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Experimental study of flammability limits of oxy-methane mixture and calculation based on thermal theory
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Experimental study of flammability limits of oxy-methane mixture and calculation based on thermal theory Xianzhong Hu, Qingbo Yu*, Nan Sun, Qin Qin School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning 110819, PR China
article info
abstract
Article history:
The flammability limits of CH4/CO2/O2 were studied using theory calculations and exper-
Received 25 January 2014
imental investigation. A calculation model was re-deduced based on thermal theory. In
Received in revised form
this model, the chemical effects of CO2 on flammability limits are concerned. Results show
18 March 2014
that the chemical effects of CO2 decrease the upper flammability limits (UFL) of mixtures,
Accepted 26 March 2014
while have little effect on the lower flammability limits (LFL). Experimental measurements
Available online 24 April 2014
were performed using a cylinder reactor. The measurements of the LFL of CH4/O2/CO2 increase from 5% to 8.25% with the increase of CO2 concentration, and the UFL of mixture
Keywords:
CH4/O2/CO2 decrease from 61% to 8.25% accordingly. The model predicted the LFL very
Flammability limits
well, but the calculated values are an average 10.7% higher than the measurements at the
Thermal theory
UFL points. The differences between the predictions and measurements are mainly due to
Carbon dioxide
the two assumptions of this model.
Chemical effect
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With more concerns about carbon dioxide reduction, the technologies using pure oxygen are increasing in industry such as oxy-fuel combustion technology. However, fire and explosion safety issues [1] appeared, which caused by such mixtures. Generally, adding inert gas is always used to decrease the reaction temperature of gas mixtures using pure oxygen. The inert gas includes CO2, N2, He, Argon even or steam. Among them, CO2 is usually used as inert gas for oxyfuel combustion, because the main products are CO2 and H2O, and there is no N2 anymore. It is very easy to separate CO2 from H2O by condensation. The pure CO2 can be stored or used in other industrial field, which reduce the emission of
greenhouse gas. Hence, special attention should be paid for explosion safety issues of these mixtures. Flammability limit is one of main index to judge flame stability and explosion characteristics of fuel gases. The effects of carbon dioxide on the flammability limits of combustible gas have been widely studied. Chen et al. developed a theoretical model based on mass and energy balance to predict the upper/lower flammability limits of flammable mixtures composed of fuel gas, inert carbon dioxide and air [2]. Wang et al. developed a semi-empirical model for predicating upper flammability limits for flammable gases which mixed with fuel gas, air and carbon dioxide [3]. Kondo et al. measured the flammability limits of six kinds of combustible gas mixed with carbon dioxide and air using a 20L explosion bomb [4]. Gant et al. used an equipment (8 m long and 1.04 m
* Corresponding author. P.O. Box 345, Northeastern University, No. 11, Lane 3, Wenhua Road, Heping District, Shenyang, Liaoning, PR China. Tel./fax: þ86 24 83672216. E-mail address: [email protected] (Q. Yu). http://dx.doi.org/10.1016/j.ijhydene.2014.03.202 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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diameter pipeline) to study the effect of carbon dioxide on flame stability and ignition probability of CH4/CO2/air mixture in free-jets [5]. Wu et al. studied the flammability characteristics of flammable gases (CH4/CO2/air, CH4/N2/air, CH4/Ar/air) using a 20L-Sphere [6]. Generally, all these research works focus on the flammability limits of flammable gas (including fuel and inert gas) in air. And seldom concern with the flammability limits of fuel gas at O2/CO2 atmosphere. Flammability limits of fuel/O2/CO2 are related to flame stability and fire safety of oxy-combustion technology. Thus, it is necessary to investigate the flammability limits of oxy-fuel mixture. In the mixture of CH4/CO2/O2, pure O2 is used as an oxidizing agent and CO2 is used for dilution. The properties of CO2 in flammable gases are different from N2 in air. Firstly, the transport, thermal and radiative properties of CO2 are different from N2. It is noted that the specific heat and gas absorption coefficient of CO2 is higher than N2. Secondly, CO2 is not inert but directly participates in chemical reactions [7,8]. For these differences above, the characteristics of the flammability limits of CH4 at CO2/O2 atmosphere are not alike with traditional condition. Research works [3,4] considered the effect of thermal and radiative properties of CO2 on flammability limits, but the chemical effect of CO2 did not draw enough attention. Previous research works [7,8] found that the chemical effects of CO2 on combustion reactions could not be neglected when the concentration of CO2 is high. There are several methods to calculate the upper flammability limits (UFL) and lower flammability limits (LFL) of a mixture diluted with inert gas. Le Chatelier’s rule [9] was developed and applied to predict the LFL and UFL of the mixture which consist of multiple fuel and inert gases in air. Kodo et al. [4] tried to develop an extended Le Chatelier’s rule to predict the UFL more accurately, as Le Chatelier’s rule is not usually well used for calculating the UFL. But, both the Le Chatelier’s rule and extended Le Chatelier’s rule are not fit for calculating the UFL and LFL of CH4/CO2/O2, because there is
only single fuel (methane) in the mixture and some parameters are needed from experimental measurements which are not clear now. Hansel et al. [10] tried to estimate flammability limits using adiabatic flame temperature. There are several applications of this method [10e12], but it is only suitable for calculating the LFL and not well used for predicting the UFL. Ma [13] provided a thermal theory to estimate the flammability limits without using experimental data. This method quantifies the heating and quenching potentials of each element of a mixture. The UFL and LFL are determined when the heating potentials equals quenching potentials. It is developed for predicting the flammability limits of flammable gas (including fuel and inert gas) with air. So it cannot be used for prediction of the LFL and UFL of fuel/CO2/O2 directly. In present work, we analyzed the effect of CO2 concentration on CH4/CO2/O2 explosion. The flammability limits of methane at CO2/O2 atmosphere were measured using a cylindrical glass reactor. Calculations were carried out using the thermal theory [13] for predicting the flammability limits of a mixture of CH4/O2/CO2. The calculation model based on thermal theory was re-deduced for O2/CO2 atmosphere. The chemical effects of CO2 on flammability limits were considered in this calculation model.
Experimental method The experimental investigation of flammability limits was performed in a cylindrical glass tube. The schematic representation of the experimental rig is shown in Fig. 1. The reactor consists of a 1.6 m long and 80 mm inner diameter cylindrical quartz glass tube. The environmental temperature around the reactor keeps constant (300 K) during experimental process to make sure the initial temperature of flammable gases is constant. The temperature is controlled by a system which consists of a temperature controller, a ring
Fig. 1 e Schematic representation of the experimental rig.
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heater and two thermal couples. The initial pressure of mixtures in reactor is prepared keeping ordinary pressure (1 bar) before ignition. A circulating pump is used to stir the mixture. The ignition sparks for premixed gases are produced by a 14 KV spark generator cross two copper electrodes at the bottom of the reactor. The spark duration time is 0.4 s. The flame is observed visually in the dark. The mixtures are determined to be flammable using the European standard EN 1839(T). 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 [14]. The effect of CO2 dilution is evaluated through the flammability limits measurement of methane at various concentration of CO2. In this paper, mole concentration of CO2 is defined as follows: xd ¼
CO2 CO2 þ O2 þ CH4
(1)
The gases mixtures are prepared in the reactor using partial pressure methodology. Evacuate the reactor and break vacuum, then methane is first injected into the reactor, the oxygen second and carbon dioxide last. A circulating pump is used to stir the mixtures for about 20 min to make sure that mixtures are well blended. Purity of methane and oxygen both are 99.9%. Purity of carbon dioxide was better than 99.95%. All the gases are supplied by gas cylinders.
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(methane) and HO is the heating potential of oxygen. The quenching potential is the enthalpy change at critical adiabatic flame temperature with respect to that of air, which is determined by its thermal properties. The heating potential is due to the combustion reactions of flammable gas. All of the five parameters above are dimensionless based on the quenching potential of air. The quenching potential of air is regarded as 1. xL is volumetric ratio of fuel at the lower flame limit or the LFL, and xU is volumetric ratio of fuel at the higher flame limit or the UFL. On the right side of equations (3) and (4), there are the terms of quenching potential of methane, carbon dioxide and pure oxygen respectively. On the left side of equation (3), there is the heating potential of mixture, which is determined by the heat resale of fuel, because the fuel is burned out at the lower flammability limit point. In the same way, the heating potential of mixture is determined by the amount of oxygen at the upper flammability limit point on the left side of equation (4), because the oxygen is lean at the upper flammability limit point. The basis of equations (3) and (4) is energy balance and two major assumptions mentioned above. From equations (3) and (4), HF, QF and HO can be calculated through equations (5)e(7). HF ¼ 2HO QF ¼
ð2xL xD ð1xU xD ÞÞQD þð2xL ð1xL xD ÞÞð1xU xD ÞQO2 xL ð1xU xD 2xU Þ
Theory calculation model based on thermal theory
(6)
A thermal theory for estimating the flammability limits for a mixture was developed by Ma. The theory is based on two major assumptions: One is that the calculated adiabatic flame temperature keeps constant; the other is that the reaction at the UFL point is complete and similar to the reaction at the LFL point [13]. The thermal theory is successfully used for predicting the flammability limits of the fuel with air. But the calculation equations are not suitable for methane with pure oxygen and carbon dioxide. Thus the calculation model should be rededuced based on thermal theory. The derivation process of the calculation of flammability limits of CH4/CO2/O2 is presented as follows. The combustion of CH4/CO2/O2 can be expressed by: CH4 þ 2O2 þ Cd CO2 /CO2 þ 2H2 O þ Cd CO2
(2)
Where, the subscript d stands for dilution. At the lower flammability limit point, the heat released equals to the quenching energy, so the energy balance equation is as follows: xL QF þ xD QD þ ð1 xL xD ÞQO2 ¼ xL HF
HO ¼
ðxL ð1 xU xD Þ xU ð1 xL xD ÞÞQO2 þ ðxL xD xU xD ÞQD xL ð1 xU xD Þ 2xL xU (7)
The value of QF and HO can be obtained at the critical condition for a certain fuel/dilution system, which can be found via any Material Safety Data Sheet. Referring back to equations (3) and (4), we determine the following relationships: xL ¼
QO2 ðxD 1Þ xD QD QF QO2 HF
(8)
xU ¼
HO ð1 xD Þ QO2 ð1 xD Þ xD QD QF QO2 þ HO
(9)
Using equations (8) and (9), we could calculate the flammability limits of CH4/CO2/O2. In calculation equations, the effects of thermal and radiative properties of CO2 on flammability limits are concerned in QD. But the chemical effects of CO2 on combustion have not been included. The further investigation of the chemical effects of CO2 will be discussed in Section 4.2.
(3)
Similarly, at the upper flammability limit point, the balance equation is as follows: xU QF þ xD QD þ ð1 xU xD ÞQO2 ¼ ð1 xU xD ÞHO
(5)
(4)
Where QD is the quenching potential of dilution (CO2), and QO2 is the quenching potential of pure oxygen, QF is the quenching potential of fuel (methane), HF is the heating potential of fuel
Results and discussion Experimental data and theoretical calculation The flammability of the mixture of CH4/O2/CO2 was measured in this paper. Fig. 2 shows the experimental measurements of
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Fig. 2 e Flammability diagrams for CH4/CO2/O2.
for CH4 combustion. The GRI Mech 3.0 has been the most popular mechanism for simulating CH4 flame, which has been validated by large experimental data [16,17]. In numerical calculation, a strategy proposed in previous study [8,18] to numerically isolate the chemical effects of CO2 was employed. In this strategy, CO2 was used for CH4/CO2/O2, and FCO2 was used for CH4/FCO2/O2. Here FCO2 is a fictitious species that has identical thermal and transport properties as CO2. But it is artificially excluded from chemical reactions. In addition, FCO2 was assigned the same third-body collision efficiency as CO2 in all relevant reactions. In numerical model, the temperature was set to be 1650 K and pressure was ordinary (1 bar) [13]. The critical concentrations of methane at the LFL and UFL points are from equations (8) and (9) with data list in Table 1. The chemical effects of CO2 on heating potential are quantified by the differences between the results of heat release of mixtures of CH4/CO2/O2 and CH4/FCO2/O2. The change rate of heating potential is calculated by equation (10), e¼
the UFL and LFL. The calculation results are also given in this figure for comparison. Theoretical UFL were calculated using HO (dash line). As seen in Fig. 2, the LFL of CH4/O2/CO2 increase almost linearly and slowly as the concentration of dilution gas (CO2) increase. The results of LFL vary within a very narrow band, which are from 5% to 8.25%. The average absolute differences between the experimental measurements and theoretical prediction of the UFL are less than 0.6% vol%. There is a good agreement between the observed data and calculated values. The upper flammability limits of mixture CH4/O2/CO2 decrease with the increase of concentration of CO2. There is not a linear relationship between the measurements of the UFL and the concentration of CO2. With the increase of CO2, the UFL rates of decline become smaller and smaller. According to equation (9), there is a linear relationship between the prediction of the UFL and concentration of CO2. Thus the theory calculations of the UFL are not in good agreements with experimental measurements. The theoretical calculated UFL values using HO are always larger than experimental measurements. It is presumed that the heat release is overestimated in the theory prediction. Generally, CO2 is always regarded as inert gas in combustion. As mentioned above, CO2 is not inert but directly participates in chemical reactions at O2/CO2 atmosphere. Research [8] showed that high concentration of CO2 decreased the burning rate of CH4 because of the direct chemical participation of CO2. In other words, the chemical effects of CO2 reduce the amount of heat release. Thus, HO which characterizes the heating potential at the UFL point should be reduced.
Chemical effects of CO2 on flammability limits In order to give a quantitative analysis of heating potential at the UFL point, the heat release on the combustion was investigated with kinetic simulation. Numerical calculation of heat release of flammable gas is carried out using 0-D Closed Homogeneous Reactor of the CHEMKIN packages. In calculation, the GRI Mech 3.0 [15] mechanism is employed. It consists of 53 species and 325 reactions, which is originally developed
HFCO2 HCO2 100% HCO2
(10)
Where HFCO2 is the heat release of the mixture CH4/FCO2/O2 from combustion reaction, and HCO2 is the heat release of the mixture CH4/CO2/O2. Fig. 3 shows the change rate of heating potential from combustion reaction at LFL point. The change rate of heating potential is very small. The max change rate is less than 1%. So the HF changes little at the LFL point, which is no need to be modified. Generally, there is excessive oxygen and lack of fuel at the LFL point. The chemical effects of CO2 on reaction are inhibited because of too much oxygen which improved the forward reactions in combustion. Fig. 4 shows the change rate of heating potential from combustion reaction at the UFL point. The rate of change of heating potential at the UFL point is much bigger than which at the LFL point. As xD increases, the rate of change increases from 0 to the top at xD ¼ 0.51 and then decreases to nearly 0 at xD ¼ 0.8. The max value of change rate is about 10.7%. Thus, the chemical effects of CO2 on the change of heating potential should not be neglected. The terms HO should be reduced when carbon dioxide used as dilution. In this paper, it is assumed that the change rate of heating potential (e) is a function of concentration of dilution gas (xD) as followed: e ¼ AxD þ Bx3D
(11)
Where A and B are unspecified parameters whose values are determined by fitted curve from the data of numerical simulation. For methane, A ¼ 0.307, B ¼ 0.511 and the squared correlation coefficient (R2) is greater than 0.94. Through the analysis above, the heat release from the chemical effects of CO2 should be removed from the heating potential HO. In order to distinguish with HO which comes
Table 1 e Spreadsheet calculation on the flammability limits of CH4/CO2/O2. Fuel Methane
QO2
QD
QF
QAIR
HF
HO
1.046
1.75
12.276
1
8.891
14.383
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Fig. 5 e Calculated adiabatic flame temperature profile at the LFL and UFL points. Fig. 3 e Change rate of heating potential from combustion reaction at the LFL point.
from equation (7), the new heating potential was expressed as H0o . From equation 10 and 11, the new heating potential H0o at the UFL point is given below: H0O ¼ HO HFCO2 HCO2 ¼ HO ð1 eÞ ¼ HO 1 AxD þ Bx3D
flammable gas. But the average absolute difference between the measurement and calculation is 2.3% vol%, which corresponds to the average relative difference of 10.7% relative percent. The agreement between the experiment and theoretical prediction is by no means good. Besides the chemical effects of CO2, there are other factors caused the differences, which are discussed in the next section.
(12)
It is noted that equation (5) HF ¼ 2HO is not tenable for H0o here. Fig. 2 also gives the calculation results of the UFL using equations (9) and (12). Obviously, the predicted results using H0o (solid line) are much closer to the experimental measurements than which using HO (dash line). It means that equation (9) with H0o is more applicable for the UFL prediction of
Fig. 4 e Change rate of heating potential from combustion reaction at the UFL point.
Discussions In this paper, the calculation model for flame limits is very simple, which is developed to apply to engineering estimation. Although it is not very exact and precise, better prediction requires more complex models. This model’s weakness is that the prediction of the UFL using this model is slightly higher than the actual value. So some small fixes and improvements sometimes are needed. The source of the errors and suggestions for improvement of this model are discussed below. As mentioned earlier, two major assumptions are used in thermal theory. One assumption is that the calculation of flame temperature is fixed. This assumption is reasonable for the LFL points. But it is different from the combustion state at the UFL points. In order to deeply understand the deviation between the assumption and the reality, the adiabatic flame temperature was numerically calculated at the LFL and UFL points using the EQUIL code of the CHEMKIN package with GRI-Mech 3.0 kinetic mechanism. The calculation parameters including xL, xU, and xd are from the experimental measurements in Section 4.1. Fig. 5 shows the calculated adiabatic temperature (CAFT) profile at the LFL and UFL points. With the increase of the concentration of dilution (CO2), the adiabatic flame temperatures at the UFL points almost linearly decrease, but the adiabatic flame temperatures at the LFL points stay around 1550 K until the concentration of dilution greater than 0.775 which is near the inertization point. In most condition (xd < 0.675), the adiabatic flame temperatures at the UFL are higher than which at the LFL. In other words, the adiabatic
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flame temperatures which determine xU are underestimated in the assumption, because the calculated flame temperatures are assumed to be fixed in the thermal theory. Fig. 6 shows an example to present how to estimate the UFL and LFL of gas mixtures using calculated adiabatic flame temperatures (CAFT) method [19]. Once the critical temperature (Tc) is chosen, there are two intersections between the critical temperature line and calculated adiabatic flame temperature line. Then the UFL (xU) and LFL (xL) are determined by the two intersections. The upper flame limits increase with the decrease of critical temperature. In thermal theory, the Tc was set to be 1650 K for methane combustion, which is reasonable for the LFL points but underestimated for the UFL points. Thus the predictions of upper flame limits are bigger than actual values, which have been validated by experimental measurements (Fig. 2). This is one of the main error sources in prediction of upper flammability limits. The other assumption is that the flame structure at the LFL is similar to the UFL. It is well known that the fuel gas is lean reactant at the LFL point, while oxygen is lean reactant at the UFL point. There is little incomplete reaction at the LFL point, but it is different at the UFL point. In this paper, the chemical effects of CO2 on reaction have been considered, but this only could be regarded as part effects of incomplete reactions. In literature [13], incomplete reactions near the inertion point were discussed and equations of extended HO were illustrated to calculate the flammability limits concerning the incomplete reaction effects on combustion. In this paper, the same methods were also used to get a further modified HO. Equations (13) and (14) are the expressions of modified HO, which are derivate from equations (3) and (4) respectively. HO ¼
xF1 QF þ xD1 QD1 þ ð1 xF1 xD1 ÞQO2 2xF1
(13)
HO ¼
xF1 QF þ xD1 QD1 þ ð1 xF1 xD1 ÞQO2 ð1 xF1 xD1 Þ
(14)
Where, QF, QD1, QO2 keep constant, and the parameters xF1, xD1 are determined by experimental measurements. Equation (13) is used for prediction of the LFL with equation (5). And equation (14) is used for the UFL calculation. The two assumptions are still used widely and popular for estimation methods of flammability limits such as LCR and Beyler’s method [20]. Because the two assumptions make the prediction of flammability limits become possible and easy. Compared to other methods, the calculation model in this paper succeeds the advantages of thermal theory, and the chemical effects on flammability limits of CO2 are concerned in addition. So it is better than other methods which based on the assumption of a constant critical adiabatic flame temperature.
Conclusions In this work, a calculation model based on the thermal theory [13] was re-deduced to predict the flammability limits of CH4/ O2/CO2. It is note that, the chemical effects of CO2 on flammability limits were concerned in this model. Results show that the chemical effects of CO2 on lower flammability limits are little, but larger on upper flammability limits. At the UFL point, the chemical effects of CO2 reduce the heating potential of mixture, which decrease the upper flammability limits. The calculation model can be used for the flammability limits prediction of other hydrocarbon fuels at O2/CO2 atmosphere with a little modification. Experiments were also carried out to investigate the flammability limits of CH4/O2/CO2. Results show that the lower flammability limits of CH4/O2/CO2 increase from 5% to 8.25% as the concentration of CO2 increase, while the upper flammability limits of mixture CH4/O2/CO2 decrease from 61% to 8.25%. As compared to the prediction of theory calculation model, there is a good agreement between the experimental data and calculated values at the LFL point, while the calculated values are an average of 10.7% higher than the measurements at the UFL points. Though discussions, these errors are mainly due to the two major assumptions of the thermal theory.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Cantact No. 51274066), The National Key Technologies R&D Program of China (Cantact No. 2013BAA03B03).
Nomenclature HO HF
Fig. 6 e Estimation of flammability limits using calculated adiabatic flame temperatures (CAFT) method.
LCR QD
the heating potential of oxygen based on air (dimensionless) the heating potential of fuel based on air (dimensionless) Le Chatelier’s rule the quenching potential of diluent based on air (dimensionless)
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QF UFL LFL xL xU CAFT
the quenching potential of fuel based on air (dimensionless) upper flammability limit (volume ratio) lower flammability limit (volume ratio) lower flammability limit (volume ratio) upper flammability limit (volume ratio) calculated adiabatic flame temperatures
Subscripts L lower flammable limit U upper flammable limit D diluent-based potential to air potential F fuel-based potential to air potential O oxygen-based potential to air potential
references
[1] Di Benedetto A, Di Sarli V, Salzano E, Cammarota F, Russo G. Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures. Int J Hydrogen Energy 2009;3:6970e8. [2] Chen CC, Liawa HJ, Wang TC, Lin CY. Carbon dioxide dilution effect on flammability limits for hydrocarbons. J Hazard Mater 2009;163(2):795e803. [3] Wang TC, Chen CC, Chen HC. Nitrogen and carbon dioxide dilution effect on upper flammability limits for organic compound containing carbon, hydrogen and oxygen atoms. J Taiwan Inst Chem E 2010;41:453e64. [4] Kondo S, Takizawa K, Takahashi A, Tokuhashi K. Extended Le Chatelier’s formula for carbon dioxide dilution effect on flammability limits. J Hazard Mater 2006;A138(1):1e8. [5] Gant SE, Pursell MR, Lea CJ, Fletcher J, Rattigan W, Thyer AM, et al. Flammability of hydrocarbon and carbon dioxide mixtures. Process Saf 2011;89(6):472e81. [6] Wu SY, Lin NK, Shu CM. Effects of flammability characteristics of methane with three inert gases. Process Saf Prog 2010;29(4):349e52. [7] Masri AR, Dibble RW, Barlow RS. Chemical kinetic effects in nonpremixed flames of H2/CO2 fuel. Combust Flame 1992;91:285e309.
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[8] Liu FS, Guo HS, Smallwood GJ. The chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames. Combust Flame 2003;133:495e7. [9] Le Chatelier H, Boudouard O. Limits of flammability of gaseous mixtures. Bull Soc Chim (Paris) 1898;19:483e8. [10] Hansel JG, Mitchell JW, Klotz HC. Predicting and controlling flammability of multiple fuel and multiple inert mixtures. Plant Oper Prog 1992;11(4):213e7. [11] Vidal M, Wong W, Rogers WJ, Mannan MS. Evaluation of lower flammability limits of fueleairediluent mixtures using calculated adiabatic flame temperatures. J Hazard Mater 2006;130(1e2):21e7. [12] Zhao F, Rogers WJ, Mannan MS. Calculated flame temperature (CFT) modeling of fuel mixture lower flammability limits. J Hazard Mater 2010;119(1):416e23. [13] Ma TG. A thermal theory for estimating the flammability limits of a mixture. Fire Saf J 2011;46:558e67. [14] EN 1839. Determination of explosion limits of gases and vapours. European Committee for Standardization; 2003. [15] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al. GRI Mech 3.0 [last access: 03.06.14], http://www.me.berkeley.edu/gri_mech/; 2006. [16] Hall JM, Petersen EL. An optimized kinetics model for OH chemiluminescence at high temperatures and atmospheric pressures. Int J Chem Kinet 2006;38:714e24. [17] Lamoureux N, Djebaı¨li-Chaumeix N, Paillard CE. Laminar flame velocity determination for H2eaireHeeCO2 mixtures using the spherical bomb method. Exp Therm Fluid Sci 2003;27(4):385e93. [18] Liu FS, Guo HS, Smallwood GJ. The chemical effects of carbon dioxide as an additive in an ethylene diffusion flame: implications for soot and NOx formation. Combust Flame 2001;125:778e87. [19] Melhem GA. A detailed method for estimating mixture flammability limits using chemical equilibrium. Process Saf Prog 1997;16(4):203e18. [20] Beyler C. Flammability limits of premixed and diffusion flames. SFPE handbook of fire protection engineering. 3rd ed. Quincy, Massachusetts: National Fire Protection Association; 2002. 2e172-87.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Experimental study of flammability limits of oxy-methane mixture and calculation based on thermal theory Xianzhong Hu, Qingbo Yu*, Nan Sun, Qin Qin School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning 110819, PR China
article info
abstract
Article history:
The flammability limits of CH4/CO2/O2 were studied using theory calculations and exper-
Received 25 January 2014
imental investigation. A calculation model was re-deduced based on thermal theory. In
Received in revised form
this model, the chemical effects of CO2 on flammability limits are concerned. Results show
18 March 2014
that the chemical effects of CO2 decrease the upper flammability limits (UFL) of mixtures,
Accepted 26 March 2014
while have little effect on the lower flammability limits (LFL). Experimental measurements
Available online 24 April 2014
were performed using a cylinder reactor. The measurements of the LFL of CH4/O2/CO2 increase from 5% to 8.25% with the increase of CO2 concentration, and the UFL of mixture
Keywords:
CH4/O2/CO2 decrease from 61% to 8.25% accordingly. The model predicted the LFL very
Flammability limits
well, but the calculated values are an average 10.7% higher than the measurements at the
Thermal theory
UFL points. The differences between the predictions and measurements are mainly due to
Carbon dioxide
the two assumptions of this model.
Chemical effect
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With more concerns about carbon dioxide reduction, the technologies using pure oxygen are increasing in industry such as oxy-fuel combustion technology. However, fire and explosion safety issues [1] appeared, which caused by such mixtures. Generally, adding inert gas is always used to decrease the reaction temperature of gas mixtures using pure oxygen. The inert gas includes CO2, N2, He, Argon even or steam. Among them, CO2 is usually used as inert gas for oxyfuel combustion, because the main products are CO2 and H2O, and there is no N2 anymore. It is very easy to separate CO2 from H2O by condensation. The pure CO2 can be stored or used in other industrial field, which reduce the emission of
greenhouse gas. Hence, special attention should be paid for explosion safety issues of these mixtures. Flammability limit is one of main index to judge flame stability and explosion characteristics of fuel gases. The effects of carbon dioxide on the flammability limits of combustible gas have been widely studied. Chen et al. developed a theoretical model based on mass and energy balance to predict the upper/lower flammability limits of flammable mixtures composed of fuel gas, inert carbon dioxide and air [2]. Wang et al. developed a semi-empirical model for predicating upper flammability limits for flammable gases which mixed with fuel gas, air and carbon dioxide [3]. Kondo et al. measured the flammability limits of six kinds of combustible gas mixed with carbon dioxide and air using a 20L explosion bomb [4]. Gant et al. used an equipment (8 m long and 1.04 m
* Corresponding author. P.O. Box 345, Northeastern University, No. 11, Lane 3, Wenhua Road, Heping District, Shenyang, Liaoning, PR China. Tel./fax: þ86 24 83672216. E-mail address: [email protected] (Q. Yu). http://dx.doi.org/10.1016/j.ijhydene.2014.03.202 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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diameter pipeline) to study the effect of carbon dioxide on flame stability and ignition probability of CH4/CO2/air mixture in free-jets [5]. Wu et al. studied the flammability characteristics of flammable gases (CH4/CO2/air, CH4/N2/air, CH4/Ar/air) using a 20L-Sphere [6]. Generally, all these research works focus on the flammability limits of flammable gas (including fuel and inert gas) in air. And seldom concern with the flammability limits of fuel gas at O2/CO2 atmosphere. Flammability limits of fuel/O2/CO2 are related to flame stability and fire safety of oxy-combustion technology. Thus, it is necessary to investigate the flammability limits of oxy-fuel mixture. In the mixture of CH4/CO2/O2, pure O2 is used as an oxidizing agent and CO2 is used for dilution. The properties of CO2 in flammable gases are different from N2 in air. Firstly, the transport, thermal and radiative properties of CO2 are different from N2. It is noted that the specific heat and gas absorption coefficient of CO2 is higher than N2. Secondly, CO2 is not inert but directly participates in chemical reactions [7,8]. For these differences above, the characteristics of the flammability limits of CH4 at CO2/O2 atmosphere are not alike with traditional condition. Research works [3,4] considered the effect of thermal and radiative properties of CO2 on flammability limits, but the chemical effect of CO2 did not draw enough attention. Previous research works [7,8] found that the chemical effects of CO2 on combustion reactions could not be neglected when the concentration of CO2 is high. There are several methods to calculate the upper flammability limits (UFL) and lower flammability limits (LFL) of a mixture diluted with inert gas. Le Chatelier’s rule [9] was developed and applied to predict the LFL and UFL of the mixture which consist of multiple fuel and inert gases in air. Kodo et al. [4] tried to develop an extended Le Chatelier’s rule to predict the UFL more accurately, as Le Chatelier’s rule is not usually well used for calculating the UFL. But, both the Le Chatelier’s rule and extended Le Chatelier’s rule are not fit for calculating the UFL and LFL of CH4/CO2/O2, because there is
only single fuel (methane) in the mixture and some parameters are needed from experimental measurements which are not clear now. Hansel et al. [10] tried to estimate flammability limits using adiabatic flame temperature. There are several applications of this method [10e12], but it is only suitable for calculating the LFL and not well used for predicting the UFL. Ma [13] provided a thermal theory to estimate the flammability limits without using experimental data. This method quantifies the heating and quenching potentials of each element of a mixture. The UFL and LFL are determined when the heating potentials equals quenching potentials. It is developed for predicting the flammability limits of flammable gas (including fuel and inert gas) with air. So it cannot be used for prediction of the LFL and UFL of fuel/CO2/O2 directly. In present work, we analyzed the effect of CO2 concentration on CH4/CO2/O2 explosion. The flammability limits of methane at CO2/O2 atmosphere were measured using a cylindrical glass reactor. Calculations were carried out using the thermal theory [13] for predicting the flammability limits of a mixture of CH4/O2/CO2. The calculation model based on thermal theory was re-deduced for O2/CO2 atmosphere. The chemical effects of CO2 on flammability limits were considered in this calculation model.
Experimental method The experimental investigation of flammability limits was performed in a cylindrical glass tube. The schematic representation of the experimental rig is shown in Fig. 1. The reactor consists of a 1.6 m long and 80 mm inner diameter cylindrical quartz glass tube. The environmental temperature around the reactor keeps constant (300 K) during experimental process to make sure the initial temperature of flammable gases is constant. The temperature is controlled by a system which consists of a temperature controller, a ring
Fig. 1 e Schematic representation of the experimental rig.
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heater and two thermal couples. The initial pressure of mixtures in reactor is prepared keeping ordinary pressure (1 bar) before ignition. A circulating pump is used to stir the mixture. The ignition sparks for premixed gases are produced by a 14 KV spark generator cross two copper electrodes at the bottom of the reactor. The spark duration time is 0.4 s. The flame is observed visually in the dark. The mixtures are determined to be flammable using the European standard EN 1839(T). 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 [14]. The effect of CO2 dilution is evaluated through the flammability limits measurement of methane at various concentration of CO2. In this paper, mole concentration of CO2 is defined as follows: xd ¼
CO2 CO2 þ O2 þ CH4
(1)
The gases mixtures are prepared in the reactor using partial pressure methodology. Evacuate the reactor and break vacuum, then methane is first injected into the reactor, the oxygen second and carbon dioxide last. A circulating pump is used to stir the mixtures for about 20 min to make sure that mixtures are well blended. Purity of methane and oxygen both are 99.9%. Purity of carbon dioxide was better than 99.95%. All the gases are supplied by gas cylinders.
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(methane) and HO is the heating potential of oxygen. The quenching potential is the enthalpy change at critical adiabatic flame temperature with respect to that of air, which is determined by its thermal properties. The heating potential is due to the combustion reactions of flammable gas. All of the five parameters above are dimensionless based on the quenching potential of air. The quenching potential of air is regarded as 1. xL is volumetric ratio of fuel at the lower flame limit or the LFL, and xU is volumetric ratio of fuel at the higher flame limit or the UFL. On the right side of equations (3) and (4), there are the terms of quenching potential of methane, carbon dioxide and pure oxygen respectively. On the left side of equation (3), there is the heating potential of mixture, which is determined by the heat resale of fuel, because the fuel is burned out at the lower flammability limit point. In the same way, the heating potential of mixture is determined by the amount of oxygen at the upper flammability limit point on the left side of equation (4), because the oxygen is lean at the upper flammability limit point. The basis of equations (3) and (4) is energy balance and two major assumptions mentioned above. From equations (3) and (4), HF, QF and HO can be calculated through equations (5)e(7). HF ¼ 2HO QF ¼
ð2xL xD ð1xU xD ÞÞQD þð2xL ð1xL xD ÞÞð1xU xD ÞQO2 xL ð1xU xD 2xU Þ
Theory calculation model based on thermal theory
(6)
A thermal theory for estimating the flammability limits for a mixture was developed by Ma. The theory is based on two major assumptions: One is that the calculated adiabatic flame temperature keeps constant; the other is that the reaction at the UFL point is complete and similar to the reaction at the LFL point [13]. The thermal theory is successfully used for predicting the flammability limits of the fuel with air. But the calculation equations are not suitable for methane with pure oxygen and carbon dioxide. Thus the calculation model should be rededuced based on thermal theory. The derivation process of the calculation of flammability limits of CH4/CO2/O2 is presented as follows. The combustion of CH4/CO2/O2 can be expressed by: CH4 þ 2O2 þ Cd CO2 /CO2 þ 2H2 O þ Cd CO2
(2)
Where, the subscript d stands for dilution. At the lower flammability limit point, the heat released equals to the quenching energy, so the energy balance equation is as follows: xL QF þ xD QD þ ð1 xL xD ÞQO2 ¼ xL HF
HO ¼
ðxL ð1 xU xD Þ xU ð1 xL xD ÞÞQO2 þ ðxL xD xU xD ÞQD xL ð1 xU xD Þ 2xL xU (7)
The value of QF and HO can be obtained at the critical condition for a certain fuel/dilution system, which can be found via any Material Safety Data Sheet. Referring back to equations (3) and (4), we determine the following relationships: xL ¼
QO2 ðxD 1Þ xD QD QF QO2 HF
(8)
xU ¼
HO ð1 xD Þ QO2 ð1 xD Þ xD QD QF QO2 þ HO
(9)
Using equations (8) and (9), we could calculate the flammability limits of CH4/CO2/O2. In calculation equations, the effects of thermal and radiative properties of CO2 on flammability limits are concerned in QD. But the chemical effects of CO2 on combustion have not been included. The further investigation of the chemical effects of CO2 will be discussed in Section 4.2.
(3)
Similarly, at the upper flammability limit point, the balance equation is as follows: xU QF þ xD QD þ ð1 xU xD ÞQO2 ¼ ð1 xU xD ÞHO
(5)
(4)
Where QD is the quenching potential of dilution (CO2), and QO2 is the quenching potential of pure oxygen, QF is the quenching potential of fuel (methane), HF is the heating potential of fuel
Results and discussion Experimental data and theoretical calculation The flammability of the mixture of CH4/O2/CO2 was measured in this paper. Fig. 2 shows the experimental measurements of
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Fig. 2 e Flammability diagrams for CH4/CO2/O2.
for CH4 combustion. The GRI Mech 3.0 has been the most popular mechanism for simulating CH4 flame, which has been validated by large experimental data [16,17]. In numerical calculation, a strategy proposed in previous study [8,18] to numerically isolate the chemical effects of CO2 was employed. In this strategy, CO2 was used for CH4/CO2/O2, and FCO2 was used for CH4/FCO2/O2. Here FCO2 is a fictitious species that has identical thermal and transport properties as CO2. But it is artificially excluded from chemical reactions. In addition, FCO2 was assigned the same third-body collision efficiency as CO2 in all relevant reactions. In numerical model, the temperature was set to be 1650 K and pressure was ordinary (1 bar) [13]. The critical concentrations of methane at the LFL and UFL points are from equations (8) and (9) with data list in Table 1. The chemical effects of CO2 on heating potential are quantified by the differences between the results of heat release of mixtures of CH4/CO2/O2 and CH4/FCO2/O2. The change rate of heating potential is calculated by equation (10), e¼
the UFL and LFL. The calculation results are also given in this figure for comparison. Theoretical UFL were calculated using HO (dash line). As seen in Fig. 2, the LFL of CH4/O2/CO2 increase almost linearly and slowly as the concentration of dilution gas (CO2) increase. The results of LFL vary within a very narrow band, which are from 5% to 8.25%. The average absolute differences between the experimental measurements and theoretical prediction of the UFL are less than 0.6% vol%. There is a good agreement between the observed data and calculated values. The upper flammability limits of mixture CH4/O2/CO2 decrease with the increase of concentration of CO2. There is not a linear relationship between the measurements of the UFL and the concentration of CO2. With the increase of CO2, the UFL rates of decline become smaller and smaller. According to equation (9), there is a linear relationship between the prediction of the UFL and concentration of CO2. Thus the theory calculations of the UFL are not in good agreements with experimental measurements. The theoretical calculated UFL values using HO are always larger than experimental measurements. It is presumed that the heat release is overestimated in the theory prediction. Generally, CO2 is always regarded as inert gas in combustion. As mentioned above, CO2 is not inert but directly participates in chemical reactions at O2/CO2 atmosphere. Research [8] showed that high concentration of CO2 decreased the burning rate of CH4 because of the direct chemical participation of CO2. In other words, the chemical effects of CO2 reduce the amount of heat release. Thus, HO which characterizes the heating potential at the UFL point should be reduced.
Chemical effects of CO2 on flammability limits In order to give a quantitative analysis of heating potential at the UFL point, the heat release on the combustion was investigated with kinetic simulation. Numerical calculation of heat release of flammable gas is carried out using 0-D Closed Homogeneous Reactor of the CHEMKIN packages. In calculation, the GRI Mech 3.0 [15] mechanism is employed. It consists of 53 species and 325 reactions, which is originally developed
HFCO2 HCO2 100% HCO2
(10)
Where HFCO2 is the heat release of the mixture CH4/FCO2/O2 from combustion reaction, and HCO2 is the heat release of the mixture CH4/CO2/O2. Fig. 3 shows the change rate of heating potential from combustion reaction at LFL point. The change rate of heating potential is very small. The max change rate is less than 1%. So the HF changes little at the LFL point, which is no need to be modified. Generally, there is excessive oxygen and lack of fuel at the LFL point. The chemical effects of CO2 on reaction are inhibited because of too much oxygen which improved the forward reactions in combustion. Fig. 4 shows the change rate of heating potential from combustion reaction at the UFL point. The rate of change of heating potential at the UFL point is much bigger than which at the LFL point. As xD increases, the rate of change increases from 0 to the top at xD ¼ 0.51 and then decreases to nearly 0 at xD ¼ 0.8. The max value of change rate is about 10.7%. Thus, the chemical effects of CO2 on the change of heating potential should not be neglected. The terms HO should be reduced when carbon dioxide used as dilution. In this paper, it is assumed that the change rate of heating potential (e) is a function of concentration of dilution gas (xD) as followed: e ¼ AxD þ Bx3D
(11)
Where A and B are unspecified parameters whose values are determined by fitted curve from the data of numerical simulation. For methane, A ¼ 0.307, B ¼ 0.511 and the squared correlation coefficient (R2) is greater than 0.94. Through the analysis above, the heat release from the chemical effects of CO2 should be removed from the heating potential HO. In order to distinguish with HO which comes
Table 1 e Spreadsheet calculation on the flammability limits of CH4/CO2/O2. Fuel Methane
QO2
QD
QF
QAIR
HF
HO
1.046
1.75
12.276
1
8.891
14.383
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Fig. 5 e Calculated adiabatic flame temperature profile at the LFL and UFL points. Fig. 3 e Change rate of heating potential from combustion reaction at the LFL point.
from equation (7), the new heating potential was expressed as H0o . From equation 10 and 11, the new heating potential H0o at the UFL point is given below: H0O ¼ HO HFCO2 HCO2 ¼ HO ð1 eÞ ¼ HO 1 AxD þ Bx3D
flammable gas. But the average absolute difference between the measurement and calculation is 2.3% vol%, which corresponds to the average relative difference of 10.7% relative percent. The agreement between the experiment and theoretical prediction is by no means good. Besides the chemical effects of CO2, there are other factors caused the differences, which are discussed in the next section.
(12)
It is noted that equation (5) HF ¼ 2HO is not tenable for H0o here. Fig. 2 also gives the calculation results of the UFL using equations (9) and (12). Obviously, the predicted results using H0o (solid line) are much closer to the experimental measurements than which using HO (dash line). It means that equation (9) with H0o is more applicable for the UFL prediction of
Fig. 4 e Change rate of heating potential from combustion reaction at the UFL point.
Discussions In this paper, the calculation model for flame limits is very simple, which is developed to apply to engineering estimation. Although it is not very exact and precise, better prediction requires more complex models. This model’s weakness is that the prediction of the UFL using this model is slightly higher than the actual value. So some small fixes and improvements sometimes are needed. The source of the errors and suggestions for improvement of this model are discussed below. As mentioned earlier, two major assumptions are used in thermal theory. One assumption is that the calculation of flame temperature is fixed. This assumption is reasonable for the LFL points. But it is different from the combustion state at the UFL points. In order to deeply understand the deviation between the assumption and the reality, the adiabatic flame temperature was numerically calculated at the LFL and UFL points using the EQUIL code of the CHEMKIN package with GRI-Mech 3.0 kinetic mechanism. The calculation parameters including xL, xU, and xd are from the experimental measurements in Section 4.1. Fig. 5 shows the calculated adiabatic temperature (CAFT) profile at the LFL and UFL points. With the increase of the concentration of dilution (CO2), the adiabatic flame temperatures at the UFL points almost linearly decrease, but the adiabatic flame temperatures at the LFL points stay around 1550 K until the concentration of dilution greater than 0.775 which is near the inertization point. In most condition (xd < 0.675), the adiabatic flame temperatures at the UFL are higher than which at the LFL. In other words, the adiabatic
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flame temperatures which determine xU are underestimated in the assumption, because the calculated flame temperatures are assumed to be fixed in the thermal theory. Fig. 6 shows an example to present how to estimate the UFL and LFL of gas mixtures using calculated adiabatic flame temperatures (CAFT) method [19]. Once the critical temperature (Tc) is chosen, there are two intersections between the critical temperature line and calculated adiabatic flame temperature line. Then the UFL (xU) and LFL (xL) are determined by the two intersections. The upper flame limits increase with the decrease of critical temperature. In thermal theory, the Tc was set to be 1650 K for methane combustion, which is reasonable for the LFL points but underestimated for the UFL points. Thus the predictions of upper flame limits are bigger than actual values, which have been validated by experimental measurements (Fig. 2). This is one of the main error sources in prediction of upper flammability limits. The other assumption is that the flame structure at the LFL is similar to the UFL. It is well known that the fuel gas is lean reactant at the LFL point, while oxygen is lean reactant at the UFL point. There is little incomplete reaction at the LFL point, but it is different at the UFL point. In this paper, the chemical effects of CO2 on reaction have been considered, but this only could be regarded as part effects of incomplete reactions. In literature [13], incomplete reactions near the inertion point were discussed and equations of extended HO were illustrated to calculate the flammability limits concerning the incomplete reaction effects on combustion. In this paper, the same methods were also used to get a further modified HO. Equations (13) and (14) are the expressions of modified HO, which are derivate from equations (3) and (4) respectively. HO ¼
xF1 QF þ xD1 QD1 þ ð1 xF1 xD1 ÞQO2 2xF1
(13)
HO ¼
xF1 QF þ xD1 QD1 þ ð1 xF1 xD1 ÞQO2 ð1 xF1 xD1 Þ
(14)
Where, QF, QD1, QO2 keep constant, and the parameters xF1, xD1 are determined by experimental measurements. Equation (13) is used for prediction of the LFL with equation (5). And equation (14) is used for the UFL calculation. The two assumptions are still used widely and popular for estimation methods of flammability limits such as LCR and Beyler’s method [20]. Because the two assumptions make the prediction of flammability limits become possible and easy. Compared to other methods, the calculation model in this paper succeeds the advantages of thermal theory, and the chemical effects on flammability limits of CO2 are concerned in addition. So it is better than other methods which based on the assumption of a constant critical adiabatic flame temperature.
Conclusions In this work, a calculation model based on the thermal theory [13] was re-deduced to predict the flammability limits of CH4/ O2/CO2. It is note that, the chemical effects of CO2 on flammability limits were concerned in this model. Results show that the chemical effects of CO2 on lower flammability limits are little, but larger on upper flammability limits. At the UFL point, the chemical effects of CO2 reduce the heating potential of mixture, which decrease the upper flammability limits. The calculation model can be used for the flammability limits prediction of other hydrocarbon fuels at O2/CO2 atmosphere with a little modification. Experiments were also carried out to investigate the flammability limits of CH4/O2/CO2. Results show that the lower flammability limits of CH4/O2/CO2 increase from 5% to 8.25% as the concentration of CO2 increase, while the upper flammability limits of mixture CH4/O2/CO2 decrease from 61% to 8.25%. As compared to the prediction of theory calculation model, there is a good agreement between the experimental data and calculated values at the LFL point, while the calculated values are an average of 10.7% higher than the measurements at the UFL points. Though discussions, these errors are mainly due to the two major assumptions of the thermal theory.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Cantact No. 51274066), The National Key Technologies R&D Program of China (Cantact No. 2013BAA03B03).
Nomenclature HO HF
Fig. 6 e Estimation of flammability limits using calculated adiabatic flame temperatures (CAFT) method.
LCR QD
the heating potential of oxygen based on air (dimensionless) the heating potential of fuel based on air (dimensionless) Le Chatelier’s rule the quenching potential of diluent based on air (dimensionless)
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QF UFL LFL xL xU CAFT
the quenching potential of fuel based on air (dimensionless) upper flammability limit (volume ratio) lower flammability limit (volume ratio) lower flammability limit (volume ratio) upper flammability limit (volume ratio) calculated adiabatic flame temperatures
Subscripts L lower flammable limit U upper flammable limit D diluent-based potential to air potential F fuel-based potential to air potential O oxygen-based potential to air potential
references
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