NOx emissions during oxygen-enriched combustion of sewage sludge and corresponding hydrochar

Abstract

This study proposed an oxygen-enriched combustion technology for the resource utilization of hydrochar prepared from sewage sludge (SS) through hydrothermal carbonization (HTC). However, its application potential relies heavily on NOx emissions. To investigate NOx emissions during oxygen-enriched combustion of hydrochars prepared at different temperatures (200 °C, 240 °C, and 280 °C), real-time NOx concentrations were recorded under varying conditions, including different carrier gases (N2 and CO2), temperatures (600 °C, 800 °C, and 1000 °C), and oxygen concentrations (20 %, 30 %, and 40 %). In comparison to air combustion processes, the NOx emissions during the devolatilization stage were considerably depressed in oxygen-enriched combustion. A high concentration of CO2 inhibited NH3 production and HCN oxidation, and generated increased amounts of CO to facilitate NOx reduction. Elevated temperatures also reduced the NOx release during the oxygen-enriched combustion of SS and hydrochar, as the accelerated oxygen consumption weakened the oxidation reactions of HCN and NH3; the enhanced reduction reaction rate and increased CO production also promoted NOx reduction. Elevated oxygen concentrations, however, promoted NOx release owing to the increased NH3 and HCN oxidation by an increased number of oxidizing groups. Additionally, the reduction of reducing gases and rapid char combustion hindered NOx reduction. The NOx-N conversion of the hydrochars was much lower than that of SS under different conditions. HTC effectively reduced the nitrogen content of SS, with nitrogen in HC-280 existing as stable heterocyclic -N. HC-280 showed significantly less nitrogen conversion at low temperatures (9.03 % reduction at 600 °C) and high oxygen concentrations (2.01 % reduction at 40 % O2) compared to SS, highlighting the potential of combining HTC and oxygen-enriched combustion.