Analysis of Combustion Characteristics When Adding Hydrogen and

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Cite This: Energy Fuels 2019, 33, 6767−6774

Analysis of Combustion Characteristics When Adding Hydrogen and Short-Chain Hydrocarbons to RP‑3 Aviation Kerosene Based on the Variation Disturbance Method Shuhao Li,†,‡ Junjiang Guo,†,§ Zhenghe Wang,†,‡ Shuanghui Xi,†,∥ Junxing Hou,†,‡ and Zhenhua Wen*,‡ ‡

School of Aeronautical Engineering, Zhengzhou University of Aeronautics, Zhengzhou, Henan 450046, People’s Republic of China School of Chemical Engineering, Guizhou Institute of Technology, Guiyang, Guizhou 550003, People’s Republic of China ∥ School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China

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§

S Supporting Information *

ABSTRACT: Hydrogen and five short-chain hydrocarbons are mixed with RP-3 aviation kerosene (RP-3) to study their blending effects on the combustion of RP-3. Seven combustion characteristics, the ignition delay time, burnout time, adiabatic flame temperature, extinction temperature, rate of production of hydroxyl radicals, laminar flame speed, and extinction strain rate, are simulated in four different reactors. The simulated data are preprocessed to match the requirements for a variation disturbance method proposed in this paper, and then the disturbance is obtained for representing the total influence of hydrogen and five short-chain hydrocarbons blending on the combustion properties of RP-3. The results show that H2, CH4, and C2H4 have a greater degree of disturbance to RP-3. In contrast, the influence of C3H6 is the weakest. The rate of disturbance shows that H2 and C2H4 have a positive effect on each of the combustion characteristics, and especially, C2H4 plays a promoting role in the combustion performance of RP-3. The reaction paths of seven fuels are analyzed by time-integrated element flux analysis, and the viability and rationality of the variation disturbance method are supported by the calculation of branching ratios of six main reaction channels.

1. INTRODUCTION Research on the related combustion performance and reaction mechanisms of traditional aviation fuels and emerging bioalternative fuels plays a significant role in alleviating the energy crisis and environmental pollution.1,2 The combustion characteristics of fuels are a critical problem for all types of engines, and the combustion thermodynamics and kinetics studies also provide an overall understanding for combustion quality of the fuel.1,3 The fossil fuels, aviation kerosene, gasoline, and diesel, are commonly used in engines. For wider application and more stable and efficient combustion process, additives are included with the fuel for ease of flow, better spray, improved atomization, and higher combustion efficiency.3,4 For the purpose of economy and environmental protection, H2O is added to oxygen to control combustion temperatures by a conventional steam cycle, which can effectively control greenhouse gas emissions and improve the economics of fossil fuel.5 The methods for improving combustion performance usually include mixing additives or blending agents to the fuel. Unlike mixing non-fuel substances to fuel for improving non-combustion-related behavior, blending small hydrocarbons or biomass fuels with fossil fuels can effectively improve the combustion efficiency of fossil fuel. To study the mixing of different additives or blending agents with fossil fuels, the effects on the physical characteristics and combustion properties of fossil fuels need to be clarified. Moreover, more combustion characteristics need to be considered in the analysis.6,7 A comprehensive, reliable, and © 2019 American Chemical Society

quantifiable assessment method is the key to evaluating the influence of additives and blending agents to the primary fuel. Experimental research on the combustion characteristics of fossil fuels with the addition of blending agents has been conducted, and some combustion models have been developed to predict the combustion performance of various fuel blends. Wang et al.8 studied the ignition delay of kerosene cracking products/RP-3 kerosene over a relatively wide temperature range from 657 to 1333 K via a shock tube. Liu et al.9 determined the flame radius diffusion rate, Markstein length, and laminar flame speed of methane/RP-3 aviation kerosene based on studies in the combustion bomb facility. They performed a qualitative analysis for the effects of adding shortchain hydrocarbons to RP-3. Other studies mainly focused on the combustion and emissions of diesel and gasoline with different oxy-fuels, such as alcohols and methyl esters.10−27 There was also some work carried out on diesel mixing with biomass fuels and olefins.14,15,25 For alcohols, ethanol is widely used as a blending agent for practical fuels.10−13,18,19,23,26,27 In addition, there are some studies on 2-ethyl-1-hexanol,16 pentanol,17,20 and butanol.21,22 These studies analyzed and compared combustion and soot emissions of oxy-fuels/diesel blends and reported on the application of blending agents in optimizing engine performance. Results have shown that alcohols/diesel and alcohols/gasoline mixtures have higher Received: February 1, 2019 Revised: June 9, 2019 Published: June 17, 2019 6767

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Energy & Fuels combustion efficiency than diesel and gasoline, and they also show that blending with alcohol can lead to soot emission reductions. Some studies investigated the effect of blending components on the combustion properties of hydrocarbon and oxygenated fuels. Pang et al.28 studied the combustion of toluene and nheptane/toluene and isooctane/toluene blends. Zhang et al.29 explored the influence of ethane and propane on the ignition delay time of methane. Zheng et al.30 carried out a numerical and experimental study on the chemical kinetics of the isooctane/n-heptane/ethanol blend. Zhang et al.31 investigated the effects of methane addition on exhaust gas emissions and combustion efficiency of n-heptane combustion. Polat32 studied the combustion of the mixtures of diethyl ether and ethanol in homogeneous charge compression ignition (HCCI) and analyzed the engine performance and exhaust emissions. Their research also focused on a small number of combustion characteristics and compared different experimental or calculated values for qualitative analysis. In summary, there are many studies on the participation of blending agents in fossil fuels, including small hydrocarbons, alcohols, and biomass fuels. Studies also show that the impact of these blending components on typical combustion characteristics of fossil fuels is assessed by experimental measurements or numerical simulation. With the lack of quantitative characterization for evaluation of blending agents, the comparative approach can only be used for qualitative analysis of the experimental or simulation results, which is not comprehensive enough. Thus, in this work, on the basis of the combustion simulation study of RP-3 and six binary fuel blends, the variation disturbance method is employed to analyze and quantify the blending effect of hydrogen and five small hydrocarbons with RP-3 on their combustion characteristics. This work provides a scientific analysis method for evaluating the performance of additives or blending agents and has a significant application value for fuel design and optimization of combustion engines.

di =

(2) (3)

Vij is the magnitude (experimental value or simulation value) of the ith combustion characteristic of the fuel under the jth condition over a wide range of parameters. Vbij is the value of binary fuel blend combustion, and Voij is the value of original fuel. c is the difference value of the combustion characteristics of the binary fuel blend and the original fuel, and the absolute value of c is taken as the sample corresponding to the specific combustion performance. di is the disturbance amount of the ith combustion characteristic. stdev represents the standard deviation of the sample, and avg stands for the standard arithmetic mean of the sample. The degree of disturbance of the effect of blending components on the combustion performance of the primary fuel can be intuitively and numerically shown by the value of the disturbance amount. To analyze the contribution of blending agents to the primary fuel, we introduce the concept of the rate of disturbance (ROD), as shown by eqs 4−6. At the same time, the δ function is introduced to define the disturbance rate and the disturbance effect of blending components on the primary fuel could be quantified by the positive and negative values of the δ function. The calculated values demonstrate their promotion or suppression effect on the primary fuel under all conditions. δi = sgn(S+ + S−)

max(n+S+ , − 1n−S−) n+S+ − n−S−

(4)

S+ =

∑ cij

(cij > 0)

(5)

S− =

∑ cij

(cij < 0)

(6)

In the above formulas, sgn is a sign function for indicating the parameter and max is the maximum function. S is the cumulative amount of the change in the magnitude of the combustion performance, and S+ and S− are the positive and negative increments, respectively. n+ and n− are the number of positive and negative increments in the sample, respectively. 2.2. Time-Integrated Element Flux Analysis. Element flux analysis was developed by Revel et al.,33 and Androulakis et al.34,35 proposed a time-integrated element flux analysis for analyzing the reaction path during the combustion process of the fuel. The method has been widely used in the analysis of combustion dynamics. The principle of the time-integrated element flux analysis is to calculate the reaction path of the fuel combustion by tracking the flow direction of the target element (E) based on combustion simulation. Carbon is often used as a target element in practical applications. For the chemical reaction mechanism containing K species and R element reactions, the proportion of the rth reaction in all channels of k1−k2 and the ratio of the channels of k1−k2 to all of the reaction channels of k1 can be calculated, as shown by eqs 7 and 8.

2.1. Variation Disturbance Method. Combustion of binary fuel blends (blending agents/primary fuel) is subject to a variety of factors, including combustion environmental factors, such as pressure, temperature, equivalence ratio, etc. Besides, the structure and amount of blending components also have significant effects on the combustion process and combustion performance of the primary fuel. The various fuel combustion characteristics, ignition delay time, flame temperature, component concentration, component yield, heat release rate, combustion completeness, etc., have different values and dimension, resulting in an unfriendly effect on the comprehensive evaluation of the combustion performance. Therefore, the coefficient of variation is introduced in this paper to eliminate the influence of the numerical scale and dimension and quantify the influence degree of blending agents. This paper defines their influence on the combustion performance of the primary fuel as “disturbance”. On the basis of the concept of the variation coefficient, their effects on the primary fuel and the degree of influence of combustion are quantitatively evaluated by the disturbance. The overall evaluation of the disturbance of each combustion characteristic can be calculated, and the total disturbance (D) is obtained by computing the sum of the value of various combustion characteristics, as shown in eqs 1−3. In this paper, the primary fuel is RP-3 aviation kerosene and hydrogen and five short-chain hydrocarbons are used as blending agents to study their disturbance on the RP-3 combustion performance.

∑ di

avg(|ci1| : |cij|)

cij = Vijb − V ijo

2. THEORETICAL METHODOLOGY

D=

stdev(|ci1|: |cij|)

τ

Er , k1→ k 2 =

∫0 ωr (t )

NE , r

(

τ

∫0 ∑rR= 1 ωr (t ) τ

Ek1→ k 2 =

NE , k1NE , k2

(

dt

NE , k1NE , k2 NE , r

) dt ) dt É Ñ ) dt ÑÑÑÑÑÑÑÖ

NE , k1NE , k2

ÄÅ NE , k NE , k KÅ τ R ∑k ÅÅÅÅ∫ ∑r = 1 ωr (t ) N1 2 ÅÅÇ 0 E ,r

∫0 ∑rR= 1 ωr (t )

(

(7)

NE , r

(8)

In eqs 7 and 8, ωr(t) is the net reaction rate of the rth elementary reaction at time t and NE,k1,NE,k2, and NE,r represent the number of the element E in the species k1, k2, and the rth reaction, respectively. τ is the reaction time. In the calculation, important components in the mechanisms are selected as initial targets and a depth search algorithm

(1) 6768

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Energy & Fuels Table 1. Seven Combustion Characteristics of Fuel and Number of Initial Conditions combustion characteristics

ignition delay time

adiabatic flame temperature

burnout time

extinction temperature

rate of production

laminar flame speed

extinction strain rate

i j

1 10

2 6

3 6

4 2

5 6

6 8

7 2

is used to explore the reaction path of the selected important components. The key element reaction contributing to the formation and consumption of these components can be generated via eqs 7 and 8. In this work, to extract and characterize the main reaction path, the values below 5% will not be shown in the diagram.

characteristics. The results show that the combustion performances related to time are most susceptible. For example, the total disturbance to the ignition delay time of fuel is the most significant and up to 12.82. However, the temperaturedependent combustion characteristics are less sensitive, such as the total disturbance of the extinction temperature, which is only 3.69. Moreover, the degree of disturbance of the OH radical generation rate is moderate. Other reference fuels are employed to verify the feasibility of the method proposed in this paper, and the corresponding results for the disturbance data and numerical simulation of H2/gasoline blends are provided in Tables S8−15S of the Supporting Information. The disturbance effect of each blending component on the combustion performance of RP-3 is evaluated. The computed disturbance rate is used to judge their influence on the combustion performance. The results are shown in Table 3. It

3. RESULTS AND DISCUSSION 3.1. Calculation of Disturbance and ROD. In this paper, hydrogen and five short-chain hydrocarbons are separately doped into RP-3, with a molar ratio of 1:5, to obtain six binary fuel blends: BF-1 (H2/RP-3 blend), BF-2 (CH4/RP-3 blend), BF-3 (C2H4/RP-3 blend), BF-4 (C2H6/RP-3 blend), BF-5 (C3H6/RP-3 blend), and BF-6 (C3H8/RP-3 blend) (see ref 6 for specific information). Simulations of neat RP-3 and the above-mentioned six binary fuel blends are conducted with an equivalence ratio of 1.0, a pressure of 0.1−1.0 MPa, and a temperature range from 300 to 1800 K based on the RP-3 combustion model of Sichuan University.36 Seven combustion characteristics, ignition delay time, adiabatic flame temperature, burnout time, extinction temperature, rate of production (ROP), laminar flame speed, and extinction strain rate, are selected for analysis, and results of neat RP-3 are used as benchmarks. Detailed simulation results are provided in Tables 1S−7S of the Supporting Information. The differences of the simulation results between the six binary fuel blends and RP-3 under the same working conditions are the original sample c. The sample |c| obtained by taking the absolute value of the difference in these samples is the calculation the basis of the disturbance method. The disturbance quantities of the above seven combustion characteristics are d1, d2, d3, d4, d5, d6, and d7, respectively. The numbers of initial operating conditions for different combustion characteristics are shown in Table 1. The total disturbance degree (D) of each of blending component to RP-3 aviation kerosene can be obtained by summing up the seven kinds of disturbances. From Table 2, H2

Table 3. ROD of Hydrogen and Five Short-Chain Hydrocarbons on Combustion Characteristics of RP-3 Aviation Kerosene ROD H2 (%) δ1 δ2 δ3 δ4 δ5 δ6 δ7

H2

CH4

C2H4

C2H6

C3H6

C3H8

total

d1 d2 d3 d4 d5 d6 d7 D

2.56 0.90 2.31 0.79 1.18 0.56 0.76 9.06

2.23 0.55 2.06 1.00 1.12 0.81 0.67 8.44

1.53 0.89 2.29 1.03 1.10 0.52 0.82 8.18

2.36 0.80 1.36 0.60 1.12 0.50 0.48 7.21

1.53 0.43 1.99 0.25 1.08 0.82 0.70 6.81

2.62 0.42 1.74 0.02 1.12 0.98 0.83 7.73

12.82 3.99 11.76 3.69 6.72 4.19 4.25

CH4 (%)

C2H4 (%)

C2H6 (%)

C3H6 (%)

C3H8 (%)

100 −85 93 −86 −100 −82 100

−73 100 90 −86 −100 100 100

89 −61 87 71 −100 100 100

94 −100 99 −59 −100 100 100

100 −100 88 100 −100 91 100

can be seen from Table 3 that the blending of H2 and C2H4 can significantly shorten the autoignition of RP-3, while other small hydrocarbons are not conducive. For the adiabatic flame temperature, C2H4 raises the flame temperature, while C3H6 and C3H8 do not. H2 can effectively accelerate the combustion process of fuel, while small hydrocarbons have the opposite effect, especially C3H6. The extinction temperature is an important parameter of combustion stability. C2H4 and C3H6 can lower the extinction temperature. Finally, the effect on the maximum yield of OH radicals is suppressed. CH4 weakens the laminar flame speed of RP-3, and H2 and five small hydrocarbons can increase the extinction strain rate and, thus, improve the stability of the flame. These results are consistent with the conclusions in ref 6, which also shows that the idea and calculation of the disturbance rate are reasonable. 3.2. Reaction Pathway and Reaction Channel Analysis. It can be seen from the conclusions above that H2 and C2H4 have positive effects on the combustion performance of RP-3. Because of the lack of systematic experimental results to provide information on the variation disturbance method, this work employs a mature dynamic analysis methodtimeintegrated element flux analysis to verify its rationality. Through the study of the combustion path, the rationality of the two parameters proposed in the method can be verified. The combustion reaction paths of three surrogate components,36 including n-dodecane (s0C12H26), 1,3,5-trimethylcyclohexane (s1C9H18), and n-propylbenzene (PHC3H7), are

Table 2. Disturbance Value of Hydrogen and Five Short -Chain Hydrocarbons on Combustion Characteristics of RP-3 Aviation Kerosene disturbance

−100 −56 −100 100 −100 100 100

has the strongest disturbance to the combustion performance of RP-3, with a total disturbance value of 9.06. The next are CH4 and C2H4, and the total disturbance values are 8.44 and 8.18, respectively. C 3 H6 shows the weakest, and the disturbance value is only 6.81. In addition, this paper also summarizes the cumulative disturbance of hydrogen and five short-chain hydrocarbons for the following seven combustion 6769

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element flux value (%) of RP-3, BF-1, BF-2, BF-3, BF-4, BF-5, and BF-6, respectively. From Figure 1, the initial reactions of s0C12H26 are mainly hydrogen abstraction reactions by H, OH, and O radicals to form C12H25 radicals, and then some of these radicals will undergo β-scission reactions to form light olefins and macromolecular radicals, while the others will occur with an isomerization reaction and a C−C bond-breaking reaction. It can be concluded from the reaction path that the participation of hydrogen and five short-chain hydrocarbons has little effect on the reaction path of RP-3 but their influence on the reaction channel is distinct. In this paper, three main reaction channels in the combustion process of seven fuels under the same combustion conditions are selected.

used for systematic analysis. The carbon element is selected as the target element to track the reaction path of the three components. The initial operating pressure is 1.0 MPa; the equivalent ratio is 1.0; and the initial temperature is 1400 K. Figure 1 shows the reaction path of n-dodecane (s0C12H26). The seven values in each path represent the time-integrated

s0C12H 26 → s3C12H 25 → s10C9H19 + C3H6 + s20C12H 24 (R1)

s0C12H 26 → s5C12H 25 → s13C7H15 + s25C5H10 + s26C10H 20 + C2H5

(R2)

s0C12H 26 → s6C12H 25 → s29C9H18 + nC3H 7 + s14C6H13 + s28C6H12

(R3)

The branching ratios of each reaction channel can be obtained by multiplying the element flux values of each stage. The initial composition is considered to be 100%, and then each reaction path channel has a percentage of the total channel of the initial component. When RP-3 aviation kerosene combustion is taken

Figure 1. Analysis of the initial combustion reaction path of ndodecane in different fuels.

Figure 2. Branching ratios of three main reaction channels of n-dodecane in seven combustion processes. 6770

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Figure 3. Analysis of the initial combustion reaction path of 1,3,5-trimethylcyclohexane in different fuels.

Figure 4. Branching ratios of three main reaction channels of 1,3,5-trimethylcyclohexane in seven combustion processes.

two-step path channels; that is, 16.10% × 75.05% ≈ 12.08%. This method is used in the calculation of other channel branching ratios. The computed results of the above three channels are shown in Figure 2. It can be seen that the three reaction channel branching ratios of BF-1 and BF-3 are larger than RP-3 when mixing H2 and C2H4 with RP-3, indicating that H2 and C2H4 have a positive effect on the main combustion reaction path of RP-3. On the contrary, the effect of the mixing of C3H6 is

as an example, the first step of the R1 reaction channel branching ratios is s0C12H26 → s3C12H25. As seen from Figure 1, the channel accounts for 16.10% of the total reaction channel of s0C12H26. The second step is s3C12H25 → s10C9H19 + C3H6 + s20C12H24. Similarly, the percentage of the total reaction channel of s3C12H25 is 75.05%, in which s3C12H25 → s10C9H19 + C3H6 accounts for 68.72% and s3C12H25 → s20C12H24 accounts for 6.33%. Finally, the branch ratio of reaction channel R1 is the product of the percentage of above 6771

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Figure 5. Analysis of the initial combustion reaction path of n-propylbenzene in different fuels.

accelerating the main reaction channels of 1,3,5-trimethylcyclohexane. In particular, the promotion of C2H4 in the R4 and R6 reaction channels is the most significant, as shown in panels a and c of Figure 4. In addition, by comparison of RP-3 and BF-5, it can be found that the ratio of the three reaction channels is lower than that of neat RP-3, indicating that the contribution of C3H6 is still inhibitive and not conducive to the three main reaction channel paths of 1,3,5-trimethylcyclohexane. n-Propylbenzene (PHC3H7) is one of alternative surrogates of aromatic hydrocarbons and also a main component in RP-3 aviation kerosene. Its main reaction path is more complicated than that of naphthenes. About 50% of n-propylbenzene will undergo hydrogen abstraction reactions to form the PHC3H6 radical, and another part will undergo C−C bond breaking to directly form C2H5 and PHCH2 radicals. PHC3H6 radicals are mainly consumed by the β-breaking bond, resulting in smaller benzene ring radicals and light olefins, such as C2H4 and C3H6. In addition, a small amount of BPHC3H6 radicals will be isomerized. Similarly, the main consumption of n-propylbenzene is analyzed, with the results that C2H4 still raises the branching ratios of the reaction channel of PHC3H7 → PHCH2 + C2H5 (Figure 5).

negative. The results are consistent with the calculation of the total disturbance of each blending component in section 3.1, indicating that the concept and formula of the variation disturbance method are reasonable. The reaction pathway of 1,3,5-trimethylcyclohexane is shown in Figure 3. The initial reaction is also to extract hydrogen from different positions of methyl and carbon rings mainly by OH and H active radicals to form initial C9H17 radicals. Most of the hydrogen abstraction reactions occur on the carbocyclic ring. A large part of C9H17 radicals (s15C9H17, s16C9H17, and s17C9H17) participate in the isomerization reaction, while a few undergo β-scission reaction paths. Methyl is detached from the carbocyclic ring to form CH3 and C8H14 radicals, or the C−H bond on methyl of C9H17 is broken during the β-scission processes. Subsequently, s49C9H17, s51C9H17, s50C9H16, s52C9H17, and s53C8H14 radicals are mainly involved in the isomerization reaction. In addition, CH3 will continue extracting hydrogen from some of the free radicals. The participation of six blending agents also does not have a great effect on the combustion path of naphthenes. Therefore, we select the following three main reaction channels of 1,3,5-trimethylcyclohexane for supplementary analysis. s1C9H18 → s15C9H17 → s49C9H17

(R4)

4. CONCLUSION This paper proposes a variation disturbance method based on the concept of the coefficient of variation. With the method, the effect of blending agents on the combustion performance of the primary fuel can be comprehensively evaluated and the influence can be quantified clearly. The computed results show that the degree of influence of hydrogen and five short-chain hydrocarbons on RP-3 aviation kerosene is significant. H2,

s1C9H18 → s16C9H17 → s51C9H17 + s50C9H16 + H (R5)

s1C9H18 → s17C9H17 → s52C9H17 + s53C8H14 + CH3 (R6)

The computed results of the three channels are shown in Figure 4. From Figure 4, H2 and C2H4 still contribute more, 6772

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CH4, and C2H4 have a greater degree of disturbance to RP-3, but C3H6 has a weaker influence. The calculated disturbance rate shows that H2 and C2H4 have a positive effect on combustion characteristics of RP-3, and especially, C2H4 promotes the performance of auto ignition and combustion temperature. In addition, to further verify the rationality of the variation disturbance method and its related calculation formula proposed in this work, the time-integrated element flux analysis method combined with the calculation of branch ratios of the reaction channel are used to analyze three important alternative components of RP-3. The reaction pathway shows that the blending of hydrogen and five small hydrocarbons retains the main reaction path of RP-3 but H2 and C2H4 with larger total disturbance values increase branch ratios of the main reaction channels. Thus, this indicates that they can promote the combustion performance of RP-3 and verify the scientificity and rationality of the proposed variation disturbance analysis method. The method can be further extended and verified on the basis of the experimental results of the fuel mixture combustion. The application of the variation disturbance method can also provide comprehensive and high-reference information for fuel design and combustion chamber optimization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00344. Combustion simulation results for different combustion characteristics of seven fuels and calculation of disturbance and combustion simulation for gasoline and H2/gasoline (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuhao Li: 0000-0001-8077-6017 Author Contributions †

Shuhao Li, Junjiang Guo, Zhenghe Wang, Shuanghui Xi, and Junxing Hou contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Scientific Research Project of Henan Higher Education Institutions of China (18A590001, 19A460030, and 17A470004) and partly supported by the aeronautical science foundation of China (2018ZD55008) and the Henan Natural Science Foundation (182300410186). The authors wish to thank these organizations for their financial support. Natural Science Foundation (182300410186). The authors thank Sichuan University for the combustion model.

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DOI: 10.1021/acs.energyfuels.9b00344 Energy Fuels 2019, 33, 6767−6774