Effect of Ultrafine Water Mist with K₂CO₃ Additives on the Combustion and Explosion Characteristics of Methane/Hydrogen/Air Premixed Flames

Abstract

To ensure the safe utilization of hydrogen-enriched natural gas (HENG), it is essential to explore effective explosion suppressants to prevent and mitigate potential explosions. This study experimentally investigates the impact of ultrafine water mist containing K₂CO₃ additives on the explosion characteristics of methane/hydrogen/air premixed combustion. The influence of varying K₂CO₃ concentrations on pressure rise rates and flame propagation was analyzed across different hydrogen blending ratios. The results demonstrate that the addition of K₂CO₃ to ultrafine water mist significantly enhances its suppression effects. The peak overpressure decreased by 41.60%, 56.15%, 64.94%, and 72.98%, the flame speed decreased by 30.66%, 70.56%, 46.72%, and 65.65%, and the flame propagation time was prolonged by 25%, 20.83%, 22.92%, and 18.75%, respectively, for different hydrogen blending ratios, showing a similar trend. However, the suppression effectiveness diminishes under high hydrogen blending ratios and low K₂CO₃ concentrations. Further analysis using thermogravimetric infrared spectroscopy and chemical kinetics simulations revealed that the heat release rate and the generation rate of active free radicals significantly decrease after the addition of K₂CO₃ to the ultrafine water mist. The recombination cycle of KOH → K → KOH, formed by reactions (R211: K + OH + M = KOH + M) and (R259: H + KOH = K + H₂O), continuously combines active free radicals (·O, ·OH) into stable product molecules, such as H₂O. However, at low K₂CO₃ concentrations, reaction R211, which suppresses laminar combustion sensitivity and consumes a larger quantity of active free radicals, does not dominate, leading to a reduced suppression effect of K₂CO₃ ultrafine water mist. Several factors during the reaction process also adversely affect the performance of K₂CO₃-containing ultrafine water mist. These factors include the premature onset of laminar flame instability at low K₂CO₃ concentrations, the increased flame-front propagation speed due to the addition of hydrogen to methane, which shortens the residence time of K₂CO₃ in the reaction zone, and the turbulence caused by unvaporized droplets.

1. Introduction

The development of clean energy has become a global consensus and is crucial for the advancement of energy systems worldwide [1,2]. Blending hydrogen into natural gas networks holds immense potential and has attracted global attention as a promising approach for reducing carbon emissions and transitioning toward greener energy solutions [3]. However, the development of hydrogen-enriched natural gas (HENG) faces significant challenges due to its inherent risks. The addition of hydrogen to methane alters the combustion characteristics, resulting in faster flame propagation, higher explosion pressures, and lower ignition energy [4,5,6]. Furthermore, the highly reactive physicochemical properties of hydrogen make HENG prone to leakage during storage and transportation, leading to more severe leakage and explosion incidents [7,8]. Therefore, there is an urgent need to develop effective explosion suppression methods for HENG to mitigate potential risks and economic losses.

Water mist is a cost-effective and environmentally friendly explosion suppressant, making it a promising option for gas explosion suppression [9,10]. It primarily suppresses combustible gas explosions by diluting the concentration of combustible gases, absorbing heat through evaporation, and blocking thermal radiation, thereby reducing the pressure and flame propagation speed [11,12,13]. However, water mist also has its limitations. Its suppression performance is influenced by factors such as the mist concentration and droplet size [14,15,16]. Van et al. [17] noted that the turbulence generated by droplets at the flame front, along with the disturbances caused by the spray, can accelerate flame propagation. Similarly, Xia et al. [18] indicated that the introduction of water mist can distort and amplify the flame surface, resulting in an increased flame speed and pressure. Gieras et al. [19] also observed that the turbulence generated by droplets enhances the flame surface area and thickness. Ultrafine water mist presents an advanced solution to these limitations. Cao et al. [20] conducted simulations that showed that ultrafine water mist with droplet diameters smaller than 50 μm can fully vaporize in the reaction zone, achieving more effective heat absorption. Shimizu et al. [21], through experimental and simulation studies, found that the smaller droplet size of ultrafine water mist increases its specific surface area, leading to faster vaporization rates upon contact with flames, thus quickly diluting the oxygen concentration at the flame front. Additionally, Zhang et al. [22] discovered that ultrafine water mist with droplet sizes smaller than 10 μm can evaporate directly in front of the flame without generating turbulence effects.

Ultrafine water mist effectively addresses the limitations of water mist while enhancing the suppression efficiency. However, since water mist relies solely on physical suppression, it is essential to improve its effectiveness to meet the heightened risks associated with hydrogen blending. Therefore, researchers have been exploring water mist technologies that incorporate additives. Liu et al. [23] found that the incorporation of metal chlorides enhances the flame suppression capability of water mist. Mitani et al. [24] demonstrated that adding alkali metals to ultrafine water mist inhibits the premixed combustion of H₂/O₂/N₂ mixtures. Shmakova et al. [25] evaluated 21 phosphorus-containing compounds and identified organophosphorus compounds, alkali metal salts, and transition metal compounds as effective additives. However, due to toxicity and solubility issues, phosphorus- and iron-containing compounds are unsuitable for large-scale use [26]. In contrast, alkali metal salts like potassium compounds effectively overcome these drawbacks. Cao et al. [27,28] found that the suppression effectiveness of different additives for methane explosions follows the order: K₂CO₃ > KCl > KHCO₃ > Na₂CO₃ > NaCl. Joseph et al. [29] found that a 10% KHCO₃ solution reduced the extinguishing time by 96% compared to a pure water mist. Zhang et al. [22] discovered that potassium vapor extinguishes flames by generating reactive intermediates like KOH in the reaction zone, which capture free radicals. Slack et al. [30] determined the rate constants for reactions between KOH and flame free radicals, finding good agreement with numerical simulations.

In summary, incorporating additives can significantly enhance the suppression performance of water mist, with potassium salts being particularly effective due to their high efficiency, non-toxicity, and high solubility. However, research on suppressing multi-component gas mixtures is limited, and the mechanisms through which ultrafine water mist containing potassium salts affects the deflagration behavior of methane/hydrogen/air premixed flames remain unclear. Therefore, this study aims to investigate the suppression effect of ultrafine water mist with K₂CO₃ additives at different concentrations on methane/hydrogen/air explosions with varying hydrogen blending ratios. Thermogravimetric infrared analysis and chemical kinetics simulations are used to explore the suppression mechanisms. The findings will offer theoretical guidance for the safe application of potassium-based ultrafine water mist suppression technology in hydrogen-enriched natural gas systems.

2. Experimental and Simulation Methods

2.1. Experimental Setup and Procedure

Figure 1 illustrates the small-scale explosion suppression test platform employed in this study. The experimental setup primarily comprises a transparent experimental pipeline (dimensions: 100 mm × 100 mm × 1000 mm), a pressure measurement system, an image acquisition system, a gas mixture preparation system, an ultrafine water mist injection system, and an ignition mechanism. The pressure measurement system includes a high-frequency pressure sensor from PCB, boasting an accuracy of 0.25% FS, a measurement range of 0–2 MPa, and a sampling frequency of 5000 Hz. This sensor is installed 875 mm away from the ignition point. Data collection is facilitated by a Blast-PRO shock testing instrument provided by TSI Technology. For visual analysis, the image acquisition system utilizes a Phantom 710L high-speed camera set to a resolution of 1280 × 800 pixels and operating at 4000 fps. The experiments were conducted using methane and hydrogen gases with a purity of 99.99%. The gas mixture preparation system consists of methane cylinders, hydrogen cylinders, an air compressor, connecting pipelines, and three mass flow meters (ALICAT 20 series) with an accuracy of ±0.4%. The ignition system employs a high-frequency pulse device that generates an electric spark to initiate the premixed deflagration. The spark generator features a copper wire electrode with a diameter of 0.1 mm and an electrode gap of 2 mm, powered by a pulsed DC generator operating at 0.5 kV. The ignition energy is calculated to be 490.87 mJ. The ventilation outlet is located 905 mm from the ignition source and is sealed with a 0.05 mm thick PTFE film. The ultrafine water mist is generated using compressed air at 0.5 MPa and delivered via a diaphragm pump with a flow rate of 600 mL/min. Positioned 375 mm to the right of the ignition point, the nozzle is a MISTEC ultrasonic air atomizing model with a circular spray pattern and an atomization angle of 80°. Ultrapure water is used to produce the mist. The droplet size distribution of the ultrafine water mist was measured using an OMECDP-02 spray particle-size analyzer, and the results are presented in

Table 1. Ultrafine water mist particle size distribution.

Additives Concentration	Particle Sizes/μm
	D10	D25	D50	D75	D90	D(3, 2)	D(4, 3)
Pure water	25.03	25.88	26.65	27.27	27.72	26.25	26.31
3% K2CO3	23.73	24.56	25.31	25.92	26.38	25.34	25.46
9% K2CO3	23.24	24.03	24.74	25.32	25.75	24.70	24.85
15% K2CO3	24.06	24.90	25.66	26.27	26.72	25.64	25.74

.

The ignition energy (E) is calculated using the energy formula for a capacitor:

$$\mathit{E}={\displaystyle \frac{1}{2}}\mathit{C}{\mathit{U}}^2$$

where E is the ignition energy in joules (J); C represents the capacitance in farads (F); and U is the voltage applied across the capacitor in volts (V).

The capacitance (C) is calculated by:

$$\mathit{C}={\displaystyle \frac{\mathit{\epsilon }\mathit{S}}{\mathit{d}}}$$

where ε is the relative permittivity (dielectric constant), which is 1.0 in this context; S denotes the area of the electrode surfaces facing each other, measured in square meters (m²); and d is the separation distance between the electrodes in meters (m).

In this experiment, the effects of the ultrafine water mist with added K₂CO₃ at concentrations of 3%, 9%, and 15% on the deflagration characteristics of methane/air mixtures containing hydrogen at 0%, 10%, 20%, and 30% under stoichiometric conditions were examined. The equivalence ratio (Φ) and the hydrogen volume fraction (${\mathit{X}}_{{\mathit{H}}_2}$) are defined as:

$$\mathit{\Phi }={\displaystyle \frac{(\mathit{F}/\mathit{A})}{{(\mathit{F}/\mathit{A})}_{\mathit{stioch}}}}$$

$${\mathit{X}}_{{\mathit{H}}_2}={\displaystyle \frac{{\mathit{V}}_{{\mathit{H}}_2}}{{\mathit{V}}_{{\mathit{H}}_2}+{\mathit{V}}_{{\mathit{CH}}_4}}}$$

where $F$ is the mass of the fuel; A is the mass of air; and ${\mathit{V}}_{{\mathit{H}}_2}$ and ${\mathit{V}}_{{\mathit{CH}}_4}$ are the volumes of hydrogen and methane in the fuel mixture, respectively.

The flame speed (v) is calculated using the following equations [18]:

$${\displaystyle \frac{{\mathit{f}}_{{\mathit{t}}_{\mathit{i}}}-{\mathit{f}}_{{\mathit{t}}_{\mathit{i}+1}}}{{\mathit{S}}_{{\mathit{t}}_{\mathit{i}}}-{\mathit{S}}_{{\mathit{t}}_{\mathit{i}+1}}}}={\displaystyle \frac{\mathit{f}}{\mathit{l}}}$$

$$\mathit{v}={\displaystyle \frac{{\mathit{S}}_{{\mathit{t}}_{\mathit{i}}}-{\mathit{S}}_{{\mathit{t}}_{\mathit{i}+1}}}{{\mathit{t}}_{\mathit{i}}-{\mathit{t}}_{\mathit{i}+1}}}$$

where ${\mathit{f}}_{{\mathit{t}}_{\mathit{i}}}$ is the pixel coordinate of the flame front at time ${\mathit{t}}_{\mathit{i}}$; ${\mathit{S}}_{{\mathit{t}}_{\mathit{i}}}$ is the actual position of the flame front in the experimental vessel at time ${\mathit{t}}_{\mathit{i}}$, measured in meters (m); and f and l are calibration factors related to the imaging system.

To purge air from the pipeline, we used a four-volume gas filling method, maintaining the process for 8 min. The initial experimental conditions were standardized at a pressure of 1 atm and a temperature of 298 ± 2 K. After thoroughly mixing the gases, ignition was initiated one second after injecting the water mist, controlled via a relay and solenoid valve. Each experimental condition was tested at least four times to validate the accuracy and repeatability of the results.

In this study, the thermal effects of K₂CO₃ were measured using a differential thermal–thermogravimetric analyzer from Beijing Hengjiu and a Fourier-transform infrared spectrometer from Thermo Fisher SCIENTIFIC (Waltham, MA, USA). The heating rate was set to 10 K/min, with an infrared resolution of 4 cm⁻¹, and 32 scans were averaged for each measurement. Nitrogen gas served as the carrier at a flow rate of 20 mL/min.

2.2. Numerical Methods

CHEMKIN-Pro was utilized to perform the chemical reaction kinetics analysis of K₂CO₃. The initial reaction temperature was set at 298 K, and the initial pressure at 1.0 atm. The curvature parameter (CURV) and gradient parameter (GRAD) were set to 0.01 and 0.05, respectively, and over 2000 grid points were used across a calculation domain of 0–8 cm to ensure computational convergence. The mechanism file for the simulation was provided by Badhuk Pabitrah, comprising two parts: the CH₄/H₂ mechanism, which is a coupling of GRI MECH 3.0 and the San Diego mechanism, and the potassium salt mechanism, obtained from the NIST (National Institute of Standards and Technology) database, considering the effects of water phase transitions. The reliability of the mechanism has been validated in previous studies [31,32,33].

3. Results and Discussion

3.1. Explosion Pressure

The explosion pressure is a primary indicator of the explosion intensity. In this study, we focused on key parameters such as the peak overpressure, the time taken to reach this peak, and the maximum pressure rise rate. Figure 2 illustrates how an ultrafine water mist containing varying concentrations of K₂CO₃ impacts these parameters in gas mixtures with different hydrogen contents. The findings indicate that adding K₂CO₃ to the ultrafine water mist suppresses the explosion pressure characteristics of methane/hydrogen/air mixtures. As the concentration of K₂CO₃ increases, both the peak overpressure and the maximum rate of the pressure rise decrease progressively, while the time required to reach the peak overpressure becomes longer. Overall, the changes in the explosion pressure are similar across different conditions, exhibiting a smooth pressure rise phase followed by a pressure decline phase characterized by Helmholtz oscillations. This phenomenon occurs because, during the early stages of flame propagation, the expanding combustion products stabilize the flame front, counteracting acoustic disturbances. Once the flame reaches the ventilation area, the discontinuities generated—combined with the continuous reactive gases inside the tube—result in a thermoacoustic coupling effect that ultimately leads to Helmholtz oscillations [34,35]. Additionally, after spraying ultrafine water mist containing K₂CO₃, the Helmholtz oscillations are temporarily enhanced and then disappear, indicating that the ultrafine water mist initially increases the explosion reaction rate. However, this enhancing effect is significantly less than the inhibitory effect produced by the ultrafine water mist.

As shown in Figure 2a, without the addition of ultrafine water mist, the explosion overpressure reaches a maximum value of 24.13 kPa at 37.62 ms. For ultrafine water mist containing 0%, 3%, 9%, and 15% K₂CO₃, the peak overpressure is reduced by 41.60%, 56.15%, 64.94%, and 72.98%, respectively, and the time to reach the maximum overpressure is significantly delayed. This reduction and delay occur because, after the explosion shock wave passes through the droplets, the surface tension of the droplets cannot withstand the shear force of the high-speed gas, causing them to be stripped into finer droplets. This fragmentation reduces the energy of the shock wave, thereby alleviating the explosion pressure [36]. As illustrated in Figure 2b–d, regardless of the hydrogen volume fraction, ultrafine water mist with added K₂CO₃ exhibits a significant inhibitory effect on both the peak overpressure and the pressure rise rate. However, when $X_{H_2}=30\%$, the suppression effect of ultrafine water mist containing 3% K₂CO₃ is not as effective as that of ultrafine water mist without K₂CO₃. This can be attributed to the increased explosion intensity with higher hydrogen proportions, which accelerates heat and mass transfer rates in the reaction zone [37,38]. Consequently, the droplets evaporate rapidly, and the resulting vapor fills the pipeline, increasing the internal pressure [20]. Nevertheless, overall, the addition of K₂CO₃ enhances the explosion suppression effect of pure water mist, mainly manifested by a decrease in the peak explosion overpressure, a decrease in the maximum pressure rise rate, and an increase in the time to reach the peak overpressure.

3.2. Flame Propagation Velocity

Figure 3 illustrates the impact of K₂CO₃ additives at four different concentrations (0%, 3%, 9%, and 15%) on the flame speed of methane/hydrogen deflagrations. Overall, the trend in the flame propagation speed exhibits three distinct phases: a gradual increase, a significant rise, and subsequent oscillatory decline. During the early stage of flame propagation, the flame speed is relatively slow, as the droplets effectively suppress the flame by lowering the flame front temperature [39]. As the flame traverses the sprayed region, it reaches a peak speed. This peak occurs because the unvaporized droplets in the sprayed zone enhance the Darrieus–Landau instability, creating large-scale turbulence that increases the flame area and accelerates flame propagation [40,41]. Subsequently, the flame carrying partially vaporized droplets experiences a reduction in speed. This deceleration is due to the absorption of some thermal energy by the K₂CO₃-containing droplets, which also release a small amount of suppressive substances. This process extends the residence time of K₂CO₃ in the reaction zone and contributes to its effectiveness in shielding against thermal radiation and heat conduction, thereby improving the overall suppression efficiency [39]. The cooling effect of droplet evaporation and the disturbances caused by the droplets eventually lead to an oscillatory decrease in the flame speed.

As shown in Figure 3a, for ultrafine water mist with 0%, 9%, and 15% K₂CO₃, the peak flame speed decreases from 23.03 m/s to 6.78 m/s, 12.27 m/s, and 15.97 m/s, respectively. The overall suppression effect follows an inverted V-shape pattern, which might be attributed to the fact that increasing the alkali metal content lowers the saturation vapor pressure of the fine mist, thus reducing the droplet evaporation rate [42]. Notably, Figure 3d shows that with the addition of 3% K₂CO₃ to the ultrafine water mist, the peak flame speed increased by 5.16% compared to pure fine mist, corresponding to the unique observation in Figure 2d. When examining Figure 3b–d, it is observed that at hydrogen volume fractions of 0%, 20%, and 30%, the flame speed decreases by 51.53%, 41.45%, and 18.77%, respectively, with the application of a 3% K₂CO₃ ultrafine water mist. This indicates that the suppression effect of a K₂CO₃-containing mist diminishes with an increasing hydrogen volume fraction. Similarly, with 15% K₂CO₃, the peak flame speed decreases by 54.20%, 48.83%, and 32.76%. This reduction is attributed to the fact that the addition of hydrogen increases the flame speed and reduces the residence time of K₂CO₃ in the reaction zone. In summary, while the spraying of ultrafine mist accelerates flame propagation during the sprayed zone and leads to oscillatory flame speed changes, it still results in a lower peak flame speed compared to no mist spraying. This suggests that the suppressive effect of K₂CO₃ ultrafine water mist on flame propagation outweighs its promoting effect, although an increased hydrogen content diminishes the suppression efficiency of K₂CO₃.

3.3. Deflagration Flame Structure

The flame structure is closely related to its propagation speed. During the spread of non-spherical flames, an increase in the flame stretch leads to a decrease in the stretch rate, enhancing flame instability. This instability eventually causes the flame to split, forming cellular structures. The development of these cellular patterns increases the flame surface area, allowing more fuel to interact with the oxidizer, which accelerates flame propagation [43].

Figure 4a illustrates the evolution of the flame structure at a hydrogen volume fraction ${\mathit{X}}_{{\mathit{H}}_2}=30\%$ without the influence of an ultrafine water mist. The flame undergoes a classical evolution from an initial spherical flame (5 ms) to a “finger-like” shape (20 ms) due to wall friction. As reflected shock waves converge at the flame front, the planar flame gradually transforms into a “tulip flame” (50 ms). Under the influence of vortices at the front of the tulip flame, the flame near the sidewalls propagates faster than at the center, leading to distortion and the appearance of a “distorted tulip” (65 ms). Eventually, the overlapping of the distorted tulip flame reforms the tulip flame again [44]. The introduction of an ultrafine water mist significantly alters the flame’s front structure, eliminating the spherical, finger-like, tulip, and distorted tulip flame shapes. As shown in Figure 4b, the presence of an ultrafine water mist causes the smooth spherical flame front to exhibit wrinkles (20 ms), and the flame surface develops cellular structures. As the flame advances, large cellular structures at the flame front break into smaller ones (35 ms). With a continuous heat exchange between the ultrafine water mist in the spray zone and the flame, disturbances on the flame surface intensify and the cellular structures disappear. Subsequently, the flame carries unvaporized droplets and propagates to the right end of the tube (60–95 ms).

Cellular structures arise due to thermal-diffusive and hydrodynamic instabilities. As shown in Figure 4c–e, during the early stages of flame propagation, a noticeable increase in the number of cellular structures is observed. This indicates that spraying ultrafine water mist containing K₂CO₃ additives enhances flame instability. However, the time it takes for the flame to pass through the spraying zone is significantly delayed. Combined with the previously analyzed pressure and velocity profiles, it can be concluded that flame propagation is effectively suppressed at this stage. Besides the phase-change heat absorption of the ultrafine water mist, this suppression can also be attributed to the deposition of K₂CO₃ particles after droplet evaporation. Upon decomposition at the flame front, these particles generate small-molecule potassium compounds, including K, KO₂, and KOH. These compounds sequester active free radicals such as ·H and ·OH within the reaction zone, thereby diminishing the rate of the chain reaction. As the flame enters the spraying zone, unvaporized droplets enhance the flame stretch, causing the disappearance of cellular structures. As shown in Figure 4, compared to the case without water mist, the time for the flame to travel from the spraying zone to the end of the tube is extended by 25%, 20.83%, 22.92%, and 18.75%, respectively. This can be attributed to the hindering effect of non-evaporated droplets in the later stages of propagation, which increases turbulence effects. Overall, the duration from ignition to the flame reaching the tube’s end is significantly extended, indicating that the suppressive effects of the ultrafine water mist with K₂CO₃ additives surpass any promoting influences. Combining the previous analysis, it is evident that the peak flame speed with 3% K₂CO₃ additive is significantly higher than that with 0% K₂CO₃ additive. From the flame images, we observe that, compared to the 0% K₂CO₃ condition where cellular structures appear only at the flame front, cellular structures are also present within the flame under the 3% K₂CO₃ condition. This suggests that the addition of K₂CO₃ leads to an earlier occurrence of laminar flame instability [45,46,47]. Enhanced flame instability leads to a higher flame propagation speed. With increased K₂CO₃ concentrations, the water mist droplets become smaller, promoting a quicker evaporation rate and improving the explosion suppression performance of the ultrafine water mist [48]. The cellular structures within the flame passing through the spraying zone disappear, and under the influence of Richtmyer–Meshkov instability, the flame surface develops into disordered small-scale vortex structures, with noticeable flame wrinkles and a reduced flame propagation speed [9]. Similar flame structure evolution trends can be observed for other hydrogen concentrations ($X_{H_2}$ = 0%, 10%, and 20%).

3.4. Thermodynamic and Kinetic Characteristics

3.4.1. Thermogravimetric Infrared Analysis of K₂CO₃

Thermogravimetric infrared (TG-IR) analysis can determine the thermal and oxidative stability of materials and further facilitate the study of apparent reaction kinetics. Figure 5 illustrates the variation of weight with temperature during the thermal decomposition of anhydrous potassium carbonate. As shown, a weight loss peak of 0.348%/°C is observed near 50 °C on the DTG curve, mainly attributed to the removal of surface moisture from the anhydrous potassium carbonate. This is confirmed by the water peaks at 3202 cm⁻¹ and 1632 cm⁻¹ in the infrared spectrum. After 100 °C, the aforementioned substances completely volatilize, which is also evidenced by a distinct thermal weight loss plateau. Continuous heating leads to the further weight loss of anhydrous potassium carbonate. A sharp decrease in the mass fraction is observed in the temperature range of 100 °C to 200 °C, possibly related to the decomposition of anhydrous potassium carbonate, with a maximum decomposition rate of 0.951%/°C—this represents the main thermal weight loss stage of anhydrous potassium carbonate. Moreover, an endothermic decomposition peak near 146 °C on the DTA curve further confirms the decomposition of anhydrous potassium carbonate. The slow weight loss between 200 °C and 800 °C is due to hygroscopic reactions; the K₂O and KOH formed during thermal decomposition may hinder the thermal degradation process. The melting endothermic peak at 908 °C indicates that anhydrous potassium carbonate reaches its melting point. The possible chemical reactions and pathways of K₂CO₃ in the gas phase are as follows [42]:

K₂CO₃ + H₂O ⇌ 2KOH + CO₂

K₂CO₃ ⇌ K₂O + CO₂

2KOH ⇌ K₂O + H₂O

K₂O + H₂O ⇌ 2KOH

3.4.2. Sensitivity Analysis of Laminar Burning Velocity

To analyze the inhibitory effect of K₂CO₃ on premixed deflagration, the temperature sensitivity coefficients of the main elementary reactions were obtained. Positive or negative coefficients represent the corresponding reactions in the deflagration system influenced by temperature differences, indicating promotion or inhibition effects. The sensitivity analysis results of different mass fractions of the K₂CO₃ ultrafine water mist under two hydrogen blending ratios are shown in Figure 6. As illustrated, without the introduction of the ultrafine water mist, the absolute value of the sensitivity of reaction R38 (H + O₂ ⇌ O + OH) is significantly greater than that of other elementary reactions. This reaction has the greatest influence on the laminar burning velocity because it is an important chain-branching reaction that generates the active free radicals ·O and ·OH. Furthermore, the chain-transfer reaction R119 (HO₂ + CH₃ ⇌ OH + CH₃O) converts the inactive ·HO₂ radical into the highly active ·OH radical, increasing the concentration of active radicals. The chain termination reaction R52 (H + CH₃ (+M) ⇌ CH₄ (+M)) consumes the chain carriers ·H and ·CH₃ to produce CH₄ under certain pressure conditions, thereby terminating the chain reaction. Upon the addition of K₂CO₃, the sensitivity coefficients of reactions R167 (HCO + M ⇌ H + CO + M) and R178 (O + CH₃ ⇌ H + H₂ + CO) decrease, weakening their promoting effects on the laminar burning velocity. Reaction R211 (K + OH + M ⇌ KOH + M) emerges as a new elementary reaction that, along with R53 (H + CH₄ ⇌ CH₃ + H₂) and R98 (OH + CH₄ ⇌ CH₃ + H₂O), collectively inhibits the laminar burning velocity. As the mass fraction of K₂CO₃ increases, the sensitivities of R53, R98, and R221 gradually increase, enhancing their inhibitory effects on the laminar burning velocity. The newly introduced elementary reactions R259 (H + KOH ⇌ K + H₂O) and R211 (K + OH + M ⇌ KOH + M) jointly combine the active free radicals ·O and ·OH into the stable product molecule H₂O [33].

3.4.3. Analysis of Heat Release Rate

The heat release rate is an intrinsic property of combustible mixtures, reflecting the chemical reaction rate and the heat released by the flame. In order to investigate the effect of the K₂CO₃ ultrafine water mist on the thermal effects of the H₂-CH₄-air combustion system and to reveal the key reactions that influence the adiabatic flame temperature, this study examines the heat release rate of H₂-CH₄-air mixtures with varying K₂CO₃ concentrations (3%, 9%, and 15%) at a hydrogen volume fraction of XH₂ = 30%, under conditions of 298 K and 1 atm.

As shown in Figure 7a, with an increase in the K₂CO₃ concentration, the total heat release rate gradually decreases, and the peak of the heat release rate shifts downstream. This indicates that the addition of K₂CO₃ ultrafine water mist significantly suppresses the heat release during combustion, demonstrating a strong suppression effect. The heat release rate in Figure 7b shows that the main reactions contributing to the positive heat release rate are the chain propagation reaction R10 (O + CH₃ ⇌ H + CH₂O) and the chain termination reaction R52 (H + CH₃ (+M) ⇌ CH₄ (+M)). Additionally, the chain propagation reaction R38 (H + O₂ ⇌ O + OH), as an endothermic reaction, inhibits the heat release rate. Comparing Figure 7b–d, it can be seen that with the increase in the K₂CO₃ concentration, the chain termination reaction R211 (K + OH + M ⇌ KOH + M) becomes a new elementary reaction that consumes OH free radicals. Compared to other reactions, R211 has a longer duration and consumes more active free radicals. Furthermore, R211, in combination with chain termination reactions R52 (H + CH₃ (+M) ⇌ CH₄ (+M)) and R158 (2CH₃ (+M) ⇌ C₂H₆ (+M)), accelerates the consumption of active free radicals, reducing the concentration of active free radicals in the reaction pool and thereby lowering the overall reaction rate. It is worth noting that at 3% K₂CO₃ addition, reaction R211 does not play a significant dominant role in the overall heat release rate, and its suppression effect on laminar combustion is relatively small. This can explain why the suppression effect of the ultrafine water mist with a low K₂CO₃ concentration is weaker than that of the mist without K₂CO₃.

Overall, as the amount of K₂CO₃ increases, the chemical suppression gradually reduces the concentration of reactive free radicals in the combustion system, with reactions being gradually replaced as the reactants increase, resulting in a significant reduction in the overall heat release rate.

3.4.4. Analysis of Explosion Products

Figure 8 shows the effect of K₂CO₃ ultrafine water mist on the maximum generation rate of active free radicals. As seen, with the increase in the K₂CO₃ concentration, the maximum generation rate of H· radicals decreases by 13.11%, 33.76%, and 48.5%, respectively; the maximum generation rate of O· radicals decreases by 16.22%, 40.54%, and 56.94%; and the maximum generation rate of OH· radicals decreases by 12.78%, 31.71%, and 44.49%. Previous studies have shown that the key component responsible for the suppression effect of different potassium salt additives on flames is KOH, and the effectiveness of the chemical suppression of potassium salt additives is related to the concentration of gaseous KOH in the flame [32].

Figure 9 illustrates the impact of K₂CO₃ ultrafine water mist on the main products and relative yields in the hydrogen–methane–air deflagration reaction pathway. From Figure 9a, it can be observed that the molar fractions of reactants CH₄ and O₂ rapidly decrease and gradually stabilize, while the molar concentration of H₂ exhibits a slight decrease, followed by an increase and then a decline. The axial distance range where the molar fraction of O₂ drops to its stable value is wider than that of CH₄, indicating that O₂ continues to participate in chemical reactions over a broader temperature range. The molar fraction of KOH first increases, then decreases, and increases again as the axial distance changes, eventually stabilizing. This behavior is due to the fact that, under the presence of water vapor, the hydrolysis reaction of K₂CO₃ (K₂CO₃ + H₂O = 2KOH + CO₂) is more likely to occur than the decomposition reaction (K₂CO₃ = K₂O + CO₂) [49]. Additionally, K₂O rapidly transforms into KOH in the presence of water vapor (K₂O + H₂O = 2KOH), and the generated KOH participates in the chain reaction. Consequently, the molar fraction of KOH decreases; however, as the temperature and H₂O increase in the reaction zone, the K-containing reactions are promoted, generating more KOH. Therefore, the molar concentration of KOH gradually increases until it stabilizes after the reaction ceases.

In Figure 9b, it can be seen that as the reaction progresses, the generation rate of KO₂ first increases significantly and then decreases, while the generation rate of K shows the opposite trend. This is because, after a small amount of KOH is produced, it participates in the chain reaction and continuously acts on the flame front, where it combines with active free radicals such as ·OH and ·O to form KO₂. KO₂ is then converted into K through chain reactions (KO₂ + H ⇌ KO + OH) and (KO + H = K + OH), which increases the concentration of K in the reaction pool and promotes the forward progression of the chain-terminating reaction (R211: K + OH + M = KOH + M). Eventually, the molar concentrations of K and KOH stabilize after they cross each other. Studies have shown that the recombination cycle formed by reactions (R211: K + OH + M = KOH + M) and (R259: H + KOH = K + H₂O), represented as KOH → K → KOH, further continuously combines active free radicals (·O, ·OH) into stable product molecules, such as H₂O [50]. Therefore, the addition of K₂CO₃ to the ultrafine water mist reduces the concentration of key reactive free radicals in the reaction, leading to a significant decrease in the reaction rate.

4. Mechanism of Ultrafine Water Mist with K₂CO₃ Additives

This study integrates experimental observations with Chemkin Pro simulations to investigate how the ultrafine water mist containing K₂CO₃ influences the deflagration behavior of methane–hydrogen–air mixtures. The findings show that in explosions of these gas mixtures, the K₂CO₃-infused ultrafine water mist effectively slows down the flame front and reduces the peak explosion pressure, thereby diminishing the explosion rate.

Figure 10 illustrates the impact of the K₂CO₃-containing ultrafine water mist on the deflagration reaction of methane–hydrogen–air mixtures. First, a physical suppression phase occurs. The K₂CO₃ ultrafine water mist not only reduces the reaction rate by absorbing the flame temperature in the spray zone through droplet fragmentation, but also dilutes the volumetric fractions of hydrogen and methane through droplet evaporation, thereby lowering the collision probability between the methane, hydrogen, and oxygen molecules. Secondly, a chemical suppression phase is present. The decomposition of K₂CO₃ ultrafine water mist generates various small-molecule potassium compounds, such as KOH, which consume a large number of active free radicals or generate inactive free radicals, thus inhibiting the deflagration process. During suppression, reactions (R211: K + OH + M = KOH + M) and (R259: H + KOH = K + H₂O) together form a recombination cycle of KOH → K → KOH, continuously consuming free radicals and forming stable H₂O. Additionally, CO₂ produced from the thermal decomposition of K₂CO₃ acts as a new third-body diluent, further reducing the collision probability between reactants and active free radicals.

5. Conclusions

This study established a small-scale deflagration suppression test platform to compare and analyze the effects of the ultrafine water mist with different K₂CO₃ concentrations on the deflagration characteristics of methane–hydrogen mixtures. Subsequently, the thermal effects of K₂CO₃ were analyzed using a differential thermal–thermogravimetric analyzer and a Fourier-transform infrared spectrometer. The chemical kinetics characteristics were examined using Chemkin-Pro software, ultimately revealing the suppression mechanisms of K₂CO₃. The conclusions are as follows:

(a)

Ultrafine water mist containing K₂CO₃ additives exhibits a significantly better suppression performance on methane–hydrogen–air mixtures compared to ultrafine water mist alone, resulting in a reduction in the explosion overpressure, a decrease in the flame propagation speed, and an extension of the flame propagation time. Specifically, the peak overpressure decreased by 41.60%, 56.15%, 64.94%, and 72.98%, the flame speed was reduced by 30.66%, 70.56%, 46.72%, and 65.65%, and the flame propagation time was prolonged by 25%, 20.83%, 22.92%, and 18.75% at different hydrogen blending ratios. The suppression effectiveness decreases in the following order: the addition of 15% K₂CO₃ > the addition of 9% K₂CO₃ > the addition of 3% K₂CO₃ > no K₂CO₃ additive.

(b)

The ultrafine water mist with K₂CO₃ provides both physical and chemical suppression. Physical suppression includes energy absorption through droplet fragmentation and heat absorption through evaporation, thereby reducing the flame temperature. The introduction of water vapor consumes combustible gas components. Chemical suppression is reflected in the recombination cycle of KOH → K → KOH, formed by the reactions (R211: K + OH + M = KOH + M) and (R259: H + KOH = K + H₂O), which continuously combine active free radicals (·O, ·OH) into stable product molecules, such as H₂O. The CO₂ generated by the thermal decomposition of K₂CO₃ acts as a new third-body diluent, further reducing the collision probability between reactants and active free radicals.

(c)

The promoting effect of ultrafine water mist on flame propagation at high hydrogen blending ratios and low K₂CO₃ concentrations can be attributed to the following factors: Firstly, the addition of hydrogen to methane increases the flame front propagation speed and reduces the residence time of K₂CO₃ droplets in the reaction zone. Secondly, unvaporized droplets increase thermal diffusion instability and hydrodynamic instability, which, under the influence of turbulence, promotes flame propagation. Thirdly, the addition of K₂CO₃ leads to the premature onset of laminar flame instability, resulting in the formation of distinct cellular structures in the flame. Finally, at low K₂CO₃ concentrations, the absence of reaction R259 (H + KOH → K + H₂O) and the limited impact of reaction R211 (K + OH + M → KOH + M) on laminar combustion result in a weaker suppression effect of K₂CO₃ ultrafine water mist on hydrogen–methane–air mixtures.