Transformation of the Carbon Forms Present in Municipal Sludge During Pyrolysis Treatment: Pathways and Mechanisms

Abstract

Municipal sludge contains abundant amounts of carbon, with contents ranging from 14% to 38%. The various carbon forms can be converted into beneficial products, but pollutants and greenhouse gases are also released through the pyrolysis process. Ascertaining the mechanisms by which carbon is converted and transformed is crucial for addressing pollution concerns and promoting recycling. In this study, the transformation mechanisms and pathways by which carbon forms are transformed into products during the pyrolysis process of sludge were analysed. The results showed that the three major carbon forms including protein (72.8%), cellulose (10.26%), and hemicellulose (8.17%), had significantly different pyrolysis temperature of 600°C, 400 °C and 300 °C. In terms of gas pollution, most carbon was fully pyrolyzed into CO2. While the temperature raised up to 500 °C, a part of the CO2 converted into CO. Meanwhile, the carbon forms affected the gas production, which CH4 was produced more with cellulose and protein presenting in the sludge. When temperature increased to 700 °C, the 63.51% of the carbon was transformed into liquid and solid residues. The 67% of the carbon forms in the pyrolysis liquid consisted of organooxygen species (OOSs) and organonitrogen species (ONSs), with OOSs predominantly produced from cellulose and hemicellulose and ONSs derived from protein. The secondary pyrolysis stage happened for aliphatics and OOSs when temperatures were 300 °C and 400 °C, respectively. More gases were produced during this secondary pyrolysis stage, which mainly included CO2, CO, and CH4.

Keywords: Municipal sludge, pyrolysis, carbon form, transformation mechanism, pathway