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Kilometer-deep mines have become the new norm in coal mining. To study the influence of high stress-high ground temperature coupling conditions in deep mines on the oxidative exothermic characteris...

Water mist is one of the important substitutes of halon fire extinguishing agents with the advantages of clean and environmental protection. However, there will be a short time of flame intensification in the fire extinguishing progress of water mist, which threatening the safety of surrounding combustibles and fire fighters. The main objective of this study is to assess the suppression performance of N2 twin-fluid water mist containing KQ compound additive (composed of 6% fluorine surfactant and K2CO3) on ethanol pool fire, and analyze the suppressing mechanism on the flame intensification phenomenon. Experimental results showed that: the fire extinguishing time of several fire extinguishing agents from low to high was N2 twin-fluid water mist containing KQ compound additive

A gas explosion is often vented to a safe location by means of a discharge duct. However, the presence of a discharge duct increases the explosion severity in the vessel, inducing a higher explosion overpressure compared to a simply vented vessel. To reduce the explosion overpressure in the vessel, an experimental study was performed to suppress the methane-air explosions in a 5 L vessel connected to a discharge duct of different scales (e.g., length and diameter) and with various sodium bicarbonate concentrations. The results show that the initial flame propagation process in the vessel was basically similar in the simply vented vessel and in the vessel vented through a discharge duct (i.e., ducted-vessel). In the middle and late stages of flame propagation in the vessel, the flame fragmentation was more pronounced for the ducted-vessel. Moreover, the degree of flame fragmentation in the vessel increased with the duct length. The flame structure in the vessel was more irregular for a larger vent coefficient (Kv = 9.75). The more pronounced quenching in the short duct (250 mm duct) is related to the high inhibition efficiency due to the leakage of a large amount of sodium bicarbonate (SBC) powder into the discharge duct while in the long duct (750 mm duct) the disturbance is due to the strong turbulence. The appropriate SBC concentration can transform the mechanism for the pressure rise in the vessel. At high powder concentrations the maximum pressure in the vessel is dictated by the flame reaching the vessel wall, while at low concentrations the maximum pressure is dominated by the pressure (i.e., burn-up) in the discharge duct. There is an approximately linear correlation between the maximum pressure and the average flame velocity in the discharge duct for a given powder concentration, and a linear relationship between the maximum pressures in the vessel and the duct independent of the SBC concentration. Through the analysis of the flame dynamics (e.g. flame morphology, flame propagation velocity and turbulence) and friction resistance induced by the varying duct scales, the suppression efficiency of SBC powder in the vessel is higher for a longer and narrower discharge duct.

Mixing biogas with other fuels or increasing the oxygen fraction in the oxidizer can improve the combustion performance of biogas. In this study, we investigated the effects of propane addition and...

The effect of metal foam mesh on flame propagation of biomass-derived gas in a half-open duct was studied. The explanations are based essentially on the experimental investigations of premixed biomass-derived gas explosions carried out in a rectangular half-open combustion chamber. The initial temperature T0 and pressure P0 were 300 K and 1.0 atm, respectively. The key parameters of explosive characteristics, such as flame propagation images and explosive overpressure, were analyzed by changing the porosity, the pore density of porous metal foams, and the gas components. The results show that the use of porous metal foam has a significant inhibitory effect on the gas explosion. Although the combustion structure of the flames is similar, the action of the porous metal foam during the experiment also shows the characteristics consistent with the obstacles. When the porosity of the porous foam is 97%, the flame can be stimulated to produce turbulence, and then the shock–flame interaction generated by the reflection of the lead shock wave can enhance the explosion propagation and promote the explosion escalation. However, with the increase of hole density, the existence of the porous metal foam by momentum loss and heat loss to curb the spread of the explosion not only hindered the flow of not flammable but also extracted energy from the expansion of the combustion products at the same time. This study also confirms that the biological hydrogen and methane component has a vital role in the flame, and a reasonable hydrogen and methane ratio can improve the flame burning to get more economic value.

In order to evaluate the explosion characteristics of biogas (i.e., the mixture of CH4 and CO2), experiments were conducted in a duct with a length-to-width ratio of 10. The ignition was activated at the closed end while the opposite end was sealed by a PVC film of varying thickness. The effect of PVC film thickness and methane fraction in biogas was studied. The results show that there were three possible pressure peaks (Pb, Pmfa, Pext), due to the film failure, the attainment of maximum flame area and the external explosion, respectively. The pressure profile pattern was dependent on the combination of film thickness and methane fraction. Two pressure peaks Pb and Pmfa were observed for the PVC film thickness δ ≤ 0.033 mm. However, the pressure peak Pmfa was absent for the film thickness δ ≥ 0.055 mm because the PVC film did not rupture in these cases when the flame had touched the sidewall. In other words, the maximum flame area was achieved prior to the moment of venting. It signifies that in the absence of explosion venting, the flame quenching due to the sidewalls alone was insufficient to induce an individual pressure peak Pmfa. The third pressure peak Pext was low for the biogas explosion in the current configuration. The film thickness exerted a greater impact on bursting pressure Pb than did the methane fraction, at least for the range of film thicknesses and methane concentrations considered. Additionally, the individual effect of the film thickness or the methane fraction was less influenced by each other. The existing theories were accurate to predict the characteristic times of the flame propagation for δ = 0.011 mm but not for δ ≥ 0.033 mm. The PVC film thickness affected the explosion overpressure more, whereas the CH4 fraction in biogas affected the flame front velocity more remarkably.

In this work, the explosion pressure oscillation characteristics of premixed syngas/air mixtures were studied experimentally in the duct with different hydrogen concentrations and opening ratios. The results indicate that the overpressure changes as the hydrogen fraction and opening ratio varies, and the venting can mitigate the phenomenon of CRPT by increasing the opening ratio of the duct. With the increase of hydrogen volume fraction, from 10% to 50%, the rise rate of explosion overpressure increases rapidly, the maximum overpressure increases and the shortening time of reaching the maximum pressure decreases gradually. With the opening ratio increasing, the resistance, the turbulence and the pressure peak decrease gradually. Moreover, if the ratio is greater than 0.5, it has a small effect on the pressure peak; when the ratio is less than 0.5, it has a significant effect on the pressure peak. Through analyzing the dominant periods and average amplitudes of the oscillations, it can be found that the syngas combustion has an obvious combustion-induced rapid phase transition phenomenon in the opening duct. The decrement of the opening ratio and the increment of the hydrogen concentration contribute the average amplitude to increasing.

An active system was designed to detect the flame automatically and spurting out the extinguishing agent to prevent the propagation of flame after gas explosion. The effects of both nitrogen and AB...

To address the coal gangue fracture heat transfer problem, the law of heat transfers and temperature distribution in the coal gangue fracture is studied. The temperature distribution in a coal gang...

Combining the three-phase foam and simulated sea mud particle (SM-P: simulated sea mud particle) containing inert substances (SMTP: simulated sea mud three-phase foam) to engineer a new three-phase foam fire extinguishing material was a viable strategy to solve the problem of coal mine spontaneous combustion prevention and insufficient stability of mine fire extinguishing materials. Herein, based on constant temperature heating table and simulated coal spontaneous combustion fire extinguishing device, the experiments were conducted to evaluate the material properties. As expected, the SMTP-20 exhibited excellent stability (73.00 mins at 40 °C), which was 2.19 times that of ordinary three-phase foam (34.33 mins at 40 °C). The introduction of SM-P can also effectively reduce the thermal decomposition of powdered coal, the corresponding temperature of thermal decomposition peak value of raw coal was delayed 17.78 °C. The SMTP-20 could construct a thin fibrous-like foam layer with SM-P to cover the surface of burning coal and effectively seal cracks in the coal from oxygen contact, thereby preventing coal further oxidation. This work not only offers a new three-phase foam with outstanding performance, but also unravels a strategy for preparing three-phase foam with high stability and good fire extinguishing effect.

The flame propagation characteristics of hydrogen/methane/air mixtures with different hydrogen addition ratios (φ = 0, 10 %, 20 %, 30 %, 40 % and 50 %) in different variable cross-section ducts were studied. A high-speed camera and pressure sensors were used to collect flame images and determine the overpressure dynamics. The results show that the smooth flame front will be twisted and folded, when the flame propagates to the abrupt position of the cross-section area of the duct. The larger the abrupt change rate of the duct cross-section is, the more obvious the disturbance to the flame and the more severe the turbulence. With increasing hydrogen addition ratio, the flame propagation speed and overpressure in the four kinds of variable cross-section ducts studied increase. The time for the flame front to reach the downstream end is gradually shortened, and the flame propagation time when the flame propagates from the smaller cross-section tube to the larger cross-section tube is more severely shortened with increasing hydrogen addition ratio than that when the flame propagates from the larger cross-section tube to the smaller cross-section tube. The increase of the overpressure caused by the addition of hydrogen is more significant when the flame propagates from the smaller cross-section tube to the larger cross-section tube. When the flame propagates from the smaller cross-section tube to the larger cross-section tube or from the larger cross-section tube to the smaller cross-section tube, the larger the abrupt change rate of the duct cross-section is, the larger the maximum overpressure.

Self-heating of coal is a long-standing hazard and pollution source in longwall goaf and abnormal air leakage into goaf is a key yet complex drive to the hazard. To investigate such a problem with more insights, a numerical model without considering coal moisture is established based on a Shendong longwall. Abnormal air leakage into goaf mainly sources from the edge cracks resulting in presence of high level oxygen (8 %∼13 %) in start-off area of the longwall. Two heating liable regimes were identified: one is behind longwall face and another one locates in the start-off zone. Heating in the start-off zone develops more quickly than that in heating regime one. On day 25 the maximum temperature of regime two can rise to 500 K while it can only increase to approximately 340 K in regime one. The heating spot behind longwall face tends to be self-suppressed with longwall advancing while the heating in the start-off zone can develop to a spontaneous combustion incident due to constant airflow leakage from the mining induced cracks. A wide range of inertisation plans including different locations, strategies, and flowrates of nitrogen injection were conducted. An optimum inertisation plan is to proactively inject inert gas with a low flowrate (e.g. 122 m3/h) from a seal along the start-off line. A high flowrate of nitrogen stream is preferable to suppress an on-going heating and the reactive inertisation should be maintained for a long run otherwise the heating is very likely to re-develop.

In order to study the effects of chemical composite additive (CCA) on the microscopic characteristics of spontaneous combustion coal, atomic force microscope and Fourier transform infrared spectroscopy technology were used to study the microstructure and active groups of spontaneous combustion coal. The roughness, three-dimensional surface morphology, microscopic pore structure, infrared spectrum, and active group content of raw coal samples and coal samples treated with water or different concentrations of CCA were analyzed. The experimental results showed that compared with the raw coal, the roughness Rq and Ra of the CCA-treated coal samples decreased with increasing CCA concentration, and the surface topography of the microscopic structure tended to be flat and smooth, and the size becomes smaller and the depth becomes shallow of pore. In the raw coal samples and coal samples treated with water and CCA, the main types of active groups remained constant. However, the contents of these groups changed, and the order of the contents of main types of active groups is water-treated > raw coal (untreated) > CCA-1% treated > CCA-5% treated > CCA-10% treated > CCA-20% treated. In addition, the mechanism of the CCA inhibition of coal spontaneous combustion was discussed and analyzed.

Based on effects of the deep surrounding rock temperature (Tsr) on the oxidation properties of mined coal, a thermodynamic experimental system for loaded coal oxidation is employed to examine the o...

The mitigation effects of ultrafine water mist on hydrogen/methane mixture explosions with hydrogen fraction (ϕ) of the range from 0% to 60% were experimentally studied in a vented chamber with obstacles. The spraying time, droplets size of water mist and the volume ratio of hydrogen were varied in the tests, and the key parameters that reflect the explosion characteristics such as the flame propagation imagines, flame propagation velocity, and explosion overpressure were obtained. The results show that the ultrafine water mist presents a significant mitigation effect on hydrogen/methane mixture explosions. The flame propagation structures are similar under the condition of without and with ultrafine water mist while the flame temperature is declined by the physical and chemical inhibition by ultrafine water mist. In addition, the mitigation effect increases with the increase of water mist flux. As a result, the maximum flame speed and overpressure of ϕ = 30% hydrogen/methane mixture explosion are declined by 33.3% and 58.4% under the condition of spraying for 2 min with 15 μm ultrafine water mist, respectively. Besides, the mitigation effects of ultrafine water mist on ϕ = 30% hydrogen/methane mixture explosion descends evidently with the increase of the droplets size of the range from 6 μm to 25 μm, which due to the easier evaporation and the greater total droplets surface area of the smaller water mist. However, the explosion mitigation effect of ultrafine water mist on the hydrogen/methane mixture actually descends with the increase hydrogen fraction.

To study the effects of variable cross-section ducts with different configurations on the flame structure, flame propagation speed and overpressure of hydrogen/methane/air premixed gases, a series ...

Self-heating of coal mass has been a long-standing thermal dynamic hazard and also an environmental concern during coal mining, processing, and storage. Especially self-heating of coal in longwall goaf poses a great danger to underground working crew and goaf inertisation is often practiced to combat the hazard. To investigate goaf inertisation with more insights regarding two different philosophies (i.e. proactive and reactive), a three-dimensional non-equilibrium thermal CFD model was developed to study heating evolution and a range of inertisation plans in a modern longwall. The theoretical model mainly incorporated a set of governing equations (e.g. energy conservation, continuity, and momentum balance) and coal oxidation kinetics which are determined by an adiabatic coal heating test. A comprehensive inertisation plan is developed. At initial stage of mining, a proactive inertisation plan is recommended because a heating in start-up area can develop to a fire incident if precautions are not implemented. While a reactive inertisation is suggested after longwall advancing a certain distance as the heating behind longwall face can be left in the fully suffocated zone before escalating to an advanced stage.

To study the effect of chemical composite additives (CCAs) on the heat release characteristics of coal oxidation spontaneous combustion, this paper develops a CCA comprising multiple chemical reage...

This tissue developed the new environmental protection chemical composite inhibitor (CCI) for the effective inhibition of coal spontaneous combustion. CCI is combined by using sodium dodecyl sulfate, sodium laurylsulfonate, flame-retardant compound, xanthan gum and deionized water, and all the materials blend solutions with 1%, 5%, 10% and 20% concentrations. Coal samples treated with blended solutions with 1%, 5%, 10% and 20% concentrations are tested separately on the temperature-programmed oxidation test platform for loading coal samples in order to research the effectiveness and properties of solutions inhibiting oxidized spontaneous combustion of coal, which investigates and analyzes impacts on the parameters of coal spontaneous combustion, and then comparing to those of the untreated and water-treated coal samples. Experimental results showed that the developed inhibitor had remarkable inhibition effects on the CO generation and oxygen consumption rate for the treated coal samples. The individual and averaged inhibitor rates both increase with the increase in inhibitor solution concentration, and for 20% inhibitor-treated samples, these rates reached 85.92% and 67.26%, respectively. The statistical regression analysis showed that the inhibitor concentration and the averaged inhibition rate exhibited a strong logarithmic function relationship. In addition, the spontaneous combustion inhibition mechanism was discussed from both the macro- and micro-perspectives.

To study the oxidation characteristics and spontaneous combustion micro-evolution characteristics of coal unloaded under different initial stress, in this paper, a comparative study on the micro-ox...

This study discusses the use of nitrogen to prevent the propagation of an explosion in a horizontal duct in an effort to reduce the damage caused by gas explosions. Accordingly, the vent is set on the top surface of the duct. The two nitrogen nozzles are set near the vent and at a downstream location. The results show that when the nitrogen pressure reaches or exceeds 0.3 MPa, spurting nitrogen can prevent the explosion from propagating along the duct. When the nitrogen pressure is 0.1 MPa, it fails to prevent the explosion. When the nitrogen pressure is 0.2 MPa, the explosion propagation can be prevented in most experiments but not in all of them. As the nitrogen pressure increases, the position for preventing the explosion gradually shifts forward from the second nozzle to the first nozzle. At lower nitrogen pressures (i.e., 0.2 MPa), an early spurt of nitrogen can prevent the explosion, while a delayed spurt of nitrogen may not be able to prevent it. The distribution of the speed of the explosion flame along the duct exhibits two peaks. The regularity of the overpressure is similar to that of the flame speed.

Cellular instability is responsible for the self-acceleration of a flame, and such acceleration might cause considerable damage. This paper presents an experimental study on the inhibition effect of CO2 and an ultrafine water mist on the self-acceleration characteristics of a spherical flame in the initial stage of a 9.5% methane/air explosion in a constant volume combustion bomb. Results showed that insufficient water mist enhanced the self-acceleration of the spherical flame and the intensity of the explosion; nevertheless, the synergistic inhibition effect of CO2 and ultrafine water mist prevented enhancement of the explosion and significantly mitigated the self-acceleration of spherical flames, which observably delayed the appearance time of a cellular flame, and reduced the flame propagation speed, overpressure and the mean rate of pressure rise, indicating that suppression of flame self-acceleration could effectively mitigate the damage from a methane/air explosion. The reason for the synergistic effect was a result of a combination of physical suppression and chemical suppression: due to the preferential diffusion dilution effect of CO2, the initial flame speed was reduced, and the flame became thicker, which increased the evaporation time and quantity of droplets around the flame front, accordingly enhancing the cooling effect on the flame front. The increased flame thickness could withstand greater disturbance and inhibit the formation and development of a cellular flame. Meanwhile, CO2 and H2O can also reduce the concentration of active radicals (O, H and OH) and reduce the reaction rate and combustion rate of a methane/air explosion.