Article pubs.acs.org/EF
Adsorption Characteristics of Coals Pyrolyzed at Slow Heating Rates Robert L. Krumm,* Keith W. Gneshin, and Milind Deo* Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Room 3290, Salt Lake City, Utah 84112-9203, United States ABSTRACT: Underground coal pyrolysis is a process similar to in situ oil shale production wherein heat is applied to deep coal formations to produce light hydrocarbons. Like enhanced coal bed methane, the injection of CO2 post thermal treatment can improve hydrocarbon recovery and serves as a means of carbon sequestration. Little information exists pertaining to coals pyrolyzed to temperatures expected with this process. This work examines the development of meso- and micropores and their influence on methane and carbon dioxide adsorption and permeability on a thermally treated Utah bituminous coal. Residual tars were found to affect the pore size distributions in pyrolyzed coals. Generally, with increasing treatment temperature, there are more meso- and micropores. A correlation was established between the prevalence of mesopores in pyrolyzed coals and adsorption and, to a lesser extent, permeability. The treatment temperature of this particular coal is directly related to the amount of CO2 said coal can store and how the plume of injected CO2 moves through the formation.
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INTRODUCTION Anthropogenic CO2 and its implications on climate change are currently a public concern. Furthermore, growing global demands for hydrocarbons have driven interest into alternative sources such as oil shale, underground coal gasification (UCG),1 enhanced coalbed methane (ECBM),2 and underground coal pyrolysis (UCP).3 UCP draws many analogues to in situ oil shale pyrolysis and underground coal gasification in that they all use heat to recover resources without mining. With increasing regulations on conventional coal operations, UCP may serve as a viable alternative to recover energy from this immense resource. The UCP process is centered on applying heat to a deep, unmineable coal seam to pyrolyze the coal and produce hydrocarbons.4 Advantages of the UCP process include producing a product with a higher H:C ratio than the original coal, and like ECBM, the remaining coal can be used as a repository for CO2; since pyrolysis increases the porosity and surface activity of the coal, it is expected that the storage potential of the remaining material would have an improved CO2 capacity over that of untreated coal. Furthermore, the injected CO2 may also serve the purpose of displacing some of the remaining hydrocarbons. In order to determine the CO2 storage potential of a pyrolyzed coal seam, it is important to understand some of the characteristics of coal pyrolyzed with slow heating rates. There is little information regarding coals treated under conditions encountered during the UCP process.5 The scale of a UCP operation is similar to that of in situ oil shale pyrolysis where slow heating rates and low final temperatures are expected. There is going to be a gradient of treatment temperatures with the highest temperature being closest to a heating well. Due to lack of information, and since the pyrolyzed coals from UCP fall somewhere as an intermediate between a fresh coal and an activated carbon, it is important to make comparisons of the data presented in this work to other works concerned with fresh coals, chars, and activated carbons. Generally, activated carbons are heated to a greater extent than pyrolyzed coals. Products from the UCP are expected to retain some residual © XXXX American Chemical Society
hydrocarbons on the surface of the pyrolyzed coal unlike an activated carbon. Adsorption measurements on fresh coals have been extensively studied.6−15 Greaves et al.9 and Arri et al.16 were some of the first to report the preferential adsorption of CO2 over CH4. The National Energy Technology Laboratory17 released a report assessing the use of ECBM as a means of geologic sequestration of anthropogenic CO2. The data presented in the report showed that, for a variety of wet coals, CO2 adsorption is preferred over methane at ratios up to 4:1. The data also showed that the CO2/methane adsorption ratio increases when the coal is dry. Arri et al.16 performed methane−nitrogen and methane− CO2 sorption experiments on a wet Fruitland (San Juan basin) coal sample under reservoir temperatures and pressures. They found that the extended Langmuir isotherm model fits well for data at pressures less than 1000 psi. The raw data from their experiments shows that CO2 is preferably adsorbed over methane and that methane is preferably adsorbed over nitrogen. The use of the Langmuir equation to describe adsorption on coal is a common practice but does have its limitations; for example, CO2 adsorption on coal deviates from the Langmuir equation at pressures exceeding 1450 psi.12 The Dubinin−Astakov18 and Dubinin−Radushkevich19 models have been shown to provide better fits at high pressures,20,21 but since the isotherms studied in this paper were less than these conditions, the Langmuir model was sufficient. Activated carbons derived from coals for use as an adsorbent have been thoroughly studied for separation processes ranging from purification of gases to liquid phase extraction.22−29 The pyrolyzed coals produced by UCP will have similar properties to those of activated carbons produced from coal. Data from Yang and Saunders30 shows that thermal treatment of a Montana lignite and Pittsburgh bituminous increases the Received: November 22, 2016 Revised: December 19, 2016
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Energy & Fuels adsorption capacity. Moroto-Valer, Tang, and Zhang31 showed how thermal activation of anthracites increases its adsorption capacity of CO2. Moroto-Valer et al. attributes the increase in storage capacity to the increase in microporous surface area. Moroto-Valer et al. also point out that an increase in micropores does not necessarily mean that there will be an increase in adsorption capacity, but rather, the sizes of micropores also play a significant effect. Furthermore, Moroto-Valer et al. showed that pore development is highly dependent on the temperature and duration of thermal treatment; in some instances, the microporous surface increases and then decreases with duration of thermal treatment. Gavalas et al.32 showed micropore volumes before and after pyrolysis for a highly volatile bituminous coal and a subbituminous coal. Gavalas et al. used BJH to determine the micropore distribution, and measurements were conducted with nitrogen, which has been noted to produce troublesome data when used for this application.33 Thermal treatment of coal will result in increases in porosity and permeability.34 Mastalerz et al.35 investigated the effects of meso- and micropores on fresh coal and how they affect CO2 adsorption. The International Union of Pure and Applied Chemistry standards36 define mesopores as having an aperture of 2−50 nm and micropores as having a diameter 0.03 bar) that pyrolyzed coal provides approximately twice the CO2 adsorption capacity of fresh coals. They also showed that the surface area of the pyrolyzed coals increased by 50−60% over the fresh coals, an effect also seen in early adsorption experiments with the UCP project. This work focuses on the adsorption of CH4 and CO2 on a Utah bituminous coal that has undergone thermal treatment similar to that encountered during UCP. The fresh coal and pyrolyzed products were analyzed for surface area and pore size distributions with the purpose of correlating micropores and/or mesopores to adsorptive capacity. Solvent extraction on the coal was done to determine the effects of residual tars on pore sizes. Permeability of the thermally treated coal was also measured. All of the aforementioned measurements were done to relate some of the physical properties of thermally treated coals to heating rate and treatment temperature with the ultimate goal of providing a better understanding of the UCP process and the CO2 sequestration potential of the remaining material.
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EXPERIMENTAL PROCEDURE
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RESULTS AND DISCUSSION
Coal from the Skyline mine near Helper, Utah, was used for all experiments. The original coal was sent to Huffman Laboratories in Golden, CO, for elemental and moisture analysis. All coals were crushed and sieved to about a 2.54 cm diameter. A tube furnace with a sweeping flow of nitrogen was used to thermally treat the coal. The tube furnace is similar in construction to other tube furnaces used for coal pyrolysis.38,39 Coal samples were thermally treated to final temperatures of 325, 450, or 600 °C. Treatment temperatures were chosen as a range of probable UCP treatment temperatures. The heating rate for the coals was either 10 or 0.1 °C/min. If the sample was heated at 10 °C/min, it was held at the final treatment temperature for 24 h; if the coal was heated at a rate of 0.1 °C/min, it was held at the final temperature for 4 h. The final temperature hold times were validated by thermogravimetric analysis as a reasonable amount of time for the pyrolysis reaction to come to completion. Samples for isotherm measurements were ground to approximately 105 μm (140 mesh) using a SPEX shatter box. A small particle size is beneficial when taking adsorption measurements as it reduces the amount of time required for the isotherm apparatus to come to equilibrium. As the ratio of internal surface area to external surface area of coals is about 100:1, the effect from surface area changes on adsorption due to grinding is minimal.8 All samples were dried in a vacuum oven at 80 °C for at least 36 h to ensure that all moisture was removed from the coal.7 Removing the moisture from the coal is important as moisture can have a great effect on adsorption.38 The isotherm measurement apparatus used was identical in design to that used by Mavor et al.8 Isotherm measurements were conducted at TerraTek in Salt Lake City, Utah. Due to the length of time necessary for the isotherm system to come to equilibrium (hours to days per data point), three isotherm measurements were executed simultaneously. The apparatus used at TerraTek took volumetric adsorption measurements on the pyrolyzed coals. Adsorption isotherms were measured at 50 and 70 °C using CO2 and CH4 as the adsorptive. The temperatures chosen represent the upper and lower bounds of the formation temperature after UCT. One of the primary products of coal pyrolysis is methane40 which will adsorb on the coal in increasing amounts as the formation cools; the gases chosen represent the primary gaseous product and the CO2 being injected later. Permeability measurements were also done at TerraTek using a helium pressure decay permeameter. Samples for permeability measurements were crushed and sieved between 0.85 and 1.4 mm, thereby eliminating the effects of fractures that would be found in larger coal samples. Surface area and pore size distribution measurements were taken on all the coals with a Micromeritics Tristar II BET surface area analyzer. All coal samples for BET were off gassed in a vacuum oven for at least 36 h at 80 °C. Samples for BET measurements were cone and quartered with half of the sample treated with Soxhlet extraction41 with acetone. Acetone was found to be a good solvent for removing residual pyrolysis tars after a screening of various solvents; aggressive solvents such as pyridine were avoided as to not damage the overall coal structure.42 The surface area and pore size distribution were determined on the acetone treated samples in order to determine what fraction of the surface area can be attributed to the coal structure and what is attributed to residual tars. Both density functional theory (DFT) and Barrett, Joyner, and Halenda (BJH) measurements were done to encompass a wide range of pore sizes. Carbon dioxide was used as the adsorbent in the surface area and pore size measurements as problems can occur when measuring the surface area of coals and chars with nitrogen.33,43
The results of the elemental analysis of the Skyline coal found that it is 64.7% carbon, 5.6% hydrogen, 1.1% nitrogen, and 4.0% sulfur. The fresh coal is 36.8% volatile matter and 8.0% ash. Samples were weighed before and after each pyrolysis B
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exception being the samples were heated to a final temperature of 325 °C, where the opposite is observed. Surface area measurements on the fresh coal and the thermally treated coals show that coals treated to 325 °C had less surface areas than the original coal and coals treated to higher temperatures. The reduction in surface area may be due to plastic deformation of the coal or the deposition of pyrolysis tars in pores. Gneshin et al. explored porosity development of certain coals under very slow heating rates44 and saw similar softening. Solvent extraction removes residual hydrocarbons effectively, cleaning any obstructed pores; comparing the before/after solvent extraction provides insight into the nature of the residuum and the pores contained therein. Solvent extraction on the coals treated to 325 °C and the fresh coal caused an increase in surface area meaning that some pores were obstructed by high molecular weight hydrocarbons. Coals heated to 450 and 600 °C showed an increase in surface greater than that of the original coal and coal heated to 325 °C. Solvent extraction on the coals treated with a heating rate of 10 °C/min
experiment. The percentage of mass loss due to devolatilization and pyrolysis can be found in Table 1. The mass losses in Table Table 1. Mass Lost During Pyrolysis of Skyline (Utah) Coal Treated to Different Final Temperatures with Heating Rates of 10 or 0.1°C/mina
a
pyrolysis treatment
percentage mass loss (DAF)
325 °C at 10 °C/min 450 °C at 10 °C/min 600 °C at 10 °C/min 325 °C at 0.1 °C/min 450 °C at 0.1 °C/min 600 °C at 0.1 °C/min
8.09 25.48 30.3 11.25 19.15 26.21
The results are reported on a dry, ash-free basis (DAF).
1 are reported on a dry, ash-free (DAF) basis. The samples treated with a quicker heating rate often showed a larger mass loss than samples heated with a slower heating rate with the
Figure 1. (Top) Pore size distributions of the Skyline coals treated with a heating rate of 10 °C/min. (Bottom) PSDs of Skyline coals treated with a heating rate of 0.1 °C/min. The dip occurring at about 20 Å is due to the lack of overlap from the data obtained using DFT and data obtained using BJH. C
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Figure 2. Example of adsorption isotherms for thermally treated Skyline coal. Moles of adsorptive per kilogram of coal. Note that the untreated coal has a larger adsorptive capacity than the coal treated to 325 °C and that coals that showed more pore volume in the pore size distribution also exhibit higher adsorptive capacities.
to either 450 or 600 °C resulted in an almost negligible decrease in surface area suggesting that pores were unobstructed. Solvent extraction on coal treated to 450 °C with a heating rate of 0.1 °C/min showed a slight increase in the surface area while the coal treated to 600 °C at 0.1 °C/min showed a decrease in the surface area. If eluting tars are transported to the surface of the coal and are vaporized, the residual tars may have pitted and provided additional surface area. Evidence that a portion of the surface area developed with slow heating rates is in residual tars can be seen in how solvent extraction reduces the surface area in some of the samples. Figure 1 shows the pore size distributions for all of the coal samples treated under the different conditions. The dip occurring around 20 Å is from a gap in the data from DFT and BJH. DFT measured pores less than 20 Å while BJH was used to measure pores larger than 20 Å. Pore volumes increase with treatment temperature after 325 °C. Similar to the surface area measurements, coals treated to a final temperature of 325 °C showed fewer pores than the fresh coals. The reductions in the pore sizes may be the result of the coal softening and pores collapsing or the result of produced hydrocarbons filling pores.44,45 From Figure 1 the effect of residuals extraction with a mild solvent can be noticed. For example, the solvent-extracted coal treated to 600 °C with a heating rate of 10 °C/min shows a reduction in the amount of micropores compared to a coal
treated to the same temperature with a heating rate of 0.1 °C/ min. With solvent extraction on a coal sample heated to the same final temperature at a rate of 0.1 °C/min there are reductions in pore volumes in both the meso- and micropore range. This reduction in pore volume suggests that some of the pores in the thermally treated coals exist in the surfaces of residual pyrolysis hydrocarbons, the effect of which is more noticeable with slower heating rates. The samples treated to 325 °C show increases in micro- and mesopores with solvent extraction; this lends credibility to some of the reductions in pore size being residual hydrocarbons depositing in the pores. The coals heated at 0.1 °C/min to a temperature have a similar PSD to the solvent extracted fresh coal while the coal treated at 10 °C/min still has less pore volume than the fresh coal without solvent extraction. This is likely the result of porosity development in the coal matrix itself, i.e., microfissures.44 For better conceptualizing changes in pores in coal undergoing slow pyrolysis, it is mentally beneficial to compare this process to something else that undergoes deformation when heated, Swiss cheese. Swiss cheese works well for this mental exercise as it has naturally occurring small and large pores representing the initial pores in the coal. As the cheese is heated, it softens, and some of the original pores are reduced in size or completely closed off; this is like heating the coal to 325 °C. Once a certain temperature is reached, components D
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Energy & Fuels Table 2. Langmuir Fitting Parameters for Isotherms on Coals Thermally Treated with a 10°C/min Heating Ratea coal
Langmuir fit
skyline
V∞ β V∞ β V∞ β V∞ β
325@10 450@10 600@10 a
CH4@50 °C
CH4@70 °C
CO2@50 °C
CO2@70 °C
1.174 1.972 1.034 2.473 1.404 4.884 1.957 6.289
0.885 2.425 1.072 1.341 1.393 2.952 1.742 4.434
1.590 8.600 1.267 8.938 1.666 1.237 1.947 1.392
1.213 6.490 1.742 3.561 1.742 8.366 1.990 9.679
× 10−02 × 10−02 × 10−02 × 10−02
× 10−02 × 10−02 × 10−02 × 10−02
× 10−02 × 10−02 × 10−01 × 10−01
× 10−02 × 10−02 × 10−02 × 10−02
V∞ is in moles adsorbed per kilogram; β is in 1/bar.
Table 3. Langmuir Fitting Parameters for Coals Treated with a 0.1°C/min Heating Ratea coal skyline
[email protected] [email protected] [email protected] a
Langmuir fit V∞ β V∞ β V∞ β V∞ β
CH4@50 °C
CH4@70 °C
CO2@50 °C
CO2@70 °C
1.174 1.972 0.952 2.020 1.224 3.485 1.720 6.541
0.885 2.425 0.964 1.803 1.208 2.677 1.818 5.319
1.590 8.600 1.353 8.740 1.584 1.684 2.081 2.944
1.213 6.490 1.076 8.711 1.273 1.440 1.841 1.905
× 10−02 × 10−02 × 10−02 × 10−02
× 10−02 × 10−02 × 10−02 × 10−02
× 10−02 × 10−02 × 10−01 × 10−01
× 10−02 × 10−02 × 10−01 × 10−01
V∞ is in moles adsorbed per kilogram; β is in 1/bar.
Figure 3. Micropore volume (top) and mesopore volume (bottom) versus V∞ from the Langmuir equation for CO2. A stronger correlation can be found between the adsorptive capacity and mesopore volume than with micropore volume. The trends with CH4 are very similar to the trends presented here.
volatilize creating internal pressure causing bubbles that eventually burst. If the heating rate is fast enough, some of these bubbles readily solidify, but if the heating rate is slow,
some of these bubbles are still subject to closure from plastic deformation. Oil that separated from the cheese collects on the surface and in the pores. Now imagine that this oil can dry out E
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Figure 4. Permeability measurements of fresh and thermally treated Skyline coal. The permeability of the coals increased with treatment temperature.
the formation is substantial,46 it is unlikely UCP offers a net positive CO2 sequestration compared with ECBM. Previous researchers have studied the effects of micro- and mesopores on adsorption isotherms. Mastalerz et al.35 found a correlation between the mesopore volumes of several different types of coal and CO2 adsorptive capacity contrary to Clarkson37 who claimed that micropores are the primary contributor to adsorptive capacity. While most of the previous research is focused on relating pore sizes to fresh coals, this study investigated relating pore sizes to coals that have undergone pyrolysis with slow heating rates. Plotting the micropore or mesopore volume versus the theoretical maximum adsorptive capacity from the Langmuir equation provides the correlations represented in Figure 3. From Figure 3 it can be seen that there is a stronger correlation between mesopore volume and adsorptive capacity than with micropore volume. The V∞ values used in Figure 3 are for the CO2 isotherms measured at 50 °C. The lower range of normalized mesopore volumes is equivalent to the upper range of micropore volumes; i.e., there are more mesopores acting as adsorptive sites which might explain the stronger correlation. The results of the permeability measurements on the coals showed that some of the same qualitative trends are observed with the isotherm measurements and the pore size distributions. Coals treated to a final temperature of 325 °C had a measured permeability less than that of the fresh coal, and permeability increased with treatment temperatures. Since the coals had to be crushed for the permeability measurements, the effects of cleats on the overall permeability are not accounted for. The permeability through the cleats would be orders of magnitude larger than the permeability in the coal matrix. The results of the permeability measurements can be found in Figure 4. Matrix permeability is a function of the radii of pore throats, pore connectivity, and pore size; literature values place coal permeabilities in the microdarcy range.47 It needs to be noted that in situ the permeability will be affected by stress and coal swelling upon the introduction of CO2,47−49 which are expected to reduce permeability. Figure 1 shows the prevalence of pores greater than 20 Å increases for coals heated at a 0.1 °C/min heating rate from 450 to 600 °C while there is little changes in >20 Å pores heated at 10 °C/min to the same temperatures. Assuming pore throats are geometrically propor-
and doing so creates even more pores; this is akin to pores developing in the residual tars of the coal only to have the porous media washed away with solvent extraction. Example adsorption isotherms for the coals can be found in Figure 2. It should be noted that the pressure-extremity should be considered a “medium pressure” type experiment. If we assume the adsorptive characteristics of thermally treated coals are similar to their fresh counterparts,12,38 it intuitively makes sense that the adsorptive capacity of CO2 surpassed that of CH4. Also expected was that the adsorptive capacity is greater when measured at 50 °C than at 70 °C. Adsorption values were fitted to the Langmuir adsorption equation which provides theoretical maximum adsorptive capacities and curvature of the isotherm. The Langmuir equation defines the adsorbed phase as a single layer; with coals this is not the case. The Langmuir equation, however, does fit the adsorption data with a good degree of agreement and is commonly used to model adsorption on coals. Langmuir fitting parameters for the samples can be found in Tables 2 and 3; in these tables the units of V∞ is in moles adsorbed per kilogram, and β is in 1/ bar. Adsorption isotherms for the thermally treated coals showed trends similar to what was observed with the pore size distributions and surface area measurements. Isotherms on coals treated to 325 °C showed less adsorptive capacity than the fresh coals, and capacity of the coals increased with treatment temperature. The effect of heating rate on adsorptive capacity is less pronounced than their respective pore size distributions. The increase in storage capacity for thermally treated coals shows promise for using the remnant coal from the UCP process as a CO2 repository. The final treatment temperature will decrease with distance from the heater; likewise, the CO2 storage capacity will also decrease with distance from the heater. Because the adsorptive capacity of the thermally treated coal increases, the UCP process may be advantageous over ECBM from a CO2 storage perspective; however, this claim is entirely contingent on the difference between the amount of CO2 produced from heating the coal and the adsorptive capacity of pyrolyzed coal being greater than the amount of CO2 that could be stored in the fresh coal. Because the CO2 produced heating F
DOI: 10.1021/acs.energyfuels.6b03116 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels tional to pore size, it is intuitive why coals treated at 0.1 °C/ min show a permeability increase from 450 to 600 °C while coals treated at 10 °C/min do not show the same trend. In a post UCP injection scenario, the CO2 should permeate into the coal matrix more quickly nearer to the heater than further. This would also mean that the injected CO2 would liberate adsorbed hydrocarbons faster near the wellbore. In an actual injection scenario swelling resulting from the CO2 may reduce the permeability;50 however, investigation into this phenomenon would require additional experiments.
Future studies are already planned for other coals to determine the effect of coal rank on the meso- and micropore development and, by extension, adsorptive capacity and permeability during the UCP process.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail (R.L.K.):
[email protected]. *E-mail (M.D.):
[email protected].
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ORCID
Robert L. Krumm: 0000-0003-0697-7894 Milind Deo: 0000-0002-4569-7097
CONCLUSION Properties of Skyline coal treated to temperatures of a UCP process with slow heating rates such as adsorptive capacity and permeability are related to the final treatment temperatures of the coal. The primary means of relating these properties to each other can be done using the pore size distribution; the pore size distribution can be affected by residual hydrocarbons which can fill pores or serve as an additional porous medium. Extraction of high molecular weight hydrocarbons with a mild solvent is useful for differentiating pores in the coal matrix and pores in the residual redisuum. The higher treatment temperatures during the UCP process provide a solid product more favorable for permanent CO2 sequestration. A treatment temperature of 325 °C reduces the pore volume adsorptive capacity and permeability of the coal compared to the fresh coal. Treatment temperatures exceeding 450 °C increase the pore volumes, adsorptive capacity, and permeability. The improved adsorptive capacity and permeability are advantageous from a CO2 storage perspective when compared to ECBM. The results of the surface area, pore size distribution, adsorption, and permeability measurements on a Skyline bituminous coal can be outlined as follows. • Adsorption isotherms and permeability measurements share similar trends with the pore size distributions of the coals. This is expected and has been expressed by previous researchers.37 There is a stronger correlation between mesopore volume than micropore volume; this might be because there are greater amounts of mesopores. • Fresh coal and coal treated to 325 °C show increase in pore volumes after solvent extraction while coals treated to 450 or 600 °C show decreases in pore volumes after solvent extraction. Solvent extraction removes pores in the high molecular weight hydrocarbons; ergo, we conclude that a portion of the pores in the 325 °C samples are obstructed by these hydrocarbons and an appreciable fraction of pores in coals treated to 450−600 °C exist in hydrocarbon residuum. • Coals treated to 325 °C have fewer pores, less adsorptive capacity, and less permeability than the fresh coal. This was attributed to two things: plastic deformation causing pore collapse and residuum filling or blocking pore openings. • Pore volume increased with coal treated to 450 °C and further increased with coal heated to 600 °C. Adsorptive capacity of the coals increases with treatment temperature following trends in pore size distributions. • The permeability of the thermally treated coal generally increases with treatment temperature following trends in pore size distributions.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful to Dr. John McLennan for helping to arrange the relationship with TerraTek (a Schlumberger company) that allowed us to perform some of these measurements and to Dr. Keith Gneshin for being an exemplary partner in this research. We would also like to thank Keith Greaves for his help with the measurements. This work was funded by the United States Department of Energy award number NT0005015.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b03116 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.6b03116 Energy Fuels XXXX, XXX, XXX−XXX
