Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site
LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Journal of Combustion, 2021, 2021-03-03
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC :
https://nrc-publications.canada.ca/eng/view/object/?id=f872d138-7f28-473a-86b4-f4e280e89fd8
https://publications-cnrc.canada.ca/fra/voir/objet/?id=f872d138-7f28-473a-86b4-f4e280e89fd8
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.1155/2021/6642734
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
A detailed numerical study of NOx kinetics in counterflow methane
diffusion flames: effects of fuel-side versus oxidizer-side dilution
Xu, Huanhuan; Liu, Fengshan; Wang, Zhiqiang; Ren, Xiaohan; Chen, Juan;
Li, Qiang; Zhu, Zilin
Research Article
A Detailed Numerical Study of NOx Kinetics in Counterflow
Methane Diffusion Flames: Effects of Fuel-Side versus
Oxidizer-Side Dilution
Huanhuan Xu ,
1Fengshan Liu,
2Zhiqiang Wang,
1Xiaohan Ren,
3Juan Chen,
1Qiang Li,
4and Zilin Zhu
11National Engineering Laboratory for Reducing Emissions from Coal Combustion, School of Energy and Power Engineering, Shandong University, Jinan 250061, China
2Measurement Science and Standards, National Research Council Canada, Building M-9 1200, Montreal Road, Ottawa, ON K1A 0R6, Canada
3Institute of Thermal Science and Technology, Shandong University, 17923 Jingshi Road, Jinan 250061, Shandong, China 4College 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 CO2are 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 CO2dilution 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, CO2dilution in the air-side has the most profound influence on NO reduction, and CO2dilution 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 CO2dilution.
1. Introduction
Several advanced combustion technologies have been proposed to address the challenges of both fossil fuel de-pletion and environmental pollution. Additives are widely used to enhance combustion efficiency, control nitric oxide emissions, or achieve low-cost CO2capture. For example,
humidified air turbines (HATs) [1] with a mixture of air and water as the working fluid promise high electrical efficiencies and low NOx emissions. Fossil fuels combust with pure oxygen in a Graz Cycle Gas Turbine [2] by using
CO2 to control the flame temperature, enabling the
cost-effective separation of the CO2 in flue gas physical
con-densation. In combustion technologies based on exhaust gas recirculation (EGR) [3], part of the exhaust gas (mainly H2O and CO2) is introduced back to the combustion
chamber, and NOx emissions could be substantially re-duced. Internal Combustion Rankine Cycle (ICRC) power plants [4] run under an oxy-fuel combustion model with recycled water to achieve high efficiency and specific power output. Other systems using water to provide power can be found in [5, 6].
Volume 2021, Article ID 6642734, 15 pages https://doi.org/10.1155/2021/6642734
To gain the full potential benefits of dilution combustion, it is crucial to understand the effects of the following three primary factors: (1) the type of diluents, (2) the amount of dilution, and (3) the location (oxidizer or fuel stream) where the diluents are injected.
Among the various combustion technologies mentioned above, carbon dioxide and steam (water vapor) are the most common additives. As H2O and CO2 are two main
bustion products, adding these two diluents to the com-bustion zone will not introduce new species but increase H2O
and CO2 concentrations in the flue gas. As a result, CO2
capture becomes much more manageable and economical. Besides, the high heat capacity of H2O and CO2enables many
favorable combustion properties, such as low flame tem-perature [7, 8] and low NOx emissions [9]. To grasp the dilution mechanism of H2O and CO2, many efforts have been
made. For example, Park et al. [10] simulated the flame structure and NO emission behavior in CH4diffusion flames
with different diluents (H2O, CO2, and N2) added to the air
stream. Their results showed that CO2addition reduces the
flame temperature through both the thermal and chemical effects. In contrast, the addition of H2O enhances the
re-action through chemical effect and limits the decrease of temperature thermally. They also pointed out that the NO emission index decreases with increasing the volume per-centage of diluents. The effectiveness of these diluents on NO emission reduction follows the order of CO2> H2O > N2. Xie
et al. [11] found that the elementary reaction corresponding to the peak ROP (rate of production) of OH in CO/H2/air
mixture changes from OH + H2�H + H2O to
HO2+ H � OH + OH when CO2 and H2O are added, and
CO2has a more potent chemical effect than H2O. Xu et al. [8]
investigated the effects of H2O and CO2diluted oxidizer on
the structure and shape of laminar co-flow syngas diffusion flames. They found that the thermal and radiative effects of CO2 decrease the centerline temperature to a much larger
degree than those of H2O; thus, CO2reduces the centerline
temperature more effectively than H2O. Besides, they also
found that as the dilution level increases, the maximum OH mole fraction in CO2diluted flames decreases monotonously,
whereas in H2O diluted flames, OH mole fraction first
in-creases then dein-creases. Ning et al. [12] numerically studied the radiation effect on NO formation in H2/CO/air
coun-terflow diffusion flames, and results showed that, as the CO2
content in the fuel stream increases, the radiation effect on NO emission increases first and then decreases for H2-lean
syngas. In contrast, for H2-rich syngas, the radiation effect is
monotonic. Ozturk [13] studied the effects of CO2, N2, and
H2O dilution on the adiabatic, turbulent, partially premixed
combustion of synthesis gas and pointed out that increasing the dilution rates gradationally reduces the NO concentra-tion and H2O has the best effect on reduction of NO
emissions, followed by CO2and N2. Although these studies
help fundamental understanding of the influence of diluents and dilution level on the combustion characteristics of hy-drocarbon fuels, they were conducted for either fuel-side dilution or air-side dilution. They hence did not provide a direct comparison of the relative effectiveness between fuel-side dilution and oxidizer-fuel-side dilution.
Researchers have made a few attempts to explore the difference between air- and fuel-side dilutions for jet diffusion flames. For instance, Feese and Turns [14] experimentally studied the effects of N2 dilution in the air- or fuel-side on
NOx emissions in laminar CH4 jet diffusion flames. They
found that air dilution was more effective than fuel dilution in reducing NOx emissions. They also pointed out that the visible flame height increases somewhat as a diluent is added to the airstream but does not change noticeably for fuel-side dilution, even for the same diluent fraction. Unfortunately, Feese et al. did not analyze the NO formation routes or provide kinetic explanations for their results. In addition, an experimental study by Cho and Chung [15] showed that fuel-side dilution is more effective in reducing flame temperature and NO emissions than air-side dilution. It should be noted that, although Cho and Chung also studied jet diffusion flames, they reached a different conclusion from that in [14]. This contradiction is due to the different dilution parameters used in these two studies when comparing the fuel-side di-lution and the air-side didi-lution. In Cho and Chung’s study [15], the dilution ratio was defined as the ratio of the exhaust gas flow rate to the total flow rate of the fuel (or air). Because the fuel stream’s exit area is much smaller than that of the oxidizer stream, the fuel stream velocity increases more than the oxidizer stream velocity increases. The much higher in-crease in fuel stream velocity enhances the mixing in dilution combustion. In the study of Feese and Turns, however, they introduced a new dilution parameter (Z), calculated as the mass of diluent to the stoichiometric mixture’s mass. In this way, under the same Z, the amount of diluent added to the air stream for air-side dilution is much more considerable than that added to the fuel stream for fuel-side dilution.
The effect of dilution on NO formation is one of the issues this study aimed to resolve since one of the advantages of dilution combustion is the low NO emissions, which is strongly dependent on the combustion conditions, reported in the previous papers, i.e., Refs. [16–18].
To obtain a fundamental understanding of the effect of dilution position (i.e., through air or fuel stream) on flame characteristics, attention has also been paid to one-di-mensional diffusion flames, which excludes the burner’s complexity and flow field in multidimension flames. Using spherical diffusion flames, Chernovsky et al. [19] experi-mentally investigated the impact of adding CO2 on the
oxidizer side versus the fuel side. Their results showed that CO2dilution affects the flame in these two dilution scenarios
by different mechanisms. On the oxidizer side, radiation reabsorption played an essential role in strengthening the flames. In contrast, the enhanced CO2 concentration
in-creased radiative heat losses on the fuel-side without prompting radiation reabsorption. Park et al. [20] numer-ically investigated the effects of CO2addition to the fuel and
the oxidizer streams on the structure of H2/O2 diffusion
flame in counterflow configuration, and results showed that the temperature reduction caused by the chemical effects is more remarkable when CO2is added to the oxidizer stream
than that to the fuel stream. Liu et al. [21] numerically studied the chemical effects of CO2addition to both the fuel
diffusion flame. The study of Liu et al. is highly relevant to the present study, and their results showed that the chemical effects of CO2are weaker when added to the fuel side than to
the oxidizer side. NOx formation pathway analyses are es-sential to reveal how CO2 addition affects NOx formation
mechanisms; however, Liu et al. did not conduct this work but only focused on the chemical effects of CO2.
Despite several studies that have been conducted to understand the difference between fuel- and air-side dilu-tions, there is still a lack of comprehensive comparison between H2O dilution and CO2 dilution in the fuel- and
oxidizer side. Many questions remain unanswered regarding NO emissions, such as the radiation effect on NO emissions and the chemical effect of diluents on the NO formation mechanism. To this end, the objectives of this study were to investigate the influence of H2O and CO2dilution in the air
and fuel stream on methane/air counterflow diffusion flames with an emphasis on the detailed NO formation process, including not only the formation and destruction routes, but also the reaction pathway analysis. The importance of gas radiation in modeling the methane/air diffusion flames was first demonstrated for different diluting levels and in the air and fuel stream with H2O or CO2. The effects of H2O and
CO2dilutions in the fuel and air stream on flame
temper-ature and NO concentration were next examined by adopting the DOM/SNBCK radiation model. The diluents’ chemical effects and thermal effects were isolated and an-alyzed. Finally, to reveal the impact of different diluents and dilution locations (air or fuel stream) on the NO formation mechanism, the contributions of different NO formation routes were calculated, and the NO formation pathways were revealed.
2. Mathematical Model and Numerical Methods
2.1. Simulation Methods. The OPPDIF code [22] coupled with the thermal and transport subroutines in the CHEMKIN package [23] was adopted in this study to compute the counterflow diffusion flames. This code has been extensively validated and used to model one-dimen-sional laminar flames (nonpremixed or premixed) using detailed combustion chemistry and thermal and transport properties. The methane oxidation chemistry was modeled using the GRI-Mech 2.11 mechanism [24]. Many studies [25, 26] have demonstrated that GRI-Mech 2.11 produces a better prediction for NOx formation than the GRI-Mech 3.0 mechanism when modeling methane combustion. To il-lustrate the importance of radiation heat transfer, three different treatments of thermal radiation were considered, namely, no radiation heat loss, the optically thin approxi-mation (OTA) [27], and the discrete-ordinates method (DOM) coupled with the statistical narrow-band correlated-K-based radiative property model (DOM/SNBCK) for solving the radiative transfer equation (RTE) [28]. The OTA only accounts for radiation loss due to emissions. It neglects radiation gain by absorption, while the DOM/SNBCK model considers both radiation emission and absorption, as well as the non-gray nature of gas radiative properties. Multi-component transport properties and thermal diffusion were
considered to deal with the preferential diffusion of H2and
H. The grid adaptation parameters of GRAD and CURV were both set to 0.05 to ensure that the computational meshes are sufficiently refined, and the simulation results are grid size independent. The global strain rate asdefined in
equation (1) is fixed at 50 s−1for the flames modeled in this paper unless otherwise indicated. The fuel and oxidizer streams’ outlet velocities were chosen to satisfy the mo-mentum balance [29] expressed in equation (2):
as�2 VO L 1 + VF ��ρF √ VO ��ρO� √ , (1) ρOVO 2 �ρFVF 2 , (2)
where ρ and V represent the density and velocity, respec-tively, and the subscripts “F” and “O” indicate the fuel and oxidizer streams. Symbol L is the distance between the fuel and oxidizer nozzles, specified as 2 cm in this study.
2.2. Simulation Conditions. The flame conditions investi-gated in this study are summarized in Table 1. It is worth pointing out that when a certain amount of diluent (H2O or
CO2) was added to the fuel or the oxidizer stream, the same
amount of N2was removed; therefore, the CH4and O2mole
fractions could keep unchanged at 50 vol% and 21 vol% in the fuel and oxidizer stream, respectively. The pressure was 1 atm, and the inlet temperature of both the oxidizer and fuel streams was ranged from 400 K to 800 K in this study.
2.3. NO Formation Mechanism. To identify the contribu-tions of different NO formation routes, NO is considered to be formed by four pathways and destructed through NO-reburning. First, the thermal NO is formed through the N2
triple bond break-up by the O atom, and O2 and OH
subsequently oxidize the N atom. This process has very high activation energy and is strongly affected by flame tem-perature. The main reactions related to NO formation in the GRI-Mech 2.11 mechanism are listed in Table 2. Reactions responsible for thermal NO include R178, R179, and R180. Second, the prompt NO can be prevalent in hydrocarbon flames since it is produced via reactions of CHi radicals, and R240 is usually considered as the major initiation step for the formation of prompt NO. The reactions involved in prompt NO are rate-limiting, leading to the prompt NO formation less temperature-dependent. Third, the N2O-intermediate
NO is formed through the path of N2⟶N2O⟶NO, with
N2O being the intermediate species. Fourth, the NNH NO
begins with N2 reacting with H to form NNH (R204 and
R205), which then yields NO through R208. Lastly, NO-reburning is an important destruction mechanism of NO by a group of reactions involving hydrocarbon radicals, CHi.
The calculation method of these NO formation routes has been widely employed in previous studies, e.g., Refs. [27, 30, 31].
The full GRI-Mech 2.11 mechanism contains 49 species and 279 elementary reactions, and the number of reaction steps decreases to 177 when excluding NO formation. For
clarity, we abbreviate these two mechanisms as “he full GRI-Mech 2.11 mechanism conta respectively. To quantify NO formation from the four formation routes plus one de-struction route, each simulation was performed six times 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-mation route, marked as SIM2 ∼ SIM5, respectively, and “Full chemistry” excluding NO-reburning route (SIM6). The elementary steps involved in calculating each NO formation route are identical to these in Ref. [30]. As a result, NO formations through the thermal, prompt, NNH, or N2
O-intermediate routes can be obtained by subtracting results of the corresponding SIM2∼SIM5 with that of the SIM1. NO-reburning effect on NO was calculated by the difference in results of SIM6 and SIM1.
2.4. Chemical and Thermal Effects of H2O/CO2. To
quanti-tatively investigate the chemical and thermal effects of H2O
or CO2 dilution, calculations with real H2O and CO2 and
their chemically inert counterpart were conducted as we did in the previous work [8]. Specifically, this chemical effect refers to the diluents’ impact through their participation the
Table 1: Conditions of diluted methane/air counterflow diffusion flames with and without H2O or CO2replacement of N2, 400 K, 1 atm.
No. Fuel stream composition (vol%) Oxidizer stream composition (vol%) Remark
CH4 N2 H2O CO2 O2 N2 H2O CO2
Case 1 50 50 0 0 21 79 0 0
Baseline H2O dilution (fuel-side)
Case 2 50 40 10 0 21 79 0 0 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 CO2dilution (fuel-side) Case 6 50 30 0 20 21 79 0 0 Case 7 50 10 0 40 21 79 0 0 Case 8 50 50 0 0 21 69 10 0 H2O dilution (air-side) Case 9 50 50 0 0 21 59 20 0 Case 10 50 50 0 0 21 39 40 0 Case 11 50 50 0 0 21 69 0 10 CO2dilution (air-side) Case 12 50 50 0 0 21 59 0 20 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
chemical reactions. The thermal effect means the influence caused by the difference of the diluents’ thermal properties from those of N2. The two artificial species, FH2O and FCO2,
have the same thermal, transport, and radiative properties as these of the real H2O and CO2, respectively, except that
FH2O and FCO2are chemically inert and do not participate
in chemical reactions. To be specific, when evaluating CO2
and H2O’s chemical effects, two sets of simulations were
conducted. One was using the real CO2 and H2O as the
diluent, and the other was using FCO2and FH2O. Therefore,
differences between results based on FH2O and FCO2 and
those using H2O and CO2are attributed to H2O and CO2’s
chemical effects. Similarly, to quantify H2O and CO2’s
thermal effects, another pair of artificial species, TH2O and
TCO2, having the same chemical, transport, and radiative
properties as FH2O and FCO2 but share the same thermal
properties with N2 were introduced to the reaction
mech-anisms. As a result, differences in the numerical results using TH2O (or TCO2) additions and these using FH2O (or FCO2)
additions are caused by H2O and CO2’s thermal effects.
3. Results and Discussion
3.1. Effect of Radiation Models. Since H2O and CO2are the
most critical radiative gases in hydrocarbon combustion, it is expected that H2O and CO2enhance radiation heat transfer
in the combustion zone. The results calculated using the three radiation models were first examined to investigate radiation treatment’s effect on modeling the different diluted flames. Figure 1 displays the predicted peak flame tem-peratures using the three radiation heat transfer treatments for CO2and H2O dilution added to the fuel and air streams.
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
volume [7, 8]. As expected, neglecting the thermal radiation results in overprediction of flame temperature, while OTA under predicts the peak flame temperature. Since the DOM/ SNBCK model considers both emission and absorption of radiation, it predicts the peak flame temperature lower than that of no radiation but higher than OTA. It is interesting to notice that the peak temperature predicted by DOM/SNBCK is much closer to that of no radiation, suggesting that there is significant radiation reabsorption in flames to reduce the net radiation loss. These results also show that the air-side di-lution (black lines) has a stronger influence on suppressing the flame temperature than the fuel-side dilution (red lines). Kinetics analysis shows this is attributed to that the heat release rate of reactions O + C2H2�CO + CH2 and
H + O2+ H2O � HO2+ H2O is more suppressed when
di-lution occurs at the air-side than that at the fuel-side. In particular, an interesting phenomenon observed in Figure 1 is that the impact of H2O and CO2dilution on the peak flame
temperature is nearly linear for dilution ratio up to 40%, no matter dilution occurs in the fuel side or the air-side.
The effects of radiation models on the predicted peak NO mole fractions for CO2and H2O dilution on the fuel
and air stream are shown in Figure 2. The variation of peak NO mole fraction with dilution ratio is clearly nonlinear.
Besides, the radiation model’s effect on the NO mole fraction is quite significant. Consistent with the radiation model’s effects on the peak flame temperature, when neglecting the radiation heat transfer, OTA predicts the lowest NO mole fraction. At a given flame condition, the NO mole fraction predicted by the DOM/SNBCK model falls between no radiation and OTA but closer to no ra-diation. However, the radiation model’s effect on the predicted peak NO mole fraction weakens with increase in the dilution ratio, especially for the air-side dilution. As shown in Figure 2, when dilution occurs in the air-side (black lines), both H2O dilution and CO2 dilution reduce
the peak NO mole fraction significantly; however, the reduction rate continuously weakens as the dilution ratio increases. The different influence of dilution on the peak NO mole fraction form the impact on the peak flame temperature, i.e., the nonlinear reduction of the peak NO mole fraction in Figure 2 and nearly linear reduction in the peak flame temperature in Figure 1, implies that the temperature-sensitive thermal NO route is not the primary pathway for NO formation under the conditions of this study. This point will be discussed later. Since the effect of fuel-side dilution on the NO mole fraction maximum is much weaker than that of air-side dilution, the nonlinear trend is not pronounced, especially for CO2dilution on the
Red lines: fuel-side dilution
CO2 dilution
Black lines: air-side dilution
No radiation SNBCK OTA Tem pera tur e maxim um (K) 1500 1600 1700 1800 1900 2000 2100 1500 1600 1700 1800 1900 2000 2100 10 20 30 40 0 Dilution ratio (%) H2O dilution
Figure 1: Variation of the peak flame temperature of CH4/air
counterflow diffusion flames with the dilution ratio for H2O and
fuel-side. For the fuel-side dilution, thermal NO is not the dominant route, as illustrated in Section 3.4.
3.2. Flame Temperature. From this section, the numerical results were calculated using the DOM/SNBCK radiation model since this radiation treatment provides the most accurate treatment of radiation heat transfer. In this part, the influence of H2O and CO2dilution on flame temperature is
discussed in detail. Figure 3 displays the temperature dis-tributions for H2O and CO2dilution up to 40% on the fuel
and air-side as a function of the distance from the fuel nozzle. The differences in the peak flame temperature be-tween Case 1 (no dilution) and the cases of 40% H2O and
40% CO2dilution are also indicated. Although H2O and CO2
dilution on either the air or the fuel-side lowers the flame temperature consistently, differences between H2O dilution
and CO2dilution still exist. First, CO2is more effective than
H2O in reducing the flame temperature, regardless of if it
being added to the air or the fuel side. This difference is mainly caused by the higher heat capacity of CO2. Second,
H2O dilution always slightly shifts the peak flame
temper-ature towards the air nozzle, but CO2dilution can shift the
peak temperature slightly towards either the fuel nozzle or the oxidizer nozzle when, respectively, added to the fuel or air stream. This difference is related to the relative molar weights of N2, H2O, and CO2. Nevertheless, the flame
sheet almost remains at the same position since the mo-mentum was balanced by equation (2) in all the cases.
To gain further insights into how H2O and CO2dilution
affect the flame temperature, Figure 4 shows the separate effects of 20% H2O and 20% CO2dilution to the air and fuel
side on the peak temperature. The chemical and thermal effects were analyzed in this paper since the transport and radiation effects are less important [8], though the radiation effect can become significant at low strain rates. It can be seen that the thermal effects of both H2O and CO2dilution
reduce the flame temperature. Although the chemical effect of CO2lowers the flame temperature, the chemical effect of
H2O increases it, albeit only slightly. This finding agrees with
that reported in the previous studies [7, 8]. Besides, Figure 4 also shows that the dilution position (air-side or the fuel-side) does not change the individual effects of H2O and CO2
dilution on flame temperature qualitatively, namely, the chemical effects of CO2and the thermal effects of H2O and
CO2always lower the flame temperature while the chemical
effect of H2O slightly increases it. In addition, both the
overall and individual effects of H2O and CO2dilutions to
the air-side are greater than those to the fuel-side. Kinetics analyses show that it is because that heat released by re-actions O + C2H2�CO + CH2 and H + O2+ H2O � HO2
+ H2O is more suppressed when dilution occurs at the
air-side than that at the fuel-air-side.
3.3. NO Mole Fraction. Figures 5(a) and 5(b) present the distributions of NO mole fraction as a function of the 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 stream has a stronger influence on NO reduction than the fuel side, regardless of CO2or H2O dilution. The effects of
CO2and H2O dilution on NO mole fraction correlate well
with those on flame temperature. When the air-side is di-luted (red lines), CO2 dilution suppresses NO formation
more significantly than H2O dilution, again consistent with
their effects on the peak flame temperature. While for the fuel-side dilution (blue lines), the effect of H2O dilution on
suppressing NO formation is more significant than that of CO2dilution, which deserves further investigations because
the importance of these two kinds of diluents on decreasing NO formation is opposite to that on lowering the flame temperature (Figure 3).
To understand the phenomenon that the effect of fuel-side dilution by H2O on NO reduction is greater than that
by CO2dilution, CH radical distributions with or without
40% H2O and CO2 dilution in the air- and fuel-side are
compared in Figure 6. H2O dilution on the fuel-side
in-hibits CH production more significantly than CO2, which
explains the greater suppression impact of H2O dilution
than CO2dilution on NO formation in Figure 5. Although
the temperature of fuel-side CO2 dilution is higher than
that of fuel-side H2O dilution, the significantly reduced CH
concentration of fuel-side H2O dilution slows down the
initial reaction of prompt NO, i.e., CH + N2�HCN + N
(R240). Consequently, the total NO emission is inhibited
Red lines: fuel-side dilution Black lines: air-side dilution
N O mo le f rac tio n maxim um 0.0 2.0E – 5 4.0E – 5 6.0E – 5 8.0E – 5 0.0 2.0E – 5 4.0E – 5 6.0E – 5 8.0E – 5 CO2 dilution No radiation SNBCK OTA H2O dilution 10 20 30 40 0 Dilution ratio (%)
Figure 2: Variation of the peak NO mole fraction of CH4/air
counterflow diffusion flames with the dilution ratio by H2O and
41K 129K Tem pera tur e (K) 0.4 0.6 0.8 1.0 1.2 1.4 0.2 Distance (cm) 10% H2O dilution in fuel 20% H2O dilution in fuel 40% H2O dilution in fuel No dilution (Case 1) 10% H2O dilution in air 20% H2O dilution in air 40% H2O dilution in air 400 600 800 1000 1200 1400 1600 1800 2000 Tem pera tur e (K) 1500 1600 1700 1800 1900 2000 Tem pera tur e (K) 1.00 1.05 1.10 1.15 1.20 1.25 0.95 Distance (cm) 600 650 700 750 800 850 900 0.78 0.80 0.82 0.76 Distance (cm) (a) 48 K 256 K Tem pera tur e (K) 0.4 0.6 0.8 1.0 1.2 1.4 0.2 Distance (cm) 10% CO2 dilution in fuel 20% CO2 dilution in fuel 40% CO2 dilution in fuel No dilution (Case 1) 10% CO2 dilution in air 20% CO2 dilution in air 40% CO2 dilution in air 400 600 800 1000 1200 1400 1600 1800 2000 Tem pera tur e (K) 1500 1600 1700 1800 1900 2000 1.05 1.10 1.15 1.20 1.25 1.00 Distance (cm) (b)
Figure 3: Distributions of flame temperature in CH4/air counterflow diffusion flames with (a) H2O dilution and (b) CO2dilution as a
since the prompt NO is the dominant NO formation route. This point will be discussed in more detail in Section 3.4. Most simulations in this study were conducted at T � 400 K and as�50 s−1. To show that the effects of dilution
on NO reduction under these conditions are generalizable, Figures 7 and 8 exhibit the peak NO mole fraction at dif-ferent strain rates (from 20 to 100 s−1) and inlet temperature (from 400 to 600 K), and very similar trends can be found among the displayed lines. However, the detailed analyses of
how strain rates and inlet temperature affects the NO for-mation are beyond this paper’s scope.
Figure 9 displays the individual effects of 20% H2O and
20% CO2 dilution on the peak NO mole fraction. The
di-lution effects on NO reduction in the air-side are stronger than those in the fuel-side, same as the influence on flame temperature, Figure 4. In Figure 9, the chemical effect of H2O dilution exerts a decreasing impact on NO emission
despite promoting the flame temperature in Figure 4. This
20% H2O dilution in fuel 20% CO2 dilution in air 20% CO2 dilution in fuel 20% H2O dilution in air 1800 1850 1900 1950 2000 Tem pera tur e maxim um (K) Thermal effect Chemical effect Overall effect No dilution (Case 1) TH2O / TCO2 FH2O / FCO2 Real H2O / CO2
Figure 4: The overall and individual effects of 20% CO2and 20% H2O dilution on the peak flame temperature of CH4/air counterflow
diffusion flame: as�50 s−1, L � 2 cm, 400 K. Decreased by 51.4% Decreased by 93.1% –1E – 5 0 1E – 5 2E – 5 3E – 5 4E – 5 5E – 5 6E – 5 7E – 5 N O mo le f rac tio n 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.2 Distance (cm) 10% H2O dilution in fuel 20% H2O dilution in fuel 40% H2O dilution in fuel No dilution (Case 1) 10% H2O dilution in air 20% H2O dilution in air 40% H2O dilution in air (a) Decreased by 37.8% Decreased by 95.7% –1E – 5 0 1E – 5 2E – 5 3E – 5 4E – 5 5E – 5 6E – 5 7E – 5 N O mo le f rac tio n 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.2 Distance (cm) 10% CO2 dilution in fuel 20% CO2 dilution in fuel 40% CO2 dilution in fuel No dilution (Case 1) 10% CO2 dilution in air 20% CO2 dilution in air 40% CO2 dilution in air (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,
can be easily understood from the distribution of CH radical shown in Figure 6. Although the flame temperature is en-hanced slightly by H2O dilution, the CH concentration is
reduced more considerably, which results in more signifi-cant NO reduction through the prompt NO route than the prompting effect on NO through the thermal NO route.
3.4. NO Formation Mechanism. Figure 10 displays the emission index of NO via different formation routes as introduced in Section 2.3. The emission index of NO was
calculated using equation (3), which has been widely utilized in previous studies, such as [10, 30].
EINO � L 0ϖNOMNOdx − L 0ϖFMFdx , (3)
where ωNO and ωF are the NO production rate and fuel
consumption rate, respectively, and MNOand MF are the
molecular weight of nitric oxide and fuel, respectively. Firstly, it can be observed that the effects of H2O and
CO2dilution in the air- or fuel-side on EINO correlate well
Fuel side Air side as = 50 s–1 as = 100 s–1 Strain rate = 20 Strain rate = 50 Strain rate = 100 0 1E – 5 2E – 5 3E – 5 4E – 5 5E – 5 6E – 5 7E – 5 8E – 5 9E – 5 N O mo le f rac tio n 30 20 40 0 10 Dilution ratio (%) as = 20 s–1 (a) Strain rate = 20 Strain rate = 50 Strain rate = 100 Air side Fuel side 0 1E – 5 2E – 5 3E – 5 4E – 5 5E – 5 6E – 5 7E – 5 8E – 5 9E – 5 N O mo le f rac tio n 10 20 30 40 0 Dilution ratio (%) as = 50 s–1 as = 100 s–1 as = 20 s–1 (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) H
2O dilution. (b) CO2dilution. CO2 effect in fuel-side
CO2 effect
in air-side Hin air-side2O effect H2O effect in fuel-side No dilution (Case 1) 40% H2O in air 40% H2O in fuel 40% CO2 in air 40% CO2 fuel 0.0 5.0E – 7 1.0E – 6 1.5E – 6 2.0E – 6 CH mo le f rac tio n 1.04 1.08 1.12 1.16 1.20 1.24 1.00 Distance (cm)
Figure 6: Distributions of CH mole fraction in CH4/air counterflow diffusion flames with and without 40% H2O and 40% CO2dilution on
with those on the peak NO mole fraction shown in Figures 5 and 9, namely, air-side CO2 dilution > air-side H2O
dilu-tion > fuel-side H2O dilution > fuel-side CO2 dilution. This
is because that CH4 flow rate was kept constant for all the
cases, and EINO was calculated over the entire computa-tional domain in this study. Secondly, the prompt route plays the dominant role in NO formation, followed by the NO-reburning and NNH route, while the thermal and N2
O-intermediate routes contribute negligibly. For clarity, the percentages of reduction of total NO, prompt NO, NNH NO, and NO-reburning for 20% H2O and 20% CO2dilution
are also marked in Figure 10. For example, 18.8% means adding 20% CO2into the fuel stream (Case 6) reduces 18.8%
NO emission than the no-dilution case (Case 1). It can be observed that the order of NO reduction percentage through the prompt NO route and NO-reburning for different di-lutions totally agrees well with that through the full chemistry, i.e., air-side CO2 dilution > air-side H2O
dilu-tion > fuel-side H2O dilution > fuel-side CO2 dilution.
However, H2O and CO2 dilution’s influence on the third
most important NO formation route (NNH pathway) does not follow the above order. Even so, it can still be concluded that the relative importance of H2O and CO2dilution to NO
formation reduction is determined by the dilution impact on the prompt NO formation and NO-reburning routes under the conditions of this study.
Using the method proposed by Revel et al. [32], the elemental fluxes of N through some critical reactions were calculated, and the pathways of NO formation based on the 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 worth pointing out that N’s elemental fluxes were calculated along with the whole computational domain rather than just at the flame front, indicating the average value over the entire combustion process. In Figure 11, the arrow shows the progress direction of a certain conversion, from the reactants to the products. The values on each arrow indicate the N flux over the entirely computational domain (from the fuel nozzle exit to the oxidizer nozzle exit with a distance of 2 cm in this study), and the width of each arrow distinguishes the magnitude of the N flux. The percentage after the value means the ratio of the N flux reduction relative to the baseline case (Case 1) through the reaction after it in pa-rentheses. The pathway map in Figure 11 covers the five NO formation/destruction routes, and the conversion paths can be briefly highlighted as the thermal route (N2⟶N⟶NO), prompt route (N2⟶HCN⟶CN,
NCO, NH, N, and HNO⟶NO), NNH route (N2⟶NNH
Fuel side Air side 600 K 500 K 400 K 400 K 500 K 600 K 0.0 2.0E – 5 4.0E – 5 6.0E – 5 8.0E – 5 1.0E – 4 1.2E – 4 1.4E – 4 N O mo le f rac tio n 10 20 30 40 0 Dilution ratio (%) (a) Fuel side Air side 600 K 500 K 400 K 400 K 500 K 600 K 0.0 2.0E – 5 4.0E – 5 6.0E – 5 8.0E – 5 1.0E – 4 1.2E – 4 1.4E – 4 N O mo le f rac tio n 10 20 30 40 0 Dilution ratio (%) (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.
20% H2O dilution in fuel 20% CO2 dilution in air 20% CO2 dilution in fuel 20% H2O dilution in air Thermal effect Chemical effect Overall effect No dilution (Case 1) TH2O / TCO2 FH2O / FCO2 Real H2O / CO2 –1E – 5 0 1E – 5 2E – 5 3E – 5 4E – 5 5E – 5 6E – 5 7E – 5 8E – 5 9E – 5 N O mo le f rac tio n maxim um
Figure 9: Peak NO mole fractions of CH4/air counterflow diffusion
flame with and without 20% H2O and 20% CO2dilution: as�50 s−1, L � 2 cm 400 K.
38.8% 85.1% 26.6% 84.9% 0.41 Full chemistry Thermal Prompt N2O-intermediate NNH NO-reburning 0.89 0.11 0.05 1.07 0.19 75.3% 79.5% 70.3% 18.8% 23.7% 15.3% 71.8% 75.3% 75.7% 27.8% 33.4% 26.7% 0.0 0.2 0.4 0.6 0.8 1.0 1.2 EIN O (g/kg) 20% H2O in fuel 20% CO2 in air 20% CO2 in fuel 20% H2O in air No dilution
Figure 10: The percentages of EINO reduction by 20% H2O and 20% CO2dilution in the air and fuel side of CH4/air counterflow diffusion
flame: as�50 s−1, L � 2 cm, 400 K. ~ ×10–8 ~ ×10–9 ~ ×10–10 ~ ×10–11 unit: mole cm–3 s–1 4.09, (R240) 2.56, (R273) 7.19, (R246, R250, R255) 8.08, (R249, R251) 1.01, (R270) 2.41(R190) 2.08, (R212) 1.69, (R214, R215) 1.06, (R274) (R194, R197) 1.30, (R192) 3.98, (R201) 8.55, (R223) 1.31 (R202, R203) 5.13, (R272) 1.66, (R265) 5.38, (R235) 1.43, (R232) 5.62, (R231) 3.40, (R218, R220) 3.03, (R219, R221) 9.39, (R191, R193) 4.68, (R185) 4.02, (R183) 3.64, (R199) 9.25, (R182) 3.40, (R178) 2.26 5.38, (R206, R207) (R209, R210) (R204, R205) 1.37, (R208) N NH NO HNO NNH N2O NO2 N2 NH2 CN HNCO HCNO HOCN HCN NCO 1.57, (R188, R189) 1.57, (R186, R187) 1.41, (R179, R180) 2.57, (R234) (a) Figure 11: Continued.
~ ×10–8 ~ ×10–9 ~ ×10–10 ~ ×10–11 3.06, 25.2% (R240) N2O N2 NO2 NH2 2.28, 24.8% (R219, R221) 9.69, 27.8% (R192) (R194, R197) 4.10, 27.1% (R231) 3.48, 4.4% (R199) 1.75, 31.6%, (R273) 5.06, 29.6%, (R246, R250, R255) 6.11, 24.4%, (R249, R251) 7.95, 21.3%, (R270) 1.87,22.4% (R190) 1.69, 18.8%, (R212) 1.29,23.7%,(R214, R215) 8.44, 20.4%, (R274) 3.23, 18.8%, (R201) 6.32, 26.1%, (R223) 9.52, 27.3% (R202, R203) 4.05,21.1%, (R272) 1.23, 25.9%, (R265) 3.74, 30.5%, (R235) 1.04, 27.3%, (R232) 2.46, 27.6%, (R218, R220) 6.74, 28.2% (R191, R193) 4.13, 11.8%, (R185) 3.51, 12.7%, (R183) 8.17, 11.7% (R182) 3.10, 8.8%, (R178) 1.91, 15.5% (R204, R205) 4.55, 15.4% (R206, R207) (R209, R210) 1.14, 16.%, 8(R208) N NH NO HNO NNH CN HNCO HCNO HOCN HCN NCO 1.26, 19.8% (R186, R187) 1.25, 20.4% (R188, R189) 1.02, 27.7% (R179, R180) 1.75, 31.9% (R234) unit: mole cm–3 s–1 (b) ~ ×10–8 ~ ×10–9 ~ ×10–10 ~ ×10–11 2.12, 83.7% (R192) 1.50, 58.79% (R199) 5.41, 82.2% (R219, R221) 9.12, 83.8% (R231) 2.77,89.2% (R234) 8.14,80.1%, (R240) 2.76, 89.2%, (R273) 8.92, 87.6%(R246, R250, R255) 1.40, 82.67%(R249, R251) 2.12, 79.01%, (R270) 5.17, 78.6% (R190) 6.48, 68.6%, (R212) 3.17,81.2%,(R214, R215) 2.32,78.1%, (R274) (R194, R197) 9.67, 75.7%, (R201) 1.44, 83.2%, (R223) 1.81, 86.2% (R202, R203) 1.08,79.0%, (R272) 2.61, 84.3%, (R265) 6.52, 87.9%, (R235) 2.31, 83.9%, (R232) 5.37, 84.21%, (R218, R220) 1.32, 85.9% (R191, R193) 1.77, 62.2%, (R185) 1.49, 62.9%, (R183) 3.13, 66.16% (R182) 6.47, 81.0%, (R178) 7.70, 65.9% (R204, R205) 1.84, 65.8% (R206, R207) (R209, R210) 3.88, 71.7%, (R208) N NH NO HNO NNH NH2 NO2 N2O N2 CN HNCO HCNO HOCN HCN NCO 4.06, 74.1% (R186, R187) 4.04, 74.3% (R188, R189) 2.02, 85.7% (R179, R180) unit: mole cm–3 s–1 (c) Figure 11: Continued.
~ ×10–8 ~ ×10–9 ~ ×10–10 ~ ×10–11 NH2 NO2 N2O N2 3.70, -1.6% (R199) 9.18, 29.4% (R192) 1.97, 35.0% (R219, R221) 3.38, 39.9% (R231) 1.69, 34.0%, (R273) 4.43, 38.4%, (R246, R250, R255) 5.54, 31.8%, (R249, R251) 7.16, 29.1%, (R270) 1.45, 39.8% (R190) 1.07, 48.6%, (R212) 1.16,31.4%,(R214, R215) 7.60, 28.3%, (R274) (R194, R197) 2.78, 30.2%, (R201) 5.22, 39.0%, (R223) 8.91, 32.0% (R202, R203) 3.65,28.9%, (R272) 1.12, 32.5%, (R265) 3.60, 33.1%, (R235) 8.59, 39.9%, (R232) 2.13, 37.4%, (R218, R220) 5.65, 39.8% (R191, R193) 3.99, 14.7%, (R185) 3.37, 16.2%, (R183) 7.50, 18.9% (R182) 3.72, -9.4%, (R178) 1.89, 16.4% (R204, R205) 4.36, 19.0% (R206, R207) (R209, R210) 1.01, 26.3%, (R208) N NH NO HNO NNH CN HNCO HCNO HOCN HCN NCO 1.07, 31.9% (R186, R187) 1.06, 32.5% (R188, R189) 8.76, 37.9% (R179, R180) 1.69, 34.3% (R234) 2.74. 33.0%, (R240) unit: mole cm–3 s–1 (d) 3.78, 70.9% (R192) 2.28,37.4% (R199) 6.95, 77.1% (R219, R221) 9.78, 82.6% (R231) 6.35, 75.2%, (R273) 1.35, 81.2%, (R246, R250, R255) 1.97, 75.6%, (R249, R251) 2.94, 70.9%, (R270) 3.62, 85.0% (R190) 1.26, 106.1%, (R212) 4.27,74.7%,(R214, R215) 3.20, 69.8%, (R274) (R194, R197) 1.07, 73.1%, (R201) 1.54, 82.0%, (R223) 3.40, 74.1% (R202, R203) 1.50,70.8% (R272) 4.09, 75.4%, (R265) 1.40, 74.4%, (R235) 2.48, 82.7%, (R232) 7.08, 79.2%, (R218, R220) 1.56, 83.4% (R191, R193) 2.20, 53.0%, (R185) 1.84, 54.2%, (R183) 3.44,62.8% 3.44(R182) 1.70, 50.0%, (R178) 9.93, 56.1% (R204, R205) 2.01, 62.4% (R206, R207) (R209, R210) 3.28, 76.1%, (R208) N NH NO HNO NNH CN HNCO HCNO HOCN HCN NCO 3.85, 75.5% (R186, R187) 3.83, 75.6% (R188, R189) 2.66, 81.3% (R179, R180) 6.36, 75.3% (R234) NH2 NO2 N2O N2 1.02, 75.1%, (R240) unit: mole cm–3 s–1 ~ ×10–8 ~ ×10–9 ~ ×10–10 ~ ×10–11 (e)
Figure 11: NO formation pathways for (a) no dilution (Case 1). (b) 20% CO2dilution in fuel (Case 6). (c) 20% CO2dilution in air (Case 12).
(⟶NH)⟶NO), N2O-intermediate route (N2⟶N2O
(⟶NH) ⟶NO), and part of NO-reburning route (N2⟶HCN, HCNO⟶CN, NCO, NH3, NH2,
NH,⟶N2O, NO, and N2).
By comparing Figures 11(b) and 11(c) with Figures 11(a), 11(d) and 11(e) with Figure 11(a), it can be found that the air-side dilution and fuel-side dilution affect the NO formation pathway similarly in general. To be specific, the nitrogen elemental fluxes in Figure 11(a) are the highest among all the five presented cases, indicating that the CH4 flame without dilution produces the highest
amount of NO. The fuel-side dilution weakens the nitrogen elemental fluxes to a lesser degree than the air-side dilution for both H2O and CO2diluents. Besides, as a more detailed
illustration than Figure 10, Figure 11 shows that the NO formation through the thermal route, NNH route, and N2O-intermediate route is much less influenced than that
through the prompt route, and the less affected reactions include R204 to R210 (NNH route), R178 (thermal route), and R185, R183, R182, and R199 (N2O-intermediate
route).
However, differences do exist between the effects of the two diluents. When the flame is diluted with CO2, the NO
formation pathway, NO⟶HCN (⟶HOCN)⟶ HNCO⟶NH2⟶NH⟶N⟶NO (the N flux and its
reduction percentage, as well as the involved reactions, are distinguished in blue), is most affected. While for H2O
dilution, the most affected pathway is NO⟶HCN (⟶NCO)⟶NH (⟶N)⟶NO. Besides, another re-markable change for H2O dilution can be observed for the
conversion of NO⟶HNO. When H2O is added to the
fuel-side, the nitrogen elemental flux of R212 (from the NNH NO formation route) is decreased mainly by 48.56%. This degree far exceeds that of other NO formation pathways (with reduction percentages of 14.74%–39.93%). As for H2O
di-lution on the air-side, the direction of the conversion be-tween NO and HNO is even reversed. This shift of the progress direction of R212 correlates well with the highest NO reduction percentage of air-side H2O dilution in
Fig-ure 8. All the abovementioned phenomena show that H2O
dilution has more complex influences on flame structure and NO formation reduction than CO2 dilution.
4. Conclusions
Motivated by the widespread utilization of dilution com-bustion in many advanced technologies, this work sys-tematically compared the effects of up to 40% H2O and CO2
dilutions to the fuel- and air-side on NO emissions in N2
diluted CH4/air counterflow diffusion flames. This
numer-ical study was performed using the OPPDIF code in con-junction with the GRI-Mech 2.11 kinetics mechanism at varying strain rates (20–100 s−1) and inlet temperatures range from 400 K to 800 K, 1 atm. The dilution was also realized by replacing N2 with either CO2 or H2O in the
oxidizer and the fuel stream to maintain the fuel and oxygen concentration constant. Several important findings are listed as follows:
(1) The dilutions with H2O and CO2in the oxidizer and
fuel-side all decrease the flame temperature and NO formation. Dilutions by H2O and CO2lower the peak
flame temperature nearly linearly as the dilution ratio increases. The reduction rate of peak NO mole fraction displays a decreasing trend with increasing the dilution ratio, especially when the dilution takes place on the oxidizer side. The differences between the results based on the adiabatic treatment, the optically thin approximation, and the discrete-or-dinates method coupled with the statistical narrow-band based correlated-k radiative property model decrease with increasing the dilution ratio.
(2) The chemical effects of CO2dilution lower both the
flame temperature and NO emissions while the chemical effect of H2O dilution exerts a slightly
promoting impact on the flame temperature but a suppression influence on NO formation. The ap-parently conflicting chemical effect of H2O dilution
on the flame temperature and NO formation is at-tributed to the considerably suppressed CH radical, which results in more significant NO reduction through the dominant prompt NO route than the NO promotion through the thermal NO route. (3) The importance of effects of H2O and CO2dilution
in the air- and fuel-side on NO reduction follows the order of air-side CO2 dilution > air-side H2O
dilu-tion > fuel-side H2O dilution > fuel-side CO2
dilu-tion, and the prompt NO route always plays the dominant role under the current investigation conditions.
(4) Pathway analysis shows that the air-side dilution and fuel-side dilution affect the NO formation pathway similarly in general, but H2O dilution still differs
from CO2dilution. When the flame is diluted with
CO2, the most affected NO formation pathway is
NO⟶HCN (⟶HOCN)⟶HNCO⟶NH2⟶
NH⟶N⟶NO. However, when the flame is di-luted with H2O, the most affected pathway is
NO⟶HCN (⟶NCO)⟶NH (⟶N)⟶NO.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51776112), China Postdoctoral Science Foundation (2020M672059), the Fundamental Research Funds of Shandong University, Shandong Provincial Natural Science Foundation of China (Grant no. ZR2020QE198), and the Fundamental Research Funds for the Central Universities
(18CX02129A). The authors sincerely acknowledge these fi-nancial supports.
References
[1] M. Jonsson and J. Yan, “Humidified gas turbines-a review of proposed and implemented cycles,” Energy, vol. 30, no. 7, pp. 1013–1078, 2005.
[2] H. Jericha, W. Sanz, and E. Gottlich, “Design concept for large output graz cycle gas turbines,” Turbo Expo: Power for Land, Sea, and Air, pp. 1–14, American Society of Mechanical Engineers, New York, NY, USA, 2006.
[3] G. H. Abd-Alla, “Using exhaust gas recirculation in internal combustion engines: a review,” Energy Conversion and Management, vol. 43, no. 8, pp. 1027–1042, 2002.
[4] R. W. Bilger and Z. Wu, “Carbon capture for automobiles using internal combustion Rankine cycle engines,” Journal of Engineering for Gas Turbines and Power, vol. 131, no. 3, pp. 034502–034505, 2009.
[5] S. Hoseinzadeh, S. Ghasemi, and S. Heyns, “Application of hybrid systems in solution of low power generation at hot seasons for micro hydro systems,” Renewable Energy, vol. 160, pp. 323–332, 2020.
[6] S. Hoseinzadeh and S. Heyns, “Advanced energy, exergy and environmental (3E) analyses and optimization of a coal-fired 400 MW thermal power plant,” Journal of Energy Resources Technology, vol. 143, no. 8, p. 082106, 2021.
[7] L. Wang, Z. Liu, S. Chen, C. Zheng, and J. Li, “Physical and chemical effects of CO2and H2O additives on counterflow
diffusion flame burning methane,” Energy & Fuels, vol. 27, no. 12, pp. 7602–7611, 2013.
[8] H. Xu, F. Liu, S. Sun, Y. Zhao, S. Meng, and W. Tang, “Effects of H2O and CO2diluted oxidizer on the structure and shape of
laminar coflow syngas diffusion flames,” Combustion and Flame, vol. 177, pp. 67–78, 2017.
[9] L. Zhuo, Y. Jiang, R. Qiu, J. An, and W. Xu, “Effects of fuel-side N2, CO2, H2O dilution on combustion characteristics and NOx formation of syngas turbulent nonpremixed jet flames,” Journal of Engineering for Gas Turbines and Power, vol. 136, no. 6, pp. 061505–061511, 2014.
[10] J. Park, S.-G. Kim, K.-M. Lee, and T. K. Kim, “Chemical effect of diluents on flame structure and NO emission characteristic in methane-air counterflow diffusion flame,” International Journal of Energy Research, vol. 26, no. 13, pp. 1141–1160, 2002.
[11] Y. Xie, J. Wang, N. Xu, S. Yu, and Z. Huang, “Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures,” International Journal of Hydrogen Energy, vol. 39, no. 7, pp. 3450–3458, 2014.
[12] D. Ning, A. Fan, and H. Yao, “Effect of radiation emission and reabsorption on flame temperature and NO formation in H2/ CO/air counterflow diffusion flames,” International Journal of Hydrogen Energy, vol. 42, no. 34, pp. 22015–22026, 2017. [13] S. ¨Ozt¨urk, “The effects of CO2, N2, and H2O dilutions on NO
formation of partially premixed synthesis gas combustion,” Cumhuriyet Science Journal, vol. 40, no. 4, pp. 813–818, 2019. [14] J. J. Feese and S. R. Turns, “Nitric oxide emissions from laminar diffusion flames: effects of air-side versus fuel-side diluent addition,” Combustion and Flame, vol. 113, no. 1-2, pp. 66–78, 1998.
[15] E.-S. Cho and S. H. Chung, “Characteristics of NOx emission with flue gas dilution in air and fuel sides,” KSME Interna-tional Journal, vol. 18, no. 12, pp. 2303–2309, 2004.
[16] B. Rahmanian, M. R. Safaei, S. N. Kazi, G. Ahmadi, 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, pp. 1222–1235, 2014.
[17] Z. Abdelmalek, R. Alamian, M. S. Shadloo, A. Maleki, and A. Karimipour, “Numerical study on the performance of a homogeneous charge compression ignition engine fueled with different blends of biodiesel,” Journal of Thermal Analysis and Calorimetry, pp. 1–11, 2020.
[18] R. S. Gavhane, A. M. Kate, A. Pawar et al., “Effect of zinc oxide nano-additives and soybean biodiesel at varying loads and compression ratios on VCR diesel engine characteristics,” Symmetry, vol. 12, no. 6, p. 1042, 2020.
[19] M. K. Chernovsky, A. Atreya, and H. G. Im, “Effect of CO2
diluent on fuel versus oxidizer side of spherical diffusion flames in microgravity,” Proceedings of the Combustion In-stitute, vol. 31, no. 1, pp. 1005–1013, 2007.
[20] J. Park, D.-J. Hwang, J.-G. Choi, K.-M. Lee, S.-I. Keel, and S.-H. Shim, “Chemical effects of CO2 addition to oxidizer and fuel streams on flame structure in H2-O2counterflow
diffu-sion flames,” International Journal of Energy Research, vol. 27, no. 13, pp. 1205–1220, 2003.
[21] F. Liu, H. Guo, G. J. Smallwood, and ¨O. L. G¨ulder, “The chemical effects of carbon dioxide as an additive in an ethylene diffusion flame: implications for soot and NOx formation,” Combustion and Flame, vol. 125, no. 1-2, pp. 778–787, 2001. [22] A. E. Lutz, R. J. Kee, J. F. Grcar, and F. M. Rupley, OPPDIF: a
fortran program for computing opposed-flow diffusion flames, Sandia National Laboratories, Livermore, CA, USA, 1997. [23] R. J. Kee, F. M. Rupley, and J. A. Miller, Chemkin-II: A Fortran
Chemical Kinetics Package for the Analysis of Gas-phase Chemical Kinetics, Sandia National Laboratories, Livermore, CA, USA, 1989.
[24] C. Bowman, R. Hanson, D. Davidson et al., “GRI-mech 2.11, 1995,” 1995, http://www.me.berkeley.edu/gri_mech. [25] S. H. Kim, K. Y. Huh, and B. Dally, “Conditional moment
closure modeling of turbulent nonpremixed combustion in diluted hot coflow,” Proceedings of the Combustion Institute, vol. 30, no. 1, pp. 751–757, 2005.
[26] K. W. Lee and D. H. Choi, “Prediction of NO in turbulent diffusion flames using Eulerian particle flamelet model,” Com-bustion Theory and Modelling, vol. 12, no. 5, pp. 905–927, 2008. [27] S. R. Turns, Introduction to Combustion, McGraw-Hill
Companies, New York, NY, USA, 1996.
[28] F. Liu, H. Guo, G. Smallwood, and M. El Hafi, “Effects of gas and soot radiation on soot formation in counterflow ethylene diffusion flames,” Journal of Quantitative Spectroscopy and Radiative Transfer, vol. 84, no. 4, pp. 501–511, 2004. [29] D. E. Giles, S. Som, and S. K. Aggarwal, “NOx emission
characteristics of counterflow syngas diffusion flames with airstream dilution,” Fuel, vol. 85, no. 12-13, pp. 1729–1742, 2006.
[30] F. Wang, P. Li, J. Zhang, Z. Mei, J. Mi, and J. Wang, “Routes of formation and destruction of nitrogen oxides in CH4/H2 jet flames in a hot coflow,” International Journal of Hydrogen Energy, vol. 40, no. 18, pp. 6228–6242, 2015.
[31] S. J. Klippenstein, L. B. Harding, P. Glarborg, and J. A. Miller, “The role of NNH in NO formation and control,” Combustion and Flame, vol. 158, no. 4, pp. 774–789, 2011.
[32] J. Revel, J. Boettner, M. Cathonnet, and J. Bachman, “Deri-vation of a global chemical kinetic mechanism for methane ignition and combustion,” Journal de chimie physique, vol. 91, pp. 365–382, 1994.
![Table 2: Main reactions involved in NO formation (more information can be found in the GRI-Mech 2.11 mechanism [24]).](https://thumb-eu.123doks.com/thumbv2/123doknet/13968234.453508/5.900.76.820.130.376/table-main-reactions-involved-formation-information-mech-mechanism.webp)
