Yujia Chen

Identification Model and Indicator of Outburst-Prone Coal Seams

Coal and gas outburst with a sudden and violent ejection of a large amount of coal and gas from the working face is the most harmful dynamic phenomenon during coal mining (Aguado and Nicieza 2007; Lama and Bodziony 1998; Torano et al. 2012). Outbursts have occurred in all the major coal-producing countries around the world; however, nearly one-third of the total outbursts have occurred in China (Guan et al. 2009; Skoczylas 2012; Xu et al. 2006). The process of judging the possible occurrence of coal and gas outburst in the near future is defined as the identification of outburst-prone coal seams, which is of great significance for underground mining safety. The identification of outburst-prone coal seams can be conducted before or after the occurrence of dynamic phenomenon. Since the vast majority of coal seams do not show dynamic phenomenon, it is necessary to identify outburst-prone coal seams before the occurrence of dynamic phenomenon. To this end, many dynamic characteristic indicators have been used. For example, gas content thresholds of 9 m/t for CH4 and 6 m /t for CO2 were used in the Sydney Basin to indicate outburst-prone conditions, but they were based on empirical experience from operations and may vary from one coal seam to another. The DP indicator, based on the initial rate of gas desorption from coal, has been widely adopted in Europe. However, in Australia, outbursts occurred in coal seams showing low DP values (Beamish and Crosdale 1998). Currently in China, the ‘‘Four Indicators [degree of coal fracturing (for more details, see Table 1), Protodyakonov strength index (f), initial rate of gas emission (DP), and gas pressure (P)] Method’’ given in the Chinese standard AQ 1024-2006 (State Administration of Work Safety, SAWS 2006) is used as the standard for the identification of outburst-prone coal seams. If all the measured values of the four indicators exceed their thresholds given in Table 2, the coal seam is identified as an outburst-prone coal seam. This method has been widely used in China for its simplicity. However, the predictions with the ‘‘Four Indicators Method’’ are not always consistent with the occurrence of dynamic phenomenon during underground mining, and outbursts causing large casualties still occur in some nonoutburst seams identified using this method. Thus, the accurate identification of outburst-prone coal seams is an urgent issue. In this paper, a novel identification model under the ideal condition of uncovering coal in crosscut (IUCC) is proposed, in which the initial expansion energy of released gas (IEERG) is used as an indicator to identify outburstprone coal seams. To obtain the threshold of the IEERG indicator, numerous experiments of coal and gas outburst were carried out and the IEERG of coal samples were measured. After that, the ‘‘IEERG Indicator Method’’ was used on the spot to verify its accuracy. C. Jiang (&) L. Xu X. Li J. Tang Y. Chen S. Tian H. Liu School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China e-mail: xlh0612@163.com

Quick Determination of Gas Pressure before Uncovering Coal in cross-cuts and Shafts

The determination of gas pressure before uncovering coal in cross-cuts and in shafts is one of the important steps in predicting coal and gas outbursts. However, the time spent for testing gas pressure is, at present, very long, seriously affecting the application of outburst prediction techniques in opening coal seams in cross-cuts and shafts. In order to reduce the time needed in gas pressure tests and to improve the accuracy of tests, we analyzed the process of gas pressure tests and examined the effect of the length of boreholes in coal seams in tests. The result shows that 1) the shorter the borehole, the easier the real pressure value of gas can be obtained and 2) the main factors affecting the time spent in gas pressure tests are the length of the borehole in coal seams, the gas emission time after the borehole has been formed and the quality of the borehole-sealing. The longer the length of the borehole, the longer the gas emission time and the larger the pressure-relief circle formed around the borehole, the longer the time needed for pressure tests. By controlling the length of the borehole in a test case in the Huainan mining area, and adopting a quick sealing technique using a sticky liquid method, the sealing quality was clearly improved and the gas emission time as well as the amount of gas discharged greatly decreased. Before the method described, the time required for the gas pressure to increase during the pressure test process, was more than 10 days. With our new method the required time is only 5 hours. In addition, the accuracy of the gas pressure test is greatly improved.

A Fisher’s Criterion-Based Linear Discriminant Analysis for Predicting the Critical Values of Coal and Gas Outbursts Using the Initial Gas Flow in a Borehole

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 3u2009min in a 1u2009m long borehole with a diameter of 42u2009mm 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 36.63u2009L and 80.85u2009L, 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.

A rapid prediction technology for the outburst risk of coal uncovered in crosscuts and shafts and its application in the Panyi Coal Mine in Huainan, China

Abstract In this study, we propose a set of rapid prediction technologies for the outburst risk of coal uncovering in crosscuts and shafts (CUCS). These technologies are needed because of the ideal model to uncover coal in crosscuts and the law that coal seam outburst intensity increases as the initial expansion energy of released gas (IEERG) increases. Our technologies include rapid coal seam gas pressure determination under complex geological conditions, complete coal-core coring and the IEERG measurement. We successfully applied these technologies in the Panyi Coal Mine in Huainan. The field application results demonstrated that these technologies could accurately and rapidly measure coal seam gas pressure, greatly minimize human damages to coal samples in the coal coring process, and accurately predict outburst risks in CUCS.

Design of gas drainage modes based on gas emission rate in a gob: a simulation study

In the process of working face advance in longwall coal mining, a great deal of gas relieved by the strata adjacent to the mining coal seam and the residual coal in gob migrates to gob. If the gas drainage method in gob is unreasonable, gas will accumulate in the upper corner and overrun in the return air flow. In the paper, a CFD (computational fluid mechanics) model of gob based on the actual geological conditions and gas drainage mode of 1262 working face of Dingji Coal Mine, China, was established. The gas drainage modes that should be taken to effectively control gas accumulation in the upper corner and gas overrun in the return air flow at different gas emission rates were discussed. The simulation results show that when the gas emission rate in the working face is lower than 20xa0m3/min, buried pipe drainage can effectively control gas accumulation in the upper corner and gas overrun in the return air flow. When gas emission rate there is between 20 and 30xa0m3/min, the two problems can be solved through cross-measure borehole drainage combined with buried pipe drainage. When gas emission rate there is higher than 40xa0m3/min, they can be effectively controlled through a three-dimensional drainage mode including buried pipes, cross-measure boreholes, and surface wells. Arranging surface wells within the fractured zone near the return airway can increase the gas drainage rate, and the gas concentration can reach over 85%; the gas concentrations of buried pipe drainage and cross-measure borehole drainage are 15~20% and 70~80%, respectively.

Effect of aeration on fast gas pressure tests

Abstract Given the problem of the long time required for testing gas pressure, we propose a fast-test method in which we used a technique of fast borehole sealing and air replenishing. Based on the characteristics of gas emission from boreholes to be tested, we built a theoretical model for calculating parameters during the process of increasing natural pressure and aeration. Using this model, we investigated the effect of different aeration conditions on velocity of pressure tests. The result shows that: 1) aerating air into boreholes can speed up gas pressure tests and 2) the more similar the pressure of the aerated air to the original gas pressure, the smaller the gas volume absorbed by coal and the shorter the time needed in pressure test. A case study in the Luan mining area shows that the time needed for gas pressure test is only 4 h using our method of aeration and 29 h under conditions of increasing natural pressure, saving time by 86.2%. This case study also indicates that, by using the aeration method, only one hour is needed for gas pressure to reach a stable state, which breaks the record of the shortest time needed for gas pressure tests in China.