Microcapsule Emergency Response Technology for Gas and Fire Coupling Sudden Disaster

Abstract

Aiming at the complex conditions of the coexistence of the explosive gases in the coal mines and the risk of spontaneous coal combustion, the effect of encapsulation, oxygen barrier and different microcapsules on methane and long-chain alkanes has been studied. A non-toxic microcapsule comprising the anti-explosion fire-extinguishing polymeric material with neutral pH value, biodegradability and full solubility in water has been developed. The fire-extinguishing platform system has been used to test and analyze the fire-extinguishing effect, explosion suppression efficiency and package efficiency of the oil-pan fires and solid stacks. It is revealed that the microcapsule fire-extinguishing technology has a strong fire-extinguishing effect and can better inhibit the methane explosion, owing to its effective enveloping effect on methane, thus making it difficult to reignite. The developed technology is of theoretical significance and has a practical application value for studying the flame retardation and fire-extinguishing behavior of combustible substances.

1. Introduction

The mining depth of the coal mines has increased worldwide over the years. The gas content of the coal seams increases with the coal-mining depth. In addition, the high-intensity mining causes an increase in the absolute gas emission from the mines [1]. The contradiction between the gas control and fire prevention in the compound goaf of the high gas and extremely close coal seams has become significantly prominent [2]. Moreover, the fires in the open areas such as goafs and the large areas of loose coal in the underground coal mines are often accompanied by the generation of the explosive hydrocarbons, which make the fire difficult to prevent and extinguish. Such widespread fires have the characteristics of large amounts of broken coal, sufficient air leakage, the outstanding danger of gas explosion and the difficult management of the conventional measures, thus, posing a serious threat to personal safety during rescue [3,4]. Therefore, a polymer material with the function of wrapping and cocooning the explosive and dangerous hydrocarbon molecules in the underground coal mines has been developed herein. It is essential to develop the anti-explosion fire-extinguishing agents based on the micro-cell encapsulation technology for the underground coal mines in order to achieve the fire, gas, oil and gas symbiosis as well as other complex conditions [5]. Through the chemical encapsulation of gas, volatile oil vapor and other hydrocarbon gas molecules posing the danger of explosion, the risk of combustion and explosion is reduced. Subsequently, carrying out firefighting and rescue can control the spread of disasters in time and restrain the occurrence of secondary disasters, which is of high significance for improving comprehensive fire prevention and the control technology of the coal mines [6].

2. Theoretical Analysis of Microencapsulation Technology

(1)

The formation and maintenance of the Microcystis effect

The formation and maintenance of the Microcystis effect is the biggest difference between Microcystis and common water-extinguishing agents. Due to the special molecular structure of microcysts, microcysts can quickly form and be maintained in the medium after release so that their inclusion is isolated from the surrounding system, which in turn maintains the isolation of the surrounding environment, such as combustible or harmful substances and combustors.

The main active substance in the reagent belongs to a non-ionic surfactant. The non-ionic surfactant molecule has a chain molecular structure, and the two ends of the chain molecular structure are polarized (hydropHilic) and non-polarized (hydropHobic), respectively. Additionally, there is a long distance between the two molecular ends so that the two molecular ends can participate in the formation of microcysts independently. This special molecular structure makes the polarized end of the molecule soluble in water, while the non-polarized end repels the water molecule and seeks other types of molecules. Several molecules can be arranged around a group of hydrocarbon molecules, forming a negatively charged micelle “chemical cocoon”. As shown in Figure 1, the outer surfaces of these micelles are all negatively charged and repel each other. This disperses the hydrocarbon molecules in the water, leaving them too low to burn.

(2)

The capture and annihilation of free radicals and the termination of chain reactions

In the combustion reaction, free radicals are a kind of molecular activation group with high energy. In the chain reaction, free radicals collide with fuel molecules at high speed, releasing energy to promote the reaction and forming more free radicals, thus maintaining the combustion chain reaction. Because of the specific high molecular weight of the microcysts, they can absorb the energy of free radicals during the collision process and thus play a role in inhibiting the chain reaction. By annihilating free radicals, the transmission of the reaction chain in the combustion reaction is blocked so as to quickly extinguish the flame.

(3)

Microcapsule fire-extinguishing analysis

In terms of the fire-extinguishing mechanism, the traditional explosion suppression and fire-extinguishing technologies mostly use the evaporation of water to absorb heat or the formation and maintenance of a foam covering layer to cut off the oxygen supply and temporarily wrap the vapor to achieve the goal [7]. Once the microcapsule fire-extinguishing agent and water are mixed in a certain proportion, the surface tension of the water medium is reduced to achieve a greater surface wetting of the fuel, and the penetration of the combustible micropores is strengthened, which can swiftly reduce the temperature of the fuel [8]. Meanwhile, the microcells are formed and maintained around the liquid and gaseous fuel molecules, thus, making the fuel an inert material that cannot be burned. The specific high molecular weight of the microcapsule fire-extinguishing agent can absorb the energy of the free radicals during the collision and inhibits the chain reaction. Due to the unique principle of the neutralizing alkanes, re-ignition is not easy after the fire has been extinguished, which demonstrates significant advantages as compared with traditional fire-extinguishing agents such as foam [9]. The smoke and toxic gases are reduced while extinguishing the fire, thus, improving the visibility at the fire scene. The thick black smoke produced by the fire immediately turns into white water vapor under the action of the microcapsule fire-extinguishing agent, which is helpful for the rescuers to evaluate the situation [10].

3. Preparation of Microcapsule Formulation

Based on the theoretical analysis and requirements of the microencapsulation technology, the characteristics of the target microcapsule material need to meet the following three requirements:

(1)

The material is non-toxic and harmless, with neutral pH, environmental friendliness and biodegradability;

(2)

The material has strong water solubility;

(3)

The solution is clear and stable, with a long shelf life.

Based on these performance indicators, the raw materials must be non-toxic, harmless, environmentally friendly, mild, neutral and common chemical materials [11,12]. Herein, after a number of experimental screenings, four non-ionic surfactants have been selected as the main raw materials for the preparation of the microcapsules: Span-20 (S-20), Span-80 (S-80), Tween-80 (T-80) and Tween-85 (T-85). In addition to the four main surfactants, the corresponding additives are also added to the formulation in order to ensure the stability of the formulated agent [13]. The selection conditions for the additives are the same as those of the main surfactants, and the primary criteria for selection are non-toxicity, harmless nature and environmental friendliness. Four additives are added to the formulation through compound screening: ethylene glycol, ethyl oleate, polyethylene glycol 400 (PEG-400) and a trace amount of sodium dodecylbenzene sulfonate (LAS). The content of sodium dodecyl benzene sulfonate (LAS) added is 0.25 g/100 mL.

3.1. The Selection as Well as pHysical and Chemical Properties of Microcapsule Anti-Explosion Fire-Extinguishing Materials

The preparation of the microcapsule materials adopts the compounding method to mix the raw materials [14]. The compound raw material is composed of two parts: main surface-active agent and auxiliary agent. The main surfactants are Span-20 (S-20), Span-80 (S-80), Tween-80 (T-80) and Tween-85 (T-85). The additives include ethylene glycol, ethyl oleate, polyethylene glycol 400 (PEG-400) and trace sodium dodecylbenzene sulfonate (LAS).

Table 1. The content of each material added in the formulation.

Materials	Solvent	Main Agent				Additives
	Distilled Water	S-20	S-80	T-80	T-85	Ethylene Glycol	Ethyl Oleate	PEG-400
Mass ratio (%)	17	3	1.5	42.5	3	12	4	17

Note: The content of the trace additive sodium dodecylbenzene sulfonate (LAS) is 0.25 g/100 mL.

illustrates the content of each raw material added in the formulation. As the hydropHilic–lipopHilic balance (HLB) of the surfactant mixed compound system is additive, the HLB value of the mixed system can be calculated by the HLB value of each component and its corresponding mass fraction. The HLB value of the main surfactant in the formulation is shown in

Table 2. The HLB value of the main surfactants in the formulation.

Surfactant	Product Name
	S-20	S-80	T-80	T-85
HLB value	8.6	4.3	15	11

.

The HLB value of the compound system of the formulation can be calculated by the HLB value of the corresponding raw materials and mass score.

$$\mathrm{HLB}={\displaystyle \frac{{\mathrm{W}}_{\mathrm{A}}{·\mathrm{HLB}}_{\mathrm{A}}{+\mathrm{W}}_{\mathrm{B}}{·\mathrm{HLB}}_{\mathrm{B}}{+\mathrm{W}}_{\mathrm{A}}{·\mathrm{HLB}}_{\mathrm{A}}+\dots }{{\mathrm{W}}_{\mathrm{A}}{+\mathrm{W}}_{\mathrm{B}}{+\mathrm{W}}_{\mathrm{C} }+\dots }}$$

(1)

The HLB value of the formulation is determined to be 10.43, which represents an oil-in-water surfactant with the wetting and spreading effects.

3.2. Preparation of Microcapsule Anti-Explosion Fire-Extinguishing Material

Under a moderate stirring speed and 50 °C temperature, the raw materials are added to the container in a specific addition sequence, as shown in Figure 2.

For the laboratory configuration, a certain amount of distilled water is added to a 500 mL three-necked flask. The flask is subsequently placed in a water bath at 50 °C, and a small amount of LAS is added. The contents are stirred evenly until complete dissolution is achieved. Afterwards, Tween-85 (T-85), Span-20 (S-20), Tween-80 (T-80), Span-80 (S-80), Tween-80 (T-80), ethylene glycol, ethyl oleate and polyethylene glycol 400 (PEG-400) are added. Each subsequent feeding needs to ensure that the previous material is completely dissolved. The electromagnetic stirring head is inserted in the middle port of the three-necked flask. The stirring speed is maintained to be moderate (700 r/min) during the preparation process. The left port is placed with a thermometer, and the right port is used as the feed port. The duration of the entire preparation process is about 1 h.

3.3. Determination of pHysical and Chemical Parameters of Microcapsule Anti-Explosion Fire-Extinguishing Material

For the formulated material, the preliminary pHysical and chemical analysis have been carried out in the laboratory.

3.3.1. pH Measurement

Quantitative determination of the pH value of the formulation:

(1)

Equipment and reagents

Acidity meter: pH 0–14, precision pH 0.1; thermometer: 0–50 °C, division value 0.5 °C; pH reference reagent: in line with the relevant national standards.

(2)

Experiment procedure

(a)

Calibrate the pH meter with a pH reference reagent.

(b)

Take 30 mL stock solution at 20 ± 0.5 °C and pour it into a dry and clean 50 mL beaker. Immerse the electrode in the stock solution and measure the pH value.

(c)

Repeat the test and take the average of the two tests as the measurement result. The difference between the two test results shall not exceed 0.1 pH.

The pH of the formulation is determined to be 7.0 ± 0.5 by using this method.

3.3.2. Determination of Surface Tension

The accuracy of the surface tension meter: 0.1 mN/m.

Surface tension measurement.

The test steps are as follows:

Adjust the temperature of the original solution to 20 ± 2 °C and measure its surface tension. Repeat the test and take the average of the two tests as the measurement result.

3.3.3. Determination of Interfacial Tension

The test steps are as follows:

After measuring the surface tension, the platinum ring is dropped below the liquid level. A 5–7 mm thick layer of cyclohexane is added at 20 ± 2 °C on the solution to avoid the contact between the platinum ring of the instrument and cyclohexane. After equilibrating for 6 ± 1 min, measure the interfacial tension. Repeat the test and take the average of the two tests as the measurement result.

The surface tension value of the formulation by this method is determined to be 32 dyn/cm².

3.3.4. Determination of Stability

(1)

Equipment

Freezing chamber: meet the temperature requirements specified in 5.1.2a of GB15308-1994 standard [15]; electric-heating drying box; plastic barrel: volume 3 L, 30 L, colorless and transparent.

(2)

Experimental procedure

Adjust the temperature of the freezer compartment to 5 °C below the pour point of the sample.

Pour 2 L concentrated water-based fire-extinguishing agent into a 3 L plastic bucket and pour 20 L non-concentrated water-based fire-extinguishing agent or a mixture of the concentrated water-based fire-extinguishing agent into a 30 L plastic bucket. Place the samples in the freezer and maintain the temperature at −5 °C for 24 h before taking it out. Thaw at room temperature and leave it for a period of 24–96 h. Place the sample in an electric-drying oven at 40 °C ± 3 °C and retain the temperature for 24 ± 2 h. After taking it out, subsequently place it at room temperature. The time period should be between 24 and 96 h.

Repeat the process thrice to complete four freezing, melting and high-temperature tests. The sample needs to be observed for segregation. Based on this experiment, the stability of the microcapsule medicament is noted to be good.

3.3.5. Biodegradability

Biodegradation refers to the process in which the organic matter is metabolized by the organisms (microorganisms) under aerobic conditions, leading to a complete conversion into inorganic matter. The biodegradation process is very slow; thus, specific conditions and methods are usually used to evaluate the biodegradability of the organic matter. This standard adopts a five-day biochemical oxygen demand (BOD) and chemical oxygen demand (COD) correlation comparison method.

The method is as follows: prepare a specific concentration of the formulation (200 mg/L) and measure its BOD and COD, followed by the calculation of its biodegradability evaluation index. The BOD and COD data as well as the biodegradability of the formulation are obtained. After testing, the biodegradation rate of the formulation is determined to be 35%.

Overall, the performance parameters of the microcapsule material are summarized in

Table 3. The performance parameters of microcapsule anti-explosion fire-extinguishing materials.

Number	Name	Data
1	Exterior	Yellow transparent
2	Odor	Light fruity
3	Boiling point	105 °C
4	pH value	pH = 7.0 ± 0.5
5	Surface tension	32 dyn/cm2
6	Evaporation rate	Same as water
7	Proportion	1.102 (the specific gravity of water is 1)
8	Water soluble	Completely dissolved
9	Chemical stability	Stability
10	Flash point	None
11	Freezing and thawing hazards	None
12	Storage period	3 years (unopened)
13	Environment to avoid	Strong oxidizing environment
14	Storage temperature	−5 °C < T < 40 °C
15	Biodegradability BOD5/COD/(%)	35

.

The observed data reveal that the target microcapsule anti-explosion fire-extinguishing material configured based on the formulation meets the requirements of being non-toxic, harmless, environmentally friendly and biodegradable.

4. Microcapsule Performance Test

The technical index tests are carried out for exploring the efficacy of the microencapsulation of the formulated drugs [16]. The comprehensive experimental determination of the explosion suppression and oxygen barrier capabilities of the microcystic formulations against explosive hydrocarbon encapsulation, chemical cocoon coatings and oxygen barrier encapsulation have been conducted through the three performance measurement methodologies of fire extinguishing, explosion suppression and wrapping. Pure water is used as the reference material to perform a multi-concentration-level efficacy comparison test to comprehensively and accurately evaluate the microencapsulation level of the formulation.

4.1. Fire-Extinguishing Effectiveness Analysis

The fire-extinguishing effectiveness analysis is an important component used to assess the microencapsulation level [17]. In order to reduce the impact of the environmental factors on the test and to ensure the accuracy and reliability of the test results, the test process is carried out in strict accordance with the relevant national standards and technical requirements. The specific tests include a pool-fire comparison test, solid-fire suppression test and liquid hydrocarbon inertization test.

4.1.1. Fire-Extinguishing Effectiveness of Test Materials

The liquid and solid combustibles, water and corresponding fire-extinguishing additives are used in the fire-extinguishing effectiveness test. Among these, the liquid combustible includes gasoline, which is commonly used in the production, life and fire tests, whereas the solid combustibles used in the test are the common wood sticks. The pHysicochemical characteristic parameters of the test fuels are shown in

Table 4. The pHysicochemical characteristics of the combustibles.

Number	Name	State of Matter	Density (g/cm3)	Melting Point (°C)	Boiling Point (°C)	Flash Point (°C)	Burning Point (°C)	Explosion Range (v/v)
1	Gasoline	Liquid	0.73	−60	30–190	−50	415–530	1.3–6.0
2	Wood	Solid	0.60	-	-	-	200–290	-

. The fire-extinguishing agent uses pure water, formulation medicament and an advanced medicament (F-500), followed by the comparison of the fire-extinguishing behavior by using pure water and various concentration levels of the fire-extinguishing solutions (1%, 2%, 3%, 4% and 5%). The pHysical and chemical properties of the solution are presented in

Table 5. The pHysicochemical characteristics of the fire-extinguishing agent solution.

Number	Name	Content (%)	pH	Density (g/cm3)
1	Pure water	95–99%	7	1.00
2	Formula medicament	1%	7	1.00
3	Formula medicament	2%	7	1.00
4	Formula medicament	3%	7	1.00
5	Formula medicament	4%	7	1.00
6	Formula medicament	5%	7	1.00
7	International advanced pHarmacy (F-500)	5%	7	1.00

.

4.1.2. Fire-Extinguishing Efficiency of Test Platform

As per the test, the fire-extinguishing test platform is divided into pool fire-extinguishing and solid-stacking extinguishing platforms [18]. The pool fire-extinguishing platform includes an oil-pan combustion system, a low-pressure water mist system, a fire-extinguishing recording system, an image recording system and a computer post-processing system [19]. The schematic diagram of the test platform layout and the overview diagram of the test system are shown in Figure 3, respectively. Figure 4 also demonstrates an image of the pool fire-extinguishing platform test system.

In the pool fire-extinguishing test, after gasoline has passed the 60 s pre-ignition time, the low-pressure water-mist system is initiated to spray the extinguishing agent to the oil pan 1.5 m above the center of the oil pan. Starting from the pre-ignition, the fire-extinguishing recording and video-recording systems are activated at the same time until the flame is completely extinguished. The recording system records the changes in the flame shape during the entire process, as well as the initial flame intensification pHenomenon of the applied extinguishing agent, the extinguishing time and the suppression effect of the extinguishing agent on the flame and smoke during the extinguishing process, etc. The computer post-processing system uses the processing software to perform a statistical analysis on the flame image and extinguishing time during the process, leading to the quantitative evaluation of the entire pool fire-extinguishing process.

The solid stacking fire-extinguishing platform includes the stacking combustion, low-pressure water mist, fire-extinguishing recording, image recording, infrared temperature measurement and computer post-processing systems [20]. The layout diagram of the test platform and overview diagram of the test system are shown in Figure 5, respectively. Figure 6 also presents an image of the solid stacking fire-extinguishing platform test system.

In the solid stack fire-extinguishing test platform, after the solid combustible stack is ignited, it is pre-burned for a specific period of time to form a stable solid combustion body. Subsequently, the low-pressure water mist system is activated to spray the fire-extinguishing agent to the wooden slat stack to extinguish the fire. The fire-extinguishing process employs a fire-extinguishing recording system, an image recording system and an infrared temperature measurement system to simultaneously monitor the changes in the flame shape as well as the temperature of the solid combustibles during the fire-extinguishing process. The test data and flame images are analyzed quantitatively by using a computer software. It also conducts the statistical analysis of the time and temperature of the fire-extinguishing process, thus, enabling a quantitative evaluation of the fire-extinguishing behavior of the formulated agents.

4.1.3. Fire-Extinguishing Effectiveness

The fire-extinguishing test mainly employs water and various concentration ratios of the aqueous solutions of the formulation medicaments (1%, 2%, 3%, 4% and 5%), along with a 5% concentration of the international advanced medicament (F-500). It inhibits and extinguishes the gasoline fires during the combustion process. During the fire-extinguishing process, the test at each level is carried out thrice, and the average value is calculated.

Table 6. The pool fire-extinguishing schedule.

Number	Type of Fire-Extinguishing Agent	Content (%)	Fire-Extinguishing Time (s)			Average Fire-Extinguishing Time (s)
			Experiment 1	Experiment 2	Experiment 3
1	F-500	5	52	49	54	51.67
2	Formula medicament	1	26	28	23	25.67
3		2	24	27	22	24.33
4		3	21	25	22	22.67
5		4	17	15	19	17.00
6		5	14	16	13	14.33

shows the schedule for extinguishing the pool fires.

As can be seen from Figure 7, the gasoline pool fire has a strong flame-strengthening effect at the initial atomization stage of F-500 (5% concentration). Further, the flame shows a tendency to spread to the surroundings, and the process is accompanied by a strong “black smoke”. However, after using the formulation with 1% additive, the flame-strengthening effect is slightly reduced, and the amount of “black smoke” remains basically unchanged. On using 2% and 3% formulations, the flame is noted to be obviously weakened, and a large amount of “black smoke” is produced. On using 4% and 5% formulation agents to extinguish the fire, the fire-extinguishing effect is observed to be significantly improved. Further, the flame-strengthening effect is small, and the overall flame suppression effect is significant, thus, leading to a significant reduction in the amount of the “black smoke”. However, the pure water takes a long time to extinguish the gasoline pool fire, and the effect is non-optimal.

The solid combustible fire-extinguishing test mainly explores the inhibitory and extinguishing effects of water and 5% formulation on the burning process of the wooden sticks. The same experiment has been carried out thrice, and the average value has been reported.

Table 7. The extinguishing of the wood-stacking fire.

Number	Type of Fire-Extinguishing Agent	Content (%)	Fire-Extinguishing Experiment Time (s)			Average Fire-Extinguishing Time (s)
			1	2	3
1	Pure water	-	41	32	37	36.67
2	Formula medicament	5	11	17	13	13.67

presents the extinguishing of the stacking fire of the wooden slats. Further, Figure 8 illustrates the process of extinguishing the stacking fire of the wooden slats with pure water and formulation.

The 5% concentration level of the formulation has a strong extinguishing effect on the wood sticks stacking fire, and the duration of fire extinguishing is shortened by nearly half as compared with the pure water. The formula medicine can swiftly reduce the temperature of the burning material and extinguish the flame after the unstable flame suppression in the early stage.

The ignition test has been carried out on the pool fire test platform by spraying the liquid combustibles with 5% concentration each of F-500 and formula solution for 1 min, 2 min and 3 min. The ignition effect is used to determine the inerting effect of the formulation on the combustibles. Figure 9 demonstrates the flame combustion effect after spraying for 1 min, 2 min and 3 min.

As observed from Figure 8 and Figure 9, compared to the effect of spraying a 5% concentration of F-500, the combustion effect of the gasoline flame is significantly weakened after spraying a 5% concentration of the formulation solution. After spraying for 3 min, the combustion of gasoline can be effectively inhibited. The experiment shows that after spraying the formulation solution, the combustible molecules are absorbed and wrapped by the solution. Finally, the gasoline is inerted, and the combustion is inhibited. Comparing the flame burning effect in the figure, the flame intensity after ignition is noted to have achieved an optimal control effect, which is obviously superior to the fire-extinguishing effect of F-500.

4.2. Explosion Suppression Performance Test

In industrial and mining enterprises with complex geological conditions, the advanced gas explosion prevention technologies still cannot effectively prevent the occurrences of the gas explosion accidents. Once the instantaneous gas outburst occurs and the local gas concentration is too high, it is quite likely to result in a gas explosion accident after encountering an ignition source. Therefore, to control the gas explosion, it is necessary to study the explosion suppression and flameproof technology to reduce the scope of the gas explosion [21]. Therefore, the explosion suppression performance test for the gaseous short-chain alkanes, especially methane, is the core component to explore the performance of the micro-encapsulated materials. In order to ensure the accuracy and safety of the test results, the explosion suppression performance test was conducted in an independent large-space laboratory [22].

4.2.1. Test Materials for Determining Explosion Suppression Effectiveness

The experiment uses methane gas as the test material, and the methane concentration is controlled at 7%, 10% and 12% during the experiment. The aqueous solution of the formulation agent (1% concentration) and pure water are used as the explosion suppression media to carry out the explosion suppression performance test. The pHysicochemical parameters of the combustibles and explosion suppression media selected for the test are shown in

Table 8. The pHysicochemical parameters of methane.

Name	Density (g/cm3)	Melting Point (°C)	Boiling Point (°C)	Flash Point (°C)	Burning Point (°C)	Explosion Range (v/v)	Heat of Combustion (kJ/mol)
Methane	0.42	−182.5	−161.5	−188	538	5.3–15	890.31

and

Table 9. The pHysical and chemical parameters of the explosion suppression medium.

Number	Name	Content (%)	pH	Density (g/cm3)
1	Pure water	-	7	1.00
2	Formula medicament	1%	7	1.00

.

4.2.2. Explosion Suppression Performance Test Platform

The explosion suppression test experimental system is composed of an experimental pipeline, an explosion suppression agent release device, a data acquisition device and a computer post-processing system. The experimental system measures the propagation of the different concentrations of the methane gas after the explosion in the pipeline, including determining the changes in the speed, temperature and explosion pressure during the propagation process. The experimental pipeline is a circular steel pipeline, where one end is fixed with an ignition device in the center, and the other end is movably closed. The plastic pressure-relief film is used to seal the environment, and the top of the pipeline is equipped with an explosion-suppressant release device. The test process uses a pressure sensor, temperature sensor, flame speed sensor and data acquisition instrument for recording. The experimental system also includes a gas distribution device, a high-energy ignition device, a methane-gas-concentration-detection device, a flame speed sensor, a temperature sensor, a pressure sensor and a drainage pipe. The schematic diagrams of the explosion suppression test system are shown in Figure 10. Figure 11 also demonstrates an image of the explosion-suppression test system.

4.2.3. Explosion-Suppression-Effectiveness Test Results

It is noted that the formulations based on the microcystic technology have an optimal anti-explosion effect. After casting, the maximum explosion temperature, flame propagation speed, explosion pressure rise rate and maximum explosion pressure are observed to be reduced to a certain extent. The microcapsule material exhibits an excellent explosion-suppression effect.

It can be seen from Figure 12 that under the action of the pure water mist, the explosion temperature of the methane mixtures with different concentrations rises sharply during the initial stage and reaches the highest value in about 0.5–1.0 s, followed by a slow decline during the later stage.

The application of 1% solution of the agent has a significant impact on the explosion temperature of methane. As observed from the temperature curve in Figure 11, after adding the agent, the time needed for the methane explosion temperature to reach the maximum value reduces, and the temperature rise rate decreases along with the maximum temperature. Among these, the curve of methane with a concentration of 12% becomes flatter after the explosion as compared to the other formulations in 0.5 s–1.0 s. It indicates that the formulation solution can reduce the explosion temperature of methane and play a critical role in suppressing the explosion.

As shown in Figure 13, under the action of the pure water mist, the explosion velocity of the methane mixtures with different concentrations shows a slow upward trend, reaching a maximum in the 70–90 ms time period, followed by a decline. After the addition of 1% agent, the explosion rate of methane with a concentration of 12% exhibits an upward trend. The explosion speed of methane at 7% and 10% concentrations is significantly reduced, and the initial ascent rate rises slowly and linearly, reaching the highest value at about 30 ms and 70 ms. Compared with the durations of 90 ms and 70 ms observed in the case of the pure water mist, the anti-explosion effect of the agent in pure water is increased by about 2 to 3 times in the presence of 7% methane. At 10% methane concentration, although the duration is not enhanced, the speed is decreased from about 8 m/s to 6 m/s. Such a decline is especially obvious at 7% concentration, from about 8 m/s to 2.8 m/s. The experiments show that the explosion speed of methane decreases significantly after adding the agent, and the addition of the agent can markedly promote the suppression of the methane explosion by the pure water mist.

Figure 14 illustrates explosion pressure curves under the action of pure water and 1% formulation agent. The test results show that the anti-explosion effect of the microcapsule material with 1% additive is significantly improved. After adding the agent, the explosion pressure reaches the peak value at 0.08 s. In the case of the pure water alone, the explosion pressure continues to rise up to 0.11 s. After adding the agent, the explosion pressure is nearly doubled as compared with the pure water system.

4.3. Package Performance Test

The test uses methane gas as the experimental medium, and the test has been carried out using the concentration ranges of 2%, 5% and 8% so as to investigate the package adsorption effect of the pure water and formulation agents on the explosive substances. To ensure the accuracy and safety of the test results, the test has been carried out in an independent large-space laboratory. The test quantitatively evaluates the packaging ability and technical level of the medicine by measuring the changes in the gas concentration.

4.3.1. Package Performance Test Materials

Methane, pure water and formulated pHarmaceutical solutions of different concentrations have been used in the package effectiveness test. Among these, the pure water and formulation solution are selected as the absorption coating agents, and the methane mixed gas is sprayed through the pure water and solutions with various concentrations (2%, 5%, 8% and 10%). The pHysical and chemical properties of methane and absorption coating solution are presented in

Table 10. The pHysicochemical parameters of methane.

Name	Density (g/cm3)	Melting Point (°C)	Boiling Point (°C)	Flash Point (°C)	Burning Point (°C)	Explosion Range (v/v)	Heat of Combustion (kJ/mol)
Methane	0.42	−182.5	−161.5	−188	538	5.3–15	890.31

and

Table 11. The pHysicochemical characteristics of the absorption coating solution.

Number	Name	Content (%)	pH	Density (g/cm3)
1	Pure water	-	7	1.00
2	Formula medicament	2%	7	1.00
3	Formula medicament	5%	7	1.00
4	Formula medicament	8%	7	1.00
5	Formula medicament	10%	7	1.00

.

4.3.2. Parcel Performance Test Platform

The package efficiency test uses a package test platform, including a gas distribution system, a package box, a low-pressure water mist system, a concentration recording system and a computer post-processing system. The layout and overview diagrams of the test system are shown in Figure 15. Figure 16 also presents an image of the package testing platform.

During the test, the concentration of the flammable gas filled in the package absorption box is 2% v/v, 5% v/v and 8% v/v. After standing and mixing, the low-pressure water mist system is turned on. The package solution is sprayed for 30 s, followed by analyzing the methane concentration in the box after standing for 1 min. Subsequently, the coating solution is sprayed for 30 s and circulated six times. The overall experimental results are mathematically analyzed through a computer post-processing system to quantitatively evaluate the drug’s ability to absorb and wrap.

4.3.3. Package Performance Test Results

The performance index of the package performance test is based on the change in the methane concentration. The methane gas in the parcel absorption box is sprayed, absorbed and wrapped by using the pure water and formulation solutions with different concentrations (2%, 5%, 8% and 10%). After six intermittent sprays (each spray for 30 s), the ability of the formulation to absorb and wrap methane is determined by assessing the change in the concentration of methane. The change in the concentration of methane during the package test is shown in

Table 12. The change in the concentration of methane during the package test.

	Time	0% Addition	Formula Concentration				0% Addition	Formula Concentration				0% Addition	Formula Concentration
	(s)	water	2%	5%	8%	10%	water	2%	5%	8%	10%	water	2%	5%	8%	10%
Concentration change (%)	0	2.13	2.13	2.13	2.15	2.13	5.18	5.19	5.15	5.18	5.16	8.16	8.18	8.15	8.18	8.16
	30	2.04	2.05	1.86	1.71	1.66	5.11	5.08	4.78	4.44	4.21	8.11	8.01	7.78	7.64	7.49
	60	2.06	1.95	1.73	1.65	1.51	5.06	4.98	4.21	4.1	3.72	8.05	7.81	7.42	7.13	6.82
	90	2	1.89	1.71	1.56	1.46	5	4.91	4.14	3.86	3.34	8.01	7.6	6.54	6.09	5.55
	120	1.99	1.86	1.69	1.5	1.29	4.98	4.86	4.05	3.76	3.15	8	7.36	6.35	5.86	5.02
	150	2.01	1.82	1.64	1.45	1.27	4.96	4.63	3.96	3.51	3.06	7.97	7.25	6.26	5.71	4.83
	180	2.03	1.81	1.61	1.44	1.25	4.99	4.61	3.93	3.47	3.03	7.99	7.23	6.25	5.69	4.81

.

The effect of methane absorption and wrapping is noted to increase significantly with the level of addition. In the action time interval, the effect of the formulated agent increases with the action time, and the absorption reaches a stable range. It can be observed from Table 12 that the 8% methane concentration in the explosion limit range is reduced to less than 5% within 180 s after using 10% formulation agent, and the concentration is reduced by 40%. As a result, the risk of explosion is avoided, thus, indicating that the formulated agent has an optimal enveloping effect on methane.

5. Conclusions

(1)

The experiments show that the formulated agent has a strong fire-extinguishing effect and can swiftly reduce flame temperature along with suppressing flame and wrapping oxygen. The formulated agent is noted to fulfill the requirements for designing the microcapsule fire-extinguishing agent, which is superior to the international advanced agent (F-500).

(2)

Under the action of the pure water mist, the gas explosion temperature, flame propagation rate and pressure of methane (different volume fractions) increase rapidly during the initial stage, followed by a slow decline after reaching the maximum explosion temperature. Under the action of 1% microcapsule detonator aqueous solution spray, the temperature and pressure of gas explosion exhibit different degrees of decline. The peak value is obviously reduced, and the time to reach the peak value is also shortened. However, the effect of reducing the gas explosion rate at 12% methane volume fraction is not obvious, whereas the reduction in gas explosion rate at 7% methane volume fraction is significant. The experimental results show that the water mist added with the microcapsule detonator can swiftly reduce the gas explosion temperature and pressure, and the gas explosion rate at 7% methane volume fraction is significantly reduced. Further, the effect of reducing the rate of gas explosion is not obvious.

(3)

On increasing the concentration of the medicament added to the microcapsule detonator, the encapsulation effect on methane is noted to become obvious. It shows that the microcapsule detonator has a significant effect on gas wrapping.

(4)

Through laboratory compounding and screening, a microcapsule anti-explosion fire-extinguishing material with a neutral pH value, as well as including four main agents and four auxiliary agents, has been developed.

(5)

Through the energy-efficiency tests of fire extinguishing, explosion suppression and wrapping, the developed microcapsule material is observed to have a strong encapsulation and oxygen isolation effect on methane and long-chain alkanes, and the fire-extinguishing and explosion suppression effects are obvious. The material is completely soluble in water and is non-toxic, harmless and biodegradable.

(6)

The formulation has been currently tested only under laboratory conditions, and further research is needed for engineering applications. Especially, the operation automation of the drug-delivery process, as well as the effect and stability of the fire and explosion suppression package, need to be further studied.