Article pubs.acs.org/EF
Laminar Burning Velocities of Dimethyl Carbonate with Air Maxim E. Bardin,†,* Evgenii V. Ivanov,† Elna J. K. Nilsson,‡ Vladimir A. Vinokurov,† and Alexander A. Konnov‡ †
Division of Physical and Colloid Chemistry, Department of Chemistry, Gubkin Russian State University of Oil and Gas, Moscow, Russia 119991 ‡ Division of Combustion Physics, Department of Physics, Lund University, 223 63 Lund, Sweden ABSTRACT: Laminar burning velocities of dimethyl carbonate (DMC) + air flames at initial gas mixture temperatures of 298, 318, 338, and 358 K are reported. Nonstretched flames were stabilized on a perforated plate burner at atmospheric pressure, and the laminar burning velocities were determined using the heat flux method. The overall accuracy of the burning velocities was evaluated to be typically better than ±1 cm/s. The effects of unburned mixture temperature on the laminar burning velocity of DMC were analyzed using the correlation SL = SL0 (Tu/Tu0)α. The present experimental results indicated that the power exponent α reaches a minimum in slightly rich mixtures corresponding to the maximum burning velocity. Modeling of these results has been attempted using the mechanism developed by Glaude et al. It was found that this model significantly overpredicts laminar burning velocities of methanol, ethanol, and DMC; however, it accurately reproduces the temperature power exponent α for dimethyl carbonate flames.
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INTRODUCTION
molecular wt %, its efficiency in soot suppression was found inferior to that of methanol. The model of Glaude et al.8 was developed by extension of a detailed kinetic mechanism for dimethoxy methane and dimethyl ether9 and validated using experimental species profiles obtained in opposed flow diffusion flames at atmospheric pressure.10 Glaude et al.8 concluded that their model is in reasonable agreement with these measurements. The model was later compared with species profiles obtained in premixed low-pressure (30 Torr) flames of heptane with addition of dimethoxy methane or DMC11 with overall satisfactory agreement. Many reaction rate constants in the DMC submechanism8 were estimated using analogy with other reactions or employing rate rules proposed in, for example, ref 7. Although some new theoretical12 and experimental13,14 results for pertinent rate constants appeared since then, further development and validation of the DMC combustion mechanism is hampered by the lack of accurate experimental data for combustion characteristics of DMC. For instance, such an important parameter of any combustible mixture as the laminar burning velocity has never been measured. The goals of the present work were therefore (a) to determine burning velocities of DMC in air at different initial temperatures at atmospheric pressure, using the heat flux method; (b) to analyze their temperature dependence, and (c) to compare these experiments with predictions of the only available DMC combustion mechanism.8
Most of renewable nonfossil fuels (except hydrogen and syngas) are oxygenated hydrocarbons. One of the important advantages of oxygenates is that when used as additives or alternative fuels under equivalent conditions they produce less soot as compared to hydrocarbons. Environmental concerns, however, may arise if oxygenated fuels or their intermediate products are emitted from engines due to incomplete combustion. This imposes additional constrains on the use of many oxygenates and motivates the search for alternatives. One of the attractive candidates to replace environmentally hostile antiknock compounds such as methyl-tert-butyl ether is dimethyl carbonate (DMC), C3H6O3,1 suitable as an additive because of its low toxicity and persistence. An additional benefit is that DMC can be produced from methanol and carbon dioxide1,2 providing a sink (at least partial) for the greenhouse gas, CO2. DMC is one of the widely adopted additives to gasoline and diesel1 and was tested in a wide range of concentrations from 1 to 20%,3 and 10−30%,4 up to 100%5 in diesel engines. These tests showed contradictory results in terms of NOx emissions: an increased proportion of DMC in diesel fuel was found leading to increased3,5 or decreased4 formation of NOx in exhaust. Yet, in all cases. reduction of smoke or soot particulates has been confirmed. The suppression of soot production in diesel engines by addition of oxygenates was explained by the reduction in concentration of soot precursors in fuel-rich combustion zones.6 Westbrook et al.6 used chemical kinetic modeling to demonstrate that the percentage of fuel carbon converted to soot precursors shows a close to linear decrease with the fraction (by mass) of oxygen in the fuel. A detailed kinetic mechanism employed by Westbrook et al. for combustion modeling of heptane + DMC mixtures was a combination of the model for heptane,7 a conventional surrogate for diesel, and the only existing model for DMC, by Glaude et al.8 Remarkably, although DMC has very high oxygen content, 53 © 2013 American Chemical Society
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EXPERIMENTAL DETAILS
Experiments in the present study have been performed using two setups. The first one is described in details in previous papers;15−17 the Received: June 14, 2013 Revised: August 2, 2013 Published: August 20, 2013 5513
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Figure 1. Schematic of the heat flux setup. r. The β coefficient is dependent on the unburned gas velocity, vu. By plotting β versus vu, the burning velocity can be found via interpolation at β equal to zero. The slope of this interpolation is defined as the sensitivity and indicates how the β coefficient varies with the unburned gas velocity. The new approach to the error estimation is to calculate the standard deviation of the linear fit coefficient (i.e., β) and to divide this with the sensitivity derived from the β versus flow velocity interpolation. This is the error in the burning velocity due to thermocouple scatter. The total error in the burning velocity is calculated adding the error due to thermocouple scatter and the previously mentioned error from uncertainties in gas flow. The error in concentration and equivalence ratio is solely based on uncertainties from the MFCs and calibration. Modeling Details. The Premix code from the Chemkin collection was used for the flame modeling. All default parameters of the Premix code were implemented with adaptive mesh parameter GRAD = 0.05. This value was implied to ensure accurate grid-independent solution with typical number of grid points around 400. The chemical kinetics mechanism by Glaude et al.8 includes 102 species and 442 reactions. According to the authors,8 this mechanism can treat dimethylcarbonate, dimethoxymethane, methylformate, methanol, ethanol, and dimethyl ether oxidation.
second setup is largely identical with some characteristics different, as described in the following. A schematic view of the second heat flux setup is shown in Figure 1. The heat flux burner consists of a plenum chamber and a burner plate separated by a ceramic ring for thermal insulation. They are kept at different temperatures using two separate water baths. One water bath heats the burner plate to 368 K, while the second one heats the plenum chamber and the unburned gas to the required initial temperature from 298 to maximum of 348 K. The experiments with the heat flux method are based on measuring the temperature distribution in the perforated burner plate while varying the flow rate of the fresh gas mixture. The distribution reflects the heat loss of the flame to the burner and the heat gain of the unburned gas as it passes through the burner plate. The temperature distribution is measured by thermocouples (type T) inserted into small holes in the perforated burner plate. The thermocouple readings are registered by a 16-channel thermocouple input module, National Instrument 9213. When the flame is stabilized at velocities lower than the laminar burning velocity, the burner plate will gain heat from the flame. If the flame is stabilized at a velocity higher than the laminar burning velocity, the burner plate will lose heat. By varying the gas flow until the heat loss and heat gain are equal, resulting in a uniform temperature distribution of the burner plate, the laminar burning velocity is determined. Detailed description of the principles behind the heat flux method is given by Bosschaart and de Goey.18 Mass flow controllers, MFC, (Bronkhorst High Tech) were used to control the flow rate and mixture compositions. Liquid fuel was evaporated using a controlled evaporator mixer, CEM (Bronkhorst). The capacity of the CEM employed in the second setup is 5 times larger than the one on the first setup. The fuel vapor was premixed with the air before entering the burner (see Figure 1). A heating tube between the CEM and the burner maintained the unburned mixture at the desired temperature to avoid condensation. The temperature of the heating tube was kept equal to that of the plenum chamber. Error Analysis. Previous work estimates the uncertainty of the burning velocity determined using the heat flux method to near ±0.5 cm/s at ϕ =1.0.18 This uncertainty is mainly based on errors due to uncertainties of the MFCs and thermocouple scatter. While the same type of error estimation is used for the MFC uncertainty,18 adding errors due to calibration, a different method has been used to estimate the errors related to the thermocouples in the present work. This method, described in the following, will represent the error in the burning velocity due to temperature scatter in a more rigorous way. The measured temperatures are fitted to a function T(r) = T(0)+βr2, where T(r) are the temperatures at different radial positions,
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RESULTS AND DISCUSSION As explained above, the measurements of the laminar burning velocities of DMC + air were conducted on two experimental setups. The reason for this was that the two setups have different capacities for fuel evaporation. In the absence of any literature data for the burning velocity of DMC, it was initially expected that the range of the Cori-flow mass flow controller (MFC) and CEM on the first setup is sufficient for covering the whole range of mixture composition, which then appeared not to be the case. In the following, both series of measurements are presented to illustrate consistency of the results. To ensure acceptable accuracy of the measurements the MFCs were calibrated and in addition cross-check experiments with wellcharacterized fuels were performed. This is a common procedure before investigating new fuels and is even more important when switching from experiments with one potentially corrosive fuel to another one. It was discussed in detail by Nilsson et al.17 how a chemically aggressive liquid can result in malfunction of the experimental system and thus error in experiments. In the present work, the first experimental rig 5514
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was tested using ethanol + air flames at an initial gas mixture temperature of 318 K, while the second one using methanol + air flames at 298 K. The elevated initial temperature for ethanol flames was chosen to avoid vaporization limitations at 298 K discussed elsewhere20 and thus to cover a complete range of equivalence ratios. These measurements are shown in Figure 2
compared to results from other methods at room and elevated temperatures for ethanol20,22,16,23 and methanol.16,24 The comparisons showed significant scattering (typically up to 4− 6 cm/s) and possible reasons for it were discussed in the above references. Linear correction for stretch implemented in the earlier studies employing spherical or counter-flow flames was identified as the main drawback of these methods. The heat flux method was found in a very good agreement (within 2 cm/s) with the recent data obtained using nonlinear stretch correction (e.g., by Veloo et al.).21 Also shown in Figures 2 and 3 are the modeling results that are discussed in the following. Laminar burning velocities of dimethyl carbonate + air flames have been measured at initial gas mixture temperatures of 298, 318, 338, and 358 K. The results are listed in Table 1 and shown in Figures 4 and 5 to avoid too busy graphs. The measurements performed on the different setups are shown with different symbols; the agreement is within evaluated experimental uncertainties confirming consistency of these results. The experiments at 298 K were restricted to lean mixtures due to vaporization limits of DMC; at higher temperatures, a full range of equivalence ratios from 0.7 to 1.6 was covered. As can be seen from comparison with Figures 2 and 3, DMC burns significantly slower than alcohols at the same initial temperatures. Figures 4 and 5 present also burning velocities of DMC + air calculated using the mechanism of Glaude et al.8 The modeling significantly (by 7−10 cm/s) overpredicts experimental results at all initial temperatures. To reveal the reasons for the model disagreement with the experiment a sensitivity analysis could be helpful. The sensitivity analysis indicates reactions affecting calculated burning velocity; these reactions can be re-evaluated and modified if the model performs correctly for other conditions or for other subhierarchical fuels. However, this is apparently not the case: the calculated burning velocities of methanol (Figure 3) and ethanol (Figure 2) also largely deviate from the experimental data. Most probably, the underlying problem is in incorrect description of combustion chemistry of common intermediates, such as CH3O radicals or formaldehyde. Recently, developed models available in the literature do not show the same discrepancy with the measurements: examples of the predictions using the Konnov mechanism20 or the scheme of Li et al.25 are shown in Figures 2 and 3, respectively. The temperature dependence of the laminar burning velocity is commonly described by the relation
Figure 2. Laminar burning velocities of C2H5OH + air at 318 K. Symbols, experimental data; lines, modeling; triangles, present work; circles, ref 16; squares, ref 20; solid line, model of Glaude et al.;8 dashed line, the Konnov mechanism.20
and compared with recent results from the same laboratory16 and with earlier data from a different laboratory20 using the same heat flux method. Three series of measurements for ethanol are in remarkable agreement especially in the nearstoichiometric mixtures. Larger deviations are observed in rich flames, which may be related to inaccuracies in the MFC calibration manifested in apparent “shift” of the SL dependencies along the x-axis. New measurements of the laminar burning velocity of methanol + air flames are compared with recent results from the same laboratory16 in Figure 3. That these earlier measurements for alcohols are accurately reproduced proves a good operation of the setups. The results obtained using the heat flux method have previously been
SL = SL0(T /T0)α
(1)
where T0 is the reference temperature and SL0 is the burning velocity at this temperature. The power exponent α is known to depend on equivalence ratio only (at a given pressure), which allows for independent cross-check of consistency of the results. Specifically, when plotted in a log−log scale the values of burning velocity obtained at different temperatures are expected to follow a straight line, which slope gives the power exponent α. Possible outliers can be clearly identified from such a plot and may indicate potential mistakes in experiments. Figure 6 shows these plots for selected equivalence ratios. The scattering of the experimental data points around best fit trend lines never exceeds the evaluated uncertainty and the reliability of the measurements is thus confirmed. Good linear fits indicate that the data were not corrupted by corrosion, fouling, or other malfunctioning of the equipment. This is an important conclusion in the present case
Figure 3. Laminar burning velocities of CH3OH + air at 298 K. Symbols, experimental data; lines, modeling; triangles, present work; circles, ref 16; solid line, model of Glaude et al.;8 dashed line, model of Li et al.25 5515
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Table 1. Laminar Burning Velocities of DMC + Air Flames with Associated Uncertaintiesa 298 K
a
318 K
eq. ratio
SL cm/s
±error
0.65 0.7 0.7* 0.8 0.8* 0.9 0.9* 1 1* 1.1 1.2 1.3 1.4 1.4* 1.5 1.5* 1.6*
13.05 16.45 14.32 22.43 20.89 26.34 25.09 28.96 28.07
0.35 0.42 0.83 0.42 0.97 0.46 0.77 0.47 0.74
338 K
358 K
SL cm/s
±error
SL cm/s
±error
SL cm/s
±error
18.00 16.78 24.45 23.31 29.14 28.43 32.15 31.38 32.98 31.87 29.19 24.51 24.43 19.99 19.64 15.69
0.32 0.63 0.39 0.62 0.40 0.57 0.44 0.61 0.45 0.60 0.57 0.99 0.89 1.28 1.64 2.00
20.25
0.34
23.25
0.44
27.30
0.55
30.20
0.49
32.15
0.47
35.76
0.57
35.13
0.49
39.01
0.56
36.17 35.13 31.95 27.24
0.48 0.49 0.54 0.59
39.87 38.67 35.64 29.75
0.56 0.59 0.63 0.65
21.30
0.67
24.80
0.99
Asterisk denotes measurements performed on the first setup.
Figure 4. Laminar burning velocities of DMC + air at 298 K and 338 K. Symbols, experimental data; lines, modeling; circles, results from the first setup at 298 K; diamonds, results from the second setup at 298 K; triangles, results from the second setup at 338 K; solid line, modeling at 338 K; dashed line, modeling at 298 K.
Figure 5. Laminar burning velocities of DMC + air at 318 K and 358 K. Symbols, experimental data; lines, modeling; circles, results from the first setup at 318 K; diamonds, results from the second setup at 318 K; triangles, results from the second setup at 358 K; solid line, modeling at 358 K; dashed line, modeling at 318 K.
where the results were obtained sequentially on two setups and at different temperatures over extended period of time. Experimentally derived power exponents α at different equivalence ratios are shown in Figure 7. The error bars on the experimental points in Figure 7 include the uncertainties in the individual burning velocity data points, presented in Table 1, and the uncertainty in the fit using eq 1. The uncertainties of α progressively increase from lean toward rich mixtures due to somewhat higher uncertainty in the burning velocities of rich mixtures and due to reduced temperature range, as the measurements at 298 K were not fully accessible. The present experimental results indicated that the power exponent α reaches a minimum in slightly rich mixtures corresponding to the maximum burning velocity, which is further substantiated by the modeling. The occurrence of this type of minimum has recently been validated for methane,19 ethanol,20 and acetone.17 Also shown in Figure 7 are the power exponents α derived from the modeling. Even though the laminar burning velocities
of DMC + air are significantly overpredicted (see Figures 4 and 5), the agreement with the modeling in Figure 7 is excellent and the trend is clear; the power exponent goes through a minimum for slightly rich mixtures. A similar trend was observed in ethanol flames,20 although not to the same extent: while models notably overpredicted ethanol laminar burning velocities in lean and near-stoichiometric mixtures (see Figure 2), the agreement with the power exponents was somewhat better in these flames as compared to rich mixtures. One may conclude, therefore, that the comparison of the experimental and calculated power exponent coefficients provides an independent tool (target) for model validation and development.
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CONCLUSIONS Laminar burning velocities of dimethyl carbonate + air flames at initial gas mixture temperatures of 298, 318, 338, and 358 K have been measured for the first time, using the heat flux method. Special care has been taken to ensure proper operation 5516
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support of the Ministry for Science and High Education of Russian Federation by the Federal Target Programme “Human Potential” 2009-2013 Project Grant No. 14.B37.21.1959 is gratefully acknowledged.
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Figure 6. Log−log plot of selected laminar burning velocities of DMC + air flames at atmospheric pressure and different initial temperatures. Equivalence ratio: 0.7 (open diamonds); 0.8 (solid diamonds); 0.9 (solid triangles); 1.0 (crosses); 1.1 (solid squares); 1.2 (solid circles); 1.3 (open squares); 1.4 (open circles); 1.5 (open triangles).
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Figure 7. Correlation coefficient α as a function of equivalence ratio. Diamonds, experimental data; line, modeling.
of the experimental setups, which was controlled by measuring burning velocities of two well-characterized alcohols: methanol and ethanol. Moreover, the measurements performed at two setups were demonstrated to agree within the evaluated uncertainties. The overall accuracy of the burning velocities was evaluated to be typically better than ±1 cm/s. The effects of unburned mixture temperature on the laminar burning velocity of DMC were analyzed using the correlation SL = SL0(Tu/Tu0)α. The present experimental results indicated that the power exponent α reaches a minimum in slightly rich mixtures corresponding to the maximum burning velocity. Modeling of these results has been attempted using the mechanism developed by Glaude et al.8 It was found that this model significantly overpredicts laminar burning velocities of methanol, ethanol, and DMC; however, it accurately reproduces the temperature power exponent α for dimethyl carbonate flames. It can be pointed out that since the development of the base mechanism9 understanding of reaction rates of many of the intermediate and reactive species of importance in the model have improved significantly. It is likely that an update of reaction rate constants for the fundamental reactions governing laminar burning velocities will significantly improve the performance of the model. 5517
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