Co-hydrothermal carbonization of sewage sludge and polyvinyl chloride for the production of high-quality solid fuel with low nitrogen content

Sewage sludge (SS) and polyvinyl chloride (PVC) are typical solid wastes. Their co-hydrothermal carbonization behavior was investigated in this study. The low-nitrogen solid fuel (0.94 wt%) with high heating value (9.84 MJ·Kg⁻¹) was prepared through parameter optimization at 240 °C for 1.5 h under water loading amount of 0.84 g·cm⁻³. In an acidic environment, the stubborn protein in SS could be converted into free amino acids, which were generated by the decomposition of PVC under hydrothermal conditions. The stubborn N could be translated into easy-to-remove N, such as nitrile–N and inorganic N, and the dehydration reaction was evidently promoted. The acidic environment at high temperatures caused the dissolution of ash in SS and improved the combustion performance of hydrochar. FT-IR results showed that, with increased PVC loading proportion, -C=N- was converted into -C=O-. Co-hydrothermal carbonization could effectively improve the combustion performance of hydrochar. The addition of PVC could lead to the generation of increased volatile matter, fixed carbon, and unsaturated C[dbnd]C, and the combustion temperature range shifted to a high range. However, the generation of graphite-like carbon was caused by further increasing the PVC loading proportion, which hindered the improvement of its combustion performance. In the parameter optimization study, the increased water loading amount (from 0.54 g·cm⁻³ to 0.84 g·cm⁻³) had the most evident effect on the N content in the hydrochar (from 1.50 wt% to 0.94 wt%), which promoted the denitrification efficiency (from 60.11% to 75.00%) and the conversion of -C=N- components, and prevented further polymerization of solid products.