Article pubs.acs.org/EF
Numerical Model for the Chemical Kinetics of Potassium Species in Methane/Air Cup-Burner Flames Tian W. Zhang,†,‡ Hao Liu,‡ Zhi Y. Han,*,† Zhi M. Du,† and Zi D. Guo‡ †
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian, Beijing 100081, People’s Republic of China ‡ Chinese People’s Armed Police Forces Academy, Langfang, Hebei 065000, People’s Republic of China ABSTRACT: 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.
1. INTRODUCTION Since the Montreal Convention, Halon fire-extinguishing agent has gradually been forbidden because it has destructive effects in the ozone sphere. Thus, seeking environmentally friendly Halon substitute products with high fire-extinguishing efficiency has become the focus of researchers in the fire-extinguishing agent field in various countries. A laboratory study indicates that water mist is a good substitute for Halon, and the additive with the highest additive (per unit mass) is the chemical compound of K or Fe.1 Toxicity, cost, and solubility problems result in the limited use of chemical compound Fe.2 Hence, water mist with some potassium species additive has gradually become a research hotspot. Friedman and Levy3 studied the underlying mechanism of K steam inhibiting and extinguishing methane colliding flame; results indicated that fire-extinguishing effectiveness of K steam was not good and the active intermediate substance during the extinguishing process should be KOH instead. Slack et al.4 obtained a speed constant of the reaction between the KOH radical and flame free radical through an experimental method, compared the experimental results to simulation results of CHEMKIN, and verified the correctness of the five-step reaction model of the KOH radical and flame free radical. Mchale5 compared fire-extinguishing efficiencies of four kinds of potassium species through the experimental method, conducted equilibrium calculation of the reaction between the four potassium species and flame, and found that fire-extinguishing efficiency was related to KOH quantity in equilibrium. In conclusion, there is a strong fireextinguishing efficiency in potassium species. The cup-burner experiment has been extensively applied to measure the minimum extinguishing concentration (MEC) of a gaseous fire-extinguishing agent in extinguishing flammable gas and liquid in a fire disaster.6−10 Effective modeling of the cupburner experiment is conducted to solve the consistency problem of the MEC of gaseous fire-extinguishing agents under © 2017 American Chemical Society
different experimental conditions and to perform an in-depth study of the main mechanism of different kinds of fireextinguishing agents inhibiting the combustion process. Thus, high-cost entity experimental quantity can be reduced, and fireextinguishing agents with better effectiveness can be designed. Domestic and foreign scholars have calculated MECs of some gaseous fire-extinguishing agents through multiple methods and models. A phenomenological model11−13 assumes that adiabatic flame temperatures of all fire-extinguishing agents in extinguishing cup-burner flames are consistent. Under this circumstance, when the reactant concentration variation was neglected, caused by the addition of an inert gas fireextinguishing agent, the general relationship between the MEC and heat capacity as well as the fuel nature of inert gas fire-extinguishing agents is determined. A one-dimensional (1D) activation energy gradual model14 can roughly calculate the influence of the thermal effect and chemical action of fireextinguishing agents on flame structures. In addition, the MEC can be determined by establishing a premix or non-premix flame model and using a numerical simulation method.15−17 However, whether premix or non-premix, the basis of calculating fire-extinguishing conditions is the variation of the flame shape and calculation of the MEC of fire-extinguishing agents at a high calculation cost. On account of the above reasons, a simplified model that can quantify MECs of different fire-extinguishing agents is necessary. The assumed condition of the established model is the similarity in inherent dynamics when different fluids go through the cup burner. This similarity is embodied because the transport property of the gaseous medium does not undergo fundamental changes when different fluids flow into and out of Received: January 10, 2017 Revised: March 22, 2017 Published: March 30, 2017 4520
DOI: 10.1021/acs.energyfuels.7b00106 Energy Fuels 2017, 31, 4520−4530
Article
Energy & Fuels
assumed that internal mixing is infinitely fast and internal space of the reactor is uniform, neglecting the gas-mixing process. Three characteristic parameters of the PSR model are mixture composition at the entrance, temperature, and mixture existence time. The internal status inside the reactor will be calculated by energy and matter conservation equations in numerical solution integral and differential forms. Liu et al.18 used the PSR model to calculate MECs of several kinds of inert gas fire-extinguishing agents and obtained the influence of parameters. The parameters, difficult to control in experiments, are the environmental temperature, humidity, and atmospheric pressure on MEC. On the basis of the study of Liu et al., Liu and Colket19 and Zhang and Soteriou20 added the reaction mechanism of the Halon fire-extinguishing agent in the PSR model, in which calculation results were basically identical to the experimental results, thereby certifying the feasibility of the PSR model in the calculation of the chemical fireextinguishing agent with a known chemical reaction mechanism. Previous studies have only used the PSR model to predict MECs of inert gas fire-extinguishing agents and Halon fireextinguishing agent. Few studies have discussed K salt type as a chemical fire-extinguishing agent with a MEC of gaseous KOH in a flame and fire-extinguishing mechanism. This paper used the PSR model in large-scale gas-phase dynamics software CHEMKIN to conduct effective model establishment of a methane/atmosphere cup-burner experiment, calculating MEC
the cup burner; i.e., Reynolds number, Schmidt number, and Lewis number are basically unchanged with different fluids. Hence, the reaction area of the cup burner is a perfectly stirred reactor (PSR). A PSR is typically an ideal combustion process, in which the variation of fluids in the transporting and mixing process can be basically neglected, and the reactive state depends upon the competitive relationship between the chemical reaction time within PSR and the effective dwell time of fluids. The PSR model contains a reactor, an entrance, and an exit, as shown in Figure 1.
Figure 1. Schematic representation of a PSR.
Intensively mixed fuel and oxidizing agent enter the reactor in the form of a steady-state flow from the entrance. It is
Table 1. CH4 Simplified Chemical Kinetic Model k = AtB exp(−E/RT) number
Ea
B
R1
CH4 + O2 = CH3 + HO2
5.177 × 1015
−0.33
5.796 × 104
R2
CH3 + HO2 = CH3O + OH
1.100 × 10
0.00
0.000 × 100
R3
CH4 + OH = CH3 + H 2O
1.930 × 105
2.40
2.106 × 103
R4
CH3 + OH = CH4 + O
3.557 × 104
2.21
3.920 × 103
R5
CH3 + CH3 = C2H6
9.214 × 10
−1.17
6.358 × 102
R6
C2H6 + OH = C2H5 + H 2O
5.125 × 106
2.06
8.550 × 102
R7
CH3 + OH = CH 2O + H 2
2.250 × 10
13
0.00
4.300 × 103
R8
CH4 + HO2 = CH3 + H 2O2
3.420 × 1011
0.00
1.929 × 104
R9
O + OH = H + O2
1.555 × 10
13
0.00
4.250 × 102
R10
HO2 = H + O2
5.053 × 1014
−0.07
4.996 × 104
R11
CH4 + CH 2 = CH3 + CH3
4.000 × 10
R12
O + H 2O = OH + OH
2.970 × 106
2.02
1.340 × 104
R13
OH + H 2 = H + H 2O
2.160 × 10
1.51
3.430 × 103
R14
HO2 + O = OH + O2
3.250 × 1013
0.00
0.000 × 100
R15
CH 2O + O2 = HCO + HO2
2.974 × 10
10
0.33
−3.861 × 103
R16
HCO + O2 = CO + HO2
7.580 × 1012
0.00
4.100 × 102
CO + OH = CO2 + H
1.400 × 10
1.95
−1.347 × 103
R18
HCO + OH = CO + H 2O
1.020 × 1014
0.00
0.000 × 100
R19
CO + O = CO2
1.800 × 10
10
0.00
2.384 × 103
−15
7.13
1.332 × 104
R17
a
Aa
reaction
13
16
12
8
5
0.00
−570.0
R20
CO + O2 = CO2 + O
1.068 × 10
R21
CH 2 + O2 = CO2 + H + H
3.290 × 1021
−3.30
2.868 × 103
R22
CH 2 + O2 = CO2 + H 2
1.010 × 10
21
−3.30
1.508 × 103
R23
N2 + O = N + NO
1.800 × 1014
0.00
76100.0
R24
N + O2 = NO + O
9.000 × 109
0.00
6500.0
Units: for A, reaction rates of bimolecular, cm3 mol−1 s−1; reaction rates of trimolecular, cm6 mol−2 s−1; and for E, cal/mol. 4521
DOI: 10.1021/acs.energyfuels.7b00106 Energy Fuels 2017, 31, 4520−4530
Article
Energy & Fuels
H2O2 radical; therefore, the decomposition reaction of H2O2 to OH occurs prematurely, resulting in CH4 ignition time significantly ahead of schedule. On the other hand, the calculation results show that the simplified model proposed in this paper and the Zheng et al. model are in good agreement with the GRI-Mech 3.0 mechanism and the simplified model of this work had fewer chemical reactions, which helps to reduce the simulation time of the computer. Therefore, the simulation and analysis of the next part adopts the simplified model proposed in this paper. 2.2. Kinetic Modeling Results for Potassium. The cupburner experiment can be used to evaluate the fire suppression effectiveness of different types of potassium salts.24 For the detailed experimental setup and method of the cup burner, see ref 24. The experimental results of MEC of CH4/air cup-burner flames by different kinds of 1% potassium salts are shown in Table 2.
of gaseous KOH and predicting possible chemical kinetics of potassium species inhibiting flames according to the results.
2. KINETIC MODELING RESULT FOR METHANE AND POTASSIUM 2.1. Kinetic Modeling Results for Methane. The detailed elementary reaction kinetics for CH4 named the GRI-Mech 3.0 mechanism was proposed by the American Gas Fuel Research Institute based on experimental and theoretical determinations of rate parameters, containing 53 species and 325 reactions.21 The more detailed the CH4 combustion mechanism, the higher the calculation precision, which can simulate the CH4 combustion system in a wider range. However, as a result of the particularity of chemical kinetics, a too detailed of a mechanism will spend too much working time for the computer. In addition, the reactions contain a C1, C2, C3, and C4 species, N2, and CO mixture, even if some intermediate products are all in various versions of the CH4 detailed kinetics mechanism. Especially for the particular study of a CH4 combustion, the mechanism of some intermediate products is less important. Therefore, it is necessary to simplify the detailed mechanism of the CH4 combustion process to take into account the calculation accuracy and the reaction selection. The 23 and 42 step oxidation models of the CH4 combustion process were already simplified by Warnatz22 and Zheng et al.,23 respectively. The simplified kinetics in this study are obtained by the sensitivity analysis and the rate of the 325 reactions on the basis of GRI-Mech 3.0 by CHEMKIN, mainly including the C1 and C2 species reaction and the emission of NO. However, as a result of the slow chemical reaction rate of N in the CH4 oxidation process, some C species have rapid oxidation of consumption before reaction with some N species. Therefore, the production of some N-containing species is much smaller in the whole reaction process, in which most detailed chemical reaction mechanisms containing N species could negligible. The simplified model presented in this paper consists of 4000 atoms, 21 species, and 24 reactions. The reaction is shown in Table 1. Figure 2 shows the concentrations of OH, O, and H radicals simulated by the Warnatz model, Zheng et al. model, GRIMech 3.0 detailed kinetics, and this work. From the figure, a simplified model of Warnatz, Zheng et al., and this work have removed some reactions, which can inhibit the oxidation of fuel, e.g., the consumption reaction of the
Table 2. MEC of Some 1% Potassium Species MEC
K2CO3
KNO3
KCl
KH2PO4
upper limit (%) lower limit (%)
6.98 7.56
8.6 10.8
7.13 9.34
7.68 9.16
The results of thermal equilibrium species concentrations for a CH4/air stoichiometric mixture containing 1% potassium salts are shown in Table 3. From Tables 2 and 3, the 1% potassium salts with a higher concentration in the product of equilibrium species are the gaseous KOH monomer, two gaseous dimers,25 K2O2H2 and K2(OH)2, gaseous K atoms, followed by the gaseous KO monomer, which is at least 1 order of magnitude less than gaseous K. The fire suppression effectiveness is in accordance with the number of equilibrium species of the gaseous product, which indicates that the active species for fire suppression are gaseous KOH and some gaseous species containing potassium. Therefore, it is inferred from the kinetic mechanism that the catalytic reaction of the scavenging activity radical in the CH4/ air stoichiometric mixture containing potassium species should include at least K, KOH, and KO. In previous studies, many scholars have proposed the mechanism of alkali metal salts to inhibit a fire radical, a twostep kinetic mechanism of Jensen and Jones as follows, in which X represents any alkali metal element and M is a third body participate in chain reactions: XOH(g) + H = H 2O(g) + X
(1)
X + OH + M = XOH(g) + M
(2)
26
Williams and Fleming calculated the alkali metals of potassium and sodium using this two-step kinetic mechanism, which showed that most of the calculation results can reflect the actual experimental phenomenon. Hynes et al.27 and Slack et al.4 showed that the suppression kinetic mechanism containing alkali metals should also contain XO and XO2, revealed the possible reaction path, and showed that the importance of reaction 3 is equivalent to reaction 2. Figure 2. OH, O, and H radical concentrations simulated by different models. 4522
X + O2 + M = XO2 (g) + M
(3)
XO2 (g) + H = XO(g) + OH
(4) DOI: 10.1021/acs.energyfuels.7b00106 Energy Fuels 2017, 31, 4520−4530
Article
Energy & Fuels
Table 3. Thermal Equilibrium Species Concentrations for a CH4/Air Stoichiometric Mixture Containing 1% Potassium Salts equilibrium mole fractions agent K2CO3 KNO3 KCl KH2PO4
K(g) 4.14 2.05 1.04 4.58
× × × ×
KOH(g) −4
10 10−4 10−5 10−15
K2O2H2(g)
0.785 0.395 1.99 × 10−2 3.21 × 10−8
4.58 1.16 2.95 9.48
× × × ×
K2(OH)2(g)
−3
10 10−3 10−6 10−15
2.54 6.42 1.64 9.26
× × × ×
KO(g)
−3
10 10−4 10−6 10−15
2.85 1.45 7.17 6.65
× × × ×
10−5 10−5 10−7 10−16
Table 4. Five-Step Potassium Model k = AtB exp(−E/RT) number
Aa
reaction
Ea
B −2.68
note
R25
K + O2 + M → KO2 + M
1.138 × 10
2
R(−25)
KO2 + M → K + O2 + M
7.164 × 10
13
0.5
40000
b
R26
K + OH + M → KOH + M
1.144 × 10−1
−2.0
0
c
R(−26)
KOH + M → K + OH + M
3.732 × 1026
−3.0
87600
b
R27 R(−27)
596
28
KOH + H → K + H 2O
2.209 × 10
12
0.5
0
d
K + H 2O → KOH + H
3.702 × 1013
0.5
35700
b
R28
KO2 + H → KO + OH
2.209 × 10
12
0.5
0
d
R(−28)
KO + OH → KO2 + H
1.999 × 1014
0.5
10300
b
R29
KO + H 2O → KOH + OH
6.008 × 10
11
0.5
0
d
R(−29)
KOH + OH → KO + H 2O
5.033 × 1011
0.5
5320
a
3
−1 −1
6
−2 −1
Units: for A, reaction rates of bimolecular, cm mol s ; reaction rates of trimolecular, cm mol positive reaction rate and thermodynamic equilibrium. cFrom ref 4. dPredicted by gas dynamics.
Williams and Fleming26 considered all of the researchers and put forward a suppression kinetic mechanism model containing sodium species. NaOH(g) + H = H 2O(g) + Na
(5)
Na + OH + M = NaOH(g) + M
(6)
NaOH(g) + OH = NaO(g) + H 2O(g)
(7)
b b
s ; and for E, cal/mol. Determined by the
For that reason, this paper used the N2 MEC of 31% (as measured in the experiment) as the criterion. Under this condition, 69% air and 31% N2 were mixed with methane of stoichiometric ratio, which would then create a reaction under room temperature at 298 K, 1 atm, and 50% humidity. The PSR calculation of the reaction temperature with the existence time indicated that the greater fire-extinguishing agents are added, the longer time is needed to reach chemical equilibrium. Hence, the existence time for the gas mixture in the reactor to reach chemical equilibrium was long. Figure 3 shows the variation curves of the reactor temperature with the existence time in four groups under different N2 conditions.
For the potassium salts, the kinetic mechanism to suppression is even less well studied than for sodium. This work referred to all of the above kinetic dynamic mechanisms of flames containing a Na radical, where Na in the mechanism was replaced by K. The five-step kinetic dynamic mechanisms of a K-doped flame are shown in Table 4. Then, the chemical kinetic simplified model of methane flames containing potassium should include at least the species K, KOH, and KO and reactions R1−R29.
3. INFLUENCE OF GASEOUS KOH ON THE METHANE/AIR COMBUSTION PROCESS 3.1. Verification Analysis of the Methane Chemical Kinetic Model Doped with Gaseous KOH. The “extinguishing” condition of flames using the PSR model means that the chemical reaction time of the mixture is less than the existence time. Hence, there is a critical value in which the chemical reaction time is equal to the existence time. When the existence time is less than the critical value, time is insufficient for the chemical reaction to occur. The critical value is fireextinguishing time in PSR calculation. By comparison of the fire-extinguishing time of a fire-extinguishing agent with known MEC, the extinguishing time of the unknown fire-extinguishing agent can be assumed to be equal to that of the calculated fireextinguishing agent. The minimum fire-extinguishing concentration of the unknown fire-extinguishing agent can be deduced through the quantity of the fire-extinguishing agent.
Figure 3. Reactor temperatures as a function of the residence time.
As existence time decreased, the reactor temperature gradually decreased. Reducing the existence time was equivalent to increasing the mass flow of fluids. Hence, more fluids are needed for chemical energy to be generated by combustion to heat. Because the chemical reaction was absent in the reactor when the existence time decreased to a critical value, the reactor temperature sharply dropped to the 4523
DOI: 10.1021/acs.energyfuels.7b00106 Energy Fuels 2017, 31, 4520−4530
Article
Energy & Fuels Table 5. Comparison of MECs of Various Agents predicted MEC (vol %)
cup-burner experimental MEC (vol %)
agent
integrated heat capacity (kJ/mol)
Senecal
inert PSR
reactive PSR
Moore et al.7
Senecal8
IG-01 IG-55 IG-541 IG-100 CO2 H2O KOH CH2F2 CF3Br CHF3
32.2 41.4 45.7 50.6 82.1 64.1 128.6 124.9 151.43 135.6
43.08 36.98 34.68 32.48 22.88
40.118 34.818 32.318 3118 19.718 28.06 14.79 17.619 14.820 16.819
40.118 34.818 32.318 3118 19.718 28.06 4.89 9.819 3.320 7.919
38.0 28.0
42.5 36.4 34.3 31.9
temperature value of the mixture at the entrance. The existence time corresponding to the point at which the temperature sharply dropped was the extinguishing time. The critical existence time depended upon the imported mixture composition, temperature, pressure, and relative humidity of fuel, air, and fire-extinguishing agent. For example, τ = 6 ms was the conclusion of air/N2; the mixing ratio was 69/31% under conditions of 298 K, 1 atm, and 50% relative humidity. A greater N2 concentration resulted in more easily extinguished flames and a longer corresponding existence time. The time provided by long existence time to mix gas to generate a chemical reaction was long, and the effectiveness of the fireextinguishing agent was enhanced. Components of different fire-extinguishing agents in PSR changed; extinguishing time was set as 6 ms; and MECs of different fire-extinguishing agents under conditions of 298 K room temperature, 1 atm, and 50% relative humidity were calculated. For chemical fire-extinguishing agents, two different calculation methods were adopted: (1) Only combustion mechanism of methane was added to the CHEMKIN mechanism part, where the chemical fire-extinguishing agent was regarded as an inert fire-extinguishing agent and fire was extinguished only by heat absorption through the effect of heat capacity and was defined as “inert PSR”. (2) The methane combustion mechanism and dynamic mechanism related to the chemical fire-extinguishing agent were simultaneously added to the CHEMKIN mechanism part, and it was defined as “reactive PSR”. A comparison between the calculation results and Senecal-model-predicted value as well as cup-burner experimental value in the literature is shown in Table 5. Figure 4 shows variation curves of MECs with the heat absorption capacity of different fire-extinguishing agents. Except for H2O, the calculated values of the other four fireextinguishing agents conformed well to experimental values, thereby indicating that CHEMKIN could predict the MEC of the inert fire-extinguishing agent. A numerical difference of H2O resulted from solving the CHEMKIN calculation process in the gaseous state equation. However, the heat absorption process related to evaporation of water transformation from a liquid into a gaseous state was neglected; heat capacity of vapor was only 1/2 of the heat capacity of liquid water. Hence, CHEMKIN underestimated the fire-extinguishing concentration of H2O. Experimental values of chemical fire-extinguishing agents CH2F3, CF3Br, and CHF319,20 were quite close to the results of the “reactive PSR” calculation. However, “inert PSR” could not accurately predict its MEC, thereby indicating that the three species had a chemical fire-extinguishing effect. For KOH, no
30.0 20.4
Cong and Liao29
Fisher et al.6
Saito et al.11 43.3 35.6 33.6 22.0
16.7
14.4
8.8 2.9 12.6
Figure 4. MECs as functions of integrated heat capacity for various agents.
experimental data of cup-burner MEC corresponded to it. However, the MEC data (not shown in Figure 3) of conterflow diffusion flames are found in the literature.3 MEC obtained by the “reactive PSR” calculation was higher than the numerical value in colliding diffusion flames by 10%. Research conducted previously30 indicated that it was easier for conterflow diffusion flames to be extinguished than cup-burner diffusion flames and the concentration difference value between the same kind of fire-extinguishing agents was about 8−10%. Hence, the dynamic mechanism containing the K radical in reactive PSR could correctly describe the fire-extinguishing mechanism. On the other hand, the adiabatic flame temperature as a function of the K2CO3 concentration by CHEMKIN is shown in Figure 5.
Figure 5. Adiabatic flame temperature as a function of the K2CO3 concentration. 4524
DOI: 10.1021/acs.energyfuels.7b00106 Energy Fuels 2017, 31, 4520−4530
Article
Energy & Fuels As shown in Figure 5, the adiabatic flame temperature decreases nonlinearly with the increase of the K 2 CO 3 concentration. The MEC of 1 mass % K2CO3 solution is 6.98% based on the cup-burner experiment in Table 2, and Roberts and Wuince31 have assumed that the hydrocarbon fuel flame temperatures below 1600 K will self-extinguish. Extrapolation of the curve in Figure 5 shows that the mole fraction of K2CO3 must be 7.28%, in which the predicted suppression effect is of the right magnitude. The ultrafine water mist droplet size of 1% K2CO3 solution measured by a laser particle size analyzer is 7.235 μm (D90). According to the relation between the water mist droplet size and the evaporation time by Fleming et al.,30 the droplet evaporation time of water mist is less than 1 ms at the temperature range of 1100−2100 K when the droplet size is
