyrolysis process, the decomposition of alcohols, aliphatic hydrocarbons, acids, and aromatic substances occurred, accompanied by the formation of new aromatic compounds. The addition of HDPE further disrupted the char structure, while the addition of PET formed a barrier within the co-pyrolytic char, hindering the release of volatiles. Multi-objective optimization revealed that both HDPE and PET yielded superior energy performance compared with the MS pyrolysis. Increasing HDPE content further enhanced energetic optimization, with temperature and plastic type identified as the primary factors governing energy output at a heating rate of 10 °C/min. This study introduces a novel co-pyrolytic approach for tightening the co-circularity of both MS and PET/HDPE.
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