Zhentang Liu

Study of electromagnetic characteristics of stress distribution and sudden changes in the mining of gob-surrounded coal face

The incidence of dynamic coal or rock disasters is closely related to the distribution of stress in the surrounding rock. Our experiments show that electromagnetic radiation (EMR) signals are related to the state of stress of a coal body. The higher the stress, the more intense the deformation and fractures of a coal body and the stronger the EMR signals. EMR signals reflect the degrees of concentrated stress of a coal body and danger of a rock burst. We selected EMR intensity as the test index of the No.237 gob-surrounded coal face in the Nanshan coal mine. We tested the EMR characteristics of the stress distribution on the strike, on the incline and in the interior of the coal body. The EMR rule of rock bursts, caused by sudden changes in stress, is analyzed. Our research shows that EMR technology can be not only used to test qualitatively the stress distribution of the surrounding rock, but also to predict a possible occurrence of rock burst. Based on this, effective distress measures are used to eliminate or at least weaken the incidence of rock bursts. We hope that safety in coalmines will be enhanced.

Study on Ultrasonic Response to Mechanical Structure of Coal under Loading and Unloading Condition

Ultrasonic technology can be applied to study the changes in the internal defects of coal under quantitative loading, which can provide the theoretical basis for applying the technology to determine the structural stability of coal and predict disasters related to the dynamics of coal or rock. In this paper, to investigate the propagation laws of ultrasonic signals through a coal material under various loading conditions, an ultrasonic test system for the deformation and fracture of coal rock was used and a cyclic loading and unloading pattern is adopted. In addition, changes in ultrasonic parameters such as amplitude, dominant frequency, and velocity were analyzed. At the initial loading stage, the ultrasonic amplitude, amplitude of the dominant frequency, and wave velocity slightly decrease as the loading process progresses, and these three ultrasonic parameters gradually increase to their maxima when the stress level reaches approximately 46%. When it progresses from the linear elastic stage to the elastic plastic stage, the material inside the coal distorts and fractures more drastically, the inner defects are fully developed, and the acoustic parameters decrease significantly. Therefore, the corresponding measures should be adapted to reduce the loading stress before the coal is loaded to its critical stress level.

Application of Electromagnetic Radiation (EMR) Technology in Monitoring and Warning of Coal and Rock Dynamic Disasters

Coal and rock dynamic disasters induced by the large-scale deformation and fracture of surrounding coal and rock, have significant impact on the safe and efficient extraction of coal. Effective and accurate early warning is the elementary basis for the efficient and affordable prevention and control of such kind of disasters. Laboratory experiments have demonstrated that EMR signals are generated during the process of deformation and fracturing of coal or rock under various loading conditions. EMR are paroxysmal pulse signals and increase with stress applied; and the higher the load, the larger the EMR signals. The genetic mechanism, characteristics and laws of EMR of coal and rock were studied by laboratory experiments, based on which, an EMR monitoring and warning method has been put forward. Based on the research results and the method, EMR detectors, Model KBD5 and KBD7, have been developed and employed to predict coal and rock dynamic disasters, such as coal and gas outbursts, rockbursts, monitor goaf roof stability, observe pressure of the surrounding rocks, and determine the width of relief areas. Before the occurrence of a coal and rock dynamic disaster, EMR usually has a corresponding precursor. According to the characteristics of the EMR precursor in time and space, it is possible to recognize the dangerous areas of rockbursts around underground openings and thus give early warnings of the dynamic phenomena as well.

Characterization of the Products of Explosions of Varying Concentrations of Coal Dust

ABSTRACT Coal dust explosions using various concentrations of dust were performed and the gaseous and solid explosion products were analyzed. The explosion severity parameters Pm and (dP/dt)m were found to initially increase and then decrease with increasing dust concentration, while combustion time, tc, exhibited the opposite trend. The value of tc was also found to exhibit a linear correlation with both Pm and (dP/dt)m. Volatile matter and fixed carbon were the major components of each explosion and were significantly reduced in residues. CO, CO2 and CH4 were the major gas components and were present at different levels depending on the dust concentration. Micropores and mesopores played an important role in the explosion process. The data show that an explosion involving a low dust concentration is dominated by heterogeneous combustion, while homogeneous combustion is the primary process when employing worst-case and high concentrations.

Characteristics Analysis of Post-Explosion Coal Dust Samples by X-ray Diffraction

ABSTRACT Coal dust explosion tests were performed in a 20-L spherical vessel and the post-explosion dust samples were collected for analysis. Various characteristic parameters of each explosion trial were recorded, including maximum pressure, maximum rate of pressure rise, ignition time, and deflagration index. In addition, as a new analytical technique, X-ray diffraction (XRD) was used to analyze the coal dust samples before and after each explosion, and assessments of microcrystalline structures and mineral phases were also performed. The diffraction peaks of post-explosion dust samples were shifted to larger angles and were also sharper and narrower compared to the peaks generated by the original coal dusts. Variations in microcrystalline structural parameters (d002 and L002) indicated that the post-explosion specimens had a higher degree of crystallinity and a more ordered crystal structure. Differences in mineral components between the pre-explosion and post-explosion samples were also evident, suggesting phase transformations during the explosions. The above results indicate that XRD analysis can be regarded as a reliable tool to study post-explosion coal dust and provide a new insight on further understanding of coal dust explosion characteristics.

Electromagnetic Radiation Response of Surrounding Rock Stress Relaxation Zone in Excavation Roadway and Its Application

The depth of stress relaxation zone of surrounding rock in coal mining space is of great importance on taking reasonable support technology and determining appropriate safety distance to prevent outburst. The electromagnetic radiation (EMR) will be produced during deformation and fracture process of coal, the higher the stress that coal receives, and the more intense EMR signals are produced. The stress state and deformation and fracture process of coal and rock can be qualitatively analyzed by the EMR signals that are produced from coal fracturing. In this paper, the electromagnetic radiation signals at different depth in surrounding rock of excavation roadway were tested. The range of stress relaxation zone was determined by analyzing the EMR and stress distribution in coal seam under different production process and geological conditions, and corresponding roadway supporting suggestions were put forward. The research results of the paper have important practical significance on roadway support and the outburst prevention and control of coal and gas.