Suppressive Effects of Silicon Dioxide and Diatomite Powder Aerosols on Coal Mine Gas Explosions in Highlands

A gas explosion is the severest threat to the safety of coal mines, particularly in highlands. For effectively preventing gas explosions in coal mines, a quartz tube–based gas explosion test system with an open end and a nearly spherically confined gas explosion testing device were devised, fabricated, and tested. The suppressive effects of aerosols, here silicone dioxide (SiO2) and diatomite powder aerosols, of various sizes on mine gas explosions were studied. Scanning electron microscopy, low temperature nitrogen adsorption, and thermogravimetry analysis conducted on diatomite showed the porosity, effective specific surface area, adsorbability, and excellent thermal stabilities of the aerosols. Explosion suppression experiments confirmed that a test system of SiO2 and diatomite powder aerosols remarkably prolonged the gas explosion induction period and the time of peak explosion pressure and reduced the flame propagation rate. In addition, the test system reduced the maximum explosion pressure and maximum and average explosion overpressure rising rates. The suppressive effects were clearly affected by the aerosol type and size. Microscale diatomite powder aerosol exhibited a stronger suppressive effect than microscale SiO2 powder aerosol did. The porous structure and thermal characteristics of diatomite powder was vital in the suppression efficacy because they restrained the growth rate and activity of free radicals obtained through explosion reactions and eventually eliminated them. The adsorption of formaldehyde, a major intermediary of gas explosion, by diatomite powder decreased the quantity of free radicals, eventually suppressing explosions.


INTRODUCTION
In coal mining, ground air entering the mine forms mine air because of pollution.Similar to ground air, the main components of mine air are oxygen, nitrogen, and carbon dioxide.However, because of the influence of various natural factors and production processes, mine air also contains components such as dust and gas.In China's highlands, gas explosions are the leading accidents in underground coal mining.Gas concentration monitoring, gas accumulation prevention, non-fire source technology, and explosion suppression technology that reduces the influence range of an explosion are the main gas explosion prevention methods (Zhou, 2001).The first three approaches mainly prevent the simultaneous formation of the three elements necessary for water mist can effectively reduce the pressure peak of gas explosion and the propagation speed of flame than normal water mist, and with the increase of the charged voltage, the inhibiting effect of charged water mist on gas explosion is significantly enhanced.
And in porous materials-based explosion suppression technology, Varying degrees of explosion inhibition have been realized using metal wire mesh, foamed ceramic, and porous foam metal suppressants (Ciccarelli et al., 2005;Joo et al., 2005;Zhang et al., 2012;Sun et al., 2013;Wei et al., 2013), finding bore diameters, thicknesses, densities, and components of porous materials are key factors affecting their gas explosion suppression efficacy.
Actually, the rock powder has the most wide range of application for explosion suppression in coal mines because of its' inexpensiveness and convenient to use.But its' suppressing effect is not optimal.SiO 2 powder is a typical aerosol widely used in industrial applications and is a common refractory raw material.Diatomite, a porous aerosol, is a biogenic siliceous sedimentary rock composed of the fossilized remains of diatom.Diatomite mainly contains SiO 2 , with trace amounts of Al 2 O 3 , CaO, Fe 2 O 3 , K 2 O, MgO, Na 2 O, P 2 O 5 , and organic materials, rendering the diatomite exquisite, unconsolidated, light, and with excellent adsorption and immaculate osmosis characteristics.China is abundant in diatomite resources, with broadly distributed reserves of approximately 320 million tons and prospective reserves of approximately 20 million tons.Based on the comprehensive consideration of suppressing effect and the cost of suppressant in this study, methane, the dominant mine gas, was used as the experimental medium.Nanoscale and microscale SiO 2 powder aerosols and microscale diatomite powder aerosol were used as gas explosion suppressants, aiming to find some explosion suppressing powder that more effectiveness and accessible.

Experimental Systems
A self-designed quartz tube-based gas explosion test system named XKWB-S (Type S gas explosion system of Xi'an University of Science and Technology) and an XKWB-1 (Type 1 gas explosion system of Xi'an University of Science and Technology) cylindrical experimental system were used (Figs. 2 and 3) The quartz tube-based system comprised an explosion reaction pipe, an ignition control system, a gas explosion suppressant spraying system, and a multichannel high-speed data acquisition system (i.e., a high-speed camera).The explosion reaction vessel was a quartz tube with an inner diameter of 84 mm, total length of 1,600 mm, and effective volume of 8,860 mL.An igniter was placed in the blind end of the pipe, and the other end was sealed with a highly transparent polyethylene film.The high-speed camera placed within the pipe was 143 cm long.The entire experimental process was synchronously managed using the high-speed camera, which was activated as soon as methane was ignited.At 1,000 fps, the camera accurately captured a series of images of explosion flame propagation and completely demonstrated the changes in explosion flame propagation velocity in the pipe.
The XKWB-1 cylindrical experimental system comprised an explosion reactor and gas mixing, ignition, measurement, powder aerosol injection, and cleaning systems.The explosion reactor was a 20-L cylindrical vessel with an average diameter of 30 cm and an inner height of 35 cm.The measurement system mainly comprised a pressure sensor, a controller, and a computer.Data were collected from the moment that the igniter turned on.The response and maximum collection times were 1 and 500 ms, respectively.The pressure of compressed air was 1Mpa, the ignition energy was 1 J, and the delay time was 60 seconds.The experiments were conducted at temperature of 17.4-22.4°Cand atmospheric pressure, with air humidity of 50-74% RH.
According to the experimental criteria of the American Society for Testing and Materials, an explosion occurs when the pressure increases by ≥ 7%.

Gas Explosion Induction Period
The gas explosion induction periods at different particle sizes of SiO 2 and diatomite powder aerosols are listed in Table 1.As shown in Fig. 4, the gas explosion induction periods varied with the type of aerosol.Increasing the powder aerosol concentration considerably increased the induction period.At a concentration of 11.28 mg L -1 , 10-μm diatomite, 5-μm SiO 2 , and 50-nm SiO 2 powder aerosols extended the explosion induction period of 9.5% gas explosion by approximately four-, two-, and five-fold, respectively; 56.4 mg L -1 of nanoscale SiO 2 powder aerosol extended the gas explosion induction period by approximately 10-fold.The gas explosion induction period of microscale diatomite powder aerosol was longer than that of microscale SiO 2 powder aerosol.The induction periods of different aerosols were as follows: 5-μm SiO 2 < 10-μm diatomite < 50-nm SiO 2 .

Characteristic Parameters of Explosion Flame Propagation
Using the length of the flame front between consecutive  high-speed photographs captured at 500 fps, the rates of explosive flames under different conditions were obtained.Because of wide variation in the starting points, the initiation point was unified for comparing the explosion flame propagation rate.Fig. 4 presents the effect of 11.28 mg L -1 of different aerosols on the explosion flame propagation rate.The propagation rate was considerably reduced by the addition of powder aerosols, particularly nanoscale SiO 2 powder aerosol.Changes in the explosion flame propagation rate after adding aerosols were more uniform than those in the absence of aerosols.Nanoscale and microscale powder aerosols exhibited nearly the same effect up to 42 ms; that is, explosion flame propagation rates slowly increased up to 34 ms, slowed down until 42 ms, and suddenly increased.
Particularly, the explosion flame propagation rate accelerated after adding microscale SiO 2 powder aerosol.According to the changes in the explosion flame propagation rate, the explosion suppressive effects of the three aerosol types were as follows: 5-μm SiO 2 < 10-μm diatomite < 50-nm SiO 2 .The trends in changes of the flame propagation rates after adding different aerosols revealed that the flame propagation rate initially increased, decelerated, and subsequently peaked again after an inflection point.The flame propagation speed slowed down after peaking; however, when the flame nearly reached the top of the pipe, the speed increased again because of turbulence.Moreover, with decreasing particle size, the peak flame propagation speed considerably decreased and exhibited a decreasing trend.Accordingly, the explosion suppression effects of the three aerosol types were ranked as follows: 5-μm SiO 2 < 10-μm diatomite < 50-nm SiO 2 .Peak flame speeds for 5-μm SiO 2 , 10-μm diatomite, 50-nm SiO 2 were 56.7, 55.8, and 49.6 m s -1 , respectively.Compared with 1-Quartz tube; 2-High-speed camera; 3-Displayer; 4-Data acquisition unit; 5-Igniter; 6-Controller; 7-Computer; 8-Gas cylinder; 9-Spark plug; 10-Suppressant spray entrance; 11-Fixed support   the flame speed in the absence of a powder aerosol, the speeds for 5-μm SiO 2 , 10-μm diatomite, 50-nm SiO 2 decreased by approximately 44%, 45%, and 51%, respectively.

Characteristic Parameters of Explosion Pressure
The effect of various concentrations of nanoscale SiO 2 powder aerosol on the gas explosion pressure is shown in Fig. 5.The maximum explosion pressure and the pressure rising rate decreased with increasing aerosol concentration, and the explosion induction period and the time of peak explosion pressure were prolonged at powder aerosol concentrations < 100 mg L -1 .Nevertheless, at 150 mg L -1 , the maximum explosion pressure and the pressure rising rate increased and the explosion induction period and the time of peak explosion pressure decreased.Under the experimental conditions and at the optimal concentration of 150 mg L -1 , the nanoscale SiO 2 powder aerosol exhibited the optimal explosion suppression effect.
The effect of different types and particle sizes of aerosols (at 100 mg L -1 ) on the gas explosion pressure is listed in Table 2.At 7% methane explosion, the maximum explosion pressure and the pressure rising rate apparently reduced and the explosion induction period and the time of peak explosion pressure were significantly prolonged.Therefore, among the suppressants used, nanoscale SiO 2 powder aerosol demonstrated the optimal suppressive effect for the experimental duration of 500 ms.At 7% methane explosion, nanoscale SiO 2 powder aerosol reduced the maximum explosion pressure, the pressure rising rate, and the average explosion rising rate by 71%, 76%, and 90%, respectively, and the time to reach the peak pressure was extended by more than three-fold.However, the suppressive effect of microscale diatomite powder aerosol was stronger than that of microscale SiO 2 powder aerosol.At 7% methane explosion, adding 5-μm SiO 2 powder aerosol reduced the maximum explosion pressure, the pressure rising rate, and the average explosion rising rate by 22%, 50%, and 72%, respectively, and the time to reach the peak pressure was extended by 2.8fold.However, the suppressive effects of 5-μm SiO 2 powder aerosol were lower than those of 10-μm diatomite powder aerosol (30%, 58%, and 76%, and three-fold, respectively).Thus, the suppressive effect of SiO 2 and diatomite aerosol powders at the same concentration on 7% methane explosion was as follows: 5-μm SiO 2 < 10-μm diatomite < 50-nm SiO 2 .

Suppression Mechanism
The results indicated that both SiO 2 and diatomite powder aerosols suppressed gas explosions.The suppressive effect of nanoscale SiO 2 aerosol powder was considerably stronger than that of microscale SiO 2 aerosol powder, whereas the suppressive effect of microscale diatomite aerosol powder was stronger than that of microscale SiO 2 aerosol powder.
Particle sizes and surface configurations of microscale SiO 2 and diatomite samples were recorded using a JSM-6460LV scanning electron microscope (SEM), and those of nanoscale SiO 2 sample were recorded using a JY/T010-1996 SEM.Low-temperature nitrogen adsorption experiments of the samples were performed using an ASAP2020 nitrogen adsorption instrument (Micromeritics Instrument Corp., Norcross, GA, USA), and thermogravimetric curves were   acquired using a thermogravimetry-differential scanning calorimetry simultaneous thermal analyzer (TG-DSC, Type 449C, produced by Netzsch).Fig. 6 reveals that 1-5-μm microscale SiO 2 powder aerosol demonstrated a smooth surface.By contrast, nanoscale SiO 2 powder aerosols exhibited poor agglomeration on its flocculent surface.No porous structures were present on the surface of both materials.In fact, the prepared diatomite powder aerosol showed various particle shapes, such as rings, shells, and fans.A powder aerosol with porosity < 1 µm was uniformly distributed on the surface of each construction with aforementioned, that are rings, shells, and fans.
Low-temperature nitrogen adsorption results indicated that the specific surface area, microporosity, and average pore size of diatomite was 59.68 m 2 g -1 , 16.3%, and 9.254 nm, respectively, all of which contributed to its excellent adsorption performance.were observed for microscale and nanoscale SiO 2 up to 120°C.Beyond 120°C, no obvious endothermic or exothermic peaks were observed, indicating the thermal stability of the tested materials.The thermal mass loss of nanoscale SiO 2 was more than that of microscale SiO 2 , although both of them were similar in size.Because nanoscale SiO 2 has a larger specific surface area, which enhances water adsorbability, than microscale SiO 2 does, nanoscale SiO 2 contained slightly more water.The thermal mass loss of microscale diatomite occurred up to 425°C, and the TG curve was stable beyond 425°C.Thereafter, the mass loss of the diatomite sample during the experiment changed only from 100% to 97.8%, indicating its excellent thermal stability.The DSC curve of the microscale diatomite powder aerosol showed a clear endothermic peak between 50°C and 250°C, possibly because of the endothermic evaporation of water in the sample.The water may comprise bound and adsorbed water.At high temperatures, the bound water evaporated, causing the DSC endothermic peak to subside near 250°C.Moreover, other low-molecular-weight substances were considered to have decomposed during the aforementioned process, leading to fluctuations in the DSC curve.
Gas explosion was involved in a series of chain reactions, during which numerous free radicals with high reactivity capacity, such as O, ĊH 3 , ȮH, Ḣ, HȮ 2 , and HĊO, were generated.Inert additives catalyzed the decrease in the chain reactions (Xu et al., 2004) by converting the energetic free radicals to inactive radicals, slowing stable materials, or even terminating the free radical reactions.The explosion suppression mechanism of diatomite powder aerosol is as follows:

A. Slowing the Growth Rate of the free Radicals Involved in Explosion
According to Arrhenius reaction theory, decreasing temperature alleviates the rate of methane oxidation by hindering the free radical growth rate.The thermogravimetric curves of SiO 2 and diatomite powder aerosols showed that these aerosols lose the crystal or bound water and produce volatile substances through thermal decomposition for preventing explosion at a high temperature (2,000°C).These effects were achieved by absorbing an enormous amount of heat released from the combustion and explosion reactions, reducing temperature in the combustion and explosion zones, shortening the flame duration, and inhibiting explosion flame development.Furthermore, water vapor and volatile substances generated through decomposition diluted the concentration of methane and combustion air, thus stifling flame development.Solid particles separated methane and functioned as thermal radiators and conduction shields, effectively preventing gas explosion.

B. Weakening Free Radical Activity during Explosion
Because silicon powder particles and their solid phase decomposition products are inert solid particles, they can form an isolation layer for blocking combustion and explosion propagation in the explosion reaction zone as well as a barrier between activated molecules and free radicals.After the free radicals collided with inert solid particles, the energy of the radicals was transferred to the solid particles.Subsequently, the free radical activity disappeared, the responding ability of the system decreased, and the free radicals combined to form stable molecules.By contrast, after the inert solid particles collided with the free radicals, the radicals were instantaneously adsorbed on the particle surface and consumed by the particles.Therefore, the number of free radicals decreased sharply, thereby interrupting the gas explosion chain reaction.

C. Eliminating Free Radicals during Explosion
Low-temperature nitrogen adsorption and SEM analysis results revealed that the nanoscale SiO 2 powder aerosols were small and exhibited a surface effect caused by diatomite porosity.The atomic quantity and proportion on the surface increased rapidly, generating considerable surface energy, which made the atoms highly superficial, and the specific surface area increased sharply (150 m 2 g -1 ).Moreover, because of coordination number inadequacy, many dangling bonds appeared.Particularly, porous materials adsorbed other atoms or chemically reacted with other atoms to render their surfaces in unsaturated fields for achieving a relatively stable state.Such unsaturation of nanoparticles enhanced the chemical activity and improved the adsorption capacity of the free radicals involved in explosion suppression.Small-particle powder aerosols heated up quickly and easily thermally decomposed or gasified, providing more sites for the free radicals generated in the reaction zone, which effectively reduces the quantity of free radicals and inhibits the gas explosion chain reactions.Wang and Zheng (2011) reported that diatomite excellently adsorbs formaldehyde.CH 2 O + O 2 → HȮ 2 + HĊO is a crucial branched-chain reaction in methane explosion chain reactions and generates formaldehyde at temperatures less than 626.9°C.Thus, the diatomite decreased or eliminated the formaldehyde free radicals, which are essential components in gas explosion chain reactions.This finding was confirmed using the following quasi-steady method.
Because of the presence of H 2 O, the product of gas explosion at temperatures less than 626.9°C, the total reaction rate of the explosion is as follows: According to the quasi-steady method, the reaction rates of intermediate products of gas explosion, such as ĊH 3 , ȮH, Ḣ, HȮ 2 , and HĊO, are as follows: From Eqs. ( 2)-( 6), the following equations can be acquired: Finally, Eq. ( 10) is obtained as follows: where r is the reaction rate, C is the reactant concentration, and K is the elementary constant reaction rate.Eq. ( 10) indicates that the quantity of HĊO generated during gas explosion at low temperatures has a decisive role in explosions.Accordingly, eliminating HĊO or preventing its formation effectively suppressed gas explosions.In addition, the porous diatomite powder aerosol used excellently adsorbed formaldehyde, effectively suppressing gas explosion.

CONCLUSIONS
A. Gas explosion suppression was clearly affected by the surface structure and particle size of aerosols, particularly in mines in highlands.SEM tests, low-temperature nitrogen adsorption examinations, and TG analyses were conducted on diatomite powder aerosol and its diatomite porosity, adsorbability, effective specific surface area, and excellent thermal stabilities were demonstrated.Compared with the diatomite powder aerosol, the SiO 2 powder aerosol, particularly the nanoscale SiO 2 powder aerosol with a small particle size and large specific surface area, exhibited a stronger suppressive effect.The suppressive effect of microscale diatomite powder aerosol was much stronger than that of microscale SiO 2 powder aerosol.B. Results showed that the diatomite powder aerosol clearly prolonged the gas explosion induction period and the time of peak explosion pressure and reduced the maximum explosion pressure, the pressure rising rate, and the average explosion rising rate.Experiments conducted in the XKWB-S quartz tube-based gas explosion test system showed that 10-μm diatomite powder aerosol at 100 mg L -1 prolonged the induction period of 9.5% gas explosion approximately four-fold and decreased the flame peak speed by 45%.The XKWB-1 spherical experimental system was used for studying the suppressive effects of SiO 2 and diatomite powder aerosols on the pressure characteristics of gas explosion.The 10-μm diatomite powder aerosol reduced the maximum explosion pressure, the pressure rising rate, and the average explosion rising rate of 7% gas explosion by 30%, 58%, and 76%, respectively, and extended the time to reach peak pressure by more than three-fold.C. The quasi-steady method was used for verifying the role of formaldehyde in gas explosions.The diatomite particles in aerosol excellently adsorbed formaldehyde, a reactant of the CH 2 O + O 2 → HȮ 2 + HĊO reaction that occurs in explosions.Thus, diatomite decreased or eliminated the quantity of formaldehyde free radicals.D. The unique porous structure of diatomite increased the contact area available for the free radicals and the activated surface of the reaction-generating radicals, and enhanced the adsorption of free radicals during the explosion, markedly reducing the quantity of radicals.Thus, the explosion suppression effect of diatomite powder aerosol mainly relied on retarding the free radical growth rate in the explosion, weakening the free radical activities, and eliminating free radicals in the explosion reactions.
Particle size distribution of 50-nm SiO 2 powder

Fig. 4 .
Fig. 4. Influence of 11.28 mg L -1 added amount of powder aerosol on flame propagation rate of gas explosion.

Fig. 6 .
Fig. 6.Photographs on micro-scales of SiO 2 powder aerosol and diatomite powder aerosol by SEM.

Table 1 .
Explosion induction period of series powder aerosol.

Table 2 .
Effect of different kinds and particle sizes of powder aerosol on gas explosion pressure at the concentration of 100 mg L -1 .