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Abstract

Content

Introduction

An important condition for the successful development of the coal industry is to accelerate the pace of sewage treatment works. In this regard there significance with highest priority for commissioning of preparatory excavations. Further improvement of mining operations hampered by fires, in which the operational firefighting laid considerable reserve of labor productivity growth in treating faces.

Fires in dead end mines, especially traversed by gas-bearing coal-beds, characterized by this facts: limited access to immediate impact on the fire hearth with an extinguishing resources, ability to carry out work only from the outgoing air flow with high temperature and smokiness, the risk of disturbing the ventilation pipes, the possibility of accumulation of combustible gases to explosive concentration [1].

Active fire extinguishing in these conditions is extremely dangerous operation, because at any moment there can be flash or explosion of methane-air mix. Risk of explosion complicates the conduct of emergency operations, prevents the effective use of traditional fire extinguishing resources (water, foam, powder, etc.), threatening the health and lives of rescuers. For this reason, more than half of fires occurring in the dead end works of gassy mines, liquidated way isolation by building special jumpers on the distant approaches to the fire hearth.

1. The goal, idea, scientific task of the work

The goal of this work is to develop an efficient localization method of explosive waves in the dead end workings of gassy mines, providing the opportunity to actively fighting the fire at the potential threat of an explosion of methane-air mix.

The scientific mission – establishment of the energy decay laws of wave's explosion among air-mechanical foam due to the influence of structurally mechanical and rheological properties of the dispersion condition and develop an engineering method of calculating the length of the foam plug, which provides effective localization explosion of methane-air mix in a dead-end mine working.

The idea of the work is to use an air-mechanical foam not only as a traditional firefighting resource, but also as a matter explosion stability instrument to improve the efficiency and safety of fires fighting in dead-end workings of mines, hazardous gas and dust.

2. Analysis of underground fires at enterprises of Ukraine

Of the total number of accidents in 2010, underground fires are take the greatest specific weight – 28.5 %. In comparison with the year 2009 their number decreased by 7 fires.

In 2010, the economic damage from the fire was about 1.3 million USD, its share of the damages from any accidents this year is 51.5 %. Duration elimination of underground fires in 2010 was 903.5 hours (54.8 % of the duration of the emergency response), and the complexity of their elimination – 38009 persons/hour (71.7 % of the labor costs for liquidation of accidents) [2].

Specific indicators of annual values for underground fires

Figure 1 - Specific indicators of annual values for underground fires

Of the total number of underground fires in 2010 (12), 9 were extinguished in an active way and 3 - isolation method. In 2 cases, the ignitable substance was methane, 1 fire had endogenous origin. Kinds and number of fires are shown in Figure 2.

Comparative data on the number of underground fires in 1999–2009

Figure 2 - Comparative data on the number of underground fires in 1999–2009

Total damage from the fires of exogenous (11 underground and 8 on the surface complexes mines) was 7.5 million UAH (respectively 6.3 and 1.2 million UAH), accounting for 27.6 % (53.6 % in 2009 ) total damage of all accidents and emergency situations [2]. With the elimination of exogenous fires saved material assets amounting to more than 1,229.3 million UAH.

3. The dynamics of the fires in dead-end workings of gassy mines

The extension of the coal industry of new advanced systems of mine development, advanced and high-technology coal mining equipment immediately demanded an increase in the extent of development workings and equipping them with high-speed roadheaders. As a result, dead-end workings, particularly in gassy mines, have become one of the main potential places of accidents, especially fires involving burning of methane.

Analysis of coal mine accidents in Ukraine shows that over the past 10 years, the number of fires in dead mines ranges from 2 to 10 a year, and their share of 7–9 % of the total number of underground fires [3]. During fires in dead-end workings gassy mines are usually disrupted the normal mode of ventilation due to the stop of local-ventilation fan or damage vent pipes. In this case, there are complex thermodynamic processes, that when increasing temperature of solid coal at 1–2 °C lead to an increase of methane in several times [3].

Due to changes in the thermal conditions of combustion processes, degradation and desorption of combustibles, gas composition of a dead-end production is constantly changing. For coal and wood – the main combustibles mines, characterized by the same stage of burning, as for ordinary burners: drying, sublimation of volatile, direct burning and gasification of coke. In the drying and sublimation stage wet is released and forming new organic compounds – the volatile and non-volatile. Volatiles were are emitted as a mixture of combustible gases, hydrogen, methane and other hydrocarbons at relatively low temperatures (400 °C) [10]. At the burning and gasification stages at 600–900 °C is formed oxide and carbon dioxide in various proportions depending on conditions of at which there is a process. At the same time in the fire area gassy mines is released gas adsorbed in coal and rock. Generally released gas is pure methane, but sometimes mixed with carbon dioxide and nitrogen. Sometimes with methane released small amounts of hydrogen, ethane, ethylene. At the same time at closed form of the convective flow in the working face and the location of the fire in the middle or initial portion may cause an accelerated accumulation of combustible gases.

The analysis of the mine atmosphere as the active fire fighting and emergency site in isolation shows that methane in 97.8 % of cases - the only component of combustible fire gases [4]. Its content can grow from a percent to a few tens of percent.

It is known that methane and air are mixed with each other in unlimited quantities. If the air is from 5 to 15 % of the gas, in the presence of an ignition source mixture becomes explosive. Explosion reaches its greatest strength, when the atmosphere contains 9.5  % of the gasbecause in this case the methane is completely combusted. When the methane concentration in more than 16 % air mixture burns quietly without substantially pressure increase. An explosive reaction with the small effect of pressure rise is often called flashes. As flashes are considering a substitution reaction of explosive mixtures near the lower or upper explosive limits. However, the difference in the definition of the flash and explosion kinetically not justified because the flash is an explosion of small capacity.

4. State of the safety engineering and analysis of the the modern experience of struggle with explosions of dust-gas-air mixtures when extinguishing underground fires

The practice of fighting underground fires conducted in Ukraine and abroad, scientific research has helped to find effective means and methods of doing rescue work. Along with the technical progress in the process of coal extraction and establishment of new fire fighting equipment is the improvement of existing and creation of new tools fighting fires. For example, as a result of the transition of mining operations at deeper levels become more complex geotechnical and geological conditions, increased overburden pressure much more difficult to maintain mine workings supply on them the necessary air to the treatment and preparatory faces increased methane profusion of coalbed. As a result, increased the number of fires is complicated by the explosions of methane-air mixtures [5].

The complexity of fighting fires in gassy mines dead-end workings is caused by combination of the following circumstances [6]:

  1. Reverse flow of heated fire gases complicates the conduct of rescue operations.
  2. The direct location of the fire is usually not aired due to combustion of air pipes.
  3. There is an increased risk of explosion in mind the difficulty in obtaining required intensity of airing whole develop.

At the same time, the absence of lead other workings through which fresh air can be supplied to the fire hearth, allows reliable isolation of working in small amounts limited jumpers. The choice of the method of extinguishing a fire in a dead-end workings of gassy mines caused the change of the gas situation an emergency site.

The practice of extinguishing underground fires showed that with increasing content of combustible gases above the permissible limits (2 % methane) should resort to an increase in the supply of air to the dead-end working. Currently, this is done taking measures to reduce of air leaks in the ventilation or putting into work of an additional local ventilation fan.

Found that reducing the amount of air supplied to the dead-end working, in which a fire or wholly stop its airing permissible only in the case when the content of combustible gases over the upper limit of explosion.

In the coal mines of our country for the localization of explosions gas-air mixtures are widely used slate and water barriers [9], consisting of a number of overturned shelves with an inert dust or easily breakable plastic vessel filled with water, as in the first and in the second case, on the basis 400 kg / m² cross-section of development in the place of installation. Most barriers are designed to extinguish of flame explosion, but due to the large mass (average 6-7 tons) partially reduce the energy of the shock wave, spent on their destruction.

To solve the problems related to the protection of rescuers from the harmful effects of shock waves in extinguishing fires in dead-end workings of gassy mines should:

  1. Explore the laws of the process of interaction with explosion wave of air-mechanical foam.
  2. To study the influence of structural and mechanical and rheological properties of foams, and conditions of their use on the intensity of explosion wave attenuation.
  3. Develop a physical model of the localization process explosion of methane-air mixture with foam taking into account its structural and mechanical and rheological properties.
  4. Develop a method for localization of explosive waves in fire-fighting air-mechanical foam in dead-end mines.
  5. Develop science-based recommendations for the use of air-mechanical foam for localization explosion of gases in extinguishing fires in dead-end of mines, hazardous gas and dust, on the basis of generalization of research results.

5. Automatic system of explosion suppressing-localization of explosions

Automatic explosion suppressing (see Fig. 3) – localization of explosions designed to protect mine workings from the spread on them explosions of methane air mixture and (or) coal dust, by creating a barrier in the form of a cloud of powder extinguishing suspended. It is used in mines, hazardous gas and developing coal seams, dangerous for dust explosions, as the containment barriers [11].

Automatic system of explosion suppressing

Figure 3 – Automatic system of explosion suppressing

The system is in standby mode and is driven by an air shock wave formed by the explosion of methane-dust-air mixture. From the effects of shock waves on the receiving board automatic system explosion suppressing leads to a dynamic ejection into space mine working highly extinguishing powder by utilizing compressed air, which is under high pressure in the working chamber system. As a result, in the volume of mine working around its cross section in the path of the flame front is formed by secure barrier long-lived clouds of fire extinguishing powder in suspension. This barrier liquidates approached the flame front (extinguishes it) and stops (isolates) the propagation of explosions across the mine workings network [11].

Work of automatic system of explosion suppressing

Figure 4 – Work of automatic system of explosion suppressing
(animation: 8 frames, 10 cycles of repeating, 272 kilobytes)

Conclusion

In this work, considered actual task for the coal industry, which consists in increasing the safety and efficiency of complex extinguishing underground fires in dead-end gassy workings of mines at the potential threat of explosion of methane-air mixture.

References

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