Co-pyrolysis of sewage sludge and hydrochar with coals: Pyrolytic behaviors and kinetics analysis using TG-FTIR and a discrete distributed activation energy model

Dewatered sewage sludge (DS) has been upgraded to hydrochar (HC) using hydrothermal conversion because of superior fuel quality and avoidance of energy-intensive dewatering. In order to further develop sustainable energy conversion of HC, co-pyrolysis of DS and HC with three different-rank coals have been comprehensively investigated using thermogravimetric analysis coupled with Fourier transform infrared spectrometer and a discrete distributed activation energy model. Pyrolytic behaviors, kinetics and gas releasing characteristics have been examined. Results suggest that low-rank Coal 2 and moderate-rank Coal 3 blended with DS exhibited the highest synergistic removal of N and S, respectively. Co-pyrolysis of DS with high-rank Coal 1 favored C retention and N or S removal, while N was more likely to be stabilized in co-pyrolysis of HC with lower-rank coals. A less intensive but lasting devolatilization process in higher temperature regime (362–487 °C) was expected for HC. Higher fixed carbon content in HC and coals led to higher abundance of activation energy centralized around 180 kJ/mol for coal/HC blends. At the maximum decomposition rate temperature of 293.5 °C, pyrolysis of HC generated remarkable CH₄ and other combustible gases with significantly reduced CO₂. High-rank Coal 1 showed the most remarkable synergistic effect on the yield of light hydrocarbons and elevated Coal 1/HC blending ratio favored CO₂ and CH₄ release. Overall, co-pyrolysis of coals with HC could be a more sustainable technique for sewage sludge management and utilization in terms of stable devolatilization and centralized activation energy, higher yield of combustible syngas, and obviously reduced emissions of CO₂ and nitrogen-containing gases.