Coal Spontaneous Combustion Mechanism and Prevention Measures
1 The mechanism of spontaneous combustion of coal 1.1 Overview With regard to the problem of spontaneous combustion of coal, it has long been considered that the presence of pyrite in coal is the cause of spontaneous combustion. As the oxidation of pyrite to ferric oxide and trioxide* can give off heat, In the presence of water, it can form **, which is a strong oxidant, which accelerates the oxidation of coal and promotes the spontaneous combustion of coal.
It should be pointed out that some coals containing pyrite do not necessarily ignite even though they have been placed through Longs, and coals containing no or less pyrite also have spontaneous combustion. Therefore, the spontaneous combustion of coal is not caused entirely by the inclusion of pyrite. The main reason is that it absorbs oxygen from the air, oxidizes the constituents of the coal, generates heat, and is then moistened by water, which releases more moist heat and accelerates the spontaneous combustion of coal. In addition, the spontaneous combustion of coal is also related to the nature of coal itself. Such as coal grades; coal microscopic components, moisture, minerals, joints and fissures; depth of coal seams and coal seam thickness; mining methods and ventilation methods. Spontaneous combustion of coal is essentially an oxidation process of coal.
1.2 Different stages of coal spontaneous combustion (1) Water adsorption stage. Different from other phases, this stage is only a physical process. Coal and oxygen do not react. Although coal adsorption water is not the root cause of coal spontaneous combustion, it has an important influence on coal self-heating, especially low-grade coal self-heating. When the water is adsorbed by coal, a large amount of heat is released, ie, moist heat. Therefore, in most cases, this stage plays a key role in the spontaneous combustion of coal.
(2) Chemical adsorption stage. The process of coal spontaneous combustion occurs first at this stage. The reaction temperature at this stage is from ambient temperature to 70°C. During the extension process, oxygen is produced by the coal adsorbing oxygen, which is called the chemisorption stage. In the chemical adsorption stage, the weight of coal increases slightly, and gas is produced. Among them, CO can be used as a standard gas. By monitoring the CO concentration, an early forecast of coal spontaneous combustion can be performed. A small amount of water is required for the chemical adsorption stage to participate in the reaction. According to different grades and types of coal, the exothermic heat of chemical adsorption varies between 5.04 and 6.72 J/g. If the coal temperature reaches 70°C, it will decompose, and the coal weight will decrease in magnitude, even lighter than the original coal. Evaporation of the water in the coal can take away some heat, and the heat production during this process varies between 16.8-75.6 J/g. If the coal oxidation proceeds to this stage, it is very difficult to make it not self-ignite.
(3) The formation stage of coal-oxygen composites. This stage produces a stable compound, the coal-oxygen complex. The reaction temperature range is 150-230°C. The heat generated is 25.2 to 003.4 J/g. At this stage, the weight of coal has increased again, and coal oxidation will inevitably occur spontaneously at this stage.
(4) The initial stage of combustion. This is the transitional period between the formation of coal-oxygen composites and the rapid combustion of coal. When the coal temperature reaches 230°C, the coal oxidation can proceed to a stage. At this time, the reaction heat of the coal is 42 to 243.6 J/g. This heat causes the rapid rise of coal and promotes the rapid combustion of coal.
(5) Rapid combustion stage. This is the final stage of coal self-heating and it describes the actual combustion process of coal. Depending on whether the oxygen supply is sufficient or not, carbonization, incomplete combustion or safe combustion may occur at this stage. If combustion is sufficient, the heat of reaction is equal to the calorific value of the coal.
2 Factors affecting the self-heat of coal 2.1 Coal quality of coal itself has a significant effect on the sensitivity of coal to self-heating.
(1) Grade of coal. The grade of coal indicates the degree of deterioration of coal, commonly used volatile content and coal content. The low-grade pure coal has a high sensitivity to self-heating and, as the coal product can increase its self-heat sensitivity, it decreases. Thus, the dry lignite is most easily self-heated and the bituminous coal is hardly self-heating. However, lignite, which contains the most moisture, is less susceptible to spontaneous combustion than pure lignite.
(2) The moisture content of coal. The content of water in coal has great influence on the flammability of coal. Coal with saturated moisture content, especially before lignite and sub-bituminous coal with high moisture content is mined and dried, the coal no longer adsorbs moisture, and thus cannot release the heat of wetting. The heat released by the oxidation of coal usually raises the internal moisture temperature. On the other hand, the self-heating chemical reaction requires a small amount of moisture to participate. Low-level coal moisture content is far greater than the chemical reaction requirements. Therefore, for low-grade coal, moisture is actually an inhibitor of coal self-heating.
(3) Minerals. The mineral composition in coal is also called ash. It reacts with oxygen to exothermically increase the coal temperature, and decomposes the coal to increase the surface area of ​​the coal in contact with the air, such as pyrite, which can absorb part of the heat released by the oxidation reaction to reduce the oxidation reaction process of coal; the high ash content of coal makes The unit mass of oxidation heat decreases.
2.2 Environmental factors for mining, storage and transportation Environmental factors influence the self-heating of coal as follows: the moisture content of coal can be changed; and the contact conditions of coal and oxygen can be changed: the heat generated by the production can be diffused. It can be divided into:
(1) Geological factors. Faults and crevices facilitate air and moisture contact with coal. Therefore, there is no significant increase in heat dissipation, but it increases the chance of coal oxidation and the adsorption of water. That is, faults and cracks increase the risk of coal spontaneous combustion. Deep-buried coal seams have less floor air leakage. Coal remaining in goafs (especially for thick coal seams) has increased the risk of coal spontaneous combustion due to incomplete recovery.
(2) mining factors. There are two main factors that affect mining coal spontaneous combustion, namely, ventilation and coal crushing. Where there is no ventilation or adequate ventilation, coal is less likely to be spontaneously combusted. Whereas, there is a high possibility that coal is spontaneously combusted when ventilation is insufficient. Crackle air leakage is not enough leakage, it creates conditions for further oxidation of coal, and the cooling conditions have not been improved. Therefore, any air leakage is very dangerous for spontaneous combustion of coal.
(3) storage and transportation factors. In the process of storage and transportation, the factors affecting the spontaneous combustion of coal are the insufficient ventilation and drying of low-grade coal to produce wetting heat due to rain and spray water.
3 Comprehensive Prevention and Control Measures for Coal Spontaneous Combustion 3.1 Prediction and Prediction of Spontaneous Combustion of Coal Seam (1) Given that coal produces CO during the low-temperature oxidation stage, CO is a sensitive indicator for early exposure of fire. CO sensors are installed in the coal mine working face such as coal returning face and fully-mechanized coal roadway where there is spontaneous ignition. If the CO concentration is found to be out of limits, a portable CO detector may be used to track and monitor the high temperature point.
(2) The infrared detection method is used to determine the location of the high temperature point. The basic principle of the infrared detection method is to establish the correspondence relationship between the fire source and the temperature field of the fire source according to the theory of the infrared radiation field, so as to infer the location of the fire source point.
(3) Auxiliary monitoring with borehole temperature measurement. For locations where the top coal is broken or where there is a risk of spontaneous combustion, temperature probes are buried and temperature changes are regularly monitored.
(4) Strengthen air leakage detection. Tracer gas method is regularly used to check the amount of air leakage through the slot. Strengthen observations in regions where air leakage is concentrated.
3.2 Preventive measures (1) Pressure equalization ventilation control air leakage oxygen supply. Pressure equalization ventilation is an effective measure to control air leakage in the mining of coal seams. First of all, regional pressure equalization ventilation shall be fully implemented under the requirement of diluting CH4, wind speed, temperature and per capita air volume. The pressure regulating measures include single pressure regulation and multiple measures of joint pressure adjustment, and the working surface formed in the specific implementation. The pressure equalization is gradually extended to the regional pressure equalization of the goafs in the adjacent working face.
(2) Spray plugging borehole grouting. The use of fully enclosed shotcreting and shallow grouting for suspicion of suspicious locations in coal mining or the occurrence of hidden hazards is an effective measure to prevent spontaneous combustion.
(3) Note gel to prevent fire. The use of gel injection technology to deal with hot spots or spontaneous combustion is a key measure for fire prevention in coal seam mining. The method is to inject the gel into the surrounding coal body at high temperature or fire point. Its role is to seal the air leakage channel. Can absorb heat and cool down.
It should be pointed out that some coals containing pyrite do not necessarily ignite even though they have been placed through Longs, and coals containing no or less pyrite also have spontaneous combustion. Therefore, the spontaneous combustion of coal is not caused entirely by the inclusion of pyrite. The main reason is that it absorbs oxygen from the air, oxidizes the constituents of the coal, generates heat, and is then moistened by water, which releases more moist heat and accelerates the spontaneous combustion of coal. In addition, the spontaneous combustion of coal is also related to the nature of coal itself. Such as coal grades; coal microscopic components, moisture, minerals, joints and fissures; depth of coal seams and coal seam thickness; mining methods and ventilation methods. Spontaneous combustion of coal is essentially an oxidation process of coal.
1.2 Different stages of coal spontaneous combustion (1) Water adsorption stage. Different from other phases, this stage is only a physical process. Coal and oxygen do not react. Although coal adsorption water is not the root cause of coal spontaneous combustion, it has an important influence on coal self-heating, especially low-grade coal self-heating. When the water is adsorbed by coal, a large amount of heat is released, ie, moist heat. Therefore, in most cases, this stage plays a key role in the spontaneous combustion of coal.
(2) Chemical adsorption stage. The process of coal spontaneous combustion occurs first at this stage. The reaction temperature at this stage is from ambient temperature to 70°C. During the extension process, oxygen is produced by the coal adsorbing oxygen, which is called the chemisorption stage. In the chemical adsorption stage, the weight of coal increases slightly, and gas is produced. Among them, CO can be used as a standard gas. By monitoring the CO concentration, an early forecast of coal spontaneous combustion can be performed. A small amount of water is required for the chemical adsorption stage to participate in the reaction. According to different grades and types of coal, the exothermic heat of chemical adsorption varies between 5.04 and 6.72 J/g. If the coal temperature reaches 70°C, it will decompose, and the coal weight will decrease in magnitude, even lighter than the original coal. Evaporation of the water in the coal can take away some heat, and the heat production during this process varies between 16.8-75.6 J/g. If the coal oxidation proceeds to this stage, it is very difficult to make it not self-ignite.
(3) The formation stage of coal-oxygen composites. This stage produces a stable compound, the coal-oxygen complex. The reaction temperature range is 150-230°C. The heat generated is 25.2 to 003.4 J/g. At this stage, the weight of coal has increased again, and coal oxidation will inevitably occur spontaneously at this stage.
(4) The initial stage of combustion. This is the transitional period between the formation of coal-oxygen composites and the rapid combustion of coal. When the coal temperature reaches 230°C, the coal oxidation can proceed to a stage. At this time, the reaction heat of the coal is 42 to 243.6 J/g. This heat causes the rapid rise of coal and promotes the rapid combustion of coal.
(5) Rapid combustion stage. This is the final stage of coal self-heating and it describes the actual combustion process of coal. Depending on whether the oxygen supply is sufficient or not, carbonization, incomplete combustion or safe combustion may occur at this stage. If combustion is sufficient, the heat of reaction is equal to the calorific value of the coal.
2 Factors affecting the self-heat of coal 2.1 Coal quality of coal itself has a significant effect on the sensitivity of coal to self-heating.
(1) Grade of coal. The grade of coal indicates the degree of deterioration of coal, commonly used volatile content and coal content. The low-grade pure coal has a high sensitivity to self-heating and, as the coal product can increase its self-heat sensitivity, it decreases. Thus, the dry lignite is most easily self-heated and the bituminous coal is hardly self-heating. However, lignite, which contains the most moisture, is less susceptible to spontaneous combustion than pure lignite.
(2) The moisture content of coal. The content of water in coal has great influence on the flammability of coal. Coal with saturated moisture content, especially before lignite and sub-bituminous coal with high moisture content is mined and dried, the coal no longer adsorbs moisture, and thus cannot release the heat of wetting. The heat released by the oxidation of coal usually raises the internal moisture temperature. On the other hand, the self-heating chemical reaction requires a small amount of moisture to participate. Low-level coal moisture content is far greater than the chemical reaction requirements. Therefore, for low-grade coal, moisture is actually an inhibitor of coal self-heating.
(3) Minerals. The mineral composition in coal is also called ash. It reacts with oxygen to exothermically increase the coal temperature, and decomposes the coal to increase the surface area of ​​the coal in contact with the air, such as pyrite, which can absorb part of the heat released by the oxidation reaction to reduce the oxidation reaction process of coal; the high ash content of coal makes The unit mass of oxidation heat decreases.
2.2 Environmental factors for mining, storage and transportation Environmental factors influence the self-heating of coal as follows: the moisture content of coal can be changed; and the contact conditions of coal and oxygen can be changed: the heat generated by the production can be diffused. It can be divided into:
(1) Geological factors. Faults and crevices facilitate air and moisture contact with coal. Therefore, there is no significant increase in heat dissipation, but it increases the chance of coal oxidation and the adsorption of water. That is, faults and cracks increase the risk of coal spontaneous combustion. Deep-buried coal seams have less floor air leakage. Coal remaining in goafs (especially for thick coal seams) has increased the risk of coal spontaneous combustion due to incomplete recovery.
(2) mining factors. There are two main factors that affect mining coal spontaneous combustion, namely, ventilation and coal crushing. Where there is no ventilation or adequate ventilation, coal is less likely to be spontaneously combusted. Whereas, there is a high possibility that coal is spontaneously combusted when ventilation is insufficient. Crackle air leakage is not enough leakage, it creates conditions for further oxidation of coal, and the cooling conditions have not been improved. Therefore, any air leakage is very dangerous for spontaneous combustion of coal.
(3) storage and transportation factors. In the process of storage and transportation, the factors affecting the spontaneous combustion of coal are the insufficient ventilation and drying of low-grade coal to produce wetting heat due to rain and spray water.
3 Comprehensive Prevention and Control Measures for Coal Spontaneous Combustion 3.1 Prediction and Prediction of Spontaneous Combustion of Coal Seam (1) Given that coal produces CO during the low-temperature oxidation stage, CO is a sensitive indicator for early exposure of fire. CO sensors are installed in the coal mine working face such as coal returning face and fully-mechanized coal roadway where there is spontaneous ignition. If the CO concentration is found to be out of limits, a portable CO detector may be used to track and monitor the high temperature point.
(2) The infrared detection method is used to determine the location of the high temperature point. The basic principle of the infrared detection method is to establish the correspondence relationship between the fire source and the temperature field of the fire source according to the theory of the infrared radiation field, so as to infer the location of the fire source point.
(3) Auxiliary monitoring with borehole temperature measurement. For locations where the top coal is broken or where there is a risk of spontaneous combustion, temperature probes are buried and temperature changes are regularly monitored.
(4) Strengthen air leakage detection. Tracer gas method is regularly used to check the amount of air leakage through the slot. Strengthen observations in regions where air leakage is concentrated.
3.2 Preventive measures (1) Pressure equalization ventilation control air leakage oxygen supply. Pressure equalization ventilation is an effective measure to control air leakage in the mining of coal seams. First of all, regional pressure equalization ventilation shall be fully implemented under the requirement of diluting CH4, wind speed, temperature and per capita air volume. The pressure regulating measures include single pressure regulation and multiple measures of joint pressure adjustment, and the working surface formed in the specific implementation. The pressure equalization is gradually extended to the regional pressure equalization of the goafs in the adjacent working face.
(2) Spray plugging borehole grouting. The use of fully enclosed shotcreting and shallow grouting for suspicion of suspicious locations in coal mining or the occurrence of hidden hazards is an effective measure to prevent spontaneous combustion.
(3) Note gel to prevent fire. The use of gel injection technology to deal with hot spots or spontaneous combustion is a key measure for fire prevention in coal seam mining. The method is to inject the gel into the surrounding coal body at high temperature or fire point. Its role is to seal the air leakage channel. Can absorb heat and cool down.
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