CO2 Coflow - Energy & Fuels

Article pubs.acs.org/EF

Dimensions of CH4-Jet Flame in Hot O2/CO2 Coflow Zhenfeng Mei,† Jianchun Mi,*,† Feifei Wang,† and Chuguang Zheng‡ †

State Key Laboratory of Turbulence & Complex Systems, College of Engineering, Peking University, Beijing 100871, P. R. China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China

‡

ABSTRACT: The present study has numerically simulated the oxy-fuel combustion of a methane (CH4) jet in hot coflow (JHC). The main objective is to investigate the influences of the oxygen (O2) molar fraction (XO2 * ), temperature (Tcof * ) and velocity (v*cof) of the O2/CO2 coflow on dimensions of the JHC reaction zone or flame. The simulations use the model of eddy dissipation concept (EDC) with the detailed chemical mechanism described by GRI-Mech 3.0. To validate the modeling, several air-fuel JHC flames are predicted under the same conditions of the work of Dally et al. [Proc. Combust. Inst. 2002, 29, 1147− 1154]; the predictions match well with the measurements. Results suggest that, as either XO2 * or vcof * decrease or Tcof * increases, the volume of the JHC reaction zone increases and hence the overall oxidation rate of CH4 decreases. In particular, raising the coflow speed v*cof causes the flame to be significantly thinner but only slightly longer. It is also demonstrated that the oxy-fuel reaction zone is larger, and so, the temperature is lower than the air-fuel counterpart. Besides, under identical conditions, the oxy-fuel combustion produces more carbon monoxide than does the air-fuel combustion. of high-temperature flue gases through a furnace and compared their flame lengths with those of Lille et al.1 These authors investigated the dependence of flame length on various parameters including oxygen concentration and temperature of the oxidizer, fuel temperature, fuel firing rate, and diameter of the fuel nozzle. They found that the chemical flame length increases either as the oxygen content is decreased, the oxidizer temperature is increased, or as the fuel temperature is decreased. Yang et al. also numerically investigated influences of fuel temperature on a single gas jet combustion in highly preheated and oxygen deficient air in a different furnace with different configurations.4 In that furnace, an increase of the fuel inlet temperature results in a smaller flame. More detailed characteristics of the similar flames were reported earlier by Dally et al.5 Through a device generating an open jet flame in hot coflow (JHC), these investigators could well define and control the coflow temperature and O2 concentration. They used the well-developed single-point Raman−Rayleigh-laser-induced fluorescence technique to measure instantaneously and simultaneously temperature and concentration of major species CH4, H2, H2O, CO2, N2, and O2 and minor species NO, CO, and OH. The interaction of turbulence and chemistry in the combustion zone was thus evaluated, providing highly valuable data for modelers of those flames. Following the work of Dally et al.,5 many numerical simulations, e.g., 6−10, have been conducted on the JHC and furnace MILD combustions. Christo and Dally performed a detailed CFD study on the JHC flame.6 These investigators employed the standard k−ε model for turbulence and the eddy dissipation concept (EDC) model with a detailed kinetic scheme (GRI-Mech 3.0)16 for chemistry to describe the turbulence−chemistry interactions. Their numerical results agreed quite well with the measurements.5 Later, Mardani et

1. INTRODUCTION The human use of fossil energy has rapidly increased in past decades. This has resulted in ecological environmental damage, the globe being warmer, and, thus, the climate changing. In response, poor conventional combustion technologies must be replaced by those with high efficiency and low emissions in various industry sectors. For example, oxy-fuel combustion, where air is replaced by pure oxygen (O2) or a mixture of oxygen/recycled flue gas mainly with carbon dioxide (CO2), has been increasingly used in steel and metallurgical industries and tends to be utilized in coal-fired power plants. This technology eliminates nitrogen and thus produces a flue gas mainly with CO2 so that it is ready for capture and sequestration of carbon dioxide. The general benefits of oxyfuel combustion, relative to the traditional air-fuel combustion, are higher thermal efficiency, due to lower exhaust gas volumes, and much lower NOx emissions. Close attention should thus be paid to whether traditional furnaces can be changed to oxy-fuel furnaces with or without changing the furnace size. This has stimulated the present authors to investigate various factors such as the fuel injection or in-furnace flue-gas conditions that influence dimensions of the oxy-fuel diffusion flame. It is significant that oxy-fuel combustion is being coalesced with another newly developed technology called HiTAC (high temperature air combustion) or MILD (moderate and intense low-oxygen dilution) combustion so that high efficiency and low emissions of combustion can be achieved more economically. In recent years, the flame characteristics of nonpremixed MILD combustion of gaseous fuel have been investigated to some extent.1−15 Lille et al. conducted experiments at a single jet flame furnace.1,2 The fuel jet of gasol (>95% of propane) was coaxially injected into hightemperature exhaust gases generated by a gas burner also with gasol. Their flame images show that a reduction of oxygen concentration increases the flame size but decreases its luminosity and visibility. Yang et al.3 reported on their computational results of a single jet flame of LPG in a coflow © 2012 American Chemical Society

Received: January 17, 2012 Revised: May 2, 2012 Published: May 2, 2012 3257

dx.doi.org/10.1021/ef3000938 | Energy Fuels 2012, 26, 3257−3266

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Figure 1. Structure of the JHC burner5 and present computational domain. Dimensions are in millimeters. To validate the RANS modeling, the flames from the above JHC device are at first simulated under the experimental conditions of Dally et al.,5 as given in Table 1 and Figure 2a. Then, to realize the

al. also simulated the JHC flames and found that the EDC model with the detailed mechanism can well predict the combustion characteristics.7 Very recently, Wang et al. have investigated six different global combustion mechanisms, including the four-step mechanism of Jones and Lindstedt,17 the two-step mechanism of Westbrook and Dryer,18 and several modified versions of them, for predicting the major species concentration of CH4 combustion under the MILD condition.8 From a general comparison of these mechanisms, the modified Westbrook and Dryer mechanism (WD4)18 mechanism shows the best agreement with the experiment data.5 Nevertheless all the investigations1−15 are concerning the airfuel combustion only. This stimulates the present study to examine the JHC oxy-fuel combustion by using the well-proved k−ε model and the detailed chemical kinetics (GRI-Mech 3.0) with the EDC solver. Two specific objectives of the work are (1) To investigate effects of the coflow oxygen concentration, temperature, and velocity on the dimensions of the oxy-fuel diffusion flame of a CH4 jet in hot coflow (JHC) (2) To quantify the flame-dimension differences between the oxy-fuel and air-fuel JHC combustions.

Table 1. Inlet Conditions of the Work of Dally et al.5 for CH4/H2 JHC Flames fuel jet (CH4/ H2)

oxidant coflow (mass fraction)

Re

T*(K)

T*cof (K)

Y*O2 (%)

Y*N2 (%)

* YH2O (%)

* YCO2 (%)

v*cof (m/s)

9482 9482 9482

305 305 305

1300 1300 1300

3 6 9

85 82 79

6.5 6.5 6.5

5.5 5.5 5.5

3.2 3.2 3.2

designated objective, the present study uses a modified configuration of the JHC burner which produces a single jet flame surrounded only by coflowing hot flue gases, Figure 2b, i.e., removing the outer cold air stream from the original JHC system. This removal ensures that coflow O2 concentration and temperature remain constant across the whole computational domain. For the fuel jet, the Reynolds number Re ≡ UfD/νf (where Uf is the fuel exit velocity, D is the exit diameter, and νf is the kinematic viscosity of fuel) and the inlet temperature for all the cases of the present study are respectively 9482 and 305 K, i.e., those used by Dally et al.5 Table 2 lists all the 23 cases of present computations. Note that numerical tests are performed for varying X*O2 (then varying X*CO2 or * for the oxy-fuel and air-fuel conditions respectively), Tcof * , and vcof * XN2 in the corresponding columns at the listed values when keeping other parameters constant. 2.2. Computational Models. In the present study, the standard k−ε model19 with the standard wall functions is used as the turbulent model. This model is well-known to have some weaknesses in predicting round jets, see, e.g., 20−21. As suggested in 6, 9, and 20− 21, the coefficient Cε1 in the eddy dissipation equation should be varied from the standard value of 1.44 to the value of 1.6, which has been found to obtain the best agreements with experiments. The present study has therefore taken that Cε1 = 1.6. The discrete ordinate (DO) radiation model22 is employed with weighted sum of gray gas model (WSGGM) for the air-fuel cases and * = 100%) case while a nongray gas model, also the pure oxygen (XO2 similar to 23, is used for the oxy-fuel cases where X*O2 < 100%. The WSGGM is a reasonable compromise between the oversimplified gray

2. COMPUTATIONAL DETAILS 2.1. Configurations of the JHC Flame Systems and CFD Conditions. The numerical model constructed for the present study is based on the geometry and dimensions of the experimental jet in hot coflow (JHC) burner used by Dally et al.5 or the modified system. Figure 1 shows the plan of the JHC structure5 and the region of present simulations for verification of the RANS modeling. The combustor device consists of an insulated and cooled central jet nozzle (inner diameter D = 4.25 mm) and an annulus nozzle (inner diameter = 82 mm) with an internal burner mounted upstream of the perforated plate. The internal burner provides hot combustion products which are mixed with air and nitrogen using two side oxidant inlets to control the temperature and O2 levels in the mixture. The fuel is a mixture of CH4 and H2 equally in volume. In the experiments of Dally et al.,5 the JHC burner was mounted in a wind tunnel which provided a coaxial surrounding air stream at room temperature but with the same velocity as the hot coflow (3.2 m/s). Their mass fraction of O2 in the hot coflow was operated at 3%, 6%, and 9%. 3258


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Figure 2. Computational domains and boundaries of present simulations for (a) experimental air-fuel cases5 and (b) CFD cases of Table 2 (length unit is millimeters). and CO2 are treated as gray-gases.23,25−27 In oxy-fuel conditions, higher concentrations of H2O and CO2 cause different behaviors. So, it is essential to consider a nongray gas model for the oxy-fuel combustion in order to correctly predict the absorption and the emission and subsequently the temperature in the furnace. In the present study, when calculating nongray radiation with the DO radiation model in Fluent 6.3, the radiation spectrum has been divided into 6 wavelength bands, specifying a different constant absorption coefficient for each band; see Table 3. Since the total mass fraction of H2O and CO2 produced by combustion is much smaller than that of CO2 from the coflow, the mixture absorption coefficients * /XCO2 * are approximately estimated for different simulated values of XO2 from the spectral absorptivity of CO2 reported in Figure 1 of Cumber et al.28 and shown in Table 3. The EDC model29 with detailed chemical kinetic mechanisms (GRI-Mech 3.0,16 consisting of 53 species, and a total of 325 reversible reactions) is applied for the modeling of reactions. The specific heat of species (Cp) is set to change with the temperature. The SIMPLE algorithm method is utilized to solve the pressure−velocity coupling. A total of seven transport equations (continuity, axial, and radial momentums, turbulence kinetic energy and its dissipation rate, energy, and radiative intensity) are solved using the commercial CFD package FLUENT6.3. Depending on which combustion model is used, additional transport equations, e.g., species mass fraction, are also solved. The second-order upwind scheme is employed for discretizing the equations in order to improve the accuracy of the simulations. Solution convergence is obtained when (a) the residuals are less than 10−6 for the energy and 10−5 for all the other variables and (b) the variations of the downstream outlet temperature and velocity are allowed to be within 1.0 K and 0.1 m/s, respectively. Due to the symmetry of the system, a geometrically simplified twodimensional (2D) axisymmetric computational model is constructed with the domain of computation of 4500 mm × 800 mm; see Figure 2. Use of the two-dimensional domain is to reduce the computational expense. A similar choice was made by Christo and Dally,6 Mardani et al.,7 and Frassoldati et al.,10 i.e., they all used a 2D axi-symmetric mesh, obtaining the results which agree quite well with the measurements.5 A

Table 2. Coflow Conditions for Different Air-Fuel and OxyFuel Cases of Computation oxidant coflow (volume fraction)

oxy-fuel conditions

varying X*O2

varying T*cof

varying v*cof

air-fuel conditions

varying X*O2

case

T*cof (K)

X*O2 (%)

* XCO2 (%)

X*N2 (%)

v*cof (m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

1300 1300 1300 1300 1300 1300 1300 1300 1300 1700 1600 1500 1400 1300 1300 1300 1300 1300 1300 1300 1300 1300 1300

100 85 70 50 30 21 16 12 8 30 30 30 30 30 30 30 30 30 30 30 85 21 8

0 15 30 50 70 79 84 88 92 70 70 70 70 70 70 70 70 70 70 70 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 15 79 92

3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 8.0 6.4 4.8 3.2 1.6 0.1 3.2 3.2 3.2

gas model and a complete model which takes into account particular absorption bands.24 However, the WSGGM cannot be used to specify the absorption coefficient in each band and so has been found to perform poorly in oxy-fuel conditions, due to the assumption that H2O

Table 3. Absorption Coefficients for Different Values of XO2 * /XCO2 * in Coflow interval wavelength

XO2 * /XCO2 * = 8%/92%

12%/ 88%

16%/ 84%

21%/ 79%

30%/ 70%

50%/ 50%

70%/ 30%

85%/ 15%

0−2.5 μm 2.5−3.0 μm 3.0−4.0 μm 4.0−5.0 μm 5.0−9.0 μm 9.0−20 μm

0 0.79 0 0.82 0 0.75

0 0.75 0 0.78 0 0.72

0 0.72 0 0.74 0 0.68

0 0.68 0 0.70 0 0.64

0 0.60 0 0.62 0 0.57

0 0.43 0 0.44 0 0.41

0 0.26 0 0.27 0 0.24

0 0.13 0 0.13 0 0.12

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Figure 3. Definitions of length and width of the reaction zone (case 5).

Figure 4. Comparisons of the present predictions and the previous experiments5 of (a) the temperature (T) and OH mass fraction (YOH) and (b) mass fraction of O2 and CO (YO2, YCO) at x = 30 mm. Computation: Y *O2 = (− −) 3%; (- - -) 6%; (− - −) 9%. Experiment: (○) 3%; (□) 6%; (▽) 9%. It is important to note that, different from the present study, Yang et al.3 used the contour of 0.99 of the ratio RO = Y*O2/(Y*O2+ Y+O2) (where Y+O2 is the sum of oxygen needed to complete combustion) to define the border of the chemical reaction zone. Certainly their definition cannot identify the flame lift-off. In the region upstream from the lift-off, no reaction at all occurs so that both fuel and oxygen exist and their mixing can produce RO = 0.99, thus generating a false flame border. In the same region, carbon monoxide (CO) is never present, i.e., always RCO = 0. Moreover, the occurrence of RCO = 0.01 is only possible near the lifted reaction zone. That is, the lift-off should be identified by the contour of RCO = 0.01.

primary orthogonal structured mesh about 70 000 cells is used after verifying the grid-independency of the results using a finer grid with 150 000 cells. Results (i.e., temperature, mass fractions of CO) are kept in high consistency for different grids. The minimum and maximum cells are 0.26 mm × 0.26 mm (at the position of the fuel inlet at the axis) and 43.50 mm × 40.10 mm (at the boundary in the downstream away from the axis), respectively. 2.3. Definitions of Length and Width of the Reaction Zone. For methane, the general destruction takes place mainly in the following steps:30

CH4 → CH3 → H 2CO → HCO → CO → CO2

(1)

3. VERIFICATIONS OF THE MODELING To verify the RANS modeling methods firmly, two sets of predictions have been performed for the air-fuel JHC flames under the same conditions as for (1) the measurements of Dally et al.5 firing CH4/H2 and (2) those of Lille et al.1 using LPG as fuel (refer to 1 or 3 for the configurations of the burner system and computational domain, which are somewhat different from those of the JHC system shown in Figures 1 and 2). Here below comparisons are presented between the present predictions and previous experiments.1,5 As seen below, in general, the numerical predictions of the air−fuel JHC flame characteristics are consistent with the previous measurements.1,5 This suggests that the RANS modeling used presently can approximately capture the features of the JHC diffusion flame. 3.1. Present Predictions versus Previous Measurements of Dally et al.5 for CH4/H2 Flames. See Table 1 for the experimental conditions of Dally et al.5 Figure 4 shows the

Here carbon monoxide (CO) may be regarded as the last intermediate product. It hence makes sense that the contour of the near-zero ratio of the local CO molar fraction (XCO) to its maximum over the whole computation domain (XCOmax), i.e.,

R CO =

XCO XCOmax

(2)

can approximately represent the border of the reaction zone. In the present study, a contour of RCO = 0.01 or 1% of the maximum of XCO has been used as the reaction zone border which is approximately elliptic in shape and whose length (L) and width (W) are defined in Figure 3 using the data of case 5. Here the length of the reaction zone (L) contains the lift-off distance of the flame for convenience. Note also that the rotation volume of the ellipse, i.e.,

V=

π LW 2 6

(3)

is regarded approximately as the reaction zone volume. 3260


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characteristics. Figure 6 compares the resulting lengths of the reaction zone for X*O2 = 11%, 12.8%, and 16.8% obtained by the

radial profiles of the temperatures (T) and species mass fractions (YOH, YCO, YO2) obtained at the downstream location of x = 30 mm from the nozzle exit. Overall, the radial profiles of T and YOH are predicted quite satisfactorily in the reaction zone (Figure 4a). For instance, the maximal difference between the computational and experimental peak temperatures is found to be smaller than 1.5% (about 20 K). The predicted mass fractions YCO and YO2 appear to agree reasonably well with the measurements5 at r/D < 4, but the predictions are less satisfactory at r/D > 5, perhaps due mainly to the experimental problem indicated by Dally et al.5 The identical hump in the experimental YCO for all the three cases, which appears in the oxidant stream at r/D = 5−10, might be a product of cooling and extinction of the secondary flame near the burner outer wall.5 These authors believed that the maximum of YCO ≈ 0.0025 is fairly low and so that such level would have minor effects on the reaction zone. Moreover, Figure 5 compares the

Figure 6. Comparisons of predicted (present and 3) and measured 1 flame lengths of firing LPG under the same conditions of the work of Lille et al.1

three studies. Note that the present simulations for this comparison use the same combustor system and computational domain as those of Yang et al.3 (not shown here). Yang et al.3 used the global chemical mechanism, i.e., the following threestep reaction C3H8 + 1.5O2 → 3CO + 4H2 CO + 0.5O2 → CO2 Figure 5. Comparisons of the present predictions (curves) and the previous experiments (dots)5 of the mixture fraction for Y*O2 = 3% obtained (a) on the axis and (b) at x [mm] = 30, 60, and 120.

H 2 + 0.5O2 → H 2O which is different from the present simulations of the LPG flames using the detailed chemistry mechanism by GRI-Mech 3.0. Figure 6 shows that both predictions have overestimated the flame length. However it is also evident that the present predictions agree with the measurements1 significantly better than those of Yang et al.3 For instance, the present value is 6− 9%, while that of Yang et al.3 is 12−21%, higher than the experimental value.1 This suggests that GRI-Mech 3.0 performs better than the global chemical mechanism for the modeling of LPG flames although it was designed for natural gas (mainly methane) combustion.

present predictions (curves) and the previous measurements * = 3%, which are (dots)5 of the mixture fraction (ξ) for YO2 estimated from Bilger’s formula,31 along the jet axis (Figure 5a) and the radial direction at x = 30, 60, and 120 mm (Figure 5b). Obviously, the predictions agree quite well with the measurements. Such good agreements once again provide a solid support for the present modeling of the JHC flames. 3.2. Present Predictions versus Previous Measurements of Lille et al.1 for LPG Flames. Table 4 lists the experimental conditions of Lille et al.1 for firing LPG, under which Yang et al.3 and the present authors predict the flame

4. NUMERICAL RESULTS AND DISCUSSION 4.1. Effect of Outer Cold Air on the JHC Flame Structure. Present temperature contours (distributions) of the JHC flame modeled with the presence of the outer cold air stream and that entirely with hot coflow are shown in Figure 7a and b, respectively. As expected, the temperature distributions for the two cases are very different, even in the mixing layer of the central fuel jet, in the region at x > 30 mm. In the case with outer cold air coflow, quite similar to the work of Dally et al.5 for the mass fraction YO2 * = 3% (see Figure 2 and Table 1), the temperature in the mixing layer becomes significantly higher than T*cof downstream from x = 100 mm, due to more oxygen entrained into the hot coflow. By comparison, the temperature is only slightly higher in the mixing layer of the jet without cold air. Obviously the outer cold air has a great effect on the combustion in the region at x > 30 mm. Figure 7c provides quantitative comparisons for x = 30, 100, and 400 mm. The results reported in next sections are about the effects of coflow

Table 4. Experimental Conditions of Lille et al.1 for Firing LPG fuel jet (LPG)

oxidant coflow

species (volume fraction) inner diameter of the nozzle temperature velocity species (volume fraction)

inner diameter of the nozzle temperature velocity

C3H8 94.5%, C2H6 1.1%, N2 4.4% 0.5 mm 299 K 25.5 m/s O2 11.0%, N2 89.0% O2 12.8%, N2 87.2% O2 16.8%, N2 83.2% 300 mm 1173 K 0.98 m/s 3261


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Figure 7. Predicted temperature distributions of the JHC flame at Y*O2 = 3%: (a) with outer cold air stream; (b) no outer cold air stream; and (c) radial temperature profiles in the two cases [points for a and curves for b] at x [mm] = 30, 100, and 400.

Figure 8. Temperature and CO volume fraction distributions for different XO2 * (cases 1, 6, and 9).

also evident that the lift-off distance increases as X*O2 is decreased. These observations are consistent with the visual observations by Lille et al.1,2 for the air-fuel cases. The lift-off phenomenon, however, could not be reflected in the predicted flame border of Yang et al.3 who used the “oxidation mixture ratio” RO = 0.99 to define the border of the chemical reaction zone. Certainly their definition cannot identify the flame lift-off because both fuel and oxygen exist in the region upstream from the lift-off and their mixing can produce RO = 0.99, although no reaction at all occurs there (so that always RCO = 0). Figure 9 shows the length (L) and width (W), defined in Figure 3, of the reaction zone for different X*O2. Together, on the plot, the estimation of the reaction-zone volume from eq 3 is also presented. Naturally, a reduction in oxygen concentration results in the length and width of the reaction zone and therefore the flame volume to increase; e.g., quantitatively, when X*O2 is varied from 100% to 8%, L increases by 280%, W by 151%, and V by 3160%. Quite significantly, all the data of the flame volume can be fitted very well by

O2 concentration, temperature, and velocity obtained when the outer cold air is removed. 4.2. Effect of XO2 * on the Oxy-Fuel JHC Reaction Zone Size. Figure 8a and b display, respectively, the temperature (T) and CO volume fraction (XCO) distributions over the whole computational range for X*O2 = 8%, 21%, and 100%. It is clearly demonstrated that both T and XCO in the reaction zone decrease dramatically as X*O2 is reduced. More specifically, the reduction of XO2 * from 100% to 8% causes the maximal * , where Tmax is the temperature rise ΔTmax (= Tmax − Tcof temperature maximum in the whole computational domain) to drop from ΔTmax = 1800 K to ΔTmax = 450 K and simultaneously the peak value of XCO to fall from XCOmax = 0.53 to XCOmax = 0.04. Note that the magnitude of ΔTmax or XCOmax reflects the difference in T or XCO; namely, if T and XCO vary significantly across the reaction zone, ΔTmax and XCOmax should be high. Figure 8 hence suggests that the temperature and CO concentration distributions have a tendency to be more uniform as X*O2 is decreased. Moreover, to visualize the flame border and dimensions, the contour of RCO = 0.01 is superposed on the contours of T and XCO in Figures 8a and b. As demonstrated, although the reaction zone is roughly similar in shape for different XO2 * , the * . It is flame significantly increases in size with decreasing XO2

V=

1 * )−1.3 (XO2 250

(4)

This may be explained as follows. The global reaction mechanism for methane is 3262


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Figure 9. Length (L), width (W), and volume (V) of the reaction zone versus X*O2 (cases 1−9).

CH4 + 2O2 → CO2 + 2H 2O

Figure 11. Length (L), width (W), and volume (V) of the reaction zone versus T*cof (cases 10−14).

(5) 32

The corresponding reaction rate can be expressed as * ] d[XCH4 * ]0.2 [XO2 * ]1.3 R≡ ≈ −k G[XCH4 dt

Evidently, as T*cof is increased, all the reaction zone dimensions increase. The main reasons behind the above differences are given here. First, specific heats of the flue gases such as CO2, O2, and H2O all increase with increasing Tcof * so that the same amount of heat releasing from combustion causes a less rise in temperature when T*cof is higher. Second, when the coflow is preheated to higher temperatures, its overall density is smaller and thus the fuel jet becomes heavier, entraining (and then mixing with) less O2 mass from the coflow (e.g., see ref 33). As a result, the oxidation process slows down. It turns out that, when using higher coflow temperature, the mean temperature distribution of the oxy-fuel JHC flame tends to be more uniform whereas the CO concentration becomes less uniform. 4.4. Effect of v*cof on the Oxy-Fuel JHC Reaction Zone Size. Figure 12 shows distributions of the mean temperature and CO concentration in the whole calculation domain for v*cof [m/s] = 0.1, 3.2, and 8.0. Also displayed is the reaction zone border represented by the contour of RCO = 0.01 (dashed lines). Apparently, when vcof * is high enough, the flame lifts off and the lift-off distance increases with vcof * . This effect of vcof * is more or less similar to that of the fuel-jet velocity (Uf). However, there is an important distinction between the influences of the two velocities: namely, the flame becomes * is increased, which does not happen with varying thinner as vcof Uf. Figure 12 also appears to suggest that the flame length

(6)

where the prefactor kG depends on the reactant temperature. Equation 6 indicates that R ∝ (X*O2)1.3. Since the flame volume is inversely proportional to the reaction rate, i.e., V ∝ R −1, it is thus obtained that V ∝ (XO2 * )−1.3. This agrees with eq 4 and hence is in support for the best-fit in Figure 9. On the other hand, it offers a new endorsement for the present modeling, in addition to section 3 for the experimental verifications. 4.3. Effect of Tcof * on the Oxy-Fuel JHC Reaction Zone Size. For this investigation, the coflow temperature T*cof is varied from 1300 to 1700 K with X*O2 = 30% and v*cof = 3.2 m/s. Figure 10 shows contours of the mean temperature and CO volume fraction in the whole calculation domain for Tcof * = 1300, 1500, and 1700 K, together with the reaction zone borders represented by the contour of RCO = 0.01. Very significantly, the length and volume of the reaction zone grow with increasing Tcof * . It is also obtained that, when increasing T*cof from 1300 to 1700 K, the maximal temperature rise ΔTmax drops from 1170 to 870 K and the maximal CO molar fraction grows from 0.25 to 0.31. Moreover, the quantitative dependences of the reaction zone’s length, width, and volume (L, W, V) on the coflow temperature T*cof are shown in Figure 11.

Figure 10. Temperature and CO volume fraction distribution for different Tcof * (cases 10, 12, and 14). 3263


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Figure 12. Temperature and CO volume fraction distribution for different vcof * (cases 15, 18, and 20).

remains nearly constant with vcof * if it is measured from the liftoff height. Figure 13 illustrates the dependences on vcof * of the flame length (L), width (W), and volume (V), defined in Figure 3.

for these combustions occurring in any type of combustor. This gap for the JHC flames is filled in Figure 14 and Table 5. Figure 14 shows the temperature and CO concentration distributions of the oxy-fuel and air-fuel JHC flames for varying coflow oxygen at XO2 * = 8%, 21%, and 85% (see Table 2 for other parameters). Visually from the figure, the differences in T, XCO, and V between the two cases all widen as X*O2 is decreased. Quantitatively, these differences in Tmax, XCOmax, and V are shown in Table 5. At X*O2 = 8%, the oxy-fuel flame volume is almost twice as the air-fuel one, suggesting slower overall reactions and more uniform temperature distribution in the former case. In a more general sense, when switching from airfuel condition to oxy-fuel condition, the flame always increases in dimension and lifts off more distantly, irrespective of the magnitude of X*O2. The above differences can be explained as follows. In the oxyfuel condition, oxygen is diluted mainly by CO2, while in the air-fuel case, it is diluted by N2. For the same amount of O2, the CO2 concentration is much higher in the oxy-fuel case than in the air-fuel combustion. Consequently, the backward reaction of the global one-step mechanism (CH4 + 2O2 ⇌ CO2 + 2H2O + heat) is enhanced or the forward oxidation is suppressed, thus releasing heat less rapidly, under the oxy-fuel condition. In other words, the entire oxidation process slows down and heat releases more gradually when changing from the air-fuel case to oxy-fuel case. Moreover, since the specific heat of CO2 is greater than that of N2, more heat is thus needed to heat up the flue gases to the same temperature in the oxy-fuel case than in the air-fuel case. Further, gases of triatomic molecules (e.g., CO2 and H2O) generally have much higher emissivity than those of diatomic molecules (e.g., N2, CO, and O2). Andersson et al. reported that the radiation intensity increases noticeably in the oxy-fuel tests compared to that under the air-fuel condition.34 In addition, Glarborg et al. argued that the presence of high CO2 concentration will compete with O2 for atomic hydrogen and lead to the formations of CO through the reaction CO2 + H ⇌ CO + OH. High CO2 concentration therefore has significant effects on combustion reactions.35 Overall, at the same oxygen level, a higher CO2 concentration in the oxy-fuel combustion leads to lower reaction rates and lower flame temperature but higher CO concentration.

Figure 13. Variations with v*cof of the flame’s length, width, and volume (cases 15−20).

Obviously, the variation of W is greater than that of L (even though calculating from the nozzle exit). It is also shown that the flame volume decreases, or the reaction rate increases, with raising vcof * . That is, overall, an increase in vcof * enhances the oxidation process. However, it is interesting to note that the maximal temperature rise and CO concentration (ΔTmax, XCOmax) remain almost unchanged for different vcof * ; e.g., ΔTmax = 1167.5 and 1165.7 K, whereas XCOmax = 0.254 and 0.248, for v*cof = 0.1 and 8 m/s. These observations may be understood as reasoned here. When v*cof is increased from v*cof = 0.1 to 8 m/s, the streamwise oxygen mass flux increases almost by 80 times, compared with much less reduction in the oxygen mass flux entrained into the mixing layer due to the decrease of Uf − v*cof. As a result, more O2 enters into the reaction zone and the same amount of fuel is consumed in a smaller reaction zone. Thus, the reaction zone size decreases as vcof * is increased. 4.5. Comparison between the Oxy-Fuel and Air-Fuel JHC Combustions. The oxy-fuel combustion must be quite different from the air-fuel counterpart. However, to our best knowledge, no detailed comparisons have been available so far 3264


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Figure 14. Contours of (a) the mean temperature and (b) CO concentration under the oxy-fuel (upper) (cases 2, 6, and 9) and air-fuel (lower) conditions (cases 21−23). Dashed curves correspond to RCO = 0.01.

acceleration of the oxidation by speeding up the coflow. Nevertheless, the maxima of the mean temperature and carbon monoxide concentration remain virtually unchanged for different coflow velocities. In addition, an increase in the coflow velocity also lifts the JHC flame more off the nozzle exit. (4) When switching from the air-fuel to oxy-fuel combustion, the flame increases in dimension and may lift off more as long as the coflow is not a pure oxygen stream. In other words, at the same oxygen level, the oxy-fuel combustion has lower reaction rates (thus lower flame temperature) but higher carbon monoxide concentration than does the air-fuel combustion.

Table 5. Comparison between the Oxy- and Air-Fuel Combustions XO2 *

combustion

Tmax (K)

XCOmax

Voxy/Vair

85%

oxy-fuel air-fuel oxy-fuel air-fuel oxy-fuel air-fuel

3003 3082 2234 2634 1645 2044

0.486 0.477 0.164 0.127 0.037 0.029

1.1

21% 8%

1.3 1.6

5. CONCLUSIONS The present study has numerically investigated the effects of the coflow oxygen level, temperature, and velocity on the configuration of the oxy-fuel JHC flame of firing CH4. The EDC model with detailed chemical mechanism (GRI-Mech 3.0) was implemented for all simulations. Under the air-fuel condition, the computational results agree well with the measurements of Dally et al.5 for the JHC CH4/H2 flame and Lille et al.1 for the LPG flame. On the basis of the analysis of the modeling results, several conclusions can be made below on the characteristics of the oxy-fuel JHC flame: (1) As the coflow oxygen level is lowered, the flame expands in both length and width, correspondingly the maximal mean temperature and molar fraction of CO decrease, so that the thermal field generally becomes more uniform. The reduction of oxygen also increases the flame lift-off distance. (2) As the coflow temperature is increased, the global flame volume increases and hence the mean temperature becomes more uniform whereas the CO concentration level rises. It is thus deduced that the JHC flame may develop into the MILD regime by increasing the coflow temperature no matter how high the oxygen fraction is. This deduction, if correct, should also apply for the furnace combustion. (3) As the coflow velocity is elevated, the flame becomes thinner while its length changes little, consequently reducing the volume moderately. This reflects a slight

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-10-62767074. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is gratefully supported by the Specific Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110001130014), National Natural Science Foundation of China (through Grant Nos. 10921202 and 11072005), and Major State Basic Research Development Program of China (973 Program) (No. 2011CB707301). We also acknowledge the support from Foundation of State Key Laboratory of Coal Combustion of China (No. FSKLCC1101). Here, we thank all the referees for providing insightful comments on the manuscript, the addressing of which has strengthened our paper significantly.

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REFERENCES

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