A Detailed Numerical Study of NOx Kinetics in Counterflow Methane Diffusion Flames: Effects of Fuel-Side versus Oxidizer-Side Dilution
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Hindawi Journal of Combustion Volume 2021, Article ID 6642734, 15 pages https://doi.org/10.1155/2021/6642734 Research Article A Detailed Numerical Study of NOx Kinetics in Counterflow Methane Diffusion Flames: Effects of Fuel-Side versus Oxidizer-Side Dilution Huanhuan Xu ,1 Fengshan Liu,2 Zhiqiang Wang,1 Xiaohan Ren,3 Juan Chen,1 Qiang Li,4 and Zilin Zhu 1 1 National Engineering Laboratory for Reducing Emissions from Coal Combustion, School of Energy and Power Engineering, Shandong University, Jinan 250061, China 2 Measurement Science and Standards, National Research Council Canada, Building M-9 1200, Montreal Road, Ottawa, ON K1A 0R6, Canada 3 Institute of Thermal Science and Technology, Shandong University, 17923 Jingshi Road, Jinan 250061, Shandong, China 4 College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China Correspondence should be addressed to Zilin Zhu; zilinzhu323@gmail.com Received 14 October 2020; Revised 19 February 2021; Accepted 21 February 2021; Published 3 March 2021 Academic Editor: Siamak Hoseinzadeh Copyright © 2021 Huanhuan Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dilution combustion has been widely utilized due to various merits, such as enhanced efficiency, fewer pollutants emissions, and even a promising future in alleviating global warming. Diluents can be introduced through the oxidizer or fuel side to achieve the desired combustion properties, and H2O and CO2 are the most common ones. A comprehensive comparison between the different dilution methods still lacks understanding and optimizes the dilution combustion technologies. This study numerically compared the effects of H2O and CO2 dilution in the oxidizer or fuel stream on counterflow methane diffusion flames, em- phasizing NO formation kinetics. Results showed that the impact of different radiation heat transfer models on NO emissions diminishes with increasing the dilution ratio. The calculations of radiation heat transfer were treated in three ways: radiation- neglected, optically thin, and using a nongrey radiation model. When keeping the oxygen content and methane fraction constant, CO2 dilution in the air-side has the most profound influence on NO reduction, and CO2 dilution in the fuel-side has the least. H2O dilution showed a medium impact with a larger degree on air-side than that on fuel-side. To gain a deeper understanding of this effect order, the contributions of different NO formation routes were quantified, and analyses were made based on the diluents’ chemical and thermal effects. It was found that the oxidizer-side dilution and fuel-side dilution affect the NO formation pathway similarly. Still, the influence of H2O dilution on the NO formation pathway differs from that of CO2 dilution. 1. Introduction CO2 to control the flame temperature, enabling the cost- effective separation of the CO2 in flue gas physical con- Several advanced combustion technologies have been densation. In combustion technologies based on exhaust proposed to address the challenges of both fossil fuel de- gas recirculation (EGR) [3], part of the exhaust gas (mainly pletion and environmental pollution. Additives are widely H2O and CO2) is introduced back to the combustion used to enhance combustion efficiency, control nitric oxide chamber, and NOx emissions could be substantially re- emissions, or achieve low-cost CO2 capture. For example, duced. Internal Combustion Rankine Cycle (ICRC) power humidified air turbines (HATs) [1] with a mixture of air plants [4] run under an oxy-fuel combustion model with and water as the working fluid promise high electrical recycled water to achieve high efficiency and specific power efficiencies and low NOx emissions. Fossil fuels combust output. Other systems using water to provide power can be with pure oxygen in a Graz Cycle Gas Turbine [2] by using found in [5, 6].
2 Journal of Combustion To gain the full potential benefits of dilution combustion, Researchers have made a few attempts to explore the it is crucial to understand the effects of the following three difference between air- and fuel-side dilutions for jet diffusion primary factors: (1) the type of diluents, (2) the amount of flames. For instance, Feese and Turns [14] experimentally dilution, and (3) the location (oxidizer or fuel stream) where studied the effects of N2 dilution in the air- or fuel-side on the diluents are injected. NOx emissions in laminar CH4 jet diffusion flames. They Among the various combustion technologies mentioned found that air dilution was more effective than fuel dilution in above, carbon dioxide and steam (water vapor) are the most reducing NOx emissions. They also pointed out that the common additives. As H2O and CO2 are two main com- visible flame height increases somewhat as a diluent is added bustion products, adding these two diluents to the com- to the airstream but does not change noticeably for fuel-side bustion zone will not introduce new species but increase H2O dilution, even for the same diluent fraction. Unfortunately, and CO2 concentrations in the flue gas. As a result, CO2 Feese et al. did not analyze the NO formation routes or capture becomes much more manageable and economical. provide kinetic explanations for their results. In addition, an Besides, the high heat capacity of H2O and CO2 enables many experimental study by Cho and Chung [15] showed that fuel- favorable combustion properties, such as low flame tem- side dilution is more effective in reducing flame temperature perature [7, 8] and low NOx emissions [9]. To grasp the and NO emissions than air-side dilution. It should be noted dilution mechanism of H2O and CO2, many efforts have been that, although Cho and Chung also studied jet diffusion made. For example, Park et al. [10] simulated the flame flames, they reached a different conclusion from that in [14]. structure and NO emission behavior in CH4 diffusion flames This contradiction is due to the different dilution parameters with different diluents (H2O, CO2, and N2) added to the air used in these two studies when comparing the fuel-side di- stream. Their results showed that CO2 addition reduces the lution and the air-side dilution. In Cho and Chung’s study flame temperature through both the thermal and chemical [15], the dilution ratio was defined as the ratio of the exhaust effects. In contrast, the addition of H2O enhances the re- gas flow rate to the total flow rate of the fuel (or air). Because action through chemical effect and limits the decrease of the fuel stream’s exit area is much smaller than that of the temperature thermally. They also pointed out that the NO oxidizer stream, the fuel stream velocity increases more than emission index decreases with increasing the volume per- the oxidizer stream velocity increases. The much higher in- centage of diluents. The effectiveness of these diluents on NO crease in fuel stream velocity enhances the mixing in dilution emission reduction follows the order of CO2 > H2O > N2. Xie combustion. In the study of Feese and Turns, however, they et al. [11] found that the elementary reaction corresponding introduced a new dilution parameter (Z), calculated as the to the peak ROP (rate of production) of OH in CO/H2/air mass of diluent to the stoichiometric mixture’s mass. In this mixture changes from OH + H2 � H + H2O to way, under the same Z, the amount of diluent added to the air HO2 + H � OH + OH when CO2 and H2O are added, and stream for air-side dilution is much more considerable than CO2 has a more potent chemical effect than H2O. Xu et al. [8] that added to the fuel stream for fuel-side dilution. investigated the effects of H2O and CO2 diluted oxidizer on The effect of dilution on NO formation is one of the the structure and shape of laminar co-flow syngas diffusion issues this study aimed to resolve since one of the advantages flames. They found that the thermal and radiative effects of of dilution combustion is the low NO emissions, which is CO2 decrease the centerline temperature to a much larger strongly dependent on the combustion conditions, reported degree than those of H2O; thus, CO2 reduces the centerline in the previous papers, i.e., Refs. [16–18]. temperature more effectively than H2O. Besides, they also To obtain a fundamental understanding of the effect of found that as the dilution level increases, the maximum OH dilution position (i.e., through air or fuel stream) on flame mole fraction in CO2 diluted flames decreases monotonously, characteristics, attention has also been paid to one-di- whereas in H2O diluted flames, OH mole fraction first in- mensional diffusion flames, which excludes the burner’s creases then decreases. Ning et al. [12] numerically studied complexity and flow field in multidimension flames. Using the radiation effect on NO formation in H2/CO/air coun- spherical diffusion flames, Chernovsky et al. [19] experi- terflow diffusion flames, and results showed that, as the CO2 mentally investigated the impact of adding CO2 on the content in the fuel stream increases, the radiation effect on oxidizer side versus the fuel side. Their results showed that NO emission increases first and then decreases for H2-lean CO2 dilution affects the flame in these two dilution scenarios syngas. In contrast, for H2-rich syngas, the radiation effect is by different mechanisms. On the oxidizer side, radiation monotonic. Ozturk [13] studied the effects of CO2, N2, and reabsorption played an essential role in strengthening the H2O dilution on the adiabatic, turbulent, partially premixed flames. In contrast, the enhanced CO2 concentration in- combustion of synthesis gas and pointed out that increasing creased radiative heat losses on the fuel-side without the dilution rates gradationally reduces the NO concentra- prompting radiation reabsorption. Park et al. [20] numer- tion and H2O has the best effect on reduction of NO ically investigated the effects of CO2 addition to the fuel and emissions, followed by CO2 and N2. Although these studies the oxidizer streams on the structure of H2/O2 diffusion help fundamental understanding of the influence of diluents flame in counterflow configuration, and results showed that and dilution level on the combustion characteristics of hy- the temperature reduction caused by the chemical effects is drocarbon fuels, they were conducted for either fuel-side more remarkable when CO2 is added to the oxidizer stream dilution or air-side dilution. They hence did not provide a than that to the fuel stream. Liu et al. [21] numerically direct comparison of the relative effectiveness between fuel- studied the chemical effects of CO2 addition to both the fuel side dilution and oxidizer-side dilution. side and the oxidizer side of a laminar counterflow ethylene
Journal of Combustion 3 diffusion flame. The study of Liu et al. is highly relevant to considered to deal with the preferential diffusion of H2 and the present study, and their results showed that the chemical H. The grid adaptation parameters of GRAD and CURV effects of CO2 are weaker when added to the fuel side than to were both set to 0.05 to ensure that the computational the oxidizer side. NOx formation pathway analyses are es- meshes are sufficiently refined, and the simulation results are sential to reveal how CO2 addition affects NOx formation grid size independent. The global strain rate as defined in mechanisms; however, Liu et al. did not conduct this work equation (1) is fixed at 50 s−1 for the flames modeled in this but only focused on the chemical effects of CO2. paper unless otherwise indicated. The fuel and oxidizer Despite several studies that have been conducted to streams’ outlet velocities were chosen to satisfy the mo- understand the difference between fuel- and air-side dilu- mentum balance [29] expressed in equation (2): tions, there is still a lack of comprehensive comparison √�� 2 VO VF ρ between H2O dilution and CO2 dilution in the fuel- and F as � 1 + √��� , (1) oxidizer side. Many questions remain unanswered regarding L V O O ρ NO emissions, such as the radiation effect on NO emissions and the chemical effect of diluents on the NO formation ρO VO 2 � ρF VF2 , (2) mechanism. To this end, the objectives of this study were to investigate the influence of H2O and CO2 dilution in the air where ρ and V represent the density and velocity, respec- and fuel stream on methane/air counterflow diffusion flames tively, and the subscripts “F” and “O” indicate the fuel and with an emphasis on the detailed NO formation process, oxidizer streams. Symbol L is the distance between the fuel including not only the formation and destruction routes, but and oxidizer nozzles, specified as 2 cm in this study. also the reaction pathway analysis. The importance of gas radiation in modeling the methane/air diffusion flames was 2.2. Simulation Conditions. The flame conditions investi- first demonstrated for different diluting levels and in the air gated in this study are summarized in Table 1. It is worth and fuel stream with H2O or CO2. The effects of H2O and pointing out that when a certain amount of diluent (H2O or CO2 dilutions in the fuel and air stream on flame temper- CO2) was added to the fuel or the oxidizer stream, the same ature and NO concentration were next examined by amount of N2 was removed; therefore, the CH4 and O2 mole adopting the DOM/SNBCK radiation model. The diluents’ fractions could keep unchanged at 50 vol% and 21 vol% in chemical effects and thermal effects were isolated and an- the fuel and oxidizer stream, respectively. The pressure was alyzed. Finally, to reveal the impact of different diluents and 1 atm, and the inlet temperature of both the oxidizer and fuel dilution locations (air or fuel stream) on the NO formation streams was ranged from 400 K to 800 K in this study. mechanism, the contributions of different NO formation routes were calculated, and the NO formation pathways were revealed. 2.3. NO Formation Mechanism. To identify the contribu- tions of different NO formation routes, NO is considered to 2. Mathematical Model and Numerical Methods be formed by four pathways and destructed through NO- reburning. First, the thermal NO is formed through the N2 2.1. Simulation Methods. The OPPDIF code [22] coupled triple bond break-up by the O atom, and O2 and OH with the thermal and transport subroutines in the subsequently oxidize the N atom. This process has very high CHEMKIN package [23] was adopted in this study to activation energy and is strongly affected by flame tem- compute the counterflow diffusion flames. This code has perature. The main reactions related to NO formation in the been extensively validated and used to model one-dimen- GRI-Mech 2.11 mechanism are listed in Table 2. Reactions sional laminar flames (nonpremixed or premixed) using responsible for thermal NO include R178, R179, and R180. detailed combustion chemistry and thermal and transport Second, the prompt NO can be prevalent in hydrocarbon properties. The methane oxidation chemistry was modeled flames since it is produced via reactions of CHi radicals, and using the GRI-Mech 2.11 mechanism [24]. Many studies R240 is usually considered as the major initiation step for the [25, 26] have demonstrated that GRI-Mech 2.11 produces a formation of prompt NO. The reactions involved in prompt better prediction for NOx formation than the GRI-Mech 3.0 NO are rate-limiting, leading to the prompt NO formation mechanism when modeling methane combustion. To il- less temperature-dependent. Third, the N2O-intermediate lustrate the importance of radiation heat transfer, three NO is formed through the path of N2⟶N2O⟶NO, with different treatments of thermal radiation were considered, N2O being the intermediate species. Fourth, the NNH NO namely, no radiation heat loss, the optically thin approxi- begins with N2 reacting with H to form NNH (R204 and mation (OTA) [27], and the discrete-ordinates method R205), which then yields NO through R208. Lastly, NO- (DOM) coupled with the statistical narrow-band correlated- reburning is an important destruction mechanism of NO by K-based radiative property model (DOM/SNBCK) for a group of reactions involving hydrocarbon radicals, CHi. solving the radiative transfer equation (RTE) [28]. The OTA The calculation method of these NO formation routes has only accounts for radiation loss due to emissions. It neglects been widely employed in previous studies, e.g., Refs. radiation gain by absorption, while the DOM/SNBCK model [27, 30, 31]. considers both radiation emission and absorption, as well as The full GRI-Mech 2.11 mechanism contains 49 species the non-gray nature of gas radiative properties. Multi- and 279 elementary reactions, and the number of reaction component transport properties and thermal diffusion were steps decreases to 177 when excluding NO formation. For
4 Journal of Combustion Table 1: Conditions of diluted methane/air counterflow diffusion flames with and without H2O or CO2 replacement of N2, 400 K, 1 atm. Fuel stream composition (vol%) Oxidizer stream composition (vol%) No. Remark CH4 N2 H2O CO2 O2 N2 H 2O CO2 Case 1 50 50 0 0 21 79 0 0 Case 2 50 40 10 0 21 79 0 0 Baseline H2O dilution (fuel-side) Case 3 50 30 20 0 21 79 0 0 Case 4 50 10 40 0c 21 79 0 0 Case 5 50 40 0 10 21 79 0 0 Case 6 50 30 0 20 21 79 0 0 CO2 dilution (fuel-side) Case 7 50 10 0 40 21 79 0 0 Case 8 50 50 0 0 21 69 10 0 Case 9 50 50 0 0 21 59 20 0 H2O dilution (air-side) Case 10 50 50 0 0 21 39 40 0 Case 11 50 50 0 0 21 69 0 10 Case 12 50 50 0 0 21 59 0 20 CO2 dilution (air-side) Case 13 50 50 0 0 21 39 0 40 Table 2: Main reactions involved in NO formation (more information can be found in the GRI-Mech 2.11 mechanism [24]). Reaction no. Reaction steps Reaction no. Reaction steps R178 N + NO �> N2 + O R179 N + O2 ≤ NO + O R180 N + OH � NO + H R182 N2O + O ≤ 2NO R183 N2O + H � N2 + OH R185 N2O(+M) ≤ N2 + O(+M) R186 HO2 + NO � NO2 + OH R187 NO + O + M � NO2 + M R189 NO2 + H � NO + OH R190 NH + O � NO + H R191 NH + H � N + H2 R192 NH + OH � HNO + H R193 NH + OH � N + H2O R194 NH + O2 � HNO + O R195 NH + O2 � NO + OH R197 NH + H2O � HNO + H2 R199 NH + NO � N2O + H R201 NH2 + O � H + HNO R202 NH2 + H � NH + H2 R203 NH2 + OH � NH + H2O R204 NNH � N2 + H R205 NNH + M � N2 + H + M R206 NNH + O2 � HO2 + N2 R207 NNH + O � OH + N2 R208 NNH + O � NH + NO R209 NNH + H � H2 + N2 R210 NNH + OH � H2O + N2 R212 H + NO + M � HNO + M R213 HNO + O � NO + OH R214 HNO + H � H2 + NO R215 HNO + OH � NO + H2O R216 HNO + O2 � HO2 + NO R217 CN + O � CO + N R218 CN + OH � NCO + H R219 CN + H2O � HCN + OH R220 CN + O2 � NCO + O R221 CN + H2 � HCN + H R223 NCO + H � NH + CO R231 HCN + O � NCO + H R232 HCN + O � NH + CO R234 HCN + OH � HOCN + H R235 HCN + OH � HNCO + H R236 HCN + OH � NH2 + CO R240 CH + N2 � HCN + N R246 CH + NO � HCN + O R249 CH2 + NO � H + HNCO R250 CH2 + NO � OH + HCN R251 CH2 + NO � H + HCNO R255 CH3 + NO � HCN + H2O R265 HNCO + H � NH2 + CO R270 HCNO + H � H + HNCO R272 HCNO + H � NH2 + CO R273 HOCN + H � H + HNCO R274 HCCO + NO � HCNO + CO clarity, we abbreviate these two mechanisms as “he full GRI- intermediate routes can be obtained by subtracting results of Mech 2.11 mechanism conta respectively. To quantify NO the corresponding SIM2∼SIM5 with that of the SIM1. NO- formation from the four formation routes plus one de- reburning effect on NO was calculated by the difference in struction route, each simulation was performed six times results of SIM6 and SIM1. with different NO formation kinetics models, namely, “Full chemistry” (marked as SIM1), “no NO chemistry” with only the thermal, prompt, NNH, or N2O-intermediate NO for- 2.4. Chemical and Thermal Effects of H2O/CO2. To quanti- mation route, marked as SIM2 ∼ SIM5, respectively, and tatively investigate the chemical and thermal effects of H2O “Full chemistry” excluding NO-reburning route (SIM6). The or CO2 dilution, calculations with real H2O and CO2 and elementary steps involved in calculating each NO formation their chemically inert counterpart were conducted as we did route are identical to these in Ref. [30]. As a result, NO in the previous work [8]. Specifically, this chemical effect formations through the thermal, prompt, NNH, or N2O- refers to the diluents’ impact through their participation the
Journal of Combustion 5 chemical reactions. The thermal effect means the influence 2100 CO2 dilution caused by the difference of the diluents’ thermal properties from those of N2. The two artificial species, FH2O and FCO2, 2000 have the same thermal, transport, and radiative properties as 1900 these of the real H2O and CO2, respectively, except that FH2O and FCO2 are chemically inert and do not participate 1800 in chemical reactions. To be specific, when evaluating CO2 and H2O’s chemical effects, two sets of simulations were 1700 Temperature maximum (K) conducted. One was using the real CO2 and H2O as the 1600 Black lines: air-side dilution diluent, and the other was using FCO2 and FH2O. Therefore, Red lines: fuel-side dilution differences between results based on FH2O and FCO2 and 1500 those using H2O and CO2 are attributed to H2O and CO2’s chemical effects. Similarly, to quantify H2O and CO2’s 2100 H2O dilution thermal effects, another pair of artificial species, TH2O and TCO2, having the same chemical, transport, and radiative 2000 properties as FH2O and FCO2 but share the same thermal 1900 properties with N2 were introduced to the reaction mech- anisms. As a result, differences in the numerical results using 1800 TH2O (or TCO2) additions and these using FH2O (or FCO2) additions are caused by H2O and CO2’s thermal effects. 1700 1600 3. Results and Discussion 1500 3.1. Effect of Radiation Models. Since H2O and CO2 are the 0 10 20 30 40 most critical radiative gases in hydrocarbon combustion, it is Dilution ratio (%) expected that H2O and CO2 enhance radiation heat transfer No radiation in the combustion zone. The results calculated using the SNBCK three radiation models were first examined to investigate OTA radiation treatment’s effect on modeling the different diluted flames. Figure 1 displays the predicted peak flame tem- Figure 1: Variation of the peak flame temperature of CH4/air peratures using the three radiation heat transfer treatments counterflow diffusion flames with the dilution ratio for H2O and for CO2 and H2O dilution added to the fuel and air streams. CO2 addition to the air and fuel side: as � 50 s−1, L � 2 cm, 400 K. Some common behaviors can be observed from this figure. For example, CO2 dilution has a more decisive influence than H2O dilution due to its higher specific heat capacity per Besides, the radiation model’s effect on the NO mole volume [7, 8]. As expected, neglecting the thermal radiation fraction is quite significant. Consistent with the radiation results in overprediction of flame temperature, while OTA model’s effects on the peak flame temperature, when under predicts the peak flame temperature. Since the DOM/ neglecting the radiation heat transfer, OTA predicts the SNBCK model considers both emission and absorption of lowest NO mole fraction. At a given flame condition, the radiation, it predicts the peak flame temperature lower than NO mole fraction predicted by the DOM/SNBCK model that of no radiation but higher than OTA. It is interesting to falls between no radiation and OTA but closer to no ra- notice that the peak temperature predicted by DOM/SNBCK diation. However, the radiation model’s effect on the is much closer to that of no radiation, suggesting that there is predicted peak NO mole fraction weakens with increase in significant radiation reabsorption in flames to reduce the net the dilution ratio, especially for the air-side dilution. As radiation loss. These results also show that the air-side di- shown in Figure 2, when dilution occurs in the air-side lution (black lines) has a stronger influence on suppressing (black lines), both H2O dilution and CO2 dilution reduce the flame temperature than the fuel-side dilution (red lines). the peak NO mole fraction significantly; however, the Kinetics analysis shows this is attributed to that the heat reduction rate continuously weakens as the dilution ratio release rate of reactions O + C2H2 � CO + CH2 and increases. The different influence of dilution on the peak H + O2 + H2O � HO2 + H2O is more suppressed when di- NO mole fraction form the impact on the peak flame lution occurs at the air-side than that at the fuel-side. In temperature, i.e., the nonlinear reduction of the peak NO particular, an interesting phenomenon observed in Figure 1 mole fraction in Figure 2 and nearly linear reduction in the is that the impact of H2O and CO2 dilution on the peak flame peak flame temperature in Figure 1, implies that the temperature is nearly linear for dilution ratio up to 40%, no temperature-sensitive thermal NO route is not the primary matter dilution occurs in the fuel side or the air-side. pathway for NO formation under the conditions of this The effects of radiation models on the predicted peak study. This point will be discussed later. Since the effect of NO mole fractions for CO2 and H2O dilution on the fuel fuel-side dilution on the NO mole fraction maximum is and air stream are shown in Figure 2. The variation of peak much weaker than that of air-side dilution, the nonlinear NO mole fraction with dilution ratio is clearly nonlinear. trend is not pronounced, especially for CO2 dilution on the
6 Journal of Combustion CO2 dilution sheet almost remains at the same position since the mo- 8.0E – 5 mentum was balanced by equation (2) in all the cases. To gain further insights into how H2O and CO2 dilution 6.0E – 5 affect the flame temperature, Figure 4 shows the separate effects of 20% H2O and 20% CO2 dilution to the air and fuel 4.0E – 5 side on the peak temperature. The chemical and thermal effects were analyzed in this paper since the transport and 2.0E – 5 NO mole fraction maximum radiation effects are less important [8], though the radiation effect can become significant at low strain rates. It can be 0.0 Black lines: air-side dilution seen that the thermal effects of both H2O and CO2 dilution Red lines: fuel-side dilution reduce the flame temperature. Although the chemical effect of CO2 lowers the flame temperature, the chemical effect of 8.0E – 5 H2O dilution H2O increases it, albeit only slightly. This finding agrees with that reported in the previous studies [7, 8]. Besides, Figure 4 6.0E – 5 also shows that the dilution position (air-side or the fuel- side) does not change the individual effects of H2O and CO2 4.0E – 5 dilution on flame temperature qualitatively, namely, the chemical effects of CO2 and the thermal effects of H2O and 2.0E – 5 CO2 always lower the flame temperature while the chemical effect of H2O slightly increases it. In addition, both the 0.0 overall and individual effects of H2O and CO2 dilutions to the air-side are greater than those to the fuel-side. Kinetics 0 10 20 30 40 analyses show that it is because that heat released by re- Dilution ratio (%) actions O + C2H2 � CO + CH2 and H + O2 + H2O � HO2 + H2O is more suppressed when dilution occurs at the air- No radiation side than that at the fuel-side. SNBCK OTA Figure 2: Variation of the peak NO mole fraction of CH4/air 3.3. NO Mole Fraction. Figures 5(a) and 5(b) present the counterflow diffusion flames with the dilution ratio by H2O and distributions of NO mole fraction as a function of the CO2 dilution on the air and fuel side: as � 50 s−1, L � 2 cm, 400 K. distance from the fuel nozzle for H2O and CO2 dilution, respectively. It is evident that both H2O and CO2 dilution reduces NO formation significantly and dilution to the air fuel-side. For the fuel-side dilution, thermal NO is not the stream has a stronger influence on NO reduction than the dominant route, as illustrated in Section 3.4. fuel side, regardless of CO2 or H2O dilution. The effects of CO2 and H2O dilution on NO mole fraction correlate well 3.2. Flame Temperature. From this section, the numerical with those on flame temperature. When the air-side is di- results were calculated using the DOM/SNBCK radiation luted (red lines), CO2 dilution suppresses NO formation model since this radiation treatment provides the most more significantly than H2O dilution, again consistent with accurate treatment of radiation heat transfer. In this part, the their effects on the peak flame temperature. While for the influence of H2O and CO2 dilution on flame temperature is fuel-side dilution (blue lines), the effect of H2O dilution on discussed in detail. Figure 3 displays the temperature dis- suppressing NO formation is more significant than that of tributions for H2O and CO2 dilution up to 40% on the fuel CO2 dilution, which deserves further investigations because and air-side as a function of the distance from the fuel the importance of these two kinds of diluents on decreasing nozzle. The differences in the peak flame temperature be- NO formation is opposite to that on lowering the flame tween Case 1 (no dilution) and the cases of 40% H2O and temperature (Figure 3). 40% CO2 dilution are also indicated. Although H2O and CO2 To understand the phenomenon that the effect of fuel- dilution on either the air or the fuel-side lowers the flame side dilution by H2O on NO reduction is greater than that temperature consistently, differences between H2O dilution by CO2 dilution, CH radical distributions with or without and CO2 dilution still exist. First, CO2 is more effective than 40% H2O and CO2 dilution in the air- and fuel-side are H2O in reducing the flame temperature, regardless of if it compared in Figure 6. H2O dilution on the fuel-side in- being added to the air or the fuel side. This difference is hibits CH production more significantly than CO2, which mainly caused by the higher heat capacity of CO2. Second, explains the greater suppression impact of H2O dilution H2O dilution always slightly shifts the peak flame temper- than CO2 dilution on NO formation in Figure 5. Although ature towards the air nozzle, but CO2 dilution can shift the the temperature of fuel-side CO2 dilution is higher than peak temperature slightly towards either the fuel nozzle or that of fuel-side H2O dilution, the significantly reduced CH the oxidizer nozzle when, respectively, added to the fuel or concentration of fuel-side H2O dilution slows down the air stream. This difference is related to the relative molar initial reaction of prompt NO, i.e., CH + N2 � HCN + N weights of N2, H2O, and CO2. Nevertheless, the flame (R240). Consequently, the total NO emission is inhibited
Journal of Combustion 7 2000 2000 41 K 1900 129K 1800 Temperature (K) 1800 1600 1700 Temperature (K) 1400 1600 1200 1500 0.95 1.00 1.05 1.10 1.15 1.20 1.25 Distance (cm) 1000 900 850 Temperature (K) 800 800 750 600 700 650 600 400 0.76 0.78 0.80 0.82 Distance (cm) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Distance (cm) No dilution (Case 1) 10% H2O dilution in fuel 10% H2O dilution in air 20% H2O dilution in fuel 20% H2O dilution in air 40% H2O dilution in fuel 40% H2O dilution in air (a) 2000 2000 48 K 1900 Temperature (K) 1800 256 K 1800 1600 1700 Temperature (K) 1400 1600 1200 1500 1.00 1.05 1.10 1.15 1.20 1.25 Distance (cm) 1000 800 600 400 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Distance (cm) No dilution (Case 1) 10% CO2 dilution in fuel 10% CO2 dilution in air 20% CO2 dilution in fuel 20% CO2 dilution in air 40% CO2 dilution in fuel 40% CO2 dilution in air (b) Figure 3: Distributions of flame temperature in CH4/air counterflow diffusion flames with (a) H2O dilution and (b) CO2 dilution as a function of distance from the fuel nozzle: as � 50 s−1, L � 2 cm, 400 K.
8 Journal of Combustion 20% CO2 2000 20% H2O 20% CO2 dilution 20% H2O dilution in dilution in air Temperature maximum (K) in fuel dilution in air fuel 1950 1900 1850 1800 No dilution (Case 1) Overall effect TH2O / TCO2 Chemical effect FH2O / FCO2 Thermal effect Real H2O / CO2 Figure 4: The overall and individual effects of 20% CO2 and 20% H2O dilution on the peak flame temperature of CH4/air counterflow diffusion flame: as � 50 s−1, L � 2 cm, 400 K. 7E – 5 7E – 5 6E – 5 6E – 5 Decreased Decreased by 37.8% 5E – 5 by 51.4% 5E – 5 NO mole fraction NO mole fraction 4E – 5 Decreased 4E – 5 Decreased by 93.1% by 95.7% 3E – 5 3E – 5 2E – 5 2E – 5 1E – 5 1E – 5 0 0 –1E – 5 –1E – 5 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Distance (cm) Distance (cm) No dilution (Case 1) No dilution (Case 1) 10% H2O dilution in air 10% CO2 dilution in air 20% H2O dilution in air 20% CO2 dilution in air 40% H2O dilution in air 40% CO2 dilution in air 10% H2O dilution in fuel 10% CO2 dilution in fuel 20% H2O dilution in fuel 20% CO2 dilution in fuel 40% H2O dilution in fuel 40% CO2 dilution in fuel (a) (b) Figure 5: Distributions of NO mole fraction in CH4/air counterflow diffusion flames as a function of distance from the fuel nozzle with H2O, (a) and CO2, (b) dilution: as � 50 s−1, L � 2 cm, 400 K. since the prompt NO is the dominant NO formation route. how strain rates and inlet temperature affects the NO for- This point will be discussed in more detail in Section 3.4. mation are beyond this paper’s scope. Most simulations in this study were conducted at Figure 9 displays the individual effects of 20% H2O and T � 400 K and as � 50 s−1. To show that the effects of dilution 20% CO2 dilution on the peak NO mole fraction. The di- on NO reduction under these conditions are generalizable, lution effects on NO reduction in the air-side are stronger Figures 7 and 8 exhibit the peak NO mole fraction at dif- than those in the fuel-side, same as the influence on flame ferent strain rates (from 20 to 100 s−1) and inlet temperature temperature, Figure 4. In Figure 9, the chemical effect of (from 400 to 600 K), and very similar trends can be found H2O dilution exerts a decreasing impact on NO emission among the displayed lines. However, the detailed analyses of despite promoting the flame temperature in Figure 4. This
Journal of Combustion 9 2.0E – 6 CO2 effect in fuel-side H2O effect 1.5E – 6 CH mole fraction in fuel-side CO2 effect H2O effect 1.0E – 6 in air-side in air-side 5.0E – 7 0.0 1.00 1.04 1.08 1.12 1.16 1.20 1.24 Distance (cm) No dilution (Case 1) 40% CO2 in air 40% H2O in air 40% CO2 fuel 40% H2O in fuel Figure 6: Distributions of CH mole fraction in CH4/air counterflow diffusion flames with and without 40% H2O and 40% CO2 dilution on the air and fuel sides: as � 50 s−1, L � 2 cm, 400 K. 9E – 5 9E – 5 as = 20 s–1 as = 20 s–1 8E – 5 8E – 5 7E – 5 as = 50 s –1 7E – 5 as = 50 s–1 Fuel side 6E – 5 NO mole fraction 6E – 5 NO mole fraction as = 100 s–1 Fuel side as = 100 s–1 5E – 5 5E – 5 4E – 5 4E – 5 3E – 5 3E – 5 2E – 5 Air side 2E – 5 1E – 5 1E – 5 0 Air side 0 0 10 20 30 40 0 10 20 30 40 Dilution ratio (%) Dilution ratio (%) Strain rate = 20 Strain rate = 20 Strain rate = 50 Strain rate = 50 Strain rate = 100 Strain rate = 100 (a) (b) Figure 7: The peak NO mole fraction of CH4/air counterflow diffusion flames with different strain rates: as � 20 s−1 (black), 50 s−1 (red) and 100 s−1 (blue). L � 2 cm, 400 K. (a) H2O dilution. (b) CO2 dilution. can be easily understood from the distribution of CH radical calculated using equation (3), which has been widely utilized shown in Figure 6. Although the flame temperature is en- in previous studies, such as [10, 30]. hanced slightly by H2O dilution, the CH concentration is L reduced more considerably, which results in more signifi- 0 ϖNO MNO dx EINO � L , (3) cant NO reduction through the prompt NO route than the − 0 ϖF MF dx prompting effect on NO through the thermal NO route. where ωNO and ωF are the NO production rate and fuel consumption rate, respectively, and MNO and MF are the 3.4. NO Formation Mechanism. Figure 10 displays the molecular weight of nitric oxide and fuel, respectively. emission index of NO via different formation routes as Firstly, it can be observed that the effects of H2O and introduced in Section 2.3. The emission index of NO was CO2 dilution in the air- or fuel-side on EINO correlate well
10 Journal of Combustion 1.4E – 4 600 K 1.4E – 4 600 K 1.2E – 4 1.2E – 4 1.0E – 4 500 K 1.0E – 4 500 K Fuel side NO mole fraction NO mole fraction Fuel side 8.0E – 5 8.0E – 5 400 K 400 K 6.0E – 5 6.0E – 5 4.0E – 5 4.0E – 5 Air side 2.0E – 5 2.0E – 5 0.0 Air side 0.0 0 10 20 30 40 0 10 20 30 40 Dilution ratio (%) Dilution ratio (%) 400 K 400 K 500 K 500 K 600 K 600 K (a) (b) Figure 8: The peak NO mole fraction of CH4/air counterflow diffusion flames with different inlet temperatures: T � 400 K (black), 500 K (red), and 600 K (blue). as � 50 s−1, L � 2 cm. 9E – 5 NO emission than the no-dilution case (Case 1). It can be 20% H2O 20% H2O 20% CO2 20% CO2 8E – 5 dilution in dilution in dilution dilution observed that the order of NO reduction percentage through air fuel in air in fuel the prompt NO route and NO-reburning for different di- NO mole fraction maximum 7E – 5 6E – 5 lutions totally agrees well with that through the full chemistry, i.e., air-side CO2 dilution > air-side H2O dilu- 5E – 5 tion > fuel-side H2O dilution > fuel-side CO2 dilution. 4E – 5 However, H2O and CO2 dilution’s influence on the third 3E – 5 most important NO formation route (NNH pathway) does 2E – 5 not follow the above order. Even so, it can still be concluded 1E – 5 that the relative importance of H2O and CO2 dilution to NO formation reduction is determined by the dilution impact on 0 the prompt NO formation and NO-reburning routes under –1E – 5 the conditions of this study. No dilution (Case 1) Overall effect Using the method proposed by Revel et al. [32], the TH2O / TCO2 Chemical effect elemental fluxes of N through some critical reactions were FH2O / FCO2 Thermal effect calculated, and the pathways of NO formation based on the Real H2O / CO2 full chemistry of the GRI-Mech 2.11 for Case 1, Case 3, Case 6, Case 9, and Case 12 are demonstrated in Figure 11. It is Figure 9: Peak NO mole fractions of CH4/air counterflow diffusion worth pointing out that N’s elemental fluxes were calculated flame with and without 20% H2O and 20% CO2 dilution: as � 50 s−1, along with the whole computational domain rather than just L � 2 cm 400 K. at the flame front, indicating the average value over the entire combustion process. In Figure 11, the arrow shows the with those on the peak NO mole fraction shown in Figures 5 progress direction of a certain conversion, from the reactants and 9, namely, air-side CO2 dilution > air-side H2O dilu- to the products. The values on each arrow indicate the N flux tion > fuel-side H2O dilution > fuel-side CO2 dilution. This over the entirely computational domain (from the fuel is because that CH4 flow rate was kept constant for all the nozzle exit to the oxidizer nozzle exit with a distance of 2 cm cases, and EINO was calculated over the entire computa- in this study), and the width of each arrow distinguishes the tional domain in this study. Secondly, the prompt route magnitude of the N flux. The percentage after the value plays the dominant role in NO formation, followed by the means the ratio of the N flux reduction relative to the NO-reburning and NNH route, while the thermal and N2O- baseline case (Case 1) through the reaction after it in pa- intermediate routes contribute negligibly. For clarity, the rentheses. The pathway map in Figure 11 covers the five NO percentages of reduction of total NO, prompt NO, NNH formation/destruction routes, and the conversion paths can NO, and NO-reburning for 20% H2O and 20% CO2 dilution be briefly highlighted as the thermal route are also marked in Figure 10. For example, 18.8% means (N2⟶N⟶NO), prompt route (N2⟶HCN⟶CN, adding 20% CO2 into the fuel stream (Case 6) reduces 18.8% NCO, NH, N, and HNO⟶NO), NNH route (N2⟶NNH
Journal of Combustion 11 1.2 1.07 1.0 23.7% 0.89 18.8% 0.8 27.8% 33.4% EINO (g/kg) 0.6 0.41 26.6% 79.5% 75.3% 0.4 71.8% 38.8% 75.3% 15.3% 0.05 0.19 26.7% 70.3% 75.7% 84.9% 0.11 0.2 85.1% 0.0 No 20% CO2 20% CO2 20% H2O 20% H2O dilution in air in fuel in air in fuel Full chemistry N2O-intermediate Thermal NNH Prompt NO-reburning Figure 10: The percentages of EINO reduction by 20% H2O and 20% CO2 dilution in the air and fuel side of CH4/air counterflow diffusion flame: as � 50 s−1, L � 2 cm, 400 K. 7.19, (R246, R250, R255) 8.08, (R249, R251) 2.56, (R273) 1.01, (R270) HOCN 5.38, (R235) 1.66, (R265) 5.13, (R272) HNCO NH2 HCNO (R234) 2.57, 1.31 5.62, (R231) (R202, R203) 3.98, (R201) HCN 8.55, (R223) NCO 1.43, (R232) 1.30, (R192) NH 3.40, (R218, R220) (R194, HNO (R191, R193) 2.41(R190) 3.03, (R219, R221) R197) 4.09, (R240) CN 1.69, (R214, R215) 9.39, 2.08, (R212) 1.06, (R274) 4.02, (R183) 3.64, (R199) N2O 9.25, (R182) 4.68, (R185) 1.41, N2 N (R179, R180) 5.38, 3.40, (R178) NO (R206, R207) 2.26 (R204, R205) 1.57, 1.57, (R209, R210) (R188, R189) (R186, R187) NNH 1.37, (R208) NO2 ~ ×10–11 ~ ×10–9 unit: mole cm–3 s–1 ~ ×10–10 ~ ×10–8 (a) Figure 11: Continued.
12 Journal of Combustion 5.06, 29.6%, (R246, R250, R255) 6.11, 24.4%, (R249, R251) 1.75, 31.6%, (R273) 7.95, 21.3%, (R270) HOCN 4.05,21.1%, 1.23, 25.9%, (R265) 1.75, 31.9% 3.74, 30.5%, (R235) NH (R272) HCNO HNCO (R234) 2 9.52, 27.3% 4.10, 27.1% (R202, R203) (R231) 3.23, 18.8%, (R201) HCN 6.32, 26.1%, (R223) NCO 1.04, 27.3%, (R232) 9.69, 27.8% 2.46, 27.6%, (R218, R220) NH (R192) HNO 2.28, 24.8% (R194, R197) 3.06, 25.2% (R240) (R191, R193) 1.29,23.7%,(R214, R215) 6.74, 28.2% 1.87,22.4% (R219, R221) 8.44, 20.4%, (R274) CN (R190) 1.69, 18.8%, (R212) 3.48, 4.4% (R199) 3.51, 12.7%, (R183) N2O 8.17, 11.7% 4.13, 11.8%, (R182) (R185) 1.02, 27.7% N2 N (R179, R180) 3.10, 8.8%, (R178) NO 4.55, 15.4% 1.91, 15.5% (R206, R207) (R204, R205) 1.25, 20.4% 1.26, 19.8% (R209, R210) (R188, R189) (R186, R187) NNH 1.14, 16.%, 8(R208) NO2 ~ ×10–11 ~ ×10–9 unit: mole cm–3 s–1 ~ ×10–10 ~ ×10–8 (b) 8.92, 87.6%(R246, R250, R255) 1.40, 82.67%(R249, R251) 2.76, 89.2%, (R273) 2.12, 79.01%, (R270) HOCN 1.08,79.0%, 6.52, 87.9%, (R235) 2.61, 84.3%, (R265) 2.77,89.2% HNCO NH (R272) HCNO (R234) 2 1.81, 86.2% 9.12, 83.8% (R202, R203) 9.67, 75.7%, (R201) HCN (R231) 1.44, 83.2%, (R223) NCO 2.31, 83.9%, (R232) 2.12, 83.7% 5.37, 84.21%, (R218, R220) NH (R192) HNO 5.41, 82.2% (R194, 8.14,80.1%, (R240) (R191, R193) 3.17,81.2%,(R214, R215) 2.32,78.1%, (R274) 1.32, 85.9% 5.17, 78.6% (R219, R221) R197) 6.48, 68.6%, (R212) CN (R190) 1.50, 58.79% (R199) 1.49, 62.9%, (R183) N2O 3.13, 66.16% (R182) 1.77, 62.2%, (R185) 2.02, 85.7% N2 N (R179, R180) 6.47, 81.0%, (R178) NO 1.84, 65.8% 7.70, 65.9% (R206, R207) (R204, R205) 4.04, 74.3% 4.06, 74.1% (R209, R210) (R188, R189) (R186, R187) NNH 3.88, 71.7%, (R208) NO2 ~ ×10–11 ~ ×10–9 unit: mole cm–3 s–1 ~ ×10–10 ~ ×10–8 (c) Figure 11: Continued.
Journal of Combustion 13 4.43, 38.4%, (R246, R250, R255) 5.54, 31.8%, (R249, R251) 1.69, 34.0%, (R273) 7.16, 29.1%, (R270) HOCN 3.65,28.9%, 3.60, 33.1%, (R235) 1.12, 32.5%, (R265) 1.69, 34.3% HNCO NH2 (R272) HCNO (R234) 8.91, 32.0% 3.38, 39.9% (R202, R203) (R231) 2.78, 30.2%, (R201) HCN 5.22, 39.0%, (R223) NCO 9.18, 29.4% 8.59, 39.9%, (R232) 2.13, 37.4%, (R218, R220) NH (R192) HNO 2.74. 33.0%, (R240) 1.97, 35.0% (R194, (R191, R193) 1.16,31.4%,(R214, R215) 5.65, 39.8% 7.60, 28.3%, (R274) 1.45, 39.8% (R219, R221) R197) 1.07, 48.6%, (R212) CN (R190) 3.70, -1.6% (R199) 3.37, 16.2%, (R183) N2O 7.50, 18.9% (R182) 3.99, 14.7%, (R185) 8.76, 37.9% N2 N (R179, R180) 3.72, -9.4%, (R178) NO 4.36, 19.0% 1.89, 16.4% (R206, R207) (R204, R205) 1.06, 32.5% 1.07, 31.9% (R209, R210) (R188, R189) (R186, R187) NNH 1.01, 26.3%, (R208) NO2 ~ ×10–11 ~ ×10–9 unit: mole cm–3 s–1 –10 –8 ~ ×10 ~ ×10 (d) 1.35, 81.2%, (R246, R250, R255) 1.97, 75.6%, (R249, R251) 6.35, 75.2%, (R273) 2.94, 70.9%, (R270) HOCN 1.50,70.8% 4.09, 75.4%, (R265) 6.36, 75.3% 1.40, 74.4%, (R235) HNCO NH (R272) HCNO (R234) 2 3.40, 74.1% 9.78, 82.6% (R202, R203) (R231) 1.07, 73.1%, (R201) HCN NCO 1.54, 82.0%, (R223) 3.78, 70.9% 2.48, 82.7%, (R232) (R192) 7.08, 79.2%, (R218, R220) NH HNO 1.02, 75.1%, (R240) 6.95, 77.1% (R194, (R191, R193) 4.27,74.7%,(R214, R215) 1.56, 83.4% 3.62, 85.0% (R219, R221) R197) 1.26, 106.1%, (R212) 3.20, 69.8%, (R274) CN (R190) 2.28,37.4% (R199) 1.84, 54.2%, (R183) N2O 3.44,62.8% 2.20, 53.0%, 3.44(R182) (R185) 2.66, 81.3% N2 N (R179, R180) 1.70, 50.0%, (R178) NO (R206, R207) (R209, R210) 9.93, 56.1% 2.01, 62.4% (R186, R187) 3.85, 75.5% 3.83, 75.6% (R204, R205) (R188, R189) NNH 3.28, 76.1%, (R208) NO2 ~ ×10–11 ~ ×10–9 unit: mole cm–3 s–1 ~ ×10–10 ~ ×10–8 (e) Figure 11: NO formation pathways for (a) no dilution (Case 1). (b) 20% CO2 dilution in fuel (Case 6). (c) 20% CO2 dilution in air (Case 12). (d) 20% H2O dilution in fuel (Case 3). (e) 20% H2O dilution in air (Case 9).
14 Journal of Combustion (⟶NH)⟶NO), N2O-intermediate route (N2⟶N2O (1) The dilutions with H2O and CO2 in the oxidizer and (⟶NH) ⟶NO), and part of NO-reburning route fuel-side all decrease the flame temperature and NO (N2⟶HCN, HCNO⟶CN, NCO, NH3, NH2, formation. Dilutions by H2O and CO2 lower the peak NH,⟶N2O, NO, and N2). flame temperature nearly linearly as the dilution By comparing Figures 11(b) and 11(c) with ratio increases. The reduction rate of peak NO mole Figures 11(a), 11(d) and 11(e) with Figure 11(a), it can be fraction displays a decreasing trend with increasing found that the air-side dilution and fuel-side dilution affect the dilution ratio, especially when the dilution takes the NO formation pathway similarly in general. To be place on the oxidizer side. The differences between specific, the nitrogen elemental fluxes in Figure 11(a) are the results based on the adiabatic treatment, the the highest among all the five presented cases, indicating optically thin approximation, and the discrete-or- that the CH4 flame without dilution produces the highest dinates method coupled with the statistical narrow- amount of NO. The fuel-side dilution weakens the nitrogen band based correlated-k radiative property model elemental fluxes to a lesser degree than the air-side dilution decrease with increasing the dilution ratio. for both H2O and CO2 diluents. Besides, as a more detailed (2) The chemical effects of CO2 dilution lower both the illustration than Figure 10, Figure 11 shows that the NO flame temperature and NO emissions while the formation through the thermal route, NNH route, and chemical effect of H2O dilution exerts a slightly N2O-intermediate route is much less influenced than that promoting impact on the flame temperature but a through the prompt route, and the less affected reactions suppression influence on NO formation. The ap- include R204 to R210 (NNH route), R178 (thermal route), parently conflicting chemical effect of H2O dilution and R185, R183, R182, and R199 (N2O-intermediate on the flame temperature and NO formation is at- route). tributed to the considerably suppressed CH radical, However, differences do exist between the effects of the which results in more significant NO reduction two diluents. When the flame is diluted with CO2, the NO through the dominant prompt NO route than the formation pathway, NO⟶HCN (⟶HOCN)⟶ NO promotion through the thermal NO route. HNCO⟶NH2⟶NH⟶N⟶NO (the N flux and its (3) The importance of effects of H2O and CO2 dilution reduction percentage, as well as the involved reactions, are in the air- and fuel-side on NO reduction follows the distinguished in blue), is most affected. While for H2O order of air-side CO2 dilution > air-side H2O dilu- dilution, the most affected pathway is NO⟶HCN tion > fuel-side H2O dilution > fuel-side CO2 dilu- (⟶NCO)⟶NH (⟶N)⟶NO. Besides, another re- tion, and the prompt NO route always plays the markable change for H2O dilution can be observed for the dominant role under the current investigation conversion of NO⟶HNO. When H2O is added to the fuel- conditions. side, the nitrogen elemental flux of R212 (from the NNH NO formation route) is decreased mainly by 48.56%. This degree (4) Pathway analysis shows that the air-side dilution and far exceeds that of other NO formation pathways (with fuel-side dilution affect the NO formation pathway reduction percentages of 14.74%–39.93%). As for H2O di- similarly in general, but H2O dilution still differs lution on the air-side, the direction of the conversion be- from CO2 dilution. When the flame is diluted with tween NO and HNO is even reversed. This shift of the CO2, the most affected NO formation pathway is progress direction of R212 correlates well with the highest NO⟶HCN (⟶HOCN)⟶HNCO⟶NH2⟶ NO reduction percentage of air-side H2O dilution in Fig- NH⟶N⟶NO. However, when the flame is di- ure 8. All the abovementioned phenomena show that H2O luted with H2O, the most affected pathway is dilution has more complex influences on flame structure and NO⟶HCN (⟶NCO)⟶NH (⟶N)⟶NO. NO formation reduction than CO2 dilution. Data Availability 4. Conclusions The data used to support the findings of this study are available from the corresponding author upon request. Motivated by the widespread utilization of dilution com- bustion in many advanced technologies, this work sys- Conflicts of Interest tematically compared the effects of up to 40% H2O and CO2 dilutions to the fuel- and air-side on NO emissions in N2 The authors declare that they have no conflicts of interest. diluted CH4/air counterflow diffusion flames. This numer- ical study was performed using the OPPDIF code in con- Acknowledgments junction with the GRI-Mech 2.11 kinetics mechanism at varying strain rates (20–100 s−1) and inlet temperatures This work was supported by the National Natural Science range from 400 K to 800 K, 1 atm. The dilution was also Foundation of China (51776112), China Postdoctoral Science realized by replacing N2 with either CO2 or H2O in the Foundation (2020M672059), the Fundamental Research Funds oxidizer and the fuel stream to maintain the fuel and oxygen of Shandong University, Shandong Provincial Natural Science concentration constant. Several important findings are listed Foundation of China (Grant no. ZR2020QE198), and the as follows: Fundamental Research Funds for the Central Universities
Journal of Combustion 15 (18CX02129A). The authors sincerely acknowledge these fi- [16] B. Rahmanian, M. R. Safaei, S. N. Kazi, G. Ahmadi, nancial supports. H. F. Oztop, and K. Vafai, “Investigation of pollutant re- duction by simulation of turbulent non-premixed pulverized coal combustion,” Applied Thermal Engineering, vol. 73, no. 1, References pp. 1222–1235, 2014. [17] Z. Abdelmalek, R. Alamian, M. S. Shadloo, A. Maleki, and [1] M. Jonsson and J. Yan, “Humidified gas turbines-a review of A. Karimipour, “Numerical study on the performance of a proposed and implemented cycles,” Energy, vol. 30, no. 7, homogeneous charge compression ignition engine fueled with pp. 1013–1078, 2005. different blends of biodiesel,” Journal of Thermal Analysis and [2] H. Jericha, W. Sanz, and E. Gottlich, “Design concept for large Calorimetry, pp. 1–11, 2020. output graz cycle gas turbines,” Turbo Expo: Power for Land, [18] R. S. Gavhane, A. M. Kate, A. Pawar et al., “Effect of zinc oxide Sea, and Air, pp. 1–14, American Society of Mechanical nano-additives and soybean biodiesel at varying loads and Engineers, New York, NY, USA, 2006. compression ratios on VCR diesel engine characteristics,” [3] G. H. Abd-Alla, “Using exhaust gas recirculation in internal Symmetry, vol. 12, no. 6, p. 1042, 2020. combustion engines: a review,” Energy Conversion and [19] M. K. Chernovsky, A. Atreya, and H. G. Im, “Effect of CO2 Management, vol. 43, no. 8, pp. 1027–1042, 2002. diluent on fuel versus oxidizer side of spherical diffusion [4] R. W. Bilger and Z. Wu, “Carbon capture for automobiles flames in microgravity,” Proceedings of the Combustion In- using internal combustion Rankine cycle engines,” Journal of stitute, vol. 31, no. 1, pp. 1005–1013, 2007. Engineering for Gas Turbines and Power, vol. 131, no. 3, [20] J. Park, D.-J. Hwang, J.-G. Choi, K.-M. Lee, S.-I. Keel, and pp. 034502–034505, 2009. S.-H. Shim, “Chemical effects of CO2 addition to oxidizer and [5] S. Hoseinzadeh, S. Ghasemi, and S. Heyns, “Application of fuel streams on flame structure in H2-O2 counterflow diffu- hybrid systems in solution of low power generation at hot sion flames,” International Journal of Energy Research, vol. 27, seasons for micro hydro systems,” Renewable Energy, vol. 160, no. 13, pp. 1205–1220, 2003. pp. 323–332, 2020. [21] F. Liu, H. Guo, G. J. Smallwood, and Ö. L. Gülder, “The [6] S. Hoseinzadeh and S. Heyns, “Advanced energy, exergy and chemical effects of carbon dioxide as an additive in an ethylene environmental (3E) analyses and optimization of a coal-fired diffusion flame: implications for soot and NOx formation,” 400 MW thermal power plant,” Journal of Energy Resources Combustion and Flame, vol. 125, no. 1-2, pp. 778–787, 2001. Technology, vol. 143, no. 8, p. 082106, 2021. [22] A. E. Lutz, R. J. Kee, J. F. Grcar, and F. M. Rupley, OPPDIF: a [7] L. Wang, Z. Liu, S. Chen, C. Zheng, and J. Li, “Physical and fortran program for computing opposed-flow diffusion flames, chemical effects of CO2 and H2O additives on counterflow Sandia National Laboratories, Livermore, CA, USA, 1997. diffusion flame burning methane,” Energy & Fuels, vol. 27, [23] R. J. Kee, F. M. Rupley, and J. A. Miller, Chemkin-II: A Fortran no. 12, pp. 7602–7611, 2013. Chemical Kinetics Package for the Analysis of Gas-phase [8] H. Xu, F. Liu, S. Sun, Y. Zhao, S. Meng, and W. Tang, “Effects Chemical Kinetics, Sandia National Laboratories, Livermore, of H2O and CO2 diluted oxidizer on the structure and shape of CA, USA, 1989. laminar coflow syngas diffusion flames,” Combustion and [24] C. Bowman, R. Hanson, D. Davidson et al., “GRI-mech 2.11, Flame, vol. 177, pp. 67–78, 2017. 1995,” 1995, http://www.me.berkeley.edu/gri_mech. [9] L. Zhuo, Y. Jiang, R. Qiu, J. An, and W. Xu, “Effects of fuel- [25] S. H. Kim, K. Y. Huh, and B. Dally, “Conditional moment side N2, CO2, H2O dilution on combustion characteristics closure modeling of turbulent nonpremixed combustion in and NOx formation of syngas turbulent nonpremixed jet diluted hot coflow,” Proceedings of the Combustion Institute, flames,” Journal of Engineering for Gas Turbines and Power, vol. 30, no. 1, pp. 751–757, 2005. vol. 136, no. 6, pp. 061505–061511, 2014. [26] K. W. Lee and D. H. Choi, “Prediction of NO in turbulent [10] J. Park, S.-G. Kim, K.-M. Lee, and T. K. Kim, “Chemical effect diffusion flames using Eulerian particle flamelet model,” Com- of diluents on flame structure and NO emission characteristic bustion Theory and Modelling, vol. 12, no. 5, pp. 905–927, 2008. in methane-air counterflow diffusion flame,” International [27] S. R. Turns, Introduction to Combustion, McGraw-Hill Journal of Energy Research, vol. 26, no. 13, pp. 1141–1160, Companies, New York, NY, USA, 1996. 2002. [28] F. Liu, H. Guo, G. Smallwood, and M. El Hafi, “Effects of gas [11] Y. Xie, J. Wang, N. Xu, S. Yu, and Z. Huang, “Comparative and soot radiation on soot formation in counterflow ethylene study on the effect of CO2 and H2O dilution on laminar diffusion flames,” Journal of Quantitative Spectroscopy and burning characteristics of CO/H2/air mixtures,” International Radiative Transfer, vol. 84, no. 4, pp. 501–511, 2004. Journal of Hydrogen Energy, vol. 39, no. 7, pp. 3450–3458, [29] D. E. Giles, S. Som, and S. K. Aggarwal, “NOx emission 2014. characteristics of counterflow syngas diffusion flames with [12] D. Ning, A. Fan, and H. Yao, “Effect of radiation emission and airstream dilution,” Fuel, vol. 85, no. 12-13, pp. 1729–1742, reabsorption on flame temperature and NO formation in H2/ 2006. CO/air counterflow diffusion flames,” International Journal of [30] F. Wang, P. Li, J. Zhang, Z. Mei, J. Mi, and J. Wang, “Routes of Hydrogen Energy, vol. 42, no. 34, pp. 22015–22026, 2017. formation and destruction of nitrogen oxides in CH4/H2 jet [13] S. Öztürk, “The effects of CO2, N2, and H2O dilutions on NO flames in a hot coflow,” International Journal of Hydrogen formation of partially premixed synthesis gas combustion,” Energy, vol. 40, no. 18, pp. 6228–6242, 2015. Cumhuriyet Science Journal, vol. 40, no. 4, pp. 813–818, 2019. [31] S. J. Klippenstein, L. B. Harding, P. Glarborg, and J. A. Miller, [14] J. J. Feese and S. R. Turns, “Nitric oxide emissions from “The role of NNH in NO formation and control,” Combustion laminar diffusion flames: effects of air-side versus fuel-side and Flame, vol. 158, no. 4, pp. 774–789, 2011. diluent addition,” Combustion and Flame, vol. 113, no. 1-2, [32] J. Revel, J. Boettner, M. Cathonnet, and J. Bachman, “Deri- pp. 66–78, 1998. vation of a global chemical kinetic mechanism for methane [15] E.-S. Cho and S. H. Chung, “Characteristics of NOx emission ignition and combustion,” Journal de chimie physique, vol. 91, with flue gas dilution in air and fuel sides,” KSME Interna- pp. 365–382, 1994. tional Journal, vol. 18, no. 12, pp. 2303–2309, 2004.
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