Combustion Mechanisms and Kinetics of Fuel Additives: A ReaxFF

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Combustion

Combustion Mechanisms and Kinetics of Fuel Additive: A ReaxFF Molecular Simulation Zhuojun Chen, Weizhen Sun, and Ling Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02035 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Combustion Mechanisms and Kinetics of Fuel Additive: A ReaxFF Molecular Simulation Zhuojun Chena, Weizhen Suna,*, Ling Zhaoa,b

a

State Key Laboratory of Chemical Engineering, East China University of Science

and Technology, Shanghai 200237, China b

School of Chemistry & Chemical Engineering, XinJiang University, Urumqi 830046,

China ABSTRACT: Fuel additives were widely used as the octane number improvers, oxygenates, emission depressors, corrosion inhibitors to promote combustion processes of liquid fuel. In this work, six kinds of fuel additives including ethanol, butanol, DMC, DBC, MTBE, and TTAG were studied by ReaxFF MD simulations. The bond dissociation reactions were found to be more dominant at early stage than oxidation reactions, which means the unimolecular reactions was the main pathways of primary reactions in hydrocarbons combustion. The rate constants of primary reactions of ethanol combustion were much smaller than other systems, which were in good agreement with the product distribution analysis and previous work. The main reaction pathway and relative rate constants for all systems were evaluated. Four kinds of main radicals including •CH3, ••CH2, •OH, and •HO2 were detected and the number variation with time were presented. The number of •OH radical was the largest among those four radicals and it was found to gradually increase with time except for ether systems, and the number of •CH3 and ••CH2 sharply increased first and then gradually decreased.

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Hopefully, the results obtained in this work would be helpful to future design and screening of new fuel additives. Keywords: fuel additive, diesel combustion, ReaxFF molecular simulation 1. INTRODUCTION Combustion reaction is one of the earliest reactions that human could utilize. Through several hundred years’ development, increasing the combustion efficiency and reducing the air pollution have become new challenges, and therefore technologies of fuel-processing processes have been promoting. Pulverization technology of coal could obviously increase the combustion efficiency, which has been widely used in thermal power stations, cement and iron melting industry1. Gasification is a means to convert fossil fuels, biomass and wastes into either a combustible gas or a synthesis gas for subsequent utilization2. No doubt the combustion of fossil fuel is and will still be the main source of energy for human beings within a quite long period of time. Among all of the fossil fuels, liquid fuel plays a vital role in modern life, especially in vehicles, aerospace, ships and power engineering. In addition to the atomization technology that assists for completion of burning3, 4, fuel additives were also widely concerned in practical applications and academic researches. As the method of promoting liquid fuel combustion processes, fuel additives were utilized as the octane number improvers, detergents, oxygenates, emission depressors, corrosion inhibitors, and dyes. Considerable kinds of fuel additives have been studied including magnetic nano-fluid5, nano-organic compounds6, polymers7, nanoparticle8-10 and so


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on. In these additives, hydrocarbon additives are generally used because of their low cost, comparatively good miscibility, high oxygen content, and they are free of other chemical elements. Ethanol, butanol, DMC, DBC, MTBE, and TTAG have attracted much attention among hydrocarbon additives. Ethanol11, 12 has been proved to reduce particulate matter and CO emissions, but present an obvious ignition delay. Butanol has been proved to increase the brake thermal efficiency and decrease smoke emissions13, and the formation of carbonyl compounds were inhibited14, 15. Dimethyl carbonate (DMC)16 and dibutyl carbonate (DBC)17 are considered options for meeting the oxygenate specification on gasoline. It was found that hydrocarbon emission, CO and particle matter were reduced when DMC/DBC was added. Methyl tert-butyl ether (MTBE) had been added as an oxygenated additive, not only to enhance the octane number but also to reduce exhaust emissions17. During the last decade, glycerol got a sharp rise in the market for the increasing manufacture of biodiesel. The overproduction leaded to large stocks accumulated and resulted in a considerable decrease of its market value. Because of the low energy density and high viscosity, glycerol is not the ideal biofuel. However, glycerol can be modified to biodiesel additive by etherification with light olefins, such as triether TTAG (shown in Figure 1) synthesized by Felipe Izquierdo18. According to the above researches, it can be found that though numerous efforts have been paid, the understanding of fuel additives are still remains on macro scale, such as particle emissions, CO emissions, and so on. The mechanism and pathway of fuel additives were the intrinsic reason of their characteristics, on which people should pay more attentions, but fewer papers have


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been reported. The mechanism and pathway of combustion processes are extremely complex, which contain considerable intermediates and highly active radicals, and hence bring huge difficulties for experimental researches. Computational methods such as numeric analysis and engineering simulations19-25 were also used in fuel additive studies, but these methods mainly based on pre-given reactions, and thus bring uncertain artificial factors in. Reactive molecular dynamic (ReaxFF-MD) simulation is a new computational method to investigate the reactions and intermediates under severe conditions such as high temperatures and high pressures26, 27. ReaxFF method can calculate larger system with relatively high accuracy, and hence has been treated as important supplementary method in researches of pyrolysis28-30, combustion26, 31-34, and other fields35-37.

Figure 1. Structural formula of TTAG.

In this work, the mechanisms and pathways of the combustion of fuel additives (ethanol, iso-butanol, DMC, DBC, MTBE, TTAG) were investigated through Reax-MD simulations. Characteristics of different fuel additives were compared and demonstrated. A multi-component diesel model was constructed, in which the hexadecane represents paraffin, cyclopentane represents cycloparaffin, benzene represents monocyclic aromatics, and naphthalene represents bicyclic aromatics. The combustion simulations of diesel-fuel additives blends were conducted to further


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validate the influence of fuel additives with different functional groups.

2. SIMULATION DETAILS Table 1 Detailed composition of diesel model. Name

Hexadecane

Cyclopentane

Benzene

Naphthalene

Formula

C16H34

C5H10

C 6 H6

C10H8

Classification

Paraffins

Cycloparaffins

Monocyclic

Bicyclic

aromatics

aromatics

Table 2 Detailed composition of combustion model. System

Composition (molar ratio)

Ethanol

Diesel model + 57 ethanol molecules +441 O2 molecules (1:2.85:22)

2-butanol

Diesel model + 35 2-butanol molecules +480 O2 molecules (1:1.75:24)

DMC

Diesel model + 29 DMC molecules +357 O2 molecules (1:1.45:17.85)

DBC

Diesel model + 15 DBC molecules +450 O2 molecules (1:0.75:22.5)

MTBE

Diesel model + 30 MTBE molecules +495 O2 molecules (1:1.5:24.75)

TTAG

Diesel model + 10 TTAG molecules +540 O2 molecules (1:0.5:27)

The detailed composition of combustion model was shown in Table 1. A four-component diesel model was constructed, including 6 hexadecane molecules, 6 cyclopentane molecules, 4 benzene molecules, and 4 naphthalene molecules, and they represent paraffins, cycloparaffins, monocyclic aromatics, and bicyclic aromatics respectively (shown in Table 1). The molar contents of each components were determined by others’ work38,

39

. To study the effect of fuel additives, additive

molecules were randomly added into the simulation boxes which containing the diesel model. A rule was adopted in which the mass content of additives was the same as diesel model molecules, which could highlight the influence of additives within an acceptable computational cost and at the same time, keep the mechanisms of


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combustion reactions unchanged. Enough oxygen molecules were put in the simulation boxes to support complete combustion of these hydrocarbons. The detailed composition of each system is shown in Table 2 and the snapshot of the simulation boxes are shown in Figure 2.

a).

c).

e).

b).

d).

f).

Figure 2. Molecule structures of the fuel/additive models, a). diesel/ethanol, b). diesel/2-butanol, c). diesel/DMC, d). diesel/DBC, e). diesel/MTBE, f). diesel/TTAG.

Mechanisms and pathways of combustion of additives were first investigated through ReaxFF module of ADF package40-43, and then LAMMPS package44 was utilized to obtain the product/intermediate distribution. The simulation steps are as follows. Firstly, periodic boundary simulation boxes were constructed with the initial density of 0.2 g/cm3 according to the composition shown in Table 2. The small density is adopted to avoid the overlapping of atoms. Secondly, the systems were equilibrated at 100 K firstly with NVT ensemble, and then heated to 3000 K (the temperature was determined by our previous trials28, 29) with the heating rate of 50


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K/ps. Higher temperature was adopted to facilitate the collision between species, for temperature can only change the rate of reactions rather than changing the mechanisms. Thirdly, the systems were maintained at 3000 K for 2 ns. The Nose-Hoover thermostat was applied to manage temperatures in the ReaxFF molecular simulations. The time step of 0.1 fs was used and thus could give more trajectory information35, 45. A cutoff 0.3 was adopted for bond order judgment and the temperature damping constant was 100 fs. All the simulations were conducted using CHO-2016 parameter, which was developed and validated by Ashraf et al26. Through analyzing the output file of ADF, the relative rate constant can be obtained. The bond dissociation energy was calculated by QM method at the B3LYP/6-311G++(d,p) level of theory46, 47 of Gaussian 09.

3. RESULTS AND DISCUSSION 3.1. Initial combustion mechanisms of fuel additives In order to study how the fuel additives influence the combustion processes of diesel, mechanisms and pathways of combustion reactions of fuel additives were studied. Here all the mechanisms and pathways of six kinds of fuel additives were the results obtained from the statistical analysis of the trajectories. Extremely high temperatures were set to facilitate the simulation processes and hence the absolute rate constant was meaningless, therefore the absolute rate constants were normalized to give a clearer perspective (marked in k’). Through comparing the rate constant of main reactions in all systems, the slowest reaction was chosen as the reference reaction and its normalized value was set to 1 (shown in equation (1)).


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' =1 C 2 H 5 OH  k → •CH 2 CH 2 OH + • H

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(1)

Figure 3 displays the main initial pathways of ethanol molecules oxidation. There were several intermediates directly produced by ethanol through bond dissociation reaction and H-abstraction reaction, among which the C-C bond dissociation was the fastest process producing •CH3 and •CH2OH. The ethanol molecule can react with a hydroxyl radical and produce CH2O, H2O and •CH3. The H-abstraction reaction occurring on the hydroxyl of ethanol molecule would produce CH3CH2O• radical, which further produced •CH3 radical through C-C bond decomposition reaction. Besides, hydrogen radical could also be generated through H-abstraction reaction on the terminal carbon, which was the reference reaction. The rest part •CH2CH2OH would decompose into hydroxyl and ethylene molecule. Compared to primary reactions, the secondary reactions were faster.

Figure 3 Main initial reaction pathways of ethanol in additive-fuel combustion simulations at 3000 K. (Black-unimolecular reaction, blue-bimolecular reaction, red-reference reaction. If the intermediate can be produced through more than one pathway, the largest rate constant was displayed.)

Figure 4 shows the main initial pathways of 2-butanol combustion. Different from the initial pathways of ethanol, there were more intermediates directly formed by 2-butanol including alkyl radicals, enol, hydroxyl, hydrogen radical et al., and the ACS Paragon Plus Environment

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rate constants were much larger than those of ethanol systems. Dehydrogenation, demethylation, de-ethylation, dehydroxylation were the main pathways of initial combustion reaction of 2-butanol. The bond dissociation of C-O producing hydroxyl and butyl radical was the most rapid process, which indicates that the dehydroxylation of 2-butanol was the fastest process. The butyl radical would further decompose into methyl and propene with the larger rate constant. The butyl radical can also react into butene through isomerization. Methyl radical together with CH3CH2•CHOH radical can be directly produced by 2-butanol, and the CH3CH2•CHOH radical would further decompose into methyl. The slowest reaction among all the initial reactions was the dehydrogenation reaction of hydroxyl, remaining the C4H9O• radical. Another hydroxyl radical would combine with the hydrogen radical from the hydroxyl of 2-butanol and formed a water molecule. The C4H9O• radical could decompose into ethyl and ethanol rapidly and also produce methyl propyl ether through isomerization reaction. The rate constant of secondary reactions were much faster than primary reactions.


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Figure 4 Main initial reaction pathways of 2-butanol in additive-fuel combustion simulations at 3000 K. (Black-unimolecular reaction, blue-bimolecular reaction. If the intermediate can be produced through more than one pathway, the largest rate constant was displayed.)

The main initial mechanism of DMC oxidation was shown in Figure 5. The DMC molecule would decompose into •CH3 radical and C2H3O• radical through C-O bond decomposition reaction, which was the most rapid step among all the primary reactions. The C2H3O• radical further produced •CH3 radical and •OCO2• biradical through bond decomposition. The C2H3O• radical can produce CH3O• radical and HCO2• radical by colliding with H• radical. Similarly, through the collision with H• radical, DMC molecule can also produce CH3O• radical and HCO2• radical. The rate constant of secondary reactions of DMC combustion were similar to the primary reactions, which were much larger than those of the ethanol and 2-butanol.


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Figure 5. Main initial reaction pathways for the combustion of DMC in additive-fuel combustion simulations at 3000 K. (Black-unimolecular reaction, blue-bimolecular reaction. If the intermediate can be produced through more than one pathway, the largest rate constant was displayed.)

In the combustion of DBC, the main initial reactions were all bond dissociation reactions with the normalized rate constant magnitude of 102 to 104, which were similar to the initial rate constants of DMC. The bond decomposition reaction of DBC molecule which produced •C4H9 radical and C5H9O3• radical was the fastest step. The •C4H9 radical would produce butane through isomerization reaction and decompose into ethyl radical and ethylene through bond dissociation reaction. The C5H9O3• radical can further produce several intermediates or products including CH2O, CHO2•, C4H9O•, •C4H9, •OCO2• with a relatively rapid reaction rate. The intermediates C4H9O• and •OCO2• can also be directly obtained by bond cracking of DBC molecule. The C4H9O• was an active intermediate and would decompose into CH2O and propyl radical. The C5H9O2• radical was formed by C-O bond dissociation of DBC and isomerization in succession, and it further produced CHO2• and •C4H9 radical through the collision with H• radical.


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Figure 6. Main initial reaction pathways for the combustion of DBC in additive-fuel combustion simulations at 3000 K. (Black-unimolecular reaction, blue-bimolecular reaction. If the intermediate can be produced through more than one pathway, the largest rate constant was displayed.)


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Figure 7. Main initial reaction pathways for the combustion of MTBE in additive-fuel combustion simulations at 3000 K. (Black-unimolecular reaction, blue-bimolecular reaction. If the intermediate can be produced through more than one pathway, the largest rate constant was displayed.)

Figure 7 displays the main initial pathways of MTBE combustion. It can be concluded that the reaction rate of primary reactions was much slower than those of DMC and DBC. Dehydrogenation and bond decomposition were the main pathways for MTBE combustion. The bond dissociation of C-O bond forming CH3• radical and C4H9O• radical was the fastest step among all the primary reactions. By colliding with a CH3• radical, the C4H9O• radical could produce a C4H9• (tert-butyl) and a CH3O• radical with a relatively slow rate. The MTBE could also directly produce CH3O• radical and C4H9• (tert-butyl), and the CH3O• radical further turned into CH2O


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through dehydrogenation reaction and into •CH3O radical through isomerization reaction. Similarly, the C4H9• (tert-butyl) could produce C4H8 and C4H9• (iso-butyl) through dehydrogenation and isomerization respectively. Among all the primary reactions, dehydrogenation was the slowest. Through dehydrogenation reaction, the

k'

=7 .3 9e +0 2

k' = 2.0 2e +0 2

MTBE molecule produced H• radical and •C5H11O radical.

k' 02 e+ .39 =7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. Main initial reaction pathways for the combustion of TTAG in additive-fuel combustion simulations at 3000 K. (If the intermediate can be produced through more than one pathway, the largest rate constant was displayed.)

The primary reactions of the combustion of TTAG was shown in Figure 8. All the reactions were bond dissociation reaction, and in other words all the primary reactions were unimolecular reactions. The most rapid pathway was the decomposition of C-O bond on the central tert-carbon atom, which produced C5H11O•


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radical. The C-O bond dissociation reaction could occur on different position of TTAG molecule, and when occurring on the central secondary carbon atom the C-O bond decomposition was a litter faster than that on the adjacent carbon. The above two reactions could occur together and produced a CH2=C7H14O molecule with a relatively slow reaction rate. The C5H11O• radical could decompose into •CH3 radical and 2-butanone. The normalized rate constants of primary reactions were in the magnitude of 102 to 103, which were similar to those of MTBE. Table 3 Bond dissociation energy of C-C/C-H/C-O bonds for six kinds of fuel additive structures.

Structures

Bond Dissociation Energy (kcal/mol) 1 2 3 4 5 99.61 79.95 91.60 86.80 98.50

3 1

2

5

4 8 5

3

1

2

4

7

9

6

2 1

1

2

3

4

5

98.61

82.79

95.15

74.97

87.48

6

7

8

9

89.98

78.27

99.41

99.01

1 96.72

2 77.39

3 90.94

1 89.72

2 77.99

3 80.34

4 80.75

5 82.29

1

2

3

4

5

74.28

70.14

72.13

92.05

99.02

1 70.78 6 119.82

2 77.21 7 70.78

3 64.92 8 79.15

4 66.68

5 67.41

3

2

1

1

4

5

3

5 2 3

4

1 4

2 3 6

8

5 7


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The bond dissociation energies (BDE) including C-C, C-H, and C-O bonds in six fuel additive structures were calculated by B3LYP/6-311G++(d,p)46, 47 basis set in Gaussian 09. It can be concluded that in the structures of alcohols, the bond dissociation energy follows the relation of BDEC-H> BDEC-O> BDEC-C. Because of the simpler structure of ethanol, the bond dissociation of C-H may occupied larger percentage in total reactions, and thus brought difficulties in breaking bonds when energies was gradually input into the system. However, in the structures of esters and ethers, the bond dissociation energy follows the relation of BDEC-H> BDEC-C> BDEC-O. The existence of C-O bond of lower bond dissociation energy brought easiness in primary reactions. Besides, the relative complexity of structures of 2-butanol, DMC, DBC, MTBE, and TTAG supplied more reaction sites, and thus reduced the percentage of the dissociation of C-H bond. The above reasons may explain the different primary and secondary reaction rates in different types of fuel additives. In conclusion, the bond dissociation reactions were dominant at early stage compared to oxidation reactions in hydrocarbons oxidation under high temperatures, which made the unimolecular reactions be the main pathways of primary reactions in hydrocarbons combustion. This is in accordance with Wang’s work48 that the hydrocarbons would first undergo fast thermal cracking reactions to form small molecules. Hydrogen abstraction reaction can be obviously observed in the primary pathways of ethanol and 2-butanol combustion, but it is not remarkable in the early stage of the other four systems. From the perspective of reaction rate, different from


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other systems, the secondary reactions were about 100 times faster than the primary reactions for ethanol system. Besides, the rate constants of primary reactions of ethanol and 2-butanol were much smaller than other systems, which indicates that the combustion of ethanol and 2-butanol may result in longer ignition delay. These results were in good accordance with others’ work12. The reaction rates of primary reactions of DMC and DBC systems were the largest. Some biradicals such as •OCO2• were generated under such high temperatures, and these biradicals were in good agreement with previous work45, 49.

3.2. Product distribution Reactive-MD simulations were conducted through LAMMPS software to investigate the main product distribution and validate the accuracy of mechanisms. The temperature was well controlled around the target temperature 3000 K, which was displayed in Figure S1 of Supporting Information file. The detailed composition of each system at the end of the simulation is analyzed and shown in Table 4. The products can be classified into C0, C1, C2, C3, and C4 species. The total kinds of fragments show a distinct difference between diesel system and diesel/additive systems. In diesel combustion system, there were 23 different fragments which included C0, C1, C2, C3, and C4 species. When fuel additives were added, the total kinds of fragments were reduced, i.e. 19 in ethanol/diesel system, 11 in 2-butanol/diesel system, 8 in DMC/diesel system, 13 of DBC/diesel system, 8 in MTBE/diesel system, and 9 in TTAG/diesel system. It is easy to make sense that the lighter the species are, the more complete the combustion is. From this perspective,


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the addition of additives could reduce the heavier species and promote the combustion processes, because the heavier C4 species were only existed in diesel system. Among all of the fuel additives, ethanol displayed the least promoting performance due to the existence of C3 species, while in other additive systems, there were only C0, C1, and C2 species. The addition of fuel additives in fuel combustion system could reduce the content of CO, which is in accordance with experimental results11, 12, 17. The CO, CO2, H2O, and H2 were the products of higher percentage, and these products were further discussed. Table 4 The Detailed composition of products observed at the end of the ReaxFF-MD simulations at 3000 K. System

C0

C1

C2

C3

C4

Total kinds of fragments

Diesel

44H2O(15.5%)

121CO(42.6%)

2C2H2O••(0.7%)

45H2(15.8%)

7CO2(2.5%)

1C2H4O(0.3%)

26•OH(9.2%)

15CH2O(5.3%)

1•C2H3(0.3%)

2HO2(0.7%)

3CH2••(1.1%)

2C2H2(0.7%)

2CH3•(0.7%)

1C2H3O•(0.3%)

4CHO•(1.4%)

1C2H4(0.3%)

1CH3O•(0.3%)

1•C2HO2(0.3%)

1CH4(0.3%)

1C2H4O2(0.3%)

1C3H5O•(0.3%)

1C4H3(0.3%)

23

1CH4O(0.3%) Ethanol+diesel

92H2O(18.6%)

180CO(36.4%)

5C2H4(1.0%)

97H2(19.6%)

11CO2(2.2%)

1C2H2O••(0.2%)

34•OH(6.9%)

9CHO•(1.8%)

3•C2H3(0.6%)

5HO2(1.0%)

1CHO2(0.2%)

4C2H2(0.8%)

1C3H2(0.2%)

19

5CH2••(1.0%) 5CH4(1.0%) 35CH2O(7.1%) 1CH4O(0.2%) 1CH2O2(0.2%) 3CH3•(0.6%) 2-butanol+diesel

227H2O(35.5%)

220CO(34.4%)

47H2(7.3%)

85CO2(13.3%)

49•OH(7.7%)

4CH2O(0.6%)

3HO2(0.5%)

1CH4O(0.2%)

1H2O2(0.2%)

2CH2O2(0.3%)

158H2O(31.5%)

179CO(35.7%)

39H2(7.8%)

80CO2(16.0%)

37•OH(7.4%)

2CH2O(0.4%)

11

1CH4(0.1%) DMC+diesel

8

4HO2(0.8%) 2H2O2(0.4%) DBC+diesel

194H2O(32.5%)

225CO(37.7%)


13

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43H2(7.2%)

76CO2(12.7%)

42•OH(7.0%)

7CH2O(1.2%)

3HO2(0.5%)

2CH2O3(0.3%)

1H2O2(0.2%)

1CH2O2(0.2%) 1CH3O2(0.2%) 1CHO3(0.2%) 1CH3•(0.1%)

MTBE+diesel

263H2O(38.4%)

253CO(36.9%)

48H2(7.0%)

87CO2(12.7%)

19•OH(2.8%)

1CHO•(0.1%)

8

12HO2(1.8%) 2H2O2(0.3%) TTAG+diesel

282H2O(38.8%)

267CO(36.8%)

46H2(6.3%)

97CO2(13.4%)

23•OH(3.2%)

6CH2O(0.8%)

9

3HO2(0.4%) 2H2O2(0.3%)

Figure 9 displays the relationship between the number of H2, H2O, CO, CO2, O2 and the simulation time. Compared to diesel combustion shown in graph g), the number of all kinds of products were much more for those with additives in graph a) ~ f), and in other words, the addition of these fuel additives could obviously promote the combustion processes. In terms of the O2 consumption, the addition of fuel additives could increase the oxygen consumption from 48.5% for diesel combustion without additives to different degrees. The 2-butanol showed the best assisting ability with the O2 consumption of 73.33%, while ethanol showed the lowest assisting ability with the O2 consumption of 49.21%. Other fuel additives displayed a similar O2 consumption of about 70.00%. As for the generation of CO, for the system of diesel-ethanol the number of CO was about 20, which was much less than that of other systems. In terms of the earliest time the products generated, the systems of diesel-alcohols displayed the longer ignition delay, evidenced by that the earliest products were produced at about 125 ps, while it was 70 ps for the diesel-esters systems and 100 ps for diesel-ethers systems. These results were in good accordance with our mechanisms analysis in section 3.2 and other’s work12, and further prove that


Energy & Fuels

alcohol may not be a suitable additive in terms of ignition delay. a).

Num.species

400 300

H2 H2O CO CO2 O2

600

c).

500

Oxygen consumption: 71.99%

400

H2 H2O CO CO2 O2

600

400 300

200

200

100

100

100

0

0

200

0

500

b).

500

1000 Time (ps)

1500


400

2000

0

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600

H2 H2O CO CO2 O2

d).

500

1000 1500 Time (ps)


400

0

2000

H2 H2O CO CO2 O2

0

600

300

200

200

200

100

100

100

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g).

500

1000 Time (ps)

1500


400

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0

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1000 1500 Time (ps)

1000 Time (ps)

1500


400

300

500

500

f).

500

300

0


e).

500

300

600

Num. species


Num. species

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Num. species

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

2000

0

0

500

1000 1500 Time (ps)

H2 H2O CO CO2 O2

2000

H2 H2O CO CO2 O2

2000

H2 H2O CO CO2 O2

300 200 100 0

0

500

1000 1500 Time (ps)

2000

Figure 9. Time evolution of main product distribution in combustion of a). diesel/ethanol, b). diesel/2-butanol, c). diesel/DMC, d). diesel/DBC, e). diesel/MTBE, f). diesel/TTAG, g). diesel. The error bars displayed the difference between simulations which started from different configurations.

It is obvious that CO2 molecules were generated at later time compared to other products in graph a), b), e), and f) because CO2 was generally produced by the combustion of CO. However, in the graph c) (DMC) and d) (DBC), CO2 molecules were detected at very early stage, this may be due to the C=O bond in ester structures. This phenomena demonstrates that CO2 could be preferentially produced by addition of esters. At the end of simulations, the number of CO2 were about 80 in graph b), c), d), e), and f) corresponding to 2-butanol, DMC, DBC, MTBE, and TTAG respectively, while in graph a) the number of CO2 were much less, which was nearly equal to the number of CO2 in diesel system. ACS Paragon Plus Environment

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3.3. Radical detection In the combustion of diesel, radicals play a vital role in chain propagation and it is still a high-cost matter to obtain the concentration of radicals in combustion for experimental method. In this section, the radical reactions in diesel combustion were analyzed first, and then the variation trend of the number of radicals in all additive systems were investigated. 3.3.1 Main elementary reactions of diesel combustion Equations (2) to (5) display the radical reactions of higher frequency in the combustion of diesel. The •OH radical could react with H2 molecule, forming a water molecule and a •H radical, which was the most frequent radical reaction in the diesel combustion. The •CH3 radical could react with oxygen molecule and produce a peroxyalkyl radical CH3COO•. The biradical ••CH2 was another active intermediate, which could react with •OH radical, forming formaldehyde and •H radical. The •HO2 radical can react with ethylene molecule and produce •CH3CHO radical and water molecule. Due to the higher frequency and significance, the variation trend of the number of these four radicals i.e. •OH, •CH3, ••CH2, and •HO2 were further discussed in section 3.3.2. •OH + H 2  → H 2O + H •

(2)

• CH 3 + O 2   → CH 3 OO •

(3)

• OH + • • CH 2   → CH 2 O + • H

(4)

H − O − O • + C2 H 4   → •CH 2 CHO + H 2 O

(5)

3.3.2 Radical detection


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Table 5 shows the typical reactions which occurred on these radicals (•OH, •CH3,

••CH2, and •HO2). To give a relatively common perspective, reactions containing popular products or important radicals such as ethylene, oxygen, and •H radical were chosen and displayed. The •OH radical could capture •H radical of ethanol and •CH3 radical, forming H2O molecule. The •OH radical could combine with •CH3 radical and produce methanol molecule. Ethylene can also combine with •OH radical, and there were two pathways, one producing •CH2CH2O• biradical, and the other producing CH2OCH2 (oxirane). The •CH3 radical would capture •H radical from •CH3O forming methane molecule and formaldehyde molecule. On the other hand, the •H radical from •CH3 radical can be captured by •CH3O producing ••CH2 biradical and methanol molecule. The •H radical from •CH3 can be captured by •OH radical, generating ••CH2 biradical and water molecule. Another reaction type of •CH3 radical was addition reaction, in which the •CH3 radical would react with radicals or molecules to produce ether, methane, peroxyalkyl radical, and hydrogen peroxyalkyl radical. The ••CH2 biradical could react with oxygen molecule and produce another biradical •CH2OO•, and also could react with •OH radical to produce •CH2OH radical. Besides, •HO2 radical can react with ••CH2 biradical to produce •OH radical and formaldehyde. For •HO2 radical, there was no hydrogen abstraction reaction occurring, and it could react with •CH3 radical and produce hydrogen peroxyalkyl radical. By another pathway, •OH and •CH3O radical would be obtained from •HO2 radical and •CH3


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Energy & Fuels

radical. The •HO2 radical can react with water molecule and produce hydrogen peroxide. Among all of these reactions, the reactions of •HO2 and •H radical turning into •OH radical was observed for the most times in the systems of DMC, DBC, MTBE, and TTAG, revealing the high reactivity of •HO2 radical. Table 5 Reactions of typical radicals (•OH, •CH3, ••CH2, •HO2) in combustion of additives. Radical

Reaction Type H-abstraction reaction

•OH

Addition reaction Other

H-abstraction reaction

•CH3

Addition reaction

••CH2

•HO2

Addition reaction

Reaction

System

CH 3 CH 2 OH + •OH   → CH 3 CH 2 O • + H 2 O

Ethanol

•CH 3 + •OH  → • • CH 2 + H 2O

DMC

•CH 3 + •OH  → CH 3OH

2-butanol

C2 H 4 + •OH  → • H + •CH 2CH 2O •

DBC

C 2 H 4 + •OH  → • H + CH 2 OCH 2 (ring)

DBC

•CH 3 + CH 3O •  → CH 4 + CH 2 O

DMC

•CH 3 + CH 3O •  → • • CH 2 + CH 3OH

DMC

•CH 3 + •OH  → • • CH 2 + H 2O

DMC

•CH 3 + CH 3O •  → CH 3OCH 3

DMC

•CH 3 + HOO •  → CH 3OOH

DMC

•CH 3 + H •   → CH 4

2-butanol

• CH 3 + O 2   → CH 3 OO •

MTBE

• • CH 2 + O2  → •CH 2 OO •

DMC

• • CH 2 + •OH  → •CH 2 OH

DBC

Other

• • CH 2 + •OOH  → •OH + CH 2 O

TTAG

Addition reaction

•OOH + •CH 3  → CH 3OOH

DMC

•OOH + H 2 O  → •OH + H 2 O2

2-butanol

•OOH + •CH 3   → •OH + CH 3O •

DMC

•OOH +•H  →2 • OH

2-butanol

Other

In addition to study the reaction pathways of these vital radicals, time evolution of the number of •OH, •CH3, ••CH2, and •HO2 radicals were also investigated. Figure 10 shows the number of these four radicals in six systems. It can be concluded that the number of •OH radical was the largest compared to other radicals, and it was gradually increasing with time for the esters system of DMC, DBC, and the alcohols system of 2-butanol and ethanol. Specially, for the systems of DBC and 2-butanol, the number of •OH radical reached about 50 at the end of the simulations. However, for


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the system of TTAG, the number of •OH radical was increasing until 1350 ps (shown with arrow) and then decreasing, and for the system of MTBE (ethers), the number of

•OH radical was increasing until 950 ps (shown with arrow) and then decreasing. Besides, from the perspective of time, •OH radicals were generated from 0 ps in the systems of ethanol and 2-butanol, while for other systems, •OH radicals would not be produced until about 50 ps. The possible reason should be the existence of hydroxyl in ethanol and 2-butanol molecules. In graph b), time evolution of the number of •CH3 radical was displayed. Different from the trend of •OH radical, the number of •CH3 radical sharply increased in the beginning within 250 ps, and then gradually decreased in all the systems. In the systems of TTAG, MTBE, DMC, DBC and 2-butanol, •CH3 radicals were almost exhausted, while in the system of ethanol, there were still about 7 •CH3 radicals remaining (shown with arrow). This phenomenon demonstrates that the reactivity of •CH3 radical was relatively higher. The time evolution of ••CH2 biradical was shown in graph c), in which the number of ••CH2 biradical was increasing first and then decreasing. In the systems of TTAG and MTBE, all the

••CH2 biradicals were exhausted before 1500 ps (shown with arrow), and in the systems of DBC and DMC, the ••CH2 biradicals were almost depleted. However, there were still some ••CH2 biradicals existing in the system of ethanol. Graph d) displays the number of •HO2 radical, which indicated that •HO2 radical would be produced at about 100 ps in all systems and the variation trend were almost the same except for the less number of •HO2 in ethanol/diesel system.


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Page 25 of 30

75 50 25 0 50 25 0 50 25 0 50 25 0 50 25 0 50 25 0

b)

TTAG

MTBE

DBC

DMC

isobutanol

ethanol

0

500

1000

1500

30 20 10 0 20 10 0 20 10 0 20 10 0 20 10 0 20 10 0

TTAG

MTBE

Num. CH3 radical

Num. HO radical

a)

2000

DBC

DMC

isobutanol

ethanol

0

500

30

c) 30

d)20

TTAG

MTBE

Num. HO2 radical

20 10 0 20 10 0 20 10 0 20 10 0 20 10 0 20 10 0

1000

DBC

DMC

isobutanol

ethanol

0

500

1000

1500

1500

2000

Time (ps)

Time (ps)

Num. CH2 radical

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2000

10 0 20 10 0 20 10 0 20 10 0 20 10 0 20 10 0

TTAG

MTBE

DBC

DMC

isobutanol

ethanol

0

500

Time (ps)

1000

1500

2000

Time (ps)

Figure 10. Time evolution of the number of a). •OH radical, b). •CH3 radical, c). •CH2 radical, d). •HO2 radical in diesel-additive systems. The error bars displayed the difference between simulations which started from different configurations.

4. CONCLUSIONS In this work, ReaxFF MD simulations were carried out to investigate the performance of six kinds of hydrocarbon fuel additives including ethanol, iso-butanol,


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DMC, DBC, MTBE, and TTAG in promoting diesel combustion. The analysis of mechanisms and pathways showed secondary reactions were about 100 times faster than the primary reactions for ethanol and 2-butanol systems, but the rate of secondary reactions and the primary reactions were similar for other systems. The reaction rates of primary reactions of ethanol were much slower than other systems, and the product distribution analysis showed that products were generated later in ethanol system than other systems, which give a good explanation of the ignition delay of ethanol. Due to the lowest oxygen consumption and longer ignition delay, ethanol may not be the suitable single additives. Four kinds of main radicals in diesel-additive combustion were detected and the number variation trends were displayed. The number of •OH radical was the most and it was gradually increasing along the time except for the later decline for ether systems including MTBE and TTAG, and the number of •CH3 and ••CH2 sharply increased first and then gradually decreased. The number of •HO2 fluctuated within a narrow range. This work suggests that alcohols with smaller molecular weight such as ethanol may not be a good choice for fuel additive, while esters, ethers and alcohols with larger molecular weight such as 2-butanol, possibly can be suitable fuel additive from the perspective of mechanisms and pathways. This work further suggests that ReaxFF-MD is a promising approach to deal with hydrocarbon reactions under high temperatures and high pressure.

AUTHOR INFORMATION


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Energy & Fuels

Corresponding Author *(W.S.) E-mail: [email protected]. Telephone: +86 21 64253027

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The financial support by the National Natural Science Foundation of China

(91434108) and the Shanghai Excellent Technical Leaders Program (14xd1425500) is gratefully acknowledged.

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Combustion Mechanisms and Kinetics of Fuel Additives: A ReaxFF

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