Pyrolysis of hydrothermally pretreated biowastes: The controllability on the formation of NOx precursors

Process control of the NOx precursors emitted from thermal utilization of industrial biowastes is the major objective of this study, mainly through a novel coupled approach involving an initial hydrothermal pretreatment and a subsequent high-temperature pyrolysis. Three typical industrial biowaste types (i.e., herbal tea waste-HTW, penicillin mycelia waste-PMW and sewage sludge-SS) were selected to guarantee diversity, and regulatory mechanisms of NOx precursors (i.e., NH3 and HCN) were simultaneously examined by quantitatively comparing their formation characteristics among different coupled hydrothermal-pyrolysis processes. The experimental results demonstrated that hydrothermal pretreatment affects the formation pathways related to NOx precursors at different pyrolysis stages, thereby minimizing their total yield by 36.14-85.02 % (for HTW), 56.68-92.78% (for PMW) and 66.14-95.24% (for SS), with a greater effect on NH3-N than on HCN-N. Overall, the nitrogen removal during hydrothermal pretreatment is one reason weakening the origin of NOx precursors at the primary pyrolysis stage, while the nitrogen functionality stabilization during hydrothermal pretreatment is another factor inhibiting the formation of NOx precursors at the secondary reaction stage. Complementary analyses on pyrolytic tar and nitrogen distribution evaluation throughout the whole process were also conducted to gain an in-depth understanding of the control of NOx emissions through different coupled hydrothermal-pyrolysis processes. These findings indicated an excellent controllability for the formation of NOx precursors from industrial biowastes, which is believed to offer helpful guidance on release control of N-containing gaseous pollutants during energy utilization