Fengjie Zhang

Conventional standard oxygen-consumption-rate models can retain an apparent dependence on reactor height and gas flow rate when outlet oxygen concentrations become low. To examine this issue, programmed-temperature oxidation experiments were conducted using gas-fat coal from the Zhuxianzhuang Coal Mine. Two packed coal-bed configurations were used: a full bed (coal-bed height, 20 cm; packed-coal volume, 250 cm³) and a half bed (coal-bed height, 10 cm; packed-coal volume, 125 cm³); both were operated at inlet flow rates of 80, 100, and 120 mL min⁻¹. Outlet O₂, CO, and CO₂ samples were collected near the end of each isothermal hold and analysed by gas chromatography. An empirical effective oxygen-concentration correction parameter was introduced to revise the conventional standard oxygen-consumption-rate model, and corresponding standard CO and CO₂ generation-rate and calculated heat-release-intensity models were derived. The outlet oxygen concentration exhibited four apparent stages with increasing temperature: slow decrease, rapid decrease, renewed slow decrease, and stabilization. Above approximately 195 °C, the outlet oxygen concentration approached a nonzero plateau. On the basis of this plateau and the consistency of results obtained under the tested conditions, a value of 1.21 vol.% was selected as an empirical correction parameter for the present system. The conventional model produced increasingly separated oxygen-consumption-rate curves at high temperatures and yielded R² = 0.930. After introducing the correction parameter, the curves obtained under the tested conditions showed improved convergence and R² increased to 0.993. The revised standard CO and CO₂ generation rates and calculated heat-release intensities likewise showed reduced dependence on reactor height and flow rate within the present dataset and increased exponentially with temperature. These empirical correlations describe the tested coal under the specified reactor and heating conditions. Independent validation is required before extending them to other coals or operating conditions.

Due to the complex porous media structure of the longwall gob area, it has been difficult to determine the gas dispersion coefficient of oxygen when studying spontaneous coal combustion in the gob area. In this work, we first designed an experimental device for testing the gas diffusion coefficient of porous media. Then, the distribution law of gas concentration in porous media under different particle size conditions was obtained by experiments. Subsequently, we established a dimensionless mathematical model of gas dispersion in porous media and developed a corresponding numerical simulator based on the finite volume method (FVM). The influence of the dimensionless gas dispersion coefficient on the gas concentration distribution was analyzed, and then a dimensionless inversion method of the gas dispersion coefficient was summarized and put forward. Finally, we obtained the values of the gas dispersion coefficient in the experimental device under different particle size conditions by inversion and discussed its effect on the gas dispersion behavior in porous media. The results show that (1) the distribution of gas concentration obtained from the experimental test and numerical simulation is consistent, which verifies the reliability of our work; (2) the dimensionless gas concentration is the highest near the injection point and gradually decreases along the depth and both sides of the test container; (3) with the increase of the dimensionless gas dispersion coefficient, the distance required for uniform gas mixing in the test container is gradually shortened and the gas dispersion coverage is wider; and (4) the larger pore space facilitates the dispersion behavior of the gas, and the gas dispersion coefficient shows a parabolic trend with the increase of porous medium particle size.

Coal spontaneous combustion (CSC) in gob not only leads to wasted resources and casualties, but also produces a lot of waste gas that pollutes the underground environment. Mastering the degree of CSC helps ensure that timely and effective control measures are taken. The real-time and accurate monitoring of temperature, as the primary indicator of the extent of CSC, is difficult due to the harsh and hidden environment of gob, resulting in a reduced ability to anticipate and prevent CSC. In this work, a complete distributed optical fiber temperature sensing system (DTSS) set with strong anti-damage ability was developed. The optical cable is protected by internal parallel steel cables and external protective pipes, which greatly improve the system’s reliability and integrity when used in gob. During its application in the Wangyun Coal Mine, an abnormal temperature rise in gob was discovered in time, and the effect of inhibitor spraying was monitored and evaluated. The degree of CSC in the gob was accurately identified, which shows that the work of coal mining can be targeted. This work is expected to improve early warning capability to prevent the risk of CSC in gob, and has promising applications.

Coal spontaneous combustion (CSC) is a serious threat to the safe mining of coal resources, and the selection of suitable gas indicators to predict the CSC state is crucial for the prevention and control of coal mine fires. In this paper, the temperature-programmed experiment of CSC was first carried out to analyze the gas components and compositions in the oxidative pyrolysis process of three coal samples (lignite, long-flame coal, and lean coal) with different coalification degrees. Subsequently, the spontaneous combustion tendency of these three coal samples was evaluated. Finally, through the variation of gas concentration, gas concentration ratio, and fire coefficient with coal temperature, the indicators suitable for predicting the spontaneous combustion of coal were preferred, and a multi-parameter indicator system was established to make a comprehensive judgment on the spontaneous combustion status of coal. The results show that coal rank is negatively correlated with oxygen consumption rate. The higher the coalification degree of coal, the slower the oxidation reaction and the later the characteristic temperature point appears. The lignite selected in this experiment is a type of coal that is more prone to spontaneous combustion than long-flame coal and poor coal, and the CO concentration, C₂H₆/CH₄, and second fire coefficient R₂ can be used as the main indicators for predicting CSC, while the other gases, olefin-alkane ratio and fire coefficient can be used as auxiliary indicators. To some extent, the research content can effectively and accurately determine the stage and degree of coal spontaneous combustion, which has a certain guiding role in predicting CSC.

Bone Research, Published online: 1 July 2014; | doi:10.1038/boneres.2014.12