Research on the characteristics of montmorillonite microcapsule inhibitors for suppressing coal dust explosion

2.1.1. Experimental setup

This paper presents the development of a coal dust explosion suppression agent, Na-MMT@CS@PA-Na, based on independent research and tested using self-built explosion suppression system, as illustrated in Figure 1. The experimental apparatus comprises several core components, including a vertical combustion tube, powder injection system, ignition system, synchronous control unit, image acquisition system, sensors, and an oscilloscope.

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Figure 1.

Schematic diagram of experimental setup.

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The vertical combustion tube used in the experiment consists of a steel body, quartz glass, a top cover, and sealing screws, exhibiting excellent sealing performance. The external dimensions are 140 m×140 mm×500 mm, while the internal dimensions are 90 mm×90 mm×475 mm, with a volume of 3.8 L and a rated pressure resistance of 3.0 MPa. The front section of the tube is made of transparent, high-temperature-resistant quartz glass (pressure-resistant up to 2 MPa), measuring 60 mm×300 mm with a glass wall thickness of 280 mm. The powder spraying system, controlled by a controller, injects compressed air into the powder reservoir at 2 MPa to maintain constant pressure inside the explosion chamber after spraying. The ignition system uses electric pulse ignition, with a single electrode tube with a length of 42 mm and an electrode spacing of 6 mm. The ignition position is approximately 100 millimeters above the bottom of the combustion tube. It generates a 60J electric spark. The image acquisition system captures the explosion flame structure, with a high-speed camera set at 807 fps, operating for 4 s at a resolution of 2048×1536. Thermocouples and pressure sensors are mounted on the right side of the combustion tube. Temperature is measured using a Pt13% Rh-Pt precision thermocouple, 100μm in diameter, with a range of 0°C to 800°C. The PCB 113B21 pressure sensor features a range of 1379 kPa, sensitivity of ±15% (or 3.6 mV/kPa), and a weight of 6 g. The oscilloscope YOKOGAWA DL950 records temperature and pressure variations. The CH₁ and CH₃ modules were chosen for pressure and temperature measurement respectively, with a response time of 0.02ms, a sampling rate of 15kHz, and the trigger mode set to automatic triggering of the CH₃ temperature module at 25°C and the running time was 5 s. The trigger position was set at 30% of the screen. After each explosion experiment, samples of the residual particles in the pipeline were collected, and the experiment was repeated three times under each operating condition.

The experiments are categorized into two types: empty explosion tests and inhibitor-coal dust composite explosion tests. In the absence of any anti-explosion agent, coal dust explosion tests were conducted to measure parameters such as maximum explosion pressure, serving as a control group. Each group underwent 3-5 parallel trials, with the average value taken. Subsequently, coal dust was uniformly mixed with the anti-explosion agent, and explosion tests were conducted at different inserting ratios. The explosion characteristic curves were analyzed, the maximum explosion pressure was calculated, and the interaction mechanism between microcapsule anti-explosion agents and coal dust was explored.

2.1.2. Selection of experimental coal dust

The experimental coal samples used in this study were collected from Pingzhuang Coal Mine in Chifeng City, Zerim League, Inner Mongolia. This coal mine has an excellent geographical location, with coordinates of 42.069°N and 119.219°E. Pingzhuang Coal Mine is renowned for its abundant production of lignite, and its coal quality is unique, with a carbon content ranging from 60% to 77%, heat value approximately 23.0-27.2 megajoules per kilogram (equivalent to 5500-6500 kilocalories per kilogram), density about 1.1-1.2 g/cm₃, and volatile matter exceeding 40%. The coal samples are mostly brown or dark brown in color, with a relative density ranging from 1.2 to 1.45, making them ideal materials for studying the characteristics of coal dust explosions.

The particle size characteristics of coal dust were analyzed using SEM. The particle size distribution of the coal sample was measured using a Malvern Mastersizer 2000 in accordance with international standards, with detailed data illustrated in Figure 2. The experimental coal sample contained 90% of particles smaller than 47.38 μm, 50% smaller than 13.58 μm, and 10% smaller than 2.55 μm, satisfying the explosive conditions.

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Figure 2.

Particle size analysis of coal dust.

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2.3.1. Macroscopic morphology

SEM was used to obtain microscopic images of microcapsule explosion agents and their raw materials at magnifications of 500 times, 1000 times, and 5000 times. Figure 3(a) shows the SEM images of CS, Na-MMT, PA-Na, Na-MMT@CS, and Na-MMT@CS@PA-Na. The unmodified Na-MMT displays a typical loosely structured layered silicate, in which amorphous sheets overlap due to van der Waals forces and uneven surface charges, forming a porous network. The particle sizes vary greatly, and the thickness distribution is non-uniform. This cross-fiber structure results from electrostatic equilibrium between interlayer cations and the negative charges of silicon-oxygen tetrahedra, maintaining open interlayer channels and preventing particle aggregation.

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Figure 3.

(a) Surface morphology of CS, PA-Na, Na-MMT, Na-MMT@CS and Na-MMT@CS@PA-Na. (b) Elemental mapping images of Na-MMT@CS@PA-Na.

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CS exhibits the granular morphology of polysaccharide polymers, with nearly spherical individual particles. However, due to the hydrogen bonding network formed by surface hydroxyl groups, the particles tend to aggregate into chain-like or cluster-like structures. PA-Na exhibits a hierarchical porous structure with a dense surface and uneven particle sizes, often agglomerating into clusters. This loose and porous structure is attributed to the steric hindrance of phytic acid ions. Both CS and PA-Na clearly show agglomeration tendencies, providing morphological conditions for subsequent observations of adsorption and encapsulation on the Na-MMT surface.

Compared to unmodified Na-MMT, Na-MMT@CS fibers exhibit stronger interfacial bonding. Irregular CS particles adhere to Na-MMT, resulting in shortened and thickened rod-like structure and the formation of a surface film. Additionally, Na-MMT@CS@PA-Na particles show well-defined contours, rough surfaces, and uniform coverage with numerous aggregates. This indicates the formation of a new “skin” layer on the Na-MMT@CS surface, confirming the successful synthesis of the microcapsule structure.

Figure 3(b) shows the EDX elemental mapping results of the Na-MMT@CS@PA-Na microcapsule material. Six elements, C, O, Na, Mg, Al, Si, are clearly marked in different colors, with mass percentages of 14.84%, 29.75%, 0.63%, 0.43%, 2.1%, and 52.25%, respectively. As shown in Table 1, the C and O elements are widely and uniformly distributed, which aligns with the expected distribution pattern of organic components in CS and PA-Na. Unlike the uniformly distributed organic elements, inorganic elements such as Na, Mg, Al, and Si exhibit significant localized enrichment, which reflects the typical layered structure of Na-MMT.

2.3.2. Crystal structure analysis

The material’s crystal structure was analyzed using XRD. The XRD pattern of the sample is shown in Figure 4(a). The broad peak at 2θ=19.9° corresponds to the crystalline structure of the polymer CS [28,29]. For MMT, diffraction peaks at 2θ=6.7, 14.2, 19.7, 28.7, 34.7, 40.3, 43.4, 53.0, and 61.8 are consistent with the peak positions of natural MMT. The XRD pattern of MMT@CS shows that montmorillonite partially decomposes in the acidic CS solution. The intensity of its characteristic peaks slightly decreases, but the lattice structure remains unchanged, confirming a chemical reaction between MMT and CS. Under the influence of CS, the height and width of the peak at 2θ = 6.7° increase, indicating more crystal planes and stronger diffraction intensity. PA-Na shows no distinct XRD peaks, indicating its amorphous nature. Additionally, the characteristic peaks of MMT@CS@PA-Na remain unchanged, but its crystallinity is further reduced compared to MMT@CS. These results indicate that PA-Na chemically modifies MMT@CS, and the mild synthesis method minimally affects the MMT structure.

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Figure 4.

Analysis of the spectral profiles of a) XRD pattern, b) FT-IR pattern experimental materials.

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The functional groups of the materials were analyzed by FTIR, as shown in Figure 4(b). The FTIR spectrum of CS exhibits a prominent and broad characteristic band in the vicinity of 3700-3000 cm¹, which is associated with the stretching vibrations of O-H and N-H groups [30]. The characteristic bands located around 2925 cm¹ and 1434 cm¹ correspond to the symmetric and asymmetric stretching vibrations of C–H groups [31]. The bands at 1651 cm¹ and 1024 cm¹ are attributed to the stretching of C=O in amides and the asymmetric stretching of C-O-C, respectively [32,33]. Na-MMT shows a strong and broad characteristic band around 3750-3300 cm¹, which is related to the stretching vibration of O-H groups. The sharp absorption peak at 3624 cm¹ is due to the stretching vibration of the free hydroxyl group O-H, while the broad absorption peak at 3638 cm¹ is attributed to the stretching vibration of intermolecular hydrogen-bonded O-H. The peak at 1646 cm¹ is attributed to the stretching vibration of the C=C group, and the peak at 1032 cm¹ is attributed to the stretching vibration of the Si-O-Si tetrahedral sheet group.

In the FTIR spectrum of Na-MMT@CS, a noticeable reduction in band intensity is observed in the range of 3650-3000 cm¹, and the intensity of intermolecular hydrogen-bonded O-H at 3438 cm¹ is diminished, confirming the successful grafting of CS onto the surface of Na-MMT. The Na-MMT@CS@PA-Na spectrum reveals characteristic peaks of PA-Na, with the energy bands shifting from 1105 cm¹ (P=O) and 980 cm¹ (C-O) to 1100 cm¹ and 1020 cm¹, respectively. It demonstrates the electrostatic interaction between PO₃^-4 in PA-Na and NH³⁺ in CS, indicating that PA-Na can successfully bind to CS and subsequently encapsulate the CS surface. The results validate the spontaneous microencapsulation of Na-MMT through ionic cross-linking between CS and PA-Na, synthesizing the microcapsule material Na-MMT@CS@PA-Na, successfully.

2.3.3. Pyrolysis reaction

The thermal effects of Na-MMT@CS@PA-Na at high temperatures, a HITACHI STA200 synchronous thermal analyzer was used to study the decomposition behavior of the microcapsule Na-MMT@CS@PA-Na powder using TG and DSC. Figure 5 shows that the TG-DTG-DSC curves of Na-MMT@CS@PA-Na.

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Figure 5.

TG-DTG-DSC spectra of experimental materials.

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During the first stage, an endothermic peak appears within the temperature range of 23°C–112°C, with a peak temperature of 92°C and a corresponding specific heat capacity of 197 J/g. This stage is accompanied by an 11% mass loss, primarily attributed to the accelerated evaporation of adsorbed water on the powder surface and free water present within CS and PA-Na. The second stage, spanning 112°C to 437°C, involves a vigorous reaction with a 4% mass loss. During this phase, water molecules in the sample are substantially evaporated, and the -OH groups in PA-Na decompose, releasing H₂O and initiating carbonization. Concurrently, the organic modifier chitosan begins to decompose, corresponding to the random cleavage of glycosidic bonds. Additionally, water molecules bound to Mg²⁺ cations in the octahedral coordination of montmorillonite are released. The third stage, from 437°C to 580°C, results in a 2% mass loss, during which the phytic acid groups decompose, the structure of montmorillonite is disrupted, and carbon participates in the reaction, leading to combustion and mass reduction. In the fourth stage, between 580°C and 670°C, the sample exhibits a mass loss rate of 3%. A minor endothermic peak is observed at 635°C, signifying the dehydroxylation of Na-MMT crystals and the reaction of carbon elements in PA-Na with molten NaPO₃ to produce CO₂ and gaseous phosphorus [34,35]. These reactions absorb combustion heat, resulting in the endothermic peak at 635°C. The fifth stage, occurring beyond 670°C, is characterized by negligible mass change, corresponding to the complete dehydration and dehydroxylation of Na-MMT crystals. The formation of refractory and stable products, such as MgSiO₃ and SiO₂, indicates the conclusion of the reaction process.

Throughout the decomposition process of Na-MMT@CS@PA-Na, the total mass loss amounts to 20%. This powder is capable of releasing combustion and radiant heat during the initial stages of coal dust explosion, and the residual products formed after decomposition can create a barrier that impedes heat and mass transfer.