Minimum Extinguishing Concentration Research Articles

NUMERICAL JUSTIFICATION OF THE LABORATORY TECHNIQUE FOR TESTING OF FIRE-EXTINGUISHING POWDERS

Improvement of firefighting means and methods for measuring their effectiveness are important tasks in the field of fire safety. The paper presents the results of experimental measurements of the minimum extinguishing concentration of powder mixtures that can be applied as effective explosion-suppressing barriers. Measurements of the minimum extinguishing concentration of the investigated powders were carried out using a laboratory method with their pulsed delivery to a microfire of class B using compressed air. In order to justify and assess possible errors of the mentioned laboratory method for measuring extinguishing efficiency, numerical 3D modeling of the interaction of a multiphase flow with a model combustion focus was performed. The analysis of the numerical modeling results has shown that, for the applied laboratory method, almost the entire portion of the investigated powder enters the combustion zone. Additionally, the numerical calculations indicate that under the specified experimental conditions, the particle size of the powder has no noticeable effect on their loss into the surrounding flame space. Thus, these results justified the use of the mentioned laboratory method for he comparative evaluation of fire-extinguishing powders with a wide range of dispersity. The application of this laboratory method for assessing the effectiveness of the fire-extinguishing powder allowed for the development of an optimal powder composition for explosion suppression, incorporating inert mineral particles as the main component and an additive of a chemically active potassium-containing combustion inhibitor.

A comparative on the thermal decomposition and fire‐extinguishing performance of C5F8 and C5F7Cl

AbstractCompared with fluorinated cycloalkanes and liner olefins, the degradation of chlorofluorocycloolefins is much easier with good environmental friendliness. However, the fire‐extinguishing performance of chlorofluorocycloolefins has been rarely explored. In this study, the fire‐extinguishing performance and working mechanism of octafluorocyclopentene (C5F8) and 1‐chloro‐2,3,3,4,4,5,5‐heptafluorocyclopentene (C5F7Cl) were studied by combined experimental and theoretical approach. It was found that, compared to C5F8, the asymmetry of C5F7Cl agent with the chlorine atom possesses a reduced energy barrier in pyrolysis process and its reaction with OH· radicals. Moreover, chlorine‐containing unsaturated active groups and CF2· radicals generated in C5F7Cl pyrolysis can effectively capture flame radicals. As a result, a relatively low minimum extinguishing concentrations (MECs) for suppressing methane‐air (5.37 vol%) and propane‐air flame (5.93 vol%) is achieved outperforms that of C5F8 (6.73 and 7.74 vol%), even better than reported fluorinated cycloalkanes and chlorofluoroolefins agents. This study shows that chlorofluorocycloolefins are promising Halon substitutes over perfluorocyclic olefins in fire‐extinguishing performance.

An improved method to determine the minimum extinguishing concentrations of ultrafine dry powder agents: Taking NaHCO3 and KHCO3 as examples

Ultrafine dry powder agents (UDPAs) are the most potential Halon substitutes due to the high extinguishing effectiveness and excellent environmental friendliness. Nevertheless, the method for obtaining minimum extinguishing concentration (MEC) is flawed and inaccurate. An improved method is proposed by developing the special laser measurement system without powder contamination, the concentration calibration test bench and the cup burner with uniform aerosol generating. The results show that the optimum calibration position is 0.35–0.65 m from the inlet in the y-axis coordinate. More accurate mathematical relationships between the transmittance of the laser measurement system and the NaHCO3 and KHCO3 powders are established, with the errors less than 12%. Upon reaching MECs in the modified cup burner, the flames pulsate, then fade and eventually die. Based on the phenomenon of the sudden change in the transmittance of the laser measurement system at the moment of flame extinguishment, i.e. the “step shape” curve, a more accurate critical criterion point for the MEC is proposed. The measured MECs for methane flames by NaHCO3 (32.2 g/m3) are approximately 4 times that of KHCO3 (8.2 g/m3), consistent with previous studies. The obtained MECs of NaHCO3 and KHCO3 for kerosene flame are verified by 1 m3 total flooding fire extinguishing experiments. The improved method may guide the design of industry fire protection systems based on UDPAs to achieve process safety.

Study on the minimum extinguishing concentration of C6F12O for extinguishing synthesis gas flame of lithium-ion battery

Study on the minimum extinguishing concentration of C6F12O for extinguishing synthesis gas flame of lithium-ion battery

Combustion inhibition of cup-burner flame with C2HF3Cl2 and its kinetics mechanism investigation

Fire extinguishing performance and mechanism of C2HF3Cl2 (R123) were studied by experimental and theoretical methods. The minimum extinguishing concentration (MEC) of R123 in methane/air flames is 7.31 %. The addition of R123 to flame increased first and then decreased the flame height, due to the effect of R123 on the movement of the reaction kernel. Theoretical calculations indicated that R123 and pyrolysis products affect the combustion reaction, the generated fluorine or chlorine-containing groups improved the suppression of combustion chain reactions. The in-depth experimental and theoretical study of R123 boost the development of ideal halon replacement in aircraft cargo compartment.

Dibromomethane as Promising Gaseous Fire Extinguishing Substance with Short Atmospheric Lifetime

Calculation of dibromomethane atmospheric lifetime is carried out, as well as an experimental assessment of its fire extinguishing efficiency. Calculations show that methylene bromide atmospheric lifetime is 15.8 days so it rapidly decomposes in the troposphere. The main mechanisms of the removal of CH2Br2 from the atmosphere are its reaction with hydroxyl radicals and the processes of physical removal from the atmosphere. Special experimental equipment allowing to measure fire extinguishing concentration of gaseous fire suppressant having high boiling point is described. Experimentally measured minimum extinguishing concentration of methylene bromide for n-heptane is 2.5 % vol. According to this, CH2Br2 is close to the most effective gaseous fire extinguishing agents, such as C3F7I and C2F4Br2. LOAEL value for CH2Br2 is predicted at the level of 0.3-0.4 % vol. Due to the high boiling point of methylene bromide and its relatively high toxicity, the most appropriate way to use it in fire suppression is to create fire extinguishing mixtures, in particular, with fluorinated hydrocarbons. This approach allows to diminish application of greenhouse gases in fire protection and also to solve the problem of toxicity of extinguishing substance.

Core-shell microstructured nanocomposites optimized based on Box–Behnken design for enhanced suppression of hydrogen co-flow flames

Combustion characteristics of hydrogen-containing syngas are distinct from those of traditional hydrocarbon fuels. However, the efficiency for any fire extinguishant on syngas/air flame suppression has not been validated up to now. In this study, we developed novel core-shell microstructured nanocomposites (CSMNs) with inner seawater for syngas/air fire extinguishing. To optimize the preparation conditions, we adopted a three-factor three-level Box-Behnken design (BBD). The corrosion rate and minimum extinguishing concentration of CSMNs were determined by corrosion tests and Cup-burner experiments, respectively. Results showed that the BBD method exhibits good agreement between experimental data and fitted models. By applying numerical optimization, we determined that solid mass fraction, rotation speed, and rotation time in a range of 2–10%, 4000–8000 rpm, and 60–180 s, respectively, are the optimal preparation conditions for the CSMN preparation. The degree of corrosion rate and mass concentration in co-flow gaseous fuel/air fire suppression were reduced by more than 50% compared to an aqueous seawater solution. The updated methods can be used to study the fire inhibition efficiency of the emerging class of composite materials with the core-shell structure. This does not only provide solutions for syngas/air flame inhibition, but it also gives new insight into the application of seawater for fire extinguishment.

Numerical and experimental studies of extinguishment of cup-burner flames by C6F12O

The extinguishment of propane cup-burner flames by a halon-replacement fire-extinguishing agent C6F12O (Novec 1230) added to coflowing air in normal gravity has been studied computationally and experimentally. The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (up to 141 species and 2206 reactions), molecular diffusive transport, and a radiation model, is used to reveal a unique two-zone flame structure. The peak reactivity spot (i.e., reaction kernel) at the flame base stabilizes a trailing diffusion flame, which is inclined inwardly by a buoyancy-induced entrainment flow. As the volume fraction of the agent in the coflow is increased gradually, the total heat release increases up to three times due to unwanted combustion enhancement by exothermic reactions to form HF and CF2O in the two-zone trailing flame; whereas at the base, the flame-anchoring reaction kernel weakens (the local heat release rate decreases) and eventually the flame blows off. A numerical experiment, in which the C6F12O agent decomposition reactions are turned off, indicates that for addition of inert C6F12O, the maximum flame temperature decreases rapidly due to its large molar heat capacity, and the blow-off extinguishment occurs at ≈1700 K, a value identical to that for inert gases previously studied, while the reaction kernel is still burning vigorously. The calculated minimum extinguishing concentration of C6F12O in a propane flame is 4.12 % (with full chemistry), which nearly coincides with the measured value of 4.17 ± 0.30 %.

A compound silica-trifluoroacetyl powder for fire suppressant: Preparation, characterization and suppression mechanism discussion

This paper reported the preparation, fire suppression efficiency, and suppression mechanism of a new suppression powder. In present work, the new suppressant was designed for combining the ability of terminating the chain reactions into the powder extinguishing agent. The trifluoroacetyl group (COCF3) was grafted onto the silicon dioxide (SiO2), and called the compound SiO2–F. Several tests for characterizing the structure and morphology of the compound powder were done. The results indicated that the powder was prepared successfully. Then, the burning velocity of methane-air flame with powder addition and minimum extinguishing concentration were measured to evaluate the fire suppression efficiency. In comparison with NaHCO3 and SiO2 powders, the SiO2–F powder had more pronounced fire suppression efficiency. Furthermore, the fire suppression mechanism of the powder was systematically studied. The physical and chemical suppression mechanisms worked together to suppress the flame. The physical suppression mainly stemmed from the heat absorption of solid substances, and occupied 23.1% of the total suppression effect. The chemical suppression was mainly due to the termination of chain reactions by the fluorinated species which were released when the powder was exposed to high temperature.

Investigation of the effect of dimethyl methylphosphonate (DMMP) on flame extinction limit of lithium-ion battery electrolyte solvents

An experimental and numerical study was conducted to quantify the flame retardant effects of different DMMP addition loadings on the flame extinction limits of four lithium-ion battery electrolyte solvents. The minimum extinguishing concentration (MEC) of CO2, which was measured using the cup-burner method, was applied to characterize the flame retardant effects of different DMMP loadings. Then, a model based on the perfectly stirred reactor concept was constructed to calculate the MEC and interpret the measurements established in the cup-burner test. A comparison was conducted between the measured and calculated MECs of N2 and a satisfactory agreement was obtained. Further investigation found that DMMP could significantly improve the extinction temperature by clearing the H radical, suppressing the reaction temperature and increasing the sensitivity of temperature to reaction rate constants, which caused the decrease in the flame extinction limit of solvents. The experimental results showed that DMMP has a certain saturation effect on reducing the flame extinction limits of solvents. For the four solvents, the sensibilities of the flame extinction limits to the DMMP additions were different, which were ranked in the order of diethyl carbonate > ethyl methyl carbonate > dimethyl carbonate > ethyl acetate.

Experimental study on the synergistic effect of fire extinguishing by water and potassium salts

This work experimentally studied the synergistic effect of fire extinguishing by inert extinguishing of water and chemical fire extinguishing of potassium salts. The effect of the combination of water and potassium salts on the effectiveness of the fire suppression compared with that of water and potassium salts used individually was analyzed by measuring the minimum extinguishing concentration of pure water mist, water mist with potassium salts additive and potassium salts dry powder. Results show that water mist containing potassium salts can enhance the fire extinguishing efficiency of pure water and potassium salt dry powder, and the fire extinguishing efficiency is enhanced with the increase in potassium salt content in the solution. Based on the chemical thermodynamic analysis, the essence of synergistic role of water and potassium salts in flame temperature is the existence of water vapor, which can activate potassium salts, thereby easing the generation of a large number of active substances obtained by thermal decomposition of potassium salts at flame temperature, such as gaseous KOH, for fire extinguishing in the equilibrium products.

Efficiency characterization of fire extinguishing compound superfine powder containing Mg(OH)2

To improve the fire extinguishing efficiency of existing dry powders, a new type of superfine dry powder was prepared using magnesium hydroxide as an additive. In our study, a thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to analyze the thermal decomposition of the synthesized powders. The temperature of thermal decomposition, weight loss, and other thermodynamic parameters of the fire extinguishing powders were analyzed to explain the performance advantages of the compound superfine powder. Through a small-scale fire experiment, the physical parameters of the extinguishing process—such as extinguish time, powder dosage, smoke concentration, and minimum extinguishing concentration—were quantified for the suppression of a diesel fire using the different powders; these parameters were used to evaluate the fire extinguishing capacity and toxic gas suppression ability of the powders. TGA demonstrated that the compound superfine powder decomposed more quickly and its thermal decomposition process was much shorter than those of the other powders. The DSC data indicated that the compound superfine powder could decrease the characteristic temperature at each stage and thus the powder absorbed the flame's heat more quickly and suppressed flame propagation. The fire extinguishing test demonstrated that the consumption of the three types of fire extinguishing powder decreased with an increase in the driving pressure, and the order of powder dosage was as follows: commercial dry powder > superfine powder > compound superfine powder. Similarly, the order of minimum extinguishing concentration was as follows: commercial dry powder > superfine powder > compound superfine powder. Furthermore, the compound superfine powder exhibited a greater capacity for controlling toxic and harmful gas emissions.

Experimental study of the influence of dimethyl methylphosphonate on methane/air coflow diffusion flames using OH-PLIF

Experiments were conducted to evaluate the inhibition efficiency of dimethyl methylphosphonate (DMMP) on methane/air coflow diffusion flames. An optical diagnostic system (hydroxyl planar laser-induced fluorescence) was used to measure the OH radical distribution in the flames. The minimum extinguishing concentration (MEC) of CO2, as a function of DMMP addition, was measured to characterize the inhibition efficiency of DMMP. Results show that, when the concentration of DMMP is higher than 0.6%, DMMP’s marginal inhibition efficiency reduces to almost zero. It is found that the normalized peak OH concentrations keep unchanged with a value of 0.9 for flames at conditions near extinction limit. This is firstly observed based on experimental measurements. Furthermore, using DMMP alone cannot extinguish the flame even with 4.59% DMMP addition. When the inhibition efficiency of DMMP gets saturated, a large amount of phosphorus-containing molecules agglomerate around the flame. These particles act as sinks for the active phosphorus-containing species with a high inertial force, which are hard to diffuse horizontally into the flame. Therefore, it is concluded that the generation of the particles is a reason for the saturation phenomenon of DMMP in extinguishing the coflow diffusion flames.

Extinguishment of hydrocarbon pool fires by ultrafine water mist with ammonium/amidogen compound in an improved cup burner

SummaryUltrafine water mist (UFWM) (<10 μm), as a total flooding agent, has been proven to overcome obstructions in extinguishing fire. To examine the efficacy of UFWM extinguishing pool fires and reveal the primary fire extinguishing mechanisms of heat adsorption, a predictive model of minimum extinguishing concentration (MEC) is developed based on heat extraction, and the experimental data of MECs were obtained with an improved cup burner. The contribution of heat adsorption is about 25% to 50%, and turbulence effect also plays an important role in extinguishing fire. To improve fire inhibition effectiveness of UFWM by turbulence effect, CO (NH2)2, NH4HCO3, and (NH4)2HPO4 were selected as additives; KCl and KH2PO4 were also chosen for comparison purpose. It is discovered that CO (NH2)2 and (NH4)2HPO4 in the low concentrations can improve the fire extinction effectiveness except NH4HCO3. CO (NH2)2 has the most contribution to fire extinction at the optimum concentration (about 0.01 mol L−1 for n‐heptane fire and 0.03 mol L−1 for ethanol fire). Moreover, CO (NH2)2 (at 0.03 mol L−1) was dramatically better than KH2PO4 in extinguishing ethanol fire, and its effect is very close to that of KCl (at 0.067 mol L−1) for ethanol fire.

Experimental Study on Fire Extinguishing Properties of Compound Superfine Powder

Abstract In this paper, the existing fire extinguishing powder was improved with magnesium hydroxide as additive to prepare the compound superfine powder. And the fire extinguishing performance of the compound powder was tested by the modified cup-burner, being compared with the commercial dry powder and the superfine powder. The advantages of the compound superfine powder were explained from the aspects of minimum extinguishing concentration, temperature and extinguishing time. The experimental results showed that the minimum extinguishing concentration of compound superfine powder was much lower than the other two, and the temperature drop rate was greater in the process of suppressing flame. Although the compound superfine powder was slightly lower than the superfine powder in the average extinguishing time, the standard deviation results showed that the composite powder had better stability. In addition, the experimental results showed that the fray-out of flame was very fast and the time only accounted for 17.5% in the fire extinguishing process.

Active substances study in fire extinguishing by water mist with potassium salt additives based on thermoanalysis and thermodynamics

The active substances during the process of fire extinguishing by water mist with potassium salt additives was studied. The minimum extinguishing concentration (MEC) experiment results showed that K2CO3 have the most benefit in improving the fire extinguishing efficiency of pure water with the improving rate of 37.6%, 47.2% and 64.8% which the mass percent was 1%, 2% and 5%, respectively. Other potassium salt additives followed the order by: K2C2O4>CH3COOK>KNO3>KCl>KH2PO4, and the reason attributed to the different active substances decomposed from flame temperature by different kinds of potassium salt additives. Thermoanalysis and characterization of the pyrolysis products of potassium salt solutions at flame temperature were analyzed through TG-DSC, XRD, and SEM, the results showed the KOH was the main product in the high extinguishing performance of potassium salts in flame chemical reactions. Thermodynamics analysis by HSC CHEMISTRY showed the K2CO3 could provide 4.85% KOH in equilibrium substances, which well above other potassium salts, and other active substances good for fire extinguishing benefited from the intermediate product during the process of the KOH reacted with the flame free radicals.

Numerical Model for the Chemical Kinetics of Potassium Species in Methane/Air Cup-Burner Flames

A model based on the perfectly stirred reactor (PSR) concept is used to correlate the minimum extinguishing concentration (MEC) of gaseous KOH, which was established in the cup-burner experiment. Both physical and chemical mechanisms of fire suppression are considered in PSR modeling. The combustion process of the CH4/air and gaseous KOH mixture contains a complex chain branch reaction. A high gaseous KOH concentration of type K agent in the methane flame results in strong flame inhibition. The influence of gaseous KOH on the combustion process involves capturing OH free radicals in the flames and reaching the inhibition effect by controlling key elementary reactions containing OH radicals in the combustion process. By calculation of the intermediate product production rate of the fire-extinguishing agent at a high temperature through the PSR model, the main paths of gaseous KOH inhibiting the methane combustion reaction can be obtained. An increased KOH concentration will not change the reaction mechanism.

Extinguishment of hydrogen laminar diffusion flames by water vapor in a cup burner apparatus

Transient computations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow, hydrogen diffusion flame suppressed by water vapor. As the concentration of water vapor was increased, the flame detached away from the burner brim and formed an edge flame at the flame base. Water vapor showed larger chemical inhibition effect than nitrogen when extinguishing hydrogen flame, which was attributed to its enhanced third body effect in the reaction H + O2 + M = HO2 + M. The minimum extinguishing concentration (MEC) of water vapor and nitrogen was predicted by Senecal formula and perfectly stirred reactor (PSR) model respectively. The MECs predicted by PSR model agree with the MECs calculated by Fluent, which shows that 1) the flame extinction is controlled by the flame base, and 2) radiation absorption is negligible. The measured MECs are in a reasonable agreement with the values calculated by Fluent, which demonstrates the accuracy of the CFD model. A simple model was used to investigate the relative importance of extinguishing mechanisms of water vapor. The results show that in a co-flow configuration the thermal cooling and chemical inhibition effect are the main extinguishing mechanisms in suppressing hydrogen diffusion cup burner flame.

Perfectly stirred reactor model to evaluate extinction of diffusion flame

The paper focuses on developing and justification of the flame extinction model for large eddy simulations of under-resolved turbulent diffusion flames. The model is based on the perfectly stirred reactor (PSR) concept, in which the residence time is coupled with the local strain rate, and the radiative losses from the reaction zone are taken into account. The single-step global reaction of fuel oxidation is considered. A possible way to calibrate the kinetic parameters is that by fitting measured values of flame temperature and strain rate at diffusive extinction (blow-off). By comparing the simulation results with experimental data available for methane-air and heptane-air flames and with the published predictions made by the activation energy asymptotics for the ethylene-air flame, it is demonstrated that the PSR model is capable of evaluating flammability bounds of the diffusion flame, including high-strain blow-off and low-strain quenching (i.e., diffusive and radiative extinction). The confluence of these bounds is shown to produce the minimum extinguishing concentration of an inert diluent. For the flames diluted by nitrogen, carbon dioxide, water vapor, or argon, the minimum extinguishing concentrations predicted in this way by the non-adiabatic PSR model are shown to agree with the measured values. Possibility of formulating a unified extinction criterion, the Damköhler number, is confirmed.

Treatment of local extinction in CFD fire modeling

A simplified approach capturing the major flame extinction mechanisms has been formulated and calibrated against the measurement data for critical strain rates of laminar diffusion counter-flow flames with fuel and (or) oxidizer streams diluted by nitrogen. The model is based on the perfectly stirred reactor concept, thereby assuming rapid reactant mixing in the reaction zone, where reactants are delivered at stoichiometric proportions. Model simplicity is achieved by considering the single-step global reaction model. Advancing previous studies, we demonstrate that this model is able to replicate critical strain rates at extinctions of both counter- and co-flow flames in a range of experimental configurations including opposed gaseous streams and evaporating liquid pool with a normally impinging oxidizer stream, in the entire range of gaseous diluent concentration enabling flaming combustion. The model correctly predicts the minimum extinguishing concentrations of different inert diluents (argon, nitrogen, water vapor and carbon dioxide) used in practice of fire suppression. The algorithm is simple and computationally affordable enough to be incorporated in a CFD fire model. A worked example is demonstrated in simulations of flame suppression by sprinkler sprays at different water flow rates. The proposed algorithm can be used for any practical fuel, as soon as the global kinetic model of fuel oxidation is calibrated using the procedure developed in this work.