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Coal and gas outbursts are a rapid and powerful energy-releasing process. The damage to coal and the release of gas in the initial stage play key roles in the occurrence of outbursts. Existing studies have mainly focused on the gas desorption processes in coal under specific conditions such as particle size, specified mass and exposure time of pulverized coal. However, these studies have focused less on the gas release during the initial stage, and the quantitative relationship between the outburst risk level and the test results. Seven coal samples with different metamorphic degrees were chosen for use in outburst simulation experiments to investigate trends in gas release from pulverized coal in the presence of N 2 and CO 2 . The results demonstrate a strong correlation between the gas release capacity during the first 10 s and natural desorption gas content of the onset of coal exposure during the first 120 min. In general, the higher the gas content, the larger the released gas volume, and thus the larger the limiting amount of desorptive gas. The gas release from an outburst coal sample is larger than that from a non-outburst coal sample within the first 10 s. There is a good linear relationship between the initial volume of released gas (in the first 10 s) and the initial expansion energy of released gas (IEERG), which can reflect the risk of coal outburst. Thus, according to the outburst critical value (42.98 mJ/g) of IEERG, the critical value of initial volume of released gas could be calculated to be 1.113 cm 3 /g. The results can provide a reference for further study of the effect of gas on outbursts and the investigation of coal and gas outbursts disasters. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a dynamic phenomenon with violent eruptions of coal and gas from the working coal seam, resulting in economic loss and casualties. It has been proved that rapid desorption within a short period is necessary for occurrence of an outburst. Due to limitation of the present test condition, gas desorption characterization for the first several seconds (0–10 s) hasn’t been researched sufficiently. In this study, initial desorption characterization of gas in coal is studied by experiments with methane and carbon dioxide. The most remarkable characteristic of the experimental setup is application of a self-developed real-time data acquisition system with time interval of about 1.6 ms, which achieves the goal of collecting enough pressure and temperature data for analysis and calculation. The data is used to study gas pressure variation and calculate initial amount of desorbed gas and initial expansion energy of released gas. From the experimental results, new proof has been found to verify that coal and carbon dioxide outburst is more dangerous than coal and methane outburst. [ABSTRACT FROM AUTHOR]

The risk of coal and gas outbursts can be predicted using a method that is linear and continuous and based on the initial gas flow in the borehole (IGFB); this method is significantly superior to the traditional point prediction method. Acquiring accurate critical values is the key to ensuring accurate predictions. Based on ideal rock cross-cut coal uncovering model, the IGFB measurement device was developed. The present study measured the data of the initial gas flow over 3 min in a 1 m long borehole with a diameter of 42 mm in the laboratory. A total of 48 sets of data were obtained. These data were fuzzy and chaotic. Fisher’s discrimination method was able to transform these spatial data, which were multidimensional due to the factors influencing the IGFB, into a one-dimensional function and determine its critical value. Then, by processing the data into a normal distribution, the critical values of the outbursts were analyzed using linear discriminant analysis with Fisher’s criterion. The weak and strong outbursts had critical values of 36.63 L and 80.85 L, respectively, and the accuracy of the back-discriminant analysis for the weak and strong outbursts was 94.74% and 92.86%, respectively. Eight outburst tests were simulated in the laboratory, the reverse verification accuracy was 100%, and the accuracy of the critical value was verified. [ABSTRACT FROM AUTHOR]

The advantages and disadvantages of existing methods for predicting coal and gas outbursts in coal roadways were analyzed, and the primary factors influencing coal and gas outbursts, including the expansion energy of the initially released gases, were studied. To this end, a theoretical formula for calculating the amount of gas released from boreholes during drilling was deduced. In addition, it was determined that the gas emission per unit borehole length was related to the ground stress, gas pressure, and coal strength around the borehole during drilling. By collecting these continuously released gases from boreholes during drilling, that is, by adopting line prediction technology, the influence of those parameters (ground stress, gas pressure, coal strength) on the outburst process can be determined, and the outburst hazard can be predicted. To test this approach, a specially developed line prediction device was applied to the No. 3 coal seam of the Xinyuan coal mine, Shanxi Province, China. The test results demonstrate that the gas emissions per meter of borehole, Q Lm , measured by the line prediction technique differed between outburst and non-outburst samples. Additionally, compared with existing indices used for predicting coal and gas outbursts in coal roadways, including K 1 and S max , the line prediction technique was more sensitive and could reflect the outburst hazard in front of working faces more accurately. [ABSTRACT FROM AUTHOR]

The article discusses the proposed identification model for the outburst of coal and gas. Topics include the experiments carried to obtain the threshold of the initial expansion energy of released gas (IEERG) indicator, the mechanical conditions which must be met by occurrence of coal and gas outburst, and the potential for the gas in soft coal to transport through the cracks of the barrier layer.

The law of gas initial desorption from coals is greatly important for understanding the occurrence mechanism and predicting coal and gas outburst (hereinafter referred to as 'outburst'). However, dynamic characterization of gas initial desorption remains to be investigated. In this study, by monitoring the gas pressure and temperature of tectonically deformed (TD) coal and primary-undeformed (PU) coal, we established the evolution laws of gas key parameters during the initial desorption. The results indicate that the gas pressure drop rate, mass flow rate, initial desorption rate, and gas velocity increase with increasing gas pressure, with stronger gas dynamic effect, generating a high pressure gradient on the coal surface. Under the same gas pressure, the pressure gradient formed on the TD coal surface is greater than that formed on the surface of the PU coal, resulting in easily initiating an outburst in the TD coal. Moreover, the increased gas pressure increases temperature change rates (falling rate and rising rate) of coal mass. The minimum and final stable temperatures in the TD coal are generally lower compared to the PU coal. The releasing process of gas expansion energy can be divided into two stages exhibiting two peaks which increase as gas pressure increases. The two peak values for the TD coal both are about 2–3 times of those of the PU coal. In addition, the total gas expansion energy released by TD coal is far greater than that released by PU coal. The two peaks and the total values of gas expansion energy also prove that the damage of gas pressure to coal mass increases with the increased pressure, more likely producing pulverized coals and more prone to initiate an outburst. [ABSTRACT FROM AUTHOR]

• A new method for directly determining the coal seam permeability around roadway is proposed. • The coal seam permeability around roadway is divided into four stages. • The results are able to determine the sealing depth of in-seam borehole and guide the gas disaster preventions. Changes in coal seam permeability induced by roadway excavation may lead to many engineering problems, such as low gas extraction concentration in in-seam boreholes, coal and gas outburst disasters, and gas emissions in the process of coal roadway excavation. To solve these problems, this paper proposed a new method to directly determine the distribution of coal seam permeability around roadways. For this purpose, method principle was analyzed, equipment was developed, and field experiments were carried out in Baiyangling coal mine. Finally, the engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. The results showed that the coal seam permeability around roadways was divided into four stages. In stages I and II, permeability increased, in stage III, permeability decreased, and stage IV maintained the initial permeability of the coal seam. These test results will aid in increasing the gas extraction concentration of in-seam boreholes and will guide the design of gas control areas. Using the verification experiment, it was concluded that the new method is able to accurately determine the redistribution of coal seam permeability caused by roadway excavation and provide theoretical guidance in solving engineering problems. [ABSTRACT FROM AUTHOR]

• The drilling cuttings weight (DCW) exhibits the least sensitivity to gas pressure. • The influence of gas pressure on the DCW becomes significant with higher level. • The obtained critical value of DCW for outburst risk prediction is 4.88 kg/m. To judge the coal and gas outburst (hereinafter referred to as 'outburst') risk of working faces in coal roadways according to the index of drilling cuttings weight (DCW), the influence factors and critical value of DCW were investigated. The study was conducted by deducing the theoretical calculation formula for DCW and using a set of self-designed, self-machined measuring apparatus for DCW which can be used to simulate the working face in a real coal roadway. The theoretical calculation formula for DCW indicates that the DCW increases with the increase of in situ stress and gas pressure while it also increases with decreasing coal strength. Among the three factors, with the change of gas pressure, the change in DCW is the smallest. On this basis, so that the critical value of DCW for judging the outburst risk of coal masses can better indicate the outburst risk at a working face, the tests for measuring DCWs under different outburst risk levels were carried out by using a set of apparatus for outburst risk which can be used to simulate the working face in a coal roadway by taking gas pressure as a controllable factor. The test results show that, with increasing outburst risk in a simulated coal seam, the DCW linearly increases at first and then suddenly rises. It also implies that the gas pressure exerts significant influences on DCW only when the outburst risk of a coal mass increases to a certain degree. There is a linear relationship between DCW and initial expansion energy of released gas (IEERG). On this basis, according to the critical values of IEERG indicating weak outburst and strong outburst risks, the critical values of DCW for outburst risk are judged. By using discriminant analysis, it can be obtained that the critical value of DCW when a weak outburst occurred is 4.88 kg/m. The critical value can be used to distinguish data samples with measured DCWs having weak outburst risk, whose accuracy reaches 89.47%. The critical value of DCW when a strong outburst occurred is 7.55 kg/m, which can accurately (100%) distinguish between data samples from weak and strong outbursts based on the measured DCWs. The final judgement accuracy of outburst risk is 93.75%. [ABSTRACT FROM AUTHOR]

Coal and gas outburst accidents occurring in underground coal mines generally cause great casualties and economic losses, especially in construction areas of coal seams where the outburst risks are not accurately evaluated. Therefore, accurately identifying the outburst risks of coal seams is necessary and critical to prevent and control outbursts. In order to improve the prediction accuracy for outbursts in working faces of coal roadways, a self-designed drilling device that can simulate the in-situ outburst prediction of coal roadways was used in this study. We used the coal samples with different coal ranks to adsorb CO 2 and N 2 with different pressures in the laboratory, so as to simulate the coal seams with different outburst risks. Finally, boreholes for predicting the outburst risk were drilled in the simulated coal seams to study the correlation between common dynamic phenomena (e.g. gas and coal being ejected from the borehole (GCEB) and drill pipe being stuck by coal mass in the borehole (DPSC)) and outburst risks. The results show that the greater the outburst risks of coal seams, the more frequent the occurrence of GCEB and DPSC is. The GCEB phenomenon for outburst prediction is attributed to small-sized coal and gas outbursts in boreholes, while the occurrence of DPSC phenomenon does not affirm that there definitely are outburst risks on working faces. The DPSC phenomenon indicating outburst risks is generally accompanied with GCEB, which form a linkage system and are triggered successively. [ABSTRACT FROM AUTHOR]

Coal and gas outburst (hereinafter referred to as “outburst”) is a complicated dynamic disaster formed in the temporal and spatial evolution process. To further improve our understanding of outburst mechanism and help more effective outburst prevention and control measures to be devised, a finite number of outburst simulation tests were carried out in the study. The results show that the contribution of the gas pressure to the energy required for an outburst is hundreds of times that of the in situ stress during the outburst. With increasing mining depths, the higher the in situ stress, the gas pressure, and the temperature; however, the temperature shows the lowest influence on the outburst intensity among various parameters such as gas pressure, the thickness of soft coal, and the temperature. The initial expansion energy of releasing gas (IEERG) is an index reflecting the influences of in situ stress, gas pressure, and coal strength on outburst intensity. The test results indicate that the potential amount of coal ejected can be predicted by measuring the IEERG of coal masses and the thickness of soft coal in outburst-prone zones. According to the failure characteristics of coal masses after an outburst, the larger the outburst intensity is, the smaller the volume of residual coal area relative to the outburst holes. The shapes of extrusive coal-gas flows after an outburst show that the outburst process is a chain of events occurring over multiple cycles as follows: preparation, initiation, development, and termination. The recorded video footage of an outburst also shows this. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a serious coal and rock dynamic disaster, and effective prediction is essential to control. The initial expansion energy of released gas (IEERG) is a new method to predict coal and gas outburst, which depends primarily on the gas pressure and degree of coal fragmentation. The IEERG reflects the effects of the ground stress, the gas pressure, and coal strength on the coal and gas outburst. On the basis of the single factor on the IEERG, three factors have been studied by orthogonal experiments, including gas pressure, coal particle size, and moisture. The results show that the influence of gas pressure, coal particle size, and water content on the IEERG decreases in turn; that gas expansion energy increases significantly as gas pressure increases; and that the greater the gas pressure, the faster the increase. The index W p of IEERG first increases and then decreases with an increase in coal particle size and decreases with an increase in moisture content. Through this work, the main influencing factors of gas expansion energy have been defined. This method offers an important theoretical and practical significance to predict coal and gas outbursts. [ABSTRACT FROM AUTHOR]