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Effects of CO2 on soot formation in ethylene pyrolysis
Combustion and Flame 215 (2020) 28–35
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Effects of CO2 on soot formation in ethylene pyrolysis Junyu Mei a,b, Xiaoqing You a,b,∗, Chung K. Law a,c a
Center for Combustion Energy, Tsinghua University, Beijing 100084, China Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China c Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA b
a r t i c l e
i n f o
Article history: Received 3 September 2019 Revised 28 November 2019 Accepted 15 January 2020
Keywords: CO2 addition Soot particle Laminar flow reactor SMPS PSDs
a b s t r a c t Effects of CO2 on soot formation during ethylene pyrolysis were investigated in a laminar flow reactor with the addition of various amounts of CO2 (0–99.5% in mole fraction). Based on a quantitative dilution sampling technique and a scanning mobility particle sizer, soot particle size distributions and the associated global properties, including soot volume fraction, number density, and soot induction delay time, were examined. Results show that while addition of a small amount of the CO2 (0 - 10%) tends to promote soot formation as the total number and volume of soot particles increase and the soot induction delay time decreases, its further increase, from 10% to 99.5%, leads to an obvious reduction of the soot nucleation and mass growth rates. Subsequent kinetic modeling of the gas-phase chemistry showed that with increasing CO2 concentration, the corresponding concentrations of the soot precursors, namely benzene and pyrene, first increase and then decrease, which is consistent with the observed trend in soot formation. Further sensitivity and reaction path analyses of benzene formation indicate that CO2 addition produces more hydroxyl radicals, such that while the presence of a small amount of hydroxyl radicals increases the propargyl concentration and thereby promotes the formation of soot precursors, excessive hydroxyl radicals lead to more oxidation and hence inhibit soot formation. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Soot, generated from the incomplete combustion or pyrolysis of hydrocarbons, is detrimental to the climate and human health. Consequently various technologies such as exhaust gas recirculation in diesel and gasoline engines [1] and flue gas recirculation in furnace [2] have been developed to reduce the formation and emission of soot particles and other pollutants. On the positive side, the thermal decomposition of heavy hydrocarbons is one of the major methods to manufacture carbon black, which is a widely-used functional material [3] used as, for example, pigment in printer and reinforcement additive in rubber [4,5]. It is then of interest to note that carbon dioxide is involved in both processes: in that CO2 is a major component in the exhaust gas and as such would clearly affect the extent of soot emission, while in the formation of carbon black, the liquid hydrocarbon feedstock is sprayed into the heat source generated by the combustion of gaseous fuels with pre-heated air, in which a certain amount of
∗ Corresponding author at: Center for Combustion Energy, Tsinghua University, Beijing 10 0 084, China. E-mail address: [email protected] (X. You).
CO2 is produced and participates in the fuel pyrolysis and affects carbon black formation. Therefore, it is fundamentally and practically important to understand the effects of CO2 on soot formation. Both experimental and numerical studies [6–11] have revealed that the addition of CO2 can suppress soot formation in flames in three ways: dilution effect, thermal effect, and chemical effect. Early work by Du et al. [6] attempted to separate these three effects by varying the concentrations of carbon dioxide and nitrogen and accordingly adjusting the flame temperature, and all three effects were found to play a role in reducing soot yield. The former two effects were well addressed in previous studies [12–14]: the added CO2 dilutes the reactants as an inert gas and directly reduces the reactive species concentrations [12,13], while the change of thermal capacity by adding CO2 also influences the flame temperature and consequently the soot formation [14]. The role of the chemical effect of CO2 , however, is still unclear due to the complexity of the reaction pathways and needs to be investigated thoroughly. Carbon dioxide was reported to suppress soot formation dominantly through the chemical participation in flames [7,8,10,11,15,16]. The numerical studies of co-flow diffusion flames by Liu et al. [16] found that the hydroxyl radicals formed via the
https://doi.org/10.1016/j.combustflame.2020.01.015 0010-2180/© 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
J. Mei, X. You and C.K. Law / Combustion and Flame 215 (2020) 28–35
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Fig. 1. Experimental setup.
reaction of hydrogen atoms with carbon dioxide enhanced the oxidation of soot precursors. Based on an advanced sampling technique and scanning mobility particle sizer (SMPS) measurements, Tang et al. [10] studied the effects of CO2 addition on the evolution of particle size distributions (PSDs) in premixed ethylene flames. Results showed that the added CO2 reduced both the nucleation rate and mass growth rate, leading to an exponential decline in soot formation. Later, Naseri et al. [11] used discrete sectional method with detailed gas-phase kinetic modeling to further examine the CO2 effects on PSDs in premixed flames. They found that the chemical effect was stronger than the thermal effect and soot formation was influenced mainly via suppression of nucleation and condensation processes. Nevertheless, several studies on soot formation during fuel pyrolysis or in the very fuel-rich combustion reported a different conclusion that CO2 addition could promote soot yield under certain conditions. Chang et al. [17] explored the effect of CO2 on the characteristics of soot generated from coal pyrolysis in a drop tube furnace and noticed that the sooting tendency increased in the CO2 atmosphere compared to that in the N2 atmosphere. Teini et al. [18] found that while a small amount of CO2 addition promoted soot formation in acetylene combustion with very high equivalence ratios at 10 atm and 1640 K, it otherwise had negligible effect on soot formation in rich CH4 /O2 mixtures. In addition, Abian et al. [19,20] observed an increase in soot production with a small amount of CO2 addition while soot formation was inhibited for much higher CO2 mole fraction during ethylene pyrolysis. All these studies [10,16,19,21,22] have revealed that the key reaction CO2 + H = CO + OH plays a dominant role in the chemical effect of CO2 on soot formation, but it is not clear as how this reaction causes the two different effects mentioned above. To have a comprehensive study on this subject, we investigated the effects of a wide range of CO2 addition (0 - 99.5% in mole fraction) on soot formation during ethylene pyrolysis in a laminar flow reactor both experimentally and numerically. Compared to flames, the flow reactor can provide an undisturbed high temperature environment maintained by an external heat source and a temperature feedback controller [23,24]. By substituting nitrogen with carbon dioxide equivalently, the dilution and thermal effects can be minimized. The PSDs and global properties of soot were quantitatively measured by a dilution sampling probe/SMPS technique [23,25,26]. Kinetic modeling of the detailed gas-phase chemistry was performed to analyze the experimental results.
2. Experimental and computational methods 2.1. Experimental setup The experimental setup is shown in Fig. 1. The flow reactor is a 1400 mm-long corundum tube with an inner diameter of 18 mm that is heated by an external furnace. The radial temperature gradient is negligible and the axial temperature distribution includes a high-temperature region with the length of about 700 mm in the heating zone and two sharp temperature-drop regions near the inlet and outlet of the reactor, which are cooled by an air cooler. The temperature of the heating zone is set at 1673 K by a feedback temperature controller and the axial temperature profile is shown in Fig. S1 in the Supplementary Material. The inlet gas mixture is composed of 0.5% ethylene and 99.5% diluent (nitrogen and carbon dioxide). All the gas flow rates are controlled by a Coriolis massflow meter (Model CS200A) calibrated by a BUCK soap-film flow meter (Model M-30) [27] and are well mixed before being delivered to the reactor. A stainless steel sampling tube with an outer diameter of 6.35 mm, wall thickness of 0.125 mm and a 0.165-mm diameter orifice drilled in the middle is placed vertically at the outlet of the tube, similar to the one used in previous studies involving premixed flames [25,28,29] and in flow reactors [23,24,30]. In order to prevent the influence of ambient air, the exit gas of the reactor is shielded by a shroud of nitrogen flow at 10 L/min and the sampling tube is clang to the reactor exit with a distance less than 4 mm [23]. Carbonaceous particles formed in the reactor are drawn into the sampling orifice and diluted quickly by nitrogen flow at 25 L/min (STP). The orifice is cleaned by a fine stainless steel needle after each sampling scan to avoid clogging. The dilution sampling method is used to lower the particle concentration in the sampling line, so as to avoid chemical reactions and physical loss of particles, as well as to match the detection range of the measuring instrument. The dilution ratio is determined by controlling the flow rate of the sample gas through the orifice [27,31]. The detailed information of dilution ratio determination can be found in our previous study [23]. To avoid particle loss in the sampling line, the appropriate dilution ratio is ensured for each condition, as shown in Table 1. The real concentration of the particles in the reactor is the product of the measured concentration and the dilution ratio. The diluted samples are delivered to a commercial SMPS (TSI Model 3936) where two types of DMA (nano-DMA, Model 3085
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Table 1 Summary of pyrolysis condition.a Pyrolysis
P0 P1 P2 P3 P4 P5 P6 a b
Diluentb
Sampling dilution ratio
N2 (%)
CO2 (%)
99.5 98.5 97.5 94.5 89.5 49.5 0
0 1 2 5 10 50 99.5
520 881 1662 2439 2439 1662 1113
Inlet feedstock: 0.5% C2 H4 – 99.5% diluent. The unit is mole fraction.
and long-DMA, Model 3081) are used to ensure a relatively wide particle size range [32]. The measurement ranges are 3–64 nm and 10–217 nm in the mobility diameter for nano-DMA and long-DMA, respectively. The results in the overlapping size range are nearly the same for the two types of DMA. The complete sampling period includes a 50 s up-scan and a 10 s down-scan, which is a compromise between the low scanning accuracy of SMPS for a short scan time and the orifice clogging for a long scan time [26,27,33]. All particle sizes reported hereafter are the mobility diameters based on the SMPS measurements. To ensure a suitable soot yield for measuring the nascent soot characteristics, the fuel, ethylene, (C2 H4 ) is highly diluted and kept at the constant mole fraction of 0.5%. The diluent nitrogen is gradually substituted with carbon dioxide (0–99.5% in mole fraction) to study the effects of CO2 , as shown in Table 1. The particle information at different residence times is obtained by varying the inlet flow rate, F [34–36]. According to our previous study [23], the flow only shifted the high temperature region to the downstream slightly, hence it has little effect on soot formation. The Reynolds number of the flow along the tube is within the range of 208–1181, hence verifying the laminar nature for all cases. The residence time L is determined from L 2 dL/ua , where L1 and L2 are respectively the
Fig. 2. The PSDs comparison during the ethylene pyrolysis with different CO2 addition (0%, 1%, 2%, 5% in mole fraction).
1
start and end locations where the temperatures deviate from the max temperature by less than 10%, and ua is the average gas velocity considering the gas expansion effect at the reactor temperature [23,24].
Fig. 3. The PSDs comparison during the ethylene pyrolysis with different CO2 addition (0%, 10%, 50%, 99.5% in mole fraction).
2.2. Computational method The reactor parameters and the reaction conditions are designed for axial convection and chemical reaction controlled system. First, the Peclect number (Pe), characterizing the intensity of the axial convection versus diffusion in the reactor, is around or over 10 0 0, and thus diffusion can be neglected compared to axial convection [37]. Second, the residence time for the gas passing through the high temperature region is around 150 ms, being at the time scale of soot formation. Under these conditions, a plugflow model can be applied. It is noted that previous studies have also validated the plug-flow hypothesis under similar flow reactor conditions, both theoretically and experimentally [38,39]. Specifically, Skjøth-Rasmussen et al. [39] compared the modeling results of key species profiles using the zero-, one-, and two-dimensional codes, and showed that both axial and radial diffusion had minor influence. Before applying the plug flow model to simulate our experiments, we have compared the plug flow results and the cylindrical shear flow results to make sure the plug flow model in the Chemkin-Pro software package [40] is appropriate (see details in the Supplementary Material). The reaction kinetic model (249 species and 8153 reactions) including detailed polycyclic aromatic hydrocarbons (PAHs) reactions is taken from Ranzi et al. [41]. We choose this model because it has been shown to have
good performance over a wide range of conditions in both fuel rich flames [11,41] (diffusion flames: C0 –C4 fuels, 0.5–10 atm; premixed flames: C2 H4 , ϕ = 2.07, 0–18% CO2 addition) and fuel pyrolysis [42,43] (shock tube: C2 H2 , 130 0–220 0 K, 1.1–2.6 atm; flow reactor: C2 H2 , 873–1473 K; CH4 , 1200 −1800 K; C2 H4 , 1700 K). The boundary conditions are set based on our experimental setup. Compared to the external heat source, the reaction heat release in the reactor is negligible. Consequently, all the modeling results are obtained based on the measured temperature profiles without solving the energy equations. 3. Results and discussion The PSDs at different CO2 mole fractions are compared in Figs. 2 and 3. All the measurements have been repeated three times for each sampling to ensure reproducibility, and the results are the average values under each condition. The repetitive error is less than 10%, while the total uncertainty is estimated to be ±30%, which mainly comes from the determination of the dilution ratio error [23,27]. Results show that at different CO2 mole fractions, all the PSDs exhibit a power-law function at short residence times, indicating the inception of nascent particles. As the residence time increases, a typical bimodal distribution emerges and lasts, char-
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acterizing the co-existence of a durable nucleation process and a gradually predominating mass growth process. Figure 2 shows the evolution of the PSDs for ethylene pyrolysis with 0, 1, 2, and 5% CO2 . It is seen that at short residence times, the number density of small particles increases by 1–2 orders of magnitude with 1–5% CO2 addition, indicating that the nucleation rate increases significantly with a small amount of CO2 addition. For all four cases, although the number densities are quite different, the shoulders appear at t = 126 ms and distinct bimodal distributions emerge at 136 ms, showing similar evolution processes. As the residence time is further increased (t = 149 ms), the concentration of small particles decreases and the mass growth is predominant. The median diameter increases remarkably with CO2 addition, implying higher coagulation or surface growth rates in the particle growth region [44]. Figure 3 shows the results for relatively high concentrations of CO2 addition. At short residence times, the number density of small particles increase by more than one order of magnitude. Yet the soot inception is delayed and the nucleated particles become fewer when the mole fraction of CO2 increases from 10% to 99.5%. This suppression phenomenon becomes more evident at t = 136 and 149 ms, where the median size of the second peak of the PSD is remarkably reduced as more CO2 is added, indicating an intense inhibition of particle growth, similar to the effects of CO2 addition on soot formation in premixed flames [10]. At t = 149 ms, with 10% CO2 addition, the particle median size is the largest among the seven conditions. A large amount of small particles are generated promptly at t = 117 ms with 99.5% CO2 , around two orders higher than that without CO2 . Unexpectedly, higher concentration of nucleated particles does not lead to a larger growth rate of particle size. At t = 149 ms, the median size of the second peak of the PSD with 99.5% CO2 is slightly smaller than that without any CO2 . The results indicate a higher nucleation rate but a quite lower mass growth rate with 99.5% CO2 . By integrating the number and volume of the particles over all mobility sizes measured (>3 nm) respectively, soot number density (N) and mobility volume fraction (Fv ) can be obtained. These values as functions of residence time are shown in Fig. S4 in the Supplementary Material. It clearly demonstrates that, for all conditions, as residence time increases N increases first due to particle nucleation and then decreases due to coagulation, while Fv has a two-stage increment dominated by nucleation and surface growth, respectively. To explore more about the effects of different CO2 concentrations, particle mobility volume fractions at three selected residence times were plotted as a function of the CO2 mole fraction in Fig. 4. To have a better resolution, we performed three extra measurements with 3%, 30%, and 75% CO2 respectively in the mixture. The trend demonstrating first promotion and then suppression is observed for all residence times. Nevertheless, the promotion effect becomes weaker as the reaction time is extended, which is probably because a small amount of CO2 would affect particle nucleation more than mass growth, and therefore more prominently accelerates soot formation at short residence times. The maximum soot yield occurs at around 10% CO2 addition, which is consistent with the PSD results in Figs. 2 and 3. When the CO2 mole fraction increases from 10% to 99.5%, the soot volume fraction decreases exponentially, similar to the results in the laminar premixed flames [10]. It is worth noting that in both flames and fuel pyrolysis, the CO2 addition leads to the formation of OH radicals through the reaction CO2 + H = CO + OH. However, only the inhibiting effect on soot yield was observed in previous flame studies, since the mole fraction of OH radicals is relatively quite high (> 10−5 ) and the addition of CO2 would generate even more OH radicals that promote soot oxidation [10]. By contrast, there is no oxygen in fuel pyrolysis, and 10% CO2 addition would produce a small amount of OH radicals (~10−6 ) that facilitate the
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Fig. 4. The particle volume fractions as a function of CO2 addition during ethylene pyrolysis at three selected residence time. Symbols are the experimental data and lines are fitted to the data.
Fig. 5. The maximum total number density and particle induction delay time as a function of CO2 addition during ethylene pyrolysis. Symbols are the experimental data and lines are fitted to the data.
formation of PAH and soot, which will be discussed in detail later. The maximum number density and induction delay time of soot particles are shown in Fig. 5. The particle induction delay time is defined as the time of appearance of the shoulder in PSDs, which indicates that the mass growth has become comparable and the volume fraction begins to increase rapidly. [23] The results show that the trend of the maximum number density is almost the same as that of the volume fraction. A very small amount of CO2 addition (1−3%) does not change the induction delay time within the uncertainty but intensively increases the quantity of particles. When more CO2 is added, the induction delay time declines and reaches the minimum at around 10% CO2 addition. Obviously, the experimental data show two opposite effects on soot particle formation with the addition of different amounts of CO2 . Kinetic modeling has been performed to further analyze the mechanism of the promotion and suppression. Figure 6 shows the calculated mole fractions of benzene and pyrene, which are two important soot precursors, as functions of the CO2 mole fraction at the reactor outlet. It is seen that the mole fraction of pyrene increases first then decreases as CO2 increases from 0% to 99.5%, and reaches the maximum value at 10% CO2 , which is very similar to the soot yield. It indicates that the concentration of pyrene can represent the intensity of soot formation to some extent. Pyrene is also regarded as an important species that directly contributes to the nucleation and condensation process [11,45,46]. The benzene mole fraction peaks at a smaller mole fraction of CO2 than pyrene, because CO2 not only promotes benzene formation but also
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Fig. 6. Mole fractions of benzene and pyrene at the outlet of reactor computed at F = 12 L/min as a function of CO2 addition.
the growth from benzene to pyrene, when the CO2 mole fraction is less than 10%. Figure S5 in the Supplementary Material shows that the peak value of the benzene mole fraction in the heating zone reaches the maximum with around 10% CO2 addition, and the mole fraction of pyrene increases rapidly in the meanwhile, showing that 10% CO2 indeed has the strongest promotion impact. To minimize the thermal and dilution effects of CO2 addition, we have kept a constant high temperature region and substituted N2 by an equivalent amount of CO2 . To verify if the chemical effect is dominant, we performed additional calculations by replacing CO2 with a chemical-inert carbon dioxide species (FCO2 ), which
possesses the same thermal properties as CO2 but does not participate in the reactions [10,16]. Figure S6 in the Supplementary Material compares mole fraction profiles of acetylene, propargyl, benzene, and pyrene at F = 12 L/min. We can see that with 10% FCO2 , the mole fractions of propargyl, benzene, and pyrene are only slightly lower than those with 0% CO2 due to the different thermal properties of CO2 and N2 . In contrast, the discrepancies between the cases with 10% CO2 and 0% CO2 are much more significant, demonstrating that the thermal and dilution effects of CO2 are negligible compared to the chemical effect. The chemical effect firstly acts on the reactions of small molecules and radicals, and consequently affects the formation of PAHs and soot. The mole fractions of hydrogen (H), hydroxyl (OH), methyl (CH3 ), propargyl (C3 H3 ), acetylene (C2 H2 ), and diacetylene (C4 H2 ) at F = 12 L/min are compared in Fig. 7. The addition of CO2 leads to a significant increase of the OH mole fraction and a relevant reduction of the H mole fraction as a result of the key reaction of CO2 + H = CO + OH. The mole fraction of C2 H2 species shows a monotonous decreasing trend as the inlet CO2 mole fraction is increased. Under the condition without CO2 (see the black line in Fig. 7), the inlet feedstock, ethylene, decomposes into acetylene and hydrogen radicals as soon as it enters the high temperature region, and the mole fraction of acetylene maintains at a high level along the high temperature region and the outlet region, indicating that the majority of the pyrolysis products is acetylene and very few of the carbon is converted to soot considering the carbon balance. However, when CO2 is added to the inlet feedstock, the mole fraction of C2 H2 and C4 H2 decreases obviously after the initial increase in the high temperature region. Correspondingly, the production of benzene and pyrene is promoted as shown in Fig. S5 in the Supplementary Material. Recombination of two propargyl radicals is one of the most competitive reaction pathways of benzene formation [47,48]. The
Fig. 7. Mole fractions of H, OH, CH3 , C3 H3 , C2 H2 , and C4 H2 computed at F = 12 L/min as a function of axial distance in the reactor.
J. Mei, X. You and C.K. Law / Combustion and Flame 215 (2020) 28–35
C3 H3 profiles show that a small amount of CO2 addition, e.g., 2% CO2 , increases the maximum mole fraction of C3 H3 by one order of magnitude, which corresponds to the effect on the benzene profile in Fig. 6. However, a further increase of OH from the condition of 10% CO2 to that of 99.5% suppresses the formation of all soot precursors including C2 H2 , C3 H3 , benzene, and pyrene as well as soot formation due to the strong oxidation effect. Compared to the case without CO2 , the mole fractions of key soot precursors such as propargyl, benzene, and pyrene are significantly higher with the addition of 99.5% CO2 , leading to the enhanced soot inception, which explains why much more small particles are produced in the case with 99.5% CO2 than those with 0% CO2 as shown in Fig. 3. To know more about what happened to the gas-phase chemistry, we performed sensitivity and reaction pathway analysis of benzene formation, since benzene is considered as the building block of large PAHs and soot [49]. The top 15 reactions that have the largest sensitivities at F = 12 L/min and x = 70 cm are shown in Fig. 8. It is well known that there are two major pathways to form benzene [47,50], namely the self-addition of C3 species and the combination of C2 and C4 species. The key reactions of these two pathways are shown in red and blue in Fig. 8, respectively. We can see from Fig. 8(a) that benzene formation is very sensitive to the C2 + C4 pathways represented by the reactions of C4 H3 + C2 H2 = C6 H5 or C4 H4 + C2 H2 = C6 H6 . Under the pyrolysis condition without CO2 , the odd number carbon species are difficult to form in the absence of oxygen, so the mole fraction of the C3 species is relatively low. The reaction of CO2 + H = CO + OH is essential to the chemical effects of CO2 addition (see Fig. 8(b) and (c)). With a small amount of CO2 addition (e.g., 10% in mole fraction), the hydroxyl radicals are formed, which accelerate the overall soot formation process due to its high reactivity, and consequently reduces the soot induction delay time as shown in Fig. 5. Meanwhile, the existence of OH can increase the mole fraction of the odd number carbon species through the reactions: C2 H2 + OH = CH3 + CO, C4 H2 + OH = C3 H2 + HCO, and CH3 + C2 H2 = pC3 H4 + H. Consequently, more C3 species are formed, facilitating benzene formation through the self-addition reactions. With much larger amount of CO2 , the hydroxyl radicals are excessive, demonstrating intense oxidation. As shown in Fig. 8(c), with 99.5% CO2 addition, the top ten reactions all involve oxygen-containing species, implying that more carbon is oxidized instead of being converted to soot precursors. In addition, the abundant hydroxyl radicals can also retard soot formation directly by surface oxidation [51]. Figures 9–11 show the main reaction pathways from ethylene to benzene at F = 12 L/min and x = 70 cm, for 0, 10, and 99.5% CO2 mole fraction, respectively. The thickness of the lines between species indicates the relative significance of a reaction pathway based on the absolute value of the rate of production, with the auxiliary reactants and product listed beside the lines. It is seen that there are two simultaneous and competing pathways to form benzene or phenyl as mentioned above. Under the condition of 0% CO2 , the C2 + C4 pathway is more competitive, as shown in Fig. 9, because it is difficult to generate the odd carbon number species without any oxygen. Decomposition reactions from ethane or ethyl through breaking the C–C bond are the only ways, while the mole fractions of ethane and ethyl are relatively low in this process. Previous modeling studies also reported the similar core pathways to benzene in acetylene pyrolysis, where C4 H4 and C4 H3 are the key intermediates [42]. Then the successive molecular or radical polymerization reactions lead to the formation of the first ring, such as C4 H3 + C2 H2 = C6 H5 , C4 H2 + C2 H3 = C6 H5 , and C4 H4 + C2 H2 = C6 H6 . However, the situation becomes very different as a small amount of CO2 is added to the reactants, where the formed OH accelerates the bond breaking of acetylene through the reaction of C2 H2 + OH = CH3 + CO or C2 H2 + OH = CH2 CHO, which
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Fig. 8. Normalized sensitivity of benzene formation at F = 12 L/min and x = 70 cm for CO2 mole fractions of (a) 0%, (b) 10%, and (c) 99.5%. Important reactions involved in benzene formation are marked red for C4 + C2 pathways, and blue for C3 + C3 pathways. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
directly or indirectly results in the formation of CH3 species, as shown in Fig. 10. Therefore, the C3 species can accumulate through the combination between C2 and C1 species or the oxidation of C4 species by OH. Compared to the C2 + C4 reactions, the C3 + C3 reactions represented by the self-reaction of two propargyl radicals have lower energy barriers and are much more competitive [52],
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4. Conclusion
Fig. 9. Main reaction pathways from ethylene to benzene at x = 70 cm under the condition of 0% CO2 with F = 12 L/min based on the rate of product analysis. The thickness of the lines indicates the relative significance of a reaction pathway. The most importance pathways are circled by dashed line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
To investigate the effect of CO2 addition on sooting characteristics comprehensively, in this study we have conducted experimental measurements of soot formation and kinetic modeling of gasphase reactions during ethylene pyrolysis in a laminar flow reactor with a wide range of CO2 addition (0–99.5% in mole fraction). Particle size distribution, volume fraction, number density and induction delay time were examined using the orifice dilution sampling technique and the SMPS measurement system. Results show the trend of promotion followed by suppression in the soot yield as the CO2 mole fraction increases. With a small amount of CO2 addition (from 0% to 10%), the inception of the soot particles is activated earlier followed by more intense nucleation and mass growth. This promotion effect is more obvious at shorter residence times. The chemical effect of CO2 is dominated by the reaction of CO2 + H = CO + OH. A slight increase of the hydroxyl radicals facilitates the formation of odd number carbon species (e.g., propygyl), which sequentially promotes the formation of important soot precursors, such as benzene and pyrene that are important for soot nucleation and mass growth. With further increase of CO2 (from 10% to 99.5%), the total volume fraction decreases exponentially with the CO2 mole fraction and this suppression effect is more dominant in the mass growth process. A large amount of CO2 generates redundant hydroxyl radicals that lead to a strong oxidation effect and retards the PAHs and soot formation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the National Science Foundation of China (51761125012).
Fig. 10. Main reaction pathways from ethylene to benzene at x = 70 cm under the condition of 10% CO2 with F = 12 L/min based on the rate of product analysis.
Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2020.01. 015. References
Fig. 11. Main reaction pathways from ethylene to benzene at x = 70 cm under the condition of 99.5% CO2 with F = 12 L/min based on the rate of product analysis.
thereby accelerate formation of the first ring. Figure 11 shows the main pathways to benzene as well as the parallel oxidation pathways under the condition of 99.5% CO2 . As discussed above, 99.5% CO2 produces superfluous hydroxyl radicals, which can oxidize the soot precursors, such as acetylene. Consequently, more carbon in the fuel becomes carbon monoxide through a series of oxidation reactions instead of forming PAHs or soot.
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Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Effects of CO2 on soot formation in ethylene pyrolysis Junyu Mei a,b, Xiaoqing You a,b,∗, Chung K. Law a,c a
Center for Combustion Energy, Tsinghua University, Beijing 100084, China Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China c Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA b
a r t i c l e
i n f o
Article history: Received 3 September 2019 Revised 28 November 2019 Accepted 15 January 2020
Keywords: CO2 addition Soot particle Laminar flow reactor SMPS PSDs
a b s t r a c t Effects of CO2 on soot formation during ethylene pyrolysis were investigated in a laminar flow reactor with the addition of various amounts of CO2 (0–99.5% in mole fraction). Based on a quantitative dilution sampling technique and a scanning mobility particle sizer, soot particle size distributions and the associated global properties, including soot volume fraction, number density, and soot induction delay time, were examined. Results show that while addition of a small amount of the CO2 (0 - 10%) tends to promote soot formation as the total number and volume of soot particles increase and the soot induction delay time decreases, its further increase, from 10% to 99.5%, leads to an obvious reduction of the soot nucleation and mass growth rates. Subsequent kinetic modeling of the gas-phase chemistry showed that with increasing CO2 concentration, the corresponding concentrations of the soot precursors, namely benzene and pyrene, first increase and then decrease, which is consistent with the observed trend in soot formation. Further sensitivity and reaction path analyses of benzene formation indicate that CO2 addition produces more hydroxyl radicals, such that while the presence of a small amount of hydroxyl radicals increases the propargyl concentration and thereby promotes the formation of soot precursors, excessive hydroxyl radicals lead to more oxidation and hence inhibit soot formation. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Soot, generated from the incomplete combustion or pyrolysis of hydrocarbons, is detrimental to the climate and human health. Consequently various technologies such as exhaust gas recirculation in diesel and gasoline engines [1] and flue gas recirculation in furnace [2] have been developed to reduce the formation and emission of soot particles and other pollutants. On the positive side, the thermal decomposition of heavy hydrocarbons is one of the major methods to manufacture carbon black, which is a widely-used functional material [3] used as, for example, pigment in printer and reinforcement additive in rubber [4,5]. It is then of interest to note that carbon dioxide is involved in both processes: in that CO2 is a major component in the exhaust gas and as such would clearly affect the extent of soot emission, while in the formation of carbon black, the liquid hydrocarbon feedstock is sprayed into the heat source generated by the combustion of gaseous fuels with pre-heated air, in which a certain amount of
∗ Corresponding author at: Center for Combustion Energy, Tsinghua University, Beijing 10 0 084, China. E-mail address: [email protected] (X. You).
CO2 is produced and participates in the fuel pyrolysis and affects carbon black formation. Therefore, it is fundamentally and practically important to understand the effects of CO2 on soot formation. Both experimental and numerical studies [6–11] have revealed that the addition of CO2 can suppress soot formation in flames in three ways: dilution effect, thermal effect, and chemical effect. Early work by Du et al. [6] attempted to separate these three effects by varying the concentrations of carbon dioxide and nitrogen and accordingly adjusting the flame temperature, and all three effects were found to play a role in reducing soot yield. The former two effects were well addressed in previous studies [12–14]: the added CO2 dilutes the reactants as an inert gas and directly reduces the reactive species concentrations [12,13], while the change of thermal capacity by adding CO2 also influences the flame temperature and consequently the soot formation [14]. The role of the chemical effect of CO2 , however, is still unclear due to the complexity of the reaction pathways and needs to be investigated thoroughly. Carbon dioxide was reported to suppress soot formation dominantly through the chemical participation in flames [7,8,10,11,15,16]. The numerical studies of co-flow diffusion flames by Liu et al. [16] found that the hydroxyl radicals formed via the
https://doi.org/10.1016/j.combustflame.2020.01.015 0010-2180/© 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
J. Mei, X. You and C.K. Law / Combustion and Flame 215 (2020) 28–35
29
Fig. 1. Experimental setup.
reaction of hydrogen atoms with carbon dioxide enhanced the oxidation of soot precursors. Based on an advanced sampling technique and scanning mobility particle sizer (SMPS) measurements, Tang et al. [10] studied the effects of CO2 addition on the evolution of particle size distributions (PSDs) in premixed ethylene flames. Results showed that the added CO2 reduced both the nucleation rate and mass growth rate, leading to an exponential decline in soot formation. Later, Naseri et al. [11] used discrete sectional method with detailed gas-phase kinetic modeling to further examine the CO2 effects on PSDs in premixed flames. They found that the chemical effect was stronger than the thermal effect and soot formation was influenced mainly via suppression of nucleation and condensation processes. Nevertheless, several studies on soot formation during fuel pyrolysis or in the very fuel-rich combustion reported a different conclusion that CO2 addition could promote soot yield under certain conditions. Chang et al. [17] explored the effect of CO2 on the characteristics of soot generated from coal pyrolysis in a drop tube furnace and noticed that the sooting tendency increased in the CO2 atmosphere compared to that in the N2 atmosphere. Teini et al. [18] found that while a small amount of CO2 addition promoted soot formation in acetylene combustion with very high equivalence ratios at 10 atm and 1640 K, it otherwise had negligible effect on soot formation in rich CH4 /O2 mixtures. In addition, Abian et al. [19,20] observed an increase in soot production with a small amount of CO2 addition while soot formation was inhibited for much higher CO2 mole fraction during ethylene pyrolysis. All these studies [10,16,19,21,22] have revealed that the key reaction CO2 + H = CO + OH plays a dominant role in the chemical effect of CO2 on soot formation, but it is not clear as how this reaction causes the two different effects mentioned above. To have a comprehensive study on this subject, we investigated the effects of a wide range of CO2 addition (0 - 99.5% in mole fraction) on soot formation during ethylene pyrolysis in a laminar flow reactor both experimentally and numerically. Compared to flames, the flow reactor can provide an undisturbed high temperature environment maintained by an external heat source and a temperature feedback controller [23,24]. By substituting nitrogen with carbon dioxide equivalently, the dilution and thermal effects can be minimized. The PSDs and global properties of soot were quantitatively measured by a dilution sampling probe/SMPS technique [23,25,26]. Kinetic modeling of the detailed gas-phase chemistry was performed to analyze the experimental results.
2. Experimental and computational methods 2.1. Experimental setup The experimental setup is shown in Fig. 1. The flow reactor is a 1400 mm-long corundum tube with an inner diameter of 18 mm that is heated by an external furnace. The radial temperature gradient is negligible and the axial temperature distribution includes a high-temperature region with the length of about 700 mm in the heating zone and two sharp temperature-drop regions near the inlet and outlet of the reactor, which are cooled by an air cooler. The temperature of the heating zone is set at 1673 K by a feedback temperature controller and the axial temperature profile is shown in Fig. S1 in the Supplementary Material. The inlet gas mixture is composed of 0.5% ethylene and 99.5% diluent (nitrogen and carbon dioxide). All the gas flow rates are controlled by a Coriolis massflow meter (Model CS200A) calibrated by a BUCK soap-film flow meter (Model M-30) [27] and are well mixed before being delivered to the reactor. A stainless steel sampling tube with an outer diameter of 6.35 mm, wall thickness of 0.125 mm and a 0.165-mm diameter orifice drilled in the middle is placed vertically at the outlet of the tube, similar to the one used in previous studies involving premixed flames [25,28,29] and in flow reactors [23,24,30]. In order to prevent the influence of ambient air, the exit gas of the reactor is shielded by a shroud of nitrogen flow at 10 L/min and the sampling tube is clang to the reactor exit with a distance less than 4 mm [23]. Carbonaceous particles formed in the reactor are drawn into the sampling orifice and diluted quickly by nitrogen flow at 25 L/min (STP). The orifice is cleaned by a fine stainless steel needle after each sampling scan to avoid clogging. The dilution sampling method is used to lower the particle concentration in the sampling line, so as to avoid chemical reactions and physical loss of particles, as well as to match the detection range of the measuring instrument. The dilution ratio is determined by controlling the flow rate of the sample gas through the orifice [27,31]. The detailed information of dilution ratio determination can be found in our previous study [23]. To avoid particle loss in the sampling line, the appropriate dilution ratio is ensured for each condition, as shown in Table 1. The real concentration of the particles in the reactor is the product of the measured concentration and the dilution ratio. The diluted samples are delivered to a commercial SMPS (TSI Model 3936) where two types of DMA (nano-DMA, Model 3085
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Table 1 Summary of pyrolysis condition.a Pyrolysis
P0 P1 P2 P3 P4 P5 P6 a b
Diluentb
Sampling dilution ratio
N2 (%)
CO2 (%)
99.5 98.5 97.5 94.5 89.5 49.5 0
0 1 2 5 10 50 99.5
520 881 1662 2439 2439 1662 1113
Inlet feedstock: 0.5% C2 H4 – 99.5% diluent. The unit is mole fraction.
and long-DMA, Model 3081) are used to ensure a relatively wide particle size range [32]. The measurement ranges are 3–64 nm and 10–217 nm in the mobility diameter for nano-DMA and long-DMA, respectively. The results in the overlapping size range are nearly the same for the two types of DMA. The complete sampling period includes a 50 s up-scan and a 10 s down-scan, which is a compromise between the low scanning accuracy of SMPS for a short scan time and the orifice clogging for a long scan time [26,27,33]. All particle sizes reported hereafter are the mobility diameters based on the SMPS measurements. To ensure a suitable soot yield for measuring the nascent soot characteristics, the fuel, ethylene, (C2 H4 ) is highly diluted and kept at the constant mole fraction of 0.5%. The diluent nitrogen is gradually substituted with carbon dioxide (0–99.5% in mole fraction) to study the effects of CO2 , as shown in Table 1. The particle information at different residence times is obtained by varying the inlet flow rate, F [34–36]. According to our previous study [23], the flow only shifted the high temperature region to the downstream slightly, hence it has little effect on soot formation. The Reynolds number of the flow along the tube is within the range of 208–1181, hence verifying the laminar nature for all cases. The residence time L is determined from L 2 dL/ua , where L1 and L2 are respectively the
Fig. 2. The PSDs comparison during the ethylene pyrolysis with different CO2 addition (0%, 1%, 2%, 5% in mole fraction).
1
start and end locations where the temperatures deviate from the max temperature by less than 10%, and ua is the average gas velocity considering the gas expansion effect at the reactor temperature [23,24].
Fig. 3. The PSDs comparison during the ethylene pyrolysis with different CO2 addition (0%, 10%, 50%, 99.5% in mole fraction).
2.2. Computational method The reactor parameters and the reaction conditions are designed for axial convection and chemical reaction controlled system. First, the Peclect number (Pe), characterizing the intensity of the axial convection versus diffusion in the reactor, is around or over 10 0 0, and thus diffusion can be neglected compared to axial convection [37]. Second, the residence time for the gas passing through the high temperature region is around 150 ms, being at the time scale of soot formation. Under these conditions, a plugflow model can be applied. It is noted that previous studies have also validated the plug-flow hypothesis under similar flow reactor conditions, both theoretically and experimentally [38,39]. Specifically, Skjøth-Rasmussen et al. [39] compared the modeling results of key species profiles using the zero-, one-, and two-dimensional codes, and showed that both axial and radial diffusion had minor influence. Before applying the plug flow model to simulate our experiments, we have compared the plug flow results and the cylindrical shear flow results to make sure the plug flow model in the Chemkin-Pro software package [40] is appropriate (see details in the Supplementary Material). The reaction kinetic model (249 species and 8153 reactions) including detailed polycyclic aromatic hydrocarbons (PAHs) reactions is taken from Ranzi et al. [41]. We choose this model because it has been shown to have
good performance over a wide range of conditions in both fuel rich flames [11,41] (diffusion flames: C0 –C4 fuels, 0.5–10 atm; premixed flames: C2 H4 , ϕ = 2.07, 0–18% CO2 addition) and fuel pyrolysis [42,43] (shock tube: C2 H2 , 130 0–220 0 K, 1.1–2.6 atm; flow reactor: C2 H2 , 873–1473 K; CH4 , 1200 −1800 K; C2 H4 , 1700 K). The boundary conditions are set based on our experimental setup. Compared to the external heat source, the reaction heat release in the reactor is negligible. Consequently, all the modeling results are obtained based on the measured temperature profiles without solving the energy equations. 3. Results and discussion The PSDs at different CO2 mole fractions are compared in Figs. 2 and 3. All the measurements have been repeated three times for each sampling to ensure reproducibility, and the results are the average values under each condition. The repetitive error is less than 10%, while the total uncertainty is estimated to be ±30%, which mainly comes from the determination of the dilution ratio error [23,27]. Results show that at different CO2 mole fractions, all the PSDs exhibit a power-law function at short residence times, indicating the inception of nascent particles. As the residence time increases, a typical bimodal distribution emerges and lasts, char-
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acterizing the co-existence of a durable nucleation process and a gradually predominating mass growth process. Figure 2 shows the evolution of the PSDs for ethylene pyrolysis with 0, 1, 2, and 5% CO2 . It is seen that at short residence times, the number density of small particles increases by 1–2 orders of magnitude with 1–5% CO2 addition, indicating that the nucleation rate increases significantly with a small amount of CO2 addition. For all four cases, although the number densities are quite different, the shoulders appear at t = 126 ms and distinct bimodal distributions emerge at 136 ms, showing similar evolution processes. As the residence time is further increased (t = 149 ms), the concentration of small particles decreases and the mass growth is predominant. The median diameter increases remarkably with CO2 addition, implying higher coagulation or surface growth rates in the particle growth region [44]. Figure 3 shows the results for relatively high concentrations of CO2 addition. At short residence times, the number density of small particles increase by more than one order of magnitude. Yet the soot inception is delayed and the nucleated particles become fewer when the mole fraction of CO2 increases from 10% to 99.5%. This suppression phenomenon becomes more evident at t = 136 and 149 ms, where the median size of the second peak of the PSD is remarkably reduced as more CO2 is added, indicating an intense inhibition of particle growth, similar to the effects of CO2 addition on soot formation in premixed flames [10]. At t = 149 ms, with 10% CO2 addition, the particle median size is the largest among the seven conditions. A large amount of small particles are generated promptly at t = 117 ms with 99.5% CO2 , around two orders higher than that without CO2 . Unexpectedly, higher concentration of nucleated particles does not lead to a larger growth rate of particle size. At t = 149 ms, the median size of the second peak of the PSD with 99.5% CO2 is slightly smaller than that without any CO2 . The results indicate a higher nucleation rate but a quite lower mass growth rate with 99.5% CO2 . By integrating the number and volume of the particles over all mobility sizes measured (>3 nm) respectively, soot number density (N) and mobility volume fraction (Fv ) can be obtained. These values as functions of residence time are shown in Fig. S4 in the Supplementary Material. It clearly demonstrates that, for all conditions, as residence time increases N increases first due to particle nucleation and then decreases due to coagulation, while Fv has a two-stage increment dominated by nucleation and surface growth, respectively. To explore more about the effects of different CO2 concentrations, particle mobility volume fractions at three selected residence times were plotted as a function of the CO2 mole fraction in Fig. 4. To have a better resolution, we performed three extra measurements with 3%, 30%, and 75% CO2 respectively in the mixture. The trend demonstrating first promotion and then suppression is observed for all residence times. Nevertheless, the promotion effect becomes weaker as the reaction time is extended, which is probably because a small amount of CO2 would affect particle nucleation more than mass growth, and therefore more prominently accelerates soot formation at short residence times. The maximum soot yield occurs at around 10% CO2 addition, which is consistent with the PSD results in Figs. 2 and 3. When the CO2 mole fraction increases from 10% to 99.5%, the soot volume fraction decreases exponentially, similar to the results in the laminar premixed flames [10]. It is worth noting that in both flames and fuel pyrolysis, the CO2 addition leads to the formation of OH radicals through the reaction CO2 + H = CO + OH. However, only the inhibiting effect on soot yield was observed in previous flame studies, since the mole fraction of OH radicals is relatively quite high (> 10−5 ) and the addition of CO2 would generate even more OH radicals that promote soot oxidation [10]. By contrast, there is no oxygen in fuel pyrolysis, and 10% CO2 addition would produce a small amount of OH radicals (~10−6 ) that facilitate the
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Fig. 4. The particle volume fractions as a function of CO2 addition during ethylene pyrolysis at three selected residence time. Symbols are the experimental data and lines are fitted to the data.
Fig. 5. The maximum total number density and particle induction delay time as a function of CO2 addition during ethylene pyrolysis. Symbols are the experimental data and lines are fitted to the data.
formation of PAH and soot, which will be discussed in detail later. The maximum number density and induction delay time of soot particles are shown in Fig. 5. The particle induction delay time is defined as the time of appearance of the shoulder in PSDs, which indicates that the mass growth has become comparable and the volume fraction begins to increase rapidly. [23] The results show that the trend of the maximum number density is almost the same as that of the volume fraction. A very small amount of CO2 addition (1−3%) does not change the induction delay time within the uncertainty but intensively increases the quantity of particles. When more CO2 is added, the induction delay time declines and reaches the minimum at around 10% CO2 addition. Obviously, the experimental data show two opposite effects on soot particle formation with the addition of different amounts of CO2 . Kinetic modeling has been performed to further analyze the mechanism of the promotion and suppression. Figure 6 shows the calculated mole fractions of benzene and pyrene, which are two important soot precursors, as functions of the CO2 mole fraction at the reactor outlet. It is seen that the mole fraction of pyrene increases first then decreases as CO2 increases from 0% to 99.5%, and reaches the maximum value at 10% CO2 , which is very similar to the soot yield. It indicates that the concentration of pyrene can represent the intensity of soot formation to some extent. Pyrene is also regarded as an important species that directly contributes to the nucleation and condensation process [11,45,46]. The benzene mole fraction peaks at a smaller mole fraction of CO2 than pyrene, because CO2 not only promotes benzene formation but also
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Fig. 6. Mole fractions of benzene and pyrene at the outlet of reactor computed at F = 12 L/min as a function of CO2 addition.
the growth from benzene to pyrene, when the CO2 mole fraction is less than 10%. Figure S5 in the Supplementary Material shows that the peak value of the benzene mole fraction in the heating zone reaches the maximum with around 10% CO2 addition, and the mole fraction of pyrene increases rapidly in the meanwhile, showing that 10% CO2 indeed has the strongest promotion impact. To minimize the thermal and dilution effects of CO2 addition, we have kept a constant high temperature region and substituted N2 by an equivalent amount of CO2 . To verify if the chemical effect is dominant, we performed additional calculations by replacing CO2 with a chemical-inert carbon dioxide species (FCO2 ), which
possesses the same thermal properties as CO2 but does not participate in the reactions [10,16]. Figure S6 in the Supplementary Material compares mole fraction profiles of acetylene, propargyl, benzene, and pyrene at F = 12 L/min. We can see that with 10% FCO2 , the mole fractions of propargyl, benzene, and pyrene are only slightly lower than those with 0% CO2 due to the different thermal properties of CO2 and N2 . In contrast, the discrepancies between the cases with 10% CO2 and 0% CO2 are much more significant, demonstrating that the thermal and dilution effects of CO2 are negligible compared to the chemical effect. The chemical effect firstly acts on the reactions of small molecules and radicals, and consequently affects the formation of PAHs and soot. The mole fractions of hydrogen (H), hydroxyl (OH), methyl (CH3 ), propargyl (C3 H3 ), acetylene (C2 H2 ), and diacetylene (C4 H2 ) at F = 12 L/min are compared in Fig. 7. The addition of CO2 leads to a significant increase of the OH mole fraction and a relevant reduction of the H mole fraction as a result of the key reaction of CO2 + H = CO + OH. The mole fraction of C2 H2 species shows a monotonous decreasing trend as the inlet CO2 mole fraction is increased. Under the condition without CO2 (see the black line in Fig. 7), the inlet feedstock, ethylene, decomposes into acetylene and hydrogen radicals as soon as it enters the high temperature region, and the mole fraction of acetylene maintains at a high level along the high temperature region and the outlet region, indicating that the majority of the pyrolysis products is acetylene and very few of the carbon is converted to soot considering the carbon balance. However, when CO2 is added to the inlet feedstock, the mole fraction of C2 H2 and C4 H2 decreases obviously after the initial increase in the high temperature region. Correspondingly, the production of benzene and pyrene is promoted as shown in Fig. S5 in the Supplementary Material. Recombination of two propargyl radicals is one of the most competitive reaction pathways of benzene formation [47,48]. The
Fig. 7. Mole fractions of H, OH, CH3 , C3 H3 , C2 H2 , and C4 H2 computed at F = 12 L/min as a function of axial distance in the reactor.
J. Mei, X. You and C.K. Law / Combustion and Flame 215 (2020) 28–35
C3 H3 profiles show that a small amount of CO2 addition, e.g., 2% CO2 , increases the maximum mole fraction of C3 H3 by one order of magnitude, which corresponds to the effect on the benzene profile in Fig. 6. However, a further increase of OH from the condition of 10% CO2 to that of 99.5% suppresses the formation of all soot precursors including C2 H2 , C3 H3 , benzene, and pyrene as well as soot formation due to the strong oxidation effect. Compared to the case without CO2 , the mole fractions of key soot precursors such as propargyl, benzene, and pyrene are significantly higher with the addition of 99.5% CO2 , leading to the enhanced soot inception, which explains why much more small particles are produced in the case with 99.5% CO2 than those with 0% CO2 as shown in Fig. 3. To know more about what happened to the gas-phase chemistry, we performed sensitivity and reaction pathway analysis of benzene formation, since benzene is considered as the building block of large PAHs and soot [49]. The top 15 reactions that have the largest sensitivities at F = 12 L/min and x = 70 cm are shown in Fig. 8. It is well known that there are two major pathways to form benzene [47,50], namely the self-addition of C3 species and the combination of C2 and C4 species. The key reactions of these two pathways are shown in red and blue in Fig. 8, respectively. We can see from Fig. 8(a) that benzene formation is very sensitive to the C2 + C4 pathways represented by the reactions of C4 H3 + C2 H2 = C6 H5 or C4 H4 + C2 H2 = C6 H6 . Under the pyrolysis condition without CO2 , the odd number carbon species are difficult to form in the absence of oxygen, so the mole fraction of the C3 species is relatively low. The reaction of CO2 + H = CO + OH is essential to the chemical effects of CO2 addition (see Fig. 8(b) and (c)). With a small amount of CO2 addition (e.g., 10% in mole fraction), the hydroxyl radicals are formed, which accelerate the overall soot formation process due to its high reactivity, and consequently reduces the soot induction delay time as shown in Fig. 5. Meanwhile, the existence of OH can increase the mole fraction of the odd number carbon species through the reactions: C2 H2 + OH = CH3 + CO, C4 H2 + OH = C3 H2 + HCO, and CH3 + C2 H2 = pC3 H4 + H. Consequently, more C3 species are formed, facilitating benzene formation through the self-addition reactions. With much larger amount of CO2 , the hydroxyl radicals are excessive, demonstrating intense oxidation. As shown in Fig. 8(c), with 99.5% CO2 addition, the top ten reactions all involve oxygen-containing species, implying that more carbon is oxidized instead of being converted to soot precursors. In addition, the abundant hydroxyl radicals can also retard soot formation directly by surface oxidation [51]. Figures 9–11 show the main reaction pathways from ethylene to benzene at F = 12 L/min and x = 70 cm, for 0, 10, and 99.5% CO2 mole fraction, respectively. The thickness of the lines between species indicates the relative significance of a reaction pathway based on the absolute value of the rate of production, with the auxiliary reactants and product listed beside the lines. It is seen that there are two simultaneous and competing pathways to form benzene or phenyl as mentioned above. Under the condition of 0% CO2 , the C2 + C4 pathway is more competitive, as shown in Fig. 9, because it is difficult to generate the odd carbon number species without any oxygen. Decomposition reactions from ethane or ethyl through breaking the C–C bond are the only ways, while the mole fractions of ethane and ethyl are relatively low in this process. Previous modeling studies also reported the similar core pathways to benzene in acetylene pyrolysis, where C4 H4 and C4 H3 are the key intermediates [42]. Then the successive molecular or radical polymerization reactions lead to the formation of the first ring, such as C4 H3 + C2 H2 = C6 H5 , C4 H2 + C2 H3 = C6 H5 , and C4 H4 + C2 H2 = C6 H6 . However, the situation becomes very different as a small amount of CO2 is added to the reactants, where the formed OH accelerates the bond breaking of acetylene through the reaction of C2 H2 + OH = CH3 + CO or C2 H2 + OH = CH2 CHO, which
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Fig. 8. Normalized sensitivity of benzene formation at F = 12 L/min and x = 70 cm for CO2 mole fractions of (a) 0%, (b) 10%, and (c) 99.5%. Important reactions involved in benzene formation are marked red for C4 + C2 pathways, and blue for C3 + C3 pathways. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
directly or indirectly results in the formation of CH3 species, as shown in Fig. 10. Therefore, the C3 species can accumulate through the combination between C2 and C1 species or the oxidation of C4 species by OH. Compared to the C2 + C4 reactions, the C3 + C3 reactions represented by the self-reaction of two propargyl radicals have lower energy barriers and are much more competitive [52],
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J. Mei, X. You and C.K. Law / Combustion and Flame 215 (2020) 28–35
4. Conclusion
Fig. 9. Main reaction pathways from ethylene to benzene at x = 70 cm under the condition of 0% CO2 with F = 12 L/min based on the rate of product analysis. The thickness of the lines indicates the relative significance of a reaction pathway. The most importance pathways are circled by dashed line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
To investigate the effect of CO2 addition on sooting characteristics comprehensively, in this study we have conducted experimental measurements of soot formation and kinetic modeling of gasphase reactions during ethylene pyrolysis in a laminar flow reactor with a wide range of CO2 addition (0–99.5% in mole fraction). Particle size distribution, volume fraction, number density and induction delay time were examined using the orifice dilution sampling technique and the SMPS measurement system. Results show the trend of promotion followed by suppression in the soot yield as the CO2 mole fraction increases. With a small amount of CO2 addition (from 0% to 10%), the inception of the soot particles is activated earlier followed by more intense nucleation and mass growth. This promotion effect is more obvious at shorter residence times. The chemical effect of CO2 is dominated by the reaction of CO2 + H = CO + OH. A slight increase of the hydroxyl radicals facilitates the formation of odd number carbon species (e.g., propygyl), which sequentially promotes the formation of important soot precursors, such as benzene and pyrene that are important for soot nucleation and mass growth. With further increase of CO2 (from 10% to 99.5%), the total volume fraction decreases exponentially with the CO2 mole fraction and this suppression effect is more dominant in the mass growth process. A large amount of CO2 generates redundant hydroxyl radicals that lead to a strong oxidation effect and retards the PAHs and soot formation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the National Science Foundation of China (51761125012).
Fig. 10. Main reaction pathways from ethylene to benzene at x = 70 cm under the condition of 10% CO2 with F = 12 L/min based on the rate of product analysis.
Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2020.01. 015. References
Fig. 11. Main reaction pathways from ethylene to benzene at x = 70 cm under the condition of 99.5% CO2 with F = 12 L/min based on the rate of product analysis.
thereby accelerate formation of the first ring. Figure 11 shows the main pathways to benzene as well as the parallel oxidation pathways under the condition of 99.5% CO2 . As discussed above, 99.5% CO2 produces superfluous hydroxyl radicals, which can oxidize the soot precursors, such as acetylene. Consequently, more carbon in the fuel becomes carbon monoxide through a series of oxidation reactions instead of forming PAHs or soot.
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