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 NO_x 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 NO_x control.