Molecular dynamics simulation of Wucaiwan coal pyrolysis via ReaxFF

The pyrolysis of Zhundong Wucaiwan coal was studied using the reactive molecular dynamics simulation method.First, the molecular model of Wucaiwan coal was built based on the ultimate analysis, solid-state ¹³C NMR spectroscopy and X-ray photoelectron spectroscopy of coal sample.It was found that the aromatic ring in Zhundong coal mainly exists in the form of naphthalene.The hydroxyl or etheroxy, carbonyl and carboxyl are the main oxygen-containing functional groups in Zhundong coal, accounting for 56.90%, 23.94% and 19.16%, respectively.Then, the classical Hatcher low-rank coal model was modified to construct the molecular structure of Wucaiwan coal based on the experimental characterization results.Finally, the reactive molecular dynamics simulation of coal pyrolysis was carried out to study the effect of temperature and heating rate on the pyrolysis characteristics of Wucaiwan coal.The results show that the pyrolysis process was dominated by the primary pyrolysis reaction, leading to the formation of char, tar radical fragments and gas, the potential energy of the system increased rapidly first and then kept unchanged with pyrolysis time.However, the secondary reaction of tar radical fragments was significant at high temperatures, the potential energy of the system increased continuously with pyrolysis time.Compared with the primary pyrolysis process, the secondary reaction of tar free radical fragments required longer reaction time.CO₂ was mainly derived from the primary pyrolysis of coal, while the secondary reaction of tar free radical fragments led to a large amount of CO, H₂ and C₂H₂ generation.C₅H₅ and C₂H₂O were observed as the main intermediates.By analyzing the dynamic evolution of pyrolysis products, the secondary reaction mechanism of tar radical fragments was revealed.The secondary reaction of tar was not significant at rapid heating conditions, but was significant at slow heating conditions, leading to the decrease of tar yield and the increase of gas and char yields.