Research On Macro Characteristics And Control Technology Of Spontaneous Combustion Of Waterishlogged Coal

Spontaneous combustion of coal is one of the major disasters in the mining of coal. Waterishlogged coal is a kind of coal whose physical and chemical property changes as a result of immersing in water chronically. In recent years, the exploitation of coal resources development gradually to the west of our country, but most of the mining areas are coal seams of shallow buried ? near distance and easily spontaneous combustion. The upper goaf was easy to produce cracks by mining, formed the channel of water seepage, and made the coal under soaking for a long time, leading to changes in coal; moreover, the mining of under coal seam appeared “replacement of water and air” in the water exploration process, which increase the amount of air leakage and the risk of coal spontaneous combustion.At present, the research on the mechanism and prevention technology of coal spontaneous combustion has been basically formed a relatively complete research system. However, there is little research on the effect of moisture on the coal spontaneous combustion, which can not explain the phenomenon that the number of spontaneous combustion of coal is greatly increased after water immersion; In the study, most scholars only study the single coal spontaneous combustion parameters, and the parameters of coal spontaneous combustion are not considered; Although some scholars put forward a concept that there is an optimum humidity level to make coal more easy to low temperature oxidation, most of the studies are only qualitative, and no specific criteria and methods are given. Therefore, it is very important to study the effect of water on coal spontaneous combustion.This thesis has been supported financially by several important projects, such as the National Science and Technology Pillar Program during the12 th Five-Year Plan Period of China(Study on the key technologies in the construction of million tons of coal base of Shanxi-Mongolia in deep coal mine, 2012BAB13B02)and the Technology Innovation Foundation Project of China Coal Technology & Engineering Group Co., Ltd.(Study on the safety technology for efficient mining in western Inner Mongolia mine area, 2012MS001), etc. Taking the coal samples of 2-1 coal seam in HuLu-su Mine as a subject, we study on the macro feature of spontaneous combustion and control technology of waterishlogged coal by using the method of laboratory test, theoretical analysis, numerical simulation and field monitoring. The main research contents and conclusions of this thesis are as follows:(1)The characteristics of oxygen absorption of coal samples with different water content were studied combining theoretical analysis and laboratory experiments, which revealed the rule that are more easily oxidation than at low temperature.The water absorption characteristics of coal is analyzed in theory, which indicates that water in coal includes external moisture, internal moisture and combined water and external moisture can influent low temperature oxidation of coal. The natural moisture content of 2-1 coal is 5.86%, saturated moisture content is 14.19% and designed gradient of moisture content is 2%. The moisture content of coal samples in experiment is separately 5.86%?8.04%?10.08%?12.01%?14.19%. Pore structure of coal is changed by water and true density of coal with different moisture content will be reduced with the growth of moisture content, but porosity of the coal will be advanced with the growth of moisture content. Porosity of the coal enlarges as the moisture content grows, but thermal conductivity reduces gradually, which indicates that water can impede the heat conduction. Oxygen quantity of coal is the result of physical adsorption-chemical adsorption interaction. Oxygen quantity of coal will reduce first and then grow as the temperature rises and 70? is turning point. Water is the important influence factor of oxygen quantity of coal. Oxygen quantity of coal will reduce first and then grow as the water content rises. Oxygen quantity of coal reaches maximum value when the rate of water content is 12.01%.(2)Thermal analysis of coal samples with different moisture content were studied by using TG-DSC method. And the thermal analysis method for the critical moisture content of coal spontaneous combustion was advanced.The TG-DSC curves of the coal samples with different moisture content were gained; According to the five characteristic temperatures, the coal spontaneous combustion process is divided into five phases: adsorption weight gain stage(T0-T1), dehydration weight loss stage(T1-T2), oxygen uptake weight gain stage(T2-T3), oxidative decomposition stage(T3-T4) and oxidation combustion stage(T4-T5), The reaction kinetics equation of coal spontaneous combustion was established. The reaction kinetics equation of coal spontaneous combustion was established. The correlation coefficients of ln[g(?)/T2] verus 1/T for TG data was analyzed using 9 differernt kinetics mechanism functions. The results show that reaction kinetics model of coal samples in weighing by absorbing oxygen(T2-T3) were determined for 1 grade reaction, and the corresponding apparent activation energies were calculated on the basis of the Coats-Redfern integral method. The percentage of difference value of activation energy to the activation energy of raw coal samples and waterishlogged coal samples is defined as inhibition rate, which is proposed to evaluate the impact of moisture on spontaneous combustion of coal. According to the criteria of minimum inhibition rate, the critical moisture content for spontaneous combustion of the 2-1 coal seam is about 12.01%. The model of coal spontaneous combustion is established based on activation energy index. The spontaneous combustion period of coal samples with different moisture contents( 3) The characteristics of gas induced by spontaneous combustion of waterishlogged coal with different rate of water content are studied through coal spontaneous combustion simulation system. The forecast index system was established for prediction of waterishlogged coal spontaneous combustion.Gas production characteristics of coal samples with different moisture contents were studied by the simulation system of coal spontaneous combustion. The parameters to indicate spontaneous combustion such as oxygen consumption rates, heat intensity and etc. in the oxidation process were tested for coal samples with different moisture content. There is a moisture content, which generated more CO and C2H4 than other coal samples and the results implied that the effects of moister content on oxygen consumption rate of coal spontaneous combustion could be divided into three stages: below 70 ?, oxygen consumption rate is proportional to the moisture content; at 70~110 ?, coal samples with moisture content of 12.01% achieve the maximum rate of oxygen consumption, while oxygen consumption rates of other coal samples increase with the moisture content; above 110 ?, coal samples with moisture content of 12.01% also achieve the maximum rate of oxygen consumption, oxygen consumption rates of other coal samples decreased with increasing moisture content. The effect of moisture on the critical temperature of coal spontaneous combustion is twofold. With the increase of moisture content, the critical temperature of coal spontaneous combustion increases first, the minimum value was formed at the optimum water content, and then decreases; No matter moisture content is higher or lower than the optimum water content, the spontaneous combustion of coal will be inhibited. And the greater moisture content, the more is the inhibitory effect. The critical temperature of 2-1coal seam is 80 ?, dry temperature is 150 ?. Grey correlation analysis method is used to work out the correlation degree between the various gases indices within various temperature ranges and coal temperatures. This is then taken as criteria for quantitative analysis of the reliability o f gas indices assessment. Based on the analysis of the reliability index gas, a forecasting index system was established with CO at the low temperature stage and C2H4/C2H6 for main index gas and CO for auxiliary at the high temperature.( 4) The distribution characteristic of spontaneous combustion space of waterishlogged coal is studied through numerical simulation and field monitoring.First, calculated and analyzed the float coal thickness in 2101 fully mechanized caving goaf, found that the float coal thickness were larger than the minimum, and there is a danger of spontaneous combustion; monitored the oxygen concentration, air leakage intensity distribution and temperature changes, found that there existed serious air leakage in "two roads" of goaf, the oxidation heating zone were widely distributed in inlet and return airways; used oxygen concentration and wind speed indicators to divide the spontaneous combustion "three zones" in goaf: 0~42 m of air inlet side is cooling zone, 42~125 m is oxidation heating zone, and suffocation zone is larger than 125 m; in the middle goaf, 0~39 m is cooling zone, 39~107m is oxidation heating zone, and suffocation zone is larger than 107 m; 0~28 m of air return side is cooling zone, 28~90 m is oxidation heating zone, and suffocation zone is larger than 90 m; According to the experimental shortest spontaneous combustion period as 32 days, the minimum speed for safety of working face advancing is 2.69 m/d. The tendency of oxidation temperature distribution in the goaf is asymmetric and shows an “U” type, the inlet side is much greater than the return air side for depth and width. The results provide theoretical basis for prevention of spontaneous combustion in flooding goaf of shallow buried coal seam.(5)The control technology parameters of spontaneous combustion of the waterlogged coal are optimized combining laboratory experiments and numerical simulations.The evaporation rate of extent moister in coal samples are tested through the thermostat water bath. The optimal injection rate to prevent and control fire in the goaf is 55% and the injection cirle is 4 days under normal conditions, 2 days in hot weather. First, the theoretical calculations and numerical simulation methods are combined to research the influence rule of wind amount on spontaneous combustion "three zones" in goaf, find that after increasing the wind amount, the oxidation heating zone moves to the deep goaf, and moving distance is relatively large, but the amplification of the maximum width of oxidation heating zone is not obvious, determine the reasonable wind amount for working face is 720~864 m3/min; Nitrogen injection is a very effective way to prevent spontaneous combustion in goaf, with the increase of nitrogen injection flow rate, the width of oxidation heating zone decreases gradually, the mathematical relationship between injection capacity of nitrogen and the maximum width of oxidation heating zone is, the best nitrogen injection flow rate is 800~900 m3/h; the relationship between nitrogen injection port position and the width change of oxidation heating zone is, with the enlargement of nitrogen injection port position, the maximum width of oxidation heating zone decreases first and then increases, at 75 m, the maximum width of oxidation heating zone has a minimal value, the best position of nitrogen injection port is 65~85 m far away from working face.The innovation points of this paper include the following three aspects:(1)The indexe and method of thermal analysis of the critical moisture content of coal spontaneous combustion are presented. Moisture is the main affecting factors for low temperature oxidation stage of coal. The process of weighing by absorbing oxygen followed the first order reaction, and reaction kinetics model of coal samples was g(?)=?2,whose activation energy reflected the oxidation of coal. The percentage of difference value of activation energy to the activation energy of raw coal samples and waterishlogged coal samples was defined as inhibition rate, which was proposed to determine the critical moisture content of coal spontaneous combustion.(2)The change law of the critical temperature of coal sample with different moisture content is revealed. The effect of moisture on the critical temperature of coal spontaneous combustion is twofold, including inhibition and catalysis, which is mainly determined by the content of moisture in coal. With the increase of moisture content, the critical temperature of coal spontaneous combustion increases first, the minimum value was formed at the optimum water content, and then decreases; No matter moisture content is higher or lower than the optimum water content, the spontaneous combustion of coal will be inhibited. And the greater moisture content, the more is the inhibitory effect.(3)The coal sample with critical moisture content more easily low temperature oxidation than other coal samples were put forward. It is found that the amount of oxygen of the coal sample with critical moisture and oxygen consumption is greater, ignition activation energy lower, gas generating capacity bigger, and oxidation and spontaneous combustion stronger by comparison and analysis of the characteristics of oxygen absorption, thermal analysis and gas production characteristics of different water cut coal samples.