Effect of Dimethyl Ether Addition on Soot Formation Dynamics of

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Effect of Dimethyl Ether Addition on Soot Formation Dynamics of Ethylene Opposed-Flow Diffusion Flames Pengyuan Zhang,† Yinhu Kang,*,‡ Zejun Wu,*,§ Xiaofeng Lu,‡ Quanhai Wang,‡ and Lin Mei∥

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†

Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, China ‡ Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education of China, Chongqing 400044, China § College of Aerospace Engineering, Chongqing University, Chongqing 400030, China ∥ Chongqing Special Equipment Inspection and Research Institute, Liangjiang District, Chongqing 401121, China ABSTRACT: Soot formation dynamics in C2H4/dimethyl ether (DME) opposed-flow diffusion flames was numerically studied in this paper, employing detailed gas-phase and dispersed-phase chemistries and transport models. A wide range of DME additions from pure C2H4 to pure DME was involved to systematically examine its impact on the soot formation process. It was found that the flow field had an important impact on the soot structure inside the flame. The soot volume fraction reached a maximum at the stagnation plane due to the infinite residence time, and then vanished abruptly on the fuel side of the stagnation plane. In the opposed-flow diffusion flames, enhancements of nucleation, H-abstraction−C2H2-addition (HACA) reaction, and polycyclic aromatic hydrocarbon (PAH) condensation rates in the near-stagnation region disappeared, such that the local particle size distribution function (PSDF) curve became unimodal, which was rather different from the burner-stabilized stagnation premixed flame. A synergistic effect of DME addition on soot formation of the C2H4 opposed-flow diffusion flame was reported herein. Either the summary soot number density or average particle diameter increased considerably upon 5% DME addition, and then turned to decrease with further addition; they dropped to equivalent values with that of the pure C2H4 flame at 20% DME addition. The synergistic effect due to DME addition was enhanced along the convection flow direction. In the sooting area, the synergistic effect was maximum at 5% DME addition, while in the near-stagnation region, the synergistic effect was maximum at 60% DME addition. With respect to the nonpremixed combustion systems fueled by C2H4/DME blends, at least 20% DME addition is required to effectively reduce the soot number density and particle diameter.

1. INTRODUCTION Soot is an important byproduct of the fossil fuel combustion process, due to incomplete mixing or oxidation. It exists as ultrafine solid particles with diameters ranging from 101 to 104 nm, after emission into the atmosphere. Soot is composed of abundant polycyclic aromatic hydrocarbons (PAHs), with its nanostructures containing many spherical solids which are connected by physical or chemical bonds. It is reported that soot particulates emitted from fossil fuel combustion have became the major source of atmospheric PM2.5 matter in recent years, so soot has attracted increasingly more attention due to the public awareness of its hazardous impact on human health1,2 as well as the ecosystem.3,4 For instance, ultrafine soot can easily enter the human respiratory tract and alveolus, and then dissolve in the blood circulation system, which is very harmful to human health due to its abundant surface adsorbates, for example heavy metals or PAHs, that are mostly poisonous. Moreover, soot can also exist as surface deposit on the polar ice caps, enhancing the ice melting rate due to its © XXXX American Chemical Society

large radiative heat absorption coefficient, which is ascribed to one of the most significant reasons for Arctic warming.5 Hence, fundamental studies on soot formation mechanism and control strategies are a hot and urgent topic in combustion science.6 Soot formation during combustion is a rather complicated physicochemical process. Currently, it is widely accepted that the in-flame soot particulates experience the following physicochemical processes of significance: (I) The gas-phase PAH precursors are produced in a fuel-rich decomposition zone. (II) The nascent particles are formed by PAH recombination reactions. (III) Soot mass grows continuously by surface reactions such as the H-abstraction−C2H2-addition (HACA) reaction7 or by PAH surface condensation. (IV) Particle mass grows by coagulation or aggregation reaction. Received: Revised: Accepted: Published: A

January 6, 2019 March 13, 2019 April 15, 2019 April 15, 2019 DOI: 10.1021/acs.iecr.9b00084 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

significance for understanding the soot dynamics of more complicated hydrocarbon fuels with DME addition. Besides, given the complicacy of soot dynamics in combustion, inclusion of the detailed gas-phase chemistries leading to PAH formation, high-fidelity gas−solid chemistry interaction model, and detailed aerosol soot dynamics models are crucially important for predictability of the soot models. The sectional soot aerosol dynamic model,16,21,22 which can provide detailed information about the soot dynamics process by considering detailed dispersed-phase kinetics due to soot nucleation, surface growth, PAH condensation, particle agglomeration, and oxidation, was employed for the current simulations. Additionally, the present study employed the opposed-flow diffusion flame configuration, because of its well-defined boundary condition which can provide us a flexibility to choose the soot problem type, simply by adjusting the fuel/air jet momentum ratio or composition. More specifically, if the flame front is located on the oxidizer side of the stagnation plane, the soot particles produced in the flame front will be convectively transported toward the fuel side and thus soot oxidation is absent, so this is the soot formation (SF) type flame. If the flame is located on the fuel side of the stagnation plane, soot particles will be convectively transported toward the oxidizer side, and this is the soot formation/oxidation (SFO) type flame. Additionally, our previous study13 using the BSS burner reported that the existence of water-cooled stagnation boundary layer would significantly enhance the soot formation rate. The absence of a cooling boundary layer in the opposed-flow diffusion flame facilitates our investigations into the soot formation dynamics. Moreover, the governing equation of the opposed-flow diffusion flame was the simplest spatially diffusive combustion model due to its one-dimensional (1-D) flame configuration, which enables the most reduction in the computational cost.

(V) Carbonization of soot particles takes place. (VI) Soot is consumed by oxidations with OH or O2. Although knowledge about the soot emission mechanism has been improved by many existing studies, our understanding nowadays in this field is still quite limited. In the past few years, various technologies have proved their effectiveness in inhibiting soot formation. One of the most efficient ways is to blend fuels with a fraction of additives, such as oxygenated fuels like ethanol and its isomer dimethyl ether (DME, CH3OCH3), or inert diluent agents like CO2 and H2O. However, there are still some uncertainties about the details of the soot formation kinetics,4 such as the PAHs and nascent soot formations, and subsequent mass growth.8 For instance, a study on the ethylene laminar premixed stagnation flat flame reported that it was mainly the chemical effect due to CO2 dilution, rather than the thermal effect, that was responsible for the soot inhibition.9 However, the study in ref 10 suggested that the inhibitions of soot nucleation and surface mass growth due to CO2 dilution were most probably the underlying mechanism for soot reduction. Furthermore, a study on the coflow ethylene combustion with water spray assistance suggested that a combination of chemical, thermal, and dilution effects was responsible for the soot inhibition.11 As a typical oxygenated fuel with good availability, dimethyl ether (DME, CH 3 OCH 3 ) which merits the soot-free combustion feature due to the absence of C−C bonds in its molecular structure,12,13 is promising as an additive to inhibit soot emission of the large hydrocarbons, such as diesel or jet fuels, etc. Many previous studies proved its effectiveness in soot inhibition, such as the C2H4 premixed burner-stabilized stagnation (BSS) flame13−15 where soot formation rate decreased continuously with DME addition. However, there are still some studies reporting contrary effectiveness due to DME addition to ethylene. For instance, the studies on the C2H4/DME laminar coflow diffusion flame16−18 and opposedflow diffusion flame19,20 indicated a synergistic effect of DME addition on PAHs/soot productions; i.e., the in-flame PAHs/ soot concentrations were considerably enhanced upon a small DME addition, and then began to reduce with further addition, which was rather controversial to the soot-free feature of DME combustion. However, the underlying mechanism for the synergistic effect due to DME addition was still uncertain. McEnally and Pfefferle,17 Bennett et al.,18 and Yoon et al.20 suggested that, in the ethylene coflow flames with small DME addition, CH3 concentration was enhanced which accelerated the propargyl radical (C3H3) production rate through the C1 + C2 reactions, and further enhanced benzene formation by the C3H3 recombination reaction. However, in ref 16, the synergistic effects of DME addition on soot formation were mainly attributed to the enhancement in cyclization reactions of l-C6H6 and n-C6H7 leading to benzene production, rather than the C3H3 recombination reaction, although it was the most contributing pathway to benzene. Consequently, the soot formation mechanism of the C2H4 diffusion flame with DME addition merits a deeper study. This paper focuses on the PAHs/soot formation mechanism of the C2H4 opposed-flow diffusion flames with varying DME additions, from pure C2H4 to pure DME. C2H4 is the most important pyrolysis product generated in the cracking stage of long-chain hydrocarbon fuels, and additionally, it also plays a significant role in the soot inception and surface growth pathways. Hence, its soot formation mechanism with the synergistic effect due to DME addition was of fundamental

2. NUMERICAL APPROACH 2.1. Descriptions of the Opposed-Flow Diffusion Flame Parameters. The opposed-flow diffusion flame is schematically shown in Figure 1, where the C2H4/DME fuel mixture injects toward an opposing oxidizing jet stream. In the majority of opposed-flow diffusion flame conditions, the flame front is located on the fuel side since the stoichiometric

Figure 1. Schematic diagram of the opposed-flow diffusion flame. B

DOI: 10.1021/acs.iecr.9b00084 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Flame Parameters for Current Numerical Simulationsa fuel jet stream

flame parameter

oxidizer jet stream

case no.

χDME (%)

Tf (K)

Vf (cm/s)

1 2 3 4 5 6 7 8 9 10

0 5 10 15 20 30 40 60 80 100

300 300 300 300 300 300 300 300 300 300

25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0

composition 16% 16% 16% 16% 16% 16% 16% 16% 16% 16%

O2/84% O2/84% O2/84% O2/84% O2/84% O2/84% O2/84% O2/84% O2/84% O2/84%

Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar

Tox (K)

Vox (cm/s)

Hp (cm)

P (atm)

300 300 300 300 300 300 300 300 300 300

25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

χDME, DME addition; Tf, fuel jet temperature; Vf, fuel jet velocity; Tox, oxidizer jet temperature; Vox, oxidizer jet velocity; Hp, separation distance between the fuel and oxidizer jets; P, pressure. a

considered in the energy equation. Details about the governing equations of OPPDIF and sectional soot models can be readily referred to in refs 23 and 24, respectively. The following gives a brief description of the sectional soot model. The sectional aerosol dynamics model,21,22 with advantageous capabilities to reproduce the particle size evolution and particle size distribution function (PSDF) dynamics due to soot nucleation, surface growth, PAH condensation, particle coagulation, aggregation, and oxidation reactions, was utilized for the present simulations. In this method, the whole mass space of soot was divided into a finite number of discrete sections, with each section prescribed by a representative particle diameter. All soot aggregates were assigned to a specific section, depending on their diameter. Two number density equations for the soot aggregates and primary soot particles prescribed to each sectional band were solved to capture the evolution dynamic of fractal-like soot aggregates. In this paper, the nucleation, surface growth by H-abstraction− C2H2-addition (HACA) reaction7 and PAH condensation, particle coagulation and aggregation, and oxidation were considered for the dispersed-phase kinetic mechanism. The nucleation reaction is an irreversible self-combination reaction between two pyrene (A4) gaseous molecules, which provides the initial tiny solid particle for further growth. The PAH condensation process is a soot mass growth pathway by condensation of large PAH molecules (from A1 to A4) onto the soot surfaces by collision. The coagulation and aggregation processes are particle−particle collisions which lead to soot mass growth and number density reduction. The oxidation process accounts for soot consumption by surface reaction with the oxidizers O2 and OH. The HACA reaction, PAH condensation, and coagulation and aggregation are responsible for upgrading of particle mass from a lower section to higher sections. On the contrary, the oxidation process is responsible for downgrading of particle mass from a higher section to lower sections. Additionally, the participation of some gaseous species (C2H2, H, H2, PAHs, etc.) into the above dispersedphase reactions couples the gas-phase chemistry with the dispersed-phase kinetics. When employing detailed gas-phase and soot surface chemistry sets as well as detailed transport models, the current model can reproduce the high-fidelity soot evolution dynamics during the combustion process. Furthermore, it is noted that since all the flame cases involved herein belonged to the SF type, the importance of the soot oxidation reaction was rather ignorable.

coefficient of fuel is much less than that of oxygen. Consequently, to exclude soot oxidation and emphasize the soot formation kinetics during combustion, the oxygen jet stream was highly diluted (16% O2/84% Ar) to shift the flame front location to the oxidizer side; i.e., all flames involved in the current study belong to the SF type. DME addition to the C2H4 stream was varied in a wide range, from pure C2H4 to pure DME, to systematically study its impact on the soot formation dynamics. Here the DME addition (χDME) was defined as the volumetric percentage of fuel C2H4 that was substituted by DME. In addition, many studies reported that the response of soot loading was much more sensitive with respect to DME addition in the small addition range and, thereafter, became rather insensitive in the heavy addition range.17,18 As a result, the parameter space of DME addition in the lower range was refined. As shown in Table 1, 10 flames with varying DME additions were simulated in the present study. In the flames, 0, 5, 10, 15, 20, 30, 40, 60, 80, and 100% of the fuel C2H4 stream was substituted with DME, respectively, while keeping the other flame parameters including oxidizing temperature and composition as well as fuel/oxidizing injection velocities unchanged. In addition, the separation distance of the fuel jet off the oxidizing jet was 3.0 cm, and the flame pressure was 1.0 atm in the numerical model. 2.2. Computational Models and Numerical Strategies. 2.2.1. Governing Equations, Soot Models, and Chemical Kinetics. During the current simulations, the 1-D chemically reacting flow code OPPDIF23 was integrated with the sectional soot model24 to reproduce the high-fidelity physicochemical processes of great importance inside the C2H4/DME opposed-flow diffusion flame, including the gasand dispersed-phase chemical kinetics, gas/particle interactions, and the particle interactions in different sizes. The OPPDIF code23 was used to reproduce the gas-phase chemical process inside the flame, by solving the gas-phase governing equations including mass, momentum, energy, and species. The sectional soot model, which can be available in the Particle Size-Distribution Tracking feature of the CHEMKIN-pro package,24 was used to reproduce the dispersed-phase soot evolution dynamics by solving the equations in each soot section. As presented in the later section, the gas- and dispersed-phase equations were iterated in a coupled manner until convergence. Furthermore, the radiative heat loss due to participating species (CO2, H2O, CH4, and CO) and soot particles as expressed in the optically thin formulation25 was C


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overall section number is adequate enough to ensure independency of the simulation results with respect to the section resolution. Additionally, although the gas- and dispersed-phase equations are coupled with each other, these two problems were iterated separately to reduce the overall computational cost. In this method, the gas-phase equations were first iterated with the species source term due to dispersed-phase reactions kept constant, until convergence. Then the gas-phase solutions were frozen and the soot sectional model iterations began, the converged solution of which was fed back to update the species source term due to surface reactions. As such, the gas and soot equations were repeated alternately, until the gas species solution results was converged. Additionally, in case of divergence due to strong gas−particle coupling, especially under the heavy soot loading condition, a large relaxation factor was utilized toward the species source term due to surface reactions, to speed up the convergence.

The current gas-phase chemical mechanism, consisting of 151 species and 785 reactions, was derived from an integration of a detailed DME mechanism26 with the Appel PAH mechanism27 which contains the detailed PAH growth chemistry from benzene (A1) to pyrene (A4). The detailed soot surface kinetics model28 was employed to describe the dispersed-phase kinetics, which can account for soot growth via the nucleation, HACA reaction, surface condensation, and coagulation and aggregation, as well as soot consumption via oxidation with OH and O2. Two unsaturated surface sites on the aromatic molecules, i.e., H(se) and open(se), are involved in the dispersed-phase kinetics model. Here H(se) represents a surface-bonded hydrogen atom and open(se) is an open (or empty) surface site, which are important for either acetylene addition or PAH condensation, as indicated by the surface reactions (SR2)−(SR6). Our previous simulation studies validated the accuracy of current chemistry and soot models in predicting soot formation dynamics in the C2H4/DME jet diffusion flame29 and BSS premixed flame,13 over a broad range of parameters. 2.2.2. Numerical Strategies. The CHEMKIN packages30 were integrated into the current code to estimate the chemical reaction rate as well as temperature- and componentdependent thermodynamic and transport properties. The particle velocity is estimated as the summary velocity due to gas flow, particle diffusion, and thermophoresis. Here the particle diffusion and thermophoresis velocities are formulated as a function of the sectional particle number density and temperature gradient, respectively.24 Sandia’s TWOPNT package31 with Newton’s iteration method was used to solve reacting flows with chemical stiffness. The adaptive mesh method based on the gradient and curvature of temperature and major species profiles was employed to reduce the overall computational cost, while compromising less to accuracy loss. Forty-five sections with an increasing factor 2.0 for the particle mass growth were used to discretize the entire soot mass space; i.e., the number of bulk carbon atoms and thus particle mass/volume of the (j + 1)th-section representative particle is twice that of the jth section. Therefore, the binary logarithmic mass (or volume) of the jth-section representative particle increases linearly proportionally with the section number index j, as Figure 2 illustrates. It is verified that this

3. RESULTS AND DISCUSSION 3.1. Soot Formation Mechanism and PSDF Behavior of Pure C2H4 BSS Flame. In the following content, soot formation and PSDF behaviors of the pure C2H4 flame will be first presented, and then followed by the impact of DME addition to the fuel stream on soot formation mechanism. Figure 3a shows the flame-centerline distributions of temperature, velocity, and volume fractions of soot and some important species within the near-stagnation region in case 1 condition. Here C2H2 plays as the most important gaseous species for PAH formation and soot surface growth, say, by the HACA reaction. C3H3 is most crucial to the production of PAH species A1, which represents the first benzene ring structure that is generated during the combustion process. A4 is the precursor for soot nucleation which produces the initial tiny particle solids. It is consistent with the expectation that the opposed-flow diffusion flame front, which is defined at the stoichiometric location with peaking temperature, situated itself to the oxidizer side of the stagnation plane because of the rather high oxygen deficiency in the oxidizer inlet boundary (X2,O2 = 16%), as Figure 3a shows. This oxygen deficiency condition was conducive to soot formation, and additionally, this study indicated that soot particles were mainly formed within the narrow low-temperature (T = 1000−1700 K), fuelrich region between the stagnation and stoichiometric planes. As a consequence, the soot particles produced within this narrow fuel-rich region were convectively transported to the stagnation plane, and did not experience the oxidation process, so all flames involved in the present study belong to the SF type. Furthermore, Figure 3b illustrates the net reaction rate profiles of some representative hydrocarbon species (including C2H2, C3H3, A1, and A4) with importance in the incipient soot formation stage, which indicates that productions of these incipient hydrocarbons initiated from the position x = 1.45 cm or so, where it corresponded to the local equivalence ratio of 2.0. As a result, incipient soot particles and their mass growth initiated from the position x = 1.45 cm until the stagnation plane x = 1.20 cm, as Figure 3c displays. It is also noted that the lower sooting limit in terms of equivalence ratio 2.0 for ethylene coincides fairly well with the results reported in the literature.8 In the high-temperature zone (T > 1700 K) that is outside the sooting limit (x > 1.45 cm), the incipient hydrocarbon species were consumed, and thus, it was a soot-

Figure 2. Correlation of the binary logarithmic particle volume log2(Vj) with the section number index j. D


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Figure 3. (a) Axial profiles of temperature, velocity, and mole fractions for soot (f v) and some important species in case 1. (b) Axial profiles of reaction rates for some major species in case 1. (c) Axial profiles of soot number densities for some representative sections as well as the summary soot number density (Nt) in case 1. E


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Figure 4. (a) PSDF curves at some typical positions in case 1. Dj and Nj are the representative particle diameter and number density of section j, respectively. (b) Profiles of soot growth rate due to surface reactions (SR1)−(SR6) and concentrations of surface sites (open(se) and H(se)) along the flame centerline of case 1; some variables are divided by a large integer for better illustration.

coagulation reactions in the local reduced-temperature condition. After the 27th section, the soot number density became trivial, so the sectional number density profiles above 27 are not shown in Figure 3c. As a result, the summary number density, which is defined as the sum of sectional number densities in all sections (1−45), peaked at the peaking location of the 20th sectional profile (x = 1.31 cm). By contrast, the summary soot volume fraction (f v) reached its maximum at the stagnation plane (x = 1.2 cm), as Figure 3a shows. This was because the residence time in the nearstagnation area was fairly long, especially for the stagnation location where it is characterized by an infinite magnitude of residence time. Consequently, the mass and thus volume fraction of soot particles that were convectively transported toward the stagnation plane increased increasingly more quickly, peaking at the stagnation plane. Moreover, since the velocity direction reversed on the fuel side of the stagnation plane, soot particles that were produced within the sooting area could not be transported to the fuel-side area of the stagnation plane by convection; hence either the sectional or summary soot number density vanished abruptly at the fuel side of the stagnation plane. Our previous soot formation study using the premixed ethylene BSS flame configuration13 reported that, in the low-

free area, as reported in ref 20. Hence in the next section, the kinetics and structure behaviors within the sooting region x = 1.20−1.45 cm are particularly emphasized. With regard to the current SF-type flames, the primary tiny particles constructed by the nucleation reaction have 32 bulk carbon atoms in the particle core, so it should be assigned to section 6, while the sectional soot concentrations in sections 1−5 were ignorable due to the absence of soot oxidation which is responsible for particle size reduction. Figure 3c shows the axial distributions of sectional soot number density in the representative sections 6, 9, 12, 15, 18, 21, 24, and 27, as well as the summary number density distribution for case 1, respectively. It is well expected that the sixth sectional number density which was due to the nucleation reaction (i.e., the recombination reaction of two A4 species) reached its maximum at the peaking location of A4 mole fraction (x = 1.38 cm). After the production of sixth sectional tiny particles, the particle mass increased gradually along the convection flow direction. Thus, the peaking location of the sectional number density shifted toward the stagnation plane with the increase of sectional index. More importantly, the sectional number density increased more and more quickly from section 6 until section 21, and then turned to decrease significantly which was ascribed to the rapid soot aggregation and F


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Figure 5. Axial profiles of temperature (a) and velocity (b) along the flame centerline in cases 1−10, respectively.

temperature region near the stagnation plane, soot mass growth by the HACA reaction and PAHs condensation was significantly intensified due to the enhancements in unsaturated surface site concentrations (including open(se) and H(se)) in this area. Consequently, the plot of sectional number density versus axial position was bimodal, with one peaking within the flame zone, and the other peaking just at the stagnation location. However, with regard to the current ethylene opposed-flow diffusion flame, enhancements of HACA and PAH condensation rates in the low-temperature near-stagnation region disappeared, such that the sectional and summary number density profiles became unimodal, which peaked at the fuel-rich flame zone, as Figure 3c shows. Figure 4a shows the PSDF curves at some representative positions in the case 1 flame, where x = 1.45 cm corresponds to the initiation of soot chemistry, x = 1.37 cm corresponds to the peaking of nucleation rate, x = 1.32 cm is the location with maximum summary soot number density, and x = 1.20 cm designates the stagnation location. The crest of the PSDF curve can be regarded as the median or averaged size of particles. It is clearly shown that, in the incipient sooting area,

the particle sizes were distributed within a small range with a rather tiny median size. Then the particle number density grew considerably along the flow direction until x = 1.32 cm, and then turned to decrease because of soot aggregation and coagulation. However, the median particle size grew continuously until reaching the stagnation plane, which was ascribed to soot mass growth by HACA reaction, PAHs condensation, and soot aggregation and coagulation. Figure 4b displays the flame-centerline profiles for some important dispersed-phase reaction rates and unsaturated surface site concentrations (including open(se) and H(se)). The following are the surface reactions of controlling importance for soot mass growth. Reaction SR1 is the 2A4 ⇒ 32C(B) + 20H(se) + 28.7open(se)

(SR1)

nucleation reaction involving collision of two A4 molecules. Reaction SR2 is the HACA growth reaction which indeed is open(se) + C2H 2 ⇒ H(se) + 2C(B) + H

G

(SR2)


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Figure 6. Plots of summary soot number density (a), soot volume fraction (b), and average particle diameter versus separation distance off the stagnation plane x − xstag for cases 1−10, respectively. H


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Figure 7. Plots of mole fractions of A1 (a) and A4 (b) versus separation distance off the stagnation plane x − xstag for cases 1−10, respectively.

and it was prevailing in the low-temperature, fuel-rich condition. Figure 4b shows that the surfaces sites were generated within x = 1.2−1.3 cm. Consequently, although massive incipient soot particles were nucleated via (SR1) at x = 1.39 cm, the mass growth process (SR2)−(SR6) did not start until x = 1.3 cm. As shown in Figure 4b, although the concentration of H(se) site reached a maximum at the stagnation plane, the local PAHs concentration was rather depressed; hence the soot mass growth due to PAHs condensation did not peak in the near-stagnation region. Nevertheless, because the residence time near the stagnation was infinitesimal, the soot mass growth and thus soot volume fraction increased rapidly in the near-stagnation region. Additionally, it is noted from Figure 4b that the surface growth rate especially by PAHs condensation overwhelmed that of nucleation by a few orders of magnitude, and thus it played as the most important pathway for soot formation. Figure 4b also indicates that, in the near-stagnation zone, the nucleation process basically ceased while PAHs condensation and HACA reaction dominated. As a result, the nearstagnation PSDF curve corresponding to x = 1.20 cm as shown in Figure 4a was unimodal, which was exactly distinct

due to the absorption of C2H2 molecules by the unsaturated open sites over the particle surface. Reactions SR3−SR6 represent condensation of PAHs to the particle surface by hydrogen abstraction, which produces open sites on soot surface at the meantime. A1 + 6H(se) ⇒ 6C(B) + 6open(se) + 6H 2

(SR3)

A4 + 20H(se) ⇒ 16C(B) + 20open(se) + 15H 2

(SR6)

A2 + 16H(se) ⇒ 10C(B) + 16open(se) + 12H 2

(SR4)

A3 + 20H(se) ⇒ 14C(B) + 20open(se) + 15H 2

(SR5)

The weak dependences of (SR1) and (SR2) on temperature, as well as the irrelevance relationship of PAHs condensation reactions (SR3)−(SR6) with temperature, imply that soot mass growth rate relied mostly on species and surface site concentrations, rather than on temperature. By contrast, production of the surface sites was sensitive to temperature, I


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Figure 8. Plots of axial mole fractions of C2H2 (a), C3H3 (b), n-C4H5 (c), l-C6H6 (d), n-C6H7 (e), and A1− (f) versus separation distance off the stagnation plane x − xstag for cases 1−10, respectively.

from the case in premixed BSS flame9,10,13 where its nearstagnation PSDF curve was bimodal, with one crest due to revival of nucleation and the other due to surface growth by HACA reaction and PAHs condensation. 3.2. Impact of DME Addition to C2H4 on Soot and PAH Formation Mechanism in Opposed-Flow Diffusion Flame. Figure 5 displays the response of flame-centerline temperature and velocity profiles with respect to DME addition to C2H4. It is indicated that temperature and velocity curves and thus the stagnation plane were pushed toward the oxidizer side gradually, due to the slow increase of fuel jet momentum with DME addition to fuel stream. However, the temperature and velocity magnitudes were rather insensitive to DME addition in the entire range of DME addition, so the

chemical kinetics rather than temperature variation due to DME addition was mostly responsible for the response of soot formation process in C2H4 flames. Hence, in Figure 6, the flame-centerline summary number density, soot volume fraction, and average particle diameter are plotted versus the separation distance off the stagnation plane (x − xstag, where xstag designates the stagnation location). It is shown that, regardless of the DME addition value, the sooting zone was located within an equal thickness (about 0.25 cm) by the oxidizer side of the stagnation plane. Figure 6a indicates that DME addition had a synergistic effect on soot formation of the C2H4 opposed-flow diffusion flame; i.e., the summary soot number density increased considerably upon 5% DME addition, and then turned to J


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Figure 9. Plots of CH3 mole fraction versus separation distance off the stagnation plane x − xstag for cases 1−10, respectively.

Figure 10. Plots of A1 formation rate by (R234) (a), (R369)−(R371) (b), (R348) (c), and (R354) (d) versus separation distance off the stagnation plane x − xstag for cases 1−10, respectively.

the sooting area was a maximum at 5% DME addition, while in the near-stagnation region, the synergistic effect peaked at 20% addition. Figure 6c shows that the average diameter of emitted soot particles was rather insensitive to DME addition when it was below 20%. When DME addition exceeded 20%, the average diameter turned to decrease fairly quickly.

decrease continuously with further addition. At 20% DME addition, the summary number density decreased to equivalent values with that of the pure C2H4 flame. Thereafter, the summary number density dropped rather rapidly with further increment in DME addition. With respect to the soot volume fraction curve as shown in Figure 6b, the synergistic effect in K


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Figure 11. Plots of A4 formation rate by (R461) (a), (R467) (b), and (R465) (c) versus separation distance off the stagnation plane x − xstag for cases 1−10, respectively.

As Figure 5a shows, the maximum flame temperature decreased continuously with DME addition; hence, the thermal effect due to DME addition cannot adequately explain the synergistic effect of a small DME addition on the PAHs/ soot formation. The synergistic effect of DME addition to

C2H4 opposed-flow diffusion flames on soot emission was attributed to chemical kinetics associated with PAHs formation. As commented previously, soot emission of the C2H4 opposed-flow diffusion flame was mostly dependent on PAHs condensation rates onto the particle surface. The current L


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Figure 12. PSDF curves at x − xstag = 0.00 (a), 0.10 (b), 0.20 (c), and 0.25 cm (d) for cases 1−10, respectively. Dj and Nj are the presentative particle diameter and soot number density of section j, respectively. xstag designates the stagnation location.

A1 + OH = A1− + H 2O

PAH formation mechanism considers PAH growth from A1 to A4. A1 represents the first aromatic ring and A4 plays as the precursor of particle matter by nucleation, so these two typical PAHs were emphasized in the following kinetic analysis. Figure 7 shows the axial mole fraction profiles of A1 and A4 for cases 1−10, respectively. It is seen that the PAHs concentrations inside the pure C2H4 flame increased considerably upon 5% DME addition, and then turned to decrease with further addition. At 20% DME addition or so, the PAHs concentrations reduced to equivalent levels of the pure C2H4 flame. Thereafter, the PAHs concentrations began to decrease rapidly with increasing DME addition. The synergistic response of PAHs concentration agreed well with that of soot number density and volume fraction. The reaction rate and pathway analyses revealed that the gas-phase reactions (R234), (R296), (R348), (R354), (R369), (R370), and (R371) governed A1 construction, and (R461), (R465), and (R467) governed A4 formation. A1 formation pathways: C3H3 + C3H3 = A1

(R234)

n‐C4 H5 + C2H 2 = A1 + H

(R296)

l‐C6H6 + H = A1 + H

(R348)

n‐C6H 7 = A1 + H

(R354)

−

A1 + H = A1 + H 2

−

A1 + H + (M) = A1 + (M)

(R370) (R371)

A4 formation pathways: A3−4 + C2H 2 = A4 + H −

A4 + H = A4 + H 2 −

A4 + H = A4

(R461) (R465) (R467)

The cyclization reaction (R234) was the most important pathway for A1 production, followed by (R348), (R354), and (R296) in decreasing order of importance. Additionally, (R369) and (R370) are responsible for A1 destruction and (R371) is responsible for A1 formation; the summary reaction rate through these three pathways resulted in significant consumption of A1. Figure 8 displays the mole fraction profiles of species that participate in the controlling A1 formation chemistry. It also shows the synergistic response of these participating species with respect to DME addition, except for C2H2 which decreased monotonically with DME addition. Thus, the synergistic effect of DME addition was exactly ascribed to the synergistic response of PAHs chemistry due to DME addition. Figure 9 shows the CH3 profiles along the flame centerline with and without DME additions. It can be seen that, for the pure C2H4 case, the CH3 radical was produced in the hightemperature zone. When the fuel C2H4 was mixed with DME, the global CH3 radical concentration was enhanced, and in

(R369) M


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Figure 13. Distributions of the soot mass growth rates by nucleation (SR1), HACA reaction (SR2), and A1 (SR3) and A4 (SR6) surface condensations along flame centerline for cases 1−10, respectively.

addition, another enhanced fraction of CH3 radical was also formed in the low-temperature zone by DME decomposition reaction (CH3OCH3 = CH3 + CH3O), such that the CH3 profile with DME addition was bimodal. This demonstrates that DME could provide more CH3 radicals in the lowtemperature zone as compared to the pure C2H4 flame, which could enhance the C3H3 formation by C1 + C2 reactions. The enhanced C3H3 recombination reaction will increase the benzene ring formation (via reaction R234, Figure 10a) and thus PAH growth rates inside the flame, as suggested in ref 20. Moreover, as Figure 10b−d indicates, small DME addition also had a synergistic effect on the governing reactions for A1 formation (reactions R348, R354, and R369−R371). This was the underlying reason for the synergistic effect due to small DME addition on the PAHs/soot formation of the C2H4 opposed-flow diffusion flame. Figure 10 shows the rate profiles of the governing reactions responsible for A1 formation in cases 1−10, and Figure 11 illustrates those for the A4 formation. With regard to the most significant reactions responsible for A1 and A4 formations, i.e., (R234) and (R467), respectively, the irrelevance of their rate constants with temperature implies that A1 and A4 formation rates depend exactly on the reactant concentrations. Hence the synergistic response of participating species concentrations involved in PAHs formation was responsible for the synergistic response behavior of PAHs formation rates. It is clearly shown in Figure 11 that the synergistic response with respect to DME addition still holds for the A4 formation rates via its controlling

pathways. This is the underlying physics for the synergistic effect of DME addition on soot emission. Besides, our previous study using the BSS flame configuration13 showed that DME addition to the C2H4 fuel stream resulted in continuous reduction in soot emission, which was rather different from the current opposed-flow diffusion case. It was suggested that, in the C2H4 BSS premixed flame with or without DME addition, the first benzene structure A1 was mainly constructed by the C3H3-recombination reaction (R234); contributions of the cyclization reactions of C4 + C2 species (R296) and l-C6H6 (R348) and n-C6H7 (R354) were almost trivial. Since C3H3 concentration decreased monotonically with DME addition, PAH and thus soot formations in the C2H4 BSS premixed flame decreased continuously with DME addition. This indicated that the kinetics of PAHs/soot formation and the synergistic effect on C2H4 flames due to DME addition depended on the flame configuration. 3.3. Effect of DME Addition to C2H4 on the PSDF Behavior of the Opposed-Flow Diffusion Flame. As shown in Figure 6a, the sooting chemistry area with or without DME addition was always located within a constant thickness of about 0.25 cm by the oxidizer side of the stagnation plane. Figure 5 indicates that the profiles of temperature, velocity, and thus local strain rate in all cases were basically identical with each other. Hence the difference in soot formation behavior among cases 1−10 was mainly due to the changes of soot chemistry with various DME additions. Figure 12 displays the N


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Figure 14. Concentrations of surface sites open(se) (a) and H(se) (b) for cases 1−10, respectively.

As shown in Figure 13, within the sooting area (0 < x − xstag ≤ 0.25 cm), soot mass growth rate via nucleation, HACA reaction, and PAHs condensation reached the maximum upon 5% DME addition, and then began to decrease with further increasing addition. At 20% DME addition, the soot mass growth rate dropped to the equivalent level with the pure C2H4 flame condition. Thereafter, the soot growth rate turned to decrease rapidly. This was the underlying physics for the synergistic effect due to DME addition on soot formation. However, at the stagnation plane, it is shown in Figure 13b−d that the synergistic effect due to DME addition on HACA growth and condensation was maximum at 60% DME addition. It is analyzed that the HACA reaction and PAHs condensation rates depend rarely on temperature, but on concentrations of the reactants such as C2H2, PAHs, and unsaturated surface sites open(se) and H(se), as indicated in the surface reactions (SR2)−(SR6). As Figure 14a shows, the surface site concentration of open(se) at the stagnation position reached a maximum at 60% DME addition, so the near-stagnation HACA growth and condensation rates peaked at 60% DME addition. In the engineering viewpoint, it is concluded that, with respect to the nonpremixed combustion systems fueled by

comparison of PSDF curves at x − xstag = 0.0, 0.1, 0.2, and 0.25 cm among cases 1−10. x − xstag = 0.25 cm represents the location of soot incipience, and 0.2 and 0.1 cm are the locations of soot growth and coagulation. x − xstag = 0.0 cm is the stagnation plane with peaking soot volume fraction. Figure 13 illustrates distributions of soot mass growth rate by the dispersed-phase reactions (SR1), (SR2), (SR3), and (SR6) for cases 1−10, respectively. It is agreed with the expectation that the median particle diameter grew gradually when they were convectively transported toward the stagnation position, as Figure 12 shows. Furthermore, it is more interesting to note that the synergistic effect of DME addition on soot formation was enhanced along the convection flow direction. More specifically, it did not exhibit the synergistic effect with DME addition at the soot incipience location x − xstag = 0.25 cm. Then the synergistic effect appeared and then was enhanced from x − xstag = 0.2 cm to x − xstag = 0.1 cm, where the crest PSDF peaked at 5% DME addition. As shown in Figure 12a, the synergistic effect due to DME addition reached the maximum at the stagnation plane; the crest of PSDF curves increased until 20% DME addition, and then began to decrease. O



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C2H4/DME blends, a small mass of DME addition (below 20%) to C2H4 will probably lead to enhancement of the soot loading. At least 20% DME addition is required to effectively reduce the soot density and particle diameter. Furthermore, the soot emission dropped rapidly with increasing DME addition when its blending ratio exceeded 20%.

REFERENCES

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4. CONCLUSIONS This paper studies soot formation mechanism of the C2H4/ DME opposed-flow diffusion flames over a wide range of DME additions from pure C2H4 to pure DME, by a numerical approach with detailed gas-phase and dispersed-phase chemistries and transport models. The main findings include the following: 1. The summary soot volume fraction (f v) reached its maximum at the stagnation plane, because of the infinite magnitude of residence time at this location. However, either the sectional or summary soot number density vanished abruptly at the fuel side of the stagnation plane due to the impact of flow condition. 2. In the premixed BSS flames, soot nucleation and mass growth by HACA reaction and PAHs condensation were significantly enhanced in the low-temperature near-stagnation region, with a bimodal-shape PSDF curve. However, in the current opposed-flow diffusion flames, enhancements of nucleation, HACA reaction, and PAH condensation rates in the near-stagnation region disappeared, such that the local PSDF curve became unimodal. 3. DME addition had a synergistic effect on soot formation of the C2H4 opposed-flow diffusion flame. Either the summary soot number density, volume fraction, or average particle diameter increased considerably upon 5% DME addition, and then turned to decrease with further addition. They decreased to equivalent values with that of the pure C2H4 flame at 20% DME addition. The synergistic response of PAHs formation chemistry with respect to DME addition was responsible for this synergistic effect. 4. The synergistic effect of DME addition on soot formation was enhanced along the convection flow direction. Within the sooting area, the synergistic effect was maximum at 5% DME addition, while in the near-stagnation region, the synergistic effect was maximum at 60% DME addition since the concentration of the open surface site in this area reached a maximum at 60% addition. From the engineering viewpoint, with respect to the nonpremixed combustion systems fueled by C2H4/DME blends, at least 20% DME addition is required to effectively reduce the soot number density and particle diameter.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 023-65102475. Fax: +86 023-65102475. E-mail: [email protected] (Y.K.). *E-mail: [email protected] (Z.W.). ORCID

Yinhu Kang: 0000-0002-1856-570X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was supported by the National Natural Science Foundation of China (Grant No. 51706027). P


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Effect of Dimethyl Ether Addition on Soot Formation Dynamics of

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