Thakur indicated that with a well-designed ventilation system, it is economically feasible to handle specific gas emissions up to Mine operators often try to supply maximum ventilation air based on the capacity of the system to dilute the gas concentration. Nevertheless, as time passes and the roadways become longer, ventilation capacity may decline because of leakage Schatzel et al. Furthermore, gas emissions that flow into the ventilation system may increase as mines progress into deeper and gassier coal seams, and as longwall operation parameters change e.
Consequently, it has become increasingly difficult to control the gas concentration below statutory requirements by ventilation alone, as this will require impractically large quantities of air Gillies and Wu, Pre-drainage methods aim to reduce the gas content of the coal seam before mining for the purpose of reducing gas emissions during development and longwall production. Post-drainage methods also known as goaf or gob drainage aim to capture gas during longwall phases to reduce the amount of gas managed by ventilation.
When coal seams have a sufficiently high natural permeability, gas drainage has been a positive and reliable method for controlling high gas emissions in mines. DuBois et al. However, gas drainage has met with limited success in low-permeability coal. Besides low permeability, many other factors also restrict the efficiency of in-seam gas drainage Black and Aziz, :.
A method that can offer a substantial increase in drainage time is surface-to-inseam drilling Black and Aziz, This method allows drainage to take place for several years prior to mining Thakur, However, with very low permeability strata this method has limited success in reducing the gas content Packham et al. A promising technique referred to as 'enhanced gas recovery' was firstly described by Puri and Yee This technique involves injecting a gas, which is different to the seam gas, into the coal seam to stimulate methane or other gas production Packham et al.
This technique may help to increase the production of gas from coal seams and improve the recovery rate from low-permeability coal. Although promising, this technique is still under development, and the mechanism for the stimulation is not yet fully understood. As the seams worked become deeper and gassier, gas drainage has been progressively adopted to reduce the in situ gas content and gas emission. However, gas drainage is effective only for coal seams with high permeability; besides, gas drainage needs time to become effective and it cannot solve the irregular gas emission problem in underground coal mines timeously, which may cause the stoppage of work in development headings and longwall faces.
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The limitations of both ventilation and gas drainage require new techniques to be developed to deal with irregular gas emission problems, enabling development and production activities to continue uninterrupted. Potential uses of TSLs for gas management. Gas emission control with TSLs. During longwall development, headings are driven into gassy coal and gas is released as the seam is depressurized, at a rate that is a function of gas content, pressure, and permeability Noack, When ventilation and gas drainage are not sufficient to control the gas emissions, the development headings have to be stopped for safety reasons, which is not acceptable for mining companies.
A technology is urgently needed for reducing the gas emissions rate in this situation, allowing the headings to advance.
Due to their relatively low permeability, TSLs may have the potential to deal with irregular gas emissions. Their operational benefits, such as rapid application, rapid curing, and low volume required allow the timely sealing of the irregular gas emission area.
Their appropriate mechanical properties, such as high tensile strength and elongation, can also deal with deformable ground conditions. It is worth pointing out that these benefits cannot be provided by other sealing techniques such as shotcrete.
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A schematic of TSL applied in an irregular gas emission area is shown in Figure 5. The application of TSL may reduce the gas emission rate in the development heading and allow the advance of the heading. Besides the gas management benefits, application of a TSL may also help to maintain the stability of the fresh excavations.
Based on the permeability test results of Saghafi and Roberts , Gerard conducted a theoretical analysis of the effectiveness of TSLs in reducing rib emissions. He stated that a sharp drop in rib emissions would occur once TSL spraying begins, and this process is illustrated in Figure 6. Although the study showed the potential for reducing rib emissions with TSLs, there was no field trial or data to support these assumptions. However, these theoretical results did show the potential of TSLs for reducing rib emissions and field trials are recommended to prove these assumptions.
At the same time, problems associated with the application of TSLs can be addressed. Enhancing in-seam pre-drainage with TSLs. Because of the influence of the excavation, stress-induced rock fracture near the roadway will increase the permeability of the coal mass near the roadway Zhou and Lin, , as shown in Figure 7. This will cause serious air leakage around a drainage borehole due to the high suction pressure created by gas drainage Xia et al. This air leakage may not only result in a low gas drainage concentration, but also lead to many other hazards, such as spontaneous combustion of coal, gas combustion, and gas explosion Xia et al.
Apart from the risks, Palchik stated that if the migration air can be reduced by one-half to one-third, the drained gas flow rate can increase 1. Therefore, there is an urgent need to develop an effective method to deal with ventilation air migration into the drainage boreholes. Measures need to be taken to deal with this air leakage around gas drainage boreholes.
In gas drainage practice, a sealing material such as polyurethane is used to seal the borehole; however, it can seal only the borehole itself, and the internal cracks of the coal seam are not blocked off Lu et al.
A TSL may be used to reduce the migration of ventilation air into the drainage boreholes. After spraying, the TSL material will bond firmly to the coal ribs and form a liner with low permeability. The liner can seal the cracks around the drainage holes and prevent the migration of ventilation air into the drainage holes, as shown in Figure 8b. The performance of the drainage hole is enhanced: on the one hand, the purity of the drained gas is increased as the path between the ventilation air and the drainage holes is sealed; on the other hand, the suction pressure can help desorption of gas from the coal and increase the gas production.
Furthermore, the TSL can also reduce the rib emission rate to the ventilation system, which enhances gas management in underground coal mines. A field trial of using TSL for enhancing the gas drainage efficiency was carried out by Tenney et al. The results showed that the methane purity was increased by 9.tiocredesal.tk
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However, problems are also associated with the trials, such as the variability of the methane flow rate, the interpretation of the results, and the area for spraying the TSL material, which need to be investigated in future tests. The advantages and disadvantages of gas emission control techniques are compared in Table III. Spontaneous combustion control with TSLs. Spontaneous combustion of coal has been a serious problem for coal producers and users for many years.
As a result of spontaneous combustion, the coal producers may suffer Ham, ; Simion et al. It is important for the mine ventilation engineer to be conscious of the zones in which spontaneous combustion is most likely to occur. Typical areas where spontaneous combustion can occur are along ventilation leakage paths. Leakage can occur in rib fractures around ventilation stoppings, through faults or cracks passing through a pillar, or along the bed separation in any remaining coal left in the roof Ham, ; McPherson, Oxygen may accumulate in these areas where insufficient ventilation exists, resulting in the inadequate dispersion of heat from oxidation Cliff, TSLs have the potential for controlling spontaneous combustion.
If a spontaneous combustion event was discovered in a chain pillar, then a TSL could be sprayed on the ventilation intake side to prevent any further oxygen from entering the leakage path. If detected early enough, the oxidation process may slow down and eventually be stopped due to the oxygen deficiency. In fact, if a TSL has already been sprayed onto the ribs whether for support or other gas management reasons , spontaneous combustion may have been avoided already, as shown in Figure 9. Ventilation benefits provided by TSLs. Ventilation power costs have a direct relationship with frictional head losses in mine airways.
A reduction in the airway friction factor would result in a corresponding reduction in power costs. TSLs have a very low ventilation friction factor, which gives them potential to improve mine ventilation capabilities Archibald and de Souza, ; de Souza and Archibald, Archibald and de Souza carried out wind tunnel simulations of airways, made of plywood, to evaluate the friction factor K parameters associated with the installation of a polyurethane-based TSL.
This work demonstrated that the TSL used had a ventilation friction factor about 0. The introduction of TSL material in mine airways could therefore serve to reduce system friction head losses while maintaining good environmental quality dust reduction and improving mechanical support performance. A field determination of friction factor parameters was conducted by de Souza and Archibald in conjunction with a field application.
As shown in Figure 10 , the field application was carried in a 91 m long, stable fresh air drift at an underground mine. The airway resistance, friction factor, and roughness height were calculated from the data from three ventilation surveys. The results indicated that the liner, on average, decreased the airway resistance by 7.
However, only one TSL material was tested and the benefits of TSL for ground support were not investigated in this study. This paper reviewed the current gas management challenges and presented the potential benefits of TSLs for gas management in underground coal mines. These may include reducing gas emissions into the ventilation system, enhancing the in-seam drainage performance, and controlling spontaneous combustion.
Since their introduction, TSLs have received increasing attention from the mining industry around the world due to the significant benefits they bring, such as low volume, rapid application, and rapid curing, with great potential to reduce mining costs. However, this technology is not yet mature and is still under development. Most of the products on the markets are still undergoing study and field trials. For safety reasons, reactive TSLs such as polyurethane-, polyurea-, or methacrylate-based materials are not recommended for underground applications.
Most of the products on the market are nonreactive TSLs with modifications to reduce the curing time. Motivated by the potential for using TSLs as a barrier against gas movement, many tests have been conducted to investigate the permeability characteristics of different TSLs, either with or without considering the interaction with the substrate.
Gas emissions can adversely affect safety and production in underground coal mining. Appropriate approaches and equipment are needed for controlling gas emissions in order to provide safe working conditions.