Comprehensive study on co-pyrolysis mechanisms of sewage sludge and low rank coal under rapid/slow heating conditions

Co-pyrolysis of sewage sludge (SS) and low rank bituminous coal (BC) was considered as an effective technology in realising the co-resource utilization and disposal of municipal waste. The current work focused on the co-pyrolysis characteristics and synergistic mechanisms under rapid/slow heating conditions, with experimental and analytical methods of the thermogravimetric-mass spectrometry (TG-MS), gas chromatography (GC), and Fourier transform infrared spectroscopy (FTIR). The results showed that the heating rate and mixing ratio of SS and BC caused different yields of the co-pyrolysis products. The gas products performed different releasing rules with the effect of mixing ratio of SS and BC, and the different stages during the slow pyrolysis were defined in terms of weight loss and product release. Under the rapid heating condition, the gas product yield in the experiment was found to be consistently higher than the theoretical yield, regardless of the mixing ratio. The addition of SS in the mixtures would increase the yield content of CO₂ and high calorific gases (C₂H₂, C₂H₄, C₂H₆) and reduce H₂, CO, and CH₄. The intensities and widths of the absorption peaks of functional groups such as O-H, C-H, C=O and C=C in the char decreased. The morphology, pore structure, and particle size distribution of the pyrolyzed samples were compared and analysed. Kinetic analysis was carried out according to different stages of slow pyrolysis. Finally, the synergistic effect and mechanism of the co-pyrolysis were comprehensively compared and revealed under slow/rapid heating conditions.