Brief introduction of common fire extinguishing technologies in spontaneous combustion coal gangue mines at home and abroad

As mentioned above, the spontaneous combustion of coal gangue hill depends on the oxygen supply conditions, and the pores and channels between coal gangue form oxygen supply channels, thus providing fresh oxygen to the inside of coal gangue hill. The inside of coal gangue hill is conducive to the accumulation of heat generated by oxidation reaction, so the combustion starts from here first, and the development process is very slow and hidden. At the initial stage of spontaneous combustion of coal gangue hill, the amount of coal gangue involved in combustion is very small, the combustion intensity is not great, the heat released is very small, the temperature in the spontaneous combustion area is not high, and it is in a slow fire state for a long time. Therefore, it is very difficult to find the spontaneous combustion of coal gangue in the early stage and accurately locate the spontaneous combustion area.

The control technology of spontaneous combustion coal gangue can be divided into two aspects: prevention and control. The former Soviet Union, the United States and other major foreign coal-producing countries have accumulated a lot of experience in how to prevent spontaneous combustion of coal gangue hills, but the research and practice of fire extinguishing technology in coal gangue hills in China started late. It is reported that the success rate of governance in the United States is less than 50%, while that in China is even lower. So far, there is not a successful experience and a very successful example. Most of them are early successes, late failures, and governance reappears in a few years.

China has also made a long-term exploration in the fire fighting technology of coal gangue hill. At first, the clear water fire extinguishing method was used, but now it is proved that it can not only put out the fire, but also cause more serious problems. In the gangue hill with serious spontaneous combustion, a small amount of water enters to form steam, which reacts with hot gangue to produce hydrogen. The explosion danger of hydrogen is much greater than that of carbon monoxide, and the lower explosion limit is low, so the requirement for igniting the fire source is low, and the possibility of explosion accidents is high. In addition, water can't control its flow direction inside the garbage dump, and it can't guarantee that all fire areas will be extinguished by water. Due to the action of water, the porosity of gangue hill increases and the ventilation performance is greatly improved. As long as irrigation is stopped, the residual fire area will quickly rekindle and develop under the condition of sufficient oxygen supply. Therefore, a lesson of fire fighting practice is that it is not suitable, and clear water cannot be poured out.

The fire extinguishing methods of coal gangue hill at home and abroad mainly include fire source excavation and cooling method, grouting method, covering method, controlled combustion method, foam (gel) method, low temperature inert gas method and so on. These fire extinguishing methods have their own advantages and application scope, but they also have certain limitations.

A, excavation cooling method

The cooling method of digging out the spontaneous combustion fire area is the simplest method, which is found in many foreign coal mines and domestic coal mines. As a combustible with extremely low combustion ability, coal gangue can quickly release heat to the environment as long as it destroys the heat insulation ability of the spontaneous combustion fire area, losing the balance between exothermic reaction and heat dissipation to the environment, and the temperature drops rapidly, and the fire area is gradually extinguished. The biggest disadvantage of this method is security. There have been many casualties in the United States, and a large amount of air enters the exposed gangue hill, which easily leads to the explosion limit of combustible gas inside and causes an explosion. Coal gangue at high temperature also makes the operating conditions very bad, causing serious harm to people and equipment. Secondly, there is a problem with the thoroughness of this method, which may rekindle in the local area after the fire is extinguished. In fact, this method is only suitable for the initial self-heating of waste hills, but most waste hills lack the ability to monitor the thermal state of waste hills, so this method has not been popularized.

Second, the grouting method

Grouting method is a widely used fire extinguishing technology at home and abroad. Grouting method is to achieve the purpose of fire extinguishing through the interaction of cooling and oxygen isolation. According to the different grouting depth, it can be divided into surface grouting method and deep grouting method. Generally, the surface guniting method sprays gunite on the surface of coal gangue hill or digs pits and ditches, and lime slurry is injected into the pits and ditches. The process of deep grouting method is as follows: firstly, the fire extinguishing material is made into slurry with a certain concentration, then a series of drilling holes are arranged in the coal gangue mountain fire area, and then the fire extinguishing slurry is injected into the coal gangue mountain with grouting pump. After the fire extinguishing slurry comes into contact with high-temperature coal gangue, the water in the slurry quickly evaporates, taking away a lot of heat and cooling the coal gangue; The solid substances in the mud are wrapped on the surface of coal gangue or filled in the gaps between coal gangue to block the air; If the grouting material contains alkaline substances such as lime, it can also absorb some gases such as SO2 and SO3 released by coal gangue combustion, which is helpful to reduce the pollution degree of spontaneous combustion coal gangue hill. Calcium sulfite and calcium sulfate generated by the reaction of lime with SO2 and SO3 also have a certain oxygen isolation effect. At the same time, the internal environment of coal gangue hill changed from acidic to alkaline, which inhibited the growth of thiobacillus, thus greatly reducing the oxidation rate of pyrite and the possibility of spontaneous combustion of coal gangue hill.

The biggest advantage of this method is that it can quickly cool the burning coal gangue and has good oxygen isolation effect. Russia, the United States, Ukraine, the Czech Republic and other countries spoke highly of this fire fighting method. The grouting fire extinguishing technology in different countries is similar. In Donbass Mining Company, the method adopted is to inject 5% diluted lime milk into the coal gangue pile to a depth of 2.5 ~ 3 m with an ejector. The method put forward by Dunneker Institute of Technology is to arrange an exhaust pipe next to the grouting hole, and pump air with a vacuum pump while grouting, so that negative pressure is generated locally in the coal gangue hill, which is beneficial to the infiltration of slurry. Czech scholars put forward that the following principles should be followed when arranging boreholes on coal gangue slopes: the upper end of the lower horizontal boreholes should be higher than the lower end of the upper horizontal boreholes, which can reduce the air infiltration into coal gangue hills (Figure 4- 1).

In order to inject the slurry into the deep coal gangue hill, the deep grouting fire extinguishing technology proposed by Hangzhou Environmental Protection Institute of Coal Research Institute is widely used at present. Deep grouting method injects slurry into coal gangue mountain through drilling, and the fire extinguishing effect is good. But the disadvantage of deep grouting method is that it is difficult to drill holes in fire area. Fires in gangue piles mainly occur on slopes, and the slopes of some large gangue piles often exceed 100 meters in length. It is difficult to enter the fire area after the coal gangue hill burns in a large area. The concentration of harmful gases and the ambient temperature are very high, and it is also necessary to prevent the rolling stones from hurting people, which makes the operators have certain danger.

Fig. 4- 1 Layout of Borehole in Gangue Mountain Slope

Three. Covering and coating method

Covering and coating method is to cover the surface of coal gangue hill with inert substances such as loess and fly ash to isolate the air and prevent spontaneous combustion. This method is economical and widely used at home and abroad. Suitable incombustible materials or flame retardant materials can also be selected to wrap the burning place of the gangue pile to isolate it from the air and extinguish it due to lack of oxygen. Materials used for wrapping and covering include ground limestone powder, silt and waste from loess, clay and cement kiln. These materials are relatively cheap and can be obtained locally. For the coal gangue hill where spontaneous combustion has occurred, if the temperature of coal gangue is too high, under the influence of high temperature, the original compacted topsoil will slowly dry up and produce cracks, which will eventually lead to the failure of soil sealing, so we must pay attention to maintenance. The wrapping material is polyurethane foam, which is tightly wrapped and has no cracks. However, it is necessary to spray water on the gangue pile in advance to keep the temperature of the gangue below the ignition point. However, this material is expensive and difficult to be popularized in large-scale fire fighting.

At present, the methods of covering and coating are mainly used to control the fire in coal gangue hill. The key to fire fighting is to compact and cover the soil, and the coal gangue must be cooled to a certain extent before covering the soil. Yangmei Group, Fenxi Shuiyu Coal Mine, Jincheng Gushuyuan Coal Mine, Shenyang Caitun Coal Mine, etc. This method has been used to put out fires in large or local coal gangue hills.

Fourth, the controlled combustion method.

This method was first proposed in 1980 and patented in 1983. The United States conducted an industrial test of controlled combustion method on a coal gangue hill in West Virginia. The basic idea is to make the coal gangue hill burn under controlled conditions, and the flue gas generated by the combustion is discharged after treatment, so that the heat generated can be utilized. The scale of coal gangue hill is about 9000t, and it is planned to burn it out in 9 months, generating heat 1.2× 108kJ. Before the test, a stainless steel combustion tube with a length of 43m and a diameter of 0.09m and a porous gas collector with a length of 15m were horizontally installed at the bottom of the coal gangue hill. After the test fan worked for 1600 hours, the combustion tube was destroyed due to the dual effects of high temperature and negative pressure. The test stopped here, and it is estimated that 700 tons of coal gangue was burned, resulting in 8.6× 1.06 kJ of heat (Figure 4-2).

The main problem of this method is that even if the fan is used to pump water for forced oxygen supply, the coal gangue hill is still in an incomplete combustion state. The flue gas contains a lot of toxic and harmful gases such as SO2, CO and H2S, which need to be purified by secondary combustion and other treatment methods. In addition, the flue gas temperature is high and corrosive, and the equipment, especially the pipeline, is expensive, so it is difficult to bury it for anticorrosion.

Figure 4-2 Process Flow Diagram of Controlled Combustion Method

Verb (abbreviation of verb) foam method

Foam method is to inject foam extinguishing agent into the fire area to isolate oxygen and absorb heat, so as to reduce the temperature of coal gangue and achieve the purpose of fire extinguishing. Compared with ordinary grouting method, foam, as a form of water delivery, can keep water in the gaps of coal gangue for a long time without losing it quickly. Therefore, compared with water injection method, it has better cooling and oxygen isolation effect.

Foam is mainly composed of water and foaming agent. The foaming agent has high foaming multiple and can quickly fill the gaps between coal gangue, but such foam is easy to break. Although the foam with low foaming ratio has strong durability, it may not be able to fill the gap. Generally, a foaming agent with moderate foaming ratio should be selected, and thickening agent or gel substance should be added to improve the stability of foam.

This method has been successfully used in small-scale industrial experiments in the United States, but how to maintain the long-term stability of foam between coal gangue cracks under wind and rain is still a difficult problem. Xi 'an Institute of Mining and Technology tried to put out small coal pile fires by gel method. Compared with the watering method, the gel can seal the spraying area for a longer time and the effect is obvious. However, under the action of high temperature, the gel is dehydrated, which affects the fire extinguishing effect.

Six, low temperature inert gas method

Low-temperature inert gas method is to inject inert gases such as liquid nitrogen and liquid carbon dioxide into the fire area, and make use of its huge endothermic effect during gasification to quickly cool the coal gangue hill, and at the same time, the inert gas remaining in the gap of the coal gangue hill can also play a role in isolating oxygen. After the low-temperature inert gas is injected into the gangue hill, it absorbs a lot of heat in the process of changing the liquid state into the gas state, and at the same time, its volume expands rapidly, forming a cold pressure wave, and then it spreads rapidly from the injection source, so that the low-density hot flue gas is pushed out and rises to the surface, and at the same time, it plays the role of isolating fresh air from entering the gangue hill, achieving the dual purposes of fire fighting and cooling, keeping the fire area low for a long time and cooling the ignition point inside the gangue hill below the critical temperature. Practice shows that the effect of using liquid nitrogen or solid carbon dioxide alone is not as good as mixing them, because the mixture is easy to transport and relatively low in cost. Through the field test, the cost of low-temperature inert gas method is equivalent to that of traditional grouting method, but the grouting method is affected by gravity, mainly permeating downwards, while the diffusion of this cold gas is not affected by gravity, which has a wider range of action and a more uniform effect.

Under certain conditions, low temperature inert gas method can be combined with grouting method, and the effect may be better. The fire extinguishing method adopted by No.4 Coal Mine of China Pingmei Group Company is to use CO2 to extinguish fire and cool down, and then combined with grouting method for comprehensive control, which has achieved good results. Firstly, the special KY-360 CO2 generator is used to generate CO2 gas, and the self-made special drilling equipment is used to manually drill holes at the spontaneous combustion point of the gangue hill and inject CO2 gas. Finally, the fire source is isolated from the air by grouting to consolidate the fire fighting results.

The United States conducted a small-scale experiment in this way. 5t liquid nitrogen was injected into the coal pile (l5.3m3) with a temperature of about 100℃ at a speed of 0.38 m3/h, and the temperature dropped to -68℃ after a few minutes near the grouting point, and the lowest temperature of the coal pile dropped to-170℃, and it only recovered to -20℃ after one month. The cooling effect of this method is undoubtedly very good, but cooling coal gangue requires a lot of inert gas, and the gas source and fire extinguishing cost are the main problems of this method. At present, many mining areas in the United States are carrying out large-scale industrial experiments.

Seven, coal gangue spontaneous combustion inhibitor resistance.

In order to prevent spontaneous combustion of coal gangue, improve the environmental quality of mining area, reveal and study the inhibition mechanism of inhibitor for spontaneous combustion of coal gangue, and seek efficient inhibitor for spontaneous combustion of coal gangue, in recent years, it has been found that polymer emulsion composed of polymers with adsorption groups and surfactants has good inhibition effect on spontaneous combustion of coal gangue. The research results have certain guiding significance for the control of spontaneous combustion of coal gangue in coal gangue mountain. In the polymer emulsion composed of polymer and surfactant, the polymer with adsorption groups forms multipoint adsorption, which prevents oxygen from contacting with the surface of coal gangue, while the surfactant has hydrophilic and lipophilic groups, so its lipophilic groups contact with coal gangue, and the hydrophilic groups extend into the air, which makes coal gangue hydrophilic, and its emulsion forms a water film on the surface of coal gangue, which prevents air from contacting with coal gangue and plays a role of resistance. The polymer inhibitor with NH4+ group releases NH3 gas at high temperature, which can prevent the gangue from contacting with oxygen in the air, which is beneficial to prevent spontaneous combustion. The polymer emulsion spontaneous combustion inhibitor composed of polymer with adsorption group and surfactant can prevent the oxidation heat release of coal gangue, thus achieving the purpose of preventing the spontaneous combustion of coal gangue, but this method is costly.

Based on the above analysis, the used and existing fire extinguishing methods of spontaneous combustion coal gangue hill are summarized, and their application prospects and applicable conditions are shown in Table 4- 1.

Table 4- 1 Fire extinguishing method and development prospect of spontaneous combustion coal gangue hill

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