Combustion–explosion suppression and environmental protection of typical sulfur-containing hazardous chemicals

Abstract

Sulfur, as a crucial chemical raw, poses increased combustion–explosion risks when mixed with other hazardous substances due to its dual nature as both an oxidant and a reducing agent. Additionally, sulfur-induced combustion and explosions can result in environmental pollution. Combustion–explosion suppression technology plays a crucial role in industrial production by effectively preventing hazardous chemical explosion incidents. This research investigates the combustion–explosion suppression of black powder, a common hazardous chemical containing sulfur, by utilizing two solid-based blast suppressants, NH₄H₂PO₄ and NaHCO₃. On this basis, examining changes in the oxidation states of sulfur and explaining the mechanisms of combustion–explosion suppression through the examination of combustion–explosion products. Additionally, numerical calculations are employed to analyze the evolution patterns of gaseous and solid-phase products throughout the entire combustion–explosion process. Research indicates that NaHCO₃ exhibits a more effective combustion–explosion suppression effect on black powder compared to NH₄H₂PO₄, which attributed to the valence state transformation of sulfur and the reduction of carbon oxidation. Furthermore, with the enhancement of combustion–explosion suppression effect, K₂S, which a pollutes the environment, is gradually transform converted into potassium fertilizer K₂SO₄, which benefits plants. These results offer new insights into the research of combustion–explosion suppression of sulfur-containing substances and environmental protection strategies.

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1 Introduction

Sulfur (S) is a crucial chemical raw material with a wide range of applications in various industries, including agriculture, explosives, pharmaceuticals, and food.^1,2 It can ignite when exposed to an open flame, leading to a violent fire that releases toxic gases, leading to environmental pollution, and some scholars have systematically studied the explosion hazard of sulfur dust.^3,4 Additionally, sulfur can act as both an oxidizing and reducing agent due to its multiple valence states. Therefore, when combined with other flammable substances, it can pose a greater risk.⁵

Black powder is a typical hazardous chemical containing sulfur. Due to the addition of sulfur, it has excellent combustion properties. Therefore, to this day it also has various applications such as petroleum drilling, coal mining, explosive shaping, explosion welding, and fireworks.^6,7 Unfortunately, accidents frequently occur in black powder production enterprises during the processes of production, transportation, and storage, resulting in huge property losses, casualties and environmental pollution. There is a pressing need for the development of safe and effective safety measures.

Explosion suppression technology, a method with high potential for reducing or eliminating explosion hazards, is known for its practicality, cost-effectiveness, and safety. It is used to prevent gas and dust explosions in industries such as coal mining, fuel, and metallurgy.^8–11 This technology used in the production of black powder, it is anticipated to effectively address the issue of explosions. The core of explosion suppression technology is explosion suppressants, including CO₂, N₂, NH₄H₂PO₄, water, Halon, and so on.^12–17 Gas suppressants that rely on diluted oxygen concentration may have limited effectiveness due to the self-supply of oxygen by black powder. Additionally, the use of Halon will result in the production of toxic and harmful gases during the suppression process, leading to environmental pollution and damage to the ozone layer. At the same time, under low concentration conditions, Halon may even have the unintended effect of promoting combustion.^18,19 Furthermore, the use of water to suppress sulfur-containing substances can result in the production of large quantities of H₂S and sulfuric acid, posing a serious threat to both personnel and the environment present on-site. Therefore, solid-based suppressants are a more suitable choice. The two most widely used solid-based suppressants in the field of fire extinguishing are ABC dry powder and BC dry powder. Their main ingredients, NH₄H₂PO₄ and NaHCO₃, exhibit both physical and chemical explosion suppression effects.²⁰ Consequently, this work selects these compounds as typical solid suppressants for research.

NH₄H₂PO₄ and NaHCO₃ have been extensively utilized in research on gas and dust explosions in previous studies. Jiang et al. investigated their efficacy in suppressing combustion and explosion of aluminum powder and biomass dust.^21,22 The results indicate that NH₄H₂PO₄ exhibits superior performance compared to NaHCO₃ in inhibiting aluminum powder combustion, whereas NaHCO₃ demonstrates advantages when dealing with biomass dust. Wei et al. conducted a study on the suppression of explosions in hydrogen/dimethyl ether/methane/air mixtures using a water mist containing NaHCO₃. The research revealed that the addition of NaHCO₃ significantly reduced the average flame propagation speed and the maximum explosion pressure of the combustible gases. When W_NaHCO3 = 7% and Φ = 1.0, the decreasing rates of the average flame propagation speed of CH₄-enriched flames, DME-enriched flames and H₂-enriched flames are 84.9%, 29.5% and 40.8%, respectively.²³ Pang et al. investigated the inhibitory effect of NH₄H₂PO₄ on the flame propagation of aluminum alloy dust clouds. The study unveiled that at a NH₄H₂PO₄ median diameter of 10.58 μm and a concentration of 80%, both the distance and speed of the aluminum alloy dust explosion flame reached their minimum values.²⁴ The studies mentioned above collectively demonstrate the promising blast suppression capabilities of NH₄H₂PO₄ and NaHCO₃. Applying these substances to the combustion and explosion suppression of black powder holds the potential to address existing challenges effectively.

To sum up, this study examines the combustion–explosion suppression of black powder, a typical S-containing hazardous chemical, using two solid-based blast suppressants, NH₄H₂PO₄ and NaHCO₃. The study investigates alterations in the oxidation states of sulfur, mechanisms of combustion–explosion suppression through analysis of combustion–explosion products, and the environmental impact of the combustion–explosion suppression process. Furthermore, numerical calculations are utilized to analyze the evolution patterns of gaseous and solid-phase products during the entire combustion–explosion process.

2 Experimental

2.1 Source of raw materials

The purity of all chemicals in this study higher than 98%. They are NH₄H₂PO₄, AR, Beijing Tongguang Fine Chemicals Company; NaHCO₃, AR, Beijing Tongguang Fine Chemicals Company; KNO₃, AR, Beijing Tongguang Fine Chemicals Company; S, AR, Shandong Xiya Chemical Company Limited; Chinese fir charcoal, can pass 500 mesh sieve, Jiangshan Luyi Bamboo Charcoal Company Limited. NH₄H₂PO₄ and NaHCO₃ were thoroughly ground, dried, and sieved through the sieve (300 mesh) before the experiment.

2.2 Preparation of black powder

Black powder was prepared with a ratio of KNO₃ : S : C at 75 : 10 : 15. The KNO₃ and S were thoroughly crushed and dried, and then sifted through a 300 mesh sieve. The resulting powder was carefully mixed with Chinese fir charcoal powder and dried until a constant mass was achieved, resulting in black powder for future use.

The combustion temperature of the prepared black powder was tested by thermocouples according to the previous method in our present work,²⁵ and the maximum temperature reached approximately 1100 °C, as shown in Fig. 1.

Fig. 1 Test results of combustion temperature of black powder.

2.3 Combustion–explosion pressure testing experiment

To evaluate the effectiveness of different suppressants in mitigating black powder, a closed experimental platform for explosive testing was designed, which shown in Fig. 2. This platform comprises a 12 V-powered pressure sensor (0–10 MPa), an enclosed explosive device (100 mL), and an oscilloscope (Tektronix DPO4104). The enclosed explosive device has two ends: the sampling end, which is equipped with a pressure sensor, and the excitation end, which has wiring terminals that connect to a powder package. The outermost side of the powder package is aluminum foil. When making powder packets, first flatten the aluminum foil, and then the suppressants are evenly covered on its inner surface. Next, the black powder is stacked on top of the suppressants, making the suppressants uniformly wrapped around the outer layer of the black powder during the experiment. An electric igniter is used to excite the black powder by the battery, and the oscilloscope collects data through the pressure sensor to obtain the p–t curve. Due to the intense combustion–explosion reaction process, each experimental condition was repeated 3–5 times during the experiment.

Fig. 2 Enclosed explosive experimental platform.

2.4 Characterization and analysis of combustion–explosion products

To comprehensively understand the changes during the combustion–explosion process, we analyzed pressure change data and considered the physical endothermic effects of suppressants, which were obtained from TG-DSC tests. The test conditions for NH₄H₂PO₄ were as follows: a temperature ramp of 30–800 °C, with N₂ as the carrier gas and a flow rate of 10 mL min⁻¹, using the Mettler TGA/DSC 1. Since NaHCO₃ continues to decompose at temperatures above 800 °C, we employed the Mettler TGA/DSC 3+ to conduct tests at 30–1000 °C, while maintaining the other conditions consistent with those used for NH₄H₂PO₄.

To investigate the combustion–explosion suppression mechanism of black powder, we collected explosion products, including both gaseous and solid components. For the analysis of the solid products, XRD (Rigaku SmartLab SE, Japan) was used with a copper target, scanning in the 2θ range of 5° to 90° at a speed of 2° min⁻¹. Additionally, XPS (Thermo Scientific K Alpha, USA) were employed. GC (Aglient 7890B, USA) was utilized to analyze the gaseous products. Subsequently, the results of the product characterization were then used to assess the environmental protection impact of the combustion–explosion suppression process.

2.5 Thermodynamic equilibrium product analysis

Based on the product analysis in 2.4, the equilibrium products of different suppressants and black powder at different temperatures (25–1100 °C) were calculated using the equilibrium composition model in HSC Chemistry 6.0 by Outokumpu Research Oy., which is calculated using the GIBBS solver, with a total of 200 steps. Through the analysis of the changes in equilibrium products, the combustion–explosion suppression mechanisms of NH₄H₂PO₄ and NaHCO₃ were thoroughly elucidated.

3 Result and discussion

3.1 Result of combustion–explosion pressure testing experiment

3.1.1 Suppression of black powder by NH₄H₂PO₄. The suppression effectiveness of NH₄H₂PO₄ on black powder was evaluated through the experiment. Different dosages of NH₄H₂PO₄ (0.03 g, 0.05 g, 0.07 g, 0.1 g, and 0.15 g) were used on 1 g of black powder. The results of the p–t curves are presented in Fig. 3.

Fig. 3 p–t curve of suppression of black powder under different dosages of NH₄H₂PO₄.

The p–t curve provides information on the maximum combustion–explosion pressure, time, and rate of black powder under different dosages of NH₄H₂PO₄. These results are statistically presented in Fig. 4 and Table 1.

Fig. 4 Result from p–t curve in different dosage of NH₄H₂PO₄.

Table 1 Result of combustion–explosion suppression experiment in different dosage of NH₄H₂PO₄

Dosage of NH4H2PO4 (g)	Max pressure (MPa)	Time (s)	Pressure rate (MPa s^−1)
0 g	1.701	0.157	10.834
0.03 g	1.729	0.158	10.943
0.05 g	1.615	0.182	8.874
0.07 g	1.537	0.198	7.763
0.1 g	1.446	0.304	4.757
0.15 g	1.395	0.316	4.414

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Based on the statistical data analysis, a small amount of NH₄H₂PO₄ has a marginal promotional effect on black powder. When the dosage is 0.03 g, the maximum combustion–explosion pressure increases by 1.64%, and the combustion–explosion rate rises by 1.01%. This effect is attributed to the gaseous products generated by the decomposition of NH₄H₂PO₄, a phenomenon defined as the Suppressant Enhanced Explosion Parameter (SEEP). This phenomenon is frequently observed in the explosion suppression processes associated with dust explosions. Many scholars believe that both physical and chemical factors contribute to the occurrence of SEEP, typically resulting in a slight to moderate increase in the maximum explosion pressure and the rate of explosion propagation, which aligns with our experimental findings.^26,27 Notably, the SEEP phenomenon is particularly prevalent when ammonium dihydrogen phosphate is utilized as suppressants, as documented in research concerning explosion prevention and control involving materials such as Al, Mg, and nano Ti.^28,29

As the dosages of the suppressant gradually increases, the suppressive action of NH₄H₂PO₄ becomes progressively stronger. At the dosages of 0.05 g, 0.07 g, 0.1 g and 0.15 g, the maximum combustion–explosion pressure decreased by 5.06%, 9.64%, 14.99% and 17.99%, respectively, and the combustion–explosion pressure rise rate decreased by 18.09%, 28.35%, 56.09% and 59.26%. At the same time, the suppressant had a prolonged effect on the combustion–explosion pressure rise time, adding 0.15 g of NH₄H₂PO₄ extended the time by 101.27%.

Consistent with Section 3.1.1, different dosages of NaHCO₃ (0.03 g, 0.05 g, 0.07 g, 0.1 g, and 0.15 g) were used on 1 g of black powder, and the p–t curves are presented in Fig. 5.

Fig. 5 p–t curve of suppression of black powder under different dosage of NaHCO₃.

3.1.2 Suppression of black powder by NaHCO₃. Fig. 6 and Table 2 present the maximum combustion–explosion pressure, time, and rise rate of black powder with varying types of suppressants.

Fig. 6 Result from p–t curve in different dosage of NaHCO₃.

Table 2 Result of combustion–explosion suppression experiment in different dosage of NaHCO₃

Dosage of NaHCO3	Max pressure (MPa)	Time (s)	Pressure rate (MPa s^−1)
Blank	1.701	0.157	10.834
0.03 g	1.535	0.194	7.912
0.05 g	1.487	0.223	6.668
0.07 g	1.404	0.231	6.078
0.1 g	1.219	0.269	4.532
0.15 g	1.171	0.34	3.444

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Statistical data reveals that the suppressive effects of NaHCO₃ on the combustion–explosion behavior of black powder differ from those of NH₄H₂PO₄. The combustion–explosion pressure consistently decreases as the NaHCO₃ dosage increases, and there is no promotion phenomenon observed at lower dosages. When different amounts of suppressants were used (0.03 g, 0.05 g, 0.07 g, 0.1 g, and 0.15 g), the maximum combustion–explosion pressure of black powder decreased by 9.76%, 12.58%, 17.46%, 28.33%, and 31.16% respectively, and the combustion–explosion rise rate also decreased by 26.97%, 38.45%, 43.90%, 58.17%, and 68.21%. Consistent with NH₄H₂PO₄, NaHCO₃ also significantly prolonged the combustion–explosion time, with 0.15 g of NaHCO₃ extending the combustion–explosion time by 116.56%.

3.1.3 Calculation of completely suppressed dosage. Based on the experimental data, it can be inferred that the maximum combustion–explosion pressure of black powder suppresses a decreasing trend with the addition of suppressants (after exceeding 0.03 g). However, it should be noted that when the suppressants addition surpasses 0.15 g in the experiment, a substantial amount of residues is produced. Therefore, to further analyze the anticipated changes in the pressure trend, SPSS 27 was employed to fit the data for maximum combustion–explosion pressure, and the results are shown in Fig. 7.

Fig. 7 The fitting results of different suppressants. (a) NH₄H₂PO₄ and (b) NaHCO₃.

The fitting result of NH₄H₂PO₄ is shown in Fig. 7(a) (R² = 0.960). Based on the fitting curve, the dosage at which the NH₄H₂PO₄ completely suppresses the pressure drop to 0 is defined as 0.783 g. Similarly, the fitting result of NaHCO₃ is shown in Fig. 7(b) (R² = 0.983). The corresponding completely suppressed dosage for NaHCO₃ is found to be 0.369 g.

To verify the effectiveness of the fitting, 0.79 g and 0.37 g of NN₄H₂PO₄ and NaHCO₃ were mixed with 1 g of black powder respectively to simulate the conditions under complete reaction. It was found that even with the use of an electric ignition device with high-energy ignition powder, the black powder couldn't be fully ignited, indicating the reliability of the fitting results.

Based on the above data, it can be concluded that for the black powder system, NaHCO₃ showed a more significant suppressive effect compared to NH₄H₂PO₄. According to the complete suppressed dose analysis, the required suppressive dosage for NH₄H₂PO₄ is 2.12 times that of NaHCO₃. Additionally, at the same suppressant dosage, the maximum combustion–explosion pressure and combustion–explosion rise rate for NaHCO₃ are significantly lower than those for NH₄H₂PO₄.

3.2 Analysis of combustion–explosion products

3.2.1 Heat absorption during the combustion–explosion suppression process. Both NH₄H₂PO₄ and NaHCO₃ suppress physical and chemical suppressant effects. They absorb heat and decompose in high-temperature explosion fields, serving a physical cooling function. The TG-DSC characterization of these compounds is illustrated in Fig. 8.

Fig. 8 The TG-DSC results of different suppressants. (a) NH₄H₂PO₄ and (b) NaHCO₃.

The NH₄H₂PO₄ powder decomposition process consists of the following reactions:

NH₄H₂PO₄ → H₃PO₄ + NH₃ (211.3 °C)

H₃PO₄ → HPO₃ + H₂O (478.8 °C)

2HPO₃ → P₂O₅ + H₂O (631.8 °C)

There is one obvious endothermic peak in the DSC curve at the temperature around 211.3 °C. The heat absorbed by the rapid decomposition of the NH₄H₂PO₄ into NH₃ and H₃PO₄ is −52.99 J g⁻¹.

The NaHCO₃ powder decomposition process consists of the following reactions:

NaHCO₃ → Na₂CO₃ + H₂O (159.3 °C)

Na₂CO₃ → Na₂O + CO₂ (847.3 °C)

The DSC curve exhibited two prominent endothermic peaks at approximately 159.3 °C and 847.3 °C. The heat absorbed during rapid decomposition was measured at −112 J g⁻¹ and −13.2 J g⁻¹, respectively.

In summary, the endothermic heat generated by the decomposition of the suppressants is significantly smaller than the heat emitted during the combustion of black powder.^30–32 Therefore, it is evident that the primary effects of NH₄H₂PO₄ and NaHCO₃ in suppressing combustion and explosion arise from chemical reactions that are closely related to their combustion and explosion products.

3.2.2 Analysis of combustion–explosion product. To comprehensively understand the transformation of black powder components during the suppression of combustion–explosion process, solid-phase products were collected before and after suppression and analyzed using XRD. The results revealing that black powder transforms into K₂SO₄ and K₂S after the combustion–explosion. NH₄H₂PO₄ reacts with black powder in the combustion–explosion field, leading to the formation of KH₂PO₄ and KPO₃, and NaHCO₃ evolves into Na₂SO₄ and Na₃H(CO₃)₂ (a mixture of NaHCO₃ and Na₂CO₃), which shown in Fig. 9 and 10. Notably, the suppression product of NaHCO₃ exhibited the presence of K₂CO₃ and KHCO₃ components, attributed primarily to the higher reactivity of K in comparison to Na.

Fig. 9 The XRD characterization results of the products (a) blank; (b) black powder with NH₄H₂PO₄.

Fig. 10 The XRD characterization results of the products by black powder with NaHCO₃.

In order to investigate the changes of functional groups in NN₄H₂PO₄ and NaHCO₃, the combustion–explosion products were tested by XPS, with the results as shown in Fig. 11.

Fig. 11 The XPS characterization results of the combustion–explosion products (a) XPS survey; (b) P 2p peak from black powder with NH₄H₂PO₄; (c) Na 1s peak from black powder with NaHCO₃.

Fig. 11(a) presents the full-scan spectrum of the products, revealing a composition mainly consisting of K, C, O, and S. The addition of NH₄H₂PO₄, characteristic peaks of P 2p at around 134 eV emerged, indicating the presence of two oxidation states of PO₄³⁻ and PO₃⁻ (KH₂PO₄ and KPO₃), as depicted in Fig. 11(b).

Meanwhile, the addition of NaHCO₃ resulted in the appearance of a distinctive peak of Na 1s at approximately 1071 eV, along with a prominent Auger peak at around 497 eV. Due to the valence state of Na in Na-containing products being +1, it can only exhibit a large characteristic peak, as depicted in Fig. 11(c).

In the overall combustion–explosion reaction, sulfur (S) plays a crucial role. The XRD results indicate that sulfur can act both as an oxidant, producing K₂S, and as a reducing agent, yielding K₂SO₄. To further investigate the binding states of the S element, spectral scanning and Gaussian fitting were conducted, as shown in Fig. 12.

Fig. 12 The XPS characterization results of the combustion–explosion products of S element (a) blank; (b) black powder with NH₄H₂PO₄; (c) black powder with NaHCO₃; (d) proportional distribution.

In Fig. 12(a–c), the spin–orbit split peaks (p_1/2 and p_3/2) resulting from the presence of S element can be observed, further subdivided into three sub-peaks. The peak corresponding to a binding energy around 162.5 eV is attributed to S²⁻, while the peak around 168.5 eV corresponds to SO₄²⁻. Moreover, between the aforementioned peaks, around 166 eV, a minor cross-state Sⁿ (approximately 5% in content, ranging from −2 to +6 oxidation states) is still detected. Based on its binding energy, it is likely that this corresponds to SO₃²⁻.

By calculating the peak areas corresponding to each oxidation state, the proportions of different oxidation states can be determined. Comparing the proportions of S²⁻, Sⁿ, and SO₄²⁻ substances based on their elemental compositions reveals that with increasing strength of suppression (Na > P > Blank), the proportions of S²⁻ and intermediate state Sⁿ gradually decrease, while the proportion of SO₄²⁻ gradually increases, as illustrated in Fig. 12(d). For instance, in the products suppressed by NaHCO₃, the content of S²⁻ (K₂S) decreased by 10.93%, while the content of SO₄²⁻ (K₂SO₄) increased by 12.27%. This highlights the transformation of sulfur from being reduced to oxidized during the reaction process, leading to changes in the oxidation states of S, which is crucial in affecting combustion–explosion suppression effectiveness.

Apart from solid-phase products, gas-phase products after combustion–explosion were analyzed using GC, as shown in Fig. 13. It can be observed that the generation of CO₂ and CO decreases after adding suppressants, indicating a decrease in the amount of Chinese fir charcoal powder during the combustion–explosion process, which is consistent with suppression process. Additionally, the content of N₂ in the combustion–explosion products increases significantly with the addition of NH₄H₂PO₄, attributed to the introduction of N elements.

Fig. 13 The GC result of gas-phase product.

3.2.3 Analysis of environmental protection. Through the analysis of combustion–explosion products, it has been noted that as combustion–explosion suppression effect improves, the presence of K₂S in solid products decreases gradually, while the content of K₂SO₄ increases. According to the Material Safety Data Sheet (MSDS), K₂S, a highly toxic substance, poses a significant threat to aquatic organisms due to its reaction with water. The introduction of K₂S into soil or air can result in substantial environmental harm and the release of toxic gases like H₂S. On the other hand, K₂SO₄ functions as a potent potassium fertilizer, promoting plant growth effectively. Moreover, other byproducts generated during the combustion–explosion suppression process, including KPO₃, KH₂PO₄, K₂CO₃, Na₂CO₃ from NH₄H₂PO₄ and NaHCO₃, can all serve as plant fertilizers, which are environmentally friendly.

The analysis of gaseous products shows that combustion–explosion suppression effectively reduces both CO₂ and CO, gases known for their environmental hazards. Among them, CO₂ exacerbates the greenhouse effect, while CO, when combined with human hemoglobin, poses significant toxicity. As known from our research, when 0.1 g of NaHCO₃ was used, the CO₂ content in the gas product decreased by 7% and the CO content decreased by 13%. The content of CO₂ and CO is expected to further decrease with the enhancement of combustion–explosion suppression efficiency.

Therefore, it can be concluded that using NH₄H₂PO₄ and NaHCO₃ for black powder suppression not only effectively reduces the risks associated with combustion–explosion suppression but also helps significantly in reducing the environmental impact of combustion–explosion products.

3.3 Analysis of thermodynamic equilibrium product

To further analyze the evolution process of the combustion–explosion reaction, the research results in Section 3.2 were utilized to simulate the thermodynamic equilibrium products of black powder combustion–explosion with NH₄H₂PO₄ and NaHCO₃ using the equilibrium composition model in HSC Chemistry.

To enhance the clarity of the simulation results, the experiment's pharmaceutical equivalent was scaled up from mol to kmol and calculated based on the actual proportions. The calculation outcomes are depicted in Fig. 14 and 15.

Fig. 14 Simulation results of thermodynamic equilibrium products containing NH₄H₂PO₄ (a) gas-phase products; (b) solid-phase products.

Fig. 15 Simulation results of thermodynamic equilibrium products containing NaHCO₃ (a) gas-phase products; (b) solid-phase products.

The variation of gaseous products with temperature is illustrated in Fig. 14(a). In addition to CO₂ and CO produced during the combustion–explosion of black powder, the decomposition of NH₄H₂PO₄ also gradually yields H₂O and a small amount of NH₃. With increasing temperature, NH₃ breaks down into N₂ and H₂, which is consistent with the decomposition pattern observed in TG-DSC analysis. Fig. 14(b) shows the variation of solid-phase products. It can be seen that the derivative product KH₂PO₄, formed from NH₄H₂PO₄, undergoes secondary decomposition at high temperatures, resulting in the formation of KPO₃, in line with the characterization results of the products.

The computational results of gaseous products shown in Fig. 15(a) exhibit similarities to those in Fig. 14(a). Water vapor present in the products is a byproduct of the decomposition of NaHCO₃. Within the solid-phase products, gradual decomposition of KHCO₃ and NaHCO₃ occurs as temperature increases, leading to the formation of NaHCO₃, K₂CO₃, and small quantities of Na₂SO₄. At temperatures nearing 600 °C, there is a possibility of secondary decomposition of K₂CO₃. Nevertheless, since no analogous derivative compounds were detected during product characterization, further exploration of this phenomenon is not pursued.

Upon observing both figures simultaneously, it becomes evident that at temperatures reaching up to 1100 °C during the full combustion of black powder, S will all converted into K₂S. This observation is consistent with the reaction equation of black powder.

2KNO₃ + S + 3C = K₂S + N₂ + 3CO₂

However, at lower reaction temperatures, K₂SO₄ is primarily formed initially. Only when the temperature reaches 400 °C does a notable conversion from K₂SO₄ to K₂S occur. This observation is consistent with the alteration in the S oxidation state as determined by XPS analysis. The presence of suppressants in black powder induces a physical endothermic effect that decreases the reaction temperature, causing the chemical reaction to favor the lower temperature range. Consequently, sulfur, intended for reduction to produce K₂S, undergoes oxidation to produce K₂SO₄, resulting in a decrease in combustion–explosion pressure. From this, if the combustion–explosion suppression process can effectively control the reaction temperature below 300 °C, it may prevent the formation of the highly toxic sulfide K₂S. This could lead to the complete conversion of sulfur into the beneficial K₂SO₄ for plant growth, thereby contributing significantly to environmental protection.

The variations in gas products suggest that at lower reaction temperatures, the production of CO₂ and CO increases gradually with rising reaction temperature. However, when the reaction temperature surpasses 500 °C, there is a notable increase in the production of CO₂ and CO, consistent with the results observed in gas chromatography. Thus, mitigating the combustion–explosion risks of these substances hinge on early application of suppressants, lowering the reaction temperature, decreasing the involvement of sulfur as an oxidant, and minimizing carbon oxidation.

4 Conclusion

This research examines the combustion–explosion suppression and environmental protection of a typical S-containing hazardous chemical, black powder, using two suppressants, NH₄H₂PO₄ and NaHCO₃. The conclusions are summarized as follows.

(1) When compared to NH₄H₂PO₄, NaHCO₃ demonstrates a more pronounced suppression effect on combustion–explosion in black powder. Utilizing a dosage of 0.1 g, NaHCO₃ can decrease the maximum combustion–explosion pressure by 28.33%, whereas NH₄H₂PO₄ achieves a reduction of only 14.99%. The fitted curves indicate that complete suppression of 1 g of black powder necessitates the addition of 0.783 g of NH₄H₂PO₄, while only 0.369 g of NaHCO₃ is required.

(2) The combustion–explosion suppression effect of the two suppressants primarily arises from chemical reactions rather than heat absorption. XRD analysis of the combustion–explosion products reveals that NH₄H₂PO₄ primarily forms KPO₃ and KH₂PO₄ in the combustion–explosion field, while NaHCO₃ mainly produces Na₂SO₄, KHCO₃, K₂CO₃, and Na₃H(CO₃)₂.

(3) Further analysis of the combustion–explosion products using XPS and GC reveals that as combustion–explosion suppression increases, the toxic K₂S is converted into K₂SO₄, which is beneficial to plants. In the gaseous products, the amounts of CO₂ and CO decrease as combustion–explosion suppression strengthens, leading to positive effects on environmental protection.

(4) The simulation calculation results align with the chemical characterization findings, indicating that effective suppression of these substances relies on early application of suppressants, reducing the reaction temperature, minimizing the involvement of sulfur as an oxidant and reducing charcoal oxidation.

Data availability

Date will be made available on the reader's request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (No. 2023YFC3010604), Basic Product Innovation Plan (HZYZX202301), National Natural Science Foundation of China (Grants No. 22178026, 12221002, 22308020) and State Key Laboratory of Explosive and Science Technology of China (ZDKT23-01). The authors extend their gratitude to Mr Luo Nan from Shiyanjia Lab (https://www.shiyanjia.com/) for providing invaluable assistance with the XRD and XPS test. The authors gratefully acknowledge the Analysis & Testing Center, Beijing Institute of Technology, for TG-DSC characterization.

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