Baiquan Lin

Variation in the Pore Structure of Coal after Hydraulic Slotting and Gas Drainage

The integration of hydraulic slotting and gas drainage techniques has become a mainstream technique for enhancing permeability in coal seams with low permeability. However, the mechanism of action of this process is unclear. In this paper, field experiment and laboratory tests are described that aim at elucidating this process. Given the sensitivity and accuracy of test methods and their corresponding determination principles, a combination of mercury intrusion porosimetry and nitrogen gas adsorption was proposed as a complementary technique and the pore-size distribution (PSD) was obtained. It is shown that the proportion of minipores decreases remarkably, whereas that of the macropores gradually increases with the decrease in the distance from the slotted borehole. By contrast, the mesopores and micropores present insignificant changes. Meanwhile, the adsorption pore and the seepage pore show a similar variation in tendency with the minipores and macropores, respectively. Moreover, the specific surface area decreases substantially with the decrease in borehole distances. The integration of hydraulic slotting and gas drainage can lower the gas-adsorption properties and enhance the gas-seepage capacity within the disturbed zone significantly. The paper highlights the guiding factors for improving the enhanced coal bed methane recovery.

Assessing the moisture migration during microwave drying of coal using low-field nuclear magnetic resonance

ABSTRACT Microwave drying of coal has the advantage of volumetric and selective heating, which saves energy and time. However, comprehensive assessment of the moisture migration during microwave drying is lacking. In this study, the low-field nuclear magnetic resonance (LF-NMR) was used for moisture characterization during microwave drying. Moisture migration occurs first in larger pores, whereas the smaller pores appear to remain water-saturated. As the microwave power increases, the drying capacity and drying rate both increase because high-power microwave can open the blind pores to promote moisture seepage. Furthermore, moisture migration during microwave drying is usually directional (i.e., along the bedding plane). The LF-NMR method provides a framework for the determination of moisture migration of coals.

Opposing Propagation Characteristics of Methane-Air Deflagrations in an End-to-End Pipe

Propagation characteristics of methane-air deflagrations through an end-to-end pipe were investigated using the AutoReaGas software. The results indicate that the maximum overpressure first decreases and then increases with increasing distance from ignition source, and the overpressure minimum occurs at approximately 3.4 m. When shockwaves from the intake and return airways meet, a positive superposition effect forms, generating maximum overpressure, density, and combustion rate values that are larger than the same values at adjacent points. The explosion parameters along the intake airway are similar to the parameters along the return airway. A 50% filling ratio causes nearly the same level of explosion violence as a 100% filling ratio. The characteristics of explosion propagation are similar whether the ignition source is located at the upper corner of the coal face or at the centre of the coal face. Some preventive measures may be taken to reduce the losses caused by gas explosions near superposition areas in underground coal mines.

Premixed methane/air gas deflagration simulations in closed-end and open-end tubes

The overpressures, flame velocities and flow speeds were investigated in closed-end and open-end tubes via numerical simulations. Our results suggest that the tube geometry influences the evolution of these parameters. The peak overpressure initially decreased and eventually increased with the aspect ratio in both types of tubes. The flow speed initially increased and then decreased with the propagation distance in the closed-end tube, but it increased with increasing distance in the open-end tube. Empirical equations relating the peak overpressure and the peak flow speed are presented. The flame velocities in the open-end tubes were always greater than those in the closed-end tubes, especially the maximum flame velocity. The open end promoted flame acceleration, while the closed end resulted in near-wall deceleration. The results provide a reference for future limited-space gas explosion studies.

Influences of fuel concentration, fuel volume, initial temperature, and initial pressure on flame propagation and flameproof distance of methane-air deflagrations

Purpose – The purpose of this paper is to understand a flameproof distance necessary to avoid the flame harms to underground personnel which may have great significance to the safety of underground personnel and the disaster relief of gas explosions in coal mines. Design/methodology/approach – Through a roadway with a length of 100 m and a cross-section area of 80 mm×80 mm, the flame propagation of premixed methane-air deflagrations were simulated by using AutoReaGas software for various fuel concentrations (7, 8, 9.5, 11, and 14 percent), fuel volumes (0.0128, 0.0384, 0.064, and 0.0896 m3), initial temperatures (248, 268, 288, 308, and 328 K), and initial pressures (20, 60, 101.3, 150, and 200 kPa). Findings – The maximum combustion rate for each point follows a changing trend of increasing and decreasing with the distance increasing from the ignition source, and it increases with the fuel volume increasing or the initial pressure increasing, and decreases with the initial temperature increasing. However...

Experimental study on the microscopic characteristics affecting methane adsorption on anthracite coal treated with high-voltage electrical pulses

The low gas permeability of coal formations with limited coal pores and fractures leads to difficulty in coalbed methane exploration. High-voltage electrical pulse has a potential application in enhanced coalbed methane recovery. In this study, we discuss the microscopic characteristics of anthracite coals treated by high-voltage electrical pulse. We find that C, O, and other coal elements constituting oxygenic groups, which mainly account for gas adsorption, decreased slightly after high-voltage electrical pulse treatment, indicating that elemental variation may have little influence on gas adsorption. The scanning electron microscopy and low-pressure nitrogen gas adsorption (LP-N2GA) results show that the cumulative micropore volumes of high-voltage electrical pulse-treated coals were much larger than those of original coals. The mercury intrusion porosimetry results show that the cumulative macropore volumes, which act as gas migration channels in coal increased. Additionally, high-voltage electrical pulse-treated coals were found to have smaller entrapment areas, indicating that gas migration was enhanced.

Effect of filling ratio on premixed methane/air explosion in an open-end pipe:

The propagation characteristics of premixed methane/air explosion under different filling ratios (20%, 30%, 40%, 50%, 60%, and 100%) were studied using an experimental system. The results indicate that the peak overpressure showed a decreasing trend at the initial stage but then showed an increasing trend until reaching its maximum value under different filling ratios. As the explosion propagated to the open end, the overpressure showed a downtrend. At this point, the flame speed initially increased along the pipe but then dropped dramatically. In addition, the explosion overpressure and flame speed increased with the increase of filling ratio. However, when the filling ratio reached 50%, the explosion overpressure and flame speed tended to be stable and the increase was not obvious. These results will be of great importance in evaluating the explosive damage to equipment and human personnel working in coal mines or other chemical industries.

The effect of bend positions on premixed methane–air explosion overpressures in ducts

Bend structures in process industries can enhance gas explosions. Many researchers have studied their effects on flame propagations or overpressure evolutions with experiments or numerical simulations. In this work, three bends with different angles ranging from obtuse to acute were examined, and the effect of bend positions on gas explosions was also studied in the experiments. It was found that the peak overpressure evolutions under three different angles experienced three different stages, i.e. a slight downtrend before the bend, a sudden uptrend in and after the bend, and a following downtrend. The explosion violence was enhanced due to a reflection wave and flame tip wrinkle. The peak overpressure increased when the bend angles decreased because of stronger reflection and turbulence under a smaller bend angle. Additionally, the bend shortened the distance at which the peak overpressure reached its maximum values. However, the bend positions had little effect on the maximum overpressure in the ducts.