Characteristics of Coal Partial Gasification on a Circulating Fluidized

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Characteristics of Coal Partial Gasification on a Circulating Fluidized Bed Reactor Chao Ye, Qinhui Wang,* Zhongyang Luo, Guilin Xie, Ke Jin, Muhtar Siyil, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: Experiments of coal partial gasification are conducted on a self-designed circulating fluidized bed (CFB) gasifier. The CFB gasifier system consists of a furnace, coal-feed system, air-feed system, steam generation system, gas−solid separation system, temperature- and pressure-measuring system, and material-returning system. The furnace is surrounded with electric heating wires, and the height and diameter of the furnace are 2600 and 120 mm, respectively. The experiments are conducted at temperatures between 885 and 980 °C. The coal particle size of 0.35−0.9 mm is chosen for experiments. The coal particle is gasified in the mixture of O2 and steam. The effects of different oxygen/coal ratios and steam/coal ratios on the performance of coal partial gasification are studied. As the results show, a higher oxygen/coal ratio leads to a higher gasification temperature and carbon conversion, while a slightly higher steam/coal ratio is better for the quality of syngas. The results show that the volume fraction of combustible gas and the lower calorific value reach as high as 70% and 9.1 MJ/Nm3, respectively. It can be obtained from the results of Raman and Fourier transform infrared spectra that the structure of chars becomes higher ordered as the different functional groups are gradually consumed. The chars become less reactive because higher ordered results in less reactivity.

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INTRODUCTION As a clean and efficient energy utilization technology, gasification is used for converting carbonaceous fuels into valuable energy products.1−4 There are mainly three types of gasification technologies in view of gasifier configurations and flow geometry, including the entrained flow bed gasifier, fluidized bed gasifier, and fixed bed gasifier.5 Nowadays, the most widely used is the entrained flow bed gasifier.6 It aims at high carbon conversion by improving the temperature and pressure of gasification. Therefore, the cost of equipment and operation is high because of extreme furnace conditions. It is well-known that the circulating fluidized bed (CFB) has lower investment and higher fuel flexibility, especially for low-rank coal. In this paper, the characteristics of coal partial gasification on a CFB reactor are studied. As the existed literature reports, the reaction rate first increases and then decreases as the reaction proceeds.7 The coal would become more ordered as the activated part is consumed generally by testing the variation of the functional group of coal by Zhang et al.8 The single conversion of coal is not more suitable for the modern economic and environmental development. On the basis of the property of the coal gasification, the coal partial gasification technology is proposed. The cascading utilization of coal and not overpursing high carbon conversion are core concepts of coal partial gasification. The schematic diagram of dual-CFB partial gasification technology is shown in Figure 1a. In the partial gasification, coal is ground into particles and then sent into the gasifier together with steam and oxygen. In the gasifier, the active part of coal is gasified into the syngas, and the syngas is mainly composed of CO, H2, CO2, and CH4. The syngas can be used for power generation or chemical products. The ungasified char and ash are sent into the combustor through the loop seal for combustion. The heat from the combustor is used for steam © 2017 American Chemical Society

generation, which improves the steam parameters to supercritical conditions. Therefore, the thermal efficiency of coal partial gasification is improved by the supercritical steam technology. The air and steam coal partial gasification in an atmospheric fluidized bed is studied by Zhou et al.9 Several factors on the gasification characteristics are studied, which show that air/ steam gasification lead to a low higher heating value syngas. One spout-fluid bed gasifier is constructed to study the partial gasification performance by Xiao et al.10 It is obtained from the research that a higher temperature is better for carbon conversion, gas yield, and cold gas efficiency. The combined cycle with partial gasification and fluidized bed combustion is studied by Cai et al., which has the advantage of staged energy conversion and utilization. 11 The thermal and exergy efficiencies of a combined-cycle power generation based on oxygen-blown coal partial gasification are researched using AspenPlus software by Zhang et al.12 and Li et al.13 The coproduction of hydrogen and electricity based on coal partial gasification with CO2 capture is raised by Xu et al.14 It has the advantage of high efficiency, low cost, environmental friendliness, and flexible hydrogen/electricity ratio. The thermal efficiency is 3% higher than that of the reference system, and the proposed system can realize better utilization of coal. The structure development of partial gasified char is discussed in a bench-scale fluidized bed using CO2 by Komarova et al.15 It is obtained that the total char porosity increases during char conversion and char−CO2 mostly takes place on the mesopore surface area. The structures of bituminous coal under CO2 and Received: November 3, 2016 Revised: February 3, 2017 Published: February 10, 2017 2557

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Figure 1. Schematic diagram of the (a) dual-CFB partial gasification and (b) CFB gasifier.

Table 1. Proximate and Ultimate Analyses of Huating Coal proximate

ultimate

M (%, d)

A (%, d)

V (%, d)

FC (%, d)

C (%, d)

H (%, d)

N (%, d)

S (%, d)

O (%, d)

Qnet,d (MJ/kg)

7.55

20.94

25.12

46.39

61.14

3.18

1.23

0.56

12.95

21.93

Ar in a fixed reactor are investigated by Bai et al.16 The results show that the small aromatic ring systems are preferentially consumed and larger ring systems, which have lower reactivity, are formed. There is still few literature about the coal partial gasification research on a CFB reactor. The research in existing literature is only simulated or conducted in a single reaction atmosphere or on a fixed reactor. In this paper, the experiments are carried out in a self-designed CFB gasifier. Oxygen and steam are adopted as the reactant gas. The experiments are conducted under different temperatures. The effect of different oxygen/coal ratios and steam/coal ratios on the gas composition and char characteristics are investigated. Because the coal partial gasification technology has not yet been employed in the

practical industrial application, this paper aims at providing new data for the feasibility of coal partial gasification.

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MATERIALS AND METHODS

Coal and Bed Material Preparation. Huating coal adopted for experiments is screened into 0.35−0.9 mm. The proximate and ultimate analyses of coal are summarized in Table 1. The yellow sand chosen for experiments had a particle size of 0.35−0.45 mm. Experimental Facility. The schematic diagram of the CFB reactor for coal partial gasification is shown in Figure 1. The whole system mainly consists of the CFB gasifier, coal-feed system, gas-supply system, steam generation system, gas−solid separation system, distributed control system (DCS), induced fan, and material-returning system. The furnace is made of ceramics, and the diameter and height are 120 and 2600 mm, respectively. There is an air distributor at the bottom of furnace. There are heating wires around the furnace, and the highest temperature of the heating wire can reach 880 °C. Before the 2558

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Energy & Fuels experiments, the furnace is preheated to 650 °C by the heating wire. Then, the coal and gasification gas are input to the furnace. When the temperature is growing steadily, turn off the heating wires. The temperature can be controlled and measured by the DCS system. Air and oxygen are supplied by a centrifugal fan and the oxygen cylinder, respectively. The gas is preheated to 350 °C through air preheaters before entering the furnace. The steam is generated in a steam generator, and the temperature and pressure of steam are 180 °C and 0.4 MPa, respectively. The steam is mixed with oxygen before entering the furnace. The coal particle is transported into the furnace by a screw feeder. The screw feeder is driven by an electromotor, which is controlled by the DCS. There are two-stage cyclones for gas−solid separation. The char and ash from the primary cyclone are sent back to the furnace through the material-returning tube, while char and ash from the secondary cyclone are collected by an ash hopper. The J valve is adopted as the material-returning device. When it operates in a stable condition, the volume flow rate is controlled at ∼6 Nm3/h and the velocity of the gas is ∼3−4 m/s. The flow rate of returning gas is controlled at ∼1 Nm3/h. The pressure of the gas chamber is controlled at 7000−9000 Pa. Experimental Procedure. The experiments are conducted as the following steps: (1) Turn on the heating wire of the furnace, riser, and air preheater. The set temperature of these parts are 650, 300, and 350 °C, respectively. (2) Switch on the oxygen cylinder to release some gas to make sure the bed material flows in the furnace. Turn on the steam generator but make sure the valve is closed. (3) About 750 g of yellow sand and 3000 g of coal particle are added to the furnace and coal hopper. (4) Turn on the coal feeder and adjust the flow rate of oxygen to ensure full combustion of the coal. When the bed temperature reaches the demanded gasification temperature, open the valve of the steam generator. (5) The oxygen/coal ratio and steam/coal ratio are adjusted to the required value. After the gasification is in the steady state, the gas sample is collected at a 5 min interval. (6) After 20 min, the coal feeder, heating wires, and steam are stopped and oxygen is switch to nitrogen for the furnace cooling. When the temperature of the furnace is less than 150 °C, the char samples are collected for further analysis. The pre-test is conducted before the formal experiments. The temperatures of the dense-phase zone of the furnace are shown in Figure 2. There are two stages in the coal partial gasification

Methods of Analysis. The gas sample is collected from the top of the furnace and is analyzed using gas chromatography (GC, Agilent 7890B GC). The low-nitrogen method is adopted for testing. The char sample is collected after the furnace cools. Then, the char sample is screened and analyzed by Raman spectroscopy (DXR), which is from the Thermo Fisher Corporation and Fourier transform infrared (FTIR) spectroscopy (Nicolet 5700). There are a two-staged cyclones in the CFB reactor. The mass of chars and ash separated from the first cyclone is represented by mchar 1 (g), while the mass of chars and ash from the second cyclone is represented by mchar 2 (g). The mass of chars and ash from the bottom of the reactor is represented by mchar 3 (g). The mass of feed coal is represented by mcoal (g). Then, the mass of reacted coal is expressed by the following equation:

mreacted coal = mcoal − (mchar 1 + mchar 2 + mchar 3)

(1)

The carbon conversion can be known by the following equation:

conversion = (Vsyngas ∑ X n/22.4 × 12)/(McoalC %)

(2)

where Vsyngas represents the volume of syngas, Xn represents the volume fractions of CO2, CO, and CH4, Mcoal represents the mass flow rate of coal, and C% represents the carbon content in the coal. The heating value of syngas is expressed by the following equation:

LHsyngas = (∑ VLH i i) × 100%

(3)

3

In eq 3, LHsyngas (MJ/Nm ) represents the lower heating value of syngas. LHi (MJ/Nm3) represents the lower heating value of H2, CO, CH4, C2H4, C2H6, and C3H6. Vi (%) represents the volume fractions of H2, CO, CH4, C2H4, C2H6, and C3H6. Reactions. It is well-known that coal is a complex matter. Therefore, the chemical reactions involved in the gasifier are complicated. There are mainly two types of reactions, including the gas−solid reactions and gas−gas reactions, which are summarized in the following part.6,8 All of the reactions are exothermic reactions, except for reactions 6 and 7. (1) Gas−solid reactions:

C + 0.5O2 = CO,

ΔH = − 110.6 kJ/mol

(4)

C + O2 = CO2 ,

ΔH = − 393.6 kJ/mol

(5)

ΔH = − 131.3 kJ/mol

C + 2H 2O = 2H 2 + CO2 ,

(6)

C + CO2 = 2CO,

ΔH = − 172.5 kJ/mol

(7)

C + 2H 2 = CH4 ,

ΔH = − 74.9 kJ/mol

(8)

(2) Gas−gas reactions:

H 2 + 0.5O2 = H 2O,

ΔH = − 241.9 kJ/mol

(9)

CO + 0.5O2 = CO2 ,

ΔH = − 283 kJ/mol

(10)

CO + H 2O = H 2 + CO2 , CO + 3H 2 = CH4 + H 2O,

2CO + 2H 2 = CH4 + CO2 , CO2 + 4H 2 = CH4 + 2H 2O,

ΔH = − 41.2 kJ/mol ΔH = − 206.2 kJ/mol

ΔH = − 203.3 kJ/mol

(11) (12) (13)

ΔH = − 208.9 kJ/mol

(14) As two main factors, the oxygen/coal ratio and steam/coal ratio have great influence on the results of coal gasification. In the next section, the effects of the oxygen/coal ratio and steam/coal ratio on the temperature, gas composition, and heating value of syngas are studied.

Figure 2. Temperatures of different parts of the furnace. experiments, including the adjustment stage and stabilization stage. The temperatures fluctuate violently in the adjustment stage and then stabilize gradually. Because the steam is from the small-scale steam generation, the supply of the steam flow is not stable. In the meantime, the CFB reactor is not that big. Therefore, the external conditions would have an effect on the operation, which makes the temperature fluctuate.

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RESULTS AND DISCUSSION Effects of the Oxygen/Coal Ratio on the Results of Coal Partial Gasification. In this section, the steam/coal ratio 2559

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by ∼5 and ∼10%, respectively, while the volume fraction of CO2 increases by ∼14% as the oxygen/coal ratio increases, which could account for combustible gas combustion. The decrement of H2 is much larger than that of CO. The reason why the volume fraction of CO2 almost reaches more than 37% is that the oxygen content of coal reaches 12.95%. It leads to more CO consumption. The volume fraction of combustible gas accounts for 64%. The volume fraction of CH4 differs slightly. The lower calorific value of gas deceases from 8.4 to 6.7 MJ/Nm3, which is attributed to the variation of gas composition. It is much larger than that of the air and steam coal partial gasification.9,10 Effects of the Steam/Coal Ratio on the Results of Coal Partial Gasification. In this section, the oxygen/coal ratio is kept as 0.91 to explore the effect of the steam/coal ratio on the results of coal partial gasification. The effects of the steam/coal ratio on the gas composition and the temperature, carbon conversion, and lower calorific value are shown in Figures 5 and

is kept as 0.4 to study the effect of the oxygen/coal ratio on the results of coal partial gasification. The value of the oxygen/coal ratio ranges from 0.37 to 1.15. Figure 3 shows the influence of

Figure 3. Effects of the oxygen/coal ratio on the gas composition.

the oxygen/coal ratio on gas composition, while the effect of the oxygen/coal ratio on the temperature, lower calorific value, and carbon conversion is shown in Figure 4. In the following

Figure 5. Effects of the steam/coal ratio on the gas composition.

6, respectively. The steam/coal ratio is increased from 0.32 to 0.51. As seen from Figure 5, the volume fractions of CO and H2 increase by 2 and 9%, respectively, because the scale of the experimental facility is relatively smaller than the commercial type. The heat loss is relatively high, which results a higher Figure 4. Effects of the oxygen/coal ratio on the temperature, carbon conversion, and lower calorific value of syngas.

part, the gasification temperature is represented by the value of T-9. The oxygen/coal ratio has a great effect on the gasification temperature. As seen from Figure 3, the gasification temperature increases from 890 to 987 °C with the increase of the oxygen/coal ratio. It can be explained by more coal combustion. When the oxygen/coal ratio increases from 0.37 to 0.54, the carbon conversion increases from 79 to 87%. The extent of the variation is more significant than that when the oxygen/coal ratio increases from 0.54 to 1.15. It expresses that the coal conversion becomes more difficult in the late stage of coal gasification. It is also proved the point that the reaction rate increases and then decreases in the coal gasification, which is stated in the literature.7,13 A conclusion can be drawn that, when the oxygen/coal ratio is 0.54, the performance of the coal partial gasification is relatively appropriate. In the case of the gas composition, the volume fractions of CO and H2 decrease

Figure 6. Effects of the steam/coal ratio on the temperature, carbon conversion, and lower calorific value of syngas. 2560

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and FTIR spectra. The experimental conditions are summarized in Table 3.

content of CO2. Thus, CO is increased when the steam/coal ratio increased according to reaction 7. The volume fraction of CO2 decreases from 40.92 to 29.93%. Therefore, the volume fraction of combustible gas increases from 59 to 70%, which is very considerable. It is similar to the above section that the volume fraction of CH4 differs slightly. It can be obtained that the gasification temperature keeps declining because excess steam absorbs heat. The carbon conversion increases because more carbon reacts with the steam, which increases from 88.3 to 91.2%. However, the increment of carbon conversion is not larger than that of the last section. As the volume fraction of combustible gas increases, the maximum value of gas reaches as high as 9.1 MJ/Nm3. Analysis of Carbon Balance. The analysis of carbon balance is according to eq 15 MCcoal = MCsyngas + MCchar + MCash

Table 3. Number of Different Chars under Experimental Conditions

where MCcoal (g/min) represents the carbon content in the coal, MCsyngas (g/min) represents the carbon content in the syngas, MCchar (g/min) represents the carbon content in the char, and MCash represents the carbon content in the ash. MCsyngas is calculated by the following equation MCsyngas = ((XCO2 + XCO + XCH4)Vsyngas)/22.4 × 12 (16)

where X represents the volume fractions of CO2, CO, and CH4 and Vsyngas (L/min) represents the volume of syngas. MCchar and MCash are calculated by eqs 17 and 18, respectively (17)

MCash = MashCash%

(18)

char 1

char 2

char 3

temperature (°C) oxygen/coal ratio (kg/kg) steam/coal ratio (kg/kg)

860 0.365 0.375

890 0.461 0.311

930 0.492 0.362

Raman Spectra Acquisition of Char. Raman spectroscopy is used extensively to explore the structure features of carbonaceous matter.17−19 In this section, the graphite structure (G) and disorder structure (D) are mainly adopted to investigate the characteristic structure of char. However, the quantitative connection between the Raman spectra and structural features is not acquired by only taking the G and D bands into consideration; some bands disappear.3,19,20 There are overlapped areas for highly disordered carbonaceous material between the two bands, including GI, G, Gr, VI, D, SI, S, Sr, and R.17,19,21 However, some of them are too small to observe for some kinds of chars. As a result of the reason stated above, detailed information is unable to be found to study the disordered carbon material. Therefore, it is necessary to conduct the deconvolution of Raman spectra into Gaussian bands by Origin 9.20 Detailed Raman spectra are shown in Figure 7. The spectra range from 800 to 2200 cm−1. There are

(15)

MCchar = McharCchar %

experimental condition

where Mchar (g/min) and Mash (g/min) are the mass flow rates of the char and ash, respectively, and Cchar% and Cash% are the carbon contents of the char and ash, respectively. The experimental results of two runs are chosen for analysis of carbon conversion, which is shown in Table 2. In the case of Table 2. Analysis of the Carbon Balance item Mcoal (g/min) MCcoal Vsyngas (L/min) Mchar (g/min) Mash (g/min) Cchar% Cash% MCsyngas MCchar MCash

run 1 Carbon Input 97.73 59.62 Carbon Output 146.23 13.22 0.88 19.28 1.5 49.11 2.61 0.0132

run 2 101.51 61.92 135.14 14.43 0.75 21.23 1.09 51.22 3.06 0.82

Figure 7. Deconvoluted Raman spectra of coal char.

mainly four bands that exist in the derived char samples. The D1 band around 1350 cm−1 is considered as medium to large aromatic rings, including six or more fused rings and a graphitic lattice without special order.22 The D2 band around 1620 cm−1 is attributed to a lattice vibration involving graphene layers.20 The D3 around 1530 cm−1 is related to the small ring system (less than six fused rings).20,23 The D4 band around 1150 cm−1 is considered as the mixer sp2−sp3 bond form at the periphery of crystallites or C−C and CC.24 The G band around 1580 cm−1 is related to the stretching vibration of the ideal graphitic lattice.22 The total peak area shown in Figure 8 is used as a measurement to describe the overall Raman intensity. As the literature stated, growth of aromatic rings and loss of O-

syngas, there are some gas elements that contain carbon that cannot be detected by GC and there is char and ash loss in the collecting process. In the meantime, the ash cannot be fully separated by the cyclone, which would be carried by the gas. In combination of all of the situations above, MCout is not equal to MCin precisely. Char Characterization. In this section, char samples under three kinds of experimental conditions are adopted for Raman 2561

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Figure 10. ID3/ID1 of different chars at different conditions.

Figure 8. Total peak intensity of raw coal and chars at different conditions.

ordered structure.25 It is explained that, as gasification proceeds, the reactive part is generally consumed, which is comprehensive compared to the actual situation. It provides the experimental experience and theoretical basis for coal partial gasification technology. It can be obtained from Figure 10 that ID3/ID1 first decreases and then increases slightly but is still is lower than that of sample 1. It is consistent with the previous research results.8,19 When the temperature is not very high (less than 900 °C), the smaller aromatic rings are converted into larger aromatic rings or consumed.8,24 While the temperature is higher than 900 °C, it is possible that the bigger aromatic rings turn into smaller aromatic rings again. FTIR Spectra Acquisition of Char. The FTIR spectra are adopted to explore the characteristic variations of surface chemistry during the gasification process by others.20,26,27 The char is screened into powder mixed with KBr and tableted before the sample is tested. The FTIR spectra are between 400 and 4000 cm−1. The band around 3430 cm−1 is represented as v(O−H) bond stretching vibrations of hydroxyl groups.22,23 The band around 1585 cm−1 is attributed to v(CC) vibrations.22,28 The band around 1425 cm−1 means the δ(O− H) vibrations.28 The band around 1170 cm−1 represents the v(C−O) vibrations of ether-type structures.28 The band around 1090 cm−1 is attributed to v(C−O) vibrations in secondary hydroxyl groups.20,28−30 The bands around 875, 795, 750, and 695 cm−1 are attributed to aromatic structures of isolated hydrogen, three adjacent hydrogen atoms per ring, four adjacent hydrogen atoms per ring, and five adjacent hydrogen atoms per ring, respectively.28 The FTIR spectra of char and coal are shown in Figure 11. It can be obtained that some bands disappear gradually with the gasification process. The peak area of v(CC) (∼1585 cm−1) decreases as the gasification temperature increases. It could be explained that a more stable structure, such as alkyl−aryl, develops from the CC band.29 The peak area of the band (∼1090 cm−1) is the highest among all of the bands, which means that C−O in the form of secondary hydroxyl groups accounts for most. The band (∼1035 cm−1) disappears as the temperature increases, which results from the aromatic condensation.28 In general, the band (∼695 cm−1) increases and becomes obvious as the gasification temperature increases. There are two possible explanations. One of them is that larger aromatic rings may form from smaller aromatic rings.8 The other could be attributed to the

containing functional groups result in a decrease of Raman intensity,19 while the O-containing functional group leads to higher Raman intensity. Sample 4 in Figure 8 represents the total peak intensity of raw coal, which is much larger than that of the other three samples. It indicates that raw coal is much less ordered and contains more different functional groups. As the gasification temperature increases, the reactions during the gasification become much more drastic and more compositions and functional groups are involved in the reactions, while the total peak area decreases, which implies that the char structure grows more ordered. It could be explained that there are more aromatic ring forms and O-containing functional groups consumed as the gasification condition becomes hasher and hasher. The ratio of some major bands can be used for charactering the structure of carbon crystalline.20 The ratios of ID1 and IG and ID3 and ID1 are shown in Figures 9 and 10. The ratio of ID1 and IG indicates the degree of disorder of char, while ID3/ID1 represents the ratio of small and large aromatic rings.17,19,23,28 It can be seen that ID1/IG decreases as the gasification temperature increases. As stated by other literature, the disordered carbon structure is more reactive than the high

Figure 9. ID1/IG of different chars at different conditions. 2562

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ACKNOWLEDGMENTS The work is supported by the collaboration project of the Clean Energy Research Centre on Advanced Coal Technology Consortium (CERC-ACTC).

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Figure 11. FTIR spectra of the char and coal.

smaller aromatic ring consumption, which makes the larger aromatic rings enriched. It is consistent with the trend of results of Raman spectra.

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CONCLUSION The coal particle is gasified in a self-designed CFB gasified under atmospheric pressure at 885−980 °C. The effects of the oxygen/coal ratio and steam/coal ratio on the gasification temperature, gas composition, lower calorific value, and carbon conversion are studied. The gas composition and characteristics of chars are tested by GC and Raman and FTIR spectroscopies, respectively. The main conclusions are listed as follows: (1) The gasification temperature increases, while the volume concentration of combustible gas decreases, as the oxygen/ coal ratio increases from 0.37 to 1.15. However, the carbon conversion increases and then decreases, which illustrates that 0.54 is the optimal oxygen/coal ratio in our work. As the steam/coal ratio increases from 0.32 to 0.51, the gasification temperature decreases, while the volume fraction of combustible gas, lower calorific value, and carbon conversion increase. The volume concentration of combustible gas and lower calorific value can reach as high as 70% and 9.1 MJ/Nm3,9,10 respectively, which are much higher than the results from the existing literature. (2) It can be obtained from the results of Raman and FTIR spectra that the structure of chars becomes higher ordered because of the loss of different functional groups gradually with the gasification reaction process. The higher ordered structure results in less reactivity. (3) The higher reactive part is consumed first before the lower reactive part. As the gasification process proceeds, the lower reactive part becomes more difficult to convert and the converting condition becomes more rigorous. Therefore, the cascading utilization of coal has better performance for coal utilization.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 0086-571-87952802. E-mail: [email protected]. ORCID

Qinhui Wang: 0000-0002-5190-6932 Notes

The authors declare no competing financial interest. 2563

DOI: 10.1021/acs.energyfuels.6b02889 Energy Fuels 2017, 31, 2557−2564

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DOI: 10.1021/acs.energyfuels.6b02889 Energy Fuels 2017, 31, 2557−2564