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A novel fire prevention and control plastogel to inhibit spontaneous combustion of coal: Its characteristics and engineering applications
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A novel fire prevention and control plastogel to inhibit spontaneous combustion of coal: Its characteristics and engineering applications Yi-Jin Fana, Yan-Yun Zhaoa, Xiang-Ming Hub, , Ming-Yue Wub, Di Xueb ⁎
a
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China Key Lab of Mine Disaster Prevention and Control, College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China
b
ARTICLE INFO
ABSTRACT
Keywords: Coal body spontaneous combustion Plastogel Retarder for coal body oxidation Fire prevention in working face withdrawal On-site application
In order to address the issue of spontaneous combustion in coal mines during the mining process, we have proposed a novel plastogel which can prevent and control fires in coal mines. The novel plastogel was prepared by introducing a polymer plasticizer into the conventional water glass gel (water glass/coagulant/bentonite system). The effect of different concentrations of components on the properties of plastogel was analyzed by testing the gelation time and water retention of the plastogel. The Infrared spectroscopy results indicate that plastogel can effectively inhibit the oxidation of methylene in the coal body. The SEM results illustrated that the surface morphology of the plastogel and conventional water glass gels were significantly different. The compactness and smoothness of the plastogel were significantly enhanced after the addition of plasticizer. The results of the on-site application of plastogel in the withdrawal of 1308 working face of Shandong Longyun Coal Industry Co., Ltd. suggest that the plastogel effectively inhibited the oxidation of the crushed coal body, significantly reduced the temperature of coal body, and ensured a safe withdrawal of rack on the working face. The formulated plastogel had distinct practical advantages and is proposed as an ideal fire prevention and control material.
1. Introduction As a vital global energy source, coal has been used in various industries such as steel and electric power. According to the data of BP World Energy Statistical Yearbook in 2019, the world’s coal production in 2018 was 3.917 billion tons with a yearly increase of 4.3%. However, owing to an increasing coal production, the risk of coal fire, gas accidents [1–3], and coal dust pollution [4–11] is becoming more serious in the countries with higher coal productions such as China, India, the United States, and South Africa, especially coal fire in the mining process. In China, underground mining is the primary form of coal mining. As the mining depth increases (the depth of several mines in the east China can reach up to 800–1000 m), the ground temperature also increases (some coal mining faces can reach 38–40 °C), leading to a much higher probability of spontaneous combustion. Throughout the world, the spontaneous combustion of coal has become one of the most
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critical natural disasters that can affect the safe production in coal mines. The spontaneous combustion of coal, if not eradicated immediately, may lead to coal ignition and even a fullblown fire [12,13]. Effective prevention and control of spontaneous fire at underground goafs are critical to ensure coalmine safety [14]. Once spontaneous combustion occurs, it causes a series of disasters such as casualties, property losses, production interruptions, and equipment damage [15], with the simultaneous destruction of ecological environment. Coal spontaneous combustion results from a complex reaction between coal and oxygen [16]. Even when the oxygen content is as low as 3%, the oxidation reaction of coal still exists, liberating CO and heat at the same time [17]. In view of the spontaneous combustion, the presently applied fire prevention and control technologies include grouting into the goaf [18–23], injection of three-phase foam [24–26], inert gas injection [27–31], and injection of colloidal material [32–33]. The above technologies effectively guarantee safe production in a coal mine to a cer-
Corresponding author. E-mail address: [email protected] (X.-M. Hu).
https://doi.org/10.1016/j.fuel.2019.116693 Received 4 March 2019; Received in revised form 18 October 2019; Accepted 18 November 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Yi-Jin Fan, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116693
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tain extent, but there are still some shortcomings. The slurry and water are easy to separate during the grouting process [34], which can affect the coal’s quality and the dried material is easy to crack. The threephase foam has a relatively poor stability, a high cost, and a complicated preparation process. The inert gases (CO2, N2) are easy to diffuse along with the air leakage. Therefore, it is urgently required to develop a fire prevention and control material with excellent performance. In recent years, gel-based materials have been widely used in several coal mines as they can rapidly reduce the temperature of the coal body, effectively plugging the air leak and efficiently retaining water. According to their properties and characteristics of fire prevention and control material, the current gel material can be divided into inorganic silicate gel and organic polymer gel. The inorganic silicate gels are primarily colloidal substances formed by mixing sodium silicate solution (water glass), loess (or fly ash, etc.), and coagulant. Such gels have good fluidity so they can quickly cover the coal body and effectively block the wind leak [35]. However, they are not tough and easily form powder [27]. On the other hand, the organic polymer gel material has a certain strength. It loses water slowly after being heated and has high fire extinguishing stability. However, it has higher cost, poorer fluidity, and smaller penetration range. The present study envisages to prepare a novel plastogel by combining the advantages of inorganic silicate gel and polymer gel. The plastogel is expected to have good water retention, viscoelasticity, and plugging ability against air leak, which can have great potential in the fire prevention and control applications in coal mines. In this paper, a novel type of plastogel was prepared by adding coagulant, polymer plasticizer, and bentonite into the water glass solution. The effects of different added amounts of bentonite, coagulant, and polymer plasticizer on the gelation time, water retention, and other characteristics were comparatively analyzed. The optimal formulation of plastogel was determined and the plastogel was applied to the withdrawal of 1308 working face of Shandong Longyun Coal Industry Co., Ltd., to test the fire prevention and control effect of the material.
Preparation of component A: Bentonite was slowly added into the water glass and then stirred thoroughly to ensure that the bentonite particles were uniformly dispersed in the solution. Preparation of component B: A certain amount of coagulant was completely dissolved into water under stirring. The polymer plasticizer was added into the solution and stirred to obtain a uniform mixture. Components A and B were mixed with the same weight and the mixture was stirred for 5 s at 500 rpm. The mixture was kept standing until gel formation. The preparation of plastogel is shown in Fig. 1 and the specific formulations are shown in Tables 2–4. 2.3. Performance test of plastogel 2.3.1. Gelation time The components of A and B were mixed and the mixture was thoroughly stirred for 5 s. After the stirring ended, the moment was recorded as T1 and time was recorded thereafter. The mixture was slightly shaken every 2 s and flow state of the slurry was observed until the solidification of slurry. The moment when solidification was reached was recorded as T2 and the gelation time was defined as T = T2-T1. 2.3.2. Water retention of plastogel The gel samples were placed in petri dishes and heated in a vacuum oven at 100 °C. The gel samples were taken out of the vacuum oven every 1 h and weighed and then heated until the mass no longer changed. The water retention of the gel under heating is defined as w = Mt/M, where: w (%) denotes the water retention of plastogel, M denotes the initial mass (g) of the plastogel before heating, and Mt denotes the mass (g) of the plastogel at the heating time t. 2.3.3. FTIR spectra of plastogel The plastogel and coal powder (passing through 100 mesh sieve) were mixed in a mass ratio of 2:1 and the mixture was stirred to make it uniform. 2 g of pure coal powder and 4 g of plastogel were taken as control samples. The mixture and control samples were heated for 1 h in drying oven at 50 °C, 100 °C, and 150 °C. The scanning tests were performed using an infrared spectroscopy analyzer (Nicolet 380 FT-IR) in the range of 500–4000 cm−1.
2. Materials and methods 2.1. Materials The raw materials used in this experiment include water glass with Baume degree of 40°, modulus of 3.3, and sodium silicate content of 34% (Qingdao Dongyue Soka Alkali Co., Ltd.); bentonite with MW = 360.31 (Wokai Chemical Technology Co., Ltd.); coagulant with content ≥ 99.5%, pH value (50 g/L, 25 °C) ≤ 8.6, and water insoluble matter ≤ 0.01% (Analytically pure, Tianjin Guangfu Technology Development Co., Ltd.;); polymer thickening agent with MW = 5 million, (Analytically pure, Tianjin Kaitong Chemical Reagent Co., Ltd.). The coal sample was procured from Longyan Coal Mine, Heze City, Shandong Province. Coal powder was obtained by sieving crushed coal over 100 mesh. The industrial analysis data of pulverized coal is shown in Table 1.
2.3.4. SEM of plastogel The conventional water glass gel and the new plastogel were prepared. After standing for 24 h, the gels were cut into slices with a thickness of about 5 mm and then the slices were freeze-dried for 24 h to remove the moisture. The microscopic morphology of gel slice was observed using scanning electron microscope (SEM, FEI APREO) under an acceleration voltage of 2.00 kV.
2.2. Preparation of plastogel
2.3.5. Viscoelasticity measurement of plastogel The conventional water glass gel and plastogel were both cut into slices (radius was 10 mm and thickness was about 3 mm). The slice was tested on a rotary rheometer (Anton paar MCR 301). First, a strain amplitude scan (0.01–100%) was performed for determining the linear viscoelastic region and then the gel’s loss modulus (G″) and storage modulus (G′) were tested within the angular velocity range of 0.01 to 10 Hz at a dynamic frequency of 1.0% strain amplitude. The loss tangent (G″/G′) as a function of angular velocity was calculated.
The plastogel was prepared through mixing and stirring two components: A and B. The specific process was as follows: Table 1 Industrial analysis data of pulverized coal. Mad (%)
Aad (%)
Vad (%)
FCad (%)
2.07
12.45
26.95
58.53
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Fig. 1. Preparation process of plastogel. Table 2 Formulations of plastogels with different amounts of bentonite. Group
Ab Bb Cb Db Eb Fb
Component A
Component B
Water(g)
Water glass(g)
Bentonite(g)
Coagulant(g)
Plasticizer(g)
Water(g)
40.0 38.0 36.0 34.0 32.0 30.0
10.0 10.0 10.0 10.0 10.0 10.0
0 2.0 4.0 6.0 8.0 10.0
7.3 7.3 7.3 7.3 7.3 7.3
0.7 0.7 0.7 0.7 0.7 0.7
42.0 42.0 42.0 42.0 42.0 42.0
Table 3 Formulations of plastogels with different amounts of coagulant. Group
Ab Bb Cb Db Eb
Component A
Component B
Water(g)
Water glass(g)
Bentonite(g)
Coagulant(g)
Plasticizer(g)
Water(g)
36.0 36.0 36.0 36.0 36.0
10.0 10.0 10.0 10.0 10.0
4.0 4.0 4.0 4.0 4.0
3.3 5.3 7.3 9.3 11.3
0.7 0.7 0.7 0.7 0.7
46.0 44.0 42.0 40.0 38.0
Table 4 Formulations of plastogels with different amounts of polymer plasticizer. Group
Ab Bb Cb Db Eb
Component A
Component B
Water(g)
Water glass(g)
Bentonite(g)
Coagulant(g)
Plasticizer(g)
Water(g)
36.0 36.0 36.0 36.0 36.0
10.0 10.0 10.0 10.0 10.0
4.0 4.0 4.0 4.0 4.0
7.3 7.3 7.3 7.3 7.3
0 0.2 0.4 0.7 1.4
42.7 42.5 42.3 42.0 41.3
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Fig. 2. Gelation mechanism of plastogel.
3. Results and discussion
forming reaction of plastogel. However, it has the property of adsorbing water molecules, cations, and organic molecules [36], and can fill the network structure in the state of dispersed particles, which enhances the water retention capacity and the strength of the plastogel to some extent. The polymer plasticizer is a polar molecule with excellent water absorption properties [36]. The polymer plasticizer can weakly combine with the water glass gel via hydrogen bond or van der Waals force [37], thereby enhancing the water retention and bond strength of plastogel. An inorganic/organic interpenetrating network structure is finally formed under the joint action of coagulant and polymer plasticizer (Fig. 2).
3.1. Gelation mechanism of plastogel The pH value of the water glass solution decreases continuously under the action of coagulant. H2SiO42− and H3SiO4− that are originally free in the solution gradually combine with H+ to form ortho-silicic acid. Then the formed acid reacts with H3SiO4- to form a dimer of silicic acid through oxygen-combining reaction. Afterwards, the dimer of silicic acid continues to react with H3SiO4- to form a multimer of silicic acid such as trimer or tetramer of silicic acid. And finally, the silicic acid polymer forms an inorganic network structure in the solution. As an aggregate, bentonite does not directly participate in the
Fig. 3. Effect of three factors on the gelation time of plastogel (a) bentonite, (b) coagulant, and (c) polymer plasticizer.
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Fig. 4. Effect of three factors on the water retention of plastogel (a) bentonite, (b) coagulant, and (c) polymer plasticizer.
3.2. Characterization of plastogel 3.2.1. Effect of bentonite, coagulant, and polymer plasticizer on the gelation time of plastogel Fig. 3a illustrates that as the content of bentonite increases, the gelation time of the plastogel first decreases and then increases. This is because in the mixed solution with same mass, a higher content of bentonite means a less water content. Hence, the relative concentration of coagulant is higher, leading to higher gelation rate. When the bentonite content exceeds out of a certain range, it occupies larger space in the solution and inhibits the weak linkage reaction between polymer plasticizer and silicic acid, thereby extending the gelation time of the system. As the coagulant content increases, the gelation time of the plastogel showed a downward trend (Fig. 3b). This is because as the coagulant concentration increased, the solution’s pH value rapidly decreased. As a result, the association reaction between free H2SiO42−/H3SiO4 in the solution and H+ gradually accelerated, resulting in a shortening of the gelation time. As shown in Fig. 3c, the gelation time of plastogel decreased as the content of polymer plasticizer increased. The polymer plasticizer weakly combined the water glass gel through van der Waals force or hydrogen bonding, and this effect was more pronounced with the increase in the content of polymer plasticizer [37], with a macroscopic manifestation of increasing gel viscosity. Meanwhile, when the content of polymer plasticizer increases, it adsorbs more water from the solution so that the concentration of silicic acid in the system increases, leading to a fast polymerization between the molecules of silicic acid. 3.2.2. Effect of bentonite, coagulant, and polymer plasticizer on the water retention at higher temperature As shown in Fig. 4a, the plastogel with 4% bentonite has the best water retention performance. This is because bentonite is an aluminosilicate with a layered structure. The weak cationic electrostatic attraction between the layers leads to the exchangeability of interlayer cations and is expandability in polar medium so that the bentonite can adsorb water [36]. As the content of bentonite increases, the ratio between the bentonite and polymer plasticizer gradually reaches an optimum value. At this moment, the bentonite has a high degree of swelling in water and a good water retention. As bentonite content further increases, the ratio between the bentonite and polymer plasticizer becomes larger, which weakens the binding force between the bentonite particles and the polymer plasticizer. As a result, the integrity of the network structure is worsened, the bentonite particles easily fall off, and the water retention gradually reduces [38]. Fig. 4b shows the water retention of the plastogels prepared using different types of coagulants. The variation in the coagulant concentration has no major effect on the water retention properties of the material. This is because the coagulant only affects the gelation time of plastogel and has no major effect on the physicochemical structure of the gel.
Fig. 5. FTIR spectra of samples (a. FTIR spectra of coal powder at different temperatures; b. FTIR spectra of coal powder, gel, and gel + coal powder at 100℃; c. FTIR spectra of gel + coal powder at different temperatures). 5
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As shown in Fig. 4c, the plastogel with 0.4% polymeric plasticizer has an optimum water retention. The polymer plasticizer is a polar molecule and the hydrophilic group of the polymer chain can restrain water molecules through hydrogen bonding and van der Waals force; thus, it has a strong water retention performance. Within a certain range, an increase in the content of polymer plasticizer increases the water retention of the plastogel. However, an excessive increase in polymer plasticizer content can cause: 1) Agglomeration of the polymer molecules; 2) increase in the number of carboxyl groups of the plasticizer and its repulsion with the negative charge group of the bentonite becomes more pronounced [39], which weakens the encapsulation of the plasticizer over the bentonite, so that the moisture between the bentonite layers is easily lost, and hence, the water retention of plastogel reduces. 3.2.3. FTIR analysis of plastogel The reactions between oxygen molecules and functional groups are the internal cause leading to heat release and the corresponding coal spontaneous combustion [40]. The methylene group (wavenumber of 2800–3000 cm−1) is the main functional group in the coal powder. When the temperature was raised from 50 °C to 150 °C, the methylene content of the pure coal powder gradually decreased (Fig. 5a) as the methylene groups are involved in the reaction and get oxidized at higher temperatures. After the plastogel was incorporated into the coal powder, the methylene content pf the plastogel + coal powder was higher than that of the pure coal powder under the heating conditions of 100 °C (Fig. 5b). The reason for the phenomenon is that the plastogel contains a large amount of water which exerts a cooling effect on the coal powder. When the coal powder is protected by the plastogel, it is less likely to be oxidized so that part of the methylene groups of coal powder can be retained. In addition, the FTIR spectrum of pure gel sample (see Fig. 5b) does not contain the peak of the methylene group, indicating that the methylene group contained in the plastogel + coal powder sample results from the coal powder itself. As shown in Fig. 5c, the methylene group in the plastogel + coal powder sample does not alter significantly with the increase in the ambient temperature, which indicates that the plastogel has a relatively stable ability to inhibit the oxidation of coal powder.
Fig. 7. Comparison of viscoelasticity between the two plastogels.
3.2.5. Viscoelastic behavior of plastogel The loss tangent (G″/G′) is representative of the relative extent of gel’s viscosity and elasticity [41]. The results indicate that the loss tangent of the plastogel is much higher than that of the conventional water glass gel within the strain frequency range of 0.01–10 Hz (Fig. 7), which is due to the weak linkage between the polymer plasticizer and water glass gel through hydrogen bond or van der Waals force after the incorporation of polymer plasticizer [37]. The linkage enhances the toughness of the plastogel and intensifies the adhesion of plastogel between the coal layers, which is beneficial to improve the coolingplugging effect of plastogel on the broken coal body. 3.3. On-site application and fire control effect of plastogel 3.3.1. Outline of working face Yuncheng Coal Mine is located in Yuncheng County, Heze City, Shandong Province. The elevation of 1308 working face is −971.3 ~ −863.3 m. The strike length is 970 m and the face length is 123.272 m; hence, the area is 119573.84 m2. The name of coal seam is #3 coal with a total thickness of 6.85–7.65 m and a coal type of 1/3 coking coal. It was identified that #3 coal belongs to the spontaneous combustion coal seam with the spontaneous combustion propensity grade of class II and the natural ignition period of 57 days. As per the working requirements, it was known that1308 working face would be withdrawn in Jul-Aug 2018. Due to the long period of withdrawal process, the probability of spontaneous combustion is higher. In order to prevent and control the coal’s spontaneous combustion, the plastogel
3.2.4. SEM analysis Fig. 6a shows that the surface of conventional water glass gel is rough, uneven, loose in structure, and poor in compactness. These observations are consistent with the characteristics of conventional water glass gel: a poor water retention and is easily pulvlised. As shown in Fig. 6b, the plastogel has a compact and relatively smooth surface structure without clear large pores, which is beneficial for enhancing the water retention and strength of the plastogel.
Fig. 6. Microstructure of two plastogels (a. conventional water glass gel, b. plastogel).
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was applied during the withdrawal process of the working face.
ends of working surface. Each hole between the remaining racks was injected with 3 m3 of plastogel. After a round of general injection through the drilling hole, the key drill hole was refilled and the injected amount of plastogel was determined according to the actual situation on site.
3.3.2. Layout of drill holes As the shallow coal body of the working face has a large contact area with the outside, the crushed coal is easily oxidized and spontaneously ignited. Therefore, the coal body between the underground racks was drilled in advance and then the plastogel was injected into the coal body through the drilling holes. There were a total of 127 supporting racks on the working face and each rack was equipped with 3 drill holes (Fig. 8): the front drill hole, in-rack drill hole, and rear drill hole. The front drill hole was to float coal from the rack’s front beam to the rack’s rear part. The inclination angle of the opening was 30-45° and the depth was 8 ~ 10 m. The in-rack drill hole was to float the coal from the middle part of the top beam to the upper part of the top beam and the rack’s rear part. The inclination angle of the hole was about 30° and the depth was 6–8 m. The rear drill hole was used to operate from the rear transportation road to the rear tail beam and the coal inlet. The inclination of the opening was 30-45° and the depth was 3–6 m. The casing pipe was embedded in each type of the borehole with the casing diameter of 1 in. and each section length of 1.5 m. The casing’s end was a pipe thread and the casing was connected by a pipe clamp. The first section of the casing in drill hole was a floral tube that was processed from the casing, i.e., a Φ15mm hole was drilled every 10 cm on the casing and evenly distributed along the circumferential direction of the casing. Also, 8 holes were drilled at both ends of the working face; hence, a sum of 397 holes were drilled on the working face.
3.3.4. Monitoring methods for fire prevention and control effect Four monitoring points were set at the front edge of face guard, forepoling bar, shoulder points, and rear trail of each rack. The temperature and CO concentration around all monitoring points were monitored every day via the sensors. After injecting the plastogel into the goaf during the rack-withdrawal procedure, three representative monitoring points were selected to analyze the variations in CO concentration and temperature. The selection criteria for sampling points were as follows: (1) As the return air angle is located at the edge of the return air side and the goaf area, the area is not well ventilated and the temperature is high; hence, the bracket near the return air corner was preferentially selected; (2) Priority was given to select the rear tail beam points of the brackets closest to the goaf among the four monitoring points (the front edge of face guard, forepoling bar, shoulder points, and rear trail of each rack) of the bracket. 3.3.5. CO concentration variation around the coal body during the injection of plastogel During the injection of plastogel, the CO concentration within the goaf region of working face was basically maintained below 10 ppm (Fig. 10), indicating that the plastogel had better capability of plugging air leak and a decreasing temperature, thereby, effectively inhibiting the oxidation of coal. As shown in the figure, the CO concentration increased from Aug.11th to 12th (highest concentration was 16 ppm) as the failure of the injection system occurred during this period. The fourday suspension of the injection eliminated the plugging effect of the plastogel on the clefts of coal body. Therefore, a large amount of external gas penetrated into the clefts, which initiated the oxidation of coal body and increased the CO concentration. After the injection was resumed on Aug.13th, the CO concentration at the monitoring points was decreased and returned to the normal level (below 10 ppm). The variation proves that the plastogel had a good fire prevention effect during the withdrawal process of the working face.
3.3.3. Injection of plastogel The component A of the plastogel was prepared on the ground, i.e., the bentonite and water glass were mixed in an optimal ratio in a grouting tank (9 m × 0.94 m × 1.1 m); the slurry formed was stirred with a stirrer to reach a uniformly mixed state (Fig. 9a). Component B was prepared underground. Specifically, the coagulant and polymeric plasticizer were dissolved in the water and stirred uniformly with a pneumatic stirrer (Fig. 9b). Component A in the uphole grouting tank was transported into the well through a pipe and then thoroughly mixed with the component B using a grouting pump. Subsequently, through the pre-punched hole, the mixture was injected onto the crushed coal body on the top of the working face. The grouting pump and its parameters are shown in Fig. 9c and Table 5. 5 m3 of plastogel was injected into each hole between each 20 racks of track and transport
Fig. 8. Design chart of fire prevention drill hole (A. drill hole in front of rack; B. drill hole between racks; C. drill hole behind rack).
Fig. 9. On-site application chart of plastogel (a. diagrammatic sketch of component A on the ground; b. diagrammatic sketch of underground component B; c. grouting pump). 7
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Table 5 Specific parameters of pneumatic grouting pump. Model Number
Gas consumption (m3/ min)
Air supply pressure (Mpa)
Rated pressure/maximumpressure (Mpa)
Rated flowrate/Maximum flowrate (L/min)
External dimension (m)
Ratio of two slurry
2ZBQ 70/7
1
0.3–0.63
7/9
70/85
1.3*0.6*0.7
1: 1
4. Conclusions For prevention of the spontaneous combustion of coal, we developed a novel type of plastogel through the incorporation of water glass, bentonite, coagulant, and polymer plasticizer. The gelation time, water retention, infrared spectrum, and viscoelastic behavior of the plastogel were comparatively analyzed. The plastogel was applied to the fire prevention and control in the withdrawal procedure of the integrated working face and the following results were obtained: (1) When the contents of coagulant and polymer plasticizer increased, the gelation time became shorter. The gel retention performance had little relationship with the content of coagulant. When 4% bentonite and 0.4% polymer plasticizer were added, the plastogel had better water retention. (2) Plastogel effectively inhibited the oxidation of methylene groups in coal powder. Even at 100 °C, the plastogel had a relatively stable capability to inhibit the oxidation of coal powder. Compared with the conventional water glass gel, the plastogel had better viscoelasticity, which is beneficial for enhancing the adhesion of plastogel on the surface of the coal body and strengthening the protection of plastogel on the coal body. (3) The plastogel had a decent practical effect in the withdrawal procedure of the 1308 working face of Shandong Longyun Coal Industry as it effectively inhibited the oxidation of crushed coal body occurring at the rack roof and after racks. The CO concentration at the monitoring points between the racks stabilized below 10 ppm and the temperature was controlled at 27–29 °C, which ensured a safe rack-withdrawal procedure of the working surface.
Fig. 10. Variation in CO concentration at sampling points on the working face.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
Fig. 11. Temperature variation at sampling points on the working surface.
We thank funding and supporting on the study from the National Key Technologies R&D Program of China (2018YFC0807900); the National Natural Science Foundation of China (51674038, 51874193); the Shandong Province Natural Science Foundation (ZR2018JL019, ZR2017PEE024); the Shandong Province Science and Technology Development Plan (2017GSF220003); Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2017RCJJ010, 2017RCJJ037); Shandong Province First-Class Subject Funding Project (01AQ05202).
3.3.6. Variation in coal body temperature during injection of plastogel Fig. 11 shows the variation in the coal body temperature at three monitoring points after injecting molding gel during the withdrawal procedure of the working face. During the injection of the plastogel, the coal body temperature at each monitoring point on the 1308 working face exhibited an overall decreasing trend, with the lowest value of 27 °C. There are two reasons for the reduction in temperature: 1) the plastogel provided a large amount of water on the coal surface which reduced the adsorption of oxygen on the coal’s surface; 2) the water itself has a high thermal conductivity, which accelerated the coal’s heat dissipation and inhibited the temperature rise caused by oxidation of coal, to a certain extent. Similarly, after the failure of the grouting system on Aug 9th (the injection of plastogel was stopped), the temperature of coal body increased to a maximum value of 34.2 °C. After the injection of plastogel was restored, the temperature of the coal body gradually decreased.
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Full Length Article
A novel fire prevention and control plastogel to inhibit spontaneous combustion of coal: Its characteristics and engineering applications Yi-Jin Fana, Yan-Yun Zhaoa, Xiang-Ming Hub, , Ming-Yue Wub, Di Xueb ⁎
a
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China Key Lab of Mine Disaster Prevention and Control, College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China
b
ARTICLE INFO
ABSTRACT
Keywords: Coal body spontaneous combustion Plastogel Retarder for coal body oxidation Fire prevention in working face withdrawal On-site application
In order to address the issue of spontaneous combustion in coal mines during the mining process, we have proposed a novel plastogel which can prevent and control fires in coal mines. The novel plastogel was prepared by introducing a polymer plasticizer into the conventional water glass gel (water glass/coagulant/bentonite system). The effect of different concentrations of components on the properties of plastogel was analyzed by testing the gelation time and water retention of the plastogel. The Infrared spectroscopy results indicate that plastogel can effectively inhibit the oxidation of methylene in the coal body. The SEM results illustrated that the surface morphology of the plastogel and conventional water glass gels were significantly different. The compactness and smoothness of the plastogel were significantly enhanced after the addition of plasticizer. The results of the on-site application of plastogel in the withdrawal of 1308 working face of Shandong Longyun Coal Industry Co., Ltd. suggest that the plastogel effectively inhibited the oxidation of the crushed coal body, significantly reduced the temperature of coal body, and ensured a safe withdrawal of rack on the working face. The formulated plastogel had distinct practical advantages and is proposed as an ideal fire prevention and control material.
1. Introduction As a vital global energy source, coal has been used in various industries such as steel and electric power. According to the data of BP World Energy Statistical Yearbook in 2019, the world’s coal production in 2018 was 3.917 billion tons with a yearly increase of 4.3%. However, owing to an increasing coal production, the risk of coal fire, gas accidents [1–3], and coal dust pollution [4–11] is becoming more serious in the countries with higher coal productions such as China, India, the United States, and South Africa, especially coal fire in the mining process. In China, underground mining is the primary form of coal mining. As the mining depth increases (the depth of several mines in the east China can reach up to 800–1000 m), the ground temperature also increases (some coal mining faces can reach 38–40 °C), leading to a much higher probability of spontaneous combustion. Throughout the world, the spontaneous combustion of coal has become one of the most
⁎
critical natural disasters that can affect the safe production in coal mines. The spontaneous combustion of coal, if not eradicated immediately, may lead to coal ignition and even a fullblown fire [12,13]. Effective prevention and control of spontaneous fire at underground goafs are critical to ensure coalmine safety [14]. Once spontaneous combustion occurs, it causes a series of disasters such as casualties, property losses, production interruptions, and equipment damage [15], with the simultaneous destruction of ecological environment. Coal spontaneous combustion results from a complex reaction between coal and oxygen [16]. Even when the oxygen content is as low as 3%, the oxidation reaction of coal still exists, liberating CO and heat at the same time [17]. In view of the spontaneous combustion, the presently applied fire prevention and control technologies include grouting into the goaf [18–23], injection of three-phase foam [24–26], inert gas injection [27–31], and injection of colloidal material [32–33]. The above technologies effectively guarantee safe production in a coal mine to a cer-
Corresponding author. E-mail address: [email protected] (X.-M. Hu).
https://doi.org/10.1016/j.fuel.2019.116693 Received 4 March 2019; Received in revised form 18 October 2019; Accepted 18 November 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Yi-Jin Fan, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116693
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tain extent, but there are still some shortcomings. The slurry and water are easy to separate during the grouting process [34], which can affect the coal’s quality and the dried material is easy to crack. The threephase foam has a relatively poor stability, a high cost, and a complicated preparation process. The inert gases (CO2, N2) are easy to diffuse along with the air leakage. Therefore, it is urgently required to develop a fire prevention and control material with excellent performance. In recent years, gel-based materials have been widely used in several coal mines as they can rapidly reduce the temperature of the coal body, effectively plugging the air leak and efficiently retaining water. According to their properties and characteristics of fire prevention and control material, the current gel material can be divided into inorganic silicate gel and organic polymer gel. The inorganic silicate gels are primarily colloidal substances formed by mixing sodium silicate solution (water glass), loess (or fly ash, etc.), and coagulant. Such gels have good fluidity so they can quickly cover the coal body and effectively block the wind leak [35]. However, they are not tough and easily form powder [27]. On the other hand, the organic polymer gel material has a certain strength. It loses water slowly after being heated and has high fire extinguishing stability. However, it has higher cost, poorer fluidity, and smaller penetration range. The present study envisages to prepare a novel plastogel by combining the advantages of inorganic silicate gel and polymer gel. The plastogel is expected to have good water retention, viscoelasticity, and plugging ability against air leak, which can have great potential in the fire prevention and control applications in coal mines. In this paper, a novel type of plastogel was prepared by adding coagulant, polymer plasticizer, and bentonite into the water glass solution. The effects of different added amounts of bentonite, coagulant, and polymer plasticizer on the gelation time, water retention, and other characteristics were comparatively analyzed. The optimal formulation of plastogel was determined and the plastogel was applied to the withdrawal of 1308 working face of Shandong Longyun Coal Industry Co., Ltd., to test the fire prevention and control effect of the material.
Preparation of component A: Bentonite was slowly added into the water glass and then stirred thoroughly to ensure that the bentonite particles were uniformly dispersed in the solution. Preparation of component B: A certain amount of coagulant was completely dissolved into water under stirring. The polymer plasticizer was added into the solution and stirred to obtain a uniform mixture. Components A and B were mixed with the same weight and the mixture was stirred for 5 s at 500 rpm. The mixture was kept standing until gel formation. The preparation of plastogel is shown in Fig. 1 and the specific formulations are shown in Tables 2–4. 2.3. Performance test of plastogel 2.3.1. Gelation time The components of A and B were mixed and the mixture was thoroughly stirred for 5 s. After the stirring ended, the moment was recorded as T1 and time was recorded thereafter. The mixture was slightly shaken every 2 s and flow state of the slurry was observed until the solidification of slurry. The moment when solidification was reached was recorded as T2 and the gelation time was defined as T = T2-T1. 2.3.2. Water retention of plastogel The gel samples were placed in petri dishes and heated in a vacuum oven at 100 °C. The gel samples were taken out of the vacuum oven every 1 h and weighed and then heated until the mass no longer changed. The water retention of the gel under heating is defined as w = Mt/M, where: w (%) denotes the water retention of plastogel, M denotes the initial mass (g) of the plastogel before heating, and Mt denotes the mass (g) of the plastogel at the heating time t. 2.3.3. FTIR spectra of plastogel The plastogel and coal powder (passing through 100 mesh sieve) were mixed in a mass ratio of 2:1 and the mixture was stirred to make it uniform. 2 g of pure coal powder and 4 g of plastogel were taken as control samples. The mixture and control samples were heated for 1 h in drying oven at 50 °C, 100 °C, and 150 °C. The scanning tests were performed using an infrared spectroscopy analyzer (Nicolet 380 FT-IR) in the range of 500–4000 cm−1.
2. Materials and methods 2.1. Materials The raw materials used in this experiment include water glass with Baume degree of 40°, modulus of 3.3, and sodium silicate content of 34% (Qingdao Dongyue Soka Alkali Co., Ltd.); bentonite with MW = 360.31 (Wokai Chemical Technology Co., Ltd.); coagulant with content ≥ 99.5%, pH value (50 g/L, 25 °C) ≤ 8.6, and water insoluble matter ≤ 0.01% (Analytically pure, Tianjin Guangfu Technology Development Co., Ltd.;); polymer thickening agent with MW = 5 million, (Analytically pure, Tianjin Kaitong Chemical Reagent Co., Ltd.). The coal sample was procured from Longyan Coal Mine, Heze City, Shandong Province. Coal powder was obtained by sieving crushed coal over 100 mesh. The industrial analysis data of pulverized coal is shown in Table 1.
2.3.4. SEM of plastogel The conventional water glass gel and the new plastogel were prepared. After standing for 24 h, the gels were cut into slices with a thickness of about 5 mm and then the slices were freeze-dried for 24 h to remove the moisture. The microscopic morphology of gel slice was observed using scanning electron microscope (SEM, FEI APREO) under an acceleration voltage of 2.00 kV.
2.2. Preparation of plastogel
2.3.5. Viscoelasticity measurement of plastogel The conventional water glass gel and plastogel were both cut into slices (radius was 10 mm and thickness was about 3 mm). The slice was tested on a rotary rheometer (Anton paar MCR 301). First, a strain amplitude scan (0.01–100%) was performed for determining the linear viscoelastic region and then the gel’s loss modulus (G″) and storage modulus (G′) were tested within the angular velocity range of 0.01 to 10 Hz at a dynamic frequency of 1.0% strain amplitude. The loss tangent (G″/G′) as a function of angular velocity was calculated.
The plastogel was prepared through mixing and stirring two components: A and B. The specific process was as follows: Table 1 Industrial analysis data of pulverized coal. Mad (%)
Aad (%)
Vad (%)
FCad (%)
2.07
12.45
26.95
58.53
2
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Fig. 1. Preparation process of plastogel. Table 2 Formulations of plastogels with different amounts of bentonite. Group
Ab Bb Cb Db Eb Fb
Component A
Component B
Water(g)
Water glass(g)
Bentonite(g)
Coagulant(g)
Plasticizer(g)
Water(g)
40.0 38.0 36.0 34.0 32.0 30.0
10.0 10.0 10.0 10.0 10.0 10.0
0 2.0 4.0 6.0 8.0 10.0
7.3 7.3 7.3 7.3 7.3 7.3
0.7 0.7 0.7 0.7 0.7 0.7
42.0 42.0 42.0 42.0 42.0 42.0
Table 3 Formulations of plastogels with different amounts of coagulant. Group
Ab Bb Cb Db Eb
Component A
Component B
Water(g)
Water glass(g)
Bentonite(g)
Coagulant(g)
Plasticizer(g)
Water(g)
36.0 36.0 36.0 36.0 36.0
10.0 10.0 10.0 10.0 10.0
4.0 4.0 4.0 4.0 4.0
3.3 5.3 7.3 9.3 11.3
0.7 0.7 0.7 0.7 0.7
46.0 44.0 42.0 40.0 38.0
Table 4 Formulations of plastogels with different amounts of polymer plasticizer. Group
Ab Bb Cb Db Eb
Component A
Component B
Water(g)
Water glass(g)
Bentonite(g)
Coagulant(g)
Plasticizer(g)
Water(g)
36.0 36.0 36.0 36.0 36.0
10.0 10.0 10.0 10.0 10.0
4.0 4.0 4.0 4.0 4.0
7.3 7.3 7.3 7.3 7.3
0 0.2 0.4 0.7 1.4
42.7 42.5 42.3 42.0 41.3
3
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Fig. 2. Gelation mechanism of plastogel.
3. Results and discussion
forming reaction of plastogel. However, it has the property of adsorbing water molecules, cations, and organic molecules [36], and can fill the network structure in the state of dispersed particles, which enhances the water retention capacity and the strength of the plastogel to some extent. The polymer plasticizer is a polar molecule with excellent water absorption properties [36]. The polymer plasticizer can weakly combine with the water glass gel via hydrogen bond or van der Waals force [37], thereby enhancing the water retention and bond strength of plastogel. An inorganic/organic interpenetrating network structure is finally formed under the joint action of coagulant and polymer plasticizer (Fig. 2).
3.1. Gelation mechanism of plastogel The pH value of the water glass solution decreases continuously under the action of coagulant. H2SiO42− and H3SiO4− that are originally free in the solution gradually combine with H+ to form ortho-silicic acid. Then the formed acid reacts with H3SiO4- to form a dimer of silicic acid through oxygen-combining reaction. Afterwards, the dimer of silicic acid continues to react with H3SiO4- to form a multimer of silicic acid such as trimer or tetramer of silicic acid. And finally, the silicic acid polymer forms an inorganic network structure in the solution. As an aggregate, bentonite does not directly participate in the
Fig. 3. Effect of three factors on the gelation time of plastogel (a) bentonite, (b) coagulant, and (c) polymer plasticizer.
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Fig. 4. Effect of three factors on the water retention of plastogel (a) bentonite, (b) coagulant, and (c) polymer plasticizer.
3.2. Characterization of plastogel 3.2.1. Effect of bentonite, coagulant, and polymer plasticizer on the gelation time of plastogel Fig. 3a illustrates that as the content of bentonite increases, the gelation time of the plastogel first decreases and then increases. This is because in the mixed solution with same mass, a higher content of bentonite means a less water content. Hence, the relative concentration of coagulant is higher, leading to higher gelation rate. When the bentonite content exceeds out of a certain range, it occupies larger space in the solution and inhibits the weak linkage reaction between polymer plasticizer and silicic acid, thereby extending the gelation time of the system. As the coagulant content increases, the gelation time of the plastogel showed a downward trend (Fig. 3b). This is because as the coagulant concentration increased, the solution’s pH value rapidly decreased. As a result, the association reaction between free H2SiO42−/H3SiO4 in the solution and H+ gradually accelerated, resulting in a shortening of the gelation time. As shown in Fig. 3c, the gelation time of plastogel decreased as the content of polymer plasticizer increased. The polymer plasticizer weakly combined the water glass gel through van der Waals force or hydrogen bonding, and this effect was more pronounced with the increase in the content of polymer plasticizer [37], with a macroscopic manifestation of increasing gel viscosity. Meanwhile, when the content of polymer plasticizer increases, it adsorbs more water from the solution so that the concentration of silicic acid in the system increases, leading to a fast polymerization between the molecules of silicic acid. 3.2.2. Effect of bentonite, coagulant, and polymer plasticizer on the water retention at higher temperature As shown in Fig. 4a, the plastogel with 4% bentonite has the best water retention performance. This is because bentonite is an aluminosilicate with a layered structure. The weak cationic electrostatic attraction between the layers leads to the exchangeability of interlayer cations and is expandability in polar medium so that the bentonite can adsorb water [36]. As the content of bentonite increases, the ratio between the bentonite and polymer plasticizer gradually reaches an optimum value. At this moment, the bentonite has a high degree of swelling in water and a good water retention. As bentonite content further increases, the ratio between the bentonite and polymer plasticizer becomes larger, which weakens the binding force between the bentonite particles and the polymer plasticizer. As a result, the integrity of the network structure is worsened, the bentonite particles easily fall off, and the water retention gradually reduces [38]. Fig. 4b shows the water retention of the plastogels prepared using different types of coagulants. The variation in the coagulant concentration has no major effect on the water retention properties of the material. This is because the coagulant only affects the gelation time of plastogel and has no major effect on the physicochemical structure of the gel.
Fig. 5. FTIR spectra of samples (a. FTIR spectra of coal powder at different temperatures; b. FTIR spectra of coal powder, gel, and gel + coal powder at 100℃; c. FTIR spectra of gel + coal powder at different temperatures). 5
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As shown in Fig. 4c, the plastogel with 0.4% polymeric plasticizer has an optimum water retention. The polymer plasticizer is a polar molecule and the hydrophilic group of the polymer chain can restrain water molecules through hydrogen bonding and van der Waals force; thus, it has a strong water retention performance. Within a certain range, an increase in the content of polymer plasticizer increases the water retention of the plastogel. However, an excessive increase in polymer plasticizer content can cause: 1) Agglomeration of the polymer molecules; 2) increase in the number of carboxyl groups of the plasticizer and its repulsion with the negative charge group of the bentonite becomes more pronounced [39], which weakens the encapsulation of the plasticizer over the bentonite, so that the moisture between the bentonite layers is easily lost, and hence, the water retention of plastogel reduces. 3.2.3. FTIR analysis of plastogel The reactions between oxygen molecules and functional groups are the internal cause leading to heat release and the corresponding coal spontaneous combustion [40]. The methylene group (wavenumber of 2800–3000 cm−1) is the main functional group in the coal powder. When the temperature was raised from 50 °C to 150 °C, the methylene content of the pure coal powder gradually decreased (Fig. 5a) as the methylene groups are involved in the reaction and get oxidized at higher temperatures. After the plastogel was incorporated into the coal powder, the methylene content pf the plastogel + coal powder was higher than that of the pure coal powder under the heating conditions of 100 °C (Fig. 5b). The reason for the phenomenon is that the plastogel contains a large amount of water which exerts a cooling effect on the coal powder. When the coal powder is protected by the plastogel, it is less likely to be oxidized so that part of the methylene groups of coal powder can be retained. In addition, the FTIR spectrum of pure gel sample (see Fig. 5b) does not contain the peak of the methylene group, indicating that the methylene group contained in the plastogel + coal powder sample results from the coal powder itself. As shown in Fig. 5c, the methylene group in the plastogel + coal powder sample does not alter significantly with the increase in the ambient temperature, which indicates that the plastogel has a relatively stable ability to inhibit the oxidation of coal powder.
Fig. 7. Comparison of viscoelasticity between the two plastogels.
3.2.5. Viscoelastic behavior of plastogel The loss tangent (G″/G′) is representative of the relative extent of gel’s viscosity and elasticity [41]. The results indicate that the loss tangent of the plastogel is much higher than that of the conventional water glass gel within the strain frequency range of 0.01–10 Hz (Fig. 7), which is due to the weak linkage between the polymer plasticizer and water glass gel through hydrogen bond or van der Waals force after the incorporation of polymer plasticizer [37]. The linkage enhances the toughness of the plastogel and intensifies the adhesion of plastogel between the coal layers, which is beneficial to improve the coolingplugging effect of plastogel on the broken coal body. 3.3. On-site application and fire control effect of plastogel 3.3.1. Outline of working face Yuncheng Coal Mine is located in Yuncheng County, Heze City, Shandong Province. The elevation of 1308 working face is −971.3 ~ −863.3 m. The strike length is 970 m and the face length is 123.272 m; hence, the area is 119573.84 m2. The name of coal seam is #3 coal with a total thickness of 6.85–7.65 m and a coal type of 1/3 coking coal. It was identified that #3 coal belongs to the spontaneous combustion coal seam with the spontaneous combustion propensity grade of class II and the natural ignition period of 57 days. As per the working requirements, it was known that1308 working face would be withdrawn in Jul-Aug 2018. Due to the long period of withdrawal process, the probability of spontaneous combustion is higher. In order to prevent and control the coal’s spontaneous combustion, the plastogel
3.2.4. SEM analysis Fig. 6a shows that the surface of conventional water glass gel is rough, uneven, loose in structure, and poor in compactness. These observations are consistent with the characteristics of conventional water glass gel: a poor water retention and is easily pulvlised. As shown in Fig. 6b, the plastogel has a compact and relatively smooth surface structure without clear large pores, which is beneficial for enhancing the water retention and strength of the plastogel.
Fig. 6. Microstructure of two plastogels (a. conventional water glass gel, b. plastogel).
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was applied during the withdrawal process of the working face.
ends of working surface. Each hole between the remaining racks was injected with 3 m3 of plastogel. After a round of general injection through the drilling hole, the key drill hole was refilled and the injected amount of plastogel was determined according to the actual situation on site.
3.3.2. Layout of drill holes As the shallow coal body of the working face has a large contact area with the outside, the crushed coal is easily oxidized and spontaneously ignited. Therefore, the coal body between the underground racks was drilled in advance and then the plastogel was injected into the coal body through the drilling holes. There were a total of 127 supporting racks on the working face and each rack was equipped with 3 drill holes (Fig. 8): the front drill hole, in-rack drill hole, and rear drill hole. The front drill hole was to float coal from the rack’s front beam to the rack’s rear part. The inclination angle of the opening was 30-45° and the depth was 8 ~ 10 m. The in-rack drill hole was to float the coal from the middle part of the top beam to the upper part of the top beam and the rack’s rear part. The inclination angle of the hole was about 30° and the depth was 6–8 m. The rear drill hole was used to operate from the rear transportation road to the rear tail beam and the coal inlet. The inclination of the opening was 30-45° and the depth was 3–6 m. The casing pipe was embedded in each type of the borehole with the casing diameter of 1 in. and each section length of 1.5 m. The casing’s end was a pipe thread and the casing was connected by a pipe clamp. The first section of the casing in drill hole was a floral tube that was processed from the casing, i.e., a Φ15mm hole was drilled every 10 cm on the casing and evenly distributed along the circumferential direction of the casing. Also, 8 holes were drilled at both ends of the working face; hence, a sum of 397 holes were drilled on the working face.
3.3.4. Monitoring methods for fire prevention and control effect Four monitoring points were set at the front edge of face guard, forepoling bar, shoulder points, and rear trail of each rack. The temperature and CO concentration around all monitoring points were monitored every day via the sensors. After injecting the plastogel into the goaf during the rack-withdrawal procedure, three representative monitoring points were selected to analyze the variations in CO concentration and temperature. The selection criteria for sampling points were as follows: (1) As the return air angle is located at the edge of the return air side and the goaf area, the area is not well ventilated and the temperature is high; hence, the bracket near the return air corner was preferentially selected; (2) Priority was given to select the rear tail beam points of the brackets closest to the goaf among the four monitoring points (the front edge of face guard, forepoling bar, shoulder points, and rear trail of each rack) of the bracket. 3.3.5. CO concentration variation around the coal body during the injection of plastogel During the injection of plastogel, the CO concentration within the goaf region of working face was basically maintained below 10 ppm (Fig. 10), indicating that the plastogel had better capability of plugging air leak and a decreasing temperature, thereby, effectively inhibiting the oxidation of coal. As shown in the figure, the CO concentration increased from Aug.11th to 12th (highest concentration was 16 ppm) as the failure of the injection system occurred during this period. The fourday suspension of the injection eliminated the plugging effect of the plastogel on the clefts of coal body. Therefore, a large amount of external gas penetrated into the clefts, which initiated the oxidation of coal body and increased the CO concentration. After the injection was resumed on Aug.13th, the CO concentration at the monitoring points was decreased and returned to the normal level (below 10 ppm). The variation proves that the plastogel had a good fire prevention effect during the withdrawal process of the working face.
3.3.3. Injection of plastogel The component A of the plastogel was prepared on the ground, i.e., the bentonite and water glass were mixed in an optimal ratio in a grouting tank (9 m × 0.94 m × 1.1 m); the slurry formed was stirred with a stirrer to reach a uniformly mixed state (Fig. 9a). Component B was prepared underground. Specifically, the coagulant and polymeric plasticizer were dissolved in the water and stirred uniformly with a pneumatic stirrer (Fig. 9b). Component A in the uphole grouting tank was transported into the well through a pipe and then thoroughly mixed with the component B using a grouting pump. Subsequently, through the pre-punched hole, the mixture was injected onto the crushed coal body on the top of the working face. The grouting pump and its parameters are shown in Fig. 9c and Table 5. 5 m3 of plastogel was injected into each hole between each 20 racks of track and transport
Fig. 8. Design chart of fire prevention drill hole (A. drill hole in front of rack; B. drill hole between racks; C. drill hole behind rack).
Fig. 9. On-site application chart of plastogel (a. diagrammatic sketch of component A on the ground; b. diagrammatic sketch of underground component B; c. grouting pump). 7
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Table 5 Specific parameters of pneumatic grouting pump. Model Number
Gas consumption (m3/ min)
Air supply pressure (Mpa)
Rated pressure/maximumpressure (Mpa)
Rated flowrate/Maximum flowrate (L/min)
External dimension (m)
Ratio of two slurry
2ZBQ 70/7
1
0.3–0.63
7/9
70/85
1.3*0.6*0.7
1: 1
4. Conclusions For prevention of the spontaneous combustion of coal, we developed a novel type of plastogel through the incorporation of water glass, bentonite, coagulant, and polymer plasticizer. The gelation time, water retention, infrared spectrum, and viscoelastic behavior of the plastogel were comparatively analyzed. The plastogel was applied to the fire prevention and control in the withdrawal procedure of the integrated working face and the following results were obtained: (1) When the contents of coagulant and polymer plasticizer increased, the gelation time became shorter. The gel retention performance had little relationship with the content of coagulant. When 4% bentonite and 0.4% polymer plasticizer were added, the plastogel had better water retention. (2) Plastogel effectively inhibited the oxidation of methylene groups in coal powder. Even at 100 °C, the plastogel had a relatively stable capability to inhibit the oxidation of coal powder. Compared with the conventional water glass gel, the plastogel had better viscoelasticity, which is beneficial for enhancing the adhesion of plastogel on the surface of the coal body and strengthening the protection of plastogel on the coal body. (3) The plastogel had a decent practical effect in the withdrawal procedure of the 1308 working face of Shandong Longyun Coal Industry as it effectively inhibited the oxidation of crushed coal body occurring at the rack roof and after racks. The CO concentration at the monitoring points between the racks stabilized below 10 ppm and the temperature was controlled at 27–29 °C, which ensured a safe rack-withdrawal procedure of the working surface.
Fig. 10. Variation in CO concentration at sampling points on the working face.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
Fig. 11. Temperature variation at sampling points on the working surface.
We thank funding and supporting on the study from the National Key Technologies R&D Program of China (2018YFC0807900); the National Natural Science Foundation of China (51674038, 51874193); the Shandong Province Natural Science Foundation (ZR2018JL019, ZR2017PEE024); the Shandong Province Science and Technology Development Plan (2017GSF220003); Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2017RCJJ010, 2017RCJJ037); Shandong Province First-Class Subject Funding Project (01AQ05202).
3.3.6. Variation in coal body temperature during injection of plastogel Fig. 11 shows the variation in the coal body temperature at three monitoring points after injecting molding gel during the withdrawal procedure of the working face. During the injection of the plastogel, the coal body temperature at each monitoring point on the 1308 working face exhibited an overall decreasing trend, with the lowest value of 27 °C. There are two reasons for the reduction in temperature: 1) the plastogel provided a large amount of water on the coal surface which reduced the adsorption of oxygen on the coal’s surface; 2) the water itself has a high thermal conductivity, which accelerated the coal’s heat dissipation and inhibited the temperature rise caused by oxidation of coal, to a certain extent. Similarly, after the failure of the grouting system on Aug 9th (the injection of plastogel was stopped), the temperature of coal body increased to a maximum value of 34.2 °C. After the injection of plastogel was restored, the temperature of the coal body gradually decreased.
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