Effect of inherent minerals on nitrogen migration during hydropyrolysis of brown coal

This study aims to elucidate the influence of inherent minerals on nitrogen migration during the hydropyrolysis of brown coal. Such understanding holds significant importance in establishing pollution control methods for nitrogen substances in the cascade utilization of brown coal and achieving its clean and efficient utilization. The investigation specifically focuses on the hydropyrolysis of Shengli brown coal in a fixed-bed reactor under different pressures (0.1 MPa to 3 MPa). The study discusses the role of inherent minerals in the migration of coal nitrogen to various pyrolysis products, including NH₃, HCN, N₂, tar nitrogen, and char nitrogen, as well as the formation of nitrogen functional groups within the char. Quantitative analysis of NH₃, HCN, N₂, tar nitrogen, and char nitrogen is performed using an ultraviolet-visible spectrophotometer, gas chromatography, and elemental analyzer. Additionally, nitrogen functional groups in chars are detected using X-ray photoelectron spectroscopy (XPS). The results indicate that during the hydropyrolysis of Shengli brown coal, inherent minerals significantly enhance the migration of coal nitrogen to NH₃ and N₂. Effect of inherent minerals on migration of nitrogen from coal to HCN was affected by pyrolysis pressure, the inherent minerals obviously inhibits the conversion of coal nitrogen to HCN at 0.1 MPa, while, their influence on the migration to HCN remains negligible at pressures between 1 MPa and 3 MPa. Furthermore, inherent minerals effectively suppress the generation of char nitrogen and tar nitrogen. This is attributed to the reduction of the activation energy of coal hydropyrolysis reactions by inherent minerals, which results in more free radicals attacking nitrogen-containing heterocycles within the coal. This, in turn, leads to increased more nitrogen-containing heterocycles decomposition and transformation to NH₃, N₂ and HCN. Moreover, the decrease in activation energy promotes the formation of volatile matter, prolongs its separation time from the reactor, strengthens its secondary reactions, and facilitates the generation of NH₃ from HCN and tar nitrogen within volatile matter. Inherent minerals in the char improve the reactivity of new char, speeding up the reaction between char and adsorbed H, which further promotes the secondary generation of NH₃ and N₂. These inherent minerals also facilitate the formation of pyridinic nitrogen in char while inhibiting the production of quaternary nitrogen formation. The findings of this study provide fundamental data and technical support for the development of clean and efficient brown coal utilization technologies.