Nitrogen transformations during fast pyrolysis of sewage sludge

Fast pyrolysis of a sewage sludge sample, which contains a high content of nitrogen, was investigated to understand the effects of pyrolysis temperature and sweeping gas flow rate on the yields of pyrolysis products and the distributions of carbon and nitrogen. The maximum oil yield of ca. 48.7% (daf) was achieved at a pyrolysis temperature of 500 °C and a sweeping gas flow rate of 600 cm3/min. NH3 was found to be the predominant nitrogenous gas under all the conditions, and its yield increased with raising pyrolysis temperature and decreasing sweeping gas flow rate. The significant release of NH3 at temperatures lower than 500 °C should be related to the high protein content in the sludge. The N yield in HCN was lower than 2% during pyrolysis below 550 °C, and sharply increased to 5.8% at 700 °C due to thermal cracking of volatile matter. Water-insoluble nitrogen- and carbon-containing species were significantly decomposed to water-soluble ones during secondary reactions. At high temperatures, heavy hydrocarbons were mainly cracked to gaseous products, while the nitrogen-containing species tended to form water-soluble species. This study provides a basic insight into the nitrogen transformations during fast pyrolysis of sludge, which would benefit the clean utilization of sludge as an energy source.