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| Uneven Distribution and Transformation of Nitrogens During Vitrinite- and Inertinite-Rich Coal Pyrolysis Char of Xiaobaodang Coal Mining
Area, Northern Shaanxi Province |
| LI Huan-tong1, ZOU Xiao-yan2, ZHANG Ting-ting1, ZHANG Wei-guo1, WANG Jun-qi1 |
1. College of Geology and Environment, Xi'an University of Science and Technology, Xi'an 710054, China
2. College of Urban, Rural Planning and Architectural Engineering, Shangluo University, Shangluo 726000, China
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Abstract Xiaobaodang raw coal (XR), vitrinite-rich coal (XV) and inertinite-rich coal (XI) were pyrolyzed in a closed high-temperature energy-saving furnace at a heating rate of 5 ℃·min-1 under high-purity Ar atmosphere. The final pyrolysis temperature was 300~900 ℃. Thermogravimetric-differential thermal analysis (TG-DTA) and X-ray photoelectron spectroscopy (XPS) were used to study the pyrolysis characteristics of raw coal (XR), vitrinite-rich coal (XV) and inertinite-rich coal (XI), and the occurrence of nitrogen in pyrolysis char. The results show that during the pyrolysis process of Xiaobaodang coal, its structure is decomposed and depolymerized to generate and discharge many volatiles. At 440~450 ℃, there is an obvious weight loss peak. The total weight loss rate and maximum weight loss rate of inertinite-rich coal (XI) are smaller than those of vitrinite-rich coal (XV), which is due to the high degree of the aromatization of inertinite and the short length of aromatic ring side chain. The dissociation bond energy of chemical bonds in the structure is large and the thermal stability is high. The main forms of nitrogen in Xiaobaodang coal are pyridine nitrogen (N-6), pyrrole nitrogen (N-5), quaternary nitrogen (N-Q) and nitrogen oxide (N-X). Pyrrole nitrogen (N-5) is higher than pyridine nitrogen (N-6). Pyrrole nitrogen (N-5) in vitrinite-rich coal is slightly lower than in inertinite-rich coal. The quaternary nitrogen (N-Q) content in inertinite-rich coal is slightly higher, mainly due to its high degree of aromatic ring condensation and more nitrogen elements embedded in macromolecular polycyclic aromatic structure. The main forms of nitrogen in pyrolysis char are pyridine nitrogen (N-6) and pyrrole nitrogen (N-5). When the pyrolysis temperature was 300 ℃, the relative content of pyridine nitrogen (N-6) and quaternary nitrogen (N-Q) decreased significantly. With the temperature increase, pyrrole nitrogen (N-5) was converted to pyridine nitrogen (N-6). At 700 ℃, the pyridine-type nitrogen (N-6) in the inertinite-rich coal group has an inflection point, showing a change from rise to fall, possibly caused by aromatic cyclization and polycondensation. At 900 ℃, nitrogen oxides (N-X) disappeared. The surface nitrogen of pyrolysis char is more than that of bulk nitrogen. The removal rates of surface nitrogen in vitrinite-rich coal (XV) and inertinite-rich coal (XI) are 53.49% and 31.86%, respectively. The removal rates of nitrogen in the bulk phase were 33.72% and 15.84%, respectively. At 600 ℃, an inflection point appeared in N/C, showing a change from decrease to increase, which may be caused by the unstable chemical bonds of bridge bonds or alkyl side chains in the structure.
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Received: 2024-03-21
Accepted: 2024-12-02
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