Report: APP CMHS Project 1




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3.3.5. Spontaneous Combustion Control

Of state-owned coal mines in China, 55 % are spontaneous combustion prone mines. There are about 360 spontaneous combustion incidents each year, or about 0.7 incidents per million tonnes of coal mined.

Current Technologies

Telemetric monitoring and tube bundle systems are installed in most Chinese coal mines to monitor spontaneous combustion indicative gases such as CO, O2, H2 and C2H4. The Coal Mine Safety Regulation requires that a surface-based slurry injection system has to be built in spontaneous combustion-prone mines. In addition to these systems, a suite of technologies have been developed to prevent, detect, locate and control the hazard. Among the technologies developed, the radon technique, infrared technique, and fire-suppressant materials, are quite advanced.

Among the technologies developed, the radon technique, infrared technique, and fire-suppressant materials, are quite advanced.

The radon technique is unique in that it is the only practical technique available for remotely locating underground spon com from the surface. It has proven to be very successful in China.

The infrared technique tracks the change in radiation energy fields and uses back-analysing methods to locate spon com in coal seams, as shown in Figure 35. It is more advanced than infrared cameras and thermographs. It can be used in conjunction with gas indicators to detect the onset of spontaneous combustion.

The fire-suppressant materials developed in China have special fire-controlling characteristics that control spon com.

Figure 35 Technologies for spon com prevention and control

Radon detection

The characteristics of radon (222Rn) – its radioactivity (detectable in small concentrations), inert gas geochemical properties (that allows it to accumulate and to be transported in the pore fluid), relative abundance and long half life (half-life = 3.82 days), have made it widely useful as a precursor or event indicator for earthquakes and volcanic activity. Like any other nuclear-based technique, the fundamental principle of the 222Rn technique for detecting spon com, is based on detecting and measuring the energy emitted in the radioactive decay process; then using that information and other relevant data to locate subsurface heating’s. 222Rn is a gas, but its short-lived daughters are all solid. 222Rn and its daughters are easily adsorbed by activated carbon, silica gel, polyethylene and some other materials. This property enables 222Rn and its daughters to be easily collected from the surface of a container coated with adsorbents and analysed. Figure 36 illustrates the principle of the radon detection technology.

Figure 36 Principle of radon technology for spon com detection

Since 222Rn and its short-lived daughters are alpha-emitting nuclides, the alpha cup method described by Fang and Jia (1998) is used for on-site measurements. The method employs an alpha counter and sample cups. The counter detects the alpha radiation of 222Rn and its daughters and is portable and battery powered. The sample cup is an open-end plastic cup with coating of an adsorbent on its internal surface. The alpha counter and sampling cup are shown in Figure 37.

Figure 37 Equipment used in radon technology

Radon detection has been successfully used to pin point the location of spon com in China and Australia. Figure 38 shows the location of the spon com detected with the technology in a Chinese coal mine.

Figure 38 Typical representation of spon com zone using radon technology

Infrared detection

As with all objects, a coal seam is also emitting infrared waves. If there is a spon com event in the coal seam, this should be evident in the characteristics of its infrared radiation field. So the principle of infrared detection is based establishing the relationship between the field and its source and monitoring the change in characteristics of the field, as shown in Figure 39.

Figure 39 Principle of infra-red detection

An infrared detector (Figure 40) is used to measure the strength of the energy field of infrared radiation and surface temperatures of the measurement points. If a zone of abnormal strength is recorded, then repeated measurements in that zone are undertaken. The measured data is then processed with software called RRDS. Figure 42 shows the location of the spon com in a coal mine detected with the infrared technology.

Figure 40 Infra red detector



Figure 41 Spon com detected with infra red technology

Prevention – Fire-suppression materials

Several types of innovative fire-suppression materials have been developed in China to better control spon com in coal mines because of their special characteristics. Such notable materials include gels, large-molecule colloids and compound colloids, as shown in Figure 42.

gel

large-molecule colloid

compound colloid

Figure 42 Fire suppression materials

Gels

The gels consist of three main components: a base material (solid), an additive for fast gelatinisation and water. The gels have the following characteristics:

  • Water solidification. Water makes up 90 % of the gels and as the water is driven off by the heat the gel solidifies, so it cannot flow easily and controls the spon com in a targeted area.

  • Cooling effect due to two processes: formation process of gelatinisation and vaporisation process of water in the gels. Both processes are heat absorbing processes and thus reducing the temperature of the surrounding medium.

  • Block air leakage paths. Free-flowing gel solution is pumped into the fractures of coal prior to the formation of gelatinisation, once the gel is formed it will take up its shape in the fractures of coal and block air/gas leakage paths.

  • Control of formation time of gelatinisation. The formation time of gelatinisation can be adjusted to account for various circumstances of spon com, distance of desired material transport, and permeable ranges of a borehole.

  • Thermally stable. The gels are not sensitive to high temperature. At 10,000C, the gels don’t melt or fracture.

Large molecule colloids

The large-molecule colloids developed are composed of two main components: large-molecule materials and water. The large-molecule colloids have the following characteristics:

  • Low content. The large-molecule material concentration in the colloids is only 0.3-0.5 %. In the process of controlling spon com, this small concentration greatly reduces the amount of material to be transported underground.

  • Elasticity. Large-molecule colloids belong to the category of elastic colloids. It can tightly fill the fractures and fissures in coal seams. Even in the case that coal is heavily fractured and broken, the colloids can still tightly fill the fractures and prevent the air/gas leakage.

  • Viscosity. In transportation, its viscosity is low and therefore has a low flow resistance. Once the colloid is in coal seams, its viscosity increases and this makes it easy for it to remain where it is intended to be.

Compound colloids

Compound colloids are made by adding some additives for enhancing mechanical strength in the gels or large-molecule colloids. These additives may include mud clay, tailings and fine sand. Apart from the above-described characteristics of the gels or large-molecule colloids, the compound colloids have the following additional characteristics:

  • High mechanical strength. The mechanical strength is increased because of the additives.

  • Low cost. The use of additives for enhancing mechanical strength, mean that the amounts used for base material, fast gelatinisation and large-molecule materials are reduced.

  • Better control of spon com. The gels and large-molecule colloids lose water as they absorb the heat in the area of the spon com; however the additive for enhancing mechanical strength remains in the fractures and fissures of coal and continue to play its role in controlling spon com.

Some of the mine sites faced with serious spontaneous combustion issues and have successfully implemented strategies to predict, prevent and control the spontaneous combustion include Yankuang Coal Mining Group and Shenhua Energy Group.

Application Sites

The technologies described in this section have been applied in most of sponcom-prone mines in China, such as those in Shenhua and Yankuang.

Technology Gaps/Needs

Current methods of controlling spontaneous combustion are quite successful. The future developments should focus on:

  • an early detection method of spontaneous combustion with enriched gas analyse technologies

  • development of a pre-warning system that incorporates a spontaneous combustion prediction expert system with mining scheduling to obtain analytical result of spontaneous combustion proneness.


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