A Reduced Kinetic Mechanism for the Combustion of n-Butanol

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A reduced kinetic mechanism for the combustion of n-butanol Mario Díaz-González, César Treviño, and Juan C. Prince Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03011 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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A reduced kinetic mechanism for the combustion of n-butanol Mario Díaz-Gonzáleza Cesar Treviñob Juan C. Princec, * a

Facultad de Ingeniería, Universidad Nacional Autónoma de México, Ciudad de México, México

b

UMDI Sisal, Facultad de Ciencias, Universidad Nacional Autónoma de México. Sisal, Yucatán México

c

Departamento de Metal-Mecánica, Instituto Tecnológico de Veracruz, Veracruz 91860, México

Keywords: n-Butanol; Chemical mechanism; Modeling; Low-temperature ignition; Flame.

* Corresponding author

1

E-mail address: [email protected]

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Abstract

A reduced chemical mechanism for modeling the combustion of n-butanol in air, including low temperature ignition phenomena, was obtained in the present work. To this end, only fourteen chemical-kinetic reactions and six chemical species were included to a short base mechanism (the so-called San Diego mechanism). Two important features of the reaction of the hydroxybutyl radical C4H8OH-1 radical with molecular oxygen involve a path competition of low and high-temperature reactions, and low total available heat release not promoting NTC behavior. Validation of the chemical mechanism shows an excellent agreement against experimental data of laminar flame velocities, ignition delay times and jet stirred reactors. These numerical results confirm that this reduced mechanism can be used instead of larger mechanisms, particularly when computing-time is an important fact to be considered for modeling practical combustion systems.


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1. Introduction

Currently, biofuels are having great attention as a source of energy for transportation, since they are renewable, can be produced locally and generate fewer pollutants; they are also biodegradable and can reduce greenhouse gases (1-2). n-Butanol has several advantages over ethanol, including similar energy properties to gasoline, that it can be used in modern engines with minor or no modifications (3-4). Also, because of its relatively greater resistance to water absorption and lower volatility, transport and storage, n-butanol can be easily handled by existing infrastructure. The traditional bio-butanol isomer is n-butanol, but new chemical technologies ensure that iso-butanol and 2-butanol are produced as fuels, even identifying the biological pathways for alcohols of higher molecular weights (5). Several chemical mechanisms (6-22) of butanol isomers have been developed to modeling different combustion problems, including jet-stirred reactors (6, 8-9), laminar flame speeds (13, 23-27), ignition delay times (7, 15-18, 28-31) and, ignition and flames at high-pressure conditions (21-22). Moreover, some studies on combustion in engines using only n-butanol or as a blending with diesel or gasoline (32-37), used detailed chemical mechanisms to improve predictions of combustion performance and n-butanol pollutant gases on combustion engine experiments.


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For the development of chemical mechanisms of any fuel two very different points of view can be considered. One of them is to cover all known chemical reactions, resulting in detailed chemical models. The other is to obtain reduced mechanisms considering only the main chemical reactions that influence the combustion phenomena. As many detailed chemical mechanisms of the literature are complex to modeling practical combustion processes, there is a need to obtain short mechanisms without detriment to the predictions of the wide range of experimental data. The present work has the goal of obtaining a reduced chemical mechanism for n-butanol that can be reliably used in combustion problems of interest, without the expensive time-consuming of larger detailed mechanism. Low temperature chemistry is included for covering the wider set of experiments obtained for the ignition phenomena. A sensitivity analysis is presented, to testing the influence of each elementary reaction to ignition delay time. For the sake of simplicity of the present n-butanol mechanism, pollutant emission derived from the combustion of this alcohol is not included.

2. The reaction mechanism of n-butanol

The reduced San Diego mechanism was chosen as the base mechanism (38); its full C4 scheme comprises 265 reactions with 56 chemical species. Its validation tests include laminar burning velocities and ignition times for equivalence ratios about 0.5 < φ < 3, and pressures below around 100 atm. With the addition of the n-butanol reaction scheme and the corresponding


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chemical species based mainly on (18), the original system of thousands of reactions and hundreds of species is reduced to a chemical model of 279 reactions and 62 species. Diverse detailed chemical mechanisms (6, 10-13, 16, 18), with specific selections stated below, were considered for constructing the reduced n-butanol sub-mechanism. Wherever possible, to keep consistency, the reaction rates, transport and thermodynamic data, the present study was based on a recent and extensively used detailed chemical mechanism (18). By using sensitivity analysis, lumping procedures and steady state approximations, the reduced mechanism contains the main reactions that influence the predictions of the combustion processes of interest, such as ignition times, laminar flame speed and concentration measurements in jet-stirred reactors. When analyzing each hydroxybutyl radical (C4H8OH-1, C4H8OH-2, C4H8OH-3, C4H8OH-4), as a reduction process of the kinetic mechanism, radical C4H8OH-1 was the most appropriate pathway to obtain a reduced kinetic mechanism for nbutanol; according to (18), this radical is predominant during H-abstraction reactions. Therefore, for the chemical scheme shown below, the reaction pathway for the entire temperature regime is undertaken through C4H8OH-1. The proposed kinetic mechanism for n-butanol, added to the base mechanism, is shown in Table 1, with 14 elementary reactions and 6 chemical species (NC4H9OH,

C4H8OH-1,

C4H8OH-1O2,

C4H7OH-1OOH-4,

C4H7OH-1OOH-4O2,

and

C4OHKET1-4). The mechanism in Table 1 can be summarized as follows: Reactions R1-R6 are needed to appropriately characterize the initial n-butanol breakdown for the extensive combustion phenomena of interest. The α-hydroxybutyl (C4H8OH-1) radical can break down to produce acetaldehyde and an ethyl radical (reaction R7), or it may react with oxygen, whether to * Corresponding author

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generate a hydroperoxyl, ethane and ketene (reaction R8) or experiencing molecular oxygen addition to form C4H8OH-1O2 (reaction R9), starting the chemistry for the low- temperature combustion (LTC), reactions R10-R14. Equivalent to the LTC of normal alkanes (38), the lowtemperature chemistry is initiated by a carbonyl that, for the case of the present research on nbutanol, improves the computations for low-temperature ignition, although the NTC behavior of this alcohol is not well established (31). Moreover, the uni-molecular decomposition shown in reaction R1 plays an important role for high-temperature combustion phenomena: laminar burning velocity and ignition at T > 1000 K. Reaction 2 is the basic oxidation reaction for any fuel, whereas the H-atom abstractions through O, OH, HO2 and H attacks (reactions R3-R6), for the formation of the radical 1-hydroxybutyl (C4H8OH-1), are essential for laminar flames, flow reactors and ignition delays. It is important to note that HO2 abstraction increases the system reactivity at temperatures above 700 K, improving the ignition delay time predictions. Also, for better agreements of ignition times at both low and high-temperatures, the decomposition of the radical α-hydroxybutyl (R7) must be included. Reaction R8 corresponds to the reaction pathway of O2 with the α-hydroxybutyl radical to directly yield butanal + HO2. For the α-hydroxybutyl, this is the most favored pathway to produce this aldehyde and the hydroperoxyl radical; according to (18), the low-temperature behavior of n-butanol is mainly inhibited by this reaction path, and therefore considers it as a kind of reaction at high temperature for all alcohols. By reason of the reduced mechanism, steady-state approximation is applied for the uni-molecular decomposition of butanal to form CH2CO + C2H6 (13), that appears in reaction R8. The rate parameters for the initial O2 addition * Corresponding author 6 E-mail address: [email protected]


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to form 1-RO2, reaction R9, is commonly used for the chemical mechanisms reported in the literature for the same type of O2 addition reaction. Welz et al. (20), experimentally demonstrate that the most stable products of the βa channel for the decomposition of the β-RO2 reaction are propanal, and formaldehyde via the Waddington mechanism. Considering that the αhydroxybutyl radical has the same αb/βa channel as the β-hydroxybutyl radical (not included in the present mechanism that would yield β-RO2), the formation of propanal and formaldehyde is proposed (reaction R10) as the only channel of elimination of α-RO2; the resulting rate lie between those of (16) and (18). In view of the simplified mechanism herein proposed, a lumping procedure is used for replacing the propanal by its corresponding decomposition species (CO + C2H6) without detriment of the numerical predictions. The hydroperoxyalkyl radical C4H8OH1O2 is isomerized to form the specie C4H7OH-1OOH-4 (QOOH), reaction R11, essential for the low-temperature regime, that directly competes with reaction R10. The second addition of molecular oxygen is given by reaction R12, forming the peroxy-hydroperoxy-alkyl radicals C4H7OH-1OOH-4O2. Reaction R13 corresponds to the chain branching, while the decomposition of the radical C4OHKET1-4 is shown in reaction R14.


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Table 1. Reduced set of chemical reactions for n-butanol added to the short San Diego mechanism. Nº R1

Reaction

A

N

E

Ref.

NC4H9OH(+M) ↔ NC3H7 + CH2OH(+M)

3.020E+23

-1.880

85710

18

Low pressure limit

1.416E+59

-11.93

83980

Troe: 7.6460E-01 8.3440E+09 7.2480E+02 8.2140E+09 R2

NC4H9OH+O2 ↔ C4H8OH-1+HO2

2.000E+13

0.000

46800

18

R3

NC4H9OH+HO2 ↔ C4H8OH-1+H2O2

6.000E+12

0.000

16000

12

R4

NC4H9OH+O ↔ C4H8OH-1+OH

1.450E+05

2.470

876

18

R5

NC4H9OH+OH ↔ C4H8OH-1+H2O

5.560E+10

0.500

-380

12

R6

NC4H9OH+H ↔ C4H8OH-1+H2

1.790E+05

2.530

3420

12

R7

C4H8OH-1 ↔ CH3CHO+C2H5

3.000E+11

0.000

31000

13

R8

C4H8OH-1+O2 ↔ CH2CO+C2H6+HO2

4.400E+11

0.000

5000

12

R9

C4H8OH-1+O2 ↔ C4H8OH-1O2

1.000E+12

0.000

0.000

18

R10 C4H8OH-1O2 ↔ CO+C2H6+CH2O+OH

2.500E+11

0.000

25000

18

R11 C4H8OH-1O2 ↔ C4H7OH-1OOH-4

4.688E+09

0.000

21950

18


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R12 C4H7OH-1OOH-4+O2 ↔ C4H7OH-1OOH-4O2

4.520E+12

0.000

0.000

18

R13 C4H7OH-1OOH-4O2 ↔ C4OHKET1-4+OH

1.560E+09

0.000

13650

18

R14 C4OHKET1-4 ↔ C2H4+H+CO2+CH2O+OH

1.000E+16

0.000

39000

18

With corresponding units, A [mol-cm-s-K], E [cal/mol].

Figure 1. C-H bond energies for butanol isomers in kcal (17).

3. Validation tests

For testing the reduced kinetic mechanism, the perfectly stirred reactor (PSR) and the isochoric homogeneous reactor models, from the computer program FlameMaster (41), were used to modeling the experimental data for the laminar burning velocity, ignition delay time and jet stirred reactor measurements, respectively. * Corresponding author

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Comparisons of predicted laminar flame velocities against measured data is a conventional test for any chemical mechanism to prove its applicability for the extensive range of practical combustion problems. The experimental data shown for validation, were reported in (23, 25, 27), and the laminar burning velocity predictions were carried out with the code FlameMaster (41), without thermal diffusion and using 240 grid-points. The experiments shown in Fig. 2, for laminar flame speed testing, were performed at different pressures and mixture temperatures of 343, 353, and 373 K. The laminar burning velocities are well predicted by the reduced mechanism, for the different equivalence ratio values, within the uncertainties of the experimental measurements. Also, the model captures the pressure and mixture temperature dependences of the laminar flame velocity. Thus, the reduced n-butanol mechanism developed herein exhibits agreements with burning velocities experiments, that are comparable with other published mechanisms (13, 18, 23).


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Figure 2. Laminar flame speeds of n-butanol/air mixtures at different pressures.

Figure 3(a, b) shows the predictions of the proposed kinetic mechanism for the shock tube ignition time data at initial temperatures above 1000 K at different pressures, of a stoichiometric mixture of n-butanol/oxygen in Ar (7, 30). At low-pressure of p = 1.13 bar and 1.54 atm, Fig. 3a shows very good agreement to the experimental measurements of around 1.13 bar (0.98-1.3 bar) (7) as well as for the experiments of approximately 1.54 atm (1.36-1.73 atm) (30), respectively. The comparison of the modeling results for p = 42 atm to the experimental data nearby 42 atm (39-46 atm) (30), plotted in Fig. 3b, shows excellent agreement for these high-pressure experiments. The validations were achieved employing the FlameMaster code (41), for a homogeneous isochoric reactor system. The criterion used for determining the ignition delay * Corresponding author

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time corresponds to the instant of the maximum elevation of the temperature (max dT/dt). As can be observed, the proposed kinetic model predicts with excellent agreement both low and highpressure experiments. Figure 4 shows the predictions of the proposed kinetic model, with the experimental data of the ignition delay of the shock tube, in the temperature range of 770 K to 1250 K of n-butanol in air, φ = 1.0, for pressures between 10-80 atm (16, 29, 31). The numerical simulations were carried in a homogeneous isochoric reactor system, using the FlameMaster code (41). The proposed model predicts well the experimental data, at all pressures and over the temperature range of 870-1250 K. The pressure dependence on the ignition time decrease, experimentally observed, is also well predicted by the chemical model.

Figure 3. High-temperature ignition delay time at low and high-pressure.


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Figure 4. Ignition delay time of n-butanol in air, φ = 1.0 at different pressures.

To validate the proposed kinetic mechanism in relation to low-temperature reactivity, the predictions are compared with the experimental results obtained under the well-controlled conditions of the rapid compression machine (RCM). Figures 5a and 5b show the predicted dependence of the ignition time against experimental data reported by Vranckx et al. (16), Moss et al. (7) and Heufer et al. (29), for a stoichiometric mixture of n-butanol and air at different pressures. The results of the reduced kinetic mechanism for ignition times for stoichiometric mixtures (φ = 1) at pressures of 20, 40 and 80 bars, at initial temperatures below 1000 K, agree well with the experiments for low temperature ignition. Figs. 5c and 5d show the ignition delay time comparisons for 15 and 20 atm. pressures, respectively, against the experimental data * Corresponding author

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reported by Weber et al. (15) and the recently CRV (constrained-reaction-volume) method for ignition data reported by Zhu et al. (31). Three different mixture fractions are considered. The agreements of the values and tendencies are rather good.

Figure 5. Ignition delay time at the equivalence ratios φ of 0.5, 1.0, and 2.0, different pressures.

The jet-stirred reactor (JSR) is essential for studying low and high temperature fuel reactivity. The proposed kinetic mechanism was validated against JSR experimental data by Sarathy et al. * Corresponding author

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(10) and Dagaut et al. (9) for 1.0 and 10 atm, respectively. The simulations were performed with the PSR model of FlameMaster (41). Fig. 6 shows the experimental (10) and modeling results for a 1 atm. JSR with an initial molar fraction of n-butanol of 0.1%, for different equivalence ratios with N2 as bath gas, with a residence time of 0.07 s. The agreements are good in particular for the stoichiometric mixture.

Figure 6. Concentration of n-butanol at 1 atm (10), as a function of temperature and equivalence ratio in JSR.


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For 10 atm JSR case (Fig. 7), only the stoichiometric mixture was used for the same initial mixture and residence time. Several species concentrations were measured and modeled. The predictions of the model in comparison to the experimental data are in rather good agreement. The fuel concentration is well reproduced for the entire temperature region. The tendencies of major species concentrations with temperature are well reproduced and the values exhibit discrepancies which are consistent with the experimental uncertainties.

Figure 7. Products of n-butanol oxidation in a jet-stirred reactor at 10 atm (9).

4. Heat release at low temperatures

One of the key features in the low temperature chemistry is the overall heat release, produced mainly by the low temperature kinetic mechanism showed in Table 1. There is a limited thermal * Corresponding author

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energy source available in the low temperature regime. Fig. 8 shows the thermal energy release rate of the important reactions as a function of the temperature, for a stoichiometric mixture of nbutanol for different initial temperatures and a pressure of 20 bar. Negative values represent exothermal reactions. In all plots, the principal heat source is due to overall reaction R9 (first addition of oxygen). As reaction proceeds, the heat release first increases and then decreases, reaching a minimum after a limited temperature rise. A further heat release is mainly achieved by the high temperature chemistry (HO2 → H2O2 → OH). There are other minor contributions to the heat release. The exothermic reaction R5 almost cancels the endothermic reaction R14. The total available heat release, defined as the temperature difference between that obtained in the point of minimum energy release and the initial temperature, Tf –T0, is plotted in Fig. 9, as a function of the initial temperature T0. This figure shows that the available heat release is almost uniform, around 50 K, decreasing slightly with temperature. As shown in Fig. 9, the n-butanol kinetics is unable to rise the mixture temperature to high values to switch rapidly with the high temperature kinetics. The contrary occurs for example with the n-butane for relatively high-pressure (38) (shown also in the same figure), where the heat release is enough to increase the mixture temperature close to 900 K, even at very low initial temperatures (NTC behavior). Therefore, nbutanol is unable to exhibit NTC behavior.


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Figure 8. Heat release rates at low-temperature for different initial temperatures and a pressure of 20 bar, for a stoichiometric mixture of n-butanol and air.


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Figure 9. Available heat release as a function of the initial temperature for butane, isobutane and n-butanol.

5. Sensitivity analysis

The objective of the sensitivity analysis is to evaluate, as a function of temperature, the contribution of the kinetics of each reaction to the ignition time. The sensitivity coefficient , for reaction is defined by

S =

=

,

(1).

where is the pre-exponential factor of reaction and τ0 is the ignition delay time. Here the super-index 0 indicates the correct values. Each time, the pre-exponential factor of each reaction is multiplied by a factor of two and the resulting ignition delay time τ0 is computed. A negative sensitivity coefficient (τ < τ0), indicates high reactivity, while a low reactivity is indicated by a * Corresponding author 19 E-mail address: [email protected]


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positive value. Sensitivity analyses were performed at a pressure of 20 bar at different initial temperatures. Figure 10a shows the sensitivity coefficient for a stoichiometric mixture of nbutanol and air at 800 K. The most active reaction pathway is given by the direct competition between reactions R11 and R10. Reaction R11 leads to the low temperature chain branching process, competing with direct removal of the radical C4H8OH-1O2 by reaction R10. The fuel is consumed mainly by reactions R5 and R3, with radicals OH and HO2, respectively. The relatively low sensitivity of heat production reaction R9 is because it is not a rate limiting reaction (radical C4H8OH-1 can be assumed to be in steady state). The reactions typical of the high-temperature regime are present in Fig. 10a, showing that despite an initial push from the low-temperature kinetics, it is the high-temperature kinetics that leads finally to ignition in a relatively long-time delay. Figure 10b shows the sensitivity coefficient for the same stoichiometric mixture of n-butanol in air at 875 K. The main difference with previous graph is the main fuel attack with hydroperoxy radical HO2 instead of hydroxyl radical OH, showing the relative importance of the high temperature kinetics. Basically, the same trend the most reactive pathways are given by the attack of the fuel, to obtain the radical 1-hydroxybutyl, i.e., NC4H9OH+HO2↔C4H8OH-1+H2O2, followed by the ROO↔QOOH isomerization step; both reactions show small differences between the reactivity of low and high temperature processes, that is, they generate a competition with a slighter higher sensitivity of the H abstraction reaction. Figure 10c shows the sensitivity coefficient for a stoichiometric mixture of n-butanol and air at 950 K. The reaction with the highest negative sensitivity coefficient is given by the attack of the radical HO2 to the fuel, followed by the ROO↔QOOH isomerization reaction, and * Corresponding author

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starting the predominance of the decomposition of H2O2, characteristic of high-temperature chemistry T > 900 K.

Figure 10. Sensitivity coefficients for different initial temperatures on the 1-butanol ignition time for a stoichiometric mixture.


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6. Conclusions In this work, few chemical kinetic steps were added to the San Diego mech to simulate different combustion processes of n-butanol (1-C4H9OH) mixtures. The present study shows that, n-butanol combustion processes can be modeled retaining the current reactions and reaction rates of the base mechanism. Recent detailed chemical mechanisms were considered, particularly in the low-temperature combustion regime for improving predictions for the applications of interests. To obtain a usable short mechanism, two of the reactions, R8 and R10, represent overall effects thorough steady-state approximation. The selected chemical reactions and their corresponding rate parameters taken from the literature without revision, except for two cases, where chemical species were eliminated using steady-state approximations, without detriment of the numerical results. The applicability of the mechanism covers high and low-temperature ignition delay times at low and high-pressures and laminar flame velocity predictions. The results describe correctly the premixed laminar flame velocity of n-butanol in air at different pressures and ignition at different equivalence ratios and pressures, including low-temperature ignition. The results of the simulation were compared to the several experimental studies, showing excellent agreement. The low temperature chemistry, although not enough to promote the ignition (due to low total available heat release), contributes to reduce the ignition delay time, as shown in the semi log plot Fig. 5, where the slope of the ignition time changes in the low temperature region.


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Acknowledgment MDG and JCP thanks mainly to Consejo Nacional de Ciencia y Tecnología (CONACyT), and Tecnológico Nacional de México (TNM), both from the Mexican government, for the support to carry out this work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version.

References

(1). Demirbas, Ayhan. Progress and recent trends in biofuels. Prog. Energy Combust. Sci., 2007, 33, 1-18. (2). Jones, David T.; Woods, David R. Acetone-butanol fermentation revisited. Microbiol. Rev., 1986, 50, 484. (3).

DuPont

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(2006),

available

at

http://www2.dupont.com/Biofuels/en_US/

facts/BiobutanolFactsheet.html.  


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A Reduced Kinetic Mechanism for the Combustion of n-Butanol

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