N migration and transformation during the co-combustion of sewage sludge and coal slime

The co-combustion of sewage sludge and coal slime is considered a promising technique for reducing the volume of sewage sludge, adding value, and decreasing the risks associated with these wastes. This work aimed to study N migration and transformation mechanisms and the related interactions during the co-combustion of sewage sludge (SS) and coal slime (CS) by thermogravimetric-mass spectrometry combined with X-ray photoelectron spectroscopy. The results revealed that the main N-containing gases produced during the combustion of SS and CS were NH₃ generated from Amino-N at 200-400 °C and HCN generated from heterocyclic nitrogen at 400-600 °C, respectively. The increase of CS ratio led to a decrease in the release of NH₃ and NO, but an increase in the release of HCN. Distinct interactions were observed during the co-combustion process, which promoted the production of NH₃ and inhibited the production of HCN and NO. Co-combustion inhibited the release of NO by 36.9% when the CS ratio was 50%. The interaction mechanism suggested that H radicals from SS promoted the premature decomposition of N species in CS, and increased the selectivity of N species for NH₃ formation by promoting the conversion of heterocyclic-N to Amino-N. In addition, the interaction of char (in SS) and char (in CS) enhanced the reduction of NO. Above 600 °C, co-combustion promoted the retention of N species in the ash.