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Physical and chemical effects of CO2 addition on CH4/H2 flames on a Jet in Hot Coflow (JHC) burner Yaojie Tu, Kai Su, Hao Liu, Sheng Chen, Zhaohui Liu, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02499 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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Physical and chemical effects of CO2 addition on CH4/H2 flames on a Jet in Hot Coflow (JHC) burner Yaojie Tu, Kai Su, Hao Liu*, Sheng Chen, Zhaohui Liu, Chuguang Zheng

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China

ABSTRACT: Moderate or intense low oxygen dilution (MILD) combustion is a promising technology for energy saving and pollutants emission reduction, and it has been experimentally shown that, MILD regime can be achieved more easily when using CO2 as diluent with respect to N2 [Combustion and Flame 2013, 160, (5), 933-946]. Since CO2 is different to N2 in both physical and chemical properties, the present study aims at distinguishing the physical and chemical effects of CO2 addition on the establishment of MILD combustion. A Jet in Hot Coflow (JHC) burner firing CH4/H2 blended fuel is numerically modeled coupled with detailed chemical kinetics mechanism. Following the examination of grid independency and model validation, the differences in combustion temperature, minor and major species formations as well as CH4 oxidation pathway are compared by respectively replacing N2 with CO2 or X (artificial CO2) in low oxygen coflow. An interesting phenomenon is presented that, the chemical effects of CO2 play a comparable role on the suppression of temperature rise to the physical effects. However, with the replacement of N2 by CO2,

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the contribution of chemical effects to temperature reduction is gradually weakened. Moreover, the chemical effects of CO2 are responsible for the ignition delay as well as the enhanced CO emission when CO2 replacing N2, which is due to the inhibition of CH4 oxidation through Route I (CH4 → CH3 → CH2(S)/CH2 → (CH → CH2O →) HCO → CO → CO2) and II (CH4 → CH3 → CH2O → HCO → CO → CO2) together with the enhanced CO2 dissociation by R99 (OH + CO ↔ H + CO2) and R153 (CO2 + CH2(S) ↔ CO + CH2O).

KEYWORDS: MILD combustion, oxy-fuel combustion, oxy-MILD combustion, pathway analysis

1.

INTRODUCTION

MILD (moderate or intense low oxygen dilution) combustion1, also called as HTAC (high temperature air combustion)2, or FLOX (flameless oxidation)3, or CDC (color distributed combustion)4, has been proposed for decades aiming at reducing NOX emission and enhancing heat efficiency during industrial combustion and heating process. Recently, MILD combustion is expected to be combined with oxy-fuel combustion5-7, which inherits the relative merits of the two technologies and facilitates the CO2 capture, energy-saving as well as pollutants emission control. Moreover, for steel production, the combined form was reported to improve the slab heating quality due to less scale formation with respect to the conventional heating technologies8. Currently, the studies related to MILD combustion are mainly conducted in air atmosphere. On the other hand, the knowledge to oxy-fuel


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combustion is mainly dealing with conventional burner, i.e. swirling burner using high-level oxygen stream. However, because of the low oxygen dilution by internal flue gas recirculation, the MILD combustion rate is slower and homogeneous, which differs much from the conventional operation. In particular, when MILD combustion is coupled with oxy-fuel combustion, namely under oxy-MILD combustion, the combustion characteristics still need to be further investigated.

As is known, CO2 is different from N2 not only in the physical properties (i.e. specific heat capacity, thermal conductivity), but also in the chemical properties (i.e. elementary composition, reactivity). What is more, N2 is an inert gas which basically does not participate in the chemical reactions (except for NOX related reactions), while CO2 is either the reactant or the product. For this reason, the differences between air-fired combustion and oxy-fired combustion are caused mainly from the discrepancies between N2 and CO2. From this point of view, figuring out the impacts of replacing N2 by CO2 under MILD combustion can lead to a better insight into oxy-MILD combustion, especially on the establishment of MILD regime. Although the studies on oxy-MILD combustion are quite sparse, but the related literatures can still be found as follows. On the experimental side, Li et al.9 recently conducted lab-scale tests using gaseous fuels to compare the MILD combustion characteristics in O2/N2 and O2/CO2 atmospheres. By taking photos of the combustion flame, the flame edge becomes harder to distinguish with CO2 replacing N2 at an identical equivalence ratio. Besides,


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the equivalence ratio which leads to the occurrence of MILD combustion is found to reduce by replacing N2 gradually with CO2. For coal combustion, our group10, 11 also observed a more invisible flame appearance in a 300 kW down-fired vertical furnace when N2 is replaced by recycled flue gas (mainly CO2) under MILD operation. On the numerical modeling side, Sabia et al.12 and Zhang et al.13 studied the effects of CO2 on fuel combustion process under MILD condition in a zero-dimensional well stirred reactor (WSR). They found the higher specific heat capacity (CP) of CO2 can lower the adiabatic ﬂame temperature, thus facilitate the establishment of MILD combustion, however, the chemical effects of CO2 would increase the ignition delay. Wang et al.14 compared the physical and chemical effects of CO2 addition on the counterflow diffusion flame. They reported a significant flame temperature reduction by both chemical and physical effects of CO2. Using the similar burner configuration, Liu et al.15 claimed that, CO2 is more appropriate for sustaining MILD combustion due to less fluctuated combustion temperature. Mei et al.16 studied the dimensions of CH4 jet flames in hot O2/CO2 coflow by varying the coflow parameters, and found the larger reaction zone for oxy-MILD combustion than air-MILD combustion.

From the above literatures, it both experimentally and theoretically concludes that, MILD regime is likely to be established more easily with CO2 dilution than N2 dilution. However, whether the physical effects or the chemical effects are more important is still not well understood. Undoubtedly, for conventional combustion burner, the differences between air-fired and oxy-fuel combustion are mainly resulted


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from the differences in physical properties, i.e.: specific heat capacity, between N2 and CO25-7, 17-19. While for MILD combustion burner, the local oxygen concentration is reduced, which in turn suppresses the fuel oxidation rate as well as the temperature rise. Thus, the entropy change by flue gas absorption could be reduced through T1

∆h = ∫ C p dT , and the physical effects on combustion temperature suppression would T0

not be as significant as that under conventional combustion. On the contrary, the differences in chemical properties between N2 and CO2 is expected to play a more important role.

To address the above-mentioned problem, although some progress has been achieved based on zero-dimensional or one-dimensional kinetic analysis9, 12-14, 16, 20, 21, while for a real combustor which involves both chemical reaction and turbulent mixing, no systematic study has been conducted. Furthermore, the physical and chemical effects of CO2 on MILD combustion characteristics (ignition, fuel oxidation and burnout, etc.) are still not well distinguished. To this end, in the present work, the CFD modeling of a JHC (jet in hot coflow) flame is carried out by considering both detailed chemical reactions and turbulent mixing. Here, the adopted JHC burner was originally developed by Dally et al.22, which was used for investigating the diffusion flame structure under flue gas and heat recirculation conditions, such as MILD combustion regime. The aim of the present study is to compare and analysis the physical and chemical effects of CO2 on MILD combustion, especially during the early ignition stage in a real JHC burner. An additional aim is to further reveal the respective role of physical and chemical effects of CO2 on MILD combustion under different CO2


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dilution degrees. To begin with, the modeling results are compared with the experimental measurements, i.e. temperature, major and minor species, to examine our modeling reliability, then, we compare the differences in flow field, combustion temperature and species distribution as well as CH4 oxidation pathways by replacing N2 respectively with actual CO2 and artificial CO2 (X). Note that, the artificial matter of X has the same physical properties (i.e. specific heat, molecular mass and density, etc.) as CO2, but does not participate in chemical reactions as N2. By comparing the results between N2 and X diluted cases, the physical effects of CO2 can be recognized. While by comparing the results between X and CO2 diluted cases, the chemical effects of CO2 can be recognized.

2.

COMPUTATIONAL DETAILS

2.1 Physical Configurations The JHC open flame system is described in detail in Ref.22-24. Therefore, only a brief illustration of the burner configuration is shown in Fig. 1. The fuel is a mixture of 50% CH4 and 50% H2 in volume fraction, and is injected at 60 m/s and 305 K through a 4.25 mm inlet. The outlet temperature of the annulus coflow (inner diameter = 82 mm) from the prepositive internal burner is 1300 K. The whole JHC burner is operated inside a wind tunnel, where the room temperature air (300 K) possesses the same velocity of 3.2 m/s as the coflow.


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Figure 1. Schematic diagram of the JHC burner of Dally et al.22. It was indicated that, the tunnel air can be induced by the high velocity fuel jet, and then could influence the combustion regime of the JHC flame, i.e. from partly emulated MILD mode to traditional turbulent flame above the axial position of x = 100 mm25. In addition, N2 in the tunnel air can affect the combustion process as well as the NOX formation in the downstream computational domain. In order to eliminate the potential influences, the burner configuration is modified by replacing the tunnel air with coflow gas as Fig. 2(b) shows. This modification guarantees the fuel combusts in an atmosphere close to that in a real combustor. Furthermore, the modification of the burner configuration has been proofed effective in studying the effects of initial inlet conditions on JHC flames as well as NOX formation and destruction mechanism under MILD regime16, 25, 26. The computational domains of the original and modified JHC burners have the same dimension of 520 mm in the axial direction and 210 mm in the radial direction as shown in Fig. 2(a) and Fig. 2(b), respectively. Owning to the symmetrical geometry of the two burners, the


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computational domains are simplified by 2D axisymmetric configurations. The grid sizes of the original and the modified JHC burners are respectively 46560 and 32656.

Figure 2. Computational domain and boundary conditions of (a) original JHC burner of Dally et al.22 and (b) modified JHC burner. 2.2 Computational Models The present numerical simulations are performed with the aid of the commercial CFD code Fluent, version 6.327 to solve the Reynolds Averaged Navier-Stokes (RANS) equations. The standard k-ε (SKE) model with C1ε value adjusted from 1.44 to 1.6 is considered for turbulent flow calculation23. The velocity boundary conditions are used for the three streams of fuel, coflow and tunnel air, and pressure outlet boundary condition is adopted for the downstream exit. While for the side of the computational domain, a zero-shear stress wall boundary condition is selected, since the tunnel air is


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wide enough that this boundary condition would not affect the flame structure, whereas could enhance the convergence and reduce computing time28. For the fuel stream, the mean turbulent kinetic energy (TKE) is adjusted to 60 m2/s2 to obtain the best agreement between predictions and measurements23, and the TKE values for the coflow and tunnel air are respectively set to 4 and 3.5, with which better results are obtained as compared to those from Ref.28.

It was indicated that, differential diffusion has strong influence on predictions JHC flames, and they can also affect the predicting accuracy of such flames29. Thus, the differential diffusion approach is taken into consideration by regarding the molecular diffusion coefficients as functions of temperature. The discrete ordinate (DO) model is considered for radiation calculation in the simulations despite that radiation does not have any noticeable effects on the results23. Also the weighted sum of gray gas (WSGG) model is adopted to calculate the gas mixture total emissivity as a function of the gas temperature and partial pressure. It is important to note that, when CO2 is added into the oxidants, the WSGG model needs to be modified due to the high presence of CO2 as well as its non-gray gas characteristic30. In this regard, the radiation spectrum is divided into 6 wavelength bands, and the corresponding values of each wavelength band are given in Table 1 as in Ref.16.

Table 1. Absorption coefficients for different wavelength band for CO2 added coflow. Interval wavelength (µm) 0-2.5 2.5-3.0 3.0-4.0 4.0-5.0 5.0-9.0 9.0-20 20% CO2 addition

0

0.75

0

0.78


0

0.72

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50% CO2 addition

0

0.79

0

0.82

0

0.75

97% CO2 addition

0

0.84

0

0.87

0

0.79

According to Wang et al.25, the kinetic mechanism of GRI-Mech 2.11 (containing 49 species and 279 reversible reactions) gives comparable results of the JHC flames to that from GRI-Mech 3.0 (containing 53 species and 325 reversible reactions). However, GRI-Mech 3.0 always produces a higher NO prediction, which fails to exist for GRI-Mech 2.11. Hence, GRI-Mech 2.11 is employed together with the eddy dissipation concept (EDC) model for turbulence-chemistry interactions in the present simulations. It also should be noted, the standard EDC model parameters are unreliable for flows with turbulence Reynolds number (Ret) less than 65 due to early ignition problem31. However, it can be solved by modifying the EDC model parameters. Referring to the previous literatures32-34, the volume fraction constant (Cξ) and the time scale constant (Cτ) of EDC model parameters are adjusted to 3 and 1, respectively. Besides, the ISAT algorithm is used to accelerate the computation.

The SIMPLE approach is selected for velocity-pressure coupling, and the second-order upwind scheme is used for the solution discretization to improve the calculation accuracy. The consideration of the computational convergence is depended on two criterions: (1) all residual levels are lower than 10-6; (2) the facet maximum temperature stays unchanged.

2.3 Computational Conditions When N2 is replaced by CO2, the differences are derived from two parts, i.e. the


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physical effects and the chemical effects, as stated above. To tell these two effects apart, an artificial material X is introduced, which has the same physical properties as real CO2, but is totally chemical inert. To include X in the CFD simulations, it is first added in the chemical mechanism without defining any elementary reactions, and then created as a new material in the database of CFD with the same property as CO2. By this way, the differences between N2 and X diluted cases can be identified as the physical effects, while the differences between X and CO2 diluted cases can be identified as the chemical effects. Table 2. Computational conditions of the present work. (with * indicates mass fraction, without * indicates volume fraction.) Case

Grid

Computational Coflow compositions

number Size

domain

O2 N2 H2O CO2 X

1*

46560

a

3

85 6.5

5.5

0

2*

46560

a

6

82 6.5

5.5

0

3*

46560

a

9

79 6.5

5.5

0

4*

124560 a

3

85 6.5

5.5

0

5

46560

b

3

97 0

0

0

6

46560

b

3

77 0

20

0

7

46560

b

3

77 0

0

20

8

46560

b

3

47 0

50

0

9

46560

b

3

47 0

0

50


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10

46560

b

3

0

0

97

0

11

46560

b

3

0

0

0

97

Eleven cases are modeled including four cases (1-4) using the original JHC burner (Fig. 2(a)) and seven cases (5-11) using the modified JHC burner (Fig. 2(b)), as listed in Table 2. Specifically, cases 1-3 are performed to assess the reliability of the present computational models for modeling the JHC flames, and case 4 is conducted to check the grid-independency of the computational domain with a comparison to case 1. Then, cases 5-11 are simulated using the validated models and mesh to distinguish the physical effects from the chemical effects of CO2. In detail, case 5 is the base case without CO2 or X replacement. Case 6 and 7 represent the cases with 20% (mole fraction) CO2 and X replacement of N2, respectively. Case 8 and 9 represent the cases with 50% (mole fraction) CO2 and X replacement of N2, respectively. Case 10 and 11 represent the cases with all N2 (97% mole fraction) replaced by CO2 and X, respectively. Note that, in cases 1-4, the 3% oxygen concentration in the coflow is measured by mass fraction as in the experiments22-24. While in cases 5-11, the 3% oxygen concentration in the coflow is treated in volume fraction to keep the injecting velocity consistent for all cases.

3.

VERIFICATION OF THE MODELING

3.1 Grid Independency Check In previous JHC modeling works16, 33, orthogonal structured mesh with grid size of about 30,000 cells was found to be independent to the results by comparing to a finer mesh with grid size up to about 80,000 cells. To further check the grid-independency,


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a comparison was made between a medium mesh (46560 cells) and a finer mesh (124560 cells) for the JHC flame with 3% oxygen mass fraction in the coflow (case 1 and 4). The predicted velocity, temperature and species profiles at three axial locations (x = 30, 60, 120 mm) for the two grids are presented in Fig. 3. The comparison between the two grids shows no obvious differences at varied locations, which suggests the adequacy of the medium mesh for modeling the JHC flame. Then, the meshes in Fig. 2(a) and 2(b) are adopted for model validation and objective studying individually in the following sections.

70

2000

(a)

60

V (m/s)

(b)

46560 cells 124560 cells

50


1600

T (K)

40 30

1200 800

20 400

10 0

0

2

4

6

8

10

12

0

14

0

2

4

6

r/D yCO (mass fraction)

(c)

2

10

12

14

0.030

0.20 0.15 0.10 0.05 0.00

8

r/D

0.25

yO (mass fraction)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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46560 cells 124560 cells 0

2

4

6

8

10

12

14

(d)

0.025


0.020 0.015 0.010 0.005 0.000

0

2

4

r/D

6

8

10

12

14

r/D

Figure 3. Comparisons of (a) velocity, (b) temperature, (c) O2 mass fraction and (d) CO mass fraction profiles at x = 30 mm (black), 60 mm (red), 120 mm (blue) between two grids. 3.2 Modeling Reliability Check Fig. 4 displays the comparisons of gas temperature and species between the numerical


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predictions and the experimental measurements22 for JHC flames with 3%, 6%, and 9% oxygen mass fraction in the coflow at axial location of x = 30 mm. Note that, for the measured CO hump in the coflow stream, it is explained to be the result of cooling and extinction of the secondary flame near the burner outer wall23. Nevertheless, the predictions agree reasonably well with the experimental measurements for the three JHC flames, and it suggests that the present numerical method is adequate for modeling such diffusion combustion process.

1800

0.020

(a)

(b) yCO (mass fraction)

1500

T (K)

1200 3% O2 (EXP)

900

3% O2 (CFD) 6% O2 (EXP)

600

6% O2 (CFD) 9% O2 (EXP)

300 0

9% O2 (CFD)

0

2

4

6

8

10

12

14

0.015 3% O2 (EXP) 3% O2 (CFD)

0.010

6% O2 (EXP) 6% O2 (CFD) 9% O2 (EXP)

0.005 0.000 0

9% O2 (CFD)

2

4

6

r/D

10

12

14

0.25

yO (mass fraction)

(c) 0.0015 3% O2 (EXP)

0.0010

3% O2 (CFD) 6% O2 (EXP) 6% O2 (CFD)

0.0005

9% O2 (CFD)

2

4

6

8

10

12

0.20

(d)

0.15

3% O2 (EXP) 3% O2 (CFD)

0.10

6% O2 (EXP) 6% O2 (CFD)

0.05

9% O2 (EXP)

2

9% O2 (EXP)

0.0000 0

8

r/D

0.0020

yOH (mass fraction)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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14

0.00 0

9% O2 (CFD)

2

4

r/D

6

8

10

12

14

r/D

Figure 4. Comparisons of (a) temperature, (b) CO mass fraction, (c) OH mass fraction, (d) O2 mass fraction profiles between predictions (curves) and measurements (symbols) at x = 30 mm for JHC flames with 3%, 6% and 9% O2 mass fraction in coflow.


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4.

RESULTS AND DISCUSSION

4.1 Effects of CO2 Addition on Flow Field Since the combustion process is highly subject to the flow dynamics35, it is necessary to see whether the flow dynamics are changed when replacing N2 by CO2. Hence, Fig. 5 compares the radial velocity profiles when N2 is replaced by different levels of CO2 or X at three axial locations. As observed, both the trend and the magnitude of the velocity profiles are generally consistent in various conditions at each axial location. This on one hand is resulted from the simple structure of such turbulent flow, on the other hand thanks to the low temperature increment (less than 300 K) which produces minor difference in gas volume. Although a discrepancy is noticed in the central region where cold fuel jet locates with CO2 or X addition according to Figure 5(b), it can be neglected as compared to the general velocity distribution. It follows that, the flow dynamics are not significantly influenced within the axial distance of x = 100 mm by CO2 or X addition, which inherits the merit of modifying burner geometry. On the other hand, it suggests the potential differences in combustion behaviors with CO2 or X addition are mainly derived from the physical or (and) chemical effects rather than the flow dynamics. Therefore, in the subsequent sections, the detailed differences in combustion characteristics of the JHC flames in either CO2 or X diluted conditions are evaluated.


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70

(a)

60

Velocity (m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40

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CO2=0%

(b)

35

CO2=20%

50

30

X=20% CO2=50%

40

25

30

20

20

X=50% CO2=97% 0

1

2

3

4

X=97%

5

x=30mm x=60mm x=90mm

10 0 0

10

20

30

40

50

60

Radial distance (mm) Figure 5. Radial velocity profiles at x = 30 mm, 60 mm, 90 mm in with CO2 (X) addition. 4.2 Effects of CO2 Addition on Combustion Temperature In the experiments of Dally et al.22, 24, the status of MILD combustion was partly emulated near the burner exit below x = 100 mm, especially in 3% oxygen concentration condition25. Herein, we take the radial line at x = 90 mm as a characteristic curve to make comparisons of the temperature and species distributions between the considered cases. In Fig. 6(a), the radial combustion temperature profiles with 0%, 20%, 50% and 97% CO2 or X addition are presented. It can be seen that, as N2 is replaced by either CO2 or X, the temperature rise is reduced continually. Moreover, for the CO2 dilution cases, a comparable reduction in combustion temperature is observed as compared to the X diluted cases. It implies that not only the physical effects but also the chemical effects of CO2 can play a negative role on the temperature rise for the JHC flames.


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1500

Temperature (K)

(a)

1200

CO2=0% CO2=20% X=20% CO2=50%

900

X=50% CO2=97% X=97% 600 0

10

20

30

40

50

60

Radial distance (mm) Maximum combustion temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1600

(b) 1550

31.8%

41.3%

1500

46.8%

68.2% 1450

58.7% 1400

53.2%

1350

CO2 addition

1300

X addition Physical effects Chemical effects

1250

0

20

40

60

80

100

CO2 or X mole fraction in oxidant (%)

Figure 6. (a) Radial temperature profiles at x = 90 mm, (b) Maximum combustion temperature with CO2 (X) addition. In order to quantitatively distinguish the physical effects and the chemical effects of CO2 addition on temperature reduction, the peak combustion temperature of the studied cases are shown in Fig. 6(b). Besides, the relative proportions of the physical


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effects and the chemical effects to the combined effects are also indicated. Similar to Fig. 6(a), the physical effects and the chemical effects are both found to result in lower peak combustion temperature. When CO2 or X mole fraction increases, the contribution of the physical effects to temperature reduction gradually becomes stronger, while that of the chemical effects turns weaker. However, no matter what concentration CO2 or X is, the contribution of chemical effects is always found larger than the physical effects at the same N2 replacing level. The reasons behind this can be explained as follows.

In the studied cases, the gas temperature rise is limited (less than 300 K) due to lower combustion rate and high preheating temperature of coflow. On the other hand, the present work is based on an actual burner configuration in a limited computational domain, in which the resistant time of fuel is less than 0.1 s. Thus, the mixing between the fuel and oxygen is not sufficient in the early ignition stage, and the chemical energy from the fuel is not fully released and absorbed by the flue gas. As a result, the physical effects on the suppression of temperature rise is not that obvious.

4.3 Effects of CO2 Addition on OH and CH2O Formations Since OH is considered as one of the most important free radicals in high temperature in the JHC combustion process36, Fig. 7 displays the radial OH profiles for different levels of CO2 or X addition conditions at x = 90 mm. Besides, the physical effects, chemical effects and their combined effects are indicated based on a comparison between 0% and 20% addition conditions. According to Fig. 7, it is found, when N2 is


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replaced by either CO2 or X gradually, the peak OH values of the radial profiles reduce continually. In particular, the reduction in the maximum OH formation is found mainly resulted from the chemical effects. In addition, because the luminance of a flame can be reflected by the OH distribution37, the reduced maximum OH concentration in some degree can indicate a reduced brightness of flame appearance with CO2 addition mainly by its chemical effects, which is consistent to the experimental observations9.

0.0005 (a)

CO2=0%

Physical effects

CO2=20%

0.0004

OH mole fraction

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X=20% CO2=50%

Combined effects

0.0003

X=50% CO2=97% X=97%

0.0002 Chemical effects

0.0001 0.0000 0

10

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Radial distance (mm) Figure 7. Radial OH profiles at x = 90 mm with CO2 (X) addition. Meanwhile, as observed from Fig. 7, for X addition conditions, OH peak values reduce with X addition in the central region (r < 30 mm), while increase in the outer region (r > 30 mm). However, for CO2 addition conditions, OH formation is suppressed with CO2 addition not only in the central region, but also in the outer region. This demonstrates that the physical effects of CO2 is helpful to realizing


Energy & Fuels

uniform OH distribution. From another point of view, it implies that enhancing the specific heat capacity of the diluents can promote the uniformity of the OH distribution, which is beneficial to the homogeneous flame appearance as well.

0.00025

CO2=0%

CH2O mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Physical effects

0.00020

CO2=20% X=20% CO2=50%

Chemical effects

0.00015

X=50% CO2=97%

0.00010

X=97%

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0.00005

0.00000

0

10

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Radial distance (mm) Figure 8. Radial CH2O profiles at x = 30 mm with CO2 (X) addition. Since CH2O is a first step intermediate formed in the low temperature region, especially during the early ignition stage, the CH2O distribution are extracted at an earlier position of x = 30 mm as shown in Fig. 8. Together, the physical effects, chemical effects and their combined effects are indicated based on a comparison between 0% and 20% addition conditions. It is observed, the peak CH2O concentration is improved by X addition, while reduced by CO2 addition. This seems to demonstrate that, the fuel ignition can be accelerated by the physical effects while inhibited by chemical effects of CO2. More significantly, the chemical effects plays a role over the physical effects, and lead to an overall suppression of CH2O formation


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as N2 is replaced by CO2. This observation is consistent to the previous study that fuel ignition delay can be enhanced under oxy-MILD combustion as compared to air-MILD combustion13. However, it also can be noticed a wider distribution of CH2O under CO2 diluted conditions. This implies that, with CO2 replacing N2, not only the reacting region is expanded, but also the fuel ignition takes place more homogeneously.

4.4 Effects of CO2 Addition on CO and NO Formations Fig. 9(a) compares the radial CO profiles at x = 90 mm in different atmospheres. Minor difference is found in the CO distribution for X diluted cases, while huge difference for CO2 diluted cases. It seems to indicate the physical effects of CO2 plays a negligible role than the chemical effects on the formation of CO. This is because, according to Glarborg et al.38, for hydrocarbon fuels under oxy-fuel combustion, CO2 can compete with O2 for atom hydrogen generation, and produce CO through the reaction CO2 + H ↔ CO + OH (R99 in GRI-Mech 2.11). Also under MILD condition, Zou et al.39, Mardani et al.40 and Zhang et al.13 all likewise found R99 to be the most sensitive elementary reaction to CO formation. Moreover, this can be further verified according to the reaction route (Fig. 12) in section 4.5. In this regard, since the CO2 concentrations are basically the same for X diluted cases, the CO and OH formations are expected to be similar as Figure 7 and 9(a) illustrate.

In addition to the CO distribution, the CO emission is also compared among the studied cases as Fig. 9(b) shows. Similarly, the CO emission is found hardly changes


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with X addition. While interestingly, for CO2 diluted cases, not only the CO formation (see Fig. 9(a)), but also the CO emission (see Fig. 9(b)) reach the peak value under 20% CO2 addition condition. This saturation phenomenon has also occurred in the kinetic study of Wang et al.14, namely, when the CO2 dilution degree exceeds a certain value, the CO production changes from increasing to decreasing. Note that, the saturation CO2 mole fraction in their study is about 40%, while in the present study it is around 20%. The reason behind this is probably that, as N2 is replaced by CO2, on one hand the fuel (mainly CH4) conversion is suppressed due to slower reaction rate, on the other hand the CO2 dissociation through CO2 + H ↔ CO + OH is strengthened14. When the CO2 concentration reaches a certain degree, i.e. 20%, as a result of the competition, the CO formation and emission change from increasing to reducing.

0.005

CO2=0%

(a)

CO2=20%

0.004

CO mole fraction

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X=20% CO2=50%

0.003

X=50% CO2=97% X=97%

0.002 0.001 0.000 0

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Radial distance (mm)


50

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-3

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Average CO mole fraction at exit (10 )

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8 (b) 6

4

2 CO2 addition X addition

0 0

20

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60

80

100

CO2 or X mole fraction in oxidant (%) Figure 9. (a) Radial CO profiles at x = 90 mm, (b) CO emission with CO2 (X) addition. To further study the CO formation characteristics, the CO mole fraction contours in different X or CO2 dilution degrees are displayed in Fig. 10. For X diluted cases, no significant differences are observed on the CO distribution as well as its magnitude. While under CO2 dilution conditions, the CO formation is found to be delayed as CO2 dilution degree increases. As a result, the position where maximum CO concentration occurs moves downstream. Moreover, wider CO distribution is noticed in CO2 dilution conditions with respect to X dilution conditions. This implies that using CO2 as diluent for MILD combustion can generate a broader reaction zone, which is consistent to Mei et al.16, that CO2 can lead to enhanced ignition delay and wider flame border than N2 does. Specifically, the reasons behind this can be resulted from the chemical effects of CO2 according to Fig. 10.


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Figure 10. CO contours in different CO2 (X) addition conditions. Due to the significantly reduced combustion temperature under MILD combustion, NO formation through the thermal route, namely the extended Zeldovich mechanism27, is expected to play a relatively smaller part. Actually, for the H2/CH4 blended JHC flames of Dally et al.23, the NNH route is verified to be the most important mechanism for the NO formation41, 42. What is more, in the present study, there are two means further inhibiting the NO formation, i.e.: one is the removal of the tunnel air, and the other is the replacement of N2 by CO2 in the coflow.

In Fig. 11(a) and Fig. 11(b), the radial NO distributions at x = 90 mm and the final NO emissions in different atmospheres are presented. Note that, since the cases with 97% X and CO2 addition are absence of N2, the NO formations are avoided. From Fig. 11(a), the peak values of the radial NO profiles are found to be reduced with either X addition or CO2 addition. However, under the identical N2 replacement condition, CO2 always leads to the lower NO formation. Similarly, Fig. 11(b) shows that the CO2 addition is more beneficial to reducing the final NO emission. Moreover, the respective proportion of physical and chemical effects of CO2 on NO emission


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reduction are indicated in Fig. 11(b). It is found, the role of physical effects of CO2 on NO emission reduction is becoming stronger with CO2 addition. More specifically, under low CO2 mole fraction condition, the reduction in NO emission is mainly derived from the chemical effects of CO2. While under high CO2 mole fraction condition, the NO emission is reduced mainly by the physical effects of CO2.

2.5 CO2=0%

-6

NO mole fraction (10 )

(a)

CO2=20%

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X=20% CO2=50%

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X=50%

1.0 0.5 0.0 0

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Average NO mole fraction at exit (10 )

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6 CO2 addition

(b)

X addition Physical effects Chemical effects

5 4 10.5% 3

47.5%

2

100%

89.5%

1

52.5%

0 0

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CO2 or X mole fraction in oxidant (%) ACS Paragon Plus Environment

100

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Figure 11. (a) Radial NO profiles at x = 90 mm, (b) NO emission with CO2 (X) addition. 4.5 Effects of CO2 Addition on CH4 Oxidation From the above observations of major and minor species, such as CO, OH and CH2O, it is easy to deduce that the physical effects of CO2 have a small impact on the fuel (mainly CH4) combustion, while the chemical effects can make a big difference. To figure out how the physical and chemical effects of CO2 affect the fuel combustion process, respectively, the CH4 oxidation pathways in O2/N2, O2/CO2, and O2/X atmospheres, which refers to case 5, case 10 and case 11 in Table 2, are shown in Fig. 12. Note that, the elementary reactions are in the same order as those in GRI-Mech 2.11. In addition, for each elementary reaction, the average reaction rates ri is labeled in the corresponding position according to Eq. 1, where A is the total area of the computational domain, and ri is the local reaction rate of reaction i. To conveniently observe the relative importance of the elementary reactions, the arrows of the elementary reactions are also highlighted in different width and color according to their magnitude. ri =

1 ri dA A∫

(1)

Generally, CH4 can be consumed to CO2 through four routes, i.e. Route I: CH4 → CH3 → CH2(S)/CH2 → (CH → CH2O →) HCO → CO → CO2; Route II: CH4 → CH3 → CH2O → HCO → CO → CO2; Route III: CH4 → CH3 → CH3OH/CH3O/CH2OH → CH2O → HCO → CO → CO2; Route IV: CH4 → CH3 → C2H6 → C2H5 → C2H4 → C2H3 → (HCO →) C2H2 → HCCO → CO → CO2. Since CH3 is at the startup of each


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Energy & Fuels

route, the conversion rate from CH3 to the latter species in each route can be regarded as the intensity of the corresponding route.

(a) O2/N2

(b) O2/CO2


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(c) O2/X

Figure 12. CH4 oxidation pathway in O2/N2 (top), O2/CO2 (middle) and O2/X (bottom). In Fig. 12, the CH4 oxidation processes are similar in O2/N2 and O2/X atmospheres, as expected, because CH4 is noticed to be consumed mainly through Route I and II. While in O2/CO2 atmosphere, both Route I and II are strongly inhibited, and Route IV is found to play a comparable role to Route I and II. Nevertheless, the overall conversion rate from CH4 to CH3 is reduced in O2/CO2 with respect to O2/N2 and O2/X. It is worth noting that, Route IV increases from 2.80 ×10 −7 in O2/N2 to

2.95 ×10−7 in O2/CO2, and further to 3.91× 10−7 in O2/X. It indicates that, replacing N2 with CO2 can promote the CH4 oxidation through Route IV by reforming to higher hydrocarbons (i.e. ethane), mainly from its chemical effects.

According to the CO related elementary reactions in Fig. 12, R99 (OH + CO ↔ H +


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Energy & Fuels

CO2) is found to be the dominating reaction in O2/N2 and O2/X atmospheres for CO formation and destruction, which is consistent to the findings of Glarborg et al.38. While in O2/CO2 atmosphere, the reaction rate of R99 is becoming negative. Moreover, the reaction rate of R153 (CO2 + CH2(S) ↔ CO + CH2O), which is likewise important for CO formation39, is found increased due to the high presence of CO2. However, the conversion rates to CO from HCO and HCCO are all reduced, and the sum of the reaction rates of R99 and R153 is close to that of R55, R166, R167, R168 and R176. This implies that the dissociation of CO2 by R153 accounts for half of the CO formation under CO2 diluted condition. As a result, the final CO emission in O2/CO2 is higher than that in O2/N2 and O2/X, which is consistent to the observation in Fig. 9(b). In addition, Route II (R55, R166, R167 and R168) is found to be enhanced from 8.79 ×10−7 in O2/X to 1.73 ×10−6 in O2/N2. This then explains the reason behind the evaluated CO emission when N2 is replaced by X as shown in Fig. 9(b).

It need to be mentioned here that, the pathway analysis from the present two-dimensional CFD modeling produces some differences to that from the previous zero-dimensional kinetic calculation13. For instance, the CH2O formation is not influenced in previous zero-dimensional kinetic calculation with CO2 dilution, while is suppressed in present two-dimensional CFD modeling. This because the zero-dimensional kinetic calculation assumes the reactions inside the reactor take place in an atmosphere of identical temperature and residence time. However, for two-dimensional reactions, temperature and residence time are varied in different


Energy & Fuels

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parts of the computational domain. As a result, the CH2O formation can be different as diluent changes. Nevertheless, the present pathway analysis in some degree succeeds in distinguishing the respect role of physical and chemical effects of CO2 on CH4 oxidation process, especially the CO formation characteristics.

5.

CONCLUSIONS

This paper numerically studies the jet flames in a hot low-oxygen coflow burning CH4/H2 blended fuel, to reveal the physical and chemical effects of CO2 on combustion temperature, minor and major species formations as well as CH4 oxidation process during the early ignition stage of MILD combustion. Furthermore, the respective roles of physical and chemical effects of CO2 are quantitatively distinguished at varied CO2 dilution degrees. The main findings can be concluded as follows.

1.

CO2 is superior to N2 in achieving MILD or flameless regime of the JHC flames

due to reduced temperature rise from both physical effects and chemical effects. With the increase of CO2 concentration, the contribution of the physical effects of CO2 on temperature reduction is becoming stronger, while that of the chemical effects is gradually weakened.

2.

By replacing N2 with CO2, OH and CH2O formations are both suppressed mainly

from the chemical effects of CO2, which then leads to the result of reduced flame luminosity and delayed ignition under oxy-MILD operation. On the other hand, the physical effects of CO2 is expected to generate more homogeneous flame appearance


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Energy & Fuels

due to more uniform OH distribution. In addition, for NO emission reduction, the chemical and physical effects of CO2 are responsible respectively in low and high CO2 conditions.

3.

When N2 is replaced by CO2, the physical effects of CO2 have a minor influence

on the CH4 oxidation route, while the chemical effects of CO2 not only inhibit the CH4 oxidation through Route I (CH4 → CH3 → CH2(S)/CH2 → (CH → CH2O →) HCO → CO → CO2) and II (CH4 → CH3 → CH2O → HCO → CO → CO2), but also promote CO2 dissociation to CO by R99 (OH + CO ↔ H + CO2) and R153 (CO2 + CH2(S) ↔ CO + CH2O). As a result of the competition, the CO formation is higher under oxy-MILD combustion than air-MILD combustion.

ACKNOWLEDGMENTS The authors acknowledge the support of the National Natural Science Foundation of China (Grant 51276074), State Key Development Program for Basic Research of China (Grant 2011CB707301), and Innovation Research Foundation of Huazhong University of Science and Technology (Grant 2014NY008).

REFERENCES 1.

Cavaliere, A.; de Joannon, M., Mild combustion. Progress in Energy and

Combustion science 2004, 30, (4), 329-366.

2.

Tsuji, H.; Gupta, A. K.; Hasegawa, T.; Katsuki, M.; Kishimoto, K.; Morita, M.,

High temperature air combustion: from energy conservation to pollution reduction.

CRC press: 2002.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.

Wünning, J.; Wünning, J., Flameless oxidation to reduce thermal NO-formation.

Progress in Energy and Combustion science 1997, 23, (1), 81-94.

4.

Arghode, V. K.; Gupta, A. K.; Bryden, K. M., High intensity colorless distributed

combustion for ultra low emissions and enhanced performance. Applied Energy 2012, 92, 822-830. 5.

Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D., Oxy-fuel

combustion of solid fuels. Progress in Energy and Combustion science 2010, 36, (5), 581-625. 6.

Chen, L.; Yong, S. Z.; Ghoniem, A. F., Oxy-fuel combustion of pulverized coal:

Characterization, fundamentals, stabilization and CFD modeling. Progress in Energy and Combustion science 2012, 38, (2), 156-214.

7.

Buhre, B.; Elliott, L.; Sheng, C.; Gupta, R.; Wall, T., Oxy-fuel combustion

technology for coal-fired power generation. Progress in Energy and Combustion science 2005, 31, (4), 283-307.

8.

Wikström, P.; Kalisz, S.; Brobäck, J.; Blasiak, W., Study on slab heating

processes in high temperature conditions-work in progress. IFRF Online Combustion Journal 2003, 1-5.

9.

Li, P.; Dally, B. B.; Mi, J.; Wang, F., MILD oxy-combustion of gaseous fuels in a

laboratory-scale furnace. Combustion and Flame 2013, 160, (5), 933-946. 10. Li, P.; Wang, F.; Tu, Y.; Mei, Z.; Zhang, J.; Zheng, Y.; Liu, H.; Liu, Z.; Mi, J.; Zheng, C., Moderate or intense low-oxygen dilution oxy-combustion characteristics of light oil and pulverized coal in a pilot-scale furnace. Energy & Fuels 2014, 28, (2),


Page 32 of 37

Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1524-1535. 11. Tu, Y.; Liu, H.; Li, P.; Wang, F.; Zhang, T.; Bai, C.; Zheng, Y.; Liu, Z.; Mi, J.; Zheng, C., Experimental study of MILD combustion for pulverized coal in 0.3 MW vertical furnace, In 9th Asia-Pacific Conference on Combustion, Gueongju, Korea,

2013. 12. Sabia, P.; Sorrentino, G.; Chinnici, A.; Cavaliere, A.; Ragucci, R., Dynamic Behaviors in Methane MILD and Oxy-Fuel Combustion. Chemical Effect of CO2. Energy & Fuels 2015, 29.

13. Zhang, J.; Mi, J.; Li, P.; Wang, F.; Dally, B. B., MILD Combustion of Methane Diluted by CO2 and N2. Energy Fuels 2015. 14. Lin, W.; Liu, Z.; Sheng, C.; Zheng, C.; Li, J., Physical and Chemical Effects of CO2 and H2O Additives on Counterflow Diffusion Flame Burning Methane. Energy Fuels 2013, 27, (12), 7602-7611.

15. Liu, Y.; Chen, S.; Yang, B.; Liu, K.; Zheng, C., First and second thermodynamic-law comparison of biogas MILD oxy-fuel combustion moderated by CO2 or H2O. Energy Conversion and Management 2015, 106, 625-634. 16. Mei, Z. F.; Mi, J. C.; Wang, F. F.; Zheng, C. G., Dimensions of CH4-Jet Flame in Hot O2/CO2 Coflow. Energy Fuels 2012, 26, (6), 3257-3266. 17. Seepana, S.; Jayanti, S., Flame structure investigations of oxy-fuel combustion. Fuel 2012, 93, 52-58.

18. Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; Kiga, M. K. T., Characteristics of pulverized coal-combustion in the system of oxygen/recycled flue


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

gas combusttion. Energy Conversion and Management 1997, 38, 129-134. 19. Scheffknecht, G.; Al-Makhadmeh, L.; Schnell, U.; Maier, J., Oxy-fuel coal combustion—A review of the current state-of-the-art. International Journal of Greenhouse Gas Control 2011, 5, S16-S35.

20. Liu, Y.; Chen, S.; Yang, B.; Liu, K.; Zheng, C., First and second thermodynamic-law comparison of biogas MILD oxy-fuel combustion moderated by CO2 or H2O. Energy Conversion and Management 2015, 106, 625-634. 21. Sabia, P.; Lavadera, M. L.; Giudicianni, P.; Sorrentino, G.; Ragucci, R.; de Joannon, M., CO2 and H2O effect on propane auto-ignition delay times under mild combustion operative conditions. Combustion and Flame 2015, 162, (3), 533-543. 22. Dally, B. B.; Karpetis, A. N.; Barlow, R. S., Structure of turbulent non-premixed jet flames in a diluted hot coflow. Proceedings of the Combustion Institute 2002, 29, (02), 1147-1154. 23. Christo, F. C.; Dally, B. B., Modeling turbulent reacting jets issuing into a hot and diluted coflow. Combustion & Flame 2005, 142, 117–129. 24. Dally, B. B.; Karpetis, A. N.; Barlow, R. S., The Effect of Oxygen Concentration on the Structure of Turbulent Nonpremixed Flames. Schools & Disciplines 2001. 25. Wang, F.; Mi, J.; Li, P.; Zheng, C., Diffusion flame of a CH4/H2 jet in hot low-oxygen coflow. International journal of hydrogen energy 2011, 36, (15), 9267-9277. 26. Mei, Z.; Wang, F.; Pengfei, L. I.; Jianchun, M. I., Diffusion Flame of a CH4/H2 Jet in a Hot Coflow: Effects of Coflow Oxygen and Temperature. Chinese Journal of


Page 34 of 37

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Chemical Engineering 2013, 21, (7), 787-799.

27. ANSYS, Inc., Fluent 6.3 Documentation. 2007. 28. Aminian, J.; Galletti, C.; Shahhosseini, S.; Tognotti, L., Key modeling issues in prediction of minor species in diluted-preheated combustion conditions. Applied Thermal Engineering 2011, 31, (16), 3287-3300.

29. Mardani, A.; Tabejamaat, S.; Ghamari, M., Numerical study of influence of molecular diffusion in the Mild combustion regime. Combustion Theory & Modelling

2010, 14, (5), 747-774. 30. Yin, C.; Johansen, L. C. R.; Rosendahl, L. A.; Kær, S. K., New Weighted Sum of Gray Gases Model Applicable to Computational Fluid Dynamics (CFD) Modeling of Oxy-Fuel Combustion: Derivation, Validation, and Implementation. Energy Fuels

2010, 24, (4), 6275-6282. 31. De, A.; Oldenhof, E.; Sathiah, P.; Roekaerts, D., Numerical simulation of delft-jet-in-hot-coflow (djhc) flames using the eddy dissipation concept model for turbulence–chemistry interaction. Flow, Turbulence and Combustion 2011, 87, (4), 537-567. 32. Parente, A.; Malik, M. R.; Contino, F.; Cuoci, A.; Dally, B. B., Extension of the Eddy Dissipation Concept for turbulence/chemistry interactions to MILD combustion. Fuel 2016, 163, 98-111.

33. Wang, F.; Li, P.; Mi, J.; Wang, J.; Xu, M., Chemical kinetic effect of hydrogen addition on ethylene jet flames in a hot and diluted coflow. International journal of hydrogen energy 2015, 40, (46), 16634-16648.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

34. Evans, M.; Medwell, P.; Tian, Z., Modeling Lifted Jet Flames in a Heated Coflow using an Optimized Eddy Dissipation Concept Model. Combustion Science and Technology 2015, 187, (7), 1093-1109.

35. Tu, Y.; Liu, H.; Chen, S.; Liu, Z.; Zhao, H.; Zheng, C., Effects of furnace chamber shape on the MILD combustion of natural gas. Applied Thermal Engineering 2015, 76, 64-75. 36. Medwell, P. R.; Kalt, P. A.; Dally, B. B., Simultaneous imaging of OH, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow. Combustion and Flame 2007, 148, (1), 48-61. 37. Abboud, J. E.; Jiang, N.; Zhang, Z.; Roy, S.; Gord, J. R., Spatial and temporal control of on-demand propane–air flame ignition by active photothermal effect of aluminum nanoenergetics. Combustion and Flame 2013, 160, (9), 1842-1847. 38. Glarborg, P.; Bentzen, L. L., Chemical effects of a high CO2 concentration in oxy-fuel combustion of methane. Energy & Fuels 2007, 22, (1), 291-296. 39. Zou, C.; Cao, S.; Song, Y.; He, Y.; Guo, F.; Zheng, C., Characteristics and mechanistic analysis of CO formation in MILD regime with simultaneously diluted and preheated oxidant and fuel. Fuel 2014, 130, 10-18. 40. Mardani, A.; Tabejamaat, S.; Hassanpour, S., Numerical study of CO and CO2 formation in CH4/H2 blended flame under MILD condition. Combustion and Flame

2013, 160, (9), 1636-1649. 41. Gao, X.; Duan, F.; Lim, S. C.; Yip, M. S., NOx formation in hydrogen–methane turbulent diffusion flame under the moderate or intense low-oxygen dilution


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Energy & Fuels

conditions. Energy 2013, 59, 559-569. 42. Mardani, A.; Tabejamaat, S., NOx Formation in H2-CH4 Blended Flame Under MILD Conditions. Combustion Science and Technology 2012, 184, (7-8), 995-1010.


H2 Flames on

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