Initial Mechanism and Kinetics of the Diesel Incomplete Combustion: A

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Initial Mechanism and Kinetics of the Diesel Incomplete Combustion: A ReaxFF Molecular Dynamics Based on a Multi-component Fuel Model Zhuojun Chen, Weizhen Sun, and Ling Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11078 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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The Journal of Physical Chemistry

Initial Mechanism and Kinetics of the Diesel Incomplete Combustion: A ReaxFF Molecular Dynamics Based on a Multi-component Fuel Model 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: This work attempts to investigate the incomplete combustion of a multi-component fuel model using ReaxFF-MD simulations. The main products of incomplete combustion simulation included H2, CO, H2O and CO2. Temperatures produced different effects on different products. At lower temperature, a larger increasing rate of the number of product was found at later stage, whereas the increasing rate of the number of product would be diminished over time at higher temperature. The pressure-dependent simulations indicated that the high pressure could promote the combustion process, especially on the production of H2. The analysis of mechanisms and pathways of combustion process indicated that the C-C bond dissociation dominated the early stage of combustion mechanism of paraffin, while the isomerization, Habstraction and C-C bond formation were observed in other systems. Ethylene (C2H4) was the product of β-scission of paraffin and naphthene, while ethyne (C2H2) was the product of β-scission of aromatic structures. Huge fluctuation range was observed in variation trend of •OH and ••CH2, which revealed the

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high reactivity of these two radicals. Besides, collision was the main reason for the initial formation of coke instead of thermal deposition in gas phase under extremely high temperature. This work further suggests that ReaxFF-MD is a promising approach for investigating the combustion behavior of hydrocarbon models at high temperatures. Key words: ReaxFF-MD, incomplete combustion, reaction mechanism, radical detection

1. Introduction The combustion of fuel would still be the main source of energy for human beings within a quite long period of time in future,1 that is to say, combustion would continuously play a vital role in aerospace, ships, vehicles, and power engineering, etc. Since 1970s, complex mechanism researches of combustion have been widely carried out. However, human’s understanding of the essence of combustion is still far behind human’ application toward combustion. Numerous researchers have paid attention on the incomplete combustion process. Gatts et al. investigated the combustion efficiency and emissions of the gaseous fuels in incomplete combustion and found that the emissions of unburned gaseous fuels were due to the failure of initiation of the turbulent flames and the propagations through the bulk mixture.2 Feng et al. studied the particulate matter emission from bio-oil incomplete combustion and found that atmosphere has significant influence on solid emissions.3 In realistic combustion process, the lack of oxygen and the high temperature/pressure may lead to the formation of detrimental solid deposits, which block the inner channel of engine and finally result in the decline of combustion efficient and the disability of engine. Liu et al. studied the solid deposition of n-dodecane and Chinese RP-3 jet fuel and found that the formation


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of deposits could be inhibited by delaying the aromatic ring condensation, but the deposit mechanisms were still remained unknown.4 Andrésen et al. investigated the formation of solid deposition during thermal degradation of JP-8 and found that the solid deposition was a function of the rise in the aromatic content as well as the rate of formation of the non-protonated aromatic carbons.5 Pei et al. studied the thermal oxidation and coking deposition in aviation fuel and found that the dissolved oxygen concentration would effectively change the extent of coke deposition.6 The mechanism of incomplete combustion was the intrinsic reason that causes the above phenomena, however, the reaction pathways and mechanisms in incomplete combustion process of fuel are rarely presented. As the development of research methods, ReaxFF-MD is proposed which can deal with the reactivity in computational calculations.7-8 Due to the utilization of bond order cutoff, pre-assumption of reactions are not required in ReaxFF-MD, that is to say, complex mechanisms and pathways can be directly predicted and obtained by continuous integration. Therefore, ReaxFF-MD simulation provides the possibility of discovering new reaction pathways, which is also one of the most important functions of this method. The ReaxFF describes the overall system energy including bond energy (Ebond), overcoordination energy (Eover), under-coordination energy (Eunder), valence angle energy (Eval), penalty energy (Epen), torsion energy (Etors), conjugation energy (Econj), and nonbond interactions van der Waals (EvdWaals) and Coulomb (ECoulomb). The polarization effect is managed through EEM (electronegativity equalization method).9-10 Except for revealing mechanisms in the system, some important intermediates, which may be harder to study through experimental methods, can be detected in ReaxFF-MD simulations. ReaxFFMD has been treated as an important complementary method for experiments, and it has been successfully



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applied in C/H/O fuel systems such as n-heptane,11 1-heptene,12 n-dodecane,13 methane,14 JP-10,15 hydrogen,16-17 as well as in other systems.18-24 It can be found that the single component surrogate was widely used in ReaxFF-MD simulations, however, the composition of fuel is quite complicated which should be described by a multi-component model rather than the single component model. During the past few decades, plentiful efforts have been paid on the composition analysis of diesel. The chemical compounds in diesel can be approximately divided into two main categories 25-26, they are paraffin-naphthene hydrocarbons which occupy about 60%, and arenes, which take up about 40%. The sort of paraffin-naphthene hydrocarbons can be further divided into about 30% paraffins and 30% naphthene. Because of the low cost and the wide existence in diesel, hexadecane has been chosen as the surrogate of diesel and has been investigated for a long time,27-29 therefore, hexadecane can be seen as the suitable representative of paraffins. In chemical structure of naphthenes, the bond angle greatly influence the stability of naphthenes, i.e. multiple-member ring naphthenes are relatively stable, except for cyclopropane (3-member ring) and cyclobutane (4-member ring). In arenes, monocyclic aromatics and bicyclic aromatics take up more than 90%,25 and the other multi-cyclic arenes were rarely present. Therefore, the single component diesel model was not accurate enough in the combustion research of fuel, and the multi-component diesel model which include paraffin, naphthene, monocyclic aromatic, and bicyclic aromatic should be constructed and utilized in combustion and pyrolysis simulation researches of diesel. In this work, a multi-component diesel model was constructed and the incomplete combustion process was investigated by ReaxFF molecular simulations. The incomplete and complete combustion


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was firstly compared, and then the typical products in temperature-dependent and pressure-dependent simulations were discussed in detail. Besides, mechanisms and pathways and some important radicals in diesel combustion processes were also concerned, and the formation of coke precursor was presented. This article is organized as follows. In section 2, the information of the multi-component diesel model is briefly introduced, and the simulation methods are presented. Section 3 is devoted to discuss the results of combustion simulations include the analysis of main products, the effect of temperature and pressure, mechanisms and pathways of combustion process, the formation of coke, and the radical detection. Finally, main conclusions are displayed in Section 4. 2. Simulation Details 2.1. Construction of the multi-component fuel model In order to give an integrated description of in diesel, we constructed a four-component diesel model to represent different kinds of chemical compounds, including 15 hexadecane molecules, 15 cyclopentane molecules, 10 benzene molecules, and 10 naphthalene molecules, which represent paraffin, naphthene, monocyclic aromatic and bicyclic aromatic respectively. The proportion of bicyclic arene is amplied to provide sufficient molecules to study the mechanism accurately. Table 1 displays the detailed information of diesel model used in this work. Table 1 Detailed information of multi-component diesel model Name

Chemical formula

Num. species

Hexadecane

C16H34

15

Snapshot


Type Paraffin


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Cyclopentane

C5H10

15

Naphthene

Benzene

C6H6

10

Monocyclic arene

Naphthalene

C10H8

10

Bicyclic arene

2.2. Computational methods The comparison of incomplete and complete combustion (O2/fuel ratio) was firstly studied. The multi-component diesel model was placed into simulation boxes measuring 707070 Å. Then 400 oxygen molecules (φ=1.09) and 700 oxygen molecules (φ=0.96) were put into the periodic boundary simulation boxes separately to study the two types of combustion conditions (shown in Figure 1). The simulation boxes were energy-minimized firstly and kept at a low temperature 100 K for 10 ps, and then the systems were heated up to 3000 K with the heating rate of 50 K/ps. High temperature was set to give enough collision between molecules and intermediates, and thus facilitate the simulation process. After the heating stage, the systems were maintained at 3000 K for 2 ns. The ReaxFF-MD simulations were conducted under NVT ensemble (Nose-Hoover thermostat is applied), which means constant number of atoms (N), constant volume (V), and constant temperature (T). The time step of 0.1 fs and temperature damping constant of 10 fs were utilized in the simulation stages. b).

a).

φ=0.96

φ=1.09 ACS Paragon Plus Environment

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Figure 1. Multi-component diesel model with a). 400 O2 molecules and b). 700 O2 molecules.

For the temperature plays a vital role in combustion processes, therefore temperature-dependent simulations were further conducted. The reactant molecules were randomly put in a periodic boundary box with the length of 70 Å, together with 400 oxygen molecules. The system was minimized and equilibrated at 100 K for 10 ps and then heated to 2500 K, 2750 K, 3000 K, and 3250 K respectively with the heating rate of 50 K/ps. The systems were maintained at target temperatures for 2 ns using NPT ensemble (Nose-Hoover thermostat is applied). The pressure was all set to 50 MPa, which was higher than the pressure in practical engines, thus could ensure the enough collisions between species. The time step of 0.1 fs was utilized, together with the temperature damping constant of 10 fs and pressure damping constant of 100 fs. For pressure-dependent simulations, the reactant molecules and 400 oxygen molecules were randomly put in periodic simulation boxes with the length of 70 Å. The system was firstly minimized and equilibrated at 100 K for 10 ps, and then heated to 3000 K with the heating rate of 50 K/ps. The pressures were set at 1 MPa, 10 MPa, 50 MPa, and 90 MPa respectively using NPT ensemble (Nose-Hoover thermostat is applied). The systems were maintained at target pressures for 2 ns. The time step of 0.1 fs, the temperature damping constant of 10 fs, and the pressure damping constant of 100 fs were used. The C/H/O force field (2016) utilized in this work was obtained from others’ research,30 which amended the over-reactivity of oxygen atom in parameters of C/H/O-2008 force field 8. A bond order cutoff of 0.3 was utilized in this work to recognize the species generated in combustion simulations. LAMMPS package

31

was used to investigate the species and analyze the mechanism and pathways of



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combustion processes. Each simulation was repeated for three times with different configurations to prevent uncertainties caused by the single run. 3. Results and Discussion 3.1 Incomplete & complete combustion The incomplete and complete combustion processes were firstly investigated and compared to obtain the general situation of combustion. The product species were categorized into Cn (n represents the number of carbon atom of intermediates/products). Under severe high temperatures, numerous small species would be produced, especially the C1~C3. The number of reactants in multi-component diesel model were also displayed. Figure 2 shows the time evolution of typical Cn species as well as reactant molecules in incomplete and complete combustion simulations. It can be concluded that the number of C1 products were the largest, which gradually increased from 0 to 750 ps and fluctuated with slight decline after 750 ps. The number of C1 species reached 108 in complete combustion system and 94 in incomplete combustion. The less number of C1 in incomplete combustion may be caused by the higher bond dissociation energy of unsaturated species, which were generated due to the lack of oxygen. The number of C2 species were similar in incomplete and complete combustion systems with the maximum of about 80. The number of C2 increased from 0 to 300 ps, and fluctuated during 300 to 650 ps, and then decreased. The decreasing rate of C2 in complete combustion was faster than that of incomplete combustion, because the C2 in complete combustion could produce C1 through C-C bond cleavage, but more unsaturated C2 species such as ethyne were produced in incomplete system, which are difficult to dissociate into C1 species. Therefore, the final number of C2 was 23 in complete combustion system and it was 33 in


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incomplete combustion system. The number of C3 species were the smallest with the maximum of about 35. The number of C3 species would increase during 0 ps to about 200 ps and then decreased. Almost all the C3 species were consumed in complete combustion system, and there were still 10 C3 left in incomplete combustion system. It can be demonstrated that the number of C1~C3 products would increase firstly and then decreased, and both the increasing rate and decreasing rate would follow the relations of C3> C2> C1. The final number of species would follow the relations of C1> C2> C3.

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


700 O2

400 O2

120 90 60 30 0 75 50 25 0 30 20 10 0

C1

C2

C3

C5H10

10 5 0

C6H6

10 5 0

C10H8

10 5 0

C16H34

10 5 0

0

500

1000

1500

2000

Time (ps) Figure 2. Time evolution of the number of Cn species and reactant molecules in incomplete (400 oxygen molecules) and complete (700 oxygen molecules) combustion simulations.

The reactant molecules kept decreasing during all the simulations. The hexadecane molecules



(C16H34) were the most rapidly consumed reactant, which would be totally exhausted within 50 ps at 3000 K in both systems. The number of C5H10 molecules decreased to 0 at 340 ps in incomplete combustion system and at 280 ps in complete combustion system. Compared to C16H34 and C5H10, the C6H6 and C10H8 molecules took longer time to be exhausted. The C6H6 and C10H8 molecules would not disappear until 1100 ps and 1050 ps respectively, which may due to the stable structure of aromatic rings. To obtain the overview of the combustion process, all the species were classified into Cn at the end of the simulations. Figure 3 displays the final Cn product distributions at 3000 K. In the system with 700 oxygen molecules, there were only C1, C2, and C3 species, occupying 70.90%, 27.90%, and 1.20% respectively. However, in the system with 400 oxygen molecules, the final products included not only C1 (55.88%), C2 (32.36%), C3 (7.87%), but also C4, C5, C7, and C14 due to the lack of oxygen. The molecular structures were also displayed in Figure 3. These large molecules were all highly unsaturated compounds, which could be seen as the precursor of coke. In summary, products of complete oxidation were concentrated on C1~C3, while there were precursors of coke being produced, which was in good agreement with other’s work.32 90 55

.8 8% .9 0%

400 O2

700 O2

70

80 70 60

32

.3 6

%

50

%

40

.9 0

30

27

C3

C5

C7


%

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Products

0. 90

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%

0. 90

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0. 90

C1

0. 90

0

%

10

%

7. 87

%

20

1. 20

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

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C14

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Figure 3. Cn distribution at the end of simulations at 3000 K in incomplete and complete combustion systems (Green-carbon, blue-hydrogen, white-oxygen). 400 350

400 O2 700 O2 oxygen consumption: 50.26%

300

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


250 200 150 100

oxygen consumption: 68.75%

50 0

H2

H2O

CO

Products

CO2

O2

Figure 4. Main products and oxygen molecules distributions sampling from the incomplete and complete combustion simulations at 2 ns, 3000 K.

Main products and oxygen consumption were analyzed. Figure 4 shows the product distribution and the oxygen consumption at the end of simulations at 3000 K. Main products included hydrogen, water, carbon monoxide, and carbon dioxide, among which the carbon monoxide was the dominant product. The main products found in our simulations were in good agreement with previous work.33 The number of carbon monoxide reached 248 in incomplete combustion and 302 in complete combustion system, which displays the different oxidation degree in two combustion conditions. Hydrogen and water were another two important products, whose number reached more than 100 in incomplete and complete combustion systems. However, the number of carbon dioxide was much less, and there was not obvious number difference between the two different combustion conditions. It indicates that the CO was difficult to consume under the current temperature and pressure conditions, resulting in less CO2 than CO even in the system with 700 O2 molecules. More CO than CO2 was also observed by other researchers.34 The oxygen



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consumption was also investigated, and it was defined as the percentage of oxygen molecules that the hydrocarbons consumed during the simulation in total oxygen molecules. In incomplete combustion system, it was 68.75% which was higher than 50.26% in complete combustion system. 3.2 Temperature-dependent simulation

450

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Time (ps) Figure 5. Time evolution of main product and oxygen distribution at a).2500 K, b).2750 K, c).3000 K, and d).3250 K.

Because of the importance of temperature, a series of temperatures were set in simulations, i.e. 2500 K, 2750 K, 3000 K, and 3250 K to study the temperature effect. The number variation trend of typical


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products including H2, H2O, CO, and CO2, as well as reactant O2 were sampled and displayed in Figure 5. As the simulation proceeded, the number of oxygen kept decreasing and the products molecules kept increasing. In Figure 5 a), there were about 315 oxygen molecules remaining at the end of the simulation, which revealed the mild consumption of oxygen at lower temperature. Besides, the number of H2, H2O, and CO started to grow at 250 ps, and fluctuated around 20. There were almost no CO2 generated at the end of the simulation at 2500 K. Larger consumption of oxygen was found at 2750 K shown in Figure 5 b). About 150 oxygen molecules were consumed at the end of the simulation. The variation trend of H2, H2O, and CO could be divided into two stage (marked with arrow). From 0 ps to 1250 ps, the generating rates of products were relatively slow, and from 1250 ps to 2000 ps, the formation rates of products became faster. Carbon monoxide was the dominant product, whose number reached 125 at 2000 ps, and the number of H2, H2O, and CO2 reached 87, 58, and 12 respectively. The number of carbon dioxide rose only after 1750 ps, when carbon monoxide had been accumulated to some degree. At 3000 K, the consumption of oxygen further increased, and the variation trend of H2 and CO could also be divided in two stage (marked with arrow). The number of H2 and CO steadily increased before 1000 ps, and then the generating rate decreased after 1000 ps. The number of H2O and CO2 gradually increased from the beginning to the end, and did not show obvious change in variation rate. However, the CO2 molecules were generated from 500 ps, which was earlier than that of 2750 K. At 3250 K, curves of CO and H2 could be divided into three stages, i.e. fast increasement, slow increasement, and fluctuation stage. Both the number of CO and H2 steadily rose before 750 ps, after that the rising rate decreased. After the slow increasement stage (from 750 to 1500 ps for CO and from 750 to 1150 ps for H2O), the number of CO and H2O fluctuated around 350 and 210 respectively. The number of H2O and CO2 slowly increased during the simulation. From the temperature dependent simulations, it can be concluded that H2, CO and H2O were the early products, and CO2 was later product. The number and the number variation trends of H2 and CO were obviously effected



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by temperature, i.e. more variation trends would be displayed under higher temperatures. However, the temperature effected less on the production of H2O and CO2. Under high temperatures, the increasing rate of products would gradually decrease and the number of products finally plateaued. The combustion simulation at 3250 K was further prolonged to 6 ns to explore whether the number of CO and CO2 would significantly change, and the result was provided in Supporting Information file. 3.3 Pressure-dependent simulation

Figure 6. Distribution of main product at the end of pressure-dependent combustion simulations.

Pressures of 1 MPa, 10, 50, and 90 MPa were set in the simulations using NPT ensemble to investigate the influence of pressure. Figure 6 displays the typical product distributions under these pressures. The number of all the products increased with the rising pressure, especially the carbon monoxide, whose number reached nearly 350 at 90 MPa. High pressure provides enough collision between species and intermediates and thus accelerates the combustion process. From the perspective of increasing rate with pressure (dotted lines), it can be concluded that from 1 MPa to 10 MPa all the products got the sharpest increase, as the pressure further rose, the promotion degree of generating rate that caused by the increasing pressure was diminished. This phenomenon indicated that the combustion promotion caused


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by the increasing pressure are more sensitive at lower pressure, for enough activated molecules were formed at that condition. The promotion effect of pressure on combustion process were in good agreement with other similar ReaxFF-MD simulations.13, 17 3.4 Mechanisms & pathways In order to study the detailed pathways of fuel combustion process, the output trajectories were analyzed and the initial mechanisms of the four reactants are displayed in Figure 7. Figure 7 a) presents the pathways of C16H34, in which the C-C bond dissociation reaction was the dominant reaction at the early stage. The C16H34 molecule cracked into •C11H23 and •C5H11, or •C9H19 and •C7H15, or •C10H21 and •C6H13, or two •C8H17 radicals. All of the cracked species would gradually react into smaller species by releasing ethylene molecule. Hydrogen abstraction and dehydrogenation reaction would also occur when C1-C2 species were produced. The hydrogen atom of ethyl radical was captured by oxygen molecule and formed •OOH radical and ethylene. Two hydrogen atoms of ethyl radical could directly drop from •C2H5 and form hydrogen molecule, leaving •C2H3 radical. Oxidation reaction would not occur until C1-C2 species were formed. The earliest oxidation reaction of C16H34 was the formation of formaldehyde (CH2O) from methyl radical (•CH3). These small species would underwent a series of subsequent reactions and the final products included CO, CO2, H2O, and other species. The process of fast pyrolysis of C16H34 was in good agreement with Wang’s work that the combustion of long-chain hydrocarbon would undergo fast thermal cracking to form H2 or C1-C4 before oxidation occurring.13 Figure 7 b) presents the initial mechanism of C5H10 in combustion simulation. The C-C bond dissociation reaction was also the initial reaction but would not last for such a long time like C16H34. The ring structure of C5H10 would crack into C2H4 and •C3H6•. The ethylene underwent continuous dehydrogenation and formed •C2H. The •C3H6• biradical could also lose hydrogen radical and produce •C3H5, which further decomposed to ••CH2 and •C2H3. Another pathway of •C3H6• is the formation of



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C-C bond to produce cyclopropane, which further lost a hydrogen radical and then decomposed into methyl and ethylene. Hydroxyl radical could directly react with C5H10 and capture a hydrogen to form water molecule. The •C5H9 would further decompose into cyclopropane and ethylene. The small species would be further oxidized to CO, CO2, and so on.

a)

b)


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c)

d) * *

*

* *

*

* * *

(*Because the connectivity information was not included in ReaxFF-MD simulation, single C-C bonds represent all possible bonds in the complicated structures.) Figure 7. Initial combustion mechanism and final products of a). C16H34 b). C5H10 c). C6H6 d). C10H8 of multi-component diesel model.

For monocyclic arene (C6H6) shown in Figure 7 c), isomerization reaction occurred. One hydrogen atom transferred to the adjacent carbon atom, and the ring structure turned into chain structure through the C-C bond dissociation. The C-C bond formation reaction was observed, resulting in the formation of a three-membered ring. The C-H bond fission was also observed, followed by the ring opening reaction,



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which decomposed to ••CH2 radical and C5H3 through continuous dissociation of C-C bond. The C-H bond fission was in accordance with Liu’s findings35, i.e. the pyrolysis of pyridine is a chain process initiated principally by C-H bond fission. Oxygen radical reacted with C6H6 and formed C6H5O•, which further reacted with three hydrogen radicals and shrank the carbon ring by releasing a CO molecule. The five-membered ring then decomposed into C2H4 and C3H4. Figure 7 d) shows the initial pathway of C10H8. The H-abstraction reaction was observed in the beginning of combustion mechanism of C10H8, followed by the ring opening reaction. Due to the lack of connectivity in ReaxFF-MD, the detailed bond information in complex structure was difficult to speculate, therefore, single C-C bonds were drawn to represent all the possible type of bond (marked with *). Through continuous C-C bond cleavage, ethyne molecules could be produced. Another initial pathway of C10H8 is the addition of methyl radical, followed by the opening of one of the ring structure. Then the continuous collision from hydroxyl radical, hydrogen radical, and oxygen radical would gradually capture hydrogen and carbon atom from the reactant. The five-membered ring was observed. Comparing the four mechanisms, it can be concluded that the C-C bond dissociation dominated the early stage of combustion mechanism of paraffin and naphthene, however, the isomerization and Habstraction were the primary initial pathways of aromatic structures, which was in accordance with Han’s work.36 Besides, ethylene (C2H4) was the product of β-scission of paraffin and naphthene while ethyne (C2H2) was the product of β-scission of aromatic structures. Our results were in good agreement with previous studies that Liu et al.35 found that C2H2 was one of the prominent products in pyrolysis and combustion of pyridine, and also in accordance with Ding’s result11 that C2H4 was produced through βscission in heptane system. The direct interactive reactions between reactants (C16H34, C5H10, C6H6, and C10H8) were not observed, and the possible reason is that molecules tend to decompose rather than interplay under high temperature and high pressure. However, interactions between small species were


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found such as the •H radical found in C10H8 mechanism was possibly generated from other reactants Figure 8 displays the timeline of typical oxides detected in simulations including •OH radical, •OOH radical, CO, H2O, CH2O, CO2, etc. It can be deduced that the difficulties in the production of oxides would follow the relations of C16H34< C5H10< C6H6< C10H8. For reactant C16H34, formaldehyde was the earliest oxide, which was produced at 17.5 ps. The formaldehyde molecule was an important intermediate in combustion system of hydrocarbons, which was detected in combustion system by other researchers 8, 15, 37.

Formaldehyde could also be produced from C10H8 at later period, 375 ps. From the perspective of

radicals, the •OH and •OOH radical were observed in early stage of all the four reactants, which indicated the significance of these two radicals in combustion process. From the perspective of molecules, the production of CO was earlier than that of H2O. Besides, the CO2 molecule would be formed at relatively late stages, i.e. 230 ps, 168 ps, and 462 ps in C16H34, C5H10, and C6H6 system respectively, and it would not be produced in C10H8 system within 500 ps.

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400

500


Figure 8. The timeline of typical oxides generated from four kinds of reactants (Light yellow-oxygen, blue-hydrogen, greencarbon).

3.5 Radical Detection 40

40

OH

30

30

20

20

10

10

0 0 40

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

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500 1000 1500 2000 2500 3000 3500 4000

0

CH2

30

20

20

10

10 0

120 100

80

60

60

40

40

20

20 0

500 1000 1500 2000 2500 3000 3500 4000 CHO

0 500 1000 1500 2000 2500 3000 3500 4000 0 120 O 100

80

0

0

40

30

0

OOH

500 1000 1500 2000 2500 3000 3500 4000

0

0

500 1000 1500 2000 2500 3000 3500 4000 H

500 1000 1500 2000 2500 3000 3500 4000

Time (ps) Figure 9. Time evolution of the number of typical radicals at 3000 K, 50 MPa.

Due to the vital role the radicals play in combustion process and the detection difficulties by experimental methods, the variation trends of the number of typical radicals were studied and displayed. Figure 9 shows the time evolution of radicals including •OH, •OOH, ••CH2, •CHO, •H and ••O at 3000 K, 50 MPa. The number of •OH radical sharply increased from 100 to 180 ps, and slowly increased from 180 to 2000 ps. During 2000 to 2750 ps, the number of •OH fluctuated around 30 and then gradually deceased. The fluctuation range of the number •OH radical reached more than 10 during the simulation, which revealed its high reactivity in combustion process. The number of •OOH was the least among these


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four radicals, and it fluctuated within 10. The number of ••CH2 dramatically rose from 80 to 180 ps and then gradually decreased. At the end of the simulation, ••CH2 were almost consumed. The fluctuation range of ••CH2 was similar to that of •OH, indicating the high reactivity of ••CH2. The existence of the double bond between carbon and oxygen atom in •CHO structure may be the possible reason for the slower increasing rate of the number of •CHO radical. The number of •CHO radical increased from 0 to 750 ps and then started to decrease. There were still some •CHO radicals in the system in the end. The •H and ••O radicals play vital roles in free radical chain reactions and the number of •H and ••O were the largest among all of the radicals. The number of •H and ••O radical got sharp increase within 100 ps, which reached 55 and 78 respectively. In the later period, both the number of •H and ••O radical showed higher chemical reactivity, revealed by the larger fluctuation range. At the end of the simulation, the number of •H and ••O radical was about 20 and 10 respectively. The reactions in which the •H and ••O radical participated were shown in Supporting Information file. 3.6 Coke Formation C

C C4

C

C

C C

C C5

C

C

C C

C C C

C7 C


C


C

C

C

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C C

C14

C

C C

C

C

C

C

C

C

Figure 10. The origin of carbon atoms in highly unsaturated structures.

During the incomplete combustion simulation of fuel model, highly unsaturated structures were observed (mentioned in section 3.1 and section 3.3). These structures could be treated as the precursor of coke in incomplete combustion system. In order to give a better understanding of the formation of these species, the trajectory of carbon atom was traced and shown in Figure 10. Two neighboring carbon atoms from C5H10 molecule and two carbon atoms from different C16H34 molecules formed the C4 structure found in the system. The C5 was formed by three carbon atoms from C5H10, C6H6, and C16H34 respectively, and two carbon atoms from one C10H8 molecule. Both the C7 and C14 were composed by carbon atoms from C5H10, C10H8, and C16H34. It can be clearly observed that the collision between carbon atoms was the main reason for coke formation instead of thermal deposition in gas phase under extremely high temperature. 4. Conclusions In this work, the incomplete combustion of a multi-component fuel model was simulated using ReaxFF method. The final products were classified into C1~C3, which showed the tendency of increasing first and then decreasing. The main products included H2, CO, H2O and CO2, among which H2, CO, and H2O were the earlier products while CO2 was later product. The number and the number variation trends of H2 and CO were obviously effected by temperature, however, the temperature effected less on the


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production of H2O and CO2. At lower temperature, a larger increasing rate of the number of product was found at later stage, whereas the increasing rate of the number of product would be diminished over time at higher temperature. The pressure-dependent simulations indicated that the high pressure could promote the combustion process, especially on the production of H2. The analysis of mechanisms and pathways of combustion process indicated that the C-C bond dissociation dominated the early stage of combustion mechanism of paraffin, while the isomerization, Habstraction and C-C bond formation were observed in other systems. Ethylene (C2H4) was the product of β-scission of paraffin and naphthene, while ethyne (C2H2) was the product of β-scission of aromatic structures. Through radical detection, it can be concluded that the number of •OH radical was the largest, at the same time, huge fluctuation range was observed in variation trend of •OH and ••CH2, which revealed the high reactivity of these two radicals. For the formation of highly unsaturated species observed in combustion simulations, it was found that collision was the main reason for the initial formation of coke rather than thermal deposition in gas phase under extremely high temperature. This work provides a new perspective of the combustion reaction of the different components in diesel, and promotes the knowledge of the calculation of chemical reactivity up to the atomic level. The radical evolution found in this work well makes up for the deficiencies in radical detection by experiments. Hopefully, the mechanisms and kinetic obtained in this work would be beneficial to the promotion of combustion processes and helpful to future design and modification of new fuel surrogate. This work further suggests that ReaxFF-MD is a promising approach in investigating the hydrocarbon reactions under the condition of high temperatures and high pressures.





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Acknowledgements This work was supported by the National Natural Science Foundation of China [91434108], the

Scientific Research Foundation for the Returned Overseas Chinese Scholars, and the Fundamental Research Funds for the Central Universities (WH1817014).



Reference

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Initial Mechanism and Kinetics of the Diesel Incomplete Combustion: A

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