Interaction Characteristics of Mineral Matter and Nitrogen during Sewage Sludge Pyrolysis

Abstract

The conversion of nitrogen-containing species in sludge is influenced by mineral matter. Raw sludge and demineralized sludge were pyrolyzed in a fixed-bed reactor in the temperature range of 500–800 °C to investigate the roles of the temperature and mineral matter on the nitrogen conversion in sewage sludge. To gain an in-depth knowledge and understanding of the nitrogen transformation mechanisms, we characterized the nitrogen-containing functional groups in raw sludge and chars via X-ray photoelectron spectroscopy, and the NOx precursors (HCN and NH3) were identified using a spectrophotometric method. According to the results, mineral matter has been proven to be able to affect the yields of char N, gas N, and other N (nitrogen in tar, N2, NO, and others). The interaction between mineral matter and N-containing species in sewage sludge is strongly limited by the temperature. Mineral matter inhibits char N conversion at 500–600 °C and promotes char N conversion within the range of 600–800 °C. Meanwhile, mineral matter suppresses the yield of HCN but promotes the production of NH3.