Mechanism of wet sewage sludge pyrolysis in a tubular furnace

摘要

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 ℃), the breaking of the C-H bonds of alkyl gave rise to the release of CH_4 and C_2 hydrocarbons, and a large amount of CO and CO_2 evolved as the result of C=O decreasing, both processes indicated the decomposition of volatile compounds. The increasing absorbance amount of C-O and C-H_(aromatic) demonstrated the formation of tar. As temperature increased further, the diminishing IR absorbance of C-0 and C-H_(aromatic) was accompanied by a significant reduction of tar yield and an increase of H_2. H_2 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.