Experimental Study on Explosion Characteristics of LPG/Air Mixtures Suppressed by CO₂ Synergistic Inert Powder

Abstract

In order to study the explosion suppression characteristics of LPG/air mixture by CO₂ synergistic inert powder, explosion suppression experiments were conducted in a 20 L explosion device. The results show that the explosion suppression effect of NaHCO₃ powder is prior to Al(OH)₃ powder under the condition of no CO₂ synergy. As the mass concentration of inert powder increases, the peak value of explosion pressure P_ex and the peak value of the pressure rise rate (dP/dt)_ex decrease, and the explosion suppression effect gradually enhances. Gas–solid two-phase inhibitors exhibit more significant inhibitory effects than single-phase inhibitors. Increasing the volume fraction of CO₂ or the mass concentration of inert powder can improve the explosion suppression effect. The explosion suppression effect of CO₂/NaHCO₃ is significantly better than that of CO₂/Al(OH)₃. The research results have certain significance for the prevention and control of LPG explosion accidents.

1. Introduction

With the development of social economy and the increasing demand for energy, new energy has been continuously developed and utilized [1]. Liquefied petroleum gas (LPG) is a colorless and volatile multi-component mixed gas obtained from petroleum or natural gas (including oilfield associated gas) through steps such as pressurization, cooling, and liquefaction. It has been widely used in civil, commercial, and industrial fields.

In recent years, LPG leaks and explosions have occurred frequently, causing great disasters [2,3,4]. On 4 July 2017, an explosion occurred due to a gas pipeline leak on Fanhua Road in Ningjiang District, Songyuan City, Jilin Province, resulting in 7 deaths and 85 injuries. On 3 December 2019, a gas explosion occurred in the production workshop of Beijing Jingri Dongda Food Co., Ltd. Phase I, located in Niulanshan Town, Shunyi District, Beijing, resulting in 4 deaths and 10 injuries. On 18 November 2020, a liquefied gas tank leak and explosion occurred at Ziyuan Tu Restaurant in Xinshi Town, Miluo City, Yueyang City, Hunan Province, resulting in 34 injuries.

In order to reduce the hazards of gas explosions, scholars have conducted a large number of experiments and theoretical studies on explosion prevention, venting, and suppression [5,6,7]. Compared with safety measures such as venting and explosion proofing, explosion suppression is a more proactive and efficient method. It can effectively avoid the discharge of toxic gases, unburned materials, etc., and avoid causing secondary explosions or pollution. It can spray explosion suppressive media to suppress combustion before the explosion forms destructive pressure, preventing the further expansion of the explosion, suppressing the explosion or combustion in the initial stage, and reducing the disasters and losses caused by the explosion [8,9]. At present, inert gases, inert powders, and liquid mist are mainly used for explosion suppression. Mitu et al. [10] studied the effects of inert gases on the peak pressure, pressure increase rate, and explosion time of methane explosions, and found that CO₂ efficiency was the highest, followed by N₂, Ar, and He. Luo et al. [11] studied the explosion characteristics of inert gases on LPG/air mixtures and found that the addition of N₂ and CO₂ can effectively suppress LPG explosions, reduce the explosion pressure and flame velocity. Wang et al. [12] studied the influence of inert gases on the explosion characteristics of H₂/LPG/air mixtures and found that the addition of N₂ and CO₂ reduced the maximum explosion pressure and maximum pressure rise rate of H₂–LPG–air mixtures, as well as decreased the rate of production of free radicals. Wu et al. [13] investigated the flammability and explosion characteristics of methane under three different inert gas (CO₂, N₂, and Ar) suppression conditions, and the results indicated that CO₂ has the best inert effect on methane. Li et al. [14] investigated the effectiveness of N₂ and CO₂ in suppressing gas explosions in confined spaces, with the results indicating that CO₂ exhibits significantly better suppression effects on CH₄–air explosions compared to N₂. Liu et al. [15] found that adding NaHCO₃ powder significantly inhibits flame propagation and the explosion pressure in oil shale dust explosions. Yang et al. [16] discovered that NaCl and NaHCO₃ exhibit notable inhibitory effects on the flame speed, maximum explosion pressure, and the rate of pressure rise. Jiang et al. [17] found that increasing concentrations of NaHCO₃ and NH₄H₂PO₄ significantly suppress the flame speed and flame temperature of biomass dust. NaHCO₃ exhibits better performance in inhibiting biomass dust explosions compared to NH₄H₂PO₄. Wang et al. [18] studied the inhibitory effects of Al(OH)₃ and Mg(OH)₂ dust on methane explosions, revealing the strong dependence of methane explosion parameters on the methane concentration and inert powder addition concentration. They found that Al(OH)₃ dust exhibits a better explosion suppression performance compared to Mg(OH)₂. Lin et al. [19] and Meng et al. [20] also demonstrated that Al(OH)₃ dust can effectively suppress dust explosions.

It is important to study the microscopic mechanisms of explosion and explosion suppression [21]. The Chemkin software version 17.0 is mainly used to calculate the kinetic characteristics of gas explosion reactions [22]. Zhang et al. [23] studied the sensitivity and fuel flux of combustion elementary reactions in LPG/DME mixed fuels. Zhou et al. [24] studied the effect of mixing H₂ on the elementary reaction sensitivity of DME. Pei et al. [25] studied the ROP and mole fractions of H, O, and OH radicals in each reaction step of LPG explosion under the action of N₂ and ultrafine water mist. The results showed that N₂ and ultrafine water mist could inhibit the production of H, O, and OH radicals, thereby suppressing the explosion.

The study focused on a 4% LPG–air mixture, based on our previous experimental findings [26] regarding the optimal explosive concentration of LPG. In a 20 L explosion chamber, inert powders of sodium bicarbonate and aluminum hydroxide, as well as CO₂, were separately introduced to conduct explosion suppression experiments. The changes in the explosive characteristics of LPG under the influence of inhibitors were investigated. By analyzing the thermal decomposition behavior of inert powders and the effect of CO₂ on combustion, the synergistic mechanism of CO₂ in conjunction with inert powders for suppressing LPG explosions was examined. This research provides a basis for preventing and controlling LPG explosion accidents, and is of significant practical importance for safeguarding public safety and property.

2. Experiment

2.1. Explosion Testing System

To analyze the explosion characteristics of liquefied petroleum gas and the effects of explosion suppression media on these characteristics, the HY16426B gas–dust–liquid mist explosion test apparatus was used. This apparatus, depicted in Figure 1, consists primarily of a spherical explosion vessel, pressure detection system, automatic gas distribution system, jet device, automatic ignition device, computer control system, and data acquisition and transmission system. The entire experimental system is fully automated. The computer control system manages operations such as gas distribution, ignition, and data acquisition. It displays real-time experiment progress and monitors and records the pressures within the explosion vessel, dust chamber, and automatic gas distribution system. The ignition method employed is electric spark ignition.

2.2. Experiment Material

Based on GC-9890 gas chromatograph detection, the liquefied petroleum gas used in this experiment consists of 0.0026% methane, 0.026% ethane, 0.0435% propylene, 38.164% propane, 60.706% butane, and 1.058% other hydrocarbon gases. For the explosion suppression experiment, two inert powders, sodium bicarbonate and aluminum hydroxide, both of analytical grade, were selected.

2.3. Experimental Procedure

Firstly, check the airtightness of the experimental apparatus. After completing the vacuum pumping, confirm that the pressure inside the explosion device remains stable. For the explosion suppression experiment with inert powder, the inert powder should be added to the powder chamber first. For experiments involving CO₂ and inert powder synergistic explosion suppression, connect CO₂ gas to the gas distribution system. Then, set the inert gas concentration and ignition delay time on the computer. In this experiment, set the ignition delay time to 60 ms with a pulsed ignition method. After setting up, the system should start to operate, automatically performing steps such as vacuum pumping, precise gas distribution, ignition, and data collection. After each explosion experiment, open the intake valve to bring the explosion chamber to atmospheric pressure, and clean the explosion chamber with compressed air and a vacuum cleaner.

3. Results and Discussion

3.1. Comparison of Explosion Suppression Effects of Different Inert Powders

The explosion pressure curves of 4% LPG–air mixtures inhibited by two types of inert powders at different mass concentrations were tested, as shown in Figure 2. It can be seen that the mass concentration difference of NaHCO₃ powder has a more significant impact on the explosion characteristics of LPG compared to Al(OH)₃ powder.

Figure 3 shows the effect of NaHCO₃ powder and Al(OH)₃ powder on the explosion parameters of LPG. It can be seen that the overall influence patterns of the two inert powders on the explosion characteristics of LPG are similar, both capable of suppressing LPG explosions to a certain extent. Generally, with an increase in the powder mass concentration, the effectiveness of both inert powders in suppressing LPG explosions is enhanced. When the mass concentration of inert powder increases to 500 g·m⁻³, both the NaHCO₃ and Al(OH)₃ powders exhibit a rapid decrease in P_ex and (dP/dt)_ex. However, the rate of change decreases as the powder mass concentration continues to increase. When inert powders are added at 1250 g·m⁻³, the NaHCO₃ and Al(OH)₃ powders reduce the P_ex value of LPG from 0.814 MPa to 0.735 MPa and 0.767 MPa, respectively, decreasing by 9.71% and 5.77%. The (dP/dt)_ex value decreases from 33.073 MPa/s to 23.745 MPa/s and 27.137 MPa/s, respectively, reducing by 28.20% and 17.95%. In conclusion, under otherwise identical conditions, adding inert powders of equal mass concentration, NaHCO₃ exhibits better explosion suppression effects compared to Al(OH)₃.

3.2. Synergistic Explosion Suppression Effect of CO₂ with Inert Powders

CO₂ with different mass concentrations of inert powder inhibits the pressure–time curve of LPG explosions, as shown in Figure 4. It can be seen that at the same volume fraction of CO₂, the P_ex gradually decreases with the increasing mass concentration of NaHCO₃ powder. When adding a binary inhibitor of 5% CO₂/750 g·m⁻³ NaHCO₃, the LPG explosion is completely suppressed. When adding a binary inhibitor of 10% CO₂/500 g·m⁻³ NaHCO₃, the LPG explosion is also completely suppressed. Compared to adding a single powder inhibitor, the binary inhibitors of CO₂ and NaHCO₃ powders show significant inhibitory effects on LPG explosions.

Under conditions of 5% CO₂ and 10% CO₂, the variation law of the characteristic parameters of LPG explosion with different mass concentrations of NaHCO₃ and Al(OH)₃ powders is shown in Figure 5. It can be observed that under the synergistic inhibition of CO₂ and the inert powders NaHCO₃ and Al(OH)₃, both the P_ex and (dP/dt)_ex of LPG decrease with an increasing mass concentration of inert powders and volume fraction of CO₂. Compared with experiments using only CO₂, the dual-phase inhibition of CO₂ and inert powders exhibits stronger suppression effects on LPG explosion. As the mass of Al(OH)₃ powder increases, P_ex decreases gradually, but the decrease is slight. In contrast, with an increasing mass of NaHCO₃ powder, P_ex decreases significantly. This indicates that the CO₂/NaHCO₃ system shows superior explosion suppression compared to the CO₂/Al(OH)₃ system.

When 5% CO₂ is added, increasing Al(OH)₃ from 0 to 1250 g·m⁻³, the explosion pressure decreases from 0.771 MPa to 0.738 MPa, and the pressure rise rate decreases from 31.277 MPa/s to 25.441 MPa/s. When 10% CO₂ is added, increasing Al(OH)₃ from 0 to 1250 g·m⁻³, the explosion pressure decreases from 0.753 MPa to 0.716 MPa, and the pressure rise rate decreases from 27.137 MPa/s to 24.593 MPa/s. Maintaining the mass concentration of Al(OH)₃ at 1250 g·m⁻³, when the CO₂ volume fraction increases from 5% to 10%, the explosion pressure of LPG decreases from 0.738 MPa to 0.716 MPa, and the pressure rise rate decreases from 25.441 MPa/s to 24.593 MPa/s. In terms of the influence on the reduction in explosion pressure and pressure rise rate by the suppressants, for the CO₂/Al(OH)₃ suppression system, CO₂ has a greater impact on the intensity of LPG explosion compared to Al(OH)₃ powder; for the CO₂/NaHCO₃ suppression system, NaHCO₃ powder has a greater impact on the intensity of LPG explosion compared to CO₂. The added CO₂ and the CO₂ generated by the thermal decomposition of inert powders in explosive high-temperature environments both play important roles in suppressing LPG explosions.

3.3. Mechanism Analysis of Explosion Suppression

The high temperature and rapid propagating flame can increase the explosion pressure. Figure 6 illustrates the development of LPG explosion flames under suppression conditions. It can be seen that the addition of inert powder and inert gas both decrease the propagation speed of LPG explosion flames and reduce the flame brightness. Particularly under synergistic suppression conditions with CO₂ and inert powder, the brightness and propagation speed of LPG explosion flames are significantly reduced.

Inert powders can affect the explosion of LPG through endothermic decomposition. Figure 7 and Figure 8, respectively, show the thermogravimetric and heat flow characteristics of NaHCO₃ and Al(OH)₃ powders in an air atmosphere with a heating rate of 10 K/min. It can be seen that both the onset temperature and the temperature of the maximum weight loss rate for NaHCO₃ are lower than those for Al(OH)₃. During the LPG explosion process, NaHCO₃ and Al(OH)₃ powders can suppress combustion reactions through their endothermic heat absorption. In terms of heat flow, NaHCO₃ exhibits strong endothermic peaks at 170.5 °C (−1.23 w/g) and 221.2 °C (−3.48 w/g), and Al(OH)₃ exhibits strong endothermic peaks at 301.7 °C (−4.92 w/g). The heat absorption per unit mass between the two powders shows no significant difference. NaHCO₃ exhibits better explosion suppression compared to Al(OH)₃, possibly due to its lower starting temperature for thermal decomposition and the differences in decomposition products.

As shown in Equations (1) and (2), the thermal decomposition of the NaHCO₃ powder mainly produces Na₂CO₃, CO₂, and H₂O. The thermal decomposition of the Al(OH)₃ powder primarily yields Al₂O₃ and 3H₂O. The generated CO₂ can decrease the O₂ concentration in the explosive reaction system, while the produced H₂O absorbs the heat released by the explosive reaction system. The resulting solid salts are non-combustible inorganic substances that hinder combustion reactions. From Equations (3)–(10), it can be seen that inert powders can capture radicals and combine with oxygen, suppressing explosive reactions. Furthermore, compared to the Al(OH)₃ powder, the thermal decomposition of the NaHCO₃ powder generates more CO₂, effectively reducing the oxygen concentration in the explosive system and inhibiting explosive reactions. This may explain why NaHCO₃ powder exhibits a better inhibitory effect on LPG explosions than Al(OH)₃ powder.

2NaHCO₃ = Na₂CO₃ + CO₂+ H₂O

(1)

2Al(OH)₃ = Al₂O₃ + 3H₂O

(2)

Na₂CO₃ = Na₂O + CO₂

(3)

NaO· + O· = Na· + O₂

(4)

Na· + O₂(+M) = NaO₂(M)

(5)

Na· + OH· + M = NaOH + M

(6)

NaOH + H· = Na· + H₂O

(7)

NaO₂ + OH· = NaOH + O₂

(8)

NaO· + H₂O = NaOH + OH·

(9)

Na₂O + OH· = NaOH

(10)

Using Chemkin software, a one-dimensional laminar flame speed model was employed to analyze the effect of CO₂ on the temperature and laminar burning speed of LPG flames. The calculation adopts the USC 2.0 mechanism file, and the number of grids is set to no less than 1000 to ensure the reliability of the calculation. As shown in Figure 9, with an increase in the CO₂ volume fraction, both the flame temperature and laminar flame speed decrease significantly. Rapid flame combustion releases more heat and gas; high temperatures can increase the combustion speed and rate, thereby increasing the explosion intensity. The laminar flame velocity and flame temperature of LPG under CO₂ dilution conditions are negatively correlated with the explosion pressure and pressure rise rate. Figure 10 shows the changes in the peak molar fractions of H, O, and OH radicals during the LPG/air premixed combustion process under CO₂ dilution conditions. It can be seen that CO₂ can effectively reduce the peak molar fractions of H, O, and OH radicals. Therefore, the addition of CO₂ can reduce the combustion speed, flame temperature, and radical concentration in the explosion process, weakening the explosion strength.

4. Conclusions

LPG, as a multi-component explosive gas based on the chemical production of raw materials and clean fuel, is highly prone to causing fire and explosion accidents. This paper investigated the performance of a single inert powder and a CO₂ synergistic inert powder in suppressing LPG explosions, and analyzed the suppression mechanisms. The main conclusions are as follows.

• The inert powders of NaHCO₃ and Al(OH)₃ both have inhibitory effects on LPG explosions. As the inert powder content increases, the explosion pressure gradually decreases. The inhibitory effect on explosions is better for NaHCO₃ than for Al(OH)₃.
• The explosion suppression effects of both gas/solid two-phase inhibitors increase with an increasing CO₂ volume fraction or NaHCO₃ and Al(OH)₃ mass concentrations. Among them, CO₂/NaHCO₃ exhibits better synergistic explosion suppression than CO₂/Al(OH)₃, and the synergistic suppression effect becomes more pronounced with higher CO₂ volume fractions.
• The addition of both inert powders and inert gases reduces the flame propagation speed of LPG explosions and weakens the flame brightness. CO₂ generated from the thermal decomposition of inert powders can decrease the O₂ concentration in the explosion reaction system, and the generated H₂O absorbs the heat released by the explosion reaction system. The resulting solid salts are non-flammable inorganic substances that hinder combustion reactions. CO₂ can reduce the combustion rate and flame temperature of LPG, weakening the intensity of combustion explosions.