Factors affecting spontaneous combustion of coal gangue hill

From the mechanism of coal gangue spontaneous combustion, it can be seen that coal gangue spontaneous combustion is a very complicated physical and chemical process. The main factors affecting the spontaneous combustion of coal gangue are pyrite content, water content in coal gangue, coal content and properties (analyzed by oxidation and volatilization of coal), porosity of coal gangue and oxidation time.

1. Effect of sulfide content and aggregation state in coal gangue on spontaneous combustion.

According to elemental forms, sulfur in coal gangue mainly includes sulfide, organic sulfur, elemental sulfur and sulfate sulfur. Sulfide is the main form of sulfur in coal gangue, which generally accounts for more than 80% of the total sulfur in coal gangue. Sulfate sulfur is generally nonflammable; Elemental sulfur is flammable, but its content is very small; Organic sulfur is flammable, but it is often uniformly distributed in the polycyclic structure of residual coal-based molecules of coal gangue in the form of -SH and -S-S-. Sulfide sulfur is flammable. According to its crystal structure and aggregation state, it can be divided into pyrite, white iron ore, colloidal pyrite and magnetite, with pyrite (FeS2) as the main gangue. Pyrite in gangue absorbs oxygen in the air at low temperature, and a series of redox reactions occur to release a lot of heat. If pyrite is scattered in coal, its particles are connected with carbonaceous materials, and it is easier to oxidize and spontaneously ignite. The research shows that when the sulfur content in coal gangue is 2%, sulfur is completely oxidized, and the released heat can increase the temperature of coal gangue by 120℃. Therefore, in the area where pyrite is locally concentrated, the oxidation, heat release and temperature rise of pyrite may make this area become the center of spontaneous combustion. However, some coal gangue has high pyrite content without spontaneous combustion, while others have low sulfur content but are easy to spontaneous combustion, which shows that pyrite is an important factor, not the only factor, in the spontaneous combustion of coal gangue.

Second, the influence of the metamorphic degree of residual coal in coal gangue on spontaneous combustion

The metamorphic degree of residual coal directly affects the ignition point and calorific value of coal gangue spontaneous combustion. Under the same size, accumulation form, residual coal content and other conditions, low metamorphic coal has a low ignition point and a relatively small calorific value. Medium-high metamorphic coal has high calorific value and high ignition point. But for the gangue in the gangue hill, the low metamorphic coal has a greater influence on the spontaneous combustion of gangue.

The volatile yield of low metamorphic coal is high, and a large number of combustible gases such as CH4, C2H2, C2H4, C2H6 are produced during the spontaneous combustion of coal gangue, which plays a role in supporting combustion and intensifying combustion of coal gangue at a certain temperature.

There are many active groups in the coal molecules of low metamorphic coal, which are easy to react with oxygen in the air.

Low metamorphic coal has low density, low hardness, easy crushing and large specific surface area, which accelerates the reaction with oxygen and accelerates the occurrence of spontaneous combustion.

Third, the influence of moisture.

1. Moisture is beneficial to the weathering of coal gangue.

Under the appropriate moisture and temperature, the weathering speed of coal gangue is accelerated, which makes the specific surface area of coal gangue increase, the particle size decrease and the porosity increase, which provides favorable conditions for air to penetrate into coal gangue hills. The natural weathering rate of pyrite in high humidity is several orders of magnitude higher than that in dry state, which fully shows that moisture obviously accelerates the oxidation reaction rate of FeS2 and releases more heat.

2. Moisture promotes the oxidation of coal gangue and coal, and accelerates the combustion speed.

The moisture in the air adsorbed on the surface of coal produces adsorption heat, and at the same time, the amount of oxygen absorbed increases, which promotes the oxidation of coal. The generated adsorption heat can increase the temperature of coal gangue, and the moist heat of water can also increase the temperature of coal gangue. The exothermic rate of low-temperature oxidation of sulfur-containing coal gangue increases with the increase of water content in a certain range. In the oxidation reaction of coal, water provides active H and HO. At this time, the reaction between C and CO became a branch chain reaction, which automatically accelerated the reaction speed.

3. Moisture reduces the ignition temperature of coal gangue hill.

In a certain range of water content, with the increase of water content, the ignition temperature of raw coal decreases. Generally, when the water content of coal reaches 20%, its ignition temperature is more than 80℃ lower than when it is dry.

4. Moisture contributes to coal gangue blasting.

Coal powder and sulfide in coal gangue are slowly oxidized at low temperature, and the heat generated is accumulated internally. When rainy weather is conducive to the infiltration of rainwater into high-temperature areas, some rainwater is heated and vaporized when it meets high-temperature coal gangue, and some of it reacts violently with high-temperature coal gangue, producing a large number of explosive gases in a short time, which makes the internal pressure of coal gangue mountain increase sharply. It can be seen that rainfall has a great influence on the explosion of gangue hill, so it is very necessary to do the explosion-proof work of gangue hill in rainy season.

To sum up, water plays an important role in promoting the spontaneous combustion of coal gangue piles, so when stacking coal gangue piles or pretreating coal gangue piles, water should be prevented from infiltrating into them as much as possible.

Fourthly, the influence of granularity.

The composition of coal gangue particle size determines the permeability of coal gangue to a certain extent, but at what particle size, the permeability of coal gangue is the best, and the oxidation heating and heat storage conditions are the most suitable, which is still inconclusive. Because the heat storage of coal gangue hill is not only related to particle size, but also related to its specific surface area, oxidation and other physical and chemical properties. When the particle size of gangue is small, even if the permeability is good and there is enough air to infiltrate for oxidation, the infiltrated oxygen is consumed on the surface of gangue pile because of its large specific surface area and high reactivity, and it is difficult to penetrate deep into gangue. Moreover, due to the good heat dissipation conditions on the surface of gangue, it is not easy to cause spontaneous combustion due to heat storage and temperature rise. It is considered that when the average effective diameter of coal gangue particles is 6 ~ 13 mm, the coal gangue pile has the best oxidation and heat storage conditions, and the possibility of spontaneous combustion is the greatest.

5. Influence of temperature on spontaneous combustion of coal gangue

In fact, the spontaneous combustion of coal gangue hill, like the combustion of other carbonaceous materials, conforms to the principle of combustion physics, that is, it must go through the stages of slow temperature rise, automatic accelerated oxidation and stable combustion.

The critical temperature of spontaneous combustion of coal gangue is the temperature at which the oxidation of coal gangue transits from slow heating stage to automatic acceleration stage. When the temperature in a coal gangue hill reaches the critical temperature (generally considered as 80 ~ 90℃), spontaneous combustion will occur. On the other hand, if the temperature of gangue cannot be reduced below 90℃, it is in danger of re-ignition. Therefore, the determination and application of critical temperature is of great guiding significance to the prevention and control of spontaneous combustion gangue piles.

Six, the air transport mode in the gangue pile

The external factors affecting the spontaneous combustion of coal gangue are oxygen supply and heat storage conditions, which are a pair of contradictory conditions. Good ventilation conditions can make coal gangue get sufficient oxygen supply in the oxidation process, but at the same time, it will also take away the heat generated in the self-heating stage of coal gangue. On the other hand, if the coal gangue in the closed environment has good thermal storage conditions, it will not be able to provide oxygen, and the coal gangue will not be further oxidized and spontaneously ignited. Oxygen supply conditions play an extremely important role in the spontaneous combustion of coal gangue with spontaneous combustion tendency, and can even be said to be decisive. According to the conditions of oxygen supply and heat storage, the coal gangue hill can be divided into three areas: ① non-spontaneous combustion area; (2) possible spontaneous combustion area; ③ Asphyxia area.

On the surface of coal gangue hill, although sufficient oxygen supply can be obtained, the heat exchange conditions with the outside world are good, and the heat generated by oxidation reaction is quickly dissipated to the surrounding environment. The temperature rise of coal gangue is too small to cause spontaneous combustion, so it is a non-spontaneous combustion area. In the deep layer of coal gangue hill, most of the oxygen brought by molecular diffusion or airflow has been consumed on the surface, the oxygen concentration in the airflow is very low, and the heat generated by the oxidation reaction of coal gangue is too small to further heat the coal gangue, so this area will not spontaneously ignite, so it is a suffocation area.

Between the non-spontaneous combustion zone and the suffocation zone, there is a certain oxygen supply, and all the heat generated will not be taken away. The heat generated by coal gangue oxidation is enough to raise the temperature of coal gangue, which is a possible area for spontaneous combustion. The range of possible spontaneous combustion zone is related to the oxidation capacity, particle size, accumulation form, porosity and external environmental conditions of coal gangue. A large number of experimental results show that most spontaneous combustion occurs in the range of 0.5 ~ 7 m from the surface. In this region, if oxygen can be continuously obtained, the oxidation reaction can continue. After a certain period of time, if the temperature of coal gangue rises above the ignition point, it will burn. At this stage, if the oxygen supply and heat storage conditions change, the oxidation reaction of gangue cannot continue and spontaneous combustion will not occur.

Coal gangue needs external oxygen supply during its self-heating process. The transmission mechanism of oxygen in coal gangue hill is: ① molecular diffusion caused by oxygen concentration gradient; ② Natural convection caused by natural wind; (3) Thermal convection after self-heating of coal gangue hill; ④ stack effect caused by particle size segregation; ⑤ "hot air" in coal gangue hill caused by temperature difference between day and night; ⑥ "barometric respiration" of gangue hill caused by atmospheric pressure change. The "barometric respiration" caused by atmospheric pressure change is too small to maintain the self-heating of gangue. The "heat interest" effect caused by the temperature difference between day and night only occurs on the surface of gangue, and the thermal effect will soon be lost. When coal gangue hill is self-heated, the required oxygen is mainly provided by oxygen molecular diffusion, natural convection, thermal convection and stack effect.

Seven, the influence of wind speed on spontaneous combustion of gangue hill.

The oxidation of coal gangue requires oxygen. Only when the external oxygen supply rate is greater than a certain critical value, the heat released by the oxidation reaction is greater than the heat dissipation rate, and the heat can be accumulated and the coal gangue can be heated up. If this critical value is not reached, all the heat released by the reaction will be dissipated to the surrounding environment through conduction and convection. When the heat released by the reaction is less than the heat dissipation rate, the gangue will gradually cool down. This critical value is called critical wind speed.

The role of airflow in coal gangue hill is two-sided, which not only supplies oxygen needed for coal gangue reaction, but also takes away the heat generated by coal gangue reaction. Therefore, the critical wind speed has an upper limit and a lower limit. When the wind speed exceeds the upper limit, all the heat generated by the reaction will be taken away. From the point of view of preventing spontaneous combustion of coal gangue, it is impossible to increase the permeability of coal gangue as a measure to prevent spontaneous combustion, so the key is to control the lower limit of critical wind speed. The critical wind speed is related to the physical and chemical properties of combustible materials and environmental conditions. Scholars at home and abroad have studied the critical wind speed in coal gangue, but the results are very different. It is generally believed that when the wind speed of coal gangue hill is lower than 4.4× 10-5m/s, coal gangue will not spontaneously ignite.

Eight, the influence of coal gangue stacking mode on spontaneous combustion.

At present, domestic coal mines mostly use the inverted slope to turn the head to discharge the gangue. First, the gangue is pulled to the top of the gangue hill, and then dumped, so that it naturally rolls down and exposes the gangue to the natural environment in a large area. Because the accumulated coal gangue is discharged from the top of the mountain to the bottom of the mountain, it forms an "inverted discharge", and most coal gangue hills are conical. Under the action of natural gravity, the slipping coal gangue has obvious sorting property (Figure 3-6). Between the A section and the B section of the airport slope of the gangue hill, the small broken gangue is close to the A end, and the bigger the broken gangue, the more it goes to the B end. In this way, an external environment of spontaneous combustion is formed between A and B, and the spontaneous combustion point is generally at 1/3(C) between A and B near the B end. The combustion zone is formed laterally and gradually extends to the A end.

Figure 3-6 Schematic Diagram of Inverted Waste Dumping

The composition of different parts of Yangquan coal gangue hill was determined. The results show that in the lower part of the coal gangue hill, the coal gangue with particle size greater than 25mm accounts for 83% of the lower part, many of which exceed 5cm, and some even exceed 20cm, and the coal gangue with particle size less than 6mm only accounts for 4.5% of the lower part. The middle and upper part of the hillside has a small particle size and contains more gravel and gangue scraps, of which 48% are coal gangue larger than 25mm, and 20% are coal gangue smaller than 6mm. This inverted slope type leads to the particle size classification of coal gangue on different vertical gradients. See Table 3-5 for the particle size distribution of coal gangue in different parts of coal gangue hill.

Table 3-5 Proportion unit of coal gangue with different particle sizes:%

The results of particle size classification increase the porosity of coal gangue hill and make coal gangue get better oxygen supply conditions. Because coal gangue with small particle size contains more coal, the result of particle size classification also promotes the further enrichment of combustible materials and increases the possibility of spontaneous combustion. Moreover, these gangue stones naturally roll down, forming layered accumulation with different particle sizes. Generally, the gangue located in the middle and lower part of the slope of the gangue hill is large, and there is a big gap between them. The oxygen in the air permeates through the gap, which provides good ventilation conditions for oxygen supply in the gangue hill. At the same time, due to the small particles in the middle and upper parts of the gangue hill, it is easy to be weathered and broken in the wind and rain season. The broken gangue blocked the pores and reduced the porosity, resulting in the formation of a well-sealed covering layer in the middle and upper parts of the gangue hill. This kind of covering layer covers the gangue like a hat, so that the heat generated by the oxidation of mixed coal or pyrite in the middle and lower gangue can not be effectively released, which is easy to cause local high-temperature spontaneous combustion of the gangue hill.

The seepage velocity of airflow in coal gangue hill is related to air pressure. It is found that there is low gas pressure in the gangue hill, and the position of this point is related to the height of gangue accumulation. The higher the pile, the greater the pressure. Therefore, the depth of air infiltration increases with the increase of the height of gangue hill, so the burning depth and burning area may also increase accordingly.

To sum up, the spontaneous combustion process of coal gangue hill is an extremely complicated physical, chemical and biological process, which is influenced by both internal and external factors. Only by mastering the spontaneous combustion mechanism of coal gangue hill and the factors affecting spontaneous combustion can we better grasp the spontaneous combustion law of coal gangue hill and provide the basis for fire extinguishing and implementing scientific and reasonable fire extinguishing methods in the future.