Mechanism of wet sewage sludge pyrolysis in a tubular furnace

The main objective of this work was to develop a preliminary mechanistic understanding of wet sewage sludge decomposition from starting constituents to final products, including intermediates formed during the pyrolysis process. Sewage sludge with a moisture content of 84.2 wt% was pyrolyzed at different temperatures in a tubular furnace, the pyrolysis products (hydrogen-rich fuel gas, tar and solid char) were detected by micro-GC, GC-MS, and FTIR, respectively. The high moisture content of wet sewage sludge generated a steam-rich atmosphere at high temperatures, leading to an in situ steam reforming of the volatile compounds and a partial gasification of the solid char, which contributed to the production of hydrogen-rich fuel gas. The pyrolysis process can be divided into two steps: at a relatively low temperature (<600 °C), the breaking of the C–H bonds of alkyl gave rise to the release of CH4 and C2 hydrocarbons, and a large amount of CO and CO2 evolved as the result of CO decreasing, both processes indicated the decomposition of volatile compounds. The increasing absorbance amount of C–O and C–Haromatic demonstrated the formation of tar. As temperature increased further, the diminishing IR absorbance of C–O and C–Haromatic was accompanied by a significant reduction of tar yield and an increase of H2. H2 was considered as an indicator for the occurrence of tar cracking. The Diels–Alder reaction mechanism followed by dehydrogenation was employed to explain the PAHs formation.