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Experimental and theoretical study of flammability limits of hydrocarbon–CO2 mixture
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
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Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture Hua Tian*, Mingqiang Wu, Gequn Shu, Yuewei Liu, Xueying Wang State Key Laboratory of Engines, Tianjin University, People's Republic of China
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abstract
Article history:
The hydrocarboneCO2 mixture is identified as the most advantageous working medium in
Received 18 July 2017
the medium-high temperature Organic Rankine cycle (ORC). However, the flammable
Received in revised form
zones of hydrocarboneCO2 mixtures at atmospheric condition are rare. In order to mea-
30 August 2017
sure the flammability limits of mixture, an experimental setup based on the ASTM E681-09
Accepted 9 October 2017
has been built completely. In the design of the experiment, an innovative tactic about gas
Available online xxx
distribution is proposed to improve the precision. The flammable zones of five mixtures were measured carefully at 30 C in this paper. But in reality, ORC system operates within
Keywords:
the temperature range from 87 to 300 C. Meanwhile the flammability zones of mixtures at
Flammability limits
higher temperature are not available in the existing literatures. To solve this problem, a
HydrocarboneCO2 mixture
modified critical flame temperature model is modified to estimate the flammable zones of
Thermal theory
mixtures at different temperatures. The modified model presents a higher prediction precision with average relative difference of 1.24% at LFL. Thus, it is easier to estimate the flammable zones of mixtures at different temperatures with the modified model. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Besides the advantages of low GWP (global warming potential), zero ODP (ozone depletion potential) and high decomposition temperature, hydrocarbons showed other better characteristics as working mediums in the medium-high temperature ORC (Organic Rankine Cycle) compared with refrigerants in previous studies [1,2]. Because all of these, hydrocarbons take the place of refrigerants gradually in the medium-high temperature ORC. However, flammability is the main restrictions in the practical application. As a diluent, CO2 is usually mixed with hydrocarbons to reduce the possibility of explosion. Ref. [3] revealed that the mixture of carbon dioxide and hydrocarbon had a better performance in cycle
efficiency than hydrocarbon only in Organic Rankine Cycle owing to a large temperature glide for mixture. In order to evaluate the risks of explosion and combustion in ORC, the knowledge about the flammability limits of such mixtures is necessary. However, little data about flammability limits of hydrocarboneCO2 mixtures is available in literatures except the ones measured by Kondo [4,5] and Zabetakis [6]. Zabetakis [6] measured the flammable zone of hydrocarboneCO2 mixtures using cylindrical glass vessel (diameter of 50 mm, height of 1500 mm) while Kondo [4] obtained the flammable zone of propane/CO2 mixture in dry conditions using the glass flask of 12 L. With the same experimental facility and condition, the data of mixture of isobutane and CO2 is obtained in Kondo [5]. Grabarczyk [7] investigated the effect of initial temperature on the explosion pressure of various
* Corresponding author. State Key Laboratory of Engines, Tianjin University, No. 92, Weijin Road, Nankai Region, Tianjin 300072, People's Republic of China. E-mail address: [email protected] (H. Tian). https://doi.org/10.1016/j.ijhydene.2017.10.053 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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Nomenclature ORC Mixture CAFT LFL UFL DHc t L(U) L0(U0) Ht Had DHc* y L1 L2 U1 U2
organic Rankine cycle hydrocarboneCO2 calculated adiabatic flame temperature lower flammability limit upper flammability limit heat of the combustion initial temperature lower(upper) flammability limit lower(upper) flammability limit at t0 (known parameter) enthalpy at the initial temperature t enthalpy at the calculated adiabatic flame temperature reaction heat at upper flammability limit the molar ratio of CO2 in mixture lowest volume fraction supports flame propagation highest volume fraction does not support flame propagation lowest volume fraction does not support flame propagation highest volume fraction supports flame propagates
liquid fuels in 20 L sphere. The different explosion regimes and UFL can be observed with increasing fuel-air equivalence ratio at different temperature. It was generally believed that the increasing size of the vessel bring the more accurate flammability limits [8e10] because flame quenching effect may become negligibly small in the jumbo vessel. Zlochower [11] investigated the influence of different sizes, 12 L and 120 L, of spherical glass flask on the flammability limits with different decision criterions. The excellent agreement between the observed values in the 120 L glass sphere considered as a jumbo vessel using the pressure rising standard of 7% and the observed results in the 12 L glass sphere obtained using the visual standard supported the fact that the values measured in 12 L glass sphere can be taken as a reasonable approximation to the values obtained in the jumbo vessel. According to the researches by Refs. [12,13], the water vapor as diluent inevitably affects flammability limits of fuel. Holborn [14] measured and modeled the effect of water fog on the UFL of hydrogen-nitrogen mixture. The results suggested that water fog can observably lower the flammable zone of mixture. Wang [15] considered that the water vapor impact on the LFL of H2/CO mixture from the perspective of the chemical effect. It was found that water vapor inhibits the propagation of reaction chain and increases the LFL of H2/CO mixture. Thus, properly speaking, results measured by Kondo [4,5] were flammability limits in ideal environment using the water-free air in experiment. In general, the flammable limits measured by Kondo and Zabetakis may not be exact value. Thus, it is worthy measuring flammability limits of hydrocarboneCO2 mixtures in actual environment using 12 L sphere for security reasons.
Theoretical researches as well as experiments were also extensively investigated to predict flammability limits of fuel. Kondo [4] exploited the modified Le Chatelier equation to predict the flammable zone of mixtures of hydrocarboneCO2 and hydrocarboneN2 mixtures. However, the modified Le Chatelier equation was built on the known experimental data of mixtures, which cannot estimate the flammability zone of other mixtures accurately. Critical flame temperature methods [16], which quantifies exothermicity and heat absorption capacity of material in the combustion process of hydrocarboneCO2 mixture, are generally used to estimate the flammability limits of the hydrocarboneCO2 mixture. In the viewpoint of thermal theories, a certain reaction temperature or threshold temperature value is closely linked to the flammability limit [17]. The studies using critical flame temperature to predict the flammable zone of hydrocarboneCO2 were put forward by researchers [16e21]. Hansel [18] attempted to establish a CAFT model to predict the flammable zone of mixtures. However, the CAFTs calculated by N2 EQUIV in his model are not exact value. The N2 EQUIV didn't consider the oxygen not taking part in reaction which should be included in N2 EQUIV. Chen [19] developed another CAFT model that considered the oxygen not taking part in reaction for estimating flammability limits of hydrocarboneCO2 mixtures. But the UFL estimated results are bad for using the heat of combustion to calculate the reaction heat. Ma [20] used the oxygen calorimetry instead of the heat of combustion to calculate the reaction heat at UFL in his model. To keep the model simple, CO and H2 were excluded from combustion products at UFL, which resulted in low accuracy. Hu [16] took into account the chemical effect of CO2 in the Ma model [20]to estimate flammable zone of methaneeCO2 in pure oxygen environment. The results indicated that the chemical effect of CO2 was not a major factor determining the prediction accuracy. All the above models cannot predict the UFL of mixture accurately due to incomplete combustion at UFL. Recently, the CAFT models proposed by Shu [21] can estimate the UFL of mixtures better than other CAFT models as adopting the simple chemical kinetics about oxygen consumption. The reaction heat can be calculated better because the simple chemical kinetics was adopted so that the reaction in model approached the real combustion reaction in vessel. But all the models above are only used for predicting the flammable zone at normal temperature. At the same time, the operating temperature of hydrocarboneCO2 mixture used in ORC is located in 81e225 C [22]. As we all know, the flammable zone of hydrocarbons will broaden with the increasing of temperature. That is to say, the lower flammability limit (LFL) becomes lower and the upper flammability limit (UFL) becomes higher [23,24]. Thus, using the flammability limits at room temperature to evaluate security issue at high temperature may result in a severe explosion hazard. In order to ensure the safe operation of ORC, it is urgent to obtain the flammable zones of mixtures at elevated temperature. Nevertheless, measuring flammable zones at the different temperatures of hydrocarboneCO2 is very labor-intensive and time-consuming. Thus, a developed model was built by modifying temperature parameter in the Shu model. Using this model the flammable zones of mixtures at different
Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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temperatures could be obtained easily without too much experimental data. The objective of this paper is to measure the flammability zones of mixtures of propane, isobutane, n-butane n-pentane and isopentane separately diluted with carbon dioxide at 30 C at first and then measure the flammability limits of pure hydrocarbon at 50 and 70 C. At last, to modify the CAFT proposed by Shu [21] to predict the flammable zones of mixtures at different temperatures.
Experimental method Experimental method The experimental setup is built on the basis of the ASTM E68109, which is an improved edition instead of ASTM E681-94. A schematic diagram of the experimental apparatus used to measure the flammability limits is shown in Fig. 1. The experimental setup is divided into two big systems, gas distribution system and explosion system, which consist of assembly units and subsystems such as valves and temperature control system. The gas distribution system is mainly composed of two vacuum pumps, solenoid valves (V1eV8) and the pressure control subsystem. The accuracy of flammability limits depends, to a great extent, on gas distribution system. Thus, a high precision pressure transducer with an accuracy of 0.02% of full scale (100 KPa) is used to measure pressure in vessel. An innovative tactic about distribution is proposed in order to improve the precision. The intake process of the fuel gas or CO2 is divided into several times in which the amount of airflow pumping into the glass flask each time is decided by the partial pressure method compared with the desired pressure. The intake process is dominated by the algorithm which
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controls the opening time of solenoid valve by comparing the deviation between the setting pressure and the actual pressure. The intake process of the fuel gas would not end until the measured value approaches very close to the setting value. The software finally displays the real measured value that approaches set value, which means that this method has nearly no error in the gas distribution. The advantage of this tactic is that the tactic avoids a large pressure variation which is bad for measuring the pressure in vessel precisely. For the explosion system, a spherical pressure-resistant flask with a volume of 12 L is chosen. A subsystem which is made up of heating resistor, air blower, PID temperature controller and thermal couple is designed to regulate the temperature. By heating or non-heating the air-bath, the temperature in flask can be controlled automatically at a given temperature point. The sparks using for igniting the homogeneous mixtures are generated by a 15 KV high-voltage transformer between two tungsten electrodes that are set (0.64 cm) apart and located one third the diameter of the flask from the bottom of the flask. The ignition duration is 0.4 s during which ignition energy of approximately 10 J is generated. The flange and rubber plug are fixed on the top by two springs. If the explosion occurs in the vessel, the rubber plug along with the flange is lifted up against the springs by the overpressure following the ignition process to relieve the pressure automatically.
Experimental procedure (1) Evacuate the system carefully and flush with air to ensure removal of the residual gas. The flask was vacuumed to 670 Pa or lower before gas comes in. The vessel should be isolated under vacuum theoretically, while actually it is allowed to leak at the speed of less than 0.01 Kpa/min.
Fig. 1 e The schematic diagram of measurement system for flammability limits. Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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(2) The hydrocarbon inflated into the flask first, and then carbon dioxide. The innovative tactic mentioned in section 2.1 would be applied to the intake process during which the pressure will raise slowly to approach the atmospheric pressure. (3) Mixture is mixed by a magnetic stirrer for 5 min. After that, standing about 2 min to obtain complete homogeneous mixtures. (4) By reducing spring tensioning before firing, the pressure rise resulting from the combustion process is unloaded by pressure relief device (rubber plug). Finally activate the ignition device. Please remember that the propagation of flame should be observed in a dark place. For each experiment, the flame spread was recorded with a video camera. If the flame firstly moves upward, then outward to the top of vessel and finally downward from the top of vessel to develop an extremely thin flame front forming a 90 angle which is measured from the point of ignition, the mixture is considered to be combustible. Example of that condition is shown in the Fig. 3 (4). The flammable zones of mixtures of hydrocarboneCO2 are measured. Purities of hydrocarbons are 99.9%. Purity of carbon dioxide is 99.9%. Air humidity is 50%. The flammable zones of mixtures at 30 C were measured following the procedure. Similarly, the flammability limits of pure hydrocarbon were measured at 50 and 70 C.
The model development A critical flame temperature model for estimating the flammability zones for mixtures of hydrocarboneCO2 was developed by Shu [21]. Validation of the model has been carried out
Establishing equations related to U (or L), y, t and the CAFT L=L0 (or U=U0) t=t0, y=0
Calculated Adiabatic Flame Temperature (CAFT)
Getting model between U (or L), t and y Various t and y Predicted L(U) and Examine Fig. 2 e Procedure of establishing and validation of the model.
on the observed values of methane/CO2, propane/CO2, propylene/CO2 and isobutane/CO2 at 35 C in the literature [4,5]. The mean relative deviations between the predicted results and observed data are than 7.8% and 1.5% at UFL and LFL, respectively. The Shu model was modified to predict the flammable zone of mixture at different temperatures. The derivation process of a new critical flame temperature model is presented as follow. The modeling procedure is shown in Fig. 2. In Fig. 2, establishment and validation of the model contain a procedure consisting of four steps: (1) Build the equation containing y, t and L (or U); (2) The CAFT of pure hydrocarbon can be known when the lower and upper flammability limits are given at t0 and 1 atm in air; (3) Re-formulate equations related to y, t, L (or U) and get correlation between y, U (or L) and t (4) Finally use the developed model to estimate the lower and upper flammability limits of hydrocarboneCO2 mixtures at different initial temperatures and validate the reliability of the model by comparing the estimated values with observed values available in the references; Some of the major hypotheses are shown in order to build the theoretical model to estimate the flammable zones of mixtures: (1) The combustion reaction of mixture takes place at atmospheric pressure; (2) The calculated adiabatic flame temperature deduced from pure hydrocarbon remains unchanged for each hydrocarboneCO2 mixture; (3) The chemical kinetics about oxygen consumption: oxygen is used to calculate the combustion products. The details of the chemical kinetics about oxygen consumption Oxygen are as follows: Firstly, the CO and H2 generate from combustion of hydrocarbon if oxygen is deficient; secondly if there is still residual oxygen after CO and H2 generated completely, H2 will be oxidized to H2O; and finally CO will be oxidized to CO2 if there is still oxygen remained. The UFL of n-butane measured is 7.9%. The combustion products contain 32.3% H2 based on the oxygen consumption. The reported flammable zone of hydrogen in Refs. [15,24] ranges from 4% to 76% at room temperature and atmospheric pressure. Thus, the flush process of flask with air should be given additional attention to avoid the burning of H2. As we all know, the amount of oxygen in combustion at UFL is deficient. Assuming one mole mixture of mixture and air burns at UFL, the overall reaction of mixture can be obtained easily:
UCn Hm þ
1 U yU ðO2 þ 3:773N2 Þ þ 1 Diluent 4:773 1y 1y
/a1 CO þ a2 CO2 þ a3 H2 O þ a4 H2 þ a5 N2 þ
yU Diluent þ DHc* 1y (1)
Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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Fig. 3 e The flame propagation of mixture of propane and dioxide at LFL.
Considering the following Eqs. (2)e(4): Cn Hm þ
CO þ
H2 þ
n þ m=4 m ðO2 þ 3:773N2 Þ/nCO2 þ H2 O þ DHcCn Hm 4:773 2
(2)
(3)
1=2 ðO2 þ 3:773N2 Þ/H2 O þ DHcH2 4:773
(4)
Eq. (5) will be available with the help of step reactions of (2)e(4) yU Diluent/ð1Þ 1y
(5)
Thus DHc* can be obtained by using the step reaction in a proper way: DHc* ¼ U DHcCn Hm a1 DHcCO a4 DHcH2
(6)
The Eqs. (7e10) can be obtained based on the law of conservation of mass: a5 þ a2 ¼ n U
(7)
2ða3 þ a4 Þ ¼ m U
(8)
a1 þ 2a2 þ a3 ¼
a5 ¼
2 U 1 4:773 1y
(9)
3:773 U 1 4:773 1y
(10)
Based on the condition of burning 1 mol of the mixture and air at UFL and the simple chemical kinetics, the amount ða1 a5 Þ of components is available after combustion. Supposed that the combustion of Eq. (1) takes place under adiabatic environment, Eq. (11) can be derived from the energy balance: a1 HtCO þ a2 HtCO2 þ a3 HtH2 O þ a4 HtH2 þ a5 HtN2 þ
yL t H þ DHc*L 1y D
ad ad ad ad /a6 Had CO þ a7 HCO2 þ a8 HH2 O þ a9 HH2 þ a10 HN2 þ
1=2 ðO2 þ 3:773N2 Þ/CO2 þ DHcCO 4:773
ð2Þ U ð3Þ a1 ð4Þ a4 þ
a6 HtCO þ a7 HtCO2 þ a8 HtH2 O þ a9 HtH2 þ a10 HtN2 þ
yU t H þ DHc* 1y D
ad ad ad ad /a1 Had CO þ a2 HCO2 þ a3 HH2 O þ a4 HH2 þ a5 HN2 þ
y U ad H 1y D (11)
The symbol ad and t in Eq. (11) stand for the CAFT and the initial temperature, respectively. Similarly, at LFL, the balance equation is as follows:
y L ad H 1y D (12)
Thus, the flammable zones of mixtures can be obtained conveniently at different temperatures for a given y.
Discussion and results Experimental results and discussions When the concentration of mixtures is less than the flammability limits, the flame propagates upward vertically without travelling outward (Fig. 3(2)). When the concentration of mixture approaches the flammability limits, the combustion process becomes smoother compared with the fierce stoichiometric combustion process in which the flame immediately expands in all direction to the full vessel from the point of ignition. The interesting phenomenon that the direction of flame propagation appears to move in the opposite direction with respect to the initial direction of propagation after the flame arriving at the vessel top is observed for all near-limit mixtures (Fig. 3(3)). Whether this phenomenon occurs or not seems to be determined by two factors. One is the heating and the other is quenching potential. For the given vessel and condition, the quenching potential is generally a fixed value while the heating potential highly depends upon the amount of hydrocarbon in mixture. Therefore, this phenomenon can be used as a supplementary method to decide whether the mixture is flammable or not when measuring angle is impossible. Comparison results between the flammability limits measured in this paper and the flammability limits in Refs. [4,5,11] are summarized in Table 1. The lower and upper flammability limits of propane measured in 120 L by Zlochower [11] are 2.05 vol% and 9.8 vol%, respectively which have very good coherence to the results measured in this paper. Therefore, the effectiveness of the test apparatus is validated against the flammability limits in the jumbo vessel. Experimental uncertainties of flammability limits are less than 0.05 and 0.3 vol% for LFL and UFL, respectively. The uncertainty primarily comes from the step size namely L1eL2 for LFL and U1eU2 for UFL.
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Table 1 e Comparing the flammability limits of propane and isobutane with observed values in Refs [4,5,11]. Fuel
This study L (vol%)
U (vol%)
Zlochower [11]
Kondo [4,5]
L U L U (vol%) (vol%) (vol%) (vol%)
Propane 2.071 ± 0.02 9.7 ± 0.3 2.05 Isobutane 1.65 ± 0.02 7.78 ± 0.3
9.8
2.03 1.68
10 7.8
L¼
L1 þ L2 2
(13)
U¼
U1 þ U2 2
(14)
The reason for the large step size at UFL can be explain by critical flame temperature theory. The heat released at UFL through reaction is much smaller than that of LFL due to the incomplete combustion. Meanwhile the heat capacity of mixtures at UFL is larger than that of LFL because heat capacity of fuel is much larger than that of air. Thus it may require a smaller step size at LFL to heat temperature of mixture of hydrocarboneCO2 air to CAFT than at UFL. Through the above mentioned analysis, uncertainties are closely related to the step size at LFL and UFL. As shown in Figs. 4e5, there is a linear relation between the flammability limits and temperature except for isopentane and n-pentane. The slope becomes larger when temperature approaches 30e40 C for n-pentane. It is found that the boiling point of pure n-pentane locates in the temperature range of 30e40 C. Namely, n-pentane is the liquid phase at 30 C while at 40 C it is the gas phase at atmospheric pressure. From the viewpoint of critical flame temperature theory, it may require more energy to overcome latent heat of evaporation at 36 C. The same is true for isopentane at LFL. Enough attention should be given for the LFL when the room temperature is close to the boiling point temperature of fuel. As the UFL shown in Fig. 5, the reason why the slope at corresponding boiling point does not change significantly for isopentane and
Fig. 5 e The upper flammability limits of pure hydrocarbons at different temperatures.
n-pentane is the large uncertainty at UFL so the slightly variation at corresponding boiling point is undetected. In other words, latent heat of evaporation only has marginal effects on UFL. Comparison results between the flammability limits measured in this paper and the flammability limits in Refs. [5] are shown in Table 2. As shown in Table 2, there is a good agreement between the flammability limits measured in this paper and those reported in Ref. [5]. By contrasting the results shown in Tables 1 and 2, the new experimental setup is validated. As shown in Figs.6e10, envelope curves of the LFLs and UFLs converge with the addition of the CO2. The measured results of LFL for all mixtures vary within a very narrow range, which not exceed 1% from 0% dilution (y ¼ 0) to overlap (FIP). The results of UFL for all mixtures vary within a very wide range comparing with those at LFL, which exceed 5% from 0% dilution (y ¼ 0) to FIP. The envelope curves of the LFLs and UFLs can be divided into two parts for the variation trend in Figs. 6e10. For envelope curves of the LFLs, the demarcation points locate at 75% dilution (y ¼ 0.75). Before the demarcation points, the LFLs remain unchanged. After the demarcation point, the LFLs change rapidly. For envelope curves of the UFLs, the demarcation points locate at 45% dilution (y ¼ 0.45).
Table 2 e Comparing the flammability limits of isobutane-CO2 mixture with those in Ref. [5]. Fuel
Fig. 4 e The lower flammability limits of pure hydrocarbons at different temperatures.
y
Isobutane 0 0.15 0.3 0.45 0.6 0.75 0.85
This study
Kondo
L (vol%)
U (vol%)
L (vol%)
U (vol%)
1.72 1.73 1.725 1.72 1.735 1.794 1.845
7.78 7.44 6.99 6.65 5.91 5.03 3.98
1.68 1.67 1.68 1.69 1.69 1.73 1.81
7.8 7.4 7.2 6.6 5.9 5.1 4.05
Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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Fig. 6 e Comparing the estimated results with the observed results for propaneeCO2 mixture.
Fig. 7 e Comparing the estimated results with the observed results for isobutaneeCO2 mixture.
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Fig. 9 e Comparing the estimated results with the observed results for n-pentaneeCO2 mixture.
Fig. 10 e Comparing the estimated results with the observed results for isopentaneeCO2 mixture.
Before the demarcation points, the UFLs vary in linear trend. After the demarcation point, the UFLs change rapidly.
Estimated results and discussions
Fig. 8 e Comparing the estimated results with the observed results for n-butaneeCO2 mixture.
The calculated adiabatic flame temperatures (CAFT) of hydrocarbon-air can be obtained as the critical temperatures when flammability limits of pure hydrocarbon are given [22]. Using the flammability limits at different temperatures of pure hydrocarbon and model proposed in section 3, the flammable zone at the corresponding temperature of hydrocarboneCO2 mixture can be calculated. Figs. 6e10 distinctly present the comparative result between the predicted values and the observed values at 30 C. The predicted profiles are close to the experimental ones for LFL, which suggests the model is comparatively effective at LFL. However, the predicted UFL results are not very good. Overall, the predicted results are acceptable. Besides, the flammable zones estimated by model completely contain the flammable zone measured, which means that the flammable
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zone determined by prediction could be used as safe range for applications. It can be learned from Figs. 6e10 that the LFL basically remain unchanged with CO2 adding in hydrocarbons. This is due to the complete combustion for mixtures at LFL. Furthermore, the isobaric heat capacity of air is slightly smaller than that of CO2. The reaction heat used to heat initial temperature of mixture and air to CAFT is essentially constant with the addition of CO2. Thus, the LFL basically remain unchanged. As demonstrated in Figs. 6e10, the UFL is highly sensitive to the addition of CO2. The reason is that the stoichiometric coefficient and production of the combustion reaction changed simultaneously with addition of CO2. A large predictive deviation is found when the composition of the mixture approaches that of FIP. As described in literature [6], for the majority of combustion of hydrocarboneCO2 mixtures at FIP, oxygen is deficient. Combustion products of most hydrocarbons at LFL are always H2O and CO2 on the basis of the simple chemical kinetics about oxygen consumption. Thus, the chemical kinetics about oxygen consumption has little influence on prediction at LFL for enough oxygen. But the prediction at UFL is totally different from that at LFL due to the deficiency of oxygen. Thus, it is difficult to precisely calculate combustion products without the chemical kinetics involved [25,26]. In other words, the chemical kinetics has an important influence on the prediction of UFL. So that may be a reason why the previous models have difficult in predicting the UFL of mixture composed of CO2 and hydrocarbon. There is another factor impacting strongly on the prediction accuracy apart from the kinetics. Solving the energy balance equation composed of the heat absorption and release is objective of critical flame temperature theory. The energy balance equation is presented: Qendothermic ¼ Qexothermic
(15)
Ma [20] considered that if the values of Qendothermic and Qexothermic were determined perfectly, the flammability limits calculated in Eq. (15) would be accurate. Kinetics largely determines the heat release at UFL. The heat absorption is affected by critical flame temperature. The heat absorption and heat release can be calculated exactly at LFL due to the constant combustion products for complete combustion. The combustion products change from CO and H2 to CO, H2O and CO2 with the dilution increasing at UFL. The critical flame temperatures of different products may be different. If the constant critical flame temperature is still used in model, the calculated heat absorption will not be the exact value. Thus, the assumption (2) proposed is another key to affect the prediction accuracy at UFL. In order to accurately describe the prediction error, the average relative error is adopted. The average relative error between the observed values and predicted values is 1.24% for the mixtures at LFL. The average relative error between the observed values and predicted values is 9.14% for the mixtures at UFL. The effectiveness of the model has been validated on observed data measured at 35 C in the literature [4,5]. A high forecasting accuracy has been achieved at LFL for the average relative error of 1.5%. The average relative error at UFL is 7.8% which is within the acceptable range. The development model has been validate by the experimental data measured at 30 and 35 C. According to the principle of critical flame
Table 3 e Comparing the flammability limits measured at 70 C with the estimated values. Fuel
y
Observed value
Estimated value
L (vol%)
U (vol%)
L (vol%)
U (vol%)
1.97 1.97 1.99 2.39
10.12 9.15 7.62 3.58
1.96 1.97 1.99 2.21
10.12 9.46 8.12 4.1
Propane 0 0.3 0.6 0.9
temperature, the accuracy of a model depends on the heat release and absorption. The heat release can be calculated accurately by means of the kinetics about oxygen consumption. The heat absorption calculated is acceptable. Theoretically, the development model is appropriate for estimating the flammable zones at different temperatures. In order to confirm the result, the flammability limits of propane at 70 C were measured. As shown in Table 3, the average relative errors are 2% and 6.1% for the LFL and UFL, respectively. Thus, the development model is suitable for the temperature no more than 70 C in terms of theory and practice. The flammable zone of mixture at 70 C is presented in Figs. 6e10. The flammable zone estimated at 30 C is contained in the flammable zone estimated at 70 C. Flammability limits of hydrocarbon are available in literatures. However, the flammable zones of hydrocarboneCO2 mixtures are very few. With the help of the model, flammability zones of hydrocarboneCO2 mixtures could be estimated readily using flammability limits of pure hydrocarbons. The only drawback of the model is only considering the physical effect of CO2 and ignoring the chemical effect. CO2 which is not inert in combustion takes part in a chemical reaction and reduces the heat of reaction on the basis of studies by Hu [16]and Huang [27,28]. Thus, calculating the reaction heat containing the chemical effect of CO2 will be studied in the next step.
Conclusions The flammable zones of propane, n-butane, isobutane, isopentane and n-pentane mixed, respectively, with CO2 have been measured. Meanwhile the flammability limits of hydrocarbons have also been measured at different temperatures between 30 and 70 C for five hydrocarbons. A model proposed previously [18] has been modified slightly to predict the flammable zones of the mixtures at different temperatures. Basing on the research, some significant conclusions are summarized as follow: (1) The interesting phenomenon about turning round of flame propagation has been observed when the concentration of mixture approximately approaches the flammability limits. This phenomenon can be used as a supplementary means to determine whether the mixture is flammable or not. (2) Experimental results indicate that there is a linear correlation between the flammability limits and temperature except for isopentane and pentane. As for
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these two fuels, the lower flammability limit of hydrocarbon varies comparatively great when test temperature approaches to boiling temperature. More attentions should be given to lower flammability limit at the above temperature range. (3) Based on the variation trend shown in Figs. 6e10, there are two demarcation points at envelope curves of the LFLs and UFLs. For the LFLs, the demarcation points locate at 75% dilution. For the UFLs, the demarcation points locate at 45% dilution. (4) From the viewpoint of theory, the main reasons of the predicted deviation at UFL have been analyzed in detail. It was found that kinetics and assumption (2) proposed are the key factors. Similarly, we can use the thermal theory to explain the large step size at UFL. It has been found that the step size is closely related to the heat release and absorption of heat. (5) The average relative errors between the observed values in this paper and predicted values are 9.14% and 1.24% for UFL and LFL, respectively. According to the principle of critical flame temperature, the model can be used to measure the flammable zones of mixtures for temperature range from 30 to 70 C.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements
[18]
This work was supported by a grant from the National Natural Science Foundation of China (No. 51676133).
[19]
references
[20] [21]
[1] Shu G, Li X, Tian H, Liang X, Wei H, Wang X. Alkanes as working fluids for high-temperature exhaust heat recovery of diesel engine using organic Rankine cycle. Appl Energy 2014;119(15):204e17. [2] Shu G, Gao Y, Tian H, Wei H, Liang X. Study of mixtures based on hydrocarbons used in ORC (Organic Rankine Cycle) for engine waste heat recovery. Energy 2014;74(5):428e38. [3] Garg P, Kumar P, Srinivasan K, Dutta P. Evaluation of carbon dioxide blends with isopentane and propane as working fluids for organic Rankine cycles. Appl Thermal Eng 2013;52:439e48. [4] Kondo S, Takizawa K, Takahashi A, Tokuhashi K. Extended Le Chatelier's formula for carbon dioxide dilution effect on flammability limits. J Hazardous Mater 2006;138:1e8. [5] Kondo S, Takizawa K, Takahashi A, Tokuhashi K, Sekiya A. Flammability limits of isobutane and its mixtures with various gases. J Hazardous Mater 2007;148:640e7. [6] Zabetakis M. Flammability characteristics of combustible gases and vapours. Bulletin 627. Washington: Bureau of Mines; 1965. [7] Grabarczyk M, Teodorczyk A, Sarli VD, Benedetto A. Effect of initial temperature on the explosion pressure of various liquid fuels and their blends. J Loss Prevention Process Ind 2016;44:775e9. [8] Benedetto AD. The thermal/thermodynamic theory of flammability: The adiabatic flammability limits. Chem Eng Sci 2013;99(32):265e73. [9] Coronado CJ, Andrade Jr JC, Cortez E, Carvalho F, Santos J, et al. Flammability limits: a review with emphasis on ethanol for aeronautical applications and description of the
[22]
[23]
[24]
[25]
[26]
[27]
[28]
9
experimental procedure. J Hazardous Mater 2012;241e242(1):32e54. Takahashi A, Urano Y, Tokuhashi K, Kondo S. Effect of vessel size and shape on experimental flammability limits of gases. J Hazardous Mater 2003;105(1-3):27. Zlochower IA, Green GM. The limiting oxygen concentration and flammability limits of gases and gas mixtures. J Loss Prevention Process Ind 2009;22(4):499e505. Shen X, Zhang B, Zhang X, Wu S. Explosion behaviors of mixtures of methane and air with saturated water vapor. Fuel 2016;177:15e8. Grune J, Breitung W, Kuznetsov M, Yanez J, Jang W, Shim W. Flammability limits and burning characteristics of COeH2eH2OeCO2eN2 mixtures at elevated temperatures. Int J of Hydrogen Energy 2015;40(31):9838e46. Holborn PG, Battersby P, Ingram JM, Averill A, Nolan P. Modelling the effect of water fog on the upper flammability limit of hydrogeneoxygenenitrogen mixtures. Int J Hydrogen Energy 2013;38(16):6896e903. Wang P, Zhao Y, Chen Y, Bao L, Meng S, Sun S. Study on the lower flammability limit of H2 /CO in O2 /H2O environment. Int J Hydrogen Energy 2017;42:11926e36. Hu X, Yu Q, Sun N, Qin Q. Experimental study of flammability limits of oxy-methane mixture and calculation based on thermal theory. Int J Hydrogen Energy 2014;39(17):9527e33. Shebeko YN, Fan W, Bolodian IA, Navzenya V. An analytical evaluation of flammability limits of gaseous mixtures of combustibleeoxidizerediluent. Fire Safety J 2002;37(6):549e68. Hansel JG, Mitchell JW, Klotz HC. Predicting and controlling flammability of multiple fuel and multiple inert mixtures. Plant Oper Progress 1992;11(4):213e7. Chen CC, Liaw HJ, Wang TC, Lin CY. Carbon dioxide dilution effect on flammability limits for hydrocarbons. J Hazardous Mater 2009;163(2e3):795e803. Ma T. A thermal theory for estimating the flammability limits of a mixture. Fire Safety J 2011;46(8):558e67. Shu G, Long B, Tian H, Wei H, Liang X. Flame temperature theory-based model for evaluation of the flammable zones of hydrocarbon-air-CO2 mixtures. J Hazardous Mater 2015;294:137e44. Shu G, Zhao M, Tian H, Huo Y, Zhu W. Experimental comparison of R123 and R245fa as working fluids for waste heat recovery from heavy-duty diesel engine. Energy 2016;115(1):756e69. z GA. Mendiburu AZ, Carvalho JAD, Coronado CR, Chumpita Determination of lower flammability limits of C-H-O compounds in air and study of initial temperature dependence. Chem Eng Sci 2016;144:188e200. Liu X, Zhang Q. Influence of initial pressure and temperature on flammability limits of hydrogeneair. Int J Hydrogen Energy 2014;39(12):6774e82. Vidal M, Wong W, Rogers WJ, Mannan MS. Evaluation of lower flammability limits of fuel-air-diluent mixtures using calculated adiabatic flame temperatures. J Hazardous Mater 2006;130(1-2):21. Shu G, Long B, Hua T, Wei H, Liang X. Evaluating upper flammability limit of low hydrocarbon diluted with an inert gas using threshold temperature. Chem Eng Sci 2015;138:810e3. Wang J, Huang Z, Kobayashi H, Ogami Y. Laminar burning velocities and flame characteristics of COeH2eCO2eO2 mixtures. Int J Hydrogen Energy 2012;37(24):19158e67. Xie Y, Wang J, Zhang M, Gong J, Wu J, Huang Z. Experimental and numerical study on laminar flame characteristics of methane oxy-fuel mixtures highly diluted with CO2. Energy Fuels 2013;27(10):6231e7.
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Available online at www.sciencedirect.com
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Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture Hua Tian*, Mingqiang Wu, Gequn Shu, Yuewei Liu, Xueying Wang State Key Laboratory of Engines, Tianjin University, People's Republic of China
article info
abstract
Article history:
The hydrocarboneCO2 mixture is identified as the most advantageous working medium in
Received 18 July 2017
the medium-high temperature Organic Rankine cycle (ORC). However, the flammable
Received in revised form
zones of hydrocarboneCO2 mixtures at atmospheric condition are rare. In order to mea-
30 August 2017
sure the flammability limits of mixture, an experimental setup based on the ASTM E681-09
Accepted 9 October 2017
has been built completely. In the design of the experiment, an innovative tactic about gas
Available online xxx
distribution is proposed to improve the precision. The flammable zones of five mixtures were measured carefully at 30 C in this paper. But in reality, ORC system operates within
Keywords:
the temperature range from 87 to 300 C. Meanwhile the flammability zones of mixtures at
Flammability limits
higher temperature are not available in the existing literatures. To solve this problem, a
HydrocarboneCO2 mixture
modified critical flame temperature model is modified to estimate the flammable zones of
Thermal theory
mixtures at different temperatures. The modified model presents a higher prediction precision with average relative difference of 1.24% at LFL. Thus, it is easier to estimate the flammable zones of mixtures at different temperatures with the modified model. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Besides the advantages of low GWP (global warming potential), zero ODP (ozone depletion potential) and high decomposition temperature, hydrocarbons showed other better characteristics as working mediums in the medium-high temperature ORC (Organic Rankine Cycle) compared with refrigerants in previous studies [1,2]. Because all of these, hydrocarbons take the place of refrigerants gradually in the medium-high temperature ORC. However, flammability is the main restrictions in the practical application. As a diluent, CO2 is usually mixed with hydrocarbons to reduce the possibility of explosion. Ref. [3] revealed that the mixture of carbon dioxide and hydrocarbon had a better performance in cycle
efficiency than hydrocarbon only in Organic Rankine Cycle owing to a large temperature glide for mixture. In order to evaluate the risks of explosion and combustion in ORC, the knowledge about the flammability limits of such mixtures is necessary. However, little data about flammability limits of hydrocarboneCO2 mixtures is available in literatures except the ones measured by Kondo [4,5] and Zabetakis [6]. Zabetakis [6] measured the flammable zone of hydrocarboneCO2 mixtures using cylindrical glass vessel (diameter of 50 mm, height of 1500 mm) while Kondo [4] obtained the flammable zone of propane/CO2 mixture in dry conditions using the glass flask of 12 L. With the same experimental facility and condition, the data of mixture of isobutane and CO2 is obtained in Kondo [5]. Grabarczyk [7] investigated the effect of initial temperature on the explosion pressure of various
* Corresponding author. State Key Laboratory of Engines, Tianjin University, No. 92, Weijin Road, Nankai Region, Tianjin 300072, People's Republic of China. E-mail address: [email protected] (H. Tian). https://doi.org/10.1016/j.ijhydene.2017.10.053 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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Nomenclature ORC Mixture CAFT LFL UFL DHc t L(U) L0(U0) Ht Had DHc* y L1 L2 U1 U2
organic Rankine cycle hydrocarboneCO2 calculated adiabatic flame temperature lower flammability limit upper flammability limit heat of the combustion initial temperature lower(upper) flammability limit lower(upper) flammability limit at t0 (known parameter) enthalpy at the initial temperature t enthalpy at the calculated adiabatic flame temperature reaction heat at upper flammability limit the molar ratio of CO2 in mixture lowest volume fraction supports flame propagation highest volume fraction does not support flame propagation lowest volume fraction does not support flame propagation highest volume fraction supports flame propagates
liquid fuels in 20 L sphere. The different explosion regimes and UFL can be observed with increasing fuel-air equivalence ratio at different temperature. It was generally believed that the increasing size of the vessel bring the more accurate flammability limits [8e10] because flame quenching effect may become negligibly small in the jumbo vessel. Zlochower [11] investigated the influence of different sizes, 12 L and 120 L, of spherical glass flask on the flammability limits with different decision criterions. The excellent agreement between the observed values in the 120 L glass sphere considered as a jumbo vessel using the pressure rising standard of 7% and the observed results in the 12 L glass sphere obtained using the visual standard supported the fact that the values measured in 12 L glass sphere can be taken as a reasonable approximation to the values obtained in the jumbo vessel. According to the researches by Refs. [12,13], the water vapor as diluent inevitably affects flammability limits of fuel. Holborn [14] measured and modeled the effect of water fog on the UFL of hydrogen-nitrogen mixture. The results suggested that water fog can observably lower the flammable zone of mixture. Wang [15] considered that the water vapor impact on the LFL of H2/CO mixture from the perspective of the chemical effect. It was found that water vapor inhibits the propagation of reaction chain and increases the LFL of H2/CO mixture. Thus, properly speaking, results measured by Kondo [4,5] were flammability limits in ideal environment using the water-free air in experiment. In general, the flammable limits measured by Kondo and Zabetakis may not be exact value. Thus, it is worthy measuring flammability limits of hydrocarboneCO2 mixtures in actual environment using 12 L sphere for security reasons.
Theoretical researches as well as experiments were also extensively investigated to predict flammability limits of fuel. Kondo [4] exploited the modified Le Chatelier equation to predict the flammable zone of mixtures of hydrocarboneCO2 and hydrocarboneN2 mixtures. However, the modified Le Chatelier equation was built on the known experimental data of mixtures, which cannot estimate the flammability zone of other mixtures accurately. Critical flame temperature methods [16], which quantifies exothermicity and heat absorption capacity of material in the combustion process of hydrocarboneCO2 mixture, are generally used to estimate the flammability limits of the hydrocarboneCO2 mixture. In the viewpoint of thermal theories, a certain reaction temperature or threshold temperature value is closely linked to the flammability limit [17]. The studies using critical flame temperature to predict the flammable zone of hydrocarboneCO2 were put forward by researchers [16e21]. Hansel [18] attempted to establish a CAFT model to predict the flammable zone of mixtures. However, the CAFTs calculated by N2 EQUIV in his model are not exact value. The N2 EQUIV didn't consider the oxygen not taking part in reaction which should be included in N2 EQUIV. Chen [19] developed another CAFT model that considered the oxygen not taking part in reaction for estimating flammability limits of hydrocarboneCO2 mixtures. But the UFL estimated results are bad for using the heat of combustion to calculate the reaction heat. Ma [20] used the oxygen calorimetry instead of the heat of combustion to calculate the reaction heat at UFL in his model. To keep the model simple, CO and H2 were excluded from combustion products at UFL, which resulted in low accuracy. Hu [16] took into account the chemical effect of CO2 in the Ma model [20]to estimate flammable zone of methaneeCO2 in pure oxygen environment. The results indicated that the chemical effect of CO2 was not a major factor determining the prediction accuracy. All the above models cannot predict the UFL of mixture accurately due to incomplete combustion at UFL. Recently, the CAFT models proposed by Shu [21] can estimate the UFL of mixtures better than other CAFT models as adopting the simple chemical kinetics about oxygen consumption. The reaction heat can be calculated better because the simple chemical kinetics was adopted so that the reaction in model approached the real combustion reaction in vessel. But all the models above are only used for predicting the flammable zone at normal temperature. At the same time, the operating temperature of hydrocarboneCO2 mixture used in ORC is located in 81e225 C [22]. As we all know, the flammable zone of hydrocarbons will broaden with the increasing of temperature. That is to say, the lower flammability limit (LFL) becomes lower and the upper flammability limit (UFL) becomes higher [23,24]. Thus, using the flammability limits at room temperature to evaluate security issue at high temperature may result in a severe explosion hazard. In order to ensure the safe operation of ORC, it is urgent to obtain the flammable zones of mixtures at elevated temperature. Nevertheless, measuring flammable zones at the different temperatures of hydrocarboneCO2 is very labor-intensive and time-consuming. Thus, a developed model was built by modifying temperature parameter in the Shu model. Using this model the flammable zones of mixtures at different
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temperatures could be obtained easily without too much experimental data. The objective of this paper is to measure the flammability zones of mixtures of propane, isobutane, n-butane n-pentane and isopentane separately diluted with carbon dioxide at 30 C at first and then measure the flammability limits of pure hydrocarbon at 50 and 70 C. At last, to modify the CAFT proposed by Shu [21] to predict the flammable zones of mixtures at different temperatures.
Experimental method Experimental method The experimental setup is built on the basis of the ASTM E68109, which is an improved edition instead of ASTM E681-94. A schematic diagram of the experimental apparatus used to measure the flammability limits is shown in Fig. 1. The experimental setup is divided into two big systems, gas distribution system and explosion system, which consist of assembly units and subsystems such as valves and temperature control system. The gas distribution system is mainly composed of two vacuum pumps, solenoid valves (V1eV8) and the pressure control subsystem. The accuracy of flammability limits depends, to a great extent, on gas distribution system. Thus, a high precision pressure transducer with an accuracy of 0.02% of full scale (100 KPa) is used to measure pressure in vessel. An innovative tactic about distribution is proposed in order to improve the precision. The intake process of the fuel gas or CO2 is divided into several times in which the amount of airflow pumping into the glass flask each time is decided by the partial pressure method compared with the desired pressure. The intake process is dominated by the algorithm which
3
controls the opening time of solenoid valve by comparing the deviation between the setting pressure and the actual pressure. The intake process of the fuel gas would not end until the measured value approaches very close to the setting value. The software finally displays the real measured value that approaches set value, which means that this method has nearly no error in the gas distribution. The advantage of this tactic is that the tactic avoids a large pressure variation which is bad for measuring the pressure in vessel precisely. For the explosion system, a spherical pressure-resistant flask with a volume of 12 L is chosen. A subsystem which is made up of heating resistor, air blower, PID temperature controller and thermal couple is designed to regulate the temperature. By heating or non-heating the air-bath, the temperature in flask can be controlled automatically at a given temperature point. The sparks using for igniting the homogeneous mixtures are generated by a 15 KV high-voltage transformer between two tungsten electrodes that are set (0.64 cm) apart and located one third the diameter of the flask from the bottom of the flask. The ignition duration is 0.4 s during which ignition energy of approximately 10 J is generated. The flange and rubber plug are fixed on the top by two springs. If the explosion occurs in the vessel, the rubber plug along with the flange is lifted up against the springs by the overpressure following the ignition process to relieve the pressure automatically.
Experimental procedure (1) Evacuate the system carefully and flush with air to ensure removal of the residual gas. The flask was vacuumed to 670 Pa or lower before gas comes in. The vessel should be isolated under vacuum theoretically, while actually it is allowed to leak at the speed of less than 0.01 Kpa/min.
Fig. 1 e The schematic diagram of measurement system for flammability limits. Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
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(2) The hydrocarbon inflated into the flask first, and then carbon dioxide. The innovative tactic mentioned in section 2.1 would be applied to the intake process during which the pressure will raise slowly to approach the atmospheric pressure. (3) Mixture is mixed by a magnetic stirrer for 5 min. After that, standing about 2 min to obtain complete homogeneous mixtures. (4) By reducing spring tensioning before firing, the pressure rise resulting from the combustion process is unloaded by pressure relief device (rubber plug). Finally activate the ignition device. Please remember that the propagation of flame should be observed in a dark place. For each experiment, the flame spread was recorded with a video camera. If the flame firstly moves upward, then outward to the top of vessel and finally downward from the top of vessel to develop an extremely thin flame front forming a 90 angle which is measured from the point of ignition, the mixture is considered to be combustible. Example of that condition is shown in the Fig. 3 (4). The flammable zones of mixtures of hydrocarboneCO2 are measured. Purities of hydrocarbons are 99.9%. Purity of carbon dioxide is 99.9%. Air humidity is 50%. The flammable zones of mixtures at 30 C were measured following the procedure. Similarly, the flammability limits of pure hydrocarbon were measured at 50 and 70 C.
The model development A critical flame temperature model for estimating the flammability zones for mixtures of hydrocarboneCO2 was developed by Shu [21]. Validation of the model has been carried out
Establishing equations related to U (or L), y, t and the CAFT L=L0 (or U=U0) t=t0, y=0
Calculated Adiabatic Flame Temperature (CAFT)
Getting model between U (or L), t and y Various t and y Predicted L(U) and Examine Fig. 2 e Procedure of establishing and validation of the model.
on the observed values of methane/CO2, propane/CO2, propylene/CO2 and isobutane/CO2 at 35 C in the literature [4,5]. The mean relative deviations between the predicted results and observed data are than 7.8% and 1.5% at UFL and LFL, respectively. The Shu model was modified to predict the flammable zone of mixture at different temperatures. The derivation process of a new critical flame temperature model is presented as follow. The modeling procedure is shown in Fig. 2. In Fig. 2, establishment and validation of the model contain a procedure consisting of four steps: (1) Build the equation containing y, t and L (or U); (2) The CAFT of pure hydrocarbon can be known when the lower and upper flammability limits are given at t0 and 1 atm in air; (3) Re-formulate equations related to y, t, L (or U) and get correlation between y, U (or L) and t (4) Finally use the developed model to estimate the lower and upper flammability limits of hydrocarboneCO2 mixtures at different initial temperatures and validate the reliability of the model by comparing the estimated values with observed values available in the references; Some of the major hypotheses are shown in order to build the theoretical model to estimate the flammable zones of mixtures: (1) The combustion reaction of mixture takes place at atmospheric pressure; (2) The calculated adiabatic flame temperature deduced from pure hydrocarbon remains unchanged for each hydrocarboneCO2 mixture; (3) The chemical kinetics about oxygen consumption: oxygen is used to calculate the combustion products. The details of the chemical kinetics about oxygen consumption Oxygen are as follows: Firstly, the CO and H2 generate from combustion of hydrocarbon if oxygen is deficient; secondly if there is still residual oxygen after CO and H2 generated completely, H2 will be oxidized to H2O; and finally CO will be oxidized to CO2 if there is still oxygen remained. The UFL of n-butane measured is 7.9%. The combustion products contain 32.3% H2 based on the oxygen consumption. The reported flammable zone of hydrogen in Refs. [15,24] ranges from 4% to 76% at room temperature and atmospheric pressure. Thus, the flush process of flask with air should be given additional attention to avoid the burning of H2. As we all know, the amount of oxygen in combustion at UFL is deficient. Assuming one mole mixture of mixture and air burns at UFL, the overall reaction of mixture can be obtained easily:
UCn Hm þ
1 U yU ðO2 þ 3:773N2 Þ þ 1 Diluent 4:773 1y 1y
/a1 CO þ a2 CO2 þ a3 H2 O þ a4 H2 þ a5 N2 þ
yU Diluent þ DHc* 1y (1)
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Fig. 3 e The flame propagation of mixture of propane and dioxide at LFL.
Considering the following Eqs. (2)e(4): Cn Hm þ
CO þ
H2 þ
n þ m=4 m ðO2 þ 3:773N2 Þ/nCO2 þ H2 O þ DHcCn Hm 4:773 2
(2)
(3)
1=2 ðO2 þ 3:773N2 Þ/H2 O þ DHcH2 4:773
(4)
Eq. (5) will be available with the help of step reactions of (2)e(4) yU Diluent/ð1Þ 1y
(5)
Thus DHc* can be obtained by using the step reaction in a proper way: DHc* ¼ U DHcCn Hm a1 DHcCO a4 DHcH2
(6)
The Eqs. (7e10) can be obtained based on the law of conservation of mass: a5 þ a2 ¼ n U
(7)
2ða3 þ a4 Þ ¼ m U
(8)
a1 þ 2a2 þ a3 ¼
a5 ¼
2 U 1 4:773 1y
(9)
3:773 U 1 4:773 1y
(10)
Based on the condition of burning 1 mol of the mixture and air at UFL and the simple chemical kinetics, the amount ða1 a5 Þ of components is available after combustion. Supposed that the combustion of Eq. (1) takes place under adiabatic environment, Eq. (11) can be derived from the energy balance: a1 HtCO þ a2 HtCO2 þ a3 HtH2 O þ a4 HtH2 þ a5 HtN2 þ
yL t H þ DHc*L 1y D
ad ad ad ad /a6 Had CO þ a7 HCO2 þ a8 HH2 O þ a9 HH2 þ a10 HN2 þ
1=2 ðO2 þ 3:773N2 Þ/CO2 þ DHcCO 4:773
ð2Þ U ð3Þ a1 ð4Þ a4 þ
a6 HtCO þ a7 HtCO2 þ a8 HtH2 O þ a9 HtH2 þ a10 HtN2 þ
yU t H þ DHc* 1y D
ad ad ad ad /a1 Had CO þ a2 HCO2 þ a3 HH2 O þ a4 HH2 þ a5 HN2 þ
y U ad H 1y D (11)
The symbol ad and t in Eq. (11) stand for the CAFT and the initial temperature, respectively. Similarly, at LFL, the balance equation is as follows:
y L ad H 1y D (12)
Thus, the flammable zones of mixtures can be obtained conveniently at different temperatures for a given y.
Discussion and results Experimental results and discussions When the concentration of mixtures is less than the flammability limits, the flame propagates upward vertically without travelling outward (Fig. 3(2)). When the concentration of mixture approaches the flammability limits, the combustion process becomes smoother compared with the fierce stoichiometric combustion process in which the flame immediately expands in all direction to the full vessel from the point of ignition. The interesting phenomenon that the direction of flame propagation appears to move in the opposite direction with respect to the initial direction of propagation after the flame arriving at the vessel top is observed for all near-limit mixtures (Fig. 3(3)). Whether this phenomenon occurs or not seems to be determined by two factors. One is the heating and the other is quenching potential. For the given vessel and condition, the quenching potential is generally a fixed value while the heating potential highly depends upon the amount of hydrocarbon in mixture. Therefore, this phenomenon can be used as a supplementary method to decide whether the mixture is flammable or not when measuring angle is impossible. Comparison results between the flammability limits measured in this paper and the flammability limits in Refs. [4,5,11] are summarized in Table 1. The lower and upper flammability limits of propane measured in 120 L by Zlochower [11] are 2.05 vol% and 9.8 vol%, respectively which have very good coherence to the results measured in this paper. Therefore, the effectiveness of the test apparatus is validated against the flammability limits in the jumbo vessel. Experimental uncertainties of flammability limits are less than 0.05 and 0.3 vol% for LFL and UFL, respectively. The uncertainty primarily comes from the step size namely L1eL2 for LFL and U1eU2 for UFL.
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Table 1 e Comparing the flammability limits of propane and isobutane with observed values in Refs [4,5,11]. Fuel
This study L (vol%)
U (vol%)
Zlochower [11]
Kondo [4,5]
L U L U (vol%) (vol%) (vol%) (vol%)
Propane 2.071 ± 0.02 9.7 ± 0.3 2.05 Isobutane 1.65 ± 0.02 7.78 ± 0.3
9.8
2.03 1.68
10 7.8
L¼
L1 þ L2 2
(13)
U¼
U1 þ U2 2
(14)
The reason for the large step size at UFL can be explain by critical flame temperature theory. The heat released at UFL through reaction is much smaller than that of LFL due to the incomplete combustion. Meanwhile the heat capacity of mixtures at UFL is larger than that of LFL because heat capacity of fuel is much larger than that of air. Thus it may require a smaller step size at LFL to heat temperature of mixture of hydrocarboneCO2 air to CAFT than at UFL. Through the above mentioned analysis, uncertainties are closely related to the step size at LFL and UFL. As shown in Figs. 4e5, there is a linear relation between the flammability limits and temperature except for isopentane and n-pentane. The slope becomes larger when temperature approaches 30e40 C for n-pentane. It is found that the boiling point of pure n-pentane locates in the temperature range of 30e40 C. Namely, n-pentane is the liquid phase at 30 C while at 40 C it is the gas phase at atmospheric pressure. From the viewpoint of critical flame temperature theory, it may require more energy to overcome latent heat of evaporation at 36 C. The same is true for isopentane at LFL. Enough attention should be given for the LFL when the room temperature is close to the boiling point temperature of fuel. As the UFL shown in Fig. 5, the reason why the slope at corresponding boiling point does not change significantly for isopentane and
Fig. 5 e The upper flammability limits of pure hydrocarbons at different temperatures.
n-pentane is the large uncertainty at UFL so the slightly variation at corresponding boiling point is undetected. In other words, latent heat of evaporation only has marginal effects on UFL. Comparison results between the flammability limits measured in this paper and the flammability limits in Refs. [5] are shown in Table 2. As shown in Table 2, there is a good agreement between the flammability limits measured in this paper and those reported in Ref. [5]. By contrasting the results shown in Tables 1 and 2, the new experimental setup is validated. As shown in Figs.6e10, envelope curves of the LFLs and UFLs converge with the addition of the CO2. The measured results of LFL for all mixtures vary within a very narrow range, which not exceed 1% from 0% dilution (y ¼ 0) to overlap (FIP). The results of UFL for all mixtures vary within a very wide range comparing with those at LFL, which exceed 5% from 0% dilution (y ¼ 0) to FIP. The envelope curves of the LFLs and UFLs can be divided into two parts for the variation trend in Figs. 6e10. For envelope curves of the LFLs, the demarcation points locate at 75% dilution (y ¼ 0.75). Before the demarcation points, the LFLs remain unchanged. After the demarcation point, the LFLs change rapidly. For envelope curves of the UFLs, the demarcation points locate at 45% dilution (y ¼ 0.45).
Table 2 e Comparing the flammability limits of isobutane-CO2 mixture with those in Ref. [5]. Fuel
Fig. 4 e The lower flammability limits of pure hydrocarbons at different temperatures.
y
Isobutane 0 0.15 0.3 0.45 0.6 0.75 0.85
This study
Kondo
L (vol%)
U (vol%)
L (vol%)
U (vol%)
1.72 1.73 1.725 1.72 1.735 1.794 1.845
7.78 7.44 6.99 6.65 5.91 5.03 3.98
1.68 1.67 1.68 1.69 1.69 1.73 1.81
7.8 7.4 7.2 6.6 5.9 5.1 4.05
Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
Fig. 6 e Comparing the estimated results with the observed results for propaneeCO2 mixture.
Fig. 7 e Comparing the estimated results with the observed results for isobutaneeCO2 mixture.
7
Fig. 9 e Comparing the estimated results with the observed results for n-pentaneeCO2 mixture.
Fig. 10 e Comparing the estimated results with the observed results for isopentaneeCO2 mixture.
Before the demarcation points, the UFLs vary in linear trend. After the demarcation point, the UFLs change rapidly.
Estimated results and discussions
Fig. 8 e Comparing the estimated results with the observed results for n-butaneeCO2 mixture.
The calculated adiabatic flame temperatures (CAFT) of hydrocarbon-air can be obtained as the critical temperatures when flammability limits of pure hydrocarbon are given [22]. Using the flammability limits at different temperatures of pure hydrocarbon and model proposed in section 3, the flammable zone at the corresponding temperature of hydrocarboneCO2 mixture can be calculated. Figs. 6e10 distinctly present the comparative result between the predicted values and the observed values at 30 C. The predicted profiles are close to the experimental ones for LFL, which suggests the model is comparatively effective at LFL. However, the predicted UFL results are not very good. Overall, the predicted results are acceptable. Besides, the flammable zones estimated by model completely contain the flammable zone measured, which means that the flammable
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8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
zone determined by prediction could be used as safe range for applications. It can be learned from Figs. 6e10 that the LFL basically remain unchanged with CO2 adding in hydrocarbons. This is due to the complete combustion for mixtures at LFL. Furthermore, the isobaric heat capacity of air is slightly smaller than that of CO2. The reaction heat used to heat initial temperature of mixture and air to CAFT is essentially constant with the addition of CO2. Thus, the LFL basically remain unchanged. As demonstrated in Figs. 6e10, the UFL is highly sensitive to the addition of CO2. The reason is that the stoichiometric coefficient and production of the combustion reaction changed simultaneously with addition of CO2. A large predictive deviation is found when the composition of the mixture approaches that of FIP. As described in literature [6], for the majority of combustion of hydrocarboneCO2 mixtures at FIP, oxygen is deficient. Combustion products of most hydrocarbons at LFL are always H2O and CO2 on the basis of the simple chemical kinetics about oxygen consumption. Thus, the chemical kinetics about oxygen consumption has little influence on prediction at LFL for enough oxygen. But the prediction at UFL is totally different from that at LFL due to the deficiency of oxygen. Thus, it is difficult to precisely calculate combustion products without the chemical kinetics involved [25,26]. In other words, the chemical kinetics has an important influence on the prediction of UFL. So that may be a reason why the previous models have difficult in predicting the UFL of mixture composed of CO2 and hydrocarbon. There is another factor impacting strongly on the prediction accuracy apart from the kinetics. Solving the energy balance equation composed of the heat absorption and release is objective of critical flame temperature theory. The energy balance equation is presented: Qendothermic ¼ Qexothermic
(15)
Ma [20] considered that if the values of Qendothermic and Qexothermic were determined perfectly, the flammability limits calculated in Eq. (15) would be accurate. Kinetics largely determines the heat release at UFL. The heat absorption is affected by critical flame temperature. The heat absorption and heat release can be calculated exactly at LFL due to the constant combustion products for complete combustion. The combustion products change from CO and H2 to CO, H2O and CO2 with the dilution increasing at UFL. The critical flame temperatures of different products may be different. If the constant critical flame temperature is still used in model, the calculated heat absorption will not be the exact value. Thus, the assumption (2) proposed is another key to affect the prediction accuracy at UFL. In order to accurately describe the prediction error, the average relative error is adopted. The average relative error between the observed values and predicted values is 1.24% for the mixtures at LFL. The average relative error between the observed values and predicted values is 9.14% for the mixtures at UFL. The effectiveness of the model has been validated on observed data measured at 35 C in the literature [4,5]. A high forecasting accuracy has been achieved at LFL for the average relative error of 1.5%. The average relative error at UFL is 7.8% which is within the acceptable range. The development model has been validate by the experimental data measured at 30 and 35 C. According to the principle of critical flame
Table 3 e Comparing the flammability limits measured at 70 C with the estimated values. Fuel
y
Observed value
Estimated value
L (vol%)
U (vol%)
L (vol%)
U (vol%)
1.97 1.97 1.99 2.39
10.12 9.15 7.62 3.58
1.96 1.97 1.99 2.21
10.12 9.46 8.12 4.1
Propane 0 0.3 0.6 0.9
temperature, the accuracy of a model depends on the heat release and absorption. The heat release can be calculated accurately by means of the kinetics about oxygen consumption. The heat absorption calculated is acceptable. Theoretically, the development model is appropriate for estimating the flammable zones at different temperatures. In order to confirm the result, the flammability limits of propane at 70 C were measured. As shown in Table 3, the average relative errors are 2% and 6.1% for the LFL and UFL, respectively. Thus, the development model is suitable for the temperature no more than 70 C in terms of theory and practice. The flammable zone of mixture at 70 C is presented in Figs. 6e10. The flammable zone estimated at 30 C is contained in the flammable zone estimated at 70 C. Flammability limits of hydrocarbon are available in literatures. However, the flammable zones of hydrocarboneCO2 mixtures are very few. With the help of the model, flammability zones of hydrocarboneCO2 mixtures could be estimated readily using flammability limits of pure hydrocarbons. The only drawback of the model is only considering the physical effect of CO2 and ignoring the chemical effect. CO2 which is not inert in combustion takes part in a chemical reaction and reduces the heat of reaction on the basis of studies by Hu [16]and Huang [27,28]. Thus, calculating the reaction heat containing the chemical effect of CO2 will be studied in the next step.
Conclusions The flammable zones of propane, n-butane, isobutane, isopentane and n-pentane mixed, respectively, with CO2 have been measured. Meanwhile the flammability limits of hydrocarbons have also been measured at different temperatures between 30 and 70 C for five hydrocarbons. A model proposed previously [18] has been modified slightly to predict the flammable zones of the mixtures at different temperatures. Basing on the research, some significant conclusions are summarized as follow: (1) The interesting phenomenon about turning round of flame propagation has been observed when the concentration of mixture approximately approaches the flammability limits. This phenomenon can be used as a supplementary means to determine whether the mixture is flammable or not. (2) Experimental results indicate that there is a linear correlation between the flammability limits and temperature except for isopentane and pentane. As for
Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
these two fuels, the lower flammability limit of hydrocarbon varies comparatively great when test temperature approaches to boiling temperature. More attentions should be given to lower flammability limit at the above temperature range. (3) Based on the variation trend shown in Figs. 6e10, there are two demarcation points at envelope curves of the LFLs and UFLs. For the LFLs, the demarcation points locate at 75% dilution. For the UFLs, the demarcation points locate at 45% dilution. (4) From the viewpoint of theory, the main reasons of the predicted deviation at UFL have been analyzed in detail. It was found that kinetics and assumption (2) proposed are the key factors. Similarly, we can use the thermal theory to explain the large step size at UFL. It has been found that the step size is closely related to the heat release and absorption of heat. (5) The average relative errors between the observed values in this paper and predicted values are 9.14% and 1.24% for UFL and LFL, respectively. According to the principle of critical flame temperature, the model can be used to measure the flammable zones of mixtures for temperature range from 30 to 70 C.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements
[18]
This work was supported by a grant from the National Natural Science Foundation of China (No. 51676133).
[19]
references
[20] [21]
[1] Shu G, Li X, Tian H, Liang X, Wei H, Wang X. Alkanes as working fluids for high-temperature exhaust heat recovery of diesel engine using organic Rankine cycle. Appl Energy 2014;119(15):204e17. [2] Shu G, Gao Y, Tian H, Wei H, Liang X. Study of mixtures based on hydrocarbons used in ORC (Organic Rankine Cycle) for engine waste heat recovery. Energy 2014;74(5):428e38. [3] Garg P, Kumar P, Srinivasan K, Dutta P. Evaluation of carbon dioxide blends with isopentane and propane as working fluids for organic Rankine cycles. Appl Thermal Eng 2013;52:439e48. [4] Kondo S, Takizawa K, Takahashi A, Tokuhashi K. Extended Le Chatelier's formula for carbon dioxide dilution effect on flammability limits. J Hazardous Mater 2006;138:1e8. [5] Kondo S, Takizawa K, Takahashi A, Tokuhashi K, Sekiya A. Flammability limits of isobutane and its mixtures with various gases. J Hazardous Mater 2007;148:640e7. [6] Zabetakis M. Flammability characteristics of combustible gases and vapours. Bulletin 627. Washington: Bureau of Mines; 1965. [7] Grabarczyk M, Teodorczyk A, Sarli VD, Benedetto A. Effect of initial temperature on the explosion pressure of various liquid fuels and their blends. J Loss Prevention Process Ind 2016;44:775e9. [8] Benedetto AD. The thermal/thermodynamic theory of flammability: The adiabatic flammability limits. Chem Eng Sci 2013;99(32):265e73. [9] Coronado CJ, Andrade Jr JC, Cortez E, Carvalho F, Santos J, et al. Flammability limits: a review with emphasis on ethanol for aeronautical applications and description of the
[22]
[23]
[24]
[25]
[26]
[27]
[28]
9
experimental procedure. J Hazardous Mater 2012;241e242(1):32e54. Takahashi A, Urano Y, Tokuhashi K, Kondo S. Effect of vessel size and shape on experimental flammability limits of gases. J Hazardous Mater 2003;105(1-3):27. Zlochower IA, Green GM. The limiting oxygen concentration and flammability limits of gases and gas mixtures. J Loss Prevention Process Ind 2009;22(4):499e505. Shen X, Zhang B, Zhang X, Wu S. Explosion behaviors of mixtures of methane and air with saturated water vapor. Fuel 2016;177:15e8. Grune J, Breitung W, Kuznetsov M, Yanez J, Jang W, Shim W. Flammability limits and burning characteristics of COeH2eH2OeCO2eN2 mixtures at elevated temperatures. Int J of Hydrogen Energy 2015;40(31):9838e46. Holborn PG, Battersby P, Ingram JM, Averill A, Nolan P. Modelling the effect of water fog on the upper flammability limit of hydrogeneoxygenenitrogen mixtures. Int J Hydrogen Energy 2013;38(16):6896e903. Wang P, Zhao Y, Chen Y, Bao L, Meng S, Sun S. Study on the lower flammability limit of H2 /CO in O2 /H2O environment. Int J Hydrogen Energy 2017;42:11926e36. Hu X, Yu Q, Sun N, Qin Q. Experimental study of flammability limits of oxy-methane mixture and calculation based on thermal theory. Int J Hydrogen Energy 2014;39(17):9527e33. Shebeko YN, Fan W, Bolodian IA, Navzenya V. An analytical evaluation of flammability limits of gaseous mixtures of combustibleeoxidizerediluent. Fire Safety J 2002;37(6):549e68. Hansel JG, Mitchell JW, Klotz HC. Predicting and controlling flammability of multiple fuel and multiple inert mixtures. Plant Oper Progress 1992;11(4):213e7. Chen CC, Liaw HJ, Wang TC, Lin CY. Carbon dioxide dilution effect on flammability limits for hydrocarbons. J Hazardous Mater 2009;163(2e3):795e803. Ma T. A thermal theory for estimating the flammability limits of a mixture. Fire Safety J 2011;46(8):558e67. Shu G, Long B, Tian H, Wei H, Liang X. Flame temperature theory-based model for evaluation of the flammable zones of hydrocarbon-air-CO2 mixtures. J Hazardous Mater 2015;294:137e44. Shu G, Zhao M, Tian H, Huo Y, Zhu W. Experimental comparison of R123 and R245fa as working fluids for waste heat recovery from heavy-duty diesel engine. Energy 2016;115(1):756e69. z GA. Mendiburu AZ, Carvalho JAD, Coronado CR, Chumpita Determination of lower flammability limits of C-H-O compounds in air and study of initial temperature dependence. Chem Eng Sci 2016;144:188e200. Liu X, Zhang Q. Influence of initial pressure and temperature on flammability limits of hydrogeneair. Int J Hydrogen Energy 2014;39(12):6774e82. Vidal M, Wong W, Rogers WJ, Mannan MS. Evaluation of lower flammability limits of fuel-air-diluent mixtures using calculated adiabatic flame temperatures. J Hazardous Mater 2006;130(1-2):21. Shu G, Long B, Hua T, Wei H, Liang X. Evaluating upper flammability limit of low hydrocarbon diluted with an inert gas using threshold temperature. Chem Eng Sci 2015;138:810e3. Wang J, Huang Z, Kobayashi H, Ogami Y. Laminar burning velocities and flame characteristics of COeH2eCO2eO2 mixtures. Int J Hydrogen Energy 2012;37(24):19158e67. Xie Y, Wang J, Zhang M, Gong J, Wu J, Huang Z. Experimental and numerical study on laminar flame characteristics of methane oxy-fuel mixtures highly diluted with CO2. Energy Fuels 2013;27(10):6231e7.
Please cite this article in press as: Tian H, et al., Experimental and theoretical study of flammability limits of hydrocarboneCO2 mixture, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.053