Formation of NOx precursors during the pyrolysis of coal and biomass. Part V. Pyrolysis of a sewage sludge

A sewage sludge sample from a wastewater treatment plant in China was pyrolysed in a fluidised-bed/fixed-bed reactor and in a fluidised-bed/tubular reactor. HCN was found to be the main NOx precursor, representing up to about 80% of the nitrogen present in the sludge. The thermal cracking of volatiles is the main route of HCN formation. NH3 was also an important NOx precursor formed during the pyrolysis of the sewage sludge. The experimental results indicate that there are at least two distinctive stages of NH3 formation during the pyrolysis of the sewage sludge at a fast heating rate. The formation of NH3 at temperatures lower than 400–500 °C is at least partly due to the amino structures in the sludge. The reactions of volatiles in the gas phase make negligible contributions to the observed NH3 yield.