Experimental Research on Fuel-N Transforming Characteristics of Low-Volatile Coal Under Oxy-Fuel Pyrolysis Conditions

Achieving efficient and clean use of low-volatile coal is of vital importance to China’s energy system. This study aims to elucidate how the high-concentration-CO2 atmosphere influences the migration pathways of fuel-bound nitrogen during the pyrolysis of low-volatile coal, thereby providing critical insights for the prediction and control of NOx emissions under oxy-fuel conditions. A high-temperature drop-tube furnace system capable of high heating rates (up to 104–105 °C/s) was employed to comparatively investigate the pyrolysis behavior of a typical low-volatile coal (volatile matter content of 7.44%) under Ar and pure CO2 atmospheres at 1000–1400 °C. The outcomes show that the CO2 atmosphere significantly promoted the release of volatiles, with the volatile release rate at 1400 °C reaching 2.1 times that under the Ar atmosphere. While volatile nitrogen primarily consists of HCN and NH3 with HCN dominance at lower temperatures, NH3 release exceeds HCN by more than tenfold at 1400 °C. CO2 promotes nitrogen release through enhanced gasification reactions, reducing char nitrogen proportion while increasing volatile nitrogen yield approximately fourfold at elevated temperatures. The X-ray photoelectron spectroscopy analysis reveals the transformation pathway of nitrogen functionalities from quaternary nitrogen to pyridine nitrogen and subsequently to pyridine under oxy-fuel conditions. These findings provide fundamental insights into fuel nitrogen evolution mechanisms and offer theoretical support for optimizing oxy-fuel combustion processes toward efficient NOx control.