**1. Introduction**

In recent years, the shortage of high-rank coal resources has gradually become a prominent problem in industrial development [1,2]. Lignite is widely used in energy fields, such as pyrolysis, combustion, gasification and liquefaction [3,4]. In order to make more effective use of lignite resources, many scholars carried out much research on modification treatment processes for lignite characteristics. Arash Tahmasebi [5] discovered that the content of some functional groups in pulverized coal particles decreased significantly after microwave irradiation, but the content of aromatic carbon and aromatic ring in lignite was not affected by microwave pyrolysis. Sun Qiang [6] selected coal samples were treated with water and heat treatment and found that the rate of re-absorption decreased with the increase of temperature, and the lignite quality could improve most in high temperature and low humidity. Ge Lichao [7] found the rank of lignite increased after microwave modification and the combustion reaction process moved to high temperature zone by Thermogravimetry (TG) analysis.

The existing research focuses more on the optimization of modification processes and proposes new modification processes. The mechanism of these processes were difficult to study by experimental methods due to innumerable coupling reaction pathways during the utilization of lignite [8–10]. Therefore, Huolinhe lignite (HLH) was selected as the experimental sample, using microwave modification (MM) and ultrasonic modification (UM) as improvement methods separately. The two-dimensional molecular models of HLH before and after modification were established based on the parameters obtained by a series of detection methods, and three-dimensional model is constructed based on molecular mechanics and molecular dynamics. A new method is proposed to study the

structure and physicochemical properties of lignite modification from the molecular point of view through this study.
