Micro-structure of crushed coal with different metamorphic degrees and its low-temperature oxidation
Introduction
In China, spontaneous combustion coal seams account for more than 60% of the country’s mineable coal seams. Fires caused by coal spontaneous combustion (CSC) account for more than 90% of mine fires (Li et al., 2019b; Liu and Qin, 2017; Shi and Qin, 2019; Zhou et al., 2016). CSC-induced gas explosions, coal dust explosions, and gas and CSC compound disasters, which are still viewed as major disasters in mines, pose serious threats to the life and health of the staff and safe production in mines (Lin et al., 2019; Qin et al., 2016; Tang et al., 2017, 2019). As the mining deepens, the coal seam is subject to increased mine pressure and ground stress, and the coal is thus more likely to be crushed (Alexeev et al., 2007; Wang et al., 2016b; Xu et al., 2017; Zhao et al., 2020). Affected by the mining activities, a large amount of spontaneous combustion coal is crushed to generate internal fractures. As a result, the crushed coal is exposed to the airflow and begins to engage in low-temperature oxidation. Under good heat storage conditions, CSC is likely to occur and eventually leads to major mine disasters. Therefore, from the perspective of the microstructure of coal, the study of the evolution of the chemical structures of coals of different metamorphic degrees in the crushing process, the analysis of the effect of coal particle size on the oxidation reactivity of crushed coal by taking free radicals as indicators, the analysis of the effect of coal particle size on the chemical structure of crushed coal by taking functional groups as indicators, and the analysis of the low-temperature oxidation characteristics of coal with different particle sizes by combining free radicals with functional groups are of great significance for exploring the mechanisms and prevention methods for oxidation and spontaneous combustion disasters of crushed coal.
Mechanical external forces (for example coal cutting by a shearer) lead to the crushing of coal, which causes the covalent bond rupture in the coal molecular structure and the generation of a large number of free radicals. This statement stems from the theory of free radical reaction in CSC (Li, 1996; Li et al., 2016). This theory, originally obtained by theoretical analysis, was subsequently validated by means of the electron paramagnetic resonance (EPR) technique (Liu et al., 2015; Tadyszak et al., 2015; Zhang et al., 2016; Zhou et al., 2019a). Liu confirmed that external forces such as pulverization, pyrolysis and ultraviolet irradiation could induce the production of free radicals in coal (Liu et al., 2015). Zhong confirmed that the oxidation of coal would result in free radical reaction; the higher the content of primary free radicals, the greater the increase of free radicals after oxidation. This revealed that the free radicals could indicate the coal oxidation reactivity (Zhong et al., 2010). Tang believed that completely crushed coal would produce more CO and was more susceptible to oxidation (Tang, 2015). After analyzing the CO concentration, oxygen consumption rate and heat release rate of oxidized coal, Deng indicated that coal oxidation (below 140°C) increased the propensity to spontaneous combustion(Deng et al., 2016). Zhu believed that coal with a smaller particle size and a smaller predicted value of activation energy was more prone to spontaneous combustion (Zhu et al., 2013). By Thermogravimetric analysis on coal oxidation, Saleh and Nugroho concluded that the weight loss increased with the decrease of particle size, indicating the increase of the propensity of coal to spontaneous combustion as particle size decreased(Saleh and Nugroho, 2013). Zhao confirmed that the variation of coal molecular structure caused coal spontaneous combustion and the production of gas products, and the oxygen-containing functional groups in the structure have the strongest reactivity(Zhao et al., 2019bZhao et al., 2019c). Wang assumed that the oxidation of oxygen-containing groups and side chains on the surface of low-rank coal led to coal oxidation (Wang et al., 2016a).
It has been confirmed by the above researchers that the coal crushing process would affect coal oxidation. The novelty of this article is to analyze the microstructure and oxidation active groups of crushed coal of different metamorphic degrees based on the combination of the free radical reaction and the functional group structure. In addition, that different particle sizes formed in the crushing process may greatly change the oxidation reactivity of crushed coal was also analyzed. Using electron paramagnetic resonance (EPR) and a Fourier transform infrared (FTIR) spectrometer, this study analyzed the evolution of the microstructure of coals with different metamorphic degrees in the process of coal crushing. Additionally, with the aid of a self-built experimental system for coal oxidation, the low-temperature oxidation of crushed coal of different particle sizes was studied, and the effect of the crushing process on the microstructure and oxidation characteristics of coal was further revealed. The results provide a research basis for the prevention and control of spontaneous combustion disaster in the coal crushing process.
Section snippets
Coal samples
Based on metamorphic degree, coal can be divided into lignite, bituminous coal and anthracite. As shown in Fig. 1, the research coal samples were lignite from Yunnan Province, bituminous coal from Hebei Province and anthracite from Shanxi Province, China, designated coal 1#, coal 2# and coal 3#, respectively. The results of proximate analysis of the coal samples are shown in Table 1. Due to the differences of metamorphic degrees among coal 1#, coal 2# and coal 3#, the change of coal quality
Free radical concentration in the coal crushing process
Under the influence of mining activities, coal molecular structures are subject to external mechanical forces. Lone electron structures generated from covalent bond rupture thus form free radicals. This part of free radicals has a strong chemical reaction activity and can be easily oxidized after coming in contact with oxygen in the air, which results in continuous oxidation and temperature rise of coal (Kong et al., 2018; Li et al., 2018, 2019a; Li et al., 2016; Wei et al., 2006). The
Characteristics of oxidation in the coal crushing process
Based on the theory of free radical reaction in CSC, the free radical concentration of CSC can be used to indicate the oxidation reactivity of coal (Kong et al., 2017; Li, 1996; Li et al., 2016; Patel et al., 2017; Zhou et al., 2019b). Taking the low-rank lignite as an example, which is the most prone to spontaneous combustion, an analysis was conducted on the oxidation reactivity after the coal was crushed to five different particle size ranges: 0.224−0.280 mm, 0.180−0.224 mm, 0.125−0.180 mm,
Conclusions
As the particle size continuously decreases in the coal crushing process, the active groups on coal surfaces suffer from the joint action of crushing and oxidation. Based on EPR and FTIR technology, the evolution of the microstructures of different ranks of coal in the crushing process was analyzed by combining the theory of free radical reaction with the functional group structure. With the aid of a self-built experimental system for coal oxidation, the low-temperature oxidation of crushed
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgments
This work was supported by the Future Scientists Program of “Double First Class” of China University of Mining and Technology (2019WLKXJ067).
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