Effects of High Concentrations of CO2 on the Lower Flammability

Apr 19, 2016 - The lower flammability limit (LFL) is the minimum composition limit of fuel in a fuel−oxidizer mixture above which a flame can propag...
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Effects of High Concentrations of CO2 on the Lower Flammability Limits of Oxy-methane Mixtures Xianzhong Hu, Qingbo Yu,* Nan Sun, and Junxiang Liu School of Metallurgy, Northeastern University, Shenyang, Liaoning 110819, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, the effects of high concentrations of CO2 on the lower flammability limits (LFL) of the CH4/O2/ CO2 mixture were studied. For comparison, the LFL of the CH4/O2/N2 mixture were studied in the same way. First, the LFL of gas mixtures were measured using a cylindrical quartz glass tube in the condition of various oxygen concentrations. The experimental values of LFL of CH4/O2/CO2 decreased with the increase of oxygen concentrations, but the decreasing rate was small. Then, the chemical, thermal, and radiative effects of high concentrations of CO2 on the LFL were analyzed with the energy balance analysis. The thermal property of the gas mixture played the major role in the determination of the LFL. The radiative effect of CO2 on the LFL was much smaller than the thermal effect. The chemical effect of CO2 has little impact on the LFL of CH4/O2/CO2. Finally, the LFL of the CH4/O2/CO2 mixture were well-predicted using the calculated flame temperature method with a fixed critical temperature. However, most of the research investigated the flammability limits of flammable gas diluted with CO2 in air conditions. Few studies focused on the LFL of fuels at an O2/CO2 atmosphere, except Li et al. and Di Benedetto et al.10,22 Chen et al. illustrated the flammable region of oxy-fuels with effects of the equivalence ratios, the temperature, and the dilution ratios.23 In our previous research,9 the flammability diagram of CH4/O2/ CO2 was studied, but the influencing mechanism of high concentrations of CO2 on the LFL was not discussed. In the present work, more measurements of LFL of CH4/ O2/CO2 were obtained using a cylindrical quartz glass reactor. For comparison, the LFL of the CH4/N2/O2 mixture were measured in the same way. Then, the mechanism of effects of high-concentration CO2 on the LFL was analyzed. The chemical, thermal, and radiative effects of CO2 on the LFL of the CH4/CO2/O2 mixture were investigated by energy balance analysis. Finally, the LFL of the CH4/CO2/O2 mixture were estimated by the calculated flame temperature (CFT) method.

1. INTRODUCTION Oxy-combustion technology is one of the most promising technologies to reduce CO2 emission.1−3 The concentration of CO2 is high in oxy-combustion,4,5 which is different from traditional combustion. There is a lot of N2 in the traditional combustion because air is used.6 The physical and chemical properties of CO2 are different from those of N2.7 In the first place, the specific heat of CO2 is higher than that of N2; therefore, CO2 absorbs more heat than N2. In addition, N2 is a diatomic and nonpolar molecule, and CO2 is a triatomic and polar molecule. Thus, the radiant heat loss is enhanced by CO2. Finally, CO2 is not inert but takes part in the combustion reactions directly. These differences affect the flame characteristics and combustion stability of oxy-fuel mixtures directly. The lower flammability limit (LFL) is the minimum composition limit of fuel in a fuel−oxidizer mixture above which a flame can propagate.8 It is a fundamental parameter to assess the combustion stability and safety. Recent research9,10 shows that dilution gas (such as CO2) is one of the most significant factors influencing the flammability limits of combustible gas. The LFL of the combustible gas diluted with high concentrations of CO2 have been widely studied. The LFL of the CH4/CO2/air mixture were measured by Gant et al.,11 Di Benedetto et al.,12 and Wu et al.13 The effects of three different diluent gases (CO2, Ar, and N2) on the flammability limits of the binary mixtures of dimethyl ether were studied by Zhang et al.14 Salzano et al. studied the explosion behavior of syngas/air mixtures diluted by CO2 using a 5 L cylindrical vessel.15 The LFL of eight compounds diluted by CO2 were measured by Kondo et al. using a 20 L explosion bomb.16 Several models17,18 were also developed to estimate the flammability limits of combustible gas diluted by CO2. The chemical and thermal effects of CO2 on the LFL of gas fuels were also discussed.19,20 Di Benedetto et al.21 measured the anomalous pressure versus time in a 5 L closed vessel for enriched CH4/O2 mixtures. © 2016 American Chemical Society

2. EXPERIMENTAL SECTION A description of the experimental setup is given here. The LFL of the CH4/CO2/O2 mixture were measured using a 1.6 m long and 80 mm inner diameter cylindrical quartz glass tube (Figure 1). The tube was kept in a constant temperature chamber. The gas mixtures were prepared in the reactor using the partial pressure methodology.24 After vacuum, O2, CO2, and CH4 were injected one after another. The premixed flammable gas was ignited using an electric igniter at the bottom of the tube. The electric ignition was made of two copper electrodes and connected to a high-voltage transformer by wires. The high-voltage transformer, with 14 kV and a short-circuit current of 30 mA, was used for producing the ignition spark. The ignition time is set to be 0.2−0.5 s. Coward and Jones25,26 pointed out that the real flammability of gas mixtures was only dependent upon the Received: March 1, 2016 Revised: April 18, 2016 Published: April 19, 2016 4346

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Figure 2. Measurements of LFL of CH4/O2/CO2 and CH4/O2/N2 mixtures.

concentrations. The values of LFL decreased from 7.875 to 6.875% corresponding to oxygen concentrations varying from 12 to 50%. The changes of LFL versus oxygen concentrations were in a very narrow range. In the work by Di Benedetto et al.,22 the LFL of CH4/O2/CO2 was regarded as a fixed value (about 6.5%). The present work and recent research13 show that the LFL slowly decreased with the increasing oxygen concentrations. Figure 2 also shows the measurements of the LFL of the CH4/O2/N2 mixture. The LFL slightly decreased from 5.25 to 4.75% with the oxygen concentrations increasing from 10 to 40%. The LFL of the CH4/O2/N2 mixture was smaller than that of the CH4/O2/CO2 mixture, but the changing trend of LFL was similar. The differences of the LFL between CH4/O2/ CO2 and CH4/O2/N2 were mainly due to the differences of thermal, radiative, and chemical properties between CO2 and N2. Figure 3 shows the profiles of the equivalence ratios (φ) versus oxygen concentrations. The equivalence ratios at the

Figure 1. Schematic of the reactor setup. composition of gas mixtures. In the experimental measurement, the LFL was affected by the heat release produced by the igniter. Thus, “the tube length by this time had been increased to 1.5 m to ensure that excess heat from the igniter had sufficient time to dissipate from the flame zone”.26 In the present work, a 1.6 m long tube was used to eliminate the effect from the electric igniter. The appearance of a flame could be visually observed. In the European standard EN 1839(T),27 the judgment of the flame propagation was described as follows: “If a halo reaches the top of the tube or has at least a height of 240 mm this shall be also counted as an ignition”. The LFL is determined by eq 1.

LFL = (x1 + x 2)/2

(1)

where x1 is the lowest fuel concentration flame propagating and x2 is the highest fuel concentration flame not propagating. If the CH4/O2/ CO2 mixture could be ignited, a bright blue flame travels very fast from the bottom to the top of the tube. If not, a very small halo appears around the ignition spark, which indicates that combustion reactions have taken place but flame propagation has not occurred. The CH4/ O2/CO2 or CH4/O2/N2 mixture was prepared with a fixed O2/CO2 or O2/N2 ratio and a variable CH4 content for one case. At the beginning, the CH4 content is high to make sure that the flame can propagate in the tube. Then, the concentration of CH4 slightly decreased until the flame propagation did not occur. The concentration of each gas was measured each time, and the LFL was calculated by eq 1. The oxygen concentration (mole fraction) of the O2/CO2 mixture is defined as follows: XO2 =

O2 × 100% CO2 + O2

(2)

Similarly, the oxygen concentration of O2/N2 mixtures is defined as follows:

XO2 =

O2 × 100% N2 + O2

Figure 3. Profiles of the equivalence ratios versus oxygen concentrations at the LFL points.

(3)

The initial pressure and temperature were at ordinary conditions (300 K and 1 atm). For each test, at least five runs have been performed to repeat the process and average the results.

LFL points decreased with the increase of the oxygen concentrations. Results showed that the LFL points appeared at the fuel-lean side, except the low-oxygen cases (e.g., XO2 = 12 and 14% in Figure 3). As pointed out by Oh and Noh,28 the highest value of the adiabatic flame temperature appeared at φ = 1.0−1.1. Thus, the equivalence ratio is possible larger than 1.0 at the LFL points. However, the maximum value of

3. RESULTS AND DISCUSSION 3.1. Measurements of LFL. Figure 2 shows the measurements of LFL of the CH4/O2/CO2 mixture (error bars = 1 standard deviation). The LFL of CH4/O2/CO2 decreased almost linearly and slowly with the increase of oxygen 4347

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Energy & Fuels Table 1. Specific Heat of Components Cp = 8.314(a1 + a2T + a3T2 + a4T3 + a5T4) (J mol−1 K−1) product CO2 H2O N2 O2

a1 3.857460 3.033992 2.926640 3.282538

a2 × × × ×

0

10 100 100 100

4.414370 2.176918 1.487977 1.483088

a3 × × × ×

−3

−2.214814 −1.640725 −5.684760 −7.579667

10 10−3 10−3 10−3

equivalence ratio can only reach the point of the largest adiabatic flame temperature at the LFL points. If the flame propagation does not occur at the point of the largest adiabatic flame temperature, the flame propagation will not take place anymore. In the present work, some equivalence ratios are larger than 1.1, which are mainly due to the error in the measurement of LFL. Because both the contents of CH4 and O2 were small in the condition of low oxygen concentrations, a small error in the measurement of the component concentration would lead to a large error in the calculation of the equivalence ratios. More details of the experimental errors were discussed in the next paragraph. The experimental errors of the measurements of LFL were mainly from two sources: One was the errors from the experimental equipment. As pointed out by Chen et al.,23 the LFL of oxy-fuels are influenced by the equivalence ratios, dilution ratios, and temperatures. The equivalence ratios and dilution ratios depend upon the composition of the gas mixture. In the present work, the gas mixtures were prepared using the partial pressure methodology. Thus, the main source of experimental error was from the partial pressure measurements. The error in partial pressure measurements (Eq) of the each species was from the vacuum gauge used in the gas mixing system. This error was estimated to be less than 1.65%. In addition, the error in temperature control was also included. The maximum error (Et) from the temperature deviation was estimated to be 1%, because the maximum deviation of the temperature was 3 K. The other error (Ea) was from the calculation of the LFL using the mean method according to the European standard EN 1839(T), which was estimated to be about 0.685%. Equation 4 gives the overall error of the measurement of LFL in the present work. E=

Eq 2 + Et 2 + Ea 2 = 2.05%

× × × ×

10 10−7 10−7 10−7

5.234902 −9.704199 1.009704 2.094706

Q f = n f Q L = LFL

a5 × × × ×

−10

−4.720842 1.682010 −6.753351 −2.167178

10 10−11 10−10 10−10

P0V Q RT0 L

× × × ×

10−14 10−14 10−15 10−14

(6)

where nf is the moles of fuel gas, QL is heat release per mole of gas mixture, P0 is the initial pressure (1 atm), T0 is the initial temperature (298 K), R is the gas constant, and V is the specified volume through which the halo traveled. The distance L of the halo traveling from the electric igniter is at least 240 mm in our experiments. Thus, the value of V is given by eq 7 V = πD2L/4

(7)

where D is the inner diameter of the cylinder reactor (80 mm) and L is the specified distance (240 mm). It is an isobaric process for the halo to travel from the bottom to the top of the reactor. Thus, Qt is expressed as follows: Qt =

∑ products

ni

∫T

Tf

Cpi dT (8)

0

where ni is the moles of one compound (such as H2O), Tf is the final flame temperature, and Cpi is the specific heat at constant pressure Cpi = R(a1, i + a 2, iT + a3, iT 2 + a4, iT 3 + a5, iT 4)

(9)

where a1,i, a2,i, a3,i, a4,i, and a5,i are constants that are listed in Table 1. These coefficients are from the thermal data of GRIMech 3.0.30 An optically thin model (OTM) gives the value of radiant heat flux q. Thus, Qr is calculated by eq 10. The use of the OTM did not affect the final conclusions, although the radiant heat loss was overestimated.31 The OTM model is simple and accurate enough to present the radiant heat loss at the LFL points

(4)

Q r = AqΔt = απDL 4σk p(Tf 4 − To 4)Δt

3.2. Effect of High Concentrations of CO2 on the LFL. 3.2.1. Energy Balance Analysis. The energy balance analysis was performed to investigate the influence mechanism of CO2 on the LFL. The LFL is mainly determined by the balance of the heat release rate and absorbing energy rate.29 The heat release rate equates to the absorbing energy rate at the LFL points. The energy balance equation was set up according to the first law of thermodynamics26 Q f = Q t + Qr + Qh

a4 −6

(10)

where α is the effectiveness factor. It is set to be 0.5 according to the theory analysis by Zhao et al.32 σ is the Stefan− Boltzmann constant, and Δt is the average propagation time (about 0.4 s). k̅a is the effective absorption coefficient, which was calculated by a modified weighted-sum-of-gray-gases (WSGG) model.33 The coefficients of the modified WSGG model were developed from a statistical narrow-band (SNB) model. The radiative effects of a high CO2 content were considered in this model. This model was suitable for various H2O/CO2 ratios in the present work. More details of the modified WSGG model and the calculated results of k̅a at the LFL points were listed in the Supporting Information. Qh is given by eq 11

(5)

where Qf is the heat release of fuel combustion, Qt is the energy absorbing by the increase of the gas temperature, Qr is the radiant heat loss, and Qh is the heat loss through convection. Because the heat loss was mainly from the end gas, Qt, Qr, and Qh were calculated on the basis of the components of the burned gas. Qf was calculated by eq 6

Q h = απDLh(Tf − To)Δt

(11)

where h is the convective heat-transfer coefficient. It is given as follows: 4348

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h = Nu

k D

was suitable for the kinetic simulations of oxy-fuel mixtures.3,5,37 Panels a−d of Figure 4 show the changes of the laminar flame speeds of the CH4/O2 /CO2 mixture versus CH4 concentrations in the condition of various CO2/O2 ratios. Figure 4 also presented the measurements of the laminar flame speeds of CH4/O2/CO2 in the work by Xie et al.37 and our previous work.3 The differences of laminar flame speeds between the CH4/O2/FCO2 and CH4/O2/CO2 mixtures became smaller with the decrease of the CH4 concentrations. The laminar flame speeds of CH4/O2/FCO2 were very close to those of CH4/O2/CO2 near the LFL point (because the Jacobian matrix is singular,38 the laminar flame speed of the CH4/O2/CO2 mixture cannot be obtained with the PREMIX code at the LFL point). The former was less than 5 percentage points higher than the latter. Thus, the chemical effect of CO2 had an insignificant effect on the combustion process at the LFL point. The major reason is that CH4 is burnt out and O2 is excess at the LFL points. As pointed out by Liu et al.,36 the most important reaction in which CO2 participates directly is reaction CO + OH ⇄ CO2 + H (R1). CO2 competes for the H radical with the most important chain branching reaction H + O2 ⇄ O + OH (R2) in CH4/O2/CO2 flame. There is too much O2 at the LFL point. Excess O2 will improve the reaction rate of R2 and compete with H with the reaction R1. The reaction rate of R1 decreases with the decrease of H. Thus, the chemical effect of CO2 is deeply depressed at the LFL points. Our previous study showed that the change rate of heating potential with the chemical effect of CO2 was less than 1% at the LFL points for the CH4/O2/CO2 mixture.9 Thus, the chemical effect

(12)

where Nu is the Nusselt number, which is set to be 3.65.34 k is the thermal conductivity of the gas mixture. The effect of H2O was ignored, because the content of H2O is very small. Table 2 listed the thermal conductivity for compounds that are from the gas data book.35 Table 2. Thermal Conductivity of Components k = A + BT + CT2 (W m−1 K−1) product

A

B

C

CO2 O2 N2

−0.01289 0.00121 0.00309

1.0390 × 10−4 8.6157 × 10−5 7.5930 × 10−5

−2.1445 × 10−8 −1.3346 × 10−8 −1.1014 × 10−8

3.2.2. Chemical Effect of CO2 on the LFL. Because CO2 takes part in combustion reactions directly, the high concentration of CO2 affects the combustion process directly, which could change the heat release of fuel (QL). In the present work, an artificial species FCO2 was employed to distinguish the chemical effect of CO2.36 FCO2 has the same thermal and transport properties of CO2 but does not take part in the chemical reactions. The laminar flame speed is one of the most fundamental parameters describing combustion characteristics. The laminar flame speeds of CH4/O2/CO2 and CH4/O2/FCO2 mixtures were calculated using the PREMIX code of Chemkin software with the GRI-Mech 3.0 mechanism. The GRI-Mech 3.0 mechanism is one of the most popular mechanisms for methane oxidation. Recent research showed that GRI-Mech 3.0

Figure 4. Laminar flame speeds of CH4/O2/CO2 and CH4/O2/FCO2 mixtures versus CH4 concentrations in the condition of varous CO2/O2 ratios: (a) CO2/O2 = 0.25/0.75, (b) CO2/O2 = 0.30/0.70, (c) CO2/O2 = 0.40/0.60, and (d) CO2/O2 = 0.50/0.50. 4349

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Figure 5. Proportions of Qt, Qr, and Qh for the CH4/O2/CO2 mixture.

of CO2 has little impact on the value of QL. The value of QL is approximately the low calorific value of CH4 (802 kJ/mol). The value of Qf is only related to the value of the LFL in the common condition (1 atm and 298 K) according to eq 6. Thus, the LFL are determined by Qt, Qr, and Qh as a result of the energy balance described by eq 5. 3.2.3. Thermal and Radiative Effects of CO2 on the LFL. According to eq 5, Qt and Qr presented the thermal and radiative effects of CO2 on the LFL, respectively. Figure 5 shows the proportions of Qt, Qr, and Qh in the conditions of various oxygen fractions. Here, the proportions of Qt, Qr, and Qh were calculated by eqs 13−15, respectively. Q t (%) =

Q r (%) =

Q h (%) =

Qt Q t + Qr + Qh

Qr Q t + Qr + Qh Qh Q t + Qr + Qh

× 100% (13)

× 100% (14)

Figure 6. Proportions of Qt, Qr, and Qh for the CH4/O2/N2 mixture.

× 100%

In addition, the changes of the proportions of Qt, Qr, and Qh versus various oxygen fractions were not obvious for both CH4/ O2/CO2 and CH4/O2/N2 mixtures. This indicated that the thermal and radiative effects of CO2 on the LFL were not significantly affected by the various oxygen concentrations. This also explained why the changes of LFL versus oxygen concentrations were in a very narrow range (in Figure 2). 3.3. Predictions of LFL of the CH4/O2/CO2 Mixture. The CFT is a useful method to estimate LFL of gas mixtures.32 Methods used to estimate flammability limits were divided into two classifications: chemical equilibrium methods and empirical correlations. According to the previous research, the calculated adiabatic flame temperature (CAFT) method is the most basic method in the chemical equilibrium methods.8 Some theory models29,39 are all based on the CAFT method. Recently, Di Benedetto et al.40 defined the adiabatic flammability limit to study the flammability of the fuel/air mixture. They predicted the LFL of gas mixtures with the adiabatic flame temperature. This method was based on the CAFT method. The CFT method was also developed from the CAFT method. The flame temperature was used to estimate the LFL in the CFT

(15)

Qt accounted for more than 93%; Qr accounted for about 5.1%; and Qh accounted for less than 1.1%. The proportions of Qt, Qr, and Qh of the CH4/O2/N2 mixture at the LFL points were presented in Figure 6 for comparison. Results show that Qt accounted for more than 96.5%, Qr accounted for less than 2.7%, and Qh accounted for about 0.8% in the condition of various oxygen concentrations. The proportion of Qr of CH4/ O2/CO2 was larger than that of CH4/O2/N2. This was due to the enhanced radiative effect of high levels of CO2. The proportion of Qt of the CH4/O2/CO2 mixture was smaller than that of the CH4/O2/N2 mixture as a result of the increase of Qr of the CH4/O2/CO2 mixture. However, the proportions of Qt account for more than 93% for both the CH4/O2/CO2 and CH4/O2/N2 mixtures. Thus, the LFL of the two mixtures mainly depended upon the thermal properties of gas mixtures. The radiative effect of CO2 on the LFL was a second concern and very small. In other words, the differences in LFL between CH4/O2/CO2 and CH4/O2/N2 were mainly due to the differences of thermodynamic properties between CO2 and N2. 4350

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Energy & Fuels method.32 Both of the methods used a critical temperature to predict the LFL of the gas mixture; therefore, the two methods have the same advantages and disadvantages. The adiabatic flame temperature is the temperature without heat losses. In the present work, the flame temperatures were established considering the effect of thermal and radiative effects in the previous section. The CFT method provides a more accurate prediction of LFL compared to the CAFT method. Figure 7 shows the flame temperature profiles of CH4/O2/ CO2. The flame temperatures were from 1412.1 to 1439.5 K.

Figure 9. Predictions and measurements of LFL of the CH4/O2/CO2 mixture.

predicted LFL of the CH4/O2/CO2 mixture were in a good agreement with the experimental measurements. The maximum absolute difference between the two was 0.185 vol %, and the average relative difference was 1.25%. Similarly, the critical temperature (Tc = 1411 K) of the CH4/O2/N2 mixture was also validated in Figure 10. The average relative difference

Figure 7. Flame temperature profiles of the CH4/O2/CO2 mixture at the LFL points.

The temperatures were not a fixed value but in a narrow ranges. To keep prediction of LFL simple, we assumed that the flame temperature was constant. The average value of these flame temperatures was chosen as the critical temperature (Tcr), which determines the LFL. Thus, the critical temperature of the CH4/O2/CO2 mixture was 1420 K from Figure 7. For comparison, the critical temperature (Tcr = 1411 K) of the CH4/O2/N2 mixture was obtained from Figure 8 in the same way. The LFL of the CH4/O2/CO2 mixture were estimated using the critical temperature (Tc = 1420 K). The comparisons between the predictions and measurements of LFL were performed to validate the critical temperature of the CFT method. Figure 9 shows the predictions of LFL of CH4/O2/ CO2 compared to the experimental measurements. The

Figure 10. Predictions and measurements of LFL of the CH4/O2/N2 mixture.

between measurements and predicted LFL of CH4/O2/N2 was 3.2%, while the maximum absolute difference was 0.415 vol %. These errors are from the assumption that the flame temperature is a fixed value. The errors are within acceptable limits in engineering estimation. This assumption makes the estimation of LFL become simple. In the present work, the LFL of CH4/O2/CO2 and CH4/O2/N2 mixtures were well-predicted using the CFT method. This method is simple and appropriate to apply in engineering estimation.

4. CONCLUSION In the present work, the LFL of the CH4/O2/CO2 mixture were investigated with experimental measurements first. Then the chemical, thermal, and radiative effects of high concentrations of CO2 on the LFL were analyzed. Finally, the LFL were predicted using the CFT method. The major conclusions are as follows: (1) The experimental LFL of CH4/O2/CO2 varied from 6.875 to 7.875% corresponding to the oxygen concentrations varying from 50 to 12%. The LFL of the CH4/ O2/CO2 mixture were higher than that of the CH4/O2/N2

Figure 8. Flame temperature profiles of the CH4/O2/N2 mixture at the LFL points. 4351

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(16) Kondo, S.; Takizawa, K.; Takahashi, A.; Tokuhashi, K. J. Hazard. Mater. 2006, 138, 1−8. (17) Chen, C.-C.; Liaw, H.-J.; Wang, T.-C.; Lin, C.-Y. J. Hazard. Mater. 2009, 163, 795−803. (18) Wang, T. C.; Chen, C. C.; Chen, H. C. J. Taiwan Inst. Chem. Eng. 2010, 41, 453−464. (19) Ma, T.; Wang, Q.; Larrañaga, M. D. Fire Saf. J. 2013, 56, 9−19. (20) Jung, S. W.; Park, J.; Kwon, O. B.; et al. Fuel 2014, 136, 69−78. (21) Di Benedetto, A.; Cammarota, F.; Di Sarli, V.; Salzano, E.; Russo, G. Combust. Flame 2011, 158, 2214−9. (22) Di Benedetto, A.; Cammarota, F.; Di Sarli, V.; Salzano, E.; Russo, G. Chem. Eng. Sci. 2012, 84, 142−7. (23) Chen, L.; Yong, S. Z.; Ghoniem, A. F. Prog. Energy Combust. Sci. 2012, 38, 156−214. (24) International Organization for Standardization (ISO). ISO 6141:1979, Gas AnalysisCalibration Gas MixturesCertificate of Mixture Preparation; ISO: Geneva, Switzerland, 1979. (25) Coward, H. F.; Jones, G. W. Limits of Flammability of Gases and Vapors; U.S. Bureau of Mines: Washington, D.C., 1952; Bulletin 503. (26) Britton, L. G. Process Saf. Prog. 2002, 21, 1−11. (27) European Committee for Standardization (CEN). EN 1839 2003, Determination of Explosion Limits of Gases and Vapours; CEN: Brussels, Belgium, 2003. (28) Oh, J.; Noh, D. Energy 2012, 45, 669−75. (29) Ma, T. Fire Saf. J. 2011, 46, 558−67. (30) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C., Jr.; Lissianski, V. V.; Qin, Z. http://www.me.berkeley. edu/gri_mech (accessed April 15, 2016). (31) Chen, Z.; Qin, X.; Xu, B.; Ju, Y.; Liu, F. Proc. Combust. Inst. 2007, 31, 2693−700. (32) Zhao, F.; Rogers, W. J.; Mannan, M. S. J. Hazard. Mater. 2010, 174, 416−23. (33) Johansson, R.; Leckner, B.; Andersson, K.; Johnsson, F. Combust. Flame 2011, 158, 893−901. (34) Mayer, E. Combust. Flame 1957, 1, 438−52. (35) Yaws, C. Matheson Gas Data Book, 7th ed.; McGraw-Hill Professional: Parsippany, NJ, 2001. (36) Liu, F. S.; Guo, H. S.; Smallwood, G. J. Combust. Flame 2003, 133, 495−7. (37) Xie, Y. L.; Wang, J. H.; Zhang, M.; Gong, J.; Jin, W.; Huang, Z. H. Energy Fuels 2013, 27, 6231−7. (38) Nishioka, M.; Law, C. K.; Takeno, T. Combust. Flame 1996, 104, 328−42. (39) Britton, L. G.; Harrison, B. K. Process Saf. Prog. 2014, 33, 314− 28. (40) Di Benedetto, A. Chem. Eng. Sci. 2013, 99, 265−73.

mixture in the same oxygen concentrations. (2) The LFL of CH4/O2/CO2 mainly depend upon the thermal property of gas mixtures. The radiative effect of CO2 on the LFL was much smaller than the thermal effect, although the radiative effect was enhanced by the high level of CO2. The chemical effect of CO2 on the LFL was highly depressed by excess O2. (3) The CFT method was used for estimating the LFL of the CH4/O2/CO2 mixture. The critical temperature for predicting LFL of the CH4/O2/CO2 mixture was 1422 K, while the critical temperature for predicting LFL of the CH4/O2/N2 mixture was 1411 K. The predicted values of LFL were in good agreement with the experimental data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00492. Details of the modified WSGG model and the calculated effective absorption coefficients (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-24-83672216. E-mail: [email protected]. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51274066) an the National Key Technologies R&D Program of China (2013BAA03B03).

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b00492 Energy Fuels 2016, 30, 4346−4352