Hydrothermal carbonization of combined sodium citrate-thermal pretreated sewage sludge for the production of low-nitrogen clean solid fuels

The high nitrogen content of hydrochar produced from the hydrothermal carbonization (HTC) of sewage sludge (SS) leads to significant NO_x emissions when used as a solid fuel. To reduce the nitrogen content in hydrochar, extracellular polymeric substances (EPS) and cellular structures that hinder nitrogen migration during HTC of SS must be disrupted. Sodium citrate-thermal (SC-T) combined pretreatment has been shown to effectively break down the EPS and cell structure, resulting in the release and hydrolysis of organic matter. Therefore, the combination of SC-T and HTC is a novel strategy to produce low-nitrogen hydrochar. The results showed that SS pretreated under optimal conditions (0.7 g SC/g SS, heated at 121 °C for 2 h) and then treated at 240 °C for 1 h during HTC achieved the lowest nitrogen content (0.94 wt%). This provides a new method to produce low nitrogen and high quality hydrochar for solid fuels.