Abstract
Coal is a porous medium. Oxygen molecules in the air penetrate through the pores of coal and are adsorbed on the coal surface. Low-temperature oxidation of coal then occurs, by which coal spontaneous combustion is promoted. Given this process, the authors analysed the physisorption characteristics of O2 in pulverized coal from the perspective of nanopore structure. In this study, five different kinds of coal samples (two lignites, one bituminous coal, and two anthracites) were selected, and the surface morphology, pore structure parameters and oxygen physisorption capacity of the pulverized coals were determined by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and oxygen adsorption with chromatography (OAC), respectively. The experimental results of SEM and MIP show that with the development of coal, the surface folds increase, and the pores increase in number and shrink, which leads to the nanopores of anthracite and bituminous coal being smaller and more complex than those of lignite. The experimental results of OAC show that adsorbed oxygen is physisorbed by pulverized coal in the order lignite > bituminous coal > anthracite. Analysis of the oxygen desorption curves shows that the oxygen desorption rates of the anthracites and bituminous coal are slower than those of the lignites. The results show that the amount of oxygen physisorbed by pulverized coal is proportional to the fractal dimension of the coal pores, proportional to the pore volume of the nanoscale pores, and inversely proportional to the number of closed pores in the coal. Based on the results of the analyses mentioned above, it is important to analyse the process of coal-oxygen chemisorption and the mechanism for low-temperature oxidation of coal to prevent coal spontaneous combustion.
Highlights
The confirmed worldwide reserves of coal stood at approximately 1,055 billion tons in 2018
Jingyu Zhao et al analysed the critical points of the low-temperature oxidation stage of coal, including the critical temperature (97.45 ± 7.15 °C) and crack temperature (149.28 ± 8.32 °C)[17]; they analysed the exothermic characteristics of the functional groups of the coal during the oxygen adsorption and mass-increasing stages and believed that these two stages required the most caution and carried the most risks[18]
The number of surface pores in the anthracite coal samples (AN and HA) and bituminous coal sample (PI) is more than two, but the pore diameter is less than 1 μm, and many nanoscale powders are attached to the surface
Summary
The confirmed worldwide reserves of coal stood at approximately 1,055 billion tons in 2018. The significance of the coal low-temperature oxidation mechanism in the physisorption, chemisorption and diffusion of oxygen in coal nanopores has been studied. Tao et al found that the heat generated by the decomposition of oxygen-containing functional groups in coal is the main reason for the initial temperature rise in coal spontaneous combustion oxidation[14]. Cai et al analysed the variation in gaseous products and reaction characteristics of the main functional groups of different metamorphic coals during low-temperature oxidation, and the critical temperature for coal spontaneous combustion was determined[15]. This study is of significance for analysing the coal low-temperature oxidation process and preventing coal spontaneous combustion
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