The impact of struvite presence on the thermal decomposition of sewage sludge, including formation of NOx emissions

Abstract

Monoincineration of sewage sludge (SS) is becoming a prevailing type of SS treatment due to its contamination and the ability to achieve enriched concentration of valuable inorganics in the end ash. In order to improve the recovery potential, particularly of inorganic, phosphorous (P) bearing species, thermal transformations of organic as well as inorganic SS compounds have to be considered. While important gains in predictive capability have already been made through extensive description of organic part decomposition, P is mostly present in the inorganic part of SS, together with notable amount of nitrogen (N). Current models consider these species in a simplified manner, making the ability to predict P and N fate during thermal treatment limited. The present work focuses on development and implementation of a one step reaction kinetics mechanism for struvite, a common P and N bearing compound in SS, which also presents an important route for P and N recovery. The new mechanism is implemented into an existing SS organic fraction decomposition mechanism and validated using 0D multiphase reactor simulations in Chemkin software. Results reveal up to 10 s faster temperature increase and decomposition of SS with the presence of struvite due to emission of NH3 and H2O occurring already from 350 °C onwards. The NH3 emissions are especially important as they lead to higher NOx formation. The work presents large potential for improving thermal decomposition prediction as well as emission mitigation strategies if struvite is included in the thermal decomposition models of SS. The developed methodology allows efficient implementation of also other P and N bearing species in the inorganic part of kinetic mechanism.