Effects of Inert Dilution and Preheating Temperature on Lean

Apr 24, 2014 - should be performed on the lower branch (radiative heat loss controlled) to obtain ...... Lean Blowoff of Bluff Body Stabilized Flames...
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Effects of Inert Dilution and Preheating Temperature on Lean Flammability Limit of Syngas Suhui Li,* Yang Zhang, Xiaolong Qiu, Bo Li, and Hai Zhang Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Lean flammability limits (LFL) of syngas mixtures were measured at different levels of inert dilution and unburned gas preheating temperatures using a counter-flow flame burner. LFL increases drastically when inert dilution (N2 + CO2) exceeds a critical value, at which the extinction flame temperature also increases remarkably. The abrupt change in LFL at high inert dilution cannot be predicted by Le Chatelier’s Rule, which is derived on heat balance basis. This suggests that thermal quenching is insufficient to capture the influence of inert dilution on flame extinction. Data analysis demonstrates that the dependence of LFL on inert dilution can be well captured by a flame temperature−chemical time correlation, revealing thermalkinetics coupled effects of inert dilution on flame extinction. In contrast, LFL decreases linearly with preheating temperature, resulting in a constant extinction flame temperature, which indicates only thermal effect of preheating on flame extinction. The contrast suggests distinctive effects of inert dilution and preheating on flame extinction. Both thermal and kinetics effects of inert dilution need to be incorporated for precise prediction of LFL of syngas at high inert dilution, whereas only thermal effect needs to be considered for unburned gas preheating.

1. INTRODUCTION Syngas, or synthesis gas, is an attractive alternative fuel to natural gas because of its abundant availability and clean combustion characteristics.1 Syngas can be produced from a wide range of feedstock (coal, biomass, petroleum coke, and landfill waste) and processes (such as gasification, coking, and fermentation), which inherently introduce large variation in syngas composition.2,3 Besides H2 and CO, small hydrocarbons (CH4 and C2H6) and inert diluents (N2 and CO2) are common components present in syngas. For example, H2/CO molar ratio may vary from 0.33 to 40, and inert diluents (N2 and CO2) volumetric content may vary from 1−51%, depending on the feedstock and production processes. The large variation in composition of syngas, however, is a critical barrier toward its application in modern combustion devices, which usually employ lean premixed combustion technology for NOx abatement.4 For example, dry-low-NOx combustors of gas turbine engines often operate at close to lean blowout conditions.5 Syngas and high-H2 fuels in IGCC (integrated gasification combined cycle) applications are usually intentionally diluted using N2 to decrease flame temperature and suppress thermal NOx.6 These combustion technologies require deep understanding on the lean flammability limit and flame extinction behavior of syngas under diluted conditions. Although the lean flammability limit (LFL) of the individual fuel components in air has been fairly well characterized, LFLs of syngas mixtures remain much less well understood because of the large variation in composition. Therefore, understanding the impact of composition variation on LFLs of syngas is of practical significance for the design and operation of lean premixed combustion devices as well as for the safety of fuel production and handling. By definition, LFL is the leanest limiting composition of a combustible mixture that will allow flame propagation, that is, being flammable.7,8 Flame propagation is dependent upon © 2014 American Chemical Society

chemical kinetics, molecular transport, and heat transfer. Variation in H2/CO ratio can affect the kinetics of the fuel. For example, previous studies9,10 show that H2 dominates the syngas kinetics when H2 content exceeds a significant level. In addition, N2 and CO2 can impact the thermal conductivity, heat capacity, and heating value of syngas, which have subsequent influence on the ignition, flame propagation and lean blowoff behavior. Natarajan et al.11 found that laminar flame speed of H2/CO mixture, an important parameter describing flame propagation, decreases significantly upon CO2 dilution. The complex nature of LFL of syngas mixtures, in addition to its practical significance, attracted considerable studies. Shaoshin and de Goey12 measured the LFLs of H2−CH4 mixtures and suggest that buoyancy and H2 preferential diffusion play an important factor in determining LFLs. Hustad and Sonju13 measured the LFLs of H2, CO, CH4, C4H10, and their mixtures at temperatures up to 450 °C, and used the LFLs of individual fuels to predict the LFL of mixtures based on the well-known Le Chatelier’s (L-C) Rule14 ⎛ n Y ⎞−1 Lm = ⎜⎜∑ i ⎟⎟ × 100 ⎝ i Li ⎠

(1)

where Lm is the LFL of the fuel mixture (vol %), Li is the LFL of the individual fuel component (vol %), and Yi is the concentration of the ith fuel component in the mixture (vol %). They found that the discrepancies between predicted values and experimental measurements are within ±20%. Wierzba et al.15,16 also reported that the experimentally measured LFLs of H2−CO and H2−CO−CH4 mixtures in air at atmospheric Received: January 20, 2014 Revised: April 13, 2014 Published: April 24, 2014 3442

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Table 1. Properties of Fuels A and B and Syngas Mixtures Used in This Study syngas

H2 (vol %)

CO (vol %)

CH4 (vol %)

C2H6 (vol %)

N2 (vol %)

CO2 (vol %)

sum (vol %)

LHV (kJ/Nm3)

A B natural gas SYN1 (50% A) SYN2 (55% A) SYN3 (60% A) SYN4 (65% A) SYN5 (70% A) SYN6 (75% A) SYN7 (80% A) SYN8 (85% A) SYN9 (90% A) SYN10 (92.5% A) SYN11 (95% A)

1.4 58.8 0 30.1 27.23 24.36 21.49 18.62 15.75 12.88 10.01 7.14 5.705 4.27

24.3 7.4 0 15.85 16.695 17.54 18.385 19.23 20.075 20.92 21.765 22.61 23.0325 23.455

0 23.8 93.1 11.9 10.71 9.52 8.33 7.14 5.95 4.76 3.57 2.38 1.785 1.19

0 0 5 0 0 0 0 0 0 0 0 0 0 0

56.3 7.3 1.9 31.8 34.25 36.7 39.15 41.6 44.05 46.5 48.95 51.4 52.625 53.85

18 2.7 0 10.35 11.115 11.88 12.645 13.41 14.175 14.94 15.705 16.47 16.8525 17.235

100 100 100 100 100 100 100 100 100 100 100 100 100 100

3058 15 012 34 700 9035 8437 7840 7242 6644 6047 5449 4851 4254 3955 3656

studying the flammability limit and limit temperature of lean methane/air counter flow flames. In particular, they proposed a new procedure to study the limit temperature and found it to be in closer agreement with the critical crossover temperature. In light of the thermal and kinetic effects reported by previous researchers, we expect that the variation in H2/CO/CH4 ratio and addition of N2 and CO2 inevitably affects the fuel chemical reactivity and the combustion kinetics. The burning rate and fuel conversion were found to decrease significantly at conditions near lean flammability limits,23,26 which subsequently impact heat release available for heating the unburned fuel−air mixture to flammable temperature. Therefore, numerical methods utilizing detailed kinetics mechanism and heat transfer model were developed in order to give more accurate results on calculating the LFLs of syngas mixtures. Such methods include the simulation of the 1-D planar flame27,28 and spherical flame,29 as well as the counter-flow flame.24,30,31 Comparison and validation of the spherical and tube methods were made by Van den Schoor et al.32 on predicting LFLs of H2−CH4 mixtures. However, most of the previous studies on syngas LFL do not consider the copresence of N2 and CO2, which is the case for practical syngas. The effects of inert dilution and unburned gas temperature on LFL of syngas thus remain to be elucidated. For example, it has not been validated if L-C Rule can be extended to practical syngas with both N2 and CO2 diluents. Obviously CO2 may affect the combustion kinetics of fuel species in the syngas. It remains unclear if the linear dependence of LFL on unburned gas temperature is still valid at heavily diluted conditions, although previous studies13,15,16 found that the LFLs of H2−CO and H2−CO−CH4 mixtures are linearly dependent on temperature. With the growing interest of exhaust gas recirculation (EGR), it also requires a thorough understanding of the effects of inert dilutions on LFL of fuels at elevated temperatures. In addition, kinetics mechanisms developed for syngas are usually tested against experimental data of ignition delay time and laminar flame speed.9,10 The capability of the mechanisms to predict LFL of practical syngas, however, is less well tested. Thus, experimental data of the LFLs of practical syngas is needed for validation of the numerical models and kinetics mechanisms. Motivated by the need of experimental data on practical syngas, this paper presents experimental measurements of LFLs of practical syngas fuels diluted with a wide range of N2 and CO2. Experiments were performed using the opposed-jet

pressure and temperatures up to 300 °C are consistent with values calculated by L-C Rule and decrease linearly with increasing temperature. Attempts were also made to use eq 1 to predict the LFL of syngas mixture containing inert diluents (N2 and CO2) using the LFLs of individual fuels. For example, van Tiggelen et al.17 and Kondo et al.18 found that the LFL of individual fuel at room temperature does not change upon N2 dilution. Karim et al.19 demonstrated that at room temperature, L-C Rule is also applicable for H2−CH4 mixtures diluted with N2 or CO2 upon modification by considering the LFL of inert diluent as infinity or by considering the diluent is associated with an individual fuel. Application of Karim’s method, however, is restricted to conditions where the diluents/fuel molar ratio is below a certain limit. The restriction arises from the basic assumptions of L-C Rule that the flame temperature at LFL and the heat capacities of combustion products (including diluents) are constant, and the combustion kinetics of individual components is independent of other species.20 In fact, combustion kinetics would be expected to change due to the presence of other combustion species and large quantity of inert diluents (especially CO2). Therefore, Karim’s method cannot be applied to syngas with a large amount of inert diluents because this method considers only the thermal effect of inert dilution. Nevertheless, these results encouraged other researchers21,22 to predict the LFL of syngas mixture containing inert diluents (N2 and CO2) using the constant flame temperature method based on LFLs of individual fuels. This is because LFLs of individual fuels are readily available and experimental measurement of LFL of syngas mixture is costly and time-consuming, particularly at elevated temperatures and pressures encountered in many combustion devices and industrial processes. The constant flame temperature method, however, has its inherent inaccuracy by neglecting the effects of heat capacity and thermal conductivity of inert diluents, which apparently have strong influence on the flame temperature and heat loss. Realizing this, Kumar23 developed an empirical equation to take into account the heat capacity and thermal conductivity of inert diluents and obtained improvements on predicting the LFLs of H2 with N2 and CO2 diluents. This improved method, however, was solely based on heat loss and did not consider the kinetics effect. It is well-known that chemical kinetics plays a significant role in flame propagation, particularly at conditions near flammability limits.24,25 Chen and Sohrab25 found that both thermal effect and chemical kinetic effect are essential in 3443

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Figure 1. Experimental setup of the counter-flow flame burner.

counter flow flame method33,34 at unburned gas temperatures up to 473 K. Numerical calculations were also performed using Chemkin program and the results are compared with the experimental data. Instead of supporting one specific theory or developing new theory of LFL, the paper focuses on evaluating the effects of inert diluents and preheating temperature on LFL, as well as providing new experimental data of practical syngas for model and mechanism validations.

rate, K. The local strain rate is normally defined as the negative value of the velocity gradient in the centerline of the flame, which can be determined by particle image velocimetry (PIV) measurements. For simplification, it is often represented by the global strain rate, namely, the global velocity gradient from nozzle exit to stagnation plane

K=

Uexit L/2

(2)

At a constant strain rate, flame can be extinguished by reducing the equivalence ratio of fuel−air mixture. On the other hand, flame extinction can be achieved at a fixed equivalence ratio ϕ by increasing the strain rate to a critical value, Kext. Extrapolating the Kext − ϕ plot to zero strain rate gives a theoretical equivalence ratio below which flame cannot propagate even if external fluid mechanics effect is excluded (i.e., quiescent mixture). Such an equivalence ratio is called the theoretical lean flammability limit. It should be noted that the above approach to determine the flammability limit has been questioned and explained by Sung and Law37 and Maruta et al.38 The main drawback is that when the flames are subjected to small strain rates, the flame exhibits C-shape turning point behavior and nonlinear correlation instead of linear correlation exists between equivalence ratio of the mixture and the extinction strain rate for a fuel with Lewis number less than 1, such as the syngas containing H2, CO, and CH4. The C-shape behavior was attributed to synergistic effects of heat release reduction and relative radiation enhancement as strain rate is decreased.31,37 Ideally, extrapolation should be performed on the lower branch (radiative heat loss controlled) to obtain the fundamental flammability limit. This approach, however, is hindered by the difficulty of establishing such weakly burning flames under normal gravity conditions. As an approximate, linear extrapolation is made on the upper branch (stretch controlled), which yields lower value than fundamental flammability limit. To minimize the error introduced by linear extrapolation compared to nonlinear extrapolation, the flame extinction strain rates were extended as low as possible (around 50 s−1). The detailed effect of C-shape, nonlinearity behavior at small strain rates on determining LFL is discussed later. Nevertheless, the above approach is still a satisfactory compromise between the need to exclude external effects (wall quenching, flame stretching, and igniter energy) and the difficulty of conducting experiments. Therefore, the opposed-flow flame method is adopted by this study. In comparison, the standard tube method39 for determining flammability limits suffers from the effects of wall quenching, flame stretching, and igniter

2. MATERIALS AND METHODS 2.1. Syngas Fuels. Composition variation of syngas is achieved by mixing two industrial process gases, A and B. Gas A is a low-Btu fuel that contains 56.3% N2 and 18% CO2 and is used as the diluent. Gas B is a medium-Btu fuel that contains 58.8% H2. Properties of the A and B fuels used in this study are listed in Table 1. The volumetric fraction of gas A in the mixture is set as 50−90% with 5% increment, and the resulted syngas mixtures are designated as SYN1 to SYN11, respectively. The properties of the diluted syngas fuels and a typical natural gas fuel are also included in Table 1 for comparison. The syngas mixtures feature a wide range of inert content (10−71% for N2 + CO2), which enables us to explore the LFL characteristics of syngas that have not been studied before. There is a large market for burning these industrial process fuels and their mixtures using for reducing waste gas emissions and improving plant efficiencies.35 2.2. Experimental Approach and Apparatus. Experiments were performed using the counter-flow flame method.33,34 This wellestablished method has been widely used to measure the fundamental flammability limit and laminar flame speed attributed to its simple configuration and accurate results. Two identical burners with exit diameter D = 10 mm are displaced in opposition along the centerline with a separation distance of L= 20 mm. The premixed fuel−air mixture is injected from two burners at the same velocity, Uexit. When the mixture is ignited, two symmetric flames can be established between the opposed burners. The middle plane between the two burners is called stagnation plane because the axial velocity of the mixture reduces to 0 when the mixture approaches the middle plane. In this work, the burners are of high contraction type to get uniform velocity distribution at the exit. This configuration gives a L/D ratio of 2, which is sufficiently large that the effect of finite domain on the flame measurement can be considered small.36 Because of the symmetric configuration, the flow field along the burner centerline can be considered one-dimensional and adiabatic. The flow field can be characterized by a single parameter, the strain 3444

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flame extinguishes at the turning point, which is the extinction strain rate.

energy,7,12,32 while the standard bomb method40 is also influenced by flame stretching and igniter energy.32,41 Figure 1 shows the schematic diagram of the overall experimental setup used in this study to measure LFL of syngas mixtures. The flow rates of syngas and air are controlled by sonic-nozzle type flow meters separately. Syngas mixture is obtained by mixing fuel A and fuel B flows at the desired mixing ratio. Air is preheated using an electric furnace. The experiments were carried out at fuel−air mixture temperatures of 20 to 200 °C by preheating the air flow. The syngas and air are then premixed within a mixing chamber before being injected into the counter-flow burners. The fuel−air mixture is ignited by a torch igniter. During the experiment, flame is usually established at a relatively high equivalence ratio and low air flow rate, and then the air flow rate (also strain rate) was gradually increased for a fixed fuel flow rate until the flame extinction was observed by human eyes. The flow rate right before extinction was recorded for calculation of extinction strain rate and equivalence ratio. The process is then repeated for different A/B mixing ratios at elevated temperatures. During this process, the flame is monitored and recorded by a 720 × 576 resolution CCD camera at 25 Hz frequency. 2.3. Numerical Modeling. To compare with experimental data, numerical calculations were performed using the Chemkin OPPDIF code. The OPPDIF code42 has been widely used to simulate the opposed-flow flame.11,43 The code simulates the opposed-flow flame as an axisymmetric, flat flame, and can predict the velocity, temperature, species concentration profiles at the centerline of the burner by neglecting edge, and gravimetric (buoyancy) effects. The code was modified to account for the thermal radiation44 of CH4, CO, CO2, and H2O in the optically thin limit and coupled with the Sandia thermal and transportation subroutine libraries.45 Multicomponent transport was used in the calculation to account for the Soret effect, which generally enhance the burning intensity of lean syngas flame. The extinction strain rate (turning point) was obtained by using a twopoint arc continuation approach43 and a predetermined temperature reduction within the computation domain. To simulate the opposedflow flame, all conditions set in the code matched the experimental conditions including the burner separation distance, flow exit velocity, and unburned gas temperature. The combustion reactions were described by the USC-II kinetics mechanisms.46 The USC mechanism was developed at the University of Southern California (USC) and was optimized for H2/CO combustion. It has been validated against ignition delay times and species profiles in shock tube experiments, as well as laminar flame speed measurements. The complete mechanism has 111 species and 784 reactions. Convergence of the calculation was obtained by using adaptive mesh control to set both the solution gradient and curvature to 0.1. Figure 2 shows an example of how to find the turning point (Kext) using the arc continuation approach for SYN5 mixture with an equivalence ratio of 0.38. The maximum flame temperature increases initially because straining increases burning intensity for lean syngas flame with subunity Lewis number.47 The maximum flame temperature then decreases upon further increasing strain rate because less residence time is available for the combustion reaction to complete, that is, heat release is incomplete. Eventually the

3. RESULTS AND DISCUSSION 3.1. Flame Observation. Figure 3 is an example of the change of flame appearance with increasing air flow rate (strain rate) until flame extinction for the SYN3 mixture. The timer at the lower-right corner of each image indicates the elapsed time since ignition. It shows that two separate flames existed at low air flow rate (low strain rate and high equivalence ratio), which corresponds to the relatively high temperature of the upper branch of the extinction curve in Figure 2. The two flames moved closer and gradually merged together upon increasing air flow rate (high strain rate), which corresponds to the initial temperature increase and gradual decrease of the upper branch of the extinction curve in Figure 2. The merge of the two flames at the stagnation plane is caused by the nonequidiffusive effect.47 For a flame with Lewis number less than 1, the nonequidiffusive effect increases the burning intensity (and thus flame temperature) upon positively stretch. The syngas mixture used in this study mainly contains H2, CH4, and CO fuel components, and the Lewis number is smaller than 1 for its lean flame. Such a flame will move toward the stagnation plane upon increased positive straining to balance the local flow velocity with flame speed. Further increasing strain rate leads to insufficient residence time for combustion reactions to complete, which results in lower flame temperature. The flame eventually extinguished when it reaches the stagnation plane and the flame position cannot be adjusted to adapt the flow velocity, which is the turning point of the extinction curve in Figure 2. At small strain rates (below 50 s−1), the flames started swinging because of the buoyancy effect. The natural convection outweighs the flame straining and radiative heat loss in determining flame extinction. Determination of LFL using extinction strain rates below 50 s−1 introduces large error. Therefore, experiments were performed at strain rates no smaller than 50 s−1. A Kext−ϕ curve of SYN5 mixture is shown in Figure 4 as an example of how the extinction limit varies with strain rate and how the extrapolation is performed to determine the LFL. The intercept of the linear fit is the lean flammability limit at zero strain. 3.2. Effect of Inert Dilution. To study the effect of inert dilution on LFL of syngas, the LFL measurement was performed for SYN1 to SYN11 mixtures at unburned gas temperature of 20 °C. The inert content (N2 + CO2) herein covers a wide range of 10−71% and the N2/CO2 molar ratio remains relatively constant (2.7−3.1). Because the purpose of the study is to reveal the compounded effect of multiple inert dilution, the LFL of syngas mixtures, ϕLEL, is plotted against total inert content in Figure 5. Data processing in this way considers the effects of all kinds of diluents and fuels, and the conclusion can be applied to multicomponent syngas fuel without considering the effect of a single component. Recognizing the C-shape, nonlinearity of Kext−ϕ plot at small strain rates,37,38 we must carefully interpret the LFL data presented in Figure 5. Because linear extrapolation of the upper branch of Kext−ϕ plot gives lower LFL than fundamental flammability limit for flames with Lewis number smaller than 1, the LFL data in Figure 5 are underestimated. Furthermore, the C-shape effect is mainly due to the synergistic effects of heat release reduction and relative radiation loss enhancement at small strain rates. As such, the highly diluted syngas tends to

Figure 2. Plot of maximum flame temperature against global strain rate as an example of how to find extinction strain rate (turning point). 3445

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Figure 3. Flame sequences during an extinction event by decreasing equivalence ratio.

reduction and radiation heat loss. In summary, the linear extrapolation method yields smaller LFL values than fundamental flammability limit, particularly for the highly diluted syngas mixtures. Nevertheless, the underestimation of LFL does not undermine the following analysis of the dependence of LFL on inert content because the trend remains the same. The plot in Figure 5 clears shows three regions of interest. At inert content below 50%, ϕLFL remains relatively constant. At 50−65% inert content, ϕLFL increases slightly. Namely, low to intermediate inert dilution seems to have minor effect on ϕLFL. In the third region when inert content exceeds 70%, ϕLFL increases dramatically, indicating the effect of inert dilution becomes significant. This phenomenon indicates that the L-C Rule may not be valid at high inert dilution conditions. To examine if the L-C Rule is still applicable at the copresence of N2 and CO2, the LFL of syngas mixtures is calculated using the modified L-C Rule proposed by Karim.19 The modified L-C Rule considers N2 is associated with a fuel component (such as H2), and the L-C Rule becomes

Figure 4. Plot of extinction limit against strain rate for SYN5 mixture at 20 °C.

−1 ⎛Y YCH4 YCO2 ⎞ YCO H 2 + N2 ⎟ × 100 Lm = ⎜⎜ + + + LCH4 LCO LCO2 ⎟⎠ ⎝ L H2 + N2

(3)

in which Lm, Yi, and Li have the same definition as in eq 1 and LCO2 is infinity as CO2 is considered as a quenching inert. Though the inert can also be considered being associated with CH4 or CO, eq 3 yields slightly better prediction when it is considered being associated with H2. Karim found that it yields more accurate predictions when considering N2 rather than CO2 associated with H2 because the flame extinction capability of CO2 is not negligible.19 The ϕLFL of H2−N2 associated

Figure 5. LFL of syngas mixtures as a function of inert content at unburned gas temperature of 20 °C.

suffer more from this effect because the highly diluted syngas has a relatively weaker mixture strength and weaker flame, which is more prone to be impacted by the heat release 3446

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mixture, LH2+N2, is calculated following Kondo et al.18 and Kumar23 and that the LFL of H2 in the H2−N2−air mixture is a constant regardless of N2 dilution, that is, L H2 = L H2 + N2

YH2 YH2 + N2

inert content is the critical value for L-C Rule to be applicable when N2 dominates the inert composition, above which the rule is not valid. This critical value is expected to be even lower when CO2 dominates the inert composition because the effect of CO2 on flame temperature and kinetics is more significant. It is well-known that CO2 has a larger heat capacity compared to N2, and has more capability of reducing flame temperature and quenching the flame. Karim et al.19 and Kumar23 also suggested that the modified L-C Rule, which is based on the constant flame temperature assumption, is applicable only when inert content is below a certain limit. This is because the effect of inert dilution on decreasing flame temperature is no longer negligible when the inert content exceeds a certain level, making the constant flame temperature assumption invalid. This point well explains the near-constant and slight increase of LFL at low to intermediate inert dilutions. However, it cannot explain the dramatic increase at high inert content, for example, 70%. The following analysis explains the phenomenon by considering both kinetics and thermal effects of inert dilution. Flame propagation/extinction is indeed a competition between the two key reactions46,55

= 4% (4)

The effectiveness of eq 4 was confirmed by Karim19 using the tube method. The LFL of CH4 is 4.9%, according to counterflow flame analysis by Law and Egolfopoulos34 and Sung and Law.37 A similar limit was measured by Kondo et al.48,49 It is well-known that the LFL of CO is sensitive to the water vapor content in the air,50 varying in the range 12.2−15.5%. The large variation of CO may introduce error in the calculation of mixture LFL using L-C rule, especially for SYN6−SYN11 mixtures with CO content exceeding 20%. Here, we choose 13.5% as the LFL of pure CO, which was measured by Wierzba and Kilchyk15 at absence of water vapor because the air used in the experiments is free of water vapor. The careful choice of LFL for CO avoids the error introduced by inappropriate value of LFL of CO. The calculated LFLs are converted to equivalence ratio (ϕLFL) and the results are plotted in Figure 6 to compare with experimental data.

H + O2 = OH + O

(R1)

H + O2 + (M ) = HO2 + (M )

(R2)

Reaction R1 is the main chain-branching reaction that initiates and supports flame propagation by generating OH and O radicals to attack fuel components. For syngas mixture, flame propagation is a complex process that involves of combustion of multiple fuel species, including H2, CO, and CH4. The presence of H2 promotes flame propagation by providing H radicals for reaction R1. In fact, the H2/O2 reaction is the starting point in the hierarchy of the oxidation of hydrocarbon fuels because H2 has the highest reactivity among the fuel components of syngas.56 Because of the preferential diffusion effect57 of H2, syngas flame propagation and extinction are strongly dependent on H2 kinetics. This is one of the reasons why less diluted syngas has a much lower LFL compared to highly diluted syngas. When syngas is diluted with N2 and CO2, however, reaction R1 is suppressed because less H2 is available to produce H radicals. On the other hand, reaction R2 is in fact the chain-terminating reaction that dominates the extinction of CH4 and syngas flames. Both CO2 and N2 can participate in reaction R2 as the intermediate M, competing for H radicals with R1 and thus inhibiting flame propagation. Furthermore, CO2 also has chemical inhibiting effect on the production of H radicals. Liu et al.53 and Park et al.54 conducted numerical studies of the effects of CO2 dilution on premixed methane−air flames and H2−air counter-flow flames, respectively. They both found that addition of CO2 changes the equilibrium of the CO oxidation reaction

Figure 6. LFL of syngas mixture predicted using L-C Rule in comparison with experimental data.

The ϕLFL predicted by L-C Rule follows closely with the trend of experimental data at low to intermediate inert contents. The difference between experimental data and L-C Rule predicted data can be attributed to the linear extrapolation method used in processing the experimental data. According to Sung and Law,37 the upper branch of the C-shape flame extinction curve exhibits nonlinear behavior. Linear extrapolation may yield a larger flammability limit than the rigorous theoretical limit of the upper branch. When inert content exceeds 70%, however, the rule fails to follow the abrupt increase in ϕLFL. Recall the C-shape effect discussed above, the sharp increase of ϕLFL at high inert dilution can be even more prominent for the fundamental flammability limit. The L-C Rule is based on heat balance analysis and constant extinction flame temperature assumption, and does not consider kinetics effect. Therefore, the large deviation at high inert dilution suggests that the effects of inert dilution on flame temperature and kinetics cannot be neglected at high dilution ratio. Similar to EGR, inert dilution has 3 major effects on combustion: decreasing flame temperature due to high heat capacity of CO2, lowering O2 concentration in combustion reactions, and reducing combustion reactions rates by chemically inhibiting H radicals.51−54 For the syngas mixtures used in this study, the N2/CO2 molar ratio is in the range 2.7−3.1, and thus N2 dominates the N2−CO2 inert composition. Therefore, 70%

CO + OH = CO2 + H

(R3)

Reaction R3 is inhibited by addition of CO2, which also results in less H radicals available for the chain-initiating reaction R1. Thus, flame strength is consequently undermined. From thermal perspective, reaction R2 is well-known to be particularly active at low temperatures58,59 of 1000−1200 K, which are typical for highly diluted syngas flames near ϕLFL. At low temperatures, the reaction is too slow to completely convert the fuel species in syngas mixture. Correspondingly, the heat release rate cannot compensate the heat loss to surrounding environment, and flame ceases propagation. 3447

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in which δL is the flame (reaction zone) thickness, SL is the laminar flame speed, and α is the mixture-averaged thermal diffusivity of the reactants calculated at the unburned gas temperature. Ideally, α and SL should be calculated at extinction limit. Calculation of SL at extinction limit, however, is hindered by the difficulty to achieve converging solutions at near extinction limit. Therefore, α and SL are calculated at stoichiometric conditions. Indeed, here we are just using τchem of stoichiometric mixture as a “representative measure” of chemical reactivity of syngas. As long as the stoichiometric mixture is consistently used throughout the τchem calculation, the analysis remains reasonably sound. According to eq 5, τchem is physically the time for a flame to propagate through the reaction zone, or the minimum residence time needed to complete combustion reactions. A small τchem indicates fast flame propagation, fast burning rate, and a more reactive fuel. A more reactive fuel generally requires a lower flame temperature to maintain combustion reaction compared to a less reactive fuel. This suggests that at conditions near flammability limit, highly diluted fuel needs a higher flame temperature to achieve a certain fuel conversion and to ensure flame propagation. The thermal diffusivity of the reactants mixture in eq 5 can be calculated by

Indeed, previous studies found that reaction rate22 and fuel conversion23,26 drop drastically at temperatures below a critical point. As such, inert dilution further undermines flame propagation by favoring reaction R2 at low temperatures. Therefore, it is the coupled effect of thermal quenching and kinetics inhibition that determines the ϕLFL of syngas flames diluted with inert gases. To reveal the coupled effect of thermal quenching by inert dilution, the adiabatic flame temperature at ϕLFL, TLFL, is calculated assuming full equilibrium. Note here TLFL is not the actual flame temperature at ϕLFL because lower-than-equilibrium fuel conversion exists at near-limit conditions.23 Ideally, TLFL should be the minimum real flame temperature that allows flame propagation at fuel lean conditions for a given syngas composition. The TLFL definition used in this study, however, assumes full equilibrium to make it convenient to calculate the equivalence ratio of syngas−air mixture at LFL. The flame temperature of lean syngas mixture (Lewis number smaller than 1) is known to be increased by positive stretch.37 The minimum flame temperature here, however, does not consider stretch effect because the flame temperature degenerates to adiabatic flame temperature as ϕLFL is obtained by extrapolating the Kext−ϕ curve to zero stretch rate. The calculated TLFL is plotted against inert content in Figure 7. TLFL remains relatively

α=

constant at inert content below 50% but slightly increases until about 65% inert content. However, TLFL increases dramatically at about 70% inert content, which is in good agreement with the abrupt increase of ϕLFL in Figure 5. This phenomenon suggests that the constant TLFL assumption at φLFL is no longer valid at high inert dilution. Inert dilution reduces reaction rate by promoting reaction R2. A higher flame temperature is needed to compensate the kinetics inhibition effect of inert on flame propagation. In other words, the heat release by combustion not only needs to heat up the extra inert diluents but also needs to raise the temperature to maintain a critical reaction rate to sustain flame propagation. To demonstrate the kinetics effect of inert dilution on ϕLFL, the chemical kinetic time of syngas mixtures is chosen to characterize the chemical reactivity of syngas. The chemical kinetic time, τchem, is a characteristic time scale for evaluating the kinetics of premixed flame and the resistance of the flame to sustain straining.60,61 It can be calculated based on laminar flame properties δL 2α = 2 SL SL

(6)

in which k is the thermal conductivity, ρ is the density, and Cp is the specific heat capacity (evaluated at constant pressure) of the unburned reactants. SL is calculated using the PREMIX code of Chemkin with the USC mechanism.46 PREMIX is a reactor model that simulates a 1D freely propagating planar flame. SL is defined as the velocity of the unburned gas of the 1D freely propagating planar flame. The converged solution is obtained by repeating the calculation process 3 times until the temperature and species slopes at the inlet boundaries are close to zero and both gradient (GRAD) and curvature (CURV) controls are at least 0.1 or less. The estimated flame zone width (domain of calculation) is 1 cm with estimated ending axial position of 0.3 cm. Multicomponent transport is used in the calculation to account for the Soret effect for the syngas mixture. τchem of syngas mixtures is presented in Figure 8 as a function of inert content. It clearly shows the abrupt increase of τchem at high inert content, indicating a much less reactive fuel, or a much lower burning rate. To ensure flame propagation, a certain level of fuel conversion (or heat release) is needed. For a much less reactive fuel, more fuel (essentially a higher

Figure 7. Limiting flame temperature of syngas mixtures at various inert content assuming full equilibrium.

τchem =

k ρCp

Figure 8. Chemical kinetic time of syngas mixtures with different inert content calculated at stoichiometric condition.

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defines the “flammable line” for the syngas mixtures with different inert dilutions. The TLFL−τchem can be used to predict the LFL of a syngas mixture. For a syngas with particular inert content, TLFL (or ϕLFL) can be calculated based on its τchem at stoichiometric condition. No flame propagation is possible below TLFL on the flammability line. In fact, τchem used in this correlation considers all the contributions that influence the chemical reactivity of the syngas mixture, including the fuel components (H2, CO, CH4) and inert diluents (N2, CO2). Indeed, τchem is the fingerprint information on the fuel reactivity. Therefore, the TLFL−τchem correlation captures both the thermal and kinetics effects of inert dilution on ϕLFL of syngas. The ϕLFL determined by Chemkin modeling is plotted in Figure 10 to compare with experimental data. At low to

equivalence ratio) needs to be burned to increase the flame temperature. Alternatively, more fuel is needed to compensate the lower conversion resulting from lower fuel reactivity. The increased equivalence ratio essentially results in an increased adiabatic flame temperature calculated assuming full conversion. Indeed, Kumar23 reported that the fuel conversion at near φLFL drops sharply because of the significant decrease of reaction rate at the low flame temperature. This implies that the actually burned fuel fraction that contributes to heat release decreases significantly at near flammability limits. As a result, the actual flame temperature is much lower than the one that is calculated assuming full equilibrium. The fuel conversion is expected to decrease further upon high inert dilution because the fuel reactivity decreases, implying a lower burning rate as evidenced by the increased chemical time. As such, more fuel is needed to compensate the decreased fuel conversion when inert dilution increases. This explains the abrupt increase of TLFL of the highly diluted syngas mixture at ϕLFL, as shown in Figure 7. In summary, inert dilution affects ϕLFL by decreasing fuel reactivity and reaction rate, which subsequently unburned fuel fraction during flame propagation. To compensate the unburned fuel fraction, ϕLFL needs to be increased, resulting an increase in TLFL calculated assuming full equilibrium. Analysis of data in Figure 7 and Figure 8 suggests there is a correlation between TLFL of the diluted syngas and τchem of its stoichiometric mixture. To reveal the correlation, TLFL in Figure 7 is plotted against τchem in Figure 8 and the results are displayed in Figure 9. The linear fit is also included in Figure 9,

Figure 10. Syngas lean flammability limit determined by Chemkin modeling in comparison with experimental data.

intermediate inert content, the modeling result agrees very well with the experimental data, demonstrating that Chemkin modeling is more accurate than L-C Rule. The improved accuracy can be attributed to the fact that Chemkin modeling on counter-flow flame considers both kinetics and radiation heat loss, which is more comprehensive than the L-C Rule’s assumption of constant flame temperature. As inert content increases further, the difference between Chemkin modeling result and experimental data becomes larger. This difference might be due to the linear extrapolation of Kext−ϕ curve. The Chemkin modeling was able to establish a flame at small strain rates (as low as 30 s−1) for the highly diluted mixtures, whereas in experiments flame can only be established at strain rates no smaller than 50 s−1. Extrapolation of the curve at small strain rates can generate lower limit than the value extrapolated at higher strain rates. 3.3. Effect of Preheating Temperature. To study the effects of unburned gas temperature, LFL measurement was conducted at preheating temperatures of up to 473 K for SYN6 to SYN10 mixtures. Here, the unburned gas temperature is measured at the burner exit, which is the temperature of the unburned syngas−air mixture. The inert content (N2 + CO2) varies in the range 55−68% from SYN6 to SYN10. The measured ϕLFL is presented in Figure 11 as a function of inert content. The linear fit of these data (R2 ≥ 0.96) suggests that ϕLFL decrease linearly with temperature, similar to the linear behavior of individual fuel component. This result is consistent with previous studies of Hustad13 and Wierzba et al.15,16 on mixtures of H2, CO, and CH4. Preheating essentially supplies heat to the reactants and requires less fuel to be burned to heat the surrounding unburned gas. Thus, flame propagation can be

Figure 9. Correlation between limiting flame temperature and chemical time of diluted syngas mixture.

which demonstrates a strong linear correlation (R2 = 0.97) between TLFL and τchem. The dramatic increase of TLFL coincides with the significant increase in τchem (or decrease in fuel reactivity). This correlation suggests that the increase of TLFL is due to the decrease in fuel reactivity or burning rate. The intercept on the vertical axis is the theoretic TLFL of the syngas that has the highest chemical reactivity, that is, lowest inert content. At a small τchem, the experimental data falls below the fitted line. This deviation is probably due to the preferential diffusion62−64 of H2, which is more significant when the H2 content is relatively high at low inert dilutions. The higher mass diffusivity of H2 relative to O2 makes the flame zone “locally richer” than the global equivalence ratio, enabling flame propagation at leaner than expected value. A recent study57 under microgravity shows that both the extinction equivalence ratio and flame temperature of the counter-flow flame are lower for the syngas with a higher H2 content. At high inert dilutions, the preferential diffusion effect of H2 is weakened by overwhelmingly large quantity of inert gases. The linear fit 3449

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invalid because the limiting flame temperature increases sharply. Detailed analysis reveals that the sharp increase in limiting flame temperature at high inert dilution is due to both the thermal and kinetics effects of inert diluents, which significantly affects the fuel reactivity. The preferential diffusion of H2 also plays an important role in determining the lean flammability limit and limiting flame temperature. The thermal and kinetics effects can be well captured by a flame temperature−chemical time correlation. This correlation can be used to predict the LFL of syngas at high inert dilutions. In comparison, the unburned gas temperature influences LFL mainly via thermal effect. The LFL of syngas mixtures decreases linearly with unburned gas temperature. The reason is that the limiting flame temperature remains constant upon preheating. In summary, the effect of inert dilution is more significant than unburned gas temperature on lean flammability limit of syngas mixture.

Figure 11. LFL of SYN6−SYN10 mixtures as a function of unburned gas temperature.

achieved at lower equivalence ratio compared to experiments conducted at room temperature. Wierzba et al.21 suggested that the flame temperature at ϕLFL is constant regardless of the preheating temperature for different syngas mixtures. To examine if this assumption is still valid at high inert dilution conditions, TLFL of syngas mixtures is plotted against unburned gas temperature and the results are presented in Figure 12. It can be seen that for a given

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-18601621259. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by NFSC (No. 51176095 and No. 51076081).

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REFERENCES

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