To investigate the evolution of carbon- and oxygen-containing groups during metamorphism and the influence of the structural variation of coal on its wettability, NMR and XPS were employed to study the two main elements, carbon and oxygen, in coal dust. Our results show that with an increase of the metamorphic degree, the number of highly reactive hydroxyl and carboxyl groups show a descending trend whereas the number of low reactivity ether and carbonyl groups increases. The number of carbon-containing groups gradually increases when the metamorphic degree increases, due to a clustering change process which includes an increase in the number of aromatic carbons (Ar-C, H) and a simultaneous decrease in the number of carboxyl carbons (–COOH), methylene carbons (CH₂) and ether based carbons (Ar-O, R–O). The reduction rate of carboxyl carbon (–COOH) is closest to the growth rate of aromatic carbon (Ar-C, H). The methyl (CH₃) and carboxyl (CO) carbon evolve discontinuously with the metamorphic degree, which is different from the other carbon groups. Linear-regression analysis suggests that the correlation coefficients between aromatic (Ar-C, H), ether (R–O), methylene (CH₂) and carboxyl (–COOH) carbons and wettability are 0.98, 0.97, 0.96 and 0.92, respectively, indicating that the main contribution to the wettability from carbon-containing groups comes from these groups. In contrast, hydroxyl oxygen (–OH) has a correlation coefficient as high as 0.98, and is a major contributor from the oxygen-containing groups.

In order to understand the characteristics of the spray field of a dust suppression nozzle and provide a reference for dust nozzle selection according to dust characteristics, a three-dimensional phase Doppler particle analyzer (PDPA) spray measurement system is used to analyze the droplet size and velocity characteristics in a spray field, particularly the joint particle size–velocity distribution. According to the results, after the ejection of the jet from the nozzle, the droplets initially maintained some velocity; however, the distribution of particles with different sizes was not uniform. As the spray distance increased, the droplet velocity decreased significantly, and the particle size distribution changed very little. As the distance increased further, the large droplets separated into smaller droplets, and their velocity decreased rapidly. The distributions of the particle size and velocity of the droplets then became stable. Based on the particle size-velocity distribution characteristics, the spray structure of pressure-swirl nozzles can be divided into five regions, i.e., the mixing, expansion, stabilization, decay, and rarefied regions. The expansion, stabilization, and decay regions are the effective dust fall areas. In addition, the droplet size in the stabilization region is the most uniform, indicating that this region is the best dust fall region. The conclusions can provide abundant calibration data for spray dust fall nozzles.

China is a major coal producer, with huge differences in coal production and safety situations between the South and the North. Taking province A as an example, its coal enterprises have low output, poor efficiency, backward equipment, and low-quality personnel. The output accounts for 0.08% of the country, and the number of deaths accounts for 2.2% of the country, the safety situation of coal enterprises in province A is severe. In order to study the safety risk situation of coal mines under difficult conditions, this paper screens 98 factor indexes including multiple subjects such as enterprise managers, front-line workers, government supervisors, external environment, work quality, stress factors, economic factors, and other dimensions. For different data, the indicator weights were calculated using triangular fuzzy number, entropy weight method, CRITIC method, and three rough set methods in a total of six methods. The comprehensive weights of the indicators were obtained using the proposed new comprehensive weight method. The current situation of safety work of four coal mining enterprises and three levels of government supervision departments was evaluated, and the evaluation results were compared with other existing data to verify the reliability of the evaluation system. The horizontal comparison of the evaluation results indicates the main problems of each subject; the longitudinal comparison points out the problems that need to be solved with the assistance of higher-level enterprises and the central government, and corresponding suggestions for coal mining enterprises and government departments are put forward to reduce the safety risks of troubled coal mining enterprises.