Risk Of Coal Spontaneous Combustion Research Articles

Study on the coupling evolution of air and temperature field in coal mine goafs based on the similarity simulation experiments

In order to prevent the occurrence of the coupling disaster between coal spontaneous combustion and gas explosion, the similarity simulation experiments for coal mine goaf were carried out with or without heat source on the scaled-down experimental platform. Results show that the O2 diffusion speed in strike direction at the air inlet side is significantly greater than that on the air return side, especially under the condition of high ventilation velocity. The higher the ventilation velocity, the greater the risk of coal spontaneous combustion. O2 consumption and thermal convection in the coal spontaneous combustion area make it difficult for O2 to migrate to the deeper goaf. Part of the O2 in air is affected by the chimney effect, rose into the spaces formed by the caving rocks and continuously mixes with the CH4 accumulated in these spaces. Meanwhile, high-temperature area tends to expand in the direction of fresh air leakage in the process of coal spontaneous combustion in the goaf. The formed explosive gases may explode during the migration of high-temperature area, resulting in disaster accidents. Therefore, the temperature change at the location where the air leakage is serious in the goaf should also be monitored in time to prevent the coal spontaneous combustion hazards.

Effect of multi-component gases competitive adsorption on coal spontaneous combustion characteristics under goaf conditions

Coal spontaneous combustion in the goaf usually occurs in the multi-component gases environment. In this work, the multi-component gases competitive adsorption properties and its effects on the coal oxidations were investigated by gas adsorption analyzer and in-situ FTIR. Results show that the order of gases adsorption selectivity on coal surface based on IAST is that: CO2 > CH4 > O2 > N2. For all the selected gas components, interactions between the gases and the coal surface undergo two main mechanisms: the physical competitive adsorption or desorption under lower temperature condition and the chemical adsorption or reaction under higher temperature conditions. In ambient temperature conditions, coal spontaneous combustion would be hindered due to the stronger adsorption capability of CH4 and CO2 on coal surfaces. At the late oxidation stage, irreversible chemical adsorption of O2 would be gradually strengthened replaces and O2 preferentially reacts with –CH3/–CH2– to generate much more oxygen-containing functional groups. Reactions of CH4 with O2 on the high-temperature coal surface would accelerate the coal spontaneous combustion, which increases the risk of coal spontaneous combustion.

Effect of water immersion on active functional groups and characteristic temperatures of bituminous coal

Abstract In the mining of multiple coal seams, geological fracture may cause water from aquifers to accumulate in the overlying gob. Residual coal in the overlying gob after water immersion and air exposure is regarded as soaked coal, which is at a high risk of coal spontaneous combustion. In the present study, Fourier transform infrared spectrometry and synchronous thermal analysis were performed to investigate the active functional groups, characteristic temperatures, and exothermic properties of bituminous coal (selected due to its abundance in northwestern China) immersed in water for different durations (150, 180, 210, and 240 days). On the basis of reaction mechanism equations, two thermokinetic models were used to calculate the apparent activation energy (AAE) in different stages of coal pyrolysis. Results revealed that soaked coals were more prone to spontaneous combustion than raw coal (WW0) because of changes in the numbers of functional groups and characteristic temperatures. The critical soaking duration of 180 days increased the exothermic strength of coal to its maximum. The AAE of WW0 was higher than that of soaked coals in Stages 1 and 4 of coal pyrolysis, indicating that soaked coals were more likely to react.

Comprehensive risk evaluation of coal spontaneous combustion in goafs based on systems-theoretic accident model and processes

The traditional risk evaluation of coal spontaneous combustion in goafs in China is unilateral and inaccurate, which can be attributed to the fact that only the change in coal properties is considered and the external environment of coal is ignored. According to latest research, the factors affecting coal spontaneous combustion in goafs is determined by the systems-theoretic accident model and processes, from which a risk index system is established. Then, a comprehensive risk evaluation model for coal spontaneous combustion in goafs is constructed with the analytic hierarchy process, unascertained measurement theory and combination weighting method, which divides the risk of coal spontaneous combustion into four levels. The model is tested in three goafs in different coal mines. Findings show that the model is objective and valid and can be applied to the risk evaluation of coal spontaneous combustion in goafs in coal mines.

Disaster-causing mechanism and risk area classification method for composite disasters of gas explosion and coal spontaneous combustion in deep coal mining with narrow coal pillars

In order to reduce the stress on the underlying coal seam and improve mining efficiency in deep coal mining activities, narrow coal pillars are often used. They are the main factor that increases the risk of coal spontaneous combustion in the goaf. Based on the on-site experiment and the experimental analysis in the laboratory, this paper adopted correlation coefficient criteria and safety principles to research the disaster-causing mechanism of such risk. It also classified and divided the studied area into different risk levels to effectively manage it. The results show the coal mass fractures generated by the movement of stress concentration points on the narrow coal pillars is the main factor that contribute to the increased spontaneous combustion in the goaf. The two main forms are gas explosion caused by CH4 flowing into the roadway and spontaneous combustion of residual coal caused by O2 flowing into the nearby goaf. By adopting correlation coefficient criteria and safety principles, this paper selected two indicators, gas-impermeability of narrow coal pillars and risk degree of residual coal spontaneous combustion in the goaf to divide the whole narrow coal pillars and goafs into three areas, namely “Safe Area”, “General Area” and “Dangerous Area”. This can maximize safety benefits with the lowest cost and guarantee safe and efficient coal mining.

Laboratory experimental study on water-soaked–dried bituminous coal’s thermal properties

In the process of multi-seam mining, the measure of water exploration is usually employed for the upper coal seam to prevent and control water accumulation. The residual coal immersed in water for a long time in the upper layer is dried under the action of air leakage, leading to the enhancement of the risk of coal spontaneous combustion. Therefore, it is necessary to study the water soaking–drying process (WSD) on the spontaneous ignition characteristics of coal. In this study, a laser thermal conductivity analyzer was used to investigate the changes in thermal properties of bituminous coal collected from mines in western China after WSD. The results show that compared with the raw (unsoaked) coal samples, the thermal diffusivity, specific heat capacity, and thermal conductivity of the bituminous coal are considerably reduced after WSD; the maximum reductions in the three parameters are 11.11%, 6.64%, and 12.39%, respectively. These reduced thermal parameters indicate that after WSD, bituminous coal is more prone to self-heating, resulting in an increase in the rate of coal–oxygen compound reactions and a significantly increased risk of spontaneous combustion.

Method of pre-oxidation treatment for spontaneous combustion inhibition and its application

A method named “pre-oxidation treatment for spontaneous combustion inhibition” was proposed to proactively prevent coal spontaneous combustion. In this method, coal samples are firstly oxidized under room temperature by an oxidizer, which have an oxidizing capacity equivalent to the capability of oxidizing only active functional groups in coal. In this way, functional groups residing in the coal structure with high reactivity will be oxidized and heat will be released at the same time. With a large amount of water protection set up in advance, the heat can be released safely. After the highly active functional groups are oxidized, the reaction products left contain more stable functional groups with low activity, thus reducing the risks of spontaneous combustion. Underpinned by the theory, the method was tested by using different aqueous solutions of oxidants to pre-oxidize various coal samples. Results show that the method of pre-oxidation treatment for spontaneous combustion inhibition can significantly reduce the risk of coal spontaneous combustion. Among all the tested aqueous solutions of oxidants, the sodium persulfate solution is the most appropriate one.

Experimental Study on Primal CO Desorption Effect Factors: External Causes

Carbon monoxide (CO) is a commonly used indicator gas to predict the risk of coal spontaneous combustion. However, the occurrence of primal CO in coal seam may result in error of prediction. Therefore, identifying the primal CO in coal seam is crucial to improve the accuracy of coal spontaneous combustion prediction. This paper conducts a laboratory experiment to research the regularities of primal CO desorption. Four analysis factors are introduced to represent the CO emission characteristics in different desorption stages. The result indicates that CO generated by different coal samples’ oxidation has big differences. Ambient temperature has little effect on CO initial emission concentration. CO concentration in the first one and first ten desorption hour increases regularly with the increase of crushing time, primal CO cannot be ignored especially in high temperature coal mines.

Coal spontaneous combustion prediction in gob using chaos analysis on gas indicators from upper tunnel

While gas sampling through upper tunnel is available right now, only little research has been reported for the prediction of coal spontaneous combustion in working face gob by analyzing drained gas in upper tunnel.This study investigated the characteristic of air leakage of 401 working face gob in Tianchi mine under stereo gas extraction condition. The air that flows to the deep gob and is then drained to upper tunnel makes the prediction of coal spontaneous combustion in gob possible. Sample collector was employed to collect gas samples in the drainage pipeline plugged in upper tunnel and gas chromatograph was applied to analyze the indicator gases. A total of 195 gas samples were gathered including 44 groups that containing CO, O2 and O2/N2 fluctuated in a small range and dropped dramatically when the oxidation of coal in gob was accelerated, while N2 and the depletion ratio of O2 obeyed the reverse rule.R/S analysis (Rescaled Range Analysis) was used to investigate the fractal characteristic of different gas indicator time series. The Hurst indexes of different gas indicator time series follow the same rule that they would gradually become stable with the accumulating data and could deviate their chaos characteristic when CO was detected, which means the oxidation stage of coal in gob was accelerated. Changes of Hurst indexes and fractal dimensions controlled by effective delay time was researched. The fractal dimension (or Hurst index) of O2, N2, O2/N2 and depletion ratio of O2 from upper tunnel can be equally applied to indicate the combustion condition of residual coal in gob.Taken 70 days as the effective delay time, the risk of coal spontaneous combustion in 401 working face gob were divided into two stages: low-temperature oxidation stage (Hurst index >0.8776 or fractal dimension <1.2224) and dangerous stage of coal spontaneous combustion (Hurst index ≤0.8776 or fractal dimension ≥1.2224). The chaos characteristic analysis of gas index series in upper tunnel provides a novel and effective method to predict the risk of coal spontaneous combustion in working face gob.

Research on Forecast of Coal Spontaneous Combustion in Goaf of Coal Mines

The risk of coal spontaneous combustion in goaf of fully mechanized face with high dip and hard roof is high, and forecast of coal spontaneous combustion is difficult. The division method of coal spontaneous combustion “three-zone” by temperature rise and CO concentration in goaf was presented in this paper, and the BP neural network model was used to forecast the temperature in goaf. Tubes and temperature probes were buried in goaf at NO.7162 fully mechanized coal face in Longdong Coal Mine to monitor the temperature and gas concentration. According to the temperature rise and CO concentration in goaf, the range of spontaneous combustion “three-zone” was determined. On the basis of the gas monitoring system with tubes in goaf, the BP neural network model forecasting the temperature of coal spontaneous combustion was established. Taking advantage of the values of CO concentration and CO2 concentration, the coal temperature was forecasted successfully by the neural network model. The results show that the division method of coal spontaneous combustion “three-zone” by temperature rise and CO concentration and the BP neural network model can improve the forecast accuracy of spontaneous combustion, and provide scientific basis for spontaneous combustion prevention in goaf.

Simulation on Parameters Temperature Field by Gas-Solid Coupling in Goaf

For solution the complex and rapidly changing question of temperature field in mining goaf, the simulation on dynamic temperature field of parameters coupling is carried out under different working coditions based on high-efficiency finite volume method (FVM) and the moving characteristic of working face. The theory analysis, the laboratory experiment, the simulation of software and the example confirmation, above all methods are used in order to find the change of temperature field in goaf. The result is found that there has spontaneous combustion in goaf when the moving speed of working face is below 1.2m/d; besides that the temperature in goaf fall and the high-temperature region is extend to the deep of goaf. Another result is exist that there has the linear increase relationship of the intensity of leak air and the temperature in goaf, and the high-temperature region is extend to the deep of goaf contrary to the effect of the coal oxidation properties, which increases the risk of coal spontaneous combustion in goaf. Especially , all the data of temperature is visualization by picture of 3D, so the distribution map of temperature in goaf is visualized distinctly. The research results provide a theoretical basis for understanding the real situation and preventing spontaneous combustion in goaf.