A review on migration and transformation of nitrogen during sewage sludge thermochemical treatment: Focusing on pyrolysis, gasification and combustion

Sewage sludge, a typical municipal solid waste, contains a large number of combustible components such as C and H, and has a high value of energy development and utilization. Thermochemical treatment is one of the advanced directions and developing trends in solid waste treatment. Due to the high content of nitrogen in sewage sludge, the influence of nitrogen behavior and environmental effect (especially the release of NOX precursors) on air pollution, ozone layer destruction and acid rain in the process of thermochemical treatment cannot be ignored. This paper reviewed the distribution, migration and transformation mechanism of nitrogen in sewage sludge during pyrolysis, gasification and combustion. Nitrogen is involved in the formation of char, tar, and gas in the three-phase products during sewage sludge pyrolysis, while nitrogen oxide precursors (NH3 and HCN) in the gas phase are mainly concerned in gasification, while NOX release in the gas phase is mainly concerned in combustion. Meanwhile, the factors (including temperature, reaction atmosphere, pretreatment, etc.) affecting nitrogen migration and transformation in sewage sludge during thermochemical transformation were summarized. The current control methods to reduce NOX and NOX precursors generation were also summarized. Finally, the challenges of further prevention of NOX production and the exploration of new methods for sewage sludge reclamation are presented.