Sewage sludge valorisation by hydrothermal carbonization: A new method to enhance nitrogen removal in hydrochar catalyzed with Ni–Mg–Al layered double oxides

Hydrothermal carbonization is a technology that can quickly treat sludge. However, the high nitrogen content of the sludge hydrochar will produce NOx and pollute the environment, which influences the utilization of hydrochar as a fuel. Therefore, the calcined Ni–Al and Ni–Mg–Al layered double hydroxides (LDH) catalyst, named layered double oxides (LDO), were used for catalytic nitrogen removal during hydrothermal carbonization. Compared with the sample without catalyst, the layered double oxides catalyst showed excellent nitrogen removal capability, which can reach more than 57.00% at 240 °C. LDH catalyst enhanced the destruction of sludge structure to effectively realize the transfer of nitrogen-containing substances from solid phase to aqueous phase, which decreased the content of organic-N in hydrochar and increased the content of ammonia nitrogen and total nitrogen in aqueous phase to 4184 and 6463 mg/L, respectively with Ni–Mg–Al LDO at 240 °C. Moreover, this study also adjusted the acid-base sites by changing the metal ratio of catalysts, which was the key to produce the low nitrogen hydrochar. The acid sites were beneficial to inhibit the Maillard reaction and the basic sites can promote the decomposition of extracellular polymer (EPS). The possible removal pathways were proposed in this study.