H2 on a Jet-in-Hot-Coflow Burner

Feb 2, 2017 - Since MILD combustion is featured as intense internal flue gas recirculation and invisible flame front, it is of great interest to inspe...
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Flame Characteristics of CH4/H2 on a Jet-in-Hot-Coflow Burner Diluted by N2, CO2, and H2O Yaojie Tu,†,‡ Hao Liu,*,† and Wenming Yang‡ †

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China ‡ Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore ABSTRACT: To enhance the knowledge to the combination of oxy-fuel combustion and MILD (moderate or intense lowoxygen dilution) combustion, namely oxy-MILD combustion, the present article examines the effects of different diluents (i.e., N2, CO2, and H2O) on the flame characteristics of CH4/H2 blended fuel in a JHC (jet-in-hot-coflow) burner, which can produce MILD combustion regime in the upstream and conventional combustion regime in the downstream. CFD modeling method considering RANS equations and detailed reaction mechanism (GRI-Mech 2.11) are used for such purpose. Results show that, in MILD combustion regime, peak temperature under oxy-fuel condition (diluted by CO2 and H2O) is reduced as compared to airfuel condition (diluted by N2) from both physical and chemical effects of the diluents. However, in the conventional combustion regime, chemical effect of the diluents on suppressing the peak temperature is weakened, resulting in the predominance of its physical effect. As for the flame ignition behavior, the dilution with CO2 or H2O causes flame liftoff according to OH appearance. More noticeably, the liftoff distance is even larger in H2O diluted case than CO2 diluted case. On the other hand, by calculating the Damköhler number in the computational domain, MILD combustion region is found well under reaction-controlled status. However, as compared to CO2, H2O plays a weakened role on the reaction-controlled status, suggesting that CO2 might be preferable to H2O in realizing oxy-MILD combustion.

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INTRODUCTION In order to reduce NOX emission while maintaining high thermal efficiency during industrial heating process, MILD combustion1 (or high temperature air combustion,2 or flameless oxidation,3 or colorless distributed combustion4) has attracted extensive attention for more than two decades. The principle of this technology is to create an intense flue gas recirculation inside the combustion furnace, which achieves diluting as well as preheating the reactants simultaneously. By such operation, the peak combustion temperature can be suppressed while average furnace temperature can be improved with a comparison to the conventional combustion (for example with a standard swirling burner),5 which subsequently inhibits NOX formation (mainly from thermal route) and enhances heat exchange. It is worth noting that, in recent years, MILD combustion has been expected to be applied in conjunction with another environmentally friendly technology, oxy-fuel combustion,6 for the purpose of achieving high thermal efficiency while obtaining greater potential on reducing pollutant emissions.7−9 The idea of combining oxy-fuel combustion with MILD combustion was initially taken into practice in the field of steel reheating using commercial burners, such as REBOX,9,10 and it was reported to be more cost-effective, environmentally friendly, and has greater potential of reducing scale formation than conventional reheating technologies. To extend the application of oxy-MILD combustion to solid fuels, plenty of studies have been conducted for both pulverized coal and sawdust.11−17 It was reported that oxy-MILD combustion obtains a greater potential on reducing NOX emissions from not only thermal-NOX route but also fuel-NOX route.11,16 Furthermore, © 2017 American Chemical Society

SOX emission was indicated to be lower as compared to conventional oxy-fuel operation.14 The contribution of gasification reactions of char was found enhanced due to lower local oxygen presence, and this could alter the whole combustion mechanism for solid fuels under oxy-MILD condition.12,17 However, oxy-MILD combustion also has disadvantages in decreasing the burnout ratio, which is thought mainly due to the insufficient residence time of particles inside the furnace.15 More recently, Li et al.18 carried out oxy-MILD combustion for three gaseous fuels and pointed out that CO2 is preferred rather than N2 in establishing MILD flameless regime, namely oxy-MILD combustion has a wider stability limit than air-MILD combustion. In addition to the above experimental studies, numerical simulations have been also performed to understand the fundamentals of oxy-MILD combustion. Among them, gaseous fuels, such as CH4, H2, CO, or their blends are mostly used due to the existing well-validated chemical reaction mechanisms. Besides, considering the relatively simple configuration of the burners, counterflow diffusion flame, coflow diffusion flame, and well stirred reactor (WSR) are commonly employed. From the dynamic point of view, Chen et al.19 examined the feasibility of oxy-MILD combustion for biogas (CH4 and CO2 mixture) in a counterflow diffusion flame. Moreover, they compared the biogas oxy-MILD combustion characteristics between CO2 and H2O dilution conditions, and claimed that CO2 is safer than H2O in moderating the flame temperature.20 Received: December 7, 2016 Revised: January 17, 2017 Published: February 2, 2017 3270

DOI: 10.1021/acs.energyfuels.6b03246 Energy Fuels 2017, 31, 3270−3280

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against the experimental data. Then, the JHC flames in atmospheres of O2/N2, O2/CO2, and O2/H2O are simulated and their characteristics are compared. To distinguish physical effect from chemical effect of the diluents, artificial materials are introduced as in other literatures.23,24,34

Mei et al.21 modeled oxy-MILD combustion for CH4 in a JHC (jet-in-hot-coflow) burner in O2/CO2 atmosphere, and pointed out that oxy-MILD combustion produces a larger reaction volume in contrast to air-MILD combustion. Mardani et al.22 simulated the JHC flames for CH4/H2 blended fuel, and reported a lower Damköhler number for oxy-MILD combustion than air-MILD combustion. On the same facility, Tu et al.23 suggested the chemical effect of CO2 on the oxy-MILD combustion characteristics (temperature, species formation, and reaction route) would be strengthened when it gradually replaces N2 in the coflow. From the kinetic point of view, typically in the zerodimensional WSR, Zhang et al.24 and Sabia et al.25 studied the CH4 combustion behaviors under low oxygen while high reactor temperature conditions, which can be regarded as MILD combustion regime. Zhang et al.24 claimed the physical effects of CO2 is responsible for the peak temperature reduction under oxy-MILD combustion, while the chemical effect has a significant impact on the CH4 oxidation. Sabia et al.25,26 found that a large amount of CO2 and H2O can alter the ignition process under MILD regime in dependence on temperature range and mixture composition. Such behaviors are derived from the change in the main branching mechanism since CO2 and H2O can act as third molecular species in chemical reactions. Up to now, the operation of oxy-fuel combustion is mainly based on the external flue gas (mainly includes CO2) recirculation technology. In other words, CO2 is considered as the diluent for oxy-fuel combustion mostly. As a matter of fact, H2O can also act as an alternative diluent for oxy-fuel combustion, namely the oxy-steam combustion,27 which is likely to be more cost-effective than CO2 diluted case. Moreover, it has been indicated that H2O is more active than CO2 during combustion since it can be decomposed into H and OH radicals, which are important for ignition and heat release.28 However, the knowledge to H2O diluted oxy-MILD combustion is quite scarce. Although the investigations of Chen et al.20 and Sabia et al.26 have been involved with H2O diluted oxy-MILD combustion, more information is needed to minimize the knowledge gap. Since MILD combustion is featured as intense internal flue gas recirculation and invisible flame front, it is of great interest to inspect the flame structure of MILD combustion. To this end, Dally et al.29 developed a JHC burner by issuing fuel jet into a controllable oxidizer (species composition and temperature) to emulate MILD combustion regime and study its flame structure. With this method, Dally’s group30−33 has well revealed the combustion behaviors (temperature and pollutant emission) and flame structures (OH and CH2O spatial distribution as well as liftoff behavior, etc.) for a variety of fuel types. However, their studies are mainly dealing with airMILD combustion, even though a small number of CO2 and H2O are contained in the hot coflow. In light of the above-mentioned shortages on oxy-MILD combustion, the objective of the present work is to study the impacts of different diluents (i.e., N2, CO2, and H2O) on MILD combustion characteristics, in terms of telling physical effect from chemical effect, flame ignition behavior, and flame structure. For such purpose, CFD numerical method is used together with the JHC flame device of Dally et al.,29 which can maintain a steady MILD combustion regime under controlled oxygen concentration and atmosphere temperature conditions above the burner. First, the numerical method is validated

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COMPUTATIONAL DETAILS

Burner Configuration. The JHC burner used in the present work was developed by Dally et al.29 at the University of Adelaide (see Figure 1), which is consisted of a central fuel pipe (ID = 4.25 mm) and

Figure 1. Schematic diagram of the JHC burner and grid mesh of computational domain. a surrounding annular coflow channel (ID = 82 mm). The whole burner was mounted in a wind tunnel that can provide a constant air temperature of 300 K and velocity of 3.2 m/s. In the experiment of Dally et al.,29 the fuel is a mixture of CH4 and H2 in equaled volume fraction. It has been indicated that, the addition of H2 helps to enhance the flame stability and prevent the flame from blowing off.35 The fuel is injected with a velocity of ∼70 m/s (jet Reynolds number is approximately 10 000), and the oxygen level of the coflow is varied to be 3, 6, and 9% in mass fraction with a fixed temperature of ∼1300 K and a flow rate of 4.8 × 10−3 kg/s (∼3.2 m/s). Since the combustion phenomenon takes place above the JHC burner exit, the computational domain (see Figure 1) is assumed to be 500 mm in height (x direction) and 210 mm in radius (r direction), on the premise that the flame structure is not affected by the downstream and side outlets. Owning to the axisymmetric geometry of the JHC burner, the modeling can be simplified from three-dimensional to twodimensional to reduce the computational cost. In addition, 20 mm length of the burner inlet is considered to account for the developing of the turbulent flow at the burner exit. According to Figure 1, fully structured grid method is adopted for meshing the computational domain, with enhanced density near the fuel and coflow exit regions to improve the modeling accuracy. In our previous work,23 the grid independence has been examined by comparing the flow and combustion behaviors between two grids with 46 560 cells and 124 560 cells. As no obvious discrepancy is noticed between the two grids, the smaller grid with 46 560 cells is utilized in the present work. Computational Methods. The numerical modeling is carried out with the commercial CFD software Fluent, version 6.3.36 In specific, the turbulent flow is simulated using the modified k−ε model with the constant Cε1 adjusted from the default value of 1.44 to 1.6 to compensate for round jet.37 It has been indicated that the change in turbulent intensity at the inlets can significantly affect the combustion process, thus plenty of previous work has focused on the optimization of the turbulent kinetic energy (TKE).38,39 In the present work, the 3271

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FH2O is introduced. To achieve this in Fluent, artificial materials must be included in both gas-phase kinetics and thermodynamics databases. Meanwhile, no reactions should be defined for the artificial material in the gas-phase kinetics database. By this means, the chemical effect can be differentiated from the physical effect. According to Figure 2, the differences between N2 and artificial materials (FCO2 or FH2O) are assigned to the physical effect, and those between real material (CO2 or H2O) and artificial material (FCO2 or FH2O) are the chemical effect. As a whole, the differences between N2 and real material (CO2 or H2O) are regarded as combined effect. Table 1 lists the operational conditions of coflow and tunnel wind for the present considered cases. In specific, case 1* to case 3* refer to

TKE values for fuel, coflow, and tunnel wind are set to 60, 4, and 3.5 m2/s2 respectively, to achieve comparable results with the experimental measurements. Due to the weakened reaction rate under MILD combustion, detailed chemical reaction mechanism (GRI-Mech 2.1140) is employed instead of global mechanism for species transportation coupling with EDC (eddy dissipation concept) model.41 In EDC model, the chemical reactions are assumed to take place in a fine structure. To improve the simulation accuracy, the constants of volume fraction (Cξ) and the time scale (Cτ) of the EDC model are respectively modified to 3 and 1.42 In addition, to accelerate the convergence of the iterations, the in situ adaptive tabulation (ISAT) method is adopted. Although radiation was indicated negligible for the JHC flames,37 this effect is still under consideration for the present modeling due to the largely increased amount of polar molecules under oxy-MILD condition, i.e., CO2 and H2O, which have relatively higher absorption coefficient than N2. Hence, the discrete ordinate (DO) model in conjunction with the weighed sum of gray gas (WSGG) assumption is adopted for the calculation of radiation heat transfer,43 where the spatial variation of the total flue gas emissivity is calculated as a function of temperature and concentration. Particularly, in the DO radiation model, six wavelength bands are assigned to compensate the radiation spectrum evaluation.21,23 Velocity inlet boundary conditions are considered for fuel, coflow, and tunnel wind, and pressure outlet boundary condition is used for the downstream exit of the computational domain. To improve the iteration steadiness, a zero-shear wall boundary condition instead of pressure outlet is taken for the lateral side of the computational domain, with minor influence on the flame structure due to its sufficient width.39 The final results are considered reliable when all the residuals achieve the desired levels, and the average temperature as well as velocity stays unchanged in the whole domain. Computational Conditions. As is known, N2, CO2, and H2O are different in both physical and chemical properties, such as specific heat value, diffusion coefficient, gas emissivity, and chemical activeness, etc. In specific, the larger specific heat value of CO2 and H2O than N2 makes the flue gas absorb more heat and produce lower flame temperature, while high presence of CO2 and H2O increases the overall gas emissivity. For chemical reaction, due to the slower diffusion rate of oxygen and the higher activeness of CO2 and H2O, the reaction rate is likely to be inhibited together with the altered reaction route. Generally, these discrepancies can be expressed by several effects,44 which can be referred to Figure 2. For convenience, in the present work, the different effects are classified into two sets, namely the physical effect and the chemical effect.

Table 1. Operational Conditions of Coflow and Tunnel Wind for the Modeled Casesa compositions (%)

NO.

HM1

2*

HM2

3*

HM3

3

N2 dilution CO2 dilution FCO2 dilution H2O dilution FH2O dilution

4 5 6 7 a

ID

1*

fuel (300 K, 70 m/s)

coflow (1300 K, 3.2 m/s)

tunnel wind (300 K, 3.2 m/s)

H2 = 11.1, CH4 = 88.9 H2 = 11.1, CH4 = 88.9 H2 = 11.1, CH4 = 88.9 H2 = 50, CH4 = 50 H2 = 50, CH4 = 50 H2 = 50, CH4 = 50 H2 = 50, CH4 = 50 H2 = 50, CH4 = 50

O2 = 3, N2 = 85, H2 O = 6.5, CO2 = 5.5 O2 = 6, N2 = 82, H2 O = 6.5, CO2 = 5.5 O2 = 9, N2 = 79, H2 O = 6.5, CO2 = 5.5 O2 = 7.9, N2 = 78.6, H2 O = 10, CO2 = 3.5 O2 = 7.9, H2O = 10, CO2 = 82.1 O2 = 7.9, H2O = 10, FCO2 = 82.1 O2 = 7.9, H2O = 88.6, CO2 = 3.5 O2 = 7.9, FH2O = 88.6, CO2 = 3.5

O2 = 23.3, N2 = 76.7 O2 = 23.3, N2 = 76.7 O2 = 23.3, N2 = 76.7 O2 = 21, N2 = 79 O2 = 21, CO2 = 79 O2 = 21, FCO2 = 79 O2 = 21, H2 O = 79 O2 = 21, FH2 O = 79

With * indicates mass fraction, without * indicates mole fraction.

the experimental cases of Dally et al.,29 which are used for validating the present numerical modeling. Case 3 to case 7 refer to the cases diluted by N2, CO2, FCO2, H2O, and FH2O, respectively. Note here that, case 3* and case 3 actually denote the same circumstance of HM3 in the experiments of Dally et al.29 The only difference is that case 3* is expressed based on mass fraction to keep consistent with the experimental demonstration, while case 3 is based on mole fraction to maintain the same injection velocities at inlets of fuel, coflow, and tunnel wind as those in case 4 to case 7. The identical injection velocity for each stream among case 3 to case 7 can produce similar flow structure, as a consequence, the flow dynamics will not be relevant to the differences in combustion.

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COMPUTATIONAL VALIDATION

In order to gain confidence of the present modeling for the JHC flame, validations are conducted for the three experimental cases (named as HM1, HM2, and HM3 in Table 1) of Dally et al.29 Figure 3 compares the radial distributions of temperature and species mass fraction (CO, OH, and O2) at x = 30 mm between numerical and experimental results for the three JHC flames. As can be observed, the predicted temperature, major and minor species all generally agree well with the experimental data, not only in the tendency but also in the magnitude. However, underestimations of CO concentration are obtained for all of the three JHC flames in the radial direction between r = 20 mm and r = 40 mm, where the coflow stream is located. As a matter of fact, this is expected to be caused by the cooling and extinction effects of the premixed flame inside the secondary burner, which is used to provide high temperature coflow for the JHC flames.37 Resulting from these effects, plenty of CO is contained in the coflow, while no suggestions have been ever reported to compensate for this.

Figure 2. Relationship between the combined effects, physical effects, and chemical effects. Since the physical and chemical effects are closely coupled during combustion process, it is proper to introduce an artificial material, which has the same physical properties as the real material, but is totally nonreactive, to decouple the physical effect from chemical effect. This method is applied to both CO2 and H2O. For example, FCO2 is same as CO2 in terms of density, molecular weight, specific heat value, thermal conductivity, etc., but it does not take part in any of the chemical reactions. Similarly, for H2O, the artificial material of 3272

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Figure 3. Comparison between simulations and experiments at x = 30 mm for three JHC flames of Dally et al. (a) Temperature, (b) CO mass fraction, (c) OH mass fraction, and (d) O2 mass fraction. On the other hand, Dally et al.29 have used the variable of mixture fraction (Z) to study the JHC flame structures. Note that, mixture fraction is a widely adopted parameter to inspect nonpremixed flame structure because the results are independent of the burner configurations, such as counterflow flame or coflow flame. It is calculated based on the mass balance of C, H, and O elements, which is expressed as follows:45 2(YC − YC,2)

Z=

WC 2(YC,1 − YC,2) WC

+ +

(YH − YH,2) 2WH (YH,1 − YH,2) 2WH

− −

(YO − YO,2) WO (YO,1 − YO,2) WO

(1)

where Yj and Wj respectively stands for the mass fraction and atomic mass for element j, i.e., C, H, and O. The subscripts 1 and 2 respectively represent the value extracted from either the fuel or the oxidant. Using this criteria, the mixture fraction is going to be 1 in the fuel and 0 in the oxidant. Figure 4 depicts the predicted and measured mixture fraction for the experimental case of HM1 (case 1* in Table 1) in the radial directions at x = 30, 60, and 120 mm, as well as in the axial centerline. A reasonable agreement of the mixture fraction in both radial and axial directions between numerical simulation and experimental measurements are achieved. This shows that the present numerical models are also capable in predicting the spatial JHC flame structure. After all, the present study mainly concerns flame characteristics, i.e., combustion temperature, intermediate species formation, and flame structure. The above validation work has well announced that the present numerical method is capable for achieving these goals, even though some minor differences can be noticed between the predictions and the measurements.

Figure 4. Comparison of mixture fraction between simulations and experiments for HM1 flame on radial and axial profiles.

flame, to highlight the features of oxy-MILD combustion diluted by CO2 and H2O. Due to entrainment by the central fuel jet with high velocity, the outer tunnel wind starts to vitiate the coflow in oxygen level and temperature from the burner exit (x = 0 mm), and MILD combustion region is expected to occur within the outer diameter of coflow channel, i.e., r < 41 mm. In order to focus on the MILD combustion region, in the subsequent sections, results are only presented in the radial range of 0−35 mm for consistency. Flame Temperature Spatial Distribution. Figure 5 displays the temperature contours in the central plane for case 3 to case 7 in the region of r < 35 mm and x < 250 mm. The outer cold tunnel wind is found to gradually penetrate into the hot coflow and reach the maximum radial distance at about x = 80 mm regardless of diluents. Below this height the fuel is basically oxidized in the atmosphere of low oxygen and hot

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RESULTS AND DISCUSSION In this section, the effects of different diluents on the JHC flames are evaluated in terms of flame temperature, intermediate species (OH and CH2O) formations as well as flame structure. For a more specific objective, the differences are comprehensively compared between MILD combustion region and conventional combustion region within the JHC 3273

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Figure 5. Temperature contours of the considered cases (0 < x < 250 mm, 0 < r < 35 mm).

maximum temperature decrement are indicted. At x = 35 mm where MILD combustion regime occurs, the physical effect is found to play a comparable role as the chemical effect on the maximum temperature suppression for either CO2 or H2O dilution condition. However, at x = 125 mm where conventional combustion takes place, the physical effect becomes more dominant than the chemical effect on reducing the peak temperature for both CO2 and H2O dilution conditions. This discrepancy is resulted from lower temperature increment under MILD combustion, which weakens the role of heat absorption by the flue gas. As a result, the physical effect on peak temperature reduction is not that obvious under MILD combustion with comparison to conventional combustion, and the relative role of chemical effect is enhanced. Moreover, it is interesting to note that the physical effect of H2O on the suppression of peak temperature exceeds 100%, suggesting a positive role of the chemical effect on enhancing the maximum temperature in H2O diluted oxy-fuel combustion. This is also reported by Zou et al.,28 who claimed that the maximum temperature with oxidizer of 27.5% O2/72.5% H2O is approximately 20 K higher than the case of 27.5% O2/72.5% FH2O for methane opposed diffusion flame, which is mainly generated from the chemical effects of H2O. OH and CH2O Spatial Distributions. For methane, the main consumption route is via the hydrogen atom abstraction reactions, i.e., CH4 → CH3 → CH2O → CHO → CO → CO2.46 During this process, the abstracted H atom combined with O atom to produce OH radical, which serves as an important intermediate for heat release and fuel consumption, by elementary reactions, such as CH2O + OH ↔ CHO + H2O and CO + OH ↔ CO2 + H. Moreover, for jet flames of hydrocarbon fuels, Gordon et al.47 defined a three-step autoignition mechanism: formation of ignition precursor (such as CH2O), initiation of ignition (high presence of OH), and the presence of steady flame (consumption of CH2O and stable OH). In this regard, CH2O is treated as an important intermediate for autoignition. Therefore, in the following, CH2O and OH spatial distributions are examined to obtain better insight into the combustion characteristics of JHC flames under different dilution conditions. Figure 8 compares the radial profiles of OH and CH2O for case 3 to case 7 at x = 35 mm and 125 mm. Regardless of diluents or combustion regime, each case has shown a similar trend that CH2O is formed in a region closer to the central fuel

coflow, while above this height the mixing between coflow and outer tunnel wind makes the fuel consumed in a richer oxygen and lower temperature condition. According to the experimental observation of Dally et al.,29,30 the flame appearance is basically invisible below x = 100 mm, where MILD combustion is believed to happen. While above this position, because of the improved mixing between fuel and oxygen, the combustion temperature gets relatively higher and subsequently makes the flame edge distinguishable from the atmosphere. Therefore, Dally et al.29 regarded the combustion regime above x = 100 mm as under conventional combustion. To compare the differences between MILD and conventional combustion, the locations of x = 35 mm and x = 125 mm are extracted from the JHC flame to represent the regions of the two combustion regimes. Figure 6 shows the radial temperature

Figure 6. Temperature profiles (a) x = 35 mm, (b) x = 125 mm of the considered cases.

profiles at x = 35 mm and x = 125 mm for case 3 to case 7. At x = 35 mm, the difference in maximum temperature among the cases is within 300 K, while that at x = 125 mm is increased to as high as nearly 550 K. This indicates that the flame peak temperature under MILD combustion regime is much less sensitive to the change in diluents with comparison to that under conventional combustion regime. More specifically, Figure 7 compares the maximum temperature at x = 35 mm and x = 125 mm for the considered cases, and also the physical and chemical contribution to the 3274

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Figure 7. Comparisons of maximum temperature on profiles at (a) x = 35 mm, (b) x = 125 mm, for case 3 to case 7.

Figure 8. Profiles of OH and CH2O at x = 35 mm (left) and x = 125 mm (right) for case 3 to case 7.

Figure 9. Comparisons between JHC flame images and simulated OH contours in the experiment of Dally et al.29,30 (Difference in flame image color is caused by varied exposure time.)

jet than OH. Moreover, combined with the temperature profiles in Figure 6, the locations of maximum OH and temperature are matched quite well. This observation demonstrates that CH2O is mainly the intermediate product

of CH4 oxidation in the fuel rich region, while OH is generated after CH2O as an indication of the high temperature region. The elementary reaction of CH3 + O ↔ CH2O + H plays an important role on CH2O formation. The abstracted H atom 3275

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Energy & Fuels further combines with O atom to form OH free radical through the third body reaction of H + O ↔ OH. In addition, the H2 molecule in the fuel stream can react with O atom through H2 + O ↔ OH + H. This in some degree explains why the addition of H2 can improve the ignition stability. However, according to Figure 8, different diluents are found to have a significant influence on the maximum concentrations of OH and CH2O. In specific, OH concentration is higher at x = 125 mm than x = 35 mm for each diluent, which follows the same tendency as the temperature profiles in Figure 6. In addition, by comparing CO2 and FCO2 diluted cases to N2 diluted case, the chemical effect plays a more important part in reducing the OH formation under MILD combustion regime (x = 35 mm), while the physical effect acts as a predominant role only under conventional combustion regime (x = 125 mm). However, no obvious difference is noticed between H2O and FH2O diluted cases on OH formation (see Figure 8) as well as temperature profiles (see Figure 6). This observation is also recognized by Wang et al.48 in methane oxy-fuel combustion diluted by H2O, who claimed that the chemical effect the H2O plays a comparable role as physical effect while at the opposite direction. As a result, the chemical effect almost offsets the physical effect. It is worth noting that, when diluted with H2O, the formation of OH not only comes from CH4 oxidation but also from the dissociation of H2O. However, as compared to N2 dilution case, the overall OH formation rate is still lower due to the reduced temperature. Because the flame edge is difficult to be distinguished by human naked eye under MILD regime, it is useful to adopt OH radial as flame maker for the JHC flames.31 To enhance this, Figure 9 shows a comparison between the practical flame images and the simulated OH fields for HM1, HM2, and HM3 flames (case 1* to case 3* in Table 1) of Dally et al.29,30 As oxygen level in the coflow increases, the flame border becomes more and more distinguishable from the surrounding atmosphere. It is also obvious that the flame brightness is gradually weakened with the reduction of oxygen level in coflow. Especially for HM1 case, an invisible flame border shows up above the burner exit, indicating the flameless regime of MILD combustion. Furthermore, according to Figure 9, the distributions of the practical flame and the simulated OH contour are matched reasonably well in dimension. Meanwhile, as oxygen level increases in the coflow, the luminance of the JHC flame as well as the OH concentration is facilitated from HM1 to HM3. According to these results, the OH contour is highly relevant to the flame structure, that it will in some degree be helpful to understand the ignition behaviors for the JHC flames. Figure 10 displays the simulated OH contours in the central plane for case 3 to case 7. It is observed that JHC flames diluted by CO2 and H2O are lifted off as compared to the N2 diluted case according to OH distributions. However, when diluted by FCO2 or FH2O, the liftoff phenomenon disappears. This demonstrates that the chemical effect of CO2 and H2O is responsible for the ignition delay for JHC flames under MILD combustion regime, while their physical effect has no significant influence on the ignition process of the JHC flames. More specifically, the liftoff distance of the JHC flames is noticed to be in the order of N2 < CO2 < H2O. One of the most probable reason is because H2O strengthens the reaction rate of CH2O + OH ↔ CHO + H2O in the reversed direction to reduce the heat release rate from inhibiting CHO formation.

Figure 10. Contours mole fraction of OH of the considered cases. (0 < x < 250 mm, 0 < r < 35 mm).

Flame Structure. For the present JHC flames, which are typically under diffusion combustion state, the stoichiometric mixture fraction (Zst) is employed to characterize the flame structure. The stoichiometric mixture fraction refers to the mixture fraction at the location of stoichiometry, where the fastest heat release rate happens, and it is used to distinguish the fuel rich region from the fuel lean region. According to Bilger’s formula in eq 1,45 the stoichiometric mixture fraction is corrected as follows: YO,2

Zst =

WO 2(YC,1 − YC,2) WC

+

(YH,1 − YH,2) 2WH

−

(YO,1 − YO,2) WO

(2)

By eq 2, the stoichiometric mixture fraction for N2, CO2, and H2O diluted cases (case 3, case 4, and case 6) are respectively calculated to be 0.0489, 0.0352, and 0.0672. Figure 11 plots the

Figure 11. OH mole fraction versus mixture fraction.

mole fraction of OH radical against the mixture fraction at x = 35 mm and 125 mm for N2, CO2, and H2O diluted cases, together with their stoichiometric mixture fraction values in vertical dashed lines. As mentioned above, the heat release rate is expected to be the largest at the stoichiometric condition. Since the heat release rate is highly relevant to OH formation, the OH peaks under both MILD and conventional combustion are found located in the vicinity of stoichiometric mixture fraction in the corresponding dilution conditions. Meanwhile, for the same dilution condition, OH concentration under conventional combustion is not always higher than that under MILD combustion at an identical mixture fraction. In other 3276

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Figure 12 shows a joint distribution of temperature with an isoline of Zst on the top and OH mole fraction with an isoline of Da = 1 on the bottom. Consistent with the above-mentioned results, the high temperature region, maximum OH region and the stoichiometric mixture fraction (Zst) profile match with each other. Moreover, these regions are surrounded by the profile of Da = 1 in the downstream. The distribution of Da = 1 profile reveals that, combustion regime is totally under reactioncontrolled status, namely MILD combustion mode, in the region close to the burner exit. However, in the relatively further downstream (x > 70 mm), reaction-controlled status only appears in the central region, while the outer region is under mixing-controlled status. Figure 13 plots the detailed Da profiles for the N2 diluted case (case 3) at x = 35 mm and 125 mm. It is found under

words, the OH concentration under MILD combustion can become higher than that under conventional combustion in the fuel rich region. On the other hand, since turbulent flame is highly coupled by turbulent mixing and chemical reaction, the Damköhler number (Da) is introduced to further examine the flame structure.49 When Da > 1, the region is dominated by turbulent mixing due to quick chemical reaction. While when Da < 1, the chemical reaction time scale is comparable or longer with respect to turbulent time scale, and the region is regarded as under reaction-controlled regime, such as MILD combustion. Here, the Damköhler number is defined as the ratio of turbulence time scale (τf) and chemical reaction time scale (τch), namely, τ k Da = τ f . The turbulence time scale is calculated by τf = ε , ch

where k is the turbulent kinetic energy, and ε is the dissipation rate of k. The chemical reaction time scale is calculated by the formula of Kumar et al.,50 which is expressed as follows: ⎡ e(−Ea / RTad,ref ) ⎤ τch = τch,ref (YO2)a1 (Yf )a2 ⎢ (−E / RT ) ⎥ ⎣ e a ad ⎦

(3)

where τch,ref is the chemical reaction time scale at a reference condition, i.e., 300 K and methane-air mixture. Yj is the mass fraction for species, Ea is activation energy, and Tad is adiabatic temperature. Exponents of a1 and a2 are used to consider the change of mass fraction in both oxidant and fuel streams, and Tad is introduced to include the effect of temperature variation in coflow. As suggested in ref 50, the above parameters for methane contained fuel are set to values listed in Table 2. Table 2. Parameters for Calculating Chemical Reaction Time Scale for Methane−Air Mixture parameters

values

a1 a2 Ea/R τch,ref

−2.0 −0.5 20000 0.06546

Figure 13. Comparison of Da profiles at x = 35 mm and 125 mm for N2 diluted case (case 3).

unit

MILD combustion (x = 35 mm), the maximum Da is less than 0.5, while under conventional combustion (x = 125 mm), the maximum Da is higher than 3 at nearly r = 30 mm. This confirms that, MILD combustion is characterized as slower reaction rate, which generates relatively longer chemical reaction time scale, and subsequently results in smaller Da (less than 1). It should also be noted that, for conventional combustion, the central region (r < 20 mm) is likewise under reaction-controlled regime. The width of this region is gradually expanded with the axial distance according to Figure 12. However, this region essentially can not be regarded as MILD combustion because the coflow has been dramatically vitiated

K ms

It should be noted, the above suggested values are obtained from the experiments of methane-air combustion. Thus, it is believed the values are only valid for the N2 diluted case in the present study, since no corrections have been made to consider the change of diluents in either fuel or oxidant in eq 3. Therefore, the calculation of Da is only carried out for N2 diluted case, as a reference of the typical JHC flame structure.

Figure 12. Contours of (top) temperature, (bottom) OH and profiles of (top) Zst, (bottom) Da = 1 for N2 diluted case (case 3). 3277

DOI: 10.1021/acs.energyfuels.6b03246 Energy Fuels 2017, 31, 3270−3280

Energy & Fuels

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CONCLUSIONS In this article, a numerical study is presented to reveal the effects of different diluents (N2, CO2, and H2O) on the CH4/ H2 JHC flame, which is typically divided into the upstream MILD combustion region and the downstream conventional combustion region. Special attentions are put on the combustion temperature, intermediate species formation, and flame structure. In addition, artificial materials (FCO2 and FH2O) are introduced with respect to real materials (CO2 and H2O) to distinguish the chemical effect from physical effect on flame characteristics for CO2 and H2O diluted oxy-MILD combustion. The main conclusions can be drawn as follows. 1. The replacement of N2 by either CO2 or H2O will lead to a conspicuous difference in reducing the maximum flame temperature under conventional combustion in contrast to MILD combustion. Because of the different combustion mechanism between MILD combustion and conventional combustion, the relative contribution of chemical effect of either CO2 or H2O on reducing the maximum flame temperature is enhanced under MILD combustion. While under conventional combustion, the dominant role on reducing the maximum flame temperature is from the physical effect. 2. For the JHC flame, OH formation is highly sensitive to the change of diluents. For CO2, the chemical effect is the main factor on reducing OH formation in MILD combustion region, while the physical effect dominates in conventional combustion region. For H2O, no matter in MILD or conventional combustion region, the physical effect acts as the primary role on reducing OH formation. In addition, the chemical effect of CO2 and H2O results in the ignition delay of the JHC flames, and the ignition delay distance is further enlarged for H2O diluted case with comparison to CO2 diluted case. 3. The calculation of Damköhler number (Da) reveals that, for the present JHC flame, the upstream MILD combustion region is well under reaction-controlled status, while the downstream conventional combustion region is generally under mixing-controlled status. With CO2 dilution, the reaction-controlled status will be reinforced in the central region, while with H2O dilution, the reaction-controlled status will be weakened in the outer region. This suggests that CO2 is a better choice for performing oxy-MILD combustion as compared to H2O.

by the outer tunnel air in terms of oxygen level and temperature according to the experimental results.29,30 While as a matter of fact, the phenomenon of lower Da in the central region can be attributed to the high exhaustion of fuel that slows down the reaction rate and increases the reaction time. This can be supported by the axial mixture fraction profile shown in Figure 4. Even though the chemical reaction time scale can not be calculated due to the absence of values of the parameters in Table 2 for CO2 and H2O diluted cases, one can still gain an impression on how the diluents affect the chemical reaction time scales under MILD and conventional combustion. To achieve this, a dimensionless variable φ is introduced and expressed as

φ = τMILD/τconventional

Article

(4)

here, τMILD and τconventional are the chemical reaction time scales under MILD and conventional combustion, respectively. By this means, the effects of the parameters in Table 2 can be eliminated, leaving φ to be the function of only species mass fractions. Typically, τMILD and τconventional can be extracted from the locations of x = 35 mm and x = 125 mm, in agreement with the above results. Figure 14 plots the profiles of φ for JHC flames with different diluents. With comparison to N2 diluted case, minor difference

Figure 14. Comparison of φ profiles for case 3 to case 7.

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is found for artificial material diluted cases, while obvious difference is noticed for real material diluted cases. Specifically, φ profile in CO2 diluted is significantly improved before r = 15 mm than the other diluted cases, which seems to indicate a longer chemical reaction time scale under MILD combustion. While for H2O diluted case, φ is much lower in the outer space (r > 15 mm). According to Figure 8, OH formation at x = 125 mm in H2O diluted case is much higher than CO2 diluted case, implying an enhanced reaction rate under H2O diluted oxy-fuel combustion. On the contrary, τconventional in H2O diluted case is expected to be reduced. Therefore, the lower φ in the outer space (r > 15 mm) for H2O diluted case is attributed to the reduction of τMILD. Therefore, the Da in the H2O diluted oxyMILD combustion region is enhanced, which is likely to weaken the reaction-controlled status under oxy-MILD combustion regime.

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 27 87545526; E-mail: [email protected]. ORCID

Yaojie Tu: 0000-0002-0591-7411 Notes

The authors declare no competing financial interest.

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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). We would also like to acknowledge SembcorpNUS Corp Lab for the funding support. 3278

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