A study on the chemical looping combustion of sewage sludge: The emission of NOx and its precursors

Abstract

Chemical looping combustion (CLC) is a promising low-NOx combustion technology for sewage sludge disposal. Herein, the emissions of NOx and its precursors (NH3 and HCN) during the pyrolysis and CLC processes are investigated on a fixed bed reactor. In particular, the effects of reaction (gas) temperature, sludge ash accumulation, and reaction intermediates (CO2 and H2O) are studied. When temperature increased from 800 to 900 °C, the content of gas-phase carbon is shown to increase from 57.9 to 79.7%. With ash accumulation, the NH3 generation rate increases from 10.8 to 11.7%, and the HCN generation rate decreases from 9.68 to 8.25%. Sludge ash accumulation is shown to promote the release of NH3, but inhibit the release of HCN and NOx. After 8 consecutive CLC cycles, the average removal efficiencies of NH3, HCN, and total N-containing pollutants are ~61%, ≥92%, and ~ 60%, respectively. The presence of CO2 and H2O in the reaction atmosphere is shown to enhance the oxidizability, thereby promoting the formation of NOx. In addition, char gasification and the reforming reaction of N-containing pollutants are promoted, thus leading to the formation of NOx precursors. Furthermore, NO is shown to be the predominant form of NOx in the CLC of sewage sludge.