Article pubs.acs.org/EF
OH* Chemiluminescence Characteristics and Structures of the Impinging Reaction Region in Opposed Impinging Diffusion Flames Xudong Song, Qinghua Guo,* Chonghe Hu, Yan Gong, and Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of the Ministry of Education, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: OH* is an important indicator of the generation of the flame reaction region. According to the OH* characteristic emissions obtained by high-spatial-resolution UV imaging and fiber optics, the effects of the velocity, the distance between the two nozzles (L), and the O/C equivalence ratio ([O/C]e) on the flame chemiluminescence characteristics and structures in opposed impinging regions are discussed herein. The flame impinging region includes an upward stream, a downward stream, and the impinging core region. The experimental results show that the reaction area in the impinging region increases until becoming steady as the velocity increases. [O/C]e can influence the OH* distributions in the upward and downward streams of the impinging region. Under fuel-rich conditions, the reaction area in the downward stream varies greatly, whereas the reaction areas in the upward and downward streams are the same under fuel-lean conditions. The ratio of L to the equivalent diameter D has the most complicated influence on the formation of the reaction area in the impinging region. Under fuel-rich conditions, the size of the reaction area in the impinging region increases as L/D increases, but the opposite relationship exists under fuel-lean conditions. From the OH* radical distribution, the reaction area in the impinging region can be divided into four types: (I) no reaction area, where no reactions in the impinging region; (II) upward-stream reaction area, where reactions occur only in the upward stream region; (III) two-stream reaction area, where reactions occur in both the upward and downward stream regions; and (IV) impinging reaction area, where reactions occur in the whole impinging region and a clear center appears in the impinging core region. The OH* radical distribution reflects the heights of the upward and downward streams, and the height of the upward stream indicates whether reactions exist in the impinging region. Moreover, the height of the upward stream in flames was predicted according to the values of [O/C]e, the flame Froude number (Fr), and L/D. Marchese et al.12 suggested that the OH* distribution could characterize the flame-front position because of the corresponding relationship between the maximum OH* emission and the maximum flame temperature. Song et al.13 analyzed the differences between normal diffusion flames and inverse diffusion flames by measuring the OH* and CH* chemiluminescence in two dimensions. Turbulent flames are widely applied in the industry. Compared with laminar flames, turbulent flames are less stable.14,15 It is difficult to determine the structure of turbulent flames because of their strong fluctuations and eddies. Nada et al.16 reported that three-dimensional flame structures caused by strong fine-scale eddies in turbulence appeared even in the laminar flamelet regime in turbulent combustion diagrams. Ikeda et al.17 studied the reaction-zone and flame-front structures by measuring the local chemiluminescence of OH* and CH* at the flame front of premixed, turbulent propane flames with Cassegrain optics. This article, focused on studies of turbulent flames, is basically the continuation of a previous work by Zhang et al.18 In that work, Zhang et al. explored the effects of velocity and the O/C equivalence ratio ([O/C]e) on the generation of the impinging reaction core area for opposed impinging laminar diffusion flames under large-separation conditions (L/D > 12, where L is the distance between the two nozzles and D is the equivalent
1. INTRODUCTION Impinging flames have many advantages, including heat transfer and mixing, and have been widely used in industrial processes, such as thse performed in coal gasifiers and boilers.1 The opposed multiburner (OMB) gasifier has been widely applied in China, and impinging flames are used in OMB gasifiers. The effective monitoring of flames is the key to achieving high energy efficiency and optimal technology. Common monitoring methods include control of the feed, measurement of the thermocouple temperature, and analysis of the fuel gas.2 However, the actual state of flames cannot be described accurately. Methods for achieving accurate results on the flame in a timely manner are a hot topic. As a rapid and nondestructive technology, flame visualization provides another way to capture information directly from flames.3 Research on flame visualization based on flame spectral diagnosis has been vigorously developed.4 Three phenomena are known to cause flames to emit radiation: blackbody spectrum produced by solid bodies, rotation-emission bands of hightemperature gas molecules (e.g., H 2 O and CO 2 ), and chemiluminescence of excited species.5 Detailed two-dimensional spatial distributions of radical radiation can directly characterize the shape and structure of flames.6−8 Zimmer et al.9 compared the flame structures at various equivalence ratios using OH* chemiluminescent emission maps. Oh and Noh10 analyzed the lengths and structures of nonpremixed oxy-methane flames using OH*, CH*, and C2* images. Ballester et al.11 used OH*, CH*, and C2* chemiluminescence signals as inputs to assess their applicability for optimization strategies in control tests. © 2016 American Chemical Society
Received: November 17, 2015 Revised: January 12, 2016 Published: January 12, 2016 1428
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Figure 1. Schematic diagram of the experimental setup.
Table 1. Experimental Conditions 1
a
2
3
L/D vCH4 (L/min)
a 0.5
4
5
1
uCH4 (m/s)
15.6
31.2
6
7
8
9
10
11
12
1.5
2
2.5
2.5
2.5
2.5
2.5
2.5
0.5
1
46.8
62.4
78.3
78.3
78.3
78.3
78.3
78.3
15.6
31.2
13
14
15
1.5
2
2.5
46.8
62.4
78.3
vO2 (L/min)
0.6
1.2
1.8
2.4
3.0
3.5
4
4.5
5
5.5
1.2
2.4
3.6
4.8
6.0
uO2 (m/s)
1.85
3.69
5.55
7.40
9.25
10.75
12.3
13.85
15.35
16.85
3.69
7.38
11.07
14.76
18.45
[O/C]e
0.60
0.60
0.60
0.60
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.20
1.20
1.20
1.20
L/D = 32, 40, and 48 for each set of conditions.
Table 2. Optical Bench and Settings for the Spectrometer grating device number
slit (μm)
grooves
blaze
wavelength range (nm)
optical resolution (nm)
integration time (ms)
1 2
5 5
1200 1200
300 nm holographic
200−420 400−605
0.16 0.15
500 500
from the environment. The O/C equivalence ratio ([O/C]e) is defined as
nozzle diameter). However, Zhang et al. focused on the generation of the reaction area in the impinging region. The emphasis in the present work is the application of OH* chemiluminescence in opposed impinging diffusion flames to analyze the reactions in the impinging diffusion flames. The specific objective is to develop an approach to explore the effects of velocity, length-to-diameter ratio (L/D), and O/C equivalence ratio ([O/C]e) on the generation of the impinging reaction core area based on opposed impinging diffusion flames.
[O/C]e = [O/C]a /[O/C]s
(1)
where [O/C]a is the actual O/C mole ratio and [O/C]s is the stoichiometric O/C mole ratio. The equivalence ratio is adjusted by the O2 flow rate. Table 1 lists the experimental conditions. Large-separation conditions (L/D > 12) were used19 in this work, where L is the distance between the two nozzles and D is the equivalent nozzle diameter. D can be calculated according to the equation
2. EXPERIMENTAL APPARATUS 2.1. Diagnostic System and Operating Conditions. The spectral diagnostics experimental platform for jet diffusion flames is shown in Figure 1 and consists of three parts: a flame generation system, a flame imaging system, and an OH* detection system. The flame image is sampled by a digital camera (Nikon D7100), and detailed information on OH* is collected by a UV imaging system and a grating spectrometer. The counterflow nozzles used for this work are the same as those described by Zhang et al.18 A three-channel nozzle was used to supply the fuel, oxidizer, and nitrogen. In the flame generation system, two concentric curtains of nitrogen isolated the flames
D = 2(mc + ma )/[πρ ̅ (Gc + Ga)]1/2
(2)
where m is the gas mass flow rate, ρ̅ is the average density of CH4 and O2, and G is the momentum flux. The subscripts c and a refer to the central and annular channels, respectively. 2.2. OH* Measurement Approach. In this study, OH* emission was investigated using a UV imaging system (Isuzu Optics) and a grating spectrometer (QE6500, Ocean Optics Inc.), as shown in Figure 1. The optical bench and settings for the spectrometer are listed in Table 2. The OH* signal obtained by the grating spectrometer has the highest intensity in the UV 1429
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Energy & Fuels region, and the most intense emission occurs at 309 nm,20 as shown in Figure 2. The spectrometer was calibrated at the Shanghai Institute of Measurement and Testing Technology (Shanghai, China), which provided the standard emission wavelength.
value.22 To provide more accurate data, the OH* chemiluminescence detection system should be calibrated at different spectral bands. This calibration was performed using an integrating sphere light source and a spectrometer (QE6500, Ocean Optics Inc.). The calibrated spectrometer and the UV imaging system were used to measure the same light source at the same position. The calibration factor was determined as the ratio of the value measured by the spectrometer to the value measured by the imaging system.
3. RESULTS AND DISCUSSION 3.1. Factors Influencing the Impinging Reaction Region. 3.1.1. Velocity. Previous research18 showed that an increase in velocity does not strengthen the reactions in the laminar flow impinging region, whereas an increase in velocity results in more CH4 and O2 in the impinging region. When the flow changes from laminar to turbulent, there are more reactions in the impinging region, which changes the distribution of OH*. Figure 3 shows the image characteristics and OH*distributions of the impinging region at L/D = 32 and [O/C]e = 0.6 (fuel-rich condition). From the visual analysis, the flame color changes from yellow to blue, and the luminescence area expands with an increase in velocity in the impinging region. Normally, yellow light is emitted by the precipitation of carbon. At low gas velocities, only the decomposition reaction of CH4 occurs because of the hypoxic conditions in the impinging region. When the velocity increases, oxidation of CH4 leads to the appearance of blue light. Comparison of the OH* distribution to the flame image indicates that the OH* distribution corresponds to the blue light distribution,5 so OH* can be treated as an indicator of whether the combustion reaction exists in the impinging region. According to the OH* distribution, the impinging region can be divided into the upward stream, downward stream, and impinging core regions. At high velocity, more OH* exists in the upward stream of the impinging region, and less exists in the downward stream under fuel-rich conditions. In the impinging region, high velocity enhances the impacting effect and promotes heat transfer and mixing of O2 and CH4. Because of the heat and the low [O/C]e value, an evident buoyancy effect causes more O2 and CH4 to flow into the upward stream. Consequently, more OH* exists in the upward stream. Figure 4 shows the image characteristics and OH* distributions of the impinging region at L/D = 32 and [O/C]e = 1.2 (fuel-lean conditions). In the impinging region, carbon precipitates and is oxidized rapidly, so less yellow light exists. The
Figure 2. UV spectrum of the CH4/O2 diffusion flame.
The UV system contained three parts: a UV CCD camera (EX-3011B), a UV lens, and a 310-nm bandpass filter. The emission information was recorded by the UV CCD camera equipped with a cooling system to reduce the dark current. The UV lens, with a focal length of 50 mm and an aperture of 3.5, was used to focus the luminescence of the flame. OH* chemiluminescence was then extracted through a 10-nm-wide bandpass filter centered at 310 nm. The full width at halfmaximum of the filter was 10 nm. The intensity at each point was determined by integration of the intensity within the wavelength range of 300−320 nm. The signal was integrated over the line of sight, and the exposure time was 3000 ms to obtain more accurate results. The image spatial resolution was 0.255 mm in both directions.. High spatial resolution can provide good resolution of the flame boundaries. The chemiluminescence signals were superimposed on the background emission. As shown in Figure 2, the background emission (mainly nonlinear background associated with CO2* emission) was so weak in the UV region for the gas flame that it could be neglected.21 The experimental data presented in previous studies were mainly focused on the OH* relative intensity, not on the absolute
Figure 3. Images and two-dimensional OH* distributions at different velocities ([O/C]e = 0.60, L/D = 32). 1430
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Figure 4. Images and two-dimensional OH* distributions at different velocities ([O/C]e = 1.20, L/D = 32).
Figure 5. OH* distributions at L/D = 32 and vCH4 = 78.3 m/s.
Figure 6. OH* distributions at vCH4 = 78.3 m/s.
under fuel-rich conditions, the flame has an upward tendency, and OH* mainly exists in the upward stream because of the buoyancy effect in the impinging region. 3.1.2. [O/C]e. Figure 5 shows the OH* distributions under turbulent conditions. Under fuel-rich conditions, OH* exists in the impinging region. With increasing [O/C]e value, the impinging core region is generated and then becomes larger. Finally, the downward stream appears. The area of the impinging region increases with the increase in [O/C]e value and then decreases under fuel-lean conditions. With the increase of O2, the position of the reaction center where the OH* peak exists in the impinging region also changes. There is no OH* peak at the impinging center under fuel-rich conditions, and more reactions occur in the upward stream. However, under fuel-lean conditions, the distribution of OH* in the upward and downward streams is symmetrical. With the increase in [O/ C]e value, the effect of buoyancy becomes weak, so more OH* exists in the impinging core region.
size of the impinging region increases with increasing velocity. According to the OH* distribution, the upward stream and downward stream are symmetrical, and a clear reaction center occurs in the impinging core region where the OH* intensity peak appears. The main factors affecting the distribution of the upward stream and downward stream are momentum and buoyancy. The influence of buoyancy on the structure and instability of single-nozzle flames has been discussed by Azzoni et al.,23 Kim et al.,24 and Tang et al.25 In the impinging region of the flame, the change in structure and instability are similar to those in a single-nozzle flame. Under fuel-rich conditions, the length of the flame increases because of buoyancy. Under fuel-lean conditions, the buoyancy has little influence, and the mixture of CH4 and O2 becomes uniform at high velocity. Therefore, the distributions of the upward and downward streams are similar. Under fuel-rich conditions, OH* mainly exists in the upward stream, whereas under fuel-lean conditions, OH* distributes in the upward and downward streams symmetrically. Under fuellean conditions, there is sufficient O2 reacting with CH4, whereas 1431
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Figure 7. OH* axial distributions at different velocities.
Figure 8. OH* radial distributions at different velocities in the impinging region.
3.1.3. L/D Ratio. Figure 6 shows the OH* distribution at different L/D values in turbulent diffusion flames. L/D influences the generation of the impinging region. Under fuel-rich conditions, the OH* intensity in the impinging region is high, and the intensity near the nozzle is low for L/D < 40. Because of the low L/D value, the velocity attenuation is slow before the impinging occurs. The effect of impinging changes the structure of the flame and influences the whole reaction region. For L/D ≥ 48, the reaction region of impinging flames can be divided into two parts. One is the impinging region, where the reactions are centralized in the impinging center. The other is the core jet region, where the reactions exist centrally near the nozzle. Under fuel-lean conditions, more fuel reacts in the impinging area, so the reactions concentrate in the impinging region. Meanwhile, the size of the reaction region decreases with increasing L/D value under fuel-lean conditions, which is different from the behavior under fuel-rich conditions. According to the preceding analysis, the reaction area in the impinging region increases until becoming steady with the increase of the velocity. The [O/C]e value influences the OH* distribution in the upward and downward streams of the impinging region. Under fuel-rich conditions, the downward stream is smaller than the upward stream, whereas the upward and downward streams have the same shape under fuel-lean
conditions. L/D has the most complicated influence on the size of the impinging region. Under large-separation conditions (L/D > 12 ), some fuel and oxygen are present in the impinging region. In fuel-rich flames, with an increase in L/D, less reaction occurs in the impinging region, and the instability becomes higher, so the size of the impinging region increases with L/D. In contrast, in fuel-lean flames, more reactions occur in the impinging region, nearly no fuel exists, and the instability is small, so the size of the impinging region decreases with L/D. 3.2. OH* Distribution in the Impinging Reaction Region. Figure 7 shows the OH* axial distributions for different velocities. The OH* axial distributions under fuel-rich and fuellean conditions for different velocities have different features. When the flow changes from laminar to turbulent, the OH* intensity under fuel-rich conditions increases, but no apparent reaction center occurs until the velocity reaches 78.3 m/s at the impinging center. The OH* peak intensity exists near the nozzle exit, and with increasing velocity, the peak position moves to the impinging center. When the velocity increases to a certain value, the two peaks combine into a single peak. The high velocity causes more fuel and oxygen to flow into the impinging region, so more reactions occur. Under fuel-lean conditions, a ternary peak appears in the OH* axial distribution in laminar flow. With increasing velocity, the ternary peak also becomes a single peak 1432
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Figure 9. OH* axial distributions at different L/D values for vCH4 = 78.3 m/s.
Figure 10. OH* radial distributions at different L/D values for vCH4 = 78.3 m/s.
turbulent flow. Along the flame propagation direction, the OH* intensity increases toward the impinging region. With an increase in [O/C]e value, a clear reaction center forms, and the intensity of the reaction center increases. Around the impinging center, there is a fluctuation of the position where the OH* peak exists in the impinging region. The peak position is approximately ±5 mm from the impinging center. The stagnation point has the characteristics of migration in the opposed flame,19 which is mainly caused by the impaction of the flames and changes the OH* peak position. For L/D = 48, OH* presents a ternary peak distribution, and the reaction region of the flame can be divided into two parts according to the axial distribution: the jet core reaction region, where a clear OH* peak exists near the nozzle, and the impinging reaction region, where an OH* peak exists in the impinging region. Because of the impingement effects, the OH* intensity changes at different [O/C]e values in the impinging reaction region and has no regularity. Figure 10 shows the OH* radial distributions for different L/D values at vCH4 = 78.3 m/s. The OH* radical distributions show the different performances of the reaction in the impinging region. For L/D = 32, the upward and downward streams at different [O/C]e values change significantly. With increasing [O/C]e value, the length of the upward stream decreases,
with high intensity. The high velocity results in more fuel and oxygen in the impinging zone, which benefits the reaction in the impinging region. Figure 8 shows the OH* radial distributions for different velocities in the impinging region. The OH* radial distributions in the impinging region are different at various velocities. Compared with low velocity, high velocity concentrates more CH4 and O2 in the impinging region. For [O/C]e = 0.6, more reactions occur in the upward stream, where a clear reaction center forms. However, the reactions in the downward stream are weak, and the intensity increases with velocity. For [O/C]e = 1.2, the OH* distribution is symmetric because the reactions in the upward and downward streams are similar. A clear reaction center is formed at the impinging center under fuel-lean conditions. The position where the reaction center forms is mainly influenced by the [O/C]e value rather than the velocity. For fuel-rich conditions, with increasing velocity, more reactions occur in the impinging region, whereas for fuel-lean conditions, the peak intensity of OH* becomes irregular, but the distribution area becomes larger with the velocity. The changes in OH* peak intensity in the impinging region are not comprehensive. Figure 9 shows the OH* axial distributions for different L/D values at vCH4 = 78.25 m/s. For L/D = 32, a single peak exists in 1433
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the combustion adiabatic temperature ΔTf, and the ambient temperature T∞.26 The influence factors of the generation of the upward stream are associated with the factors of single jet diffusion flames, and the distance between the two nozzles (L) is an additional factor. The total flame length can be expressed as
whereas the length of downward stream first increases and then decreases. Compared with L/D = 32, the upward stream exhibits few changes, but there is a clear downward stream under fuel-rich conditions for L/D = 48. The OH* intensity at L/D = 48 is weaker than that at L/D = 32. The high velocity concentrates more CH4 and O2 in the impinging region when L/D is too small. More reactions occur in the impinging region, and the high heat quantity causes more gas to flow into the upward stream, which generates more OH* in the upward stream. The OH* intensity indicates the generation of the reaction area in the impinging region. As shown in Figure 11, the
H = f (D , [O/C]e , vCH4 , ρ ̅ , ρ∞ , ΔTf , T∞ , g , L)
(3)
In the study of the generation of H, buoyancy is an important factor, so Fr and Re are introduced to characterize the flamedominant regime (momentum domination or buoyancy domination). The influencing factors can be calculated as26 Fr = uf̅ s 3/2 /[(ρ ̅ /ρ∞)1/4 (ΔTf /T∞gD1/2]
Re =
Du ̅ ρ ̅ μ̅
(4)
(5)
where D is the equivalent nozzle diameter, u̅ is the average velocity, and μ̅ is the average viscosity of the fuel and oxidizer. fs is the mixture fraction. The relationship between H and Fr is shown in Figure 12, where H in fuel-rich flames increases with increasing Fr, whereas
Figure 11. Subarea map of the reaction area in the impinging region.
occurrence of reactions in the impinging region is influenced by [O/C]e, velocity, and L/D. There is a combined influence of velocity and L/D on the generation of reaction area. When the ratio of the velocity to L/D is low, nearly no reactions occur in the impinging region. As the ratio increases, more reactions occur in the impinging region. The reactions in the upward stream appear first, and then the reactions in the downward stream are generated. For [O/C]e > 1.0, the reactions occur easily in the impinging region. A high [O/C]e value and a high ratio of velocity to L/D promote the reactions in the impinging region, so a clear reaction center is generated. According to the OH* radical distribution, the reaction area in the impinging region under different conditions can be divided into four types: (I) no reaction area, where no reactions occur in the impinging region; (II) upward-stream reaction area, where reactions occur only in the upward stream region; (III) two-stream reaction area, where reactions occur in both the upward and downward stream regions; and (IV) impinging reaction area, where reactions occur in the whole impinging region and a clear center forms in the impinging core area. 3.3. Height of the Reaction Area in the Upward Stream. From Figure 11, the OH* radical distribution reflects the height of the reaction areas in the upward and downward streams, and the height (H) indicates whether the impinging reaction region exists. H is the distance from the impinging stagnation point to the point where OH* disappears in the upward stream. (The emission can be regarded as the background emission on condition that the intensity is less than 0.8 μW·Sr−1·m−2 in the emission region.) The height of a single jet diffusion flame depends on the equivalent diameter of the nozzle (D), the [O/C]e value, the velocity of the fuel (vCH4), the average density of the oxidant and fuel ρ̅, the ambient density ρ∞, the gravitational acceleration (g),
Figure 12. Relationship between Fr and H.
H has a vertex in fuel-lean flames. The relationship between the height of the upward stream (H) and Re is the same as that with Fr (Figure 13). Fr and Re play similar roles in the generation of H, as shown in Figure 14, and the relationship between Re and Fr is Re = 135.2Fr 0.89
(6)
L is another factor that influences H. According to Figure 15, H decreases with increasing L/D value. From the analysis above, Fr, L/D, and [O/C]e are the main factors affecting H, as shown in Figure 16 H = 5.4Fr 0.62([O/C]e )−0.52 (L /D)−0.19
(7)
4. CONCLUSIONS OH* is a good indicator of whether reactions exist in a flame. In this work, the radial distributions and two-dimensional profiles of the OH* emission intensity were obtained by high-spatialresolution UV imaging. The effects of the velocity, the L/D ratio, and the O/C equivalence ratio ([O/C]e) on the generation of the impinging reaction region were discussed. The results are as follows: 1434
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Figure 16. Comparison of the experimental and fitted values.
Figure 13. Relationship between Re and H.
The occurrence of reactions in the upward and downward streams is influenced by the [O/C]e value, the velocity, and the L/D value. There is a combined influence of velocity and L/D on the generation of reaction area. When the ratio of velocity to L/D is low, nearly no reactions occur in the impinging region. As the ratio increases, more reactions occur in the impinging region. The reactions in the upward stream appear first, and then the reactions in the downward stream are generated. A high [O/C]e value and a high ratio of velocity to L/D promote the reactions in the impinging region, and a clear reaction center is generated. The OH* radical distribution also reflects the height of the reaction area in the upward and downward streams, and the height indicates whether the impinging reaction region exists. The height of the reaction area in the upward stream in impinging flames depends on [O/C]e, the flame Froude number (Fr), and L/D. Based on the existing experimental conditions, the experimental fit is H = 5.4Fr0.62[O/C]e−0.52(L/D)−0.19. According to the findings of this research, the trends of the impinging reaction area and the calculation of flame height provide further information on impinging flames and can be employed in industrial applications.
Figure 14. Relationship between Fr and Re.
■
AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86-21-64252974. Fax: +86-21-64251312. E-mail: gsyu@ ecust.edu.cn. *Tel.: +86-21-64252974. Fax: +86-21-64251312. E-mail: gqh@ ecust.edu.cn. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (51406056), the National High Technology Research and Development of China (863 Program, 2012AA053101), the Fundamental Research Funds for the Central Universities (222201414030), and the Shanghai Pujiang Program (15PJD011).
Figure 15. Relationship between L/D and H at vCH4 = 78.3 m/s.
■
The OH* intensity indicates the generation of the reaction area in the impinging region. From the OH* radical distribution, the reaction area in the impinging region can be divided into four types: (I) no reaction area, (II) upward-stream reaction area, (III) two-stream reaction area, and (IV) impinging reaction area.
REFERENCES
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DOI: 10.1021/acs.energyfuels.5b02721 Energy Fuels 2016, 30, 1428−1436
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DOI: 10.1021/acs.energyfuels.5b02721 Energy Fuels 2016, 30, 1428−1436
