Thermal Characteristics of a CH4 Jet Flame in Hot Oxidant Stream

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Combustion

Thermal Characteristics of a CH4 Jet Flame in Hot Oxidant Stream: Dilution Effects of CO2 and H2O Chong Dai, Bo Wang, Ziyun Shu, and Jianchun Mi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01460 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Thermal Characteristics of a CH4 Jet Flame in Hot Oxidant Stream: Dilution Effects of CO2 and H2O C. Dai, B. Wang, Z. Shu and J. Mi* Department of Energy & Resource Engineering, College of Engineering, Peking University, Beijing 100871, China *

Corresponding author: E-mail: [email protected]; Fax: +86-10-62767074

Abstract This study investigates the diluent-dependent diffusion flames of an axisymmetric methane jet in annular hot coflow (JHC) of oxidizer diluted by ‘inert’ N2, CO2 and H2O, respectively. The fuel jet issues at the exit Reynolds number ≈ 10,000 while the hot (1300K) coflow oxygen level (molar fraction) varies between 6% and 23%. To identify the chemical and physical factors, simulations of the diffusion flames diluted by fictitious gases XH2O and XCO2 are also conducted. Inspections and analyses to combustion radicals and heat-release-rates are made on the stoichiometric sheet. Results show that the stoichiometric length, temperature and volumetric heat-release-rate vary drastically with coflow oxygen level or dilution extent. The flame volume increases greatly when replacing the diluent N2 with CO2 but reduces substantially under the H2O dilution. Such discrepancies are found to stem mainly from specific physical properties of N2, CO2 and H2O. The CO2 and H2O dilutions show more chemical impact on the formations of key intermediate species. Besides, the dilution by CO2 weakens all the main reactions, thus producing the mildest flame, whereas that by H2O promotes the formations of radicals OH and H2, enhancing the heat-release-rate. At last, the global heat contributions of C-included and C-excluded (H2-O2) reactions are examined so as to understand the roles of the H2-O2 kinetics in the oxy-combustion.

Keywords Oxy-combustion; Fast reaction zone; MILD; Reaction rate; Heat-release-rate.

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1. Introduction The environmental deterioration and global warming due to consumption of fossil energy has remained grim to our human being. Consequently, good combustion technologies with high efficiency and low emissions of pollutants and CO2 are still in high demand. As one of the most practicable technologies to battle climate change, the CCUS (CO2 capture, utilization and sequestration) technology has attracted significant attention in recent years1. The oxy-fuel combustion technology, first proposed by Abraham et al.2, is considered as the key technology for CCUS to be commercialized, which can be applied to steel and coal-fired power plants1. Unlike the traditional combustion occurring with air (79% N2 and 21% O2 in volume), the mixture of O2 and CO2 is used as the oxidant to realize the oxy-fuel combustion, whose final flue gas consists of almost purely H2O and CO2. After removing the steam by condensation, the concentration of CO2 in the dry exhaust gas can exceed 90% (mass fraction). The oxy-fuel combustion was proposed for the ‘near zero-emission’ of combustion of fossil fuels but has faced some challenges. For instance, air-ingress causes high NOx emissions3 while delayed ignition and decreased burning rate lead to poor flame stability4. To resolve these problems, in recent years, the oxy-fuel combustion has applied in conjunction with MILD combustion5, another environmentally friendly technology. This hybrid combustion technology6 has been named “MILD oxy-fuel combustion” and widely utilized in metallurgical and steel industries7. More recently, another ‘zero-emission’ system named oxy-steam combustion8 is developed by CanmetENERGY9. This steam-moderated combustion is actually an altered form of the traditional oxy-fuel combustion and can avoid the recirculation of flue gases. Nevertheless, the oxy-fuel and oxy-steam systems are expected to produce very different flame characteristics. The reason is that their recycled exhaust gas contains different fractions of CO2 and H2O and so has distinct physical and chemical properties at elevated temperatures, which should affect the heat transfer and chemical reactions in the combustion furnace10. Indeed, the previous works8,11 have validated that some special thermodynamic properties of steam can enable the oxy-steam


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system to have numerous advantages over the oxy-fuel system, such as energy saving, higher heat radiation and compact configuration. The oxy-fuel and oxy-steam combustions are collectively termed as oxy-combustion hereafter. To distinguish the oxy-combustion and air-combustion, numerous fundamental studies have been conducted recently. The flame characteristics, such as ignition delay, peak temperature, propagating velocity, etc., have been found to change greatly under oxy-combustion conditions. These changes are related to thermal, diffusive, and chemical properties of H2O and CO2 in the oxidant. Andersson et al.12 performed an experimental study on the radiation properties of a propane flame under O2/CO2 atmosphere. Relative to the air combustion, similar temperature and intermediate species concentration levels but significantly higher intensity of flame radiation were found in the case of 27%O2 and 73%CO2. Hu et al.13 numerically analyzed the chemical, dilution, and thermal-diffusion effects of diluents (He, Ar, N2 and CO2) on the laminar flame velocity of the premixed methane/air/diluent mixtures. They found that CO2 suppresses the laminar flame velocity most significantly, where the dilution effect of CO2 takes a governing place. Further, Xie et al.14 revealed that the laminar flame velocity of the CH4 oxy-fuel flame decreases with increasing the CO2 fraction. This can be understood because CO2 directly enhances the reverse process of reaction OH + CO H + CO2 but inhibits the reaction H + O2 O + OH. Based on the non-isothermal thermogravimetric analysis, Zou et al.11 presented an experimental study on the flame properties of pulverized coal in O2/H2O mixtures. It was showed that the oxidant of O2/H2O delays the oxidation process of coal, compared with that of O2/N2. By simulating the JHC flames of CH4 moderated by N2 and CO2, Mei et al.15 found that the CO2-diluted flame corresponds to substantially lower temperature, larger reaction zone, and more CO compared to its N2 counterpart. Then, Tu et al.16 investigated how the CO2 addition affect the CH4/H2 jet diffusion flame under MILD condition and claimed that the increased ignition delay and CO emission are mainly due to the chemical activations of CO2. These authors also calculated the Damköhler number across the calculation domain and reported that the H2O addition weakens the reaction-controlled state when comparing to its CO2 counterpart17. In addition, Wang et al.18 conducted a numerical study to better understand the effects of H2O and CO2 dilutions on the CH4-O2/N2/H2O/CO2 opposed-flow flame. It revealed that both thermal and chemical properties of CO2 addition help to diminish the flame


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temperature whereas the H2O dilution has minor effect on the temperature. So far, numerous experimental and numerical investigations have been made into the flame characteristics using air or O2/CO2 as the oxidant. Unfortunately, the essential studies on the oxy-steam combustion are relatively sparse. Besides, most of the previous studies focused mainly, from a macro view, on the flame characteristics, such as flame speed, ignition delay time, and intermediate radical distributions. There is still insufficient research into kinetic effects of different diluents on the thermal characteristics of flame, a key problem to develop flame chemistry. In addition, the existing chemical mechanisms for air combustion were found unable to predict the oxy-combustion behaviors with enough accuracy19,20. Sabia et al.19 experimentally and numerically studied the auto-ignition process of propane/oxygen flames over a wide range of mixture compositions and temperatures under MILD conditions. Their experimental results showed that the dilutions of H2O and CO2 significantly slow the ignition process from that of N2. It is recommended that further studies are required to extend the applicability of the chemical schemes and improve the prediction accuracy of the models to non-standard conditions. Persis et al.20 measured laminar flame speeds of CH4/O2/N2/CO2 mixtures at various mole fractions of N2 and CO2 in the mixture. And they observed discrepancies between the experimental results and calculations obtained using the PREMIX code with the detailed mechanism, for the CO2 cases. As one of the most important global properties, heat release rate can be used to characterize the combustion chemistry. Compared to the analysis of laminar flame speed, it is more convenient to analyze the heat release details so as to identify the dominant reactions. This is because the releasing heat from flame can be regarded as the result of all the endothermic and exothermic elementary reactions21. Zhang et al.22 numerically investigated the effects of O2 enrichment on premixed methane/air flames, which indicates that the O2 enrichment can enhance the heat-release-rate. Using the opposed diffusion flame model with a detailed chemistry mechanism, Zou et al.23 studied the chemical effects of H2O addition on the methane oxy-steam flame temperature. They found that high steam concentration enhances the three-body reaction, increases the concentration of radical OH, but reduces the concentration of


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radical H, which differs from traditional combustion. Reactions supplying the maximum heat-release-rate vary with the steam concentration. Besides, measurements and simulations were conducted by Zhang et al.24 to analyze the effects of CO2 and N2 dilutions on methane/air flames. Based on the flame kinetic analyses, it was indicated that CO2 dilution shows substantial effects on the thermal characteristics but has weak effect on the heat contribution of dominant reaction. However, the above studies in terms of the kinetic effects of CO2 or H2O on combustion reactions paid little attention to the turbulent non-premixed flames in a real combustor. Zero-dimensional or one-dimensional reactors including WSR, counterflow diffusion flame and freely propagating premixed laminar flame are commonly employed. No systematic studies have been performed on the chemical and physical actions of H2O and CO2 additions on the thermal characteristics including heat contributions across the entire reaction zone. To address the above deficits, the aim of this numerical work is to examine how the H2O and CO2 dilutions affect the thermal kinetics of a methane jet flame in hot oxidizer coflow (JHC). The focus is on the combustion characteristics on the stoichiometric sheet at which the mixture fraction (ξ) is stoichiometric, i.e. ξ = ξst. A great number of RANS simulations are conducted on the JHC flames, several of which are under the same experimental conditions of Dally et al.25 to validate the modeling. The coflow temperature is 1300K, i.e., Tcof = 1300K. the coflow oxygen ranges between XO2 = 6% and XO2 = 23% in molar fraction for each diluent to obtain the three typical combustion modes, i.e., MILD combustion (XO2 = 6%), traditional combustion (XO2 = 21%), and oxy-enriched combustion (XO2 = 23%)22. The case of XO2 = 6% represents the MILD combustion because its temperature rise (∆T) caused by combustion is well below the auto-ignition temperature (Tai). According to Cavaliere and de Joannon5, such a JHC combustion may be defined as the MILD mode when it satisfies: Tcof > Tai and ∆T < Tai. More specifically, the present paper will present the variations in the stoichiometric sheet length, temperature, reaction rate and heat-release-rate versus XO2 and different diluents. It will also perform kinetic analyses on the reactive species formation and heat contribution ratios to identify the key radicals and the dominant reactions during


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the burning process of the oxy-fuel and oxy-steam methane flames.

Fig. 1 The combustor and grid meshes of present simulations for (a) experiment by Dally et al.25 and (b) the modified JHC configuration.

2. CFD simulation details 2.1 JHC configurations The physical model of the present CFD simulations, see Fig. 1(b), is modified from the experimental counterpart, Fig. 1(a), of Dally et al.25 The JHC device25 is composed of a 82 mm (inner diameter) insulated annulus nozzle and 4.25 mm (D, inner diameter) cooled central jet nozzle, which was installed at the center of a perforated plate. In the experiment25, the fuel consisting of equal CH4 and H2 in volume was discharged at the speed of about 69.5 m/s. The hot flue gas from the internal burner, was the mixture of nitrogen and air from the oxidant inlets that could be regulated and controlled for the coflow O2 concentration and temperature. Moreover, the co-axial surrounding air stream in the wind tunnel had the same velocity (≈ 3.2 m/s) as the hot coﬂow but with a room temperature (≈ 300K from Christo et al.26). Table 1 lists the experimental inlet conditions25. For all the cases, the


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fuel-jet Reynolds number Re ≡ Uf D/νf ≈ 9482, where νf is the kinematic viscosity of fuel. Table 1 Entrance conditions of the JHC flame25.

CH4/H2 jet

Coflow (mixture compositions are shown by mass fraction)

Tf (K)

Re

Tcof (K)

Ucof (m/s)

YO2 (%) YN2 (%) YCO2 (%)

YH2O (%)

305

9482

1300

3.2

3

85

5.5

6.5

305

9482

1300

3.2

6

82

5.5

6.5

305

9482

1300

3.2

9

79

5.5

6.5

2.2 Computational domain and models The commercial software code (Fluent 16.2) is used to deal with the related transport equations in terms of the mean mass conservation, radial and axial momenta, energy, radiative intensity, turbulence kinetic energy and its dissipation rate, as well as the local mass fraction for each species. To verify the Reynolds-Averaged Navier-Stokes (RANS) model, the experiment conditions in Table 1 are firstly simulated based on the computing domain in Fig. 1(a), whose results are reported in Section 3. However, the entrainment of the central jet to the cold tunnel air was demonstrated to strongly affect the JHC flame downstream from the axial position of x ≈ 100 mm, where the flame has become a traditional visible flame, especially for the case of YO2 = 3%15,27. To overcome the problem, the current study utilizes a modified JHC configuration where the outer cold stream is replaced and the central jet flame is enclosed only by hot coflow25. This modification, see Fig. 1(b), ensures that the coflow O2 levels (YO2) and temperature (Tcof) in the downstream calculation domain remain constant and is thus able to accurately predict the effects of oxygen and dilution levels of the hot coflow on the flame characteristics. According to Galletti et al.28, a simplified two-dimensional (2D) axisymmetric computational domain (4500 mm × 800 mm) is constructed, due to the symmetry of the JHC burner. About 50000 fully structured grids are used to divide the computational domain after the grid-independency check with a finer mesh of about 100000 cells. The grid density near the fuel and coflow inlets is enhanced to be 0.30


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mm × 0.35 mm, to improve the prediction accuracy. To identify the chemical and physical effects of the dilutions on the methane flame characteristics, fictitious gases XH2O and XCO2 with the same physical properties as for H2O and CO2 are utilized. Since the fictitious materials are totally nonreactive, the differences of flame characteristics between the N2-case and X-cases can indicate the physical effect caused by the dilutions of H2O and CO2. Similarly, the variances between the fictitious and real cases can further reflect chemical actions due to the dilutions of H2O and CO2. For the designated objectives, calculations of the 16 cases in Table 2 are executed with varying fractions of oxygen and diluent in the coflow, whose temperature and velocity are still 1300K and 3.2m/s, respectively. Besides, the fuel-jet Reynolds number and temperature are also taken at 9482 and 305K for all the simulations. Table 2 Operating conditions for the present study.

Fuel jet (CH4) Case

Hot coflow (mole fraction)

Tf (K)

Uf (m/s)

Tcof (K)

Ucof (m/s)

O2 (%)

N2 (%)

CO2 (%)

H2O (%)

XCO2 (%)

XH2O (%)

1

305

40.7

1300

3.2

6

94

0

0

0

0

2

305

40.7

1300

3.2

15

85

0

0

0

0

3

305

40.7

1300

3.2

21

79

0

0

0

0

4

305

40.7

1300

3.2

23

77

0

0

0

0

5

305

40.7

1300

3.2

6

0

94

0

0

0

6

305

40.7

1300

3.2

15

0

85

0

0

0

7

305

40.7

1300

3.2

21

0

79

0

0

0

8

305

40.7

1300

3.2

23

0

77

0

0

0

9

305

40.7

1300

3.2

6

0

0

94

0

0

10

305

40.7

1300

3.2

15

0

0

85

0

0

11

305

40.7

1300

3.2

21

0

0

79

0

0

12

305

40.7

1300

3.2

23

0

0

77

0

0

13

305

40.7

1300

3.2

6

0

0

0

94

0

14

305

40.7

1300

3.2

21

0

0

0

79

0

15

305

40.7

1300

3.2

6

0

0

0

0

94

16

305

40.7

1300

3.2

21

0

0

0

0

79

Like the previous studies15,26,29,30, a modified k-ε model with the standard wall


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function is adopted to model the turbulent flow, with the coefficient Cε1 in the eddy dissipation equation adjusted to 1.6. The previous works26,31 indicated that the turbulent kinetic energy at the inlet boundaries impacts significantly on the accuracy of the numerical results. Therefore, to achieve a good-enough match between the predicted and measured results, the mean turbulent kinetic energy (k) is set to be 1.8 m2/s2 and 40 m2/s2 at the hot coflow and fuel inlets27, respectively. The side-boundary and downstream conditions of the numerical simulation domain are, respectively, set as zero-shear stress and the pressure outlet32. The present work applies the EDC model coupled with DRM-22, i.e., the reduced mechanism of GRI-Mech 1.233, to calculate the interactions of the turbulence and chemistry. In the EDC model, reactions are presumed to occur in the small constant pressure reactors, whose time scale constant Cτ and volume fraction constant Cξ are both set to default34,35. The DRM-22 schemes comprise 22 species (in addition to N2 and Ar) and 104 reversible reactions and have been proved to perform well in predicting the oxidation kinetics, laminar flame speed and ignition time delay of methane flame by the previous studies33,36-38. Apart from the NOx emission, the simulated results of the intermediate radicals and temperature profiles with DRM-22 schemes are certified27 as almost the same accuracy as those with GRI-Mech 3.039 and GRI-Mech 2.1140 mechanisms. The characteristics of NOx emission are not investigated presently. Therefore, the DRM-22 mechanism, excluding the reactions involving N, is adopted to reduce the computing time. Besides, the in situ adaptive tabulation (ISAT) method proposed by Pope41 is used to upgrade the efficiency of integral calculation. In order to ensure the accuracy of the predicted results, the error tolerance of ISAT is set to 10-4. Densities of all species involved are calculated according to the incompressible ideal gas law. The molecular diffusion coefficient and specific heat (Cp) of each species are both set as polynomials of temperature, to better simulate the temperature and concentrations of CO2, CO and H2O15,26,34,42. The discrete ordinate (DO) radiation model is applied to solve the radiation of the flames17,42,43. Different from the weighted sum of gray gas model (WSGGM) of Smith


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et al.44, a refined WSGGM45 is implemented via a user-defined function, to correctly predict the emission and absorption coefficients of oxy-combustion flames with much higher concentrations of CO2 and H2O, which can lead to much stronger radiative heat transfer. The DO model solves the radiative transfer equation for 72 discrete solid angles in total across the calculation domain, considering 3×3 solid angles that are divided for each octant of the space angular. Moreover, the second-order upwind scheme is adopted to discretize the transport equations in order to achieve more accurate calculations. The pressure-velocity coupling is solved using the SIMPLE algorithm. The two critical conditions of solution convergence are: (a) the residuals of the radiation intensity and energy must be less than 10-6 and those of all the other variables must be less than 10-5, and (b) the residuals of the average area-weighted total pressure and outlet temperature must be less than 0.001 Pa and 1.0K, respectively.

3. Validation of the modeling


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Fig. 2 Comparisons between the predictions and measurements25 of the mean temperature and CO, OH and O2 mass fractions obtained at x = 30 mm for YO2 =3%, 6%, and 9%.

Fig. 3 Predicted and measured25,26 mixture fractions along (a) radial directions at x = 30, 60 and 120 mm and (b) axial directions, for YO2 = 9%. For the modeling validation by experiment, a number of simulations are firstly conducted under the same experimental conditions of Dally et al. 25 As shown in Table 1, the average YO2 in the coflow are about 3%, 6% and 9%. The measured radical profiles25 of the oxygen mass fraction at x = 4 mm from the jet exit are imported as the inlet coflow conditions of calculation via user-defined functions. Figs. 2(a-d) compare the measured and predicted mean temperature (T) and OH, CO and O2 mass fractions (i.e., YOH, YCO and YO2) obtained at x = 30 mm. Although not perfect, the predicted radial profiles overall agree quite well with the experimental data25. Particularly, the predictions of YOH and YO2 are very satisfactory over the whole domain. On the contrary, the obvious differences in T and YCO between the measured and predicted results occur for all the three cases at r/D = 6 ~ 11 and r/D = 4 ~12, respectively. Over that interval of r/D, YCO should be close to 0, yet the measurement is not so. As Fig. 1(a) illustrates, an internal burner was applied to produce the flue gas that wase mixed with oxidants via two side inlets to moderate the temperature and O2 levels of the coflow25. Due to the cooling and extinguishing effects of the internal and primary burner outer walls on the secondary flame, the CO gas initially existed in the coflow26. However, the present simulations do not use any


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carbon monoxide in coflow and so YCO = 0 at r/D = 4 ~12. Such amount of YCO ≈ 0.0025 is believed to have little effects on the combustion zone26. Besides, our careful inspection has also found that the cooling and extinction effects caused a small temperature drop in the vicinity of the outer wall. This is considered to be responsible for the discrepancies of T at r/D = 6 ~ 11; the present simulations set the fixed temperatures of 305K, 1300K, and 300K, respectively, for the fuel jet, coﬂow, and shroud air. Moreover, Fig. 3 shows the radial and axial distributions of mean mixture fraction (ξ)46 at different locations for YO2 = 9%. It is demonstrated that the simulation has a good prediction ability for the mixture fraction. Hence, the present CFD methods can model the investigated JHC flames with good accuracy.

4. Results and discussion 4.1 Stoichiometric sheet For the present study, the stoichiometric mixture fraction (ξst) is used to describe the “stoichiometric sheet” of JHC flames. Referring to Bilger et al.46, the mixture fraction is defined by ξ ≡ (β－βo) / (βf－βo), with β ≡ 2YC/MC + 0.5YH/MH － YO/MO, where Yi and Mi are the mass fractions and molecular weights of the elements: carbon (i = C), hydrogen (H) and oxygen (O). The subscripts f and o refer to the fuel and oxidant streams. The stoichiometric mixture fractions for the cases 3, 7 and 11 of N2, CO2 and H2O dilutions in Table 2 are 0.0550, 0.0389 and 0.0743, respectively. Our study will focus on the stoichiometric sheet where ξ = ξst. First, the length (Lst) and width (Wst) of the reaction zone bounded by the stoichiometric sheet are determined by the locations where ξ (r = 0, x = Lst) = ξst, and ξ (rmax = Wst/2) = ξst. Then, to understand the JHC flames under different diluents, we will examine the ensemble-averaged variable taken on the stoichiometric sheet as


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=

(1)

∑

where Zk is the predicted value of the variable obtained at the kth node and nj is the total node number for case j in Table 2.

Fig. 4 Contours of (a, b) the normalized temperature (θ) and (c, d) volumetric heat-release-rate (Γ) for the N2, CO2 and H2O-diluted JHC flames at XO2 = 21%. The axial length and radial width of the reaction zone bounded by the stoichiometric sheet are normalized by Lst, N2 and Wst, N2, respectively. Fig. 4 illustrates the contours of the normalized temperature (θ) and volumetric heat-release-rate (Γ) for the N2, CO2 and H2O-diluted JHC flames at XO2 = 21%. Here, θ = (T - Tf) / (Tmax, N2 - Tf), and Γ = (Q - Qmin, N2) / (Qmax, N2 - Qmin, N2), where Q is the volumetric heat-release-rate. The subscripts ‘max, N2’ and ‘min, N2’ respectively represent the maximum and minimum for the N2-diluted case at XO2 = 21%. Figs. 4(a-d) intuitively shows that the high temperature and volumetric heat-release-rate

zones

distribute

just

near

the

iso-surface

of

ξst,

i.e.

stoichiometric-flame sheet. It is also noted that the sheet becomes larger when the diluent N2 is changed to CO2 but become smaller as N2 is replaced by H2O. From our


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previous studies15,34, these variations result mainly from physical factors, see Fig. 5. That is, compared to the N2-moderated coflow, a lower density (ρ) of H2O plays a dominant role in entraining more oxygen so as to burn the fuel more rapidly and consequently to decrease the combustion zone, whereas higher density and thermal capacity (ρCp) of CO2 lead to a lower burning rate and a longer reaction zone.

1.0

(a)

H2O

1.0

(b) -4

6.0x10

CO2 N2 O2

0.6 0.4

0.8

ρ Cp (kj/m3K)

ρ (kg/m3)

0.8

0.2

-4

4.0x10

0.6 0.4

-4

2.0x10

υ (m2/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

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0.2

0.0

0.0 500

1000

1500

2000

500

1000

1500

0.0 2000

T (K)

T (K)

Fig. 5 Physical properties of (a) fluid density (ρ), and (b) thermal capacity (ρCp) and kinematic viscosity (ν) of gases H2O, CO2, N2 and O2 versus temperature (500K ~ 2000K). To quantify the effects of CO2 and H2O on the stoichiometric sheet, Fig. 6(a) shows the stoichiometric JHC flame lengths versus the coflow oxygen fraction XO2 under N2, CO2 and H2O dilutions. It is obvious that, in general, the CO2-diluted flame has the longest reaction zone while Lst is shortest for the H2O case. Besides, as XO2 reduces from 23% to 6%, the reaction zone grows dramatically in size; particularly, Lst increases by 236%, 256% and 208%, respectively, in the O2/N2, O2/CO2 and O2/H2O coflows.


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Fig. 6 Stoichiometric flame sheet length (Lst) varying with coflow oxygen fraction (XO2). (a) Lst ~ XO2; (b) Lst/D ~ XO2-1.3. Further, Fig. 6(b) unambiguously demonstrates that the normalized stoichiometric flame length (Lst/D) is correlated linearly with XO2-1.3, i.e., Lst/D ∝ XO2-1.3, for all the three diluents. Interestingly, this relationship might be associated with the global reaction: CH4 + 2O2 => CO2 + 2H2O, from which the fuel consumption rate can be expressed as47 RCH4 ≡ d [CH4]/dt = －kG[CH4]-0.3[O2]1.3 where kG is a temperature-dependent prefactor.

4.2 Ensemble-averaged temperature and heat release over the stoichiometric flame sheet As shown in Figs. 4(a-d), the peak temperature and volumetric heat-release-rate occur on the stoichiometric flame sheet, which can represent the major characters of flames.

Fig. 7 Profiles of (a) , (b) and (c) / versus the oxygen mole fraction XO2 in the coflow. Figs. 7(a-c) show the ensemble-averaged temperature , volumetric heat-release-rate , and the ratio of unit heat storage to versus coflow oxygen fraction, respectively; note that the ensemble-averages are taken on the stoichiometric flame sheet, see Eq. (1). Evidently, is highest in the N2-diluted case and lowest in the CO2 case, consistent with their flame luminance illustrated in Fig. 4. Besides, as XO2 increases from 6% to 23%, grows by about 632K, 521K and 619K, respectively, under the N2, CO2 and H2O dilutions. This is mainly


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attributed to their different thermal capacities, see Fig. 5(b). Significantly, the maximum temperature rises (∆Tmax) for XO2 = 6% under the N2, CO2 and H2O dilutions are approximately 453K, 251K and 332K, respectively, which are all well less than the corresponding auto-ignition temperatures (Tai) of methane (≈ 1035K, 1100K and 1075K, according to the method of Wang et al.43). Namely, the conditions of Tc > Tai and ∆Tmax < Tai for the MILD combustion5 are satisfied in these low oxygen cases. However, for XO2 = 15% ~ 23%, ∆Tmax is greater than Tai. In other words, the MILD combustion only takes place in the cases of XO2 = 6% but not in those of XO2 = 15% ~ 23%. It is well known that a higher rapid heat-release-rate always corresponds to a faster burning speed. Fig. 7(b) indicates that the coflow oxygen growth enhances combustion reactions. When XO2 increases from 6% to 23%, the average stoichiometric heat-release-rate under the N2, CO2 and H2O dilutions increase by ≈ 9 times, 25 times and 21 times, respectively. Fig. 7(b) also compares for the different diluents and finds that the order of magnitude of is: N2 > H2O > CO2 under the MILD regime at XO2 = 6%, but H2O > N2 > CO2 for XO2 = 15% ~ 23%, or even perhaps for XO2 > 8% as deduced from the plot. That is, out of the MILD regime, is largest for the O2/H2O coflow and smallest for the O2/CO2 case. Moreover, Fig. 7(c) indirectly shows that, as XO2 increases, the characteristic time of the chemical reactions occurring on the stoichiometric sheet decreases. Note that the ratio / roughly represent the time to reach the final state heated by combustion; so, an increase in reflects a reduction of the reaction time. Fig. 7(c) demonstrates that chemical reactions occur most slowly in the CO2-diluted JHC flame and most rapidly in the N2-diluted case at XO2 = 6%. The reasons for the distinct heat-release-rates of the MILD and traditional flames will be discussed in section 4.4.

4.3 Ensemble-averaged concentrations of main intermediate species over the stoichiometric flame sheet The intermediate species OH, H, H2 and CO have been confirmed to be very important for the hydrocarbon oxidation systems in fuel consumption and heat release.


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Fig. 8 compares the average concentrations of O2, OH, CO, O, H and H2 on the stoichiometric-flame sheet versus XO2 (= 6% ~ 23%) of the coflow diluted by N2, H2O and CO2. Evidently, all the average concentrations increase with rising XO2. This agrees with the tendency of in Fig. 7(a). Changing the diluent is found to have significant influences on these species concentrations as well. For XO2 = 6%~23%, the replacement of N2 by CO2 can promote the CO formation16 but inhibit the productions of OH, O, H and H2 and slows down the consumption of O2, which is due to the lower entrained oxygen fraction48 and reduced temperature by the O2/CO2 coflow. However, when N2 is replaced by H2O, the formations of CO, O, H and the O2 consumption are all suppressed34. In addition, it is worth noting that the sequencings of both and are N2 > H2O in the MILD regime, but H2O > N2 for XO2 = 15% ~ 23%. The result for XO2 = 15% ~ 23% can be explained here. As the diluent N2 is outplaced by H2O, the reverse reaction of R51 (2OH O + H2O) is enhanced and thus produces higher . Further, the reaction R2 (O + H2 H + OH) is inhibited effectively by the higher OH concentration and thus produces higher . Next, it will be revealed in section 4.4 that steam can promote the elementary reaction R49 (OH + H2 H + H2O) and thus accelerate the heat-release-rate significantly because of the higher and for XO2 = 15% ~ 23% in the O2/H2O coflow. This is also the reason for the profiles of and / between the air combustion and oxy-steam combustion mentioned in Figs. 7(b, c).


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Fig. 8 Comparisons of the average concentrations of O2, OH, CO, O, H and H2 along the ξst-curve versus the coflow XO2 (= 6% ~ 23%) diluted by N2, H2O and CO2.

4.4 Ensemble-averaged reaction rates and heat-release-rates of top 10 active elementary reactions over the stoichiometric flame sheet Heat-release-rate is the fundamental factor causing the variations in flame temperature and temperature gradient. Fig. 9 and Fig. 10, respectively, list the top 10 active reaction rates and top 10 averaged volumetric heat-release-rates, estimated based on elementary reaction analyses, along the stoichiometric sheet. These two diagrams show the effects of varying the coflow diluent and oxygen level. Here, the heat-release-rate of the kth reaction, qk, is determined as qk = ∑ , , ,

(2)

where h0j,k and MWj,k are the formation enthalpy and molar weight of the jth species in the kth reaction respectively; Rj,k is the corresponding kinetic reaction rate. Table 3 summarizes the top 15 active elementary reactions involved in Fig. 9 and Fig. 10.


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Fig. 9 Top 10 averaged kinetic reaction rates along the stoichiometric sheet: effects of varying the diluent and the coflow oxygen level. The rate of R49 at XO2 = 6% in the O2/N2 coflow is used as the reference for normalization.


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Fig. 10 Top 10 averaged volumetric heat-release-rates along the stoichiometric sheet: effects of varying the diluent and the coflow oxygen level. The heat-release-rate of R49 for XO2 = 6% in the O2/N2 coflow is used as the reference for normalization. One can find that the oxygen enrichment and varying diluent can significantly influence all reactions. The averaged progress rates according to Eq. (1) are increased exponentially with XO2. As shown in Fig. 9, the maximum reaction rates for N2, CO2 and H2O cases increase by ≈ 83 times, 23 times and 163 times, respectively, when XO2 grows from 6% to 23%. Consistently, the highest average volumetric heat-release of elementary reaction also grows with the same order of the averaged reaction rates, see Fig. 10. Reactions involving OH, H2, CO, H and O radicals are found to be important in heat production. Comparison of Figs. 9 and 10 indicates that the heat release profile may be regarded as a scale index of reaction rate to some extent, because the fastest reaction in each case also produce the largest heat release.


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Table 3 Top 15 active elementary reactions in the JHC flames. No.

Reactions

No.

Reactions

No.

Reactions

R2

O+H2H+OH

R24

H+O2O+OH

R58

OH+CH4CH3+H2O

R6

O+CH3H+CH2O

R35

H+CH4CH3+H2

R59

OH+COH+CO2

R7

O+CH4OH+CH3

R49

OH+H2H+H2O

R61

OH+CH2OCH3+H2O

R12

O+C2H2CH2(s)+CO

R51

2OH O+H2O

R64

OH+C2H4C2H3+H2O

R13

O+C2H2CH2+CO

R57

OH+CH3CH2(s)+H2O

R65

OH+C2H6C2H5+H2O

Our previous study34 has suggested that the governing reactions of CH4 oxidation may be changed by adding different diluents into the coflow. Besides, the concentrations of intermediate species have significant influences on the related chemical reactions and their heat release. According to the order of the reaction rates in Fig. 9, it is clearly demonstrated that R58 (OH + CH4 CH3 + H2O), R35 (H + CH4 CH3 + H2) and R7 (O + CH4 CH3 + OH) are the most significant chain-initializing reactions for the CH4 oxidation in the stoichiometric flame-sheet. Specifically, R58 plays the dominant role in the CO2 and H2O diluted CH4-flames, whose OH concentration is higher than those of O and H, see Figs. 8(b, d, e). For the N2-diluted flames, R35 takes place of R58 to be the most significant initiation reaction when XO2 exceeds 15%. This is because the average temperature along the stoichiometric sheet increases with the oxygen addition, thus the larger prefactor and temperature exponents of R35 overcome its high activation energy in the Arrhenius expression and finally gain higher kinetic reaction constants. As seen in Figs. 9 and 10, R49 (OH + H2 H + H2O) in the N2 diluted flames not only has the largest reaction rate but also provides the highest heat-release-rate, whereas R58 dominates the CO2 diluted flames against various oxygen fractions. Moreover, the oxy-steam combustion is mainly controlled by R58 & R49 in the MILD regime but dominated only by R49 when XO2 > 15%, which is different from the N2 and CO2 dilution cases. As revealed in Figs. 10(c1-c4), the heat-release-rate of R49 is higher in the H2O diluted cases than the N2 cases, except from XO2 = 6%, which is in accordance with the profiles of and in Fig. 8. In addition, the CO2 diluted flames exhibit much slower heat-release-rates than those of the N2 and H2O diluted flames, which leads to a greater temperature reduction. As confirmed in Figs. 13 and 14, this


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reduction in the heat release and temperature rise is mainly attributed to the chemical activations of CO2. The major reaction for the oxygen consumption R24 (H + O2 O + OH) is demonstrated to be the most important endothermic reaction for all the cases.

4.5 Contributions of top 10 active chemical reactions to the total heat release The heat-release-rate of each reaction is computed to gain their relative contributions to the total heat release from the flame. Integration of Eq. (2) through the computational zone yields Qk = ∫V qk dV

(3)

Q = ∑ Qk

(4)

where Qk represents the heat from reaction k and Q is the sum of the accumulated heat released from 104 endothermic and exothermic reactions of the DRM-22 kinetic schemes; V is the corresponding reaction volume. Therefore, the relative heat contribution of the kth reaction to the total heat, ηk, can be estimated as ηk = Qk/Q × 100%

(5)

That is, every reaction shows its contribution to the total heat release, no matter how much heat it releases or absorbs. In the heat computation, endothermic reaction makes a minus heat contribution. Fig. 11 compares the reactions of relatively large contribution ratios for different oxygen fractions and diluents. Besides, for these important elementary reactions responsible for heat production, the mean kinetic reaction rate is solved by = ∫V Rk dV/V

(6)

where Rk is the local reaction rate. For the N2-diluted cases, it is shown in Fig. 11 that the reaction R49 contributes the most to the total heat production whereas R24 is the most endothermic reaction. In the MILD regime, about 53% and -12% heat is released from reactions R49 and R24 respectively, while the total contribution ratios significantly increase as the oxygen fraction rises, i.e. about 98% and -23% for XO2 = 23%. Influence of XO2 on the volume-averaged reaction rates are roughly the same as above. Reactions R12, R13, ACS Paragon Plus Environment

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R58 and R59 can be regarded as the secondary reactions that contribute to the total heat release of flame. They jointly provide about 52% contribution to the total heat at XO2 = 6%. However, their contribution ratios decrease as XO2 increases. When 23% oxygen is added in the coflow, the total contribution ratios of reactions R12, R13, R58 and R59 are less than 28%, see Fig. 11(d).

Fig. 11 Variations of the top 10 active reactions and their relative contribution (ηk) to the total heat release of the N2, CO2 and H2O-diluted JHC flames for XO2 = 6% ~ 23%. The rate of R49 under O2/N2 with XO2 = 6% was used as the reference for normalization. Compared to the N2-diluted flames, combustion under the CO2 diluents behaves


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much milder. In the MILD regime, Fig. 11(a) illustrates that the initiation reaction R58, in terms of the OH attack on CH4, makes the largest heat contribution 42% whereas reactions R6, R12, R13, R49, R59 and R61 jointly provide 47% of the total heat. That is, the heat from chemical energy is released by more elementary reactions under the O2/CO2 circumstances, which is different from that of O2/N2 cases. Reaction R24 is still the most significant endothermic reaction, taking a -7% heat contribution. As temperature rises with XO2 increasing, rates of the exothermic reactions R49 and R59 gradually exceed R58. Then at XO2 = 23%, as Fig. 11(d) shows, they become the first and second important roles in the heat release, i.e. contributing 45% and 23%, respectively, larger than the 18% contribution of R58. Nevertheless, R58 keeps the most significant reaction in heat release along the ξst-curve, which indicates that there are discrepancies in the predicted characteristics between the local and global kinetic analysis. Besides, for all the oxygen concentrations, the replacement of N2 by CO2 weakens reactions R49 and R24 but strengthens the importance of reactions R6, R12, R13, R58, R59 and R61, which produces relatively more homogeneous flames. However, the global heat released from the oxy-steam flames is mainly controlled by R49 and R58 in the MILD regime but only by R49 when XO2 > 15%, which follows the judgement about the heat-release-rate along the ξst-curve. When XO2 = 6%, R49 makes a little greater heat contribution of 52% than R58, which occupies 40% contribution. Moreover, the most important global endothermic reaction in this regime is R51 (-6%) involving two OH radicals, rather than R24 (-3%) that dominates the heat release in the stoichiometric flame-sheet, as indicated in Fig. 10(b1). Interestingly, the exothermic reaction R49 in the O2/H2O flames is slightly diminished compared with the N2 cases but is greatly enhanced compared to the oxy-fuel flames. On the contrary, R58 is greatly promoted with respect to the N2 cases but is slightly weakened compared to the oxy-fuel flames. Besides, R59 suffers a notable dent when changing the CO2 diluents to steam. Compared with the N2 and CO2 flames, the heat of the oxy-steam flames seems to be released more centrally, mainly from reaction R49. Note that R49 supplies 90% ~ 98% heat contribution as XO2 varies from 15% to


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23%, indicating that the H2O addition can enhance the OH and H2 formations and promote the heat-release-rate globally. In addition, for the endothermic reactions, the steam addition can inhibit R24 but enhance R51, compared to the other dilutions.

4.6 Global heat contributions of C-excluded and C-included reactions The hydrogen-oxygen (H2-O2) system is important as a subsystem in the oxidation of hydrocarbons49. The present study does not intend to penetrate into the H2-O2 kinetics, but will reveal the influences of various oxygen concentrations and diluents on the global heat contributions made by the H2-O2 reactions in DRM-22. It is known that the H2-O2 reactions generally do not contain carbons. Thus, we classify the 22 reactions without carbons in DRM-22 schemes as C-excluded reactions, as Table 4 displays. Consequently, the remaining 82 reactions with carbons are grouped to be C-included reactions. Table 4 List of C-excluded reactions (22 steps) in DRM-22. R1

O+H+MOH+M

R2

O+H2H+OH

R3

O+HO2OH+O2

R19

H+O2+MHO2+M

R20

H+2O2HO2+O2

R21

H+O2+H2OHO2+H2O

R22

H+O2+N2HO2+N2

R23

H+O2+ARHO2+AR

R24

H+O2O+OH

R25

2H+MH2+M

R26

2H+H22H2

R27

2H+H2OH2+H2O

R29

H+OH+MH2O+M

R30

H+HO2O2+H2

R31

H+HO22OH

R49

OH+H2H+H2O

R50

2OH(+M)H2O2(+M)

R51

2OHO+H2O

R52

OH+HO2O2+H2O

R53

OH+H2O2HO2+H2O

R66

2HO2O2+H2O2

R67

2HO2O2+H2O2

Fig. 12 Relative contribution ratios of C-include reactions and C-excluded reactions to the total heat release of JHC flames for XO2 = 6% ~ 23% under different diluents.


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Fig. 12 illustrates the relative heat contribution profiles of C-included and C-excluded reactions versus oxygen fractions under different dilutions. All the investigated cases have the same combustion heat, due to their same fuels input. It clearly shows that oxygen addition is beneficial to the heat release of C-excluded reactions, like R49, most of whose pre-exponential factors are higher than the C-included reactions, such as R58 and R59. Compared to the N2 cases, heat released from the C-excluded reactions is inhibited by the CO2 substitution but is promoted by the steam replacement, which agrees well with the above predictions on the sequence of reactions in heat release. That is, the H2-O2 kinetics play a greater role in the oxy-steam combustion whereas the reactions in terms of carbons are more prominent in the oxy-fuel combustion. Moreover, it also confirms again that the steam addition can promote the formation of OH and H2 globally, since R49 is the dominant C-excluded reaction involving these two radicals.

5. Further discussion: physical and chemical effects of CO2 & H2O Results discussed above enlighten us that the influences of various diluents on the thermal characteristics of MILD and conventional JHC flames may be distinct. To quantitatively distinguish the chemical and physical effects of CO2 and H2O additions on the JHC flames, the analysis for the principle thermal characteristics along the ξst-curve and the global kinetics and heat release of the flames at XO2 = 6% and 21% were conducted. Figs. 13 and 14 quantify the physical and chemical actions of CO2 and H2O versus N2 dilution on , Lst/D, , , , and along the flame-sheet, respectively. As a supplement, Fig. 15 presents the difference in the volume-weighted kinetic rates and the heat release contributions of the several important elementary reactions between the CO2 and H2O and their corresponding X additions.


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Fig. 13 Percentages of chemical and physical effects of CO2 and H2O diluents on , Lst/D, , , , , and for XO2 = 6%. Fig. 13(a) shows that the variations of the temperature and length of the reaction zone result mainly from the physical effect while the chemical action is relatively little. The chemical action is more noteworthy in the presence of volumetric heat-release-rate and active radicals other than OH, especially on promoting the formation of CO and H radicals; as the diluent CO2 replaces N2, physically drops 2% but chemically grows 151%, while physically grows 31% but chemically drops 115%. This is mainly because the higher CO2 concentration inhibits the globally forward reaction of R59 (OH + CO H + CO2) (see Fig. 15(a)) and enhance R88 (CH2(s) + CO2 CO + CH2O), which jointly produces more CO but less H radicals. Consequently, the forward reactions of R24 (H + O2 O + OH)


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and R35 (H + CH4 CH3 + H2) slow down and thus produce lower O, OH and H2 concentrations. As a result, the major exothermic reactions R58 and R49 shown in Fig. 9(b1) are inhibited by the CO2 diluent, which causes a 51% chemical reduction for . However, in the MILD oxy-steam flames, the chemical action of H2O plays a comparable role as the physical effect in diminishing the flame-sheet temperature and forming OH radical at the opposite direction18. Similar to the oxy-fuel cases, the physical effect is more important on the fast reaction zone length but is relatively slight on the volumetric heat-release-rate and other active radicals. In specific, as N2 is outplaced by H2O, and grow 149% and 86% physically but drop 235% and 181% chemically, respectively. This is primarily owing to the higher H2O concentration and lower flame temperature, which inhibits the forward process of R49 (OH + H2 H + H2O), thus producing less H radicals. Meanwhile, the forward reactions of R24 (H + O2 O + OH), R12 and R13 also slow down and hence produce lower OH, O and CO concentrations. Although the oxy-steam achieves an overall growth of 3% compared to the N2 cases, the forward reaction of R49 is still greatly weakened due to the large consumption of OH radicals by R58. Consequently, 40% lower volumetric heat-release-rate is produced with the chemical drop of 104% for the oxy-steam flames. In addition, the 64% physical growth of in Fig. 13(b) is mainly attributed to the stronger entrainment of steam, thus sucking more oxygen and producing a faster heat-release-rate with higher O, H, OH and H2 concentrations in the flames. As the coflow oxygen increased from 6% to 21%, it is instantly observed from Figs. 13(a) and 14(a) that the chemical and physical factors in Lst/D, and for the oxy-fuel combustion experience great changes. Similar changes also occur in the oxy-steam flames, comparing the results of , and from Figs. 13(b) and 14(b). As Fig. 14(a) shows, the physical effect occupies a more significant position in inhibiting the OH and O formation, while the chemical action plays a comparable role under O2/CO2 atmosphere. Likewise, in Fig. 14(b), a 42% overall growth of is produced from the steam flames, where the physical effect


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of H2O is relatively more significantly. However, the chemical effect of H2O becomes the leading role in the formations of OH and H2. Besides, it is worth noting that, the formation of OH not only comes from CH4 decomposition but also from the exothermic dissociation of H2O when diluted with H2O, for instance, the revers reaction of R51.

Fig. 14 Percentages of chemical and physical effects of CO2 and H2O diluents on , Lst/D, , , , , and for XO2 = 21%. Comparing the global heat contributions of elementary reactions in Figs. 15(a) and 15(c), one can find that the chemical properties of CO2 play dominant roles in weakening R49 and R24 heat contributions but promoting the contribution ratios of R58 and R59. In the MILD regime, the reduced heat contributions of R12 and R13 and the enhanced contribution of R6 are mainly from the chemical action of CO2. ACS Paragon Plus Environment

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Conversely, in the conventional flame, heat contributions of R12 and R13 are raised mainly attributed to the physical properties of CO2. On the other hand, Figs. 15(b) and 15(d) reveal that the steam’s chemical properties have greater effect on the overall reduction in heat contributions of R24, R12, R13 and R59. For the increased contribution of R58 to the total heat release, the steam’s physical effect is minor for the MILD combustion but become comparable importance with the chemical effect in conventional combustion. Importantly, the physical and chemical properties of steam play comparable roles with opposite direction in the effects on the heat contributions of R49, for both the MILD and traditional flames.


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Fig. 15 Chemical and physical effects of varying the diluent N2 to (a, c) CO2 and (b, d) H2O on the global volume-weighted kinetic reaction rates and their heat contribution ratios for the top 10 active reactions of the JHC flames at XO2 = 6% and 21%. The kinetic rate of R49 under O2/N2 with XO2 = 6% was used as the reference for normalization.

6. Conclusions This paper has presented the thermal characteristics on the stoichiometric sheet of a methane JHC flame moderated by N2, CO2 and H2O, respectively. The fuel jet issues at the exit Reynolds number ≈ 10,000 while the hot coflow oxygen level varies between XO2 = 6% and 23%. To decouple the chemical and physical effects of varying ACS Paragon Plus Environment

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N2 to CO2 or H2O, we used the fictitious gases XCO2 and XH2O that have the same physical properties of CO2 and H2O but are totally nonreactive. To summarize from the preceding sections, four concluding remarks are provided below: 1.

The stoichiometric flame sheet length (Lst) is related with the coflow oxygen fraction (XO2) as Lst ~ XO2-1.3, no matter which diluent is used for dilution. The size of reaction zone bonded by the stoichiometric flame sheet raises greatly when varying the diluent N2 to CO2 whereas it reduces substantially to H2O. Such discrepancies are mainly from distinct physical properties of N2, CO2 and H2O. The overall reaction temperature decreases for the oxy-combustion, because of higher thermal capacities of CO2 and H2O than that of N2. As the coflow oxygen level drops, the physical effect gradually dominates in Lst whereas the chemical actions of CO2 and H2O on the temperature reduction become relatively stronger.

2.

On the stoichiometric flame sheet, the intermediate species OH and H2 are critical in controlling reactions R49, R58 and R24 and their heat release. As the coflow oxygen increases, R49 plays a more important role in both air and oxy-steam combustion while the CO2-diluted combustion is gradually monopolized by R58 and R49. When replacing the diluent N2 by CO2, the OH and H2 formations are both suppressed, reducing the oxidation rate and thus the volumetric heat-release-rate. However, in the oxy-steam combustion, steam can chemically promote the OH and H2 formations at XO2 > 15%, thereby enhancing the reaction rate of R49 and increasing the volumetric heat-release-rate. At XO2 = 6%, lower OH and H2 concentrations slow down the oxy-steam combustion with respect to the air combustion.

3.

R49 contributes the most to the global heat production in the cases diluted by N2 and H2O, whereas R24 is the most important endothermic reaction for all the cases. For example, the heat release from R49 takes more than 52% out of the total when XO2 is increased from 6% to 23% for the N2-diluted and H2O-diluted flames. For the CO2-diluted cases, R58 dominates the heat release with a 42% contribution in the MILD regime but is gradually displaced by R49 and R59. At ACS Paragon Plus Environment

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XO2 = 23% in the O2/CO2 coflow, R49 and R59 respectively make 45% and 23% contributions, larger than the contribution of R58 (≈ 18%). The physical and chemical effects of steam play comparable roles, but in the opposite direction, in varying the heat contributions of R49 for both the MILD and traditional flames. However, the chemical activations of CO2 are more important in weakening the heat contributions from R49 and R24 and promoting the contribution ratio of R58 and R59. 4.

The steam dilution enhances the heat contribution released from the H2-O2 kinetical reactions while the CO2 dilution plays the opposite role. Namely, the H2-O2 kinetics without carbon reactions are more vital in the oxy-steam combustion whereas those reactions containing carbons are more prominent in the CO2-diluted combustion.

Acknowledgement The support of Nature Science Foundation of China (Nos. 51276002 and 51776003) is gratefully acknowledged.

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