Effects of oxygen-enrichment and fuel unsaturation on soot and NOx emissions in ethylene, propane, and propene flames

Combustion and Flame 187 (2018) 217–229

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Effects of oxygen-enrichment and fuel unsaturation on soot and NOx emissions in ethylene, propane, and propene flames Krishna C. Kalvakala a, Viswanath R. Katta b, Suresh K. Aggarwal a,∗ a b

Department of Mechanical & Industrial Engineering, UIC, Chicago, IL, USA Innovative Scientific Solutions, Inc., Dayton, OH 45440, USA

a r t i c l e

i n f o

Article history: Received 7 July 2017 Revised 29 July 2017 Accepted 13 September 2017

Keywords: Oxygenation Fuel unsaturation NOx PAH Soot emissions

a b s t r a c t We have performed a computational study on the effects of oxygen enrichment1 and fuel unsaturation on the flame structure, PAHs, soot, and NOx emissions. Counterflow flames burning ethylene, propane, and propene are simulated with CHEMKIN-Pro, using a validated mechanism with 197 species and around 50 0 0 reactions. The stoichiometric mixture fraction (ζ st ) is varied by simultaneously using O2 -enriched airstream and N2 -diluted fuel stream such that the adiabatic flame temperature is nearly constant. Dominant reaction paths are analyzed to examine the relative roles of hydrodynamics and changes in flame structure on PAHs and soot emissions. As ζ st is increased, results indicate a significant reduction in acetylene and PAHs formation, and with additional soot oxidation in the post flame region, it leads to a nearly non-sooting flame. The drastic reduction in PAHs and soot formation can be attributed to both the hydrodynamic and the flame structure effects. At moderate oxygenation levels, changes in flame structure seems to play a more prominent role, while at higher oxygenation levels, the hydrodynamic effect seems to be more important. With the increase in ζ st , the O, OH, and H radical pool is enhanced, and, consequently, the intermediate species (propargyl, allene, and propyne) are reduced to smaller hydrocarbons, decreasing the formation of PAHs and soot. With further increase in ζ st , the flame location shifts from oxidizer to fuel side, and, consequently, PAH species and soot get oxidized in the oxygen rich region, leading to nearly soot free flames. However, as ζ st is increased, NO emissions increase monotonically. At low ζ st values, the prompt route contributes more to NO formation, while at high ζ st values, the thermal route contributes more. The rate of production analysis indicates that the presence of double bond promotes reactions which produce higher amounts of allyl and propargyl species, and thus higher amounts of soot precursors; benzene and pyrene. Consequently, propene and ethylene flames produce significantly higher amount of soot compared to propane flames. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction PM (particulate matter) emissions from the combustion of fossil fuels represent a major health and environment concern. Fine particles, especially 10 μm and smaller (PM10) get trapped in the respiratory tract, while finer particles (PM2.5) can penetrate deep into lungs and subsequently into blood stream, causing serious health problems, including cardiovascular and lung diseases [1,2]. Such particles are also known to contribute to global warming [1] and adversely affect our aquatic and terrestrial ecosystems. In particular, as reported by Hansen and Nazarenko, the efficacy of soot cli-

∗

Corresponding author. E-mail address: [email protected] (S.K. Aggarwal). 1 In this paper, the term oxygen enrichment (or oxygenation) is used to indicate simultaneous O2 enhancement of the oxidizer stream and N2 dilution of the fuel stream.

mate forcing is twice that of CO2 [2]. As a consequence, the emission regulations have become increasingly stringent over the years, and soot production in flames continues to be an important area of research. Soot formation represents a complex phenomenon involving many physical and chemical processes [3]. Various processes include pyrolysis and formation of smaller hydrocarbons, such as acetylene and propargyl, followed by the formation and growth of polycyclic aromatic hydrocarbon (PAH) species, nucleation, growth of particle size and mass through surface reactions, PAH condensation, particle agglomeration and coagulation, and finally soot oxidation. There has been extensive research dealing with these processes. This research has identified a number of important parameters affecting soot emission. Essentially, the amount of soot produced is found to be strongly influenced by the fuel structure [4–7], flame temperature and structure [8–11], residence time [4,8,12], and compositions of fuel and oxidizer streams [13–19].

https://doi.org/10.1016/j.combustflame.2017.09.015 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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Many fundamental studies have considered laminar nonpremixed and partially premixed flames in a counterflow configuration, which provides nearly 1-D flow field, and thus facilitates wellresolved measurements and simulations. Results from such studies are also relevant to turbulent flames in the context of laminar flamelet modeling. Previous research on soot emissions has also identified new combustion technologies for reducing NOx and soot emissions from various combustion systems. These include high temperature air combustion (HiTAC) [20], flameless combustion [21,22], mild combustion [23], low temperature combustion [24] and oxygenenhanced combustion [25]. One effective approach of employing oxygen-enhanced combustion (OEC) for soot reduction in nonpremixed flames is to use the oxygen enrichment of air stream along with the nitrogen dilution of fuel stream. As discussed by Du and Axelbaum [13] and Skeen et al. [14], this alters the stoichiometric mixture fraction (ζ st ), while keeping the flame temperature nearly the same, and can significantly affect on the flame structure and soot formation. Skeen et al. [14] demonstrated the effectiveness of this approach and reported a dramatic reduction in the concentration of PAH species (i.e., benzene and phenyl) in ethylene counterflow flames. They postulated that the formation of soot precursors is inhibited due to the prominent changes in the flame structure, which makes the H abstraction reactions very active in soot forming regions, thus decreasing the rate of production of soot precursors. The literature review indicates that most previous studies dealing with nonpremxied flames have considered either fuel stream dilution [8,9,11,13–15] or oxidizer stream dilution/enrichment [11,18,25,26] on PAH and soot emissions. As noted earlier, Skeen et al. [14] used simultaneously the oxygen enrichment of air stream and nitrogen dilution of fuel stream, and examined its effect on the formation of PAH species. The present study utilizes a similar approach for modifying the flame structure, and characterizes its effect on NOx , PAH and soot emissions. The paper has two major objectives. One is to examine comprehensively the effect of altering ζ st on NOx , PAH and soot emissions in nonpremixed flames. Previous researchers have demonstrated that by sufficiently increasing ζ st , one can obtain nearly soot-free diffusion flames. They attributed this soot-inhibiting phenomenon to either hydrodynamics [27,28] or flame structure effect [14,29]. The present study examines both of these effects by analyzing the detailed flame structure and the dominant reaction paths at different ζ st . Moreover, previous computational studies have considered the effect of ζ st on the formation of PAH species, while the present study examines its effect on NOx , PAH and soot emissions. In the context of hydrodynamic effect, it is important to consider soot emission, since soot oxidation plays a key role. Another objective is to characterize the effect of unsaturation on NOx , PAH and soot formation. Thus simulations are performed for counterflow flames burning small hydrocarbon fuels, namely, ethylene, propane and propene. For all three fuels, major routes for the formation of soot precursors are also identified. Ethylene is chosen since it has been identified as an important species in the combustion of larger hydrocarbons, and also plays a significant role in the PAH and soot formation. In addition, propane and propene are used to characterize the effect of unsaturation in smaller hydrocarbons on emissions. Most previous studies dealing with fuel unsaturation have considered larger hydrocarbons [5–7]. While there has been some work concerning smaller biodiesel surrogates, such as methyl butanoate, the focus has been mainly on the development of kinetic models [30–32]. 2. Numerical model Simulations are performed using OPPDIF in the CHEMKIN-Pro package [33] in an opposed-jet configuration, shown schematically

Fig. 1. Schematic of counterflow configuration used for establishing diffusion flames.

in Fig. 1. It consists of two opposing jets that are being issued from two coaxial nozzles. The distance between the nozzles is 20 mm, and the inlet temperatures of both fuel and air jets are maintained at 300 K. A diffusion flame is established by specifying a global strain rate [34], expressed as:



aG =

2vo 1+ L

v f √ρ f √ v o ρo



(1)

Here L is the distance between the nozzles, v and ρ the inlet velocity and density, respectively, and the subscripts f and o represent fuel and oxidizer streams, respectively. A global strain rate of aG = 100 s−1 is used in the present study. The velocities at the fuel and oxidizer inlets are specified by equating the momenta of the two streams for a given aG . The grid independence was achieved by using successively finer grids and varying the GRAD and CURV parameters so that the solution is nearly independent of the grid system. Detailed models are employed to compute the flame structure, NOx and soot emissions. The soot processes considered include nucleation, surface growth, coagulation, and oxidation. The soot model is combined with a detailed fuel oxidation and NOx chemistry model [17] involving 198 species and 4932 reactions, with coronene (C24 H12 ) being the largest species. The NOx mechanism includes the thermal NO, prompt NO, intermediate N2 O, and intermediate NNH routes. Details of these routes including relevant reactions have been provided in a previous investigation [44]. Soot modeling has been particularly challenging as it involves a number of complex physical and chemical processes. However, significant progress has been reported in this regard [35–39]. Soot formation process in the current study involves formation of nuclei through nucleation and polymerization of a primary particle, which subsequently grows through coagulation and surface reactions, while also undergoing oxidation. The nucleation reaction which defines the particle size and surface coverage is considered to be irreversible, and based on pyrene (C16 H10 ) recombination reaction, which forms one soot nucleus with 32C atoms, as indicated by Eq. (2). The nuclei formed interact with each other through coagulation as well as with gaseous species on its surface, whose dynamics is modeled by solving particle size distribution functions (PSDFs) using method of moments approach developed by Frenklach et al. [39,40]. This approach is computationally economical, while modeling soot processes in sufficient detail, and thus providing overall/average soot properties, such as average particle size, soot volume fraction and number density, using the particle size moments. Further the surface growth includes coagulation and surface reactions, which are defined through the HACA mechanism [38]. The soot oxidation reaction is represented by Eq. (7), while

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the soot aggregation is not considered in the present study. The soot model is summarized in the following equations: Nucleation:

2C16 H10 ↔ 32C + 20Csoot –H + 28.75Csoot .

(2)

HACA mechanism:

H + Csoot –H ↔ Csoot . + H2

(3)

OH + Csoot –H ↔ Csoot . + H2 O

(4)

Csoot . + H → Csoot –H

(5)

Csoot + C2 H2 → Csoot –H + 2C + H

(6)

.

Oxidation:

OH + Csoot . + C ↔ Csoot –H + CO

(7)

The fuel oxidation model has been extensively validated using species measurements (including benzene) in n-heptane partially premixed flames [4,17]. Additional validation for ethylene partially premixed flames is presented in the next section. While this mechanism was initially built for n-heptane, the additional validation further supports its use for smaller fuels as well. Moreover, results reported by Skeen et al. [14] on ethylene diffusion flames show similar trends for the formation of propargyl and benzene. Validation of the soot model is also discussed in the next section. Regarding soot modeling, it is important to recognize that this is a topic of active research, and new models for various soot formation and oxidation processes are being reported [41,42]. Therefore, while we provide validation for the soot model, results concerning soot volume fraction, etc. may be considered qualitative. 3. Results and discussion We provide herein additional validation using the measurements of Gomez et al. [9] in counterflow ethylene diffusion and partially premixed flames. For these experiments, the global strain rate was 52.6 s−1 , and the separation distance between the nozzles was 7.8 mm. The fuel stream was diluted with 67%N2 , and oxidizer stream contained 23%O2 and 77%N2 by volume. Figure 2 presents a comparison of the predicted and measured temperature and several relevant species profiles for both diffusion and partially premixed flames. Results indicate fairly good agreement between predictions and measurements. The computational model is able to reproduce the measured flame structures including the acetylene and benzene profiles, which are important for predicting emission characteristics. Some differences should also be noted, especially with respect to temperature profiles, indicating the computed flame to be narrower than the measured one. Similar discrepancies between predictions and measurement have been observed in previous studies; see, for example, Gomez et al. [9]. We had also compared (not included) predictions of the current mechanism with three other known mechanisms, and found the comparison satisfactory. The soot model is further validated using experimental data of Hwang and Chung [8] for counterflow ethylene/air diffusion flames. Figure 3 compares the predicted and measured soot volume fraction profiles for two flames with 20% and 28% oxygen (by volume) in the air stream. For these flames, velocities at the two boundaries are 19.5 cm/s each. The comparison again indicates a fairly good agreement between predictions and measurements, except that the predicted soot volume fraction values are lower compared to experiments.

219

We now discuss the effect of oxygenation, i.e., varying the stoichiometric mixture fraction (ζ st ), on the flame structure and PAH, soot and NO formation for some representative cases. As mentioned earlier, this is implemented by using an oxygen enriched air stream along with a nitrogen diluted fuel stream, i.e., by removing certain amount of N2 from the air stream and adding it to the fuel stream. Five cases are considered with the base case containing pure fuel in the fuel stream and standard air in the oxidizer stream, and the last case with pure oxygen in the oxidizer stream. Mixture fraction in general is defined as the elemental mass fraction that originated from the fuel stream, and can be written as:

Z (F ) =



NC × Yi ×

 M  C

Mi

+

 M   h Nh × Yi × Mi

(8)

Here NC and Nh are, respectively, the number of carbon and hydrogen atoms in species i, Yi the mass fraction of species i, M the molecular weight, and the sum is over all the species. Thus the elemental mass fraction here is based on C and H atomic species. In order to accommodate a broader set of conditions, Z is normalized as:

ζ=

[Z (F ) − Z (F , l )] [Z (F , r ) − Z (F , l )]

(9)

where subscripts r and l correspond to fuel and oxidizer streams, respectively. Since for calculating the stoichiometric mixture fraction (ζ st ), we need only the mass fractions of fuel and oxidizer under stoichiometric proportions, ζ st can also be calculated using [13]:



ζst =

1+



Y f Mo ϑ o YoM f ϑ f

−1

(10)

Here Yf and Yo are the mass fractions of fuel and oxygen at the respective boundaries, and ν f and ν o are the stoichiometric coefficients for fuel and oxygen. Various parameters including boundary values for the five flames and three fuels (ethylene, propane and propene) are listed in Tables 1a–1c. It should be noted that for a given fuel, the adiabatic flame temperatures for all the five flames calculated using CHEMKIN Pro were the same. Their values under constant pressure were 2369 K, 2266 K and 2334 K for ethylene, propane and propene, respectively. Figure 4a and b presents the flame structures for two propene flames with ζ st = 0.064 and 0.423, respectively. As indicated in Table 1, ζ st = 0.423 corresponds to a flame with 50% of N2 (compared to the base case of ζ st = 0.0640) removed from the oxidizer stream and added to the fuel stream. For each case, the flame structure is shown in terms of profiles of temperature and several important species including soot precursors (acetylene, benzene and pyrene). The corresponding flame structures in mixture fraction coordinate are presented in Fig. 4c and d. As expected, for the case with ζ st = 0.0640, the flame is located on the oxidizer side, and the peaks in soot precursors’ profiles occur in the fuel pyrolysis zone. An important take away from these figures is that with oxygenation, the flame shifts towards the fuel side, and the region of formation of soot precursors is also located on the fuel side with significantly reduced amounts of soot precursors being formed. For instance, the peak pyrene mole fraction decreases from about 200 ppm to 10 ppm. 3.1. Effect on flame temperature Figure 5 presents the peak flame temperature and peak acetylene mole fraction for the five flames corresponding to different stoichiometric mixture fractions (ζ st ) and the three fuels – ethylene, propane and propene. Results indicate small variation in peak flame temperatures for a given fuel, although the adiabatic flame

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Fig. 2. Predicted (solid lines) and measured profiles of temperature and several relevant species including benzene in counterflow ethylene/air diffusion flame (a) and partially premixed flame with ф = 5 (b).

Table 1a Input parameters: species mole fractions and axial velocities for ethylene/air counterflow diffusion flames at a strain rate of 100 s−1 . Cases – Percentage of N2 removed from air 0% 25% 50% 75% 100%

Fuel stream

Air stream

C2 H4 mole fraction

N2 mole fraction

Velocity (cm/s)

Velocity (cm/s)

O2 mole fraction

N2 mole fraction

1 0.262 0.151 0.106 0.081

0.0 0.738 0.849 0.894 0.919

50.745 50.926 51.225 51.808 53.452

50 50 50 50 50

0.21 0.262 0.347 0.515 1.0

0.79 0.738 0.653 0.485 0.0

Stoichiometric mixture fraction 0.064 0.243 0.423 0.602 0.782

Table 1b Input parameters: species mole fractions and axial velocities for propane/air counterflow diffusion flames at a strain rate of 100 s−1 . Cases – Percentage of N2 removed from air 0% 25% 50% 75% 100%

Fuel stream

Air stream

C3 H8 mole fraction

N2 mole fraction

Velocity (cm/s)

Velocity (cm/s)

O2 mole fraction

N2 mole fraction

1 0.175 0.096 0.066 0.051

0.0 0.825 0.904 0.934 0.949

40.480 48.551 49.873 50.855 52.697

50 50 50 50 50

0.21 0.262 0.347 0.515 1.0

0.79 0.738 0.653 0.485 0.0

Stoichiometric mixture fraction 0.0602 0.2404 0.4206 0.6008 0.7809

Table 1c Input parameters: species mole fractions and axial velocities for propene/air counterflow diffusion flames at a strain rate of 100 s−1 . Cases – Percentage of N2 removed from air 0% 25% 50% 75% 100%

Fuel stream

Air stream

C3 H6 mole fraction

N2 mole fraction

Velocity (cm/s)

Velocity (cm/s)

O2 mole fraction

N2 mole fraction

1 0.191 0.106 0.073 0.056

0.0 0.809 0.894 0.927 0.944

41.433 48.654 48.923 50.887 52.722

50 50 50 50 50

0.21 0.262 0.347 0.515 1.0

0.79 0.738 0.653 0.485 0.0

Stoichiometric mixture fraction 0.064 0.243 0.423 0.602 0.782

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221

Fig. 5. Peak flame temperature (solid lines) and peak C2 H2 mole fraction (dotted lines) versus ζ st for three fuels. Fig. 3. Comparison of measured and predicted profiles of soot volume fraction in counterflow ethylene/air diffusion flames with air containing 20% and 28% oxygen.

temperatures from equilibrium calculations were found to be the same for the five cases. This difference can be attributed to N2 availability at the flame location. Although the total amount of N2 in the system was constant, its availability at the flame was different for each case, indicating the effect of transport processes. As expected, increased amount of N2 at the flame location implies lower temperature. The variations in the peak flame temperature can also be attributed to the transport of other species, which participate in establishing the diffusion flame at a particular location. More significant differences in the peak flame temperatures are due to the fuel effect. As indicated in Fig. 5, ethylene and propane flames have the highest and lowest flame temperatures, and the

difference is mainly related to the different enthalpies of formation of fuels. Note that it is important to highlight the differences in peak flame temperature, which plays a critical role in the formation of NO, PAH species, and soot. Another important observation from Fig. 5 is that as ζ st is increased, it leads to a significant decrease in the peak C2 H2 mole fraction. With regards to the fuel effect, the ethylene flame has the highest C2 H2 value, followed by propene and propane flames. The effect of fuel unsaturation is further discussed through path analysis in the next section. Before proceeding to examine the chemical effect of oxygenation on the formation of soot precursors, we first discuss the effect of residence time. Fu et al. [4] and Wang and Chung [12] have examined the effect of strain rate on the formation of soot precursors

Fig. 4. Computed structures of two propene flames for ζ st = 0.064 (a and c) and 0.423 (b and d). Profiles of temperature and mole fractions of several important species are shown in both spatial and mixture fraction coordinates.

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Fig. 6. Profiles of axial velocity (solid lines) and ethylene mole fraction (dotted lines) for three ethylene/air diffusion flames; percentage in the legend indicates the percentage of nitrogen transferred from air to fuel stream.

and soot, and observed that the increase in strain rate (i.e., shorter residence time) decreases the amount of soot formed. Although the global strain rate is fixed in the present study, the local residence time varies along the axial location due to the variation in axial velocity. Figure 6 shows the axial velocity and ethylene mole fraction profiles for three ethylene/air diffusion flames corresponding to different oxygenation levels. With increase in oxygenation, the fuel decomposition occurs in an increasingly higher velocity region, which implies lower residence time for decomposition. In contrast, the peak velocity decreases on the oxidizer side, implying increasingly higher residence time in the soot oxidation zone. As a consequence, the fuel pyrolysis is expected to be increasingly incomplete resulting in more unburned hydrocarbons due to lower residence time, while oxidation is increased due to the increased residence time on the oxidizer side. However, there is significant decrease in the pyrolysis zone with the increase in oxygenation, as indicated by the heat release rate profiles (not shown). Moreover, a more prominent effect of oxygenation is the higher concentration of O and OH radicals along with lower velocities on the oxidizer side, resulting in significantly increased oxidation of PAHs and soot. 3.2. Formation of soot precursors: reaction path analysis Figures 7 and 8 provide a summary of the path analysis for the formation of acetylene and benzene in ethylene, propane, and propene flames. The analysis is based on the reaction rates integrated over the axial length. Results are shown for the base case corresponding to undiluted fuel stream, with oxidizer stream containing 21% O2 and 79% N2 . As indicated in Fig. 7, ethylene mostly decomposes into vinyl (94%), which subsequently forms acetylene through H abstraction reaction. This is the most significant path for acetylene formation. The benzene formation in ethylene flames involves two paths, one through the pyrolysis reaction and the other through recombination of propargyl. The first path is more dominant for the pure fuel case, and involves reaction between ethylene and diacetylene (C4 H2 ), with the latter being formed through reaction between acetylene and C2 H radicals. The second significant path is through the recombination of propargyl, which is produced through several reactions, with reaction between acetylene and methylene (CH2 and CH2 singlet) species being more prominent. Figure 8 shows the major routes contributing to the formation of acetylene and benzene in propane and propene flames. The analysis indicated that for both fuels, the recombination reaction of propargyl (C3 H3 ) represents the most contributing reaction for

Fig. 7. Key acetylene and benzene formation routes in ethylene diffusion flame established at a strain rate of 100 s−1 . The air stream contains 21% O2 and 79% N2 . Dashed line indicates multiple routes.

benzene formation while acetylene is mainly formed through H abstraction of vinyl (C2 H3 ). In propene flame, fuel undergoes pyrolysis mostly to form stable allyl (C3 H5 ) radicals (70%); here C–H bond of the γ carbon is broken as it has the lowest bond dissociation energy due to the presence of double bond. Smaller amounts of n-propyl (15%), iso-propyl (10%), CH2 CCH3 (5%) are also formed through H addition. In contrast, in propane flame, the fuel reduces to form ethyl (40%), n-propyl (30%), iso-propyl (15%) and propene (15%) through pyrolysis reactions. Relatively higher amount of ethyl radicals formed is due to the lower C–C bond energy compared to C–H bond. Ethylene, which plays a significant role in the formation of acetylene, is formed only through the reduction of n-propyl in propene flame. In contrast, along with the reduction of n-propyl and iso-propyl, the reduction of ethyl radicals also contributes to the formation of ethylene, thus resulting in higher amount of ethylene in propane flame. While the amount of ethylene is higher in propane flames, the amount of acetylene formed is higher in propene flames. This is due to the fact that propene has additional routes contributing to the formation of acetylene. A portion of allyl formed in propene flame contributes to the formation of vinyl (reaction (R1)), which further reduces to form acetylene. Other reactions to form acetylene in propene flames are (R2) and (R3) through propyne and C3 H5 (structure being CH2 CCH3 ), which have no significant role in propane flames. Due to these additional paths, higher amount of acetylene is formed in propene flames than in propane flames, as indicated in Fig. 5,

Allyl + H ↔ Vinyl + CH3

(R1)

Propyne + H ↔ C2 H2 + CH3

(R2)

CH2 CCH3 ↔ C2 H2 + CH3

(R3)

3.3. Benzene formation in ethylene, propene and propane flames As mentioned earlier, propargyl recombination represents the dominant route for benzene formation in both propene and propane flames. In propene flames, propene undergoes pyrolysis to forms allyl radicals, which through various reactions produce allene and propyne isomers (reactions (R4)–(R7)). These isomers form propargyl through H abstraction reactions at intermediate

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223

Fig. 8. Key acetylene and benzene formation routes in propane and propene diffusion flames established at a strain rate of 100 s−1 . The air stream contains 21% O2 and 79% N2 . Line thickness indicates the relative contribution of each path.

temperatures (10 0 0–160 0 K). Also, for the base case (no dilution), (R5) produces small amounts of propene along with allene. Other minor path (which plays a more significant role in ethylene flames) for the production of propargyl is via reaction between acetylene and methylene. Propargyl thus formed through various routes, undergoes recombination to form benzene. While paths for benzene formation are similar in propane and propene flames, the amount of benzene formed is much higher in propene flames than that in propane flames. The difference is that propene is an intermediate species in propane flames, whereas it is the fuel itself in propene flames. In propane flames, propene is formed through two routes – one through the pyrolysis of propane and other through reduction of iso-propyl. It is also important to note that, about 30% of propene produced is converted back into n-propyl, which leads to the production of ethylene but reduces the amount of propene available for conversion into PAHs. Another reason for the increased benzene formation in propene flames is due to higher amounts of allene, propyne, and allyl, which undergo reactions (R8) and (R9) to form benzene directly (without forming propargyl). This path contributes about 20% of the total benzene produced. Hence we observe significantly higher amounts of benzene in propene flames compared to propane and ethylene flames.

H + Allyl → H2 + Allene

(R4)

2Allyl → Allene + C3 H6

(R5)

Allyl + M ↔ Allene + H

(R6)

Allene ↔ Propyne

(R7)

as the oxygenation level or ζ st is increased, the amount of acetylene formed decreases monotonically. The rate of production (ROP) analysis indicated that the increased availability of O2 enhances the O and OH radical pool, which converts acetylene into other compounds – HCCO, C2 H, C2 H3 O. Another important observation from Fig. 5 is that the amount of C2 H2 produced is the highest in ethylene flames. This is due to the fact that ethylene undergoes pyrolysis to form either vinyl (C2 H3 ) or C2 H2 . On the other hand, in propane and propene flames, C2 H2 is mostly formed from vinyl through H abstraction, while vinyl is produced mainly from ethylene, which is an intermediate species produced only from few reactions. Thus, due to the limited availability of ethylene and vinyl, the amount of acetylene formed in propane and propene flames is significantly less compared to that in ethylene flame. Further in ethylene flames as the oxygenation level is increased, the decomposition of ethylene changes somewhat which lowers acetylene formation. For instance, for the 100% dilution case, ethylene produces 85% vinyl, 10% ethyl, and 2% acetylene, with the rest being HCO and CH3 . However, for the no dilution case, as mentioned above, ethylene decomposes into vinyl accounting for 94% of vinyl formation. This is one reason we see lower acetylene but higher ethyl (which decompose into CH3 radicals at higher dilution levels). Moreover, as discussed earlier, the residence time also has an effect decreasing fuel pyrolysis and increasing reactions through O and OH radicals, and thus reducing the amount of soot precursors for all the three fuels, as the oxygenation level is increased. The effects of oxygenation and fuel on the total amounts of benzene, pyrene and NO formed can be characterized using an emission index, defined as:

Emission Index of species P :

Allene + Allyl → C6 H6 + H2 + H

(R8)

Propyne + Allyl → C6 H6 + H2 + H

(R9)

3.4. Effect of oxygenation on soot precursors Profiles of important soot precursors, i.e., acetylene and PAH species, are now examined to understand the effect of oxygenation on soot formation. As indicated in Fig. 5, for all three fuels,

EIP =

∫L0 MP ω˙ P dx

− ∫L0 M f uel ω˙ f uel dx

Here MP is the molecular weight of species P, L is the distance between the two nozzles, and ω˙ is the net species production/consumption rate. Figure 9 plots the emission indices of benzene (EI-BENZ), pyrene (EI-PYRE) and NO (EI-NO) versus ζ st for the three fuels. For all three fuels, as ζ st is increased, there is a significant decrease in the emission indices of both benzene and pyrene. While benzene is a major precursor for the formation of larger PAH

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Fig. 9. Emission indices of benzene, pyrene and NO plotted versus ζ st . Solid, dashed, and dotted lines indicate EIBENZ, EIPYRE and EINO, respectively.

species and soot, the pyrene is the key species for soot formation, since its formation directly correlates with the nucleation rate in the present soot model. In ethylene flames, the decrease in benzene formation with the increase in ζ st is due to the decrease in the amount of C2 H2 formed, since propargyl as well as diacetylene (C4 H2 ) are mostly produced from C2 H2 . In addition, as ζ st is increased, there is increased availability of O, OH and H radicals. As a consequence, along with high temperature, reactions contributing to propargyl formation are reversed [14]. Moreover, propargyl is partially consumed to form smaller hydrocarbons, such as CH2 O, C2 H, vinyl and HCO, through reactions (R13)–(R15). This further decreases benzene formation. Another factor for the decrease in benzene at higher ζ st is that due to the increased availability of H and OH radicals in the benzene formation region, benzene reacts with OH and is reduced into smaller hydrocarbons. Also, phenyl which initially converts into benzene at lower dilution levels, reverses at higher dilution level (higher ζ st ) due to the increased availability of OH, resulting in the higher amount of phenyl being produced from benzene. Another consequence of increased availability of OH is that phenyl tends to form phenol, cyclopentadiene and HCO. Thus the benzene production is drastically reduced with the increase in ζ st . Since pyrene is formed mainly from benzene, it follows a similar trend as benzene, and decreases monotonically with the increase in ζ st .

Propyne + M ↔ C3 H3 + H + M

(R10)

C3 H3 + CH3 → C4 H6

(R11)

Allyl + C3 H3 → C2 H2 + C4 H6

(R12)

C3 H3 + H ↔ C3 H2 + H2

(R13)

C3 H3 + O ↔ CH2 O + C2 H

(R14)

C3 H3 + OH ↔ C3 H2 + H2 O

(R15)

C3 H3 + OH ↔ C2 H3 + HCO

(R16)

2C3 H3 + M ↔ C6 H6 + M

(R17)

2C3 H3 ↔ C6 H5 + H

(R18)

In propene flames, there are other intermediate species, mainly propyne and allene, which also contribute to benzene formation. Consequently, as indicated in Fig. 9, propene flames produce the

Fig. 10. Contributions of key reactions towards the consumption of propargyl formed in propene/air flames with increasing ζ st .

highest amounts of benzene and pyrene, followed by ethylene and propane flames. Moreover, as ζ st is increased, the decrease in benzene formation in propene flames is mainly due to the reduction in the contributions of allene and propyne species, since these species are broken down into smaller compounds due to the increased availability of OH. Moreover, similar to ethylene flames as discussed above, propargyl, which provides the major route for benzene production, is consumed through various reactions (i.e., (R13)–(R16)). The significance of important reactions for the consumption of propargyl (estimated using integrated rates of production/consumption of reactions involving propargyl over entire axial length) as a function of ζ st in propene flames is illustrated in Fig. 10. Some representative peak values of net reaction rates indicating the effect of radicals in two propene flames with ζ st = 0.064 and 0.782 are also provided in Table 2. Thus the reduction of propargyl into smaller hydrocarbons, mainly through O and OH radicals, increases with increase in dilution level (or ζ st ), leading to the formation of vinyl and HCO, as indicated by reactions (R14)– (R16). Note that C3 H2 formed through (R13) and (R15) also decomposes into vinyl and HCO (not shown here). Another factor affecting the formation of propargyl is through reaction (R10), which reverses as the dilution (or ζ st ) is increased. For instance, with no dilution, (R10) contributes about 7% of total propargyl produced. As dilution is increased, due to the increased availability of H in the reaction zone, this reaction reverses and consumes propargyl. In propane flames, the amount of propene formed is very important for the formation of benzene. As dilution (or ζ st ) is in-

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225

Table 2 Net rates (mol/cm3 s) of important reactions contributing towards the consumption of propargyl in two propene flames for 0% and 100% N2 dilution levels. Negative sign indicates that the reaction has reversed. Reaction#

Reactions

Net reaction rate (mol/cm3 s)

ζ st = 0.064 (0%N2 ) (R10) (R11) (R12) (R13) (R14) (R15) (R16) (R17) (R18)

Propyne + M ↔ C3 H3 + H + M C3 H3 + CH3 → C4 H6 Allyl + C3 H3 → C2 H2 + C4 H6 C3 H 3 + H ↔ C3 H 2 + H 2 C3 H3 + O ↔ CH2 O + C2 H C3 H3 + OH ↔ C3 H2 + H2 O C3 H3 + OH ↔ C2 H3 + HCO 2 C3 H 3 + M ↔ C6 H 6 + M 2 C3 H 3 ↔ C6 H 5 + H

−5

2.5 × 10 4.9 × 10−5 6.2 × 10−5 3.16 × 10−5 1.14 × 10−5 3.22 × 10−5 8.51 × 10−5 3.16 × 10−5 5.22 × 10−5

ζ st = 0.782 (100%N2 ) −2.94 × 10−5 2.77 × 10−5 8.72 × 10−6 6.0 × 10−5 4.67 × 10−5 6.77 × 10−5 13.8 × 10−5 1.09 × 10−5 1.37 × 10−5

Fig. 11. Integrated soot mass (solid lines) and peak soot volume fraction (dotted lines) plotted versus ζ st in ethylene, propane and propene flames.

creased, propene produces higher amounts of ALV1 (CHCHCH3), and thus reducing the amount of allyl formed. In addition, propyne, which plays important role in the formation of propargyl in propene and ethylene flames, plays no role in propane flames. Rather propargyl reduces to form propyne in propane flames. Consequently, there are lower amounts of benzene and pyrene formed in propane flames compared to ethylene flames, even though propane has an additional carbon. Finally, as discussed earlier, the double bond in ethylene facilitates reactions that readily form C2 H2 . Consequently there are higher amounts of all three soot precursors – acetylene, benzene and pyrene – in ethylene flames compared to propane flames.

3.5. Soot formation Soot emission here is characterized in terms of number density and soot volume fraction. Note that the soot number density correlates with pyrene concentration, since the particle nucleation rate in the present soot model is based on the rate of pyrene recombination reaction. On the other hand, the soot volume fraction depends not only on the nucleation rate, but also on the surface growth and soot oxidation rates. Figure 11 plots the integrated soot mass (based on integrated soot volume fraction) and the peak soot volume fraction (fv ) versus ζ st for the three fuels. Similar to the variation of pyrene emission index, as ζ st is increased, the integrated soot mass decreases monotonically for all three fuels. However, the peak fv decreases monotonically with ζ st in only propane flames. In ethylene and propene flames, the peak fv first increases with ζ st (for ζ st < 0.423), and then decreases as ζ st is increased

further. This may be explained by considering the reaction given by Eq. (6). This reaction is part of the HACA mechanism, and very important in converting the open spaces (Csoot . ) into bulk carbon (soot). Moreover, this reaction is highly temperature dependent. Our analysis indicated that for ζ st < 0.423, the soot formation region is characterized by high temperatures with representative values being 1100 K, 1580 K and 1990 K for ζ st = 0.064, 0.243, and 0.423, respectively and also high C2 H2 concentration, as shown in Fig. 12, which promotes the rate of reaction (R19). Consequently, the peak fv increases with ζ st for ζ st < 0.423 although the region of soot formation narrows drastically. However, as ζ st is increased further, with the sharply reduced C2 H2 concentration, the rate of surface growth rate decreases. In addition, due to the increased availability of OH radicals at higher ζ st , the soot oxidation becomes more significant. Consequently, for ζ st > 0.423, the soot volume fraction decreases as ζ st is increased. In fact, for ζ st = 0.602 and 0.782, flames for all three fuels can be assumed to be virtually soot free since the total soot mass is negligibly small, of the order 10−13 gm/cm2 . Also, as mentioned earlier, the peak fv decreases monotonically with ζ st in propane flames. This is due to low concentration of C2 H2 in the soot forming region of these flames. 3.5.1. Soot formation in propane and propene flames As discussed earlier, the presence of double bond in propene has a significant influence on the pyrolysis/oxidation chemistry, leading to increased amounts of acetylene, benzene and pyrene in propene flames. Consequently, the amount of soot formed in propene flames is expected to be higher compared to that in propene flames. Figure 13 presents the soot number density and

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Fig. 12. Profiles of acetylene mole fraction (dotted lines) and soot volume fraction (solid lines) in ethylene/air flames for different levels of oxygenation.

Fig. 13. Soot number density (a) and soot volume fraction profiles (b) in propane (dotted lines) and propene (solid lines) flames with different levels of oxygenation. Note the values for propane are significantly lower, and thus appropriate scaling factors are used for improved visibility.

soot volume fraction profiles in propane and propene flames for three different oxygenation levels. As oxygenation is increased, the soot formation region shifts from oxidizer side to the fuel side. As a consequence, the soot formed in the fuel-rich region gets oxidized in the oxygen-rich region (hydrodynamic effect), and thus the soot number density and integrated soot volume fraction decrease in both propane and propene flames. However, the decrease is more pronounced in propane flames compared to that in propene flames. This can be attributed to the increased amounts of acetylene and PAH species formed in propene flames compared to those in propane flames, as discussed earlier. 3.6. NO formation As indicated in Fig. 9, for all three fuels, EINO increases monotonically as ζ st is increased. Moreover, NO emission is the highest in ethylene flames, followed by propene and propane flames. The higher NO emission in ethylene and propene flames is due to the presence of unsaturated bonds, which is consistent with earlier studies dealing with NO formation in n-heptane and 1-heptene flames [43,44]. In order to further examine the effects of ζ st and fuel molecular structure on NO emission, contributions of

various NO formation routes (i.e., thermal, prompt, NNH and N2 O routes) in these flames were computed. The methodology has been discussed in previous studies [43–45] and therefore not repeated here. Figure 14 summarizes the contributions of different NO formation routes computed in ethylene flames. As indicated, for the base case (ζ st = 0.064), prompt NO contributes most to NO formation, followed by N2 O, NNH and thermal routes. With increase in ζ st , relative contributions from thermal and N2 O routes continuously increase, while those from prompt and NNH routes decrease. The increase in thermal and N2 O contributions is related to the increased availability of O radicals and higher local temperatures as ζ st is increased. On the other hand, the decrease in prompt and NNH contributions is due to the reduction in CH concentration with the increase in ζ st . As discussed earlier, the presence of double bond results in the increased formation of acetylene, which plays an important role in the formation of NO through the prompt route. CH radicals, which form HCN and represent the key intermediate species in the prompt route, are produced from the reduction of acetylene through the following reactions:

C2 H2 + O → CH2 + CO

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Fig. 14. Peak NO mole fractions (ppm) formed through various NO routes in ethylene/air diffusion flames. Values are shown starting from 0%N2 case (ζ st = 0.064) on the left end to 100%N2 case (ζ st = 0.782) on the right end. Thus, as indicated in Table 1, case 1 is the base case, and 2, 3, 4, 5 refer to cases with 25%, 50%, 75%, and 100% N2 removed from the oxidizer stream and added to fuel stream.

Fig. 15. Comparison of different NO formation routes in propene flames. Solid and dotted lines represent flames with air stream containing 21% and 100% O2, respectively.

CH2 + H → CH + H2 CH2 + OH → CH + H2 O As oxygenation level increases, we see a steep decrease in acetylene concentration for all the three fuels, which reduces the peak mole fractions of CH and HCN by a factor of about 1.6 and 2.5, respectively. However, due to the increased amount of O2 in the air stream, we see an increase in the concentration of OH and O radicals by a factor of about 1.3. The enhanced OH and O radical pool along with the increased temperatures in the NO formation zones promotes NO production through the thermal route. Thus, for all the three fuels, we see an increase in the amount of NO produced through the thermal and N2 O routes as dilution level is increased, as indicated in Fig. 14. Figure 15 compares the NO formation through various routes in propene flames. Results are shown for two cases corresponding to oxidizer stream containing 21%O2 and 100%O2 . As discussed earlier, with no dilution (21%O2 case), the prompt NO contributes the most to NO formation, followed by the N2 O, NNH and thermal routes. As the dilution (or ζ st ) increased, contributions from thermal and N2 O routes increase, while those from prompt and NNH routes decrease. This is consistent with the results shown in Fig. 14 for ethylene flames. As indicated, for the 100%O2 case, the thermal route contributes the most to NO formation, followed by prompt, N2 O and NNH routes. Figure 16 compares the NO formed through vari-

ous routes in propane and propene flames for the case with 21% O2 in oxidizer stream. Results for the various dilution cases in propane and propene flames are summarized in Table 3. As indicated, for all the oxygenation cases, NO formed in propene flames is higher than that in propane flames. In addition, results again indicate that for both fuels without dilution (21%O2 case), the prompt route contributes the most to NO formation. However, with the increase in dilution, contributions from thermal and N2 O routes progressively increase, while those from prompt and NNH routes decrease. Thus, for the 100% O2 case, the thermal route contributes the most to NO formation, followed by prompt, N2 O and NNH routes. 4. Conclusions A computational study has been performed to characterize the effects of oxygen enrichment and fuel unsaturation on PAHs, soot and NOx emissions. Counterflow flames burning ethylene, propane, and propene are simulated with CHEMKIN-Pro, using a validated mechanism with 197 species and around 50 0 0 reactions. The stoichiometric mixture fraction (ζ st ) is varied by simultaneously using O2 -enriched oxidizer stream and N2 -diluted fuel stream such that the adiabatic flame temperature is nearly constant. Previous researchers have observed that by increasing ζ st , one can obtain nearly soot-free diffusion flames, and attributed this phenomenon to either hydrodynamics or changes in flame structure. The present study examines both of these effects by analyzing the detailed

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Fig. 16. Comparison of different NO formation routes in propane (dotted) and propene (solid) flames with 21% O2 in the air stream.

Table 3 Total NO and relative contributions of various NO routes in propene and propane flames. The columns with grey and white fillings, respectively, indicate data for propane and propene flames. Values are in ppm. Full Mech indicates the amount of NO (peak value in ppm) produced using the complete mechanism with all the NO formation routes.

flame structure and dominant reaction paths at different ζ st . In addition, the effect of fuel unsaturation on the formation of NOx , soot precursors, and soot is investigated. Important observations are as follows.

1. As ζ st is varied, results indicate a small variation in peak flame temperature, although the adiabatic flame temperature from equilibrium calculations is found to be constant. The difference may be attributed to the variation in N2 availability at the flame location due to transport effects. 2. The amount of C2 H2 formed is the highest in ethylene flames, followed by propene and propane flames. This is due to the fact that ethylene directly decomposes into vinyl, and provides a major route for C2 H2 formation through H abstraction reaction. Higher C2 H2 in propene flame than in propane flame is due to additional routes for C2 H2 formation in propene flames. These routes involve the production of vinyl from allyl, and that of C2 H2 from propyne. The presence of double bond in propene leads to the increased production of allyl and propyne species. 3. The propargyl recombination represents the dominant route for benzene formation for all three fuels. However, propene flames produce the highest amounts of benzene and larger PAH species, followed by ethylene and propane flames. Consequently, propene flames also produce the highest amount of soot. While paths for benzene formation are similar in propane and propene flames, the difference is that propene is an intermediate species in propane flame. Another factor for more benzene production in propene flames is due to the higher amounts of allene, propyne, and allyl, which directly form benzene. Higher production of benzene in ethylene flames than in

propane flames is due to the increased amount of propargyl, and due to the direct production of benzene from ethylene. 4. As ζ st is increased, it leads to a drastic reduction in the formation of acetylene, benzene, and pyrene, and thus in soot formation. The reduction is due to both the flame structure and hydrodynamic effects. The flame structure effect implies that as ζ st is increased, the O, OH, and H radical pool is enhanced, and, consequently, the intermediate species (propargyl, allene, and propyne) are reduced to smaller hydrocarbons, decreasing the formation of PAHs and thus soot. The hydrodynamic effect is caused by the flame location shifting from air to fuel side as ζ st is increased. As a consequence, the PAHs and soot formed in the fuel rich zone oxidized in the oxygen rich region. At moderate oxygenation levels, the flame structure effect seems to play a more prominent role, while at higher oxygenation levels (ζ st > 0.423 in propene flames), the hydrodynamic effect seems to be more important. The general observations regarding the flame structure and hydrodynamic effects are consistent with the findings of Skeen et al. [14] and Faeth et al. [26], respectively. 5. For all three fuels, EINO increases monotonically as ζ st is increased. Moreover, NO emission is the highest in ethylene flames, followed by propene and propane flames. Higher NO production in ethylene and propene flames is due to the presence of unsaturated bond, which increases the prompt NO formation. Results further indicate that for the base case (no oxygenation), prompt NO contributes most to NO formation, followed by N2 O, NNH and thermal routes. With the increase in ζ st , contributions from thermal and N2 O routes increase, while those from prompt and NNH routes decrease. The increase in thermal and N2 O contributions is related to the increased avail-

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