Halogen-Mediated Partial Combustion of Methane in Molten Salts To

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Halogen-Mediated Partial Combustion of Methane in Molten Salts To Produce CO2‑Free Power and Solid Carbon D. Chester Upham,† Zachary R. Snodgrass,‡ Clarke Palmer,‡ Michael J. Gordon,‡ Horia Metiu,† and Eric W. McFarland*,‡ †

Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States Department of Chemical Engineering, University of California, Santa Barbara, California 93106-5080, United States

‡

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/21/18. For personal use only.

S Supporting Information *

ABSTRACT: The partial oxidation of methane to carbon and steam was investigated in molten salts for a process to produce CO2-free electrical power and solid carbon. Lithium iodide and lithium bromide catalysts were used in a bubble column where insoluble carbon accumulates on the melt surface and could be continuously removed. The salt acts as a heat transfer medium and reacts with oxygen to produce halogens and consume hydrogen halides in a chemical looping cycle. The halogens react with methane in gas-phase bubbles and form hydrogen halides and carbon. Hydrogen halides are then neutralized by an oxide and form steam and a halide salt. The halide salt reacts with oxygen, forming an oxide and closing both the halogen and the salt chemical looping cycles in a single vessel. Selectivities to carbon of 90% were measured for 56% methane conversion in a 12 cm bubble column reactor. The carbon was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy. Iodide and bromide salts were investigated along with the behavior of iodine, bromine, methyl iodide, and methyl bromide intermediates. KEYWORDS: Iodine, Bromine, Carbon black, Partial oxidation, Alkane

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reaction floats to the surface of molten metals where it can be continuously removed. When methane is pyrolyzed, and the resulting hydrogen is combusted, the net reaction is a two-step reaction equivalent to reaction 2, and CO2-free power can be produced. A single-step process based on reaction 2 has advantages over the two-step process. First, losses associated with integration of the high temperature endothermic pyrolysis reaction with the exothermic combustion of hydrogen are avoided in a direct, single step process. Second, methane pyrolysis to hydrogen and carbon is equilibrium limited below 1000 °C, whereas reaction 2 is not. The challenge of avoiding carbon oxide formation has been recently addressed with separate methane and oxygen feeds in a multistep chemical looping process for partial combustion.7,8 Bromine is used to activate methane and produce carbon and hydrogen bromide (reaction 3, when X = Br). Carbon is then separated in another vessel. In another reactor, hydrogen bromide reacts with oxygen to produce water and regenerate bromine (reaction 4, when X = Br). The net reaction is methane combustion to carbon and water. However, carbon separation is challenging, and the process requires multiple corrosion resistant reactors and an energy-intensive water separation between reactors.

INTRODUCTION The combustion of carbon-containing materials to produce power has provided abundant, low-cost power for centuries. Today, and for the foreseeable future, a natural gas combined cycle power station (NGCC) provides the lowest cost electricity.1 NGCC utilizes complete oxidation of methane to carbon dioxide, water, and heat (reaction 1).

If methane oxidation could be limited to forming carbon and water (reaction 2), carbon dioxide would not be produced. The amount of heat per mole of methane is less when carbon is produced instead of carbon dioxide; however, the solid carbon produced from this reaction can be sold or stored in perpetuity. In addition, carbon dioxide is not released into the atmosphere. Production of solid carbon also avoids the costs and challenges associated with carbon dioxide sequestration. Selectivity and catalyst stability are a challenge for this process. Selective partial oxidation of methane using oxygen has not been demonstrated with acceptably low selectivity to carbon oxides for any carbon-containing product.2−5 Moreover, solid catalysts with high activity would likely deactivate due to carbon deposition. The challenge of managing coproduced carbon (coke) was recently addressed using liquid catalysts for methane pyrolysis in bubble column reactors.6 The carbon produced by the © XXXX American Chemical Society

Received: August 22, 2018 Revised: October 5, 2018 Published: October 8, 2018 A

DOI: 10.1021/acssuschemeng.8b04168 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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Research Article

EXPERIMENTAL SECTION

Reactor Design and Reaction Procedure. Quartz bubble columns with an inner diameter (ID) of 12.7 mm and an inlet tube with an ID of 3 mm were used unless otherwise specified. As illustrated in Figure 2, the height of the molten salt inside the column

The use of other halogens (X) has not been reported for this reaction; however, we have some understanding of how they would behave based on other alkane dehydrogenation reactions that have been studied.2,9,10 Generally, the choice of halogen involves trade-offs in the rates of reactions 3 and 4. Cl2 reacts rapidly with methane at 300 °C, but HCl oxidation requires a catalyst, higher temperatures, and is subject to equilibrium limitations.11 I2 requires temperatures in excess of 600 °C to react with methane, but HI oxidation occurs at temperatures below 200 °C.12 Br2 reacts with methane at lower temperatures than I 2 , and HBr regeneration can be accomplished at lower temperatures than those necessary for HCl regeneration. The “partial combustion” of methane to carbon and steam (reaction 2), in a single reactor, has not been reported. In previous work, a single step process for propane oxidative dehydrogenation has been investigated using molten LiI− LiOH.13−16 In this previous process, HI oxidation (reaction 8) occurs in the same reactor as hydrocarbon iodination, and only trace carbon oxides were formed due to oxygen’s rapid reaction with LiI (reaction 5). Other salts have also been reported as chemical looping catalysts for HX oxidation in separate steps, where a metal oxide (MO) first oxidizes HX to water and a metal halide (MX).15,17−20 The metal halide then reacts with oxygen in a separate reactor to form the halogen and regenerate the metal oxide. Note that reactions 5−7 are equivalent to reaction 4 when iodine is used; reactions 3 and 4 are equivalent to reaction 2.

Figure 2. Schematic of the bubble column reactor, gas supply, and analytics. All materials in contact with halogens were made from glass, PTFE, or Hastelloy. was 12 cm. The reactor system employed an online mass spectrometer (Stanford Research Systems RGA 300) to analyze the products of reaction. All tubing was constructed from glass or Hastelloy-C with ground glass joints or graphite ferrules (Figure 2). Heated lines delivered effluent gases directly to the mass spectrometer by way of a glass capillary tube, so a complete material balance was maintained. Iodine and bromine were delivered as vapors, sourced from evaporators operating at vapor−liquid equilibrium and carried by argon gas. Argon was delivered using mass flow controllers (MKS 1179). The gases were combined and delivered to the quartz bubble column reactor, heated by contact with a stainless-steel heating block and two 350 W Omega Engineering heating cartridges. After the stream exited the hot reactor, a helium gas quench was introduced to the reactor effluent. The effluent then passed through a Hastelloy junction where a glass capillary tube (the ID was 0.025 mm) transported gases directly to the mass spectrometer. Each data point reported represents a single measurement after the product concentrations reached a pseudosteady and changed less than 10% over 30 min or more. Conversions were calculated by dividing the difference in inlet and outlet values by the inlet values as measured in a mass spectrometer (Figure 2). All values were normalized to an argon carrier gas. Space time is defined as the reactor volume in cubic centimeters divided by the total molar flow rate entering the reactor in standard cubic centimeters per minute (sccm). Selectivity to carbon oxides is defined as the molar flow rate of carbon oxides in effluent species divided by the molar flow rate of methane converted. Selectivity to carbon is defined as the difference in total moles of carbon in the effluent species and the moles of methane converted divided by the moles of methane converted. Hydrogen selectivity is defined as the difference in the atoms of H in the effluent species divided by the atoms of H in CH4 converted. Characterization. The accumulated carbon powder from the top of the melt was characterized using Raman spectroscopy and scanning

This Article describes results of methane partial combustion (reaction 2) in reactive molten salts. Our hypothesis is that rapid consumption of molecular oxygen, by reaction with LiI, will limit carbon oxide formation. Iodine will be generated and react with methane in the gas phase. The resulting hydrogen iodide will then react with LiOH, completing the catalytic halogen cycle. This combination of reactions would enable the application of methane partial combustion for CO2-free power generation as shown in Figure 1. Heat can be removed from the reactor using steam. The steam produced by the reaction is also collected, and both sources could be fed into a steam turbine, similar to power generation from molten-salt nuclear reactors.21

Figure 1. Bubble column reactor for partial combustion of methane to carbon, water, and CO2-free electrical power, mediated by halogens in a molten halide salt. Carbon is continuously separated and removed. B


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ACS Sustainable Chemistry & Engineering electron microscopy (SEM). A Horiba Jobin Yvon LabRAM Aramis Raman spectrometer was used to obtain spectra using 633 nm excitation. The D peak (1350 cm−1) and G peak (1600 cm−1) were assigned,22 where the G peak is due to the relative motions of sp2 carbon atoms, and the D peak is linked to breathing modes of rings, which are only present at disordered centers. The intensity ratio of the D peak divided by the G peak, I(D)/I(G), was calculated using the maximum intensity for each peak. For SEM, an FEI Nova Nano 650 FEG SEM was used. Microkinetic Model. The pathways for reactions of methane with halogen were described by elementary reaction steps, and a microkinetic model was constructed using the best available rate constants compiled by the National Institute of Science and Technology (NIST).23 The coupled reaction network was solved for isothermal and isobaric conditions. Activation energies and preexponentials were also taken from NIST. Sufficient data were available for reactions between iodine and methane; however, reactions between hydrocarbons and oxygen were not modeled due to insufficient data. The reactions considered are listed in the Supporting Information and in Table 1.

Figure 3. Effect of oxygen:methane ratio on methane conversion, selectivity, and H2O:H2 molar ratio. The temperature was 650 °C, the methane pressure was 0.3 bar, and the ratio of LiI:LiOH was 1:1 mol. The oxygen:methane ratio is expressed as a molar ratio, and oxygen conversion exceeds 98% in all cases.

a portion of the generated iodine did not completely react. Complete methane conversion is possible, for example, in a longer bubble column where iodine and methane can react further. On the basis of the material balance, some of this iodine left the reactor in the effluent stream and some of it dissolved in the melt, possibly as I4.2−14 The role of this dissolved iodine and other reactor configurations are discussed later. The effect of temperature on conversion and selectivity was also investigated (Figure 4). Higher temperatures resulted in

Table 1. Elementary Steps for Reactions between Methane and Iodine in the Gas Phasea no.

reaction

A (1013 cm3/mol·s)

Ea (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11

I2 → I + I I + H2 → HI + H H + I2 → HI + I H + HI → H2 + I I + I → I2 I + HI → I2 + H I + CH4 → HI + CH3 I2 + CH3 → CH3I + I CH3I + I → CH3 + I2 CH3 + HI → I + CH4 CH3I → CH3 + I

8.43 16.9 43.1 4.74 23.6 80.1 89.1 0.999 20.0 0.415 1.00

126.0 141.0 1.8 2.7 −4.8 155.0 146.0 6.3 82.8 5.4 226

a

The pre-exponentials and activation energies are taken from the National Institute of Science and Technology (NIST).

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RESULTS AND DISCUSSION I2-Mediated Methane Partial Combustion. Partial combustion was investigated in a bubble column of molten LiI−LiOH. Only methane and oxygen were fed, and the effect of oxygen-to-methane feed ratios in a 1:1 molar ratio of LiI:LiOH bubble column at 650 °C is shown in Figure 3. Oxygen reacts with LiI to generate iodine (reaction 9), which initiates C−H bond cleavage (reactions 10 and 11). At low oxygen:methane ratios, little iodine is produced, so the reaction rate is slow and methane conversion is also low. Selectivity to solid carbon is high, because there is relatively little oxygen to produce carbon oxides. Too much oxygen contributes to nonselective carbon oxide formation. At high oxygen:methane ratios, conversion is higher, and selectivity to solid carbon is lower as more CO is produced.

Figure 4. Methane conversion as a function of temperature in a bubble column with 1:1 mol LiI:LiOH, 0.3 bar methane, and 0.3 bar oxygen.

higher methane and oxygen conversions. At temperatures below 600 °C in Figure 4, oxygen does not react completely before the top of the bubble column. In this region, selectivity to carbon decreases with temperature due to increased selectivity to carbon oxides. This indicates that the rate of carbon oxide formation is a stronger function of temperature than the rate of lithium oxide formation. At temperatures above 600 °C, the selectivity to carbon and carbon oxides is independent of temperature (Figure 4). This can be explained by the fact that complete oxygen conversion occurs before the top of the bubble column, resulting in a region in which methane continues to react in the absence of oxygen. At higher temperatures, this oxygen-deficient region is larger because oxygen reacts more rapidly earlier in the

In a continuous process, oxygen:methane must be 1:1, stoichiometric, or lower, to prevent net conversion of LiI to iodine. In the experiments reported in Figure 3, oxygen conversion was always higher than methane conversion, and so C


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Figure 5. Kinetic plots for determination of activation energy and reaction orders. (a) Plot of the natural log of the reaction rate constant versus the reciprocal temperature at 0.22 bar CH4 and 0.22 bar O2, (b) methane conversion rate’s dependence on methane partial pressure at 575 °C with constant oxygen pressure and less than 10% methane conversion, and (c) methane conversion rate’s dependence on oxygen partial pressure at 575 °C with constant methane pressure. In all cases, the reaction between oxygen and methane took place in a bubble column of 1:1 mol LiI:LiOH. Argon was used to keep the total pressure at 1 bar in all cases.

hydrocarbons. In this experiment, iodine and methane were fed to a bubble column containing LiI−LiOH (Figure 6) in the absence of molecular oxygen. The major gas-phase products observed were water and smaller quantities of hydrogen. Negligible carbon oxides were observed up to 650 °C,

column. Additional methane conversion is therefore observed without increased selectivity to carbon oxides. Each data point in Figure 4 was collected after 30 min of steady reactor operation. Steady methane conversion at 650 °C was observed for 3 h (Figure S1), but at 700 °C, methane conversion began to decrease after 30 min. At the same time, selectivity to CO2 increased, possibly due to the formation of Li2O, which was observed visually at the reactor inlet. It is possible that Li2O solid forms on the reactor wall of the inlet tube and reacts with hydrocarbons to form carbon oxides. The authors hypothesize that the inner surface of the inlet tube accumulates salt, which does not mix with the bulk hydrated salt, so solid oxide cannot be converted to hydroxide liquid. Reaction Kinetics. To understand the mechanism in more detail, several experiments were performed to measure kinetic parameters. The apparent activation energy and reaction orders were found using a differential bubble column reactor to measure the temperature (Figure 5a) and pressure (Figure 5b and c) dependences of the overall reaction rate. The activation energy was determined to be 156 ±6 kJ/mol where the error bars are calculated using the method of least-squares. The activation energy for the gas-phase reaction between methane and iodine radicals is reported to be 147 ±5 kJ/ mol.23 Given the proximity of the two activation energies, it is possible that the first hydrogen abstraction from methane limits the apparent reaction rate. The reaction was found to be first order in methane over the pressure range indicated in Figure 5b. The reaction order for oxygen was also investigated and was found to be 2.5. The reaction’s dependency on oxygen to drive the critical steps of HI regeneration and I2 production is reflected in the higher reaction order. Role of LiOH. To understand the role LiOH plays in the mechanism, and how it affects conversion and selectivity, a series of experiments were conducted. First, we studied the reaction between I2 and CH4 in the presence of LiOH to determine its role in oxidizing HI and nonselectively oxidizing

Figure 6. Effect of temperature on iodine-assisted partial combustion in the presence of LiOH. (a) Methane conversion and (b) hydrogen product selectivities when 0.15:0.15 bar CH4:I2 is bubbled through a bubble column with 1:1 mol LiI:LiOH. Note: No methane conversion was observed up to 650 °C when the same feed was bubbled through LiI alone. D


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Figure 7. Conversion of methyl iodide and hydrogen atom-based selectivity to products as a function of temperature in the presence and absence of LiOH. Reaction conditions were 0.61 bar methyl iodide bubbled through 1:1 LiI:LiOH (a and b), and LiI (c and d). Argon was used as a makeup gas with a total pressure of 1 bar. The moles of salt was greater than 100 times the total moles of gas throughout the experiments, so conversion of the salt is negligible.

also the most stable of the methyl iodides and likely the longest lived intermediate. To investigate its behavior with LiI and LiOH, methyl iodide was fed into a bubble column of LiI and a bubble column of LiOH (Figure 7). The resulting conversion and selectivity to water and hydrogen were improved in the presence of LiOH. This can be explained by the removal of HI through the neutralization reaction with LiOH. When HI is removed, it is no longer available to participate in a reverse reaction with methyl iodide, which increases the conversion. In addition, HI cannot react with methyl radicals to generate methane, a nonselective product; this reaction was investigated further by kinetic modeling in the next section. Kinetic Modeling Results. The presence of LiOH reduces the HI pressure, and a kinetic model is used to investigate this effect on selectivity of the reactions occurring in the gas phase using the best available data from NIST.23 Two specific scenarios were investigated: gas-phase decomposition of methyl iodide alone (Figure 8a) and gas-phase decomposition of methyl iodide in the presence of HI (Figure 8b). HI can be produced from pyrolysis of methyl iodide, which was observed in experiments, and is thought to contribute to the methane formation from methyl iodide observed in Figure 7d. The modeling results indicate that the presence of HI results in rapid methane formation (Figure 8b). Methane is formed primarily from the reaction of HI and methyl radicals. Once methane is formed, it is relatively stable. Further dehydrogenation of methane by any radicals results in the formation of methyl radicals, which rapidly react with HI, regenerating methane. The result is that methane is produced from methyl iodide when HI is present. The results in Figure 7b indicate that more methane is produced at higher temperatures, which is consistent with the gas-phase radical mechanism. In addition, the C2 products may be produced from the

indicating that LiOH does not rapidly oxidize methane, methyl halide intermediates, or the carbon product. In the absence of molecular oxygen, water is produced by the reaction of HI and LiOH (Figure 6). The other product, hydrogen, was likely produced from radical-mediated pyrolysis of methyl halide intermediates. Similar conversion is observed when oxygen and methane are cofed (Figure 3), as expected if oxygen reacts with LiI to produce I2. In a separate experiment, iodine and methane were bubbled through LiI alone, and no conversion was detected at 650 °C, indicating that LiOH is needed to react with and remove HI. This can be explained by the fact that equilibrium methane conversion in the presence of iodine is low (Figure S2), and the reaction between HI and methyl radical is fast (see Kinetic Modeling Results). When LiOH is part of the reaction system, it rapidly reacts with HI to produce water and LiI, preventing the reverse reaction of methyl iodide with HI, and providing a thermodynamic drive forward. The conversion of lithium oxide to lithium hydroxide in the presence of water has been investigated computationally.14 Lithium iodide was found to be hygroscopic at 500 °C based on density functional theory calculations. Water is produced in the partial combustion reaction, and on the basis of the calculations, the water in the salt reacts with lithium oxide to form the more stable lithium hydroxide. In addition, lithium oxide is a solid and insoluble in lithium iodide, and we did not see the formation of a solid in the system. We therefore assume the formation of lithium hydroxide is rapid. Methyl iodide is proposed as a reaction intermediate, and its behavior in the presence of LiI versus LiOH was investigated. Radical gas-phase reactions between iodine and methane proceed through serial dehydrogenation steps, alternating with iodination steps; the first iodinated product, methyl iodide, is E


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methane alone is not converted. However, in the presence of iodine, nearly complete oxygen conversion was observed, and 30% of the methane was converted. Iodine gas was observed in the effluent, which is consistent with oxidation of HI produced in the reaction. The products were carbon oxides, instead of the solid carbon observed in the salt mediated system. We conclude that in the presence of LiI, oxygen reacts more rapidly with LiI than with the hydrocarbons, preventing carbon oxide formation. Increased methane conversion in the presence of iodine can be explained by activation of methane by iodine radicals, which initiate combustion reactions. Dissolved Iodine. Iodine dissolves in lithium iodide, and the interaction between this dissolved iodine and methane was investigated. To introduce iodine into the salt, oxygen was bubbled through molten LiI−LiOH, followed by purging with argon to remove any gaseous I2. Methane then was introduced. Methane conversion was initially observed, indicating that the iodine remaining in the salt activated methane. The conversion declined over time (Figure 10), and after 6 h, the reactor was purged with argon, and a second cycle of O2 followed by CH4 was tested with results similar to those of the first cycle.

Figure 8. Microkinetic model results at 650 °C and 0.66 bar CH3I starting with (a) CH3I and (b) CH3I and HI.

combination of two methyl radicals in the gas phase, indicating that at higher temperatures, the extent of radical reactions is higher. When partial oxidation of methane is catalyzed by iodine, hydrogenation of methyl radicals by HI is undesirable, resulting in low methane conversion. In the presence of oxygen, carbon oxides can eventually be produced, even if the forward rates for iodine-assisted dehydrogenation reactions are faster. However, LiOH reacts with hydrogen iodide, preventing hydrogenation of methyl radicals. The result is that the presence of LiOH improves conversion and selectivity by removing HI from the gas-phase reaction network. Gas-Phase I2-Assisted Combustion of CH4. The above results demonstrate that reacting HI away results in significantly improved performance, and that LiOH is capable of accomplishing this. Oxygen also reacts rapidly with HI, and to understand its role in the gas phase, reactions in the absence of salt were investigated at 650 °C (Figure 9). Under the conditions reported, oxygen and methane do not react, and

Figure 10. Effect of dissolved iodine on methane conversion. Before t = 0, 1.5:8.5 sccm O2:Ar was bubbled for 1 h at 650 °C before purging the bubble column of gas with argon. At t = 0, 1.5:8.5 sccm CH4:Ar was bubbled through 1:1 mol LiI:LiOH. The total salt mass was 22.14 g, resulting in 11 cm salt height.

Complete oxygen conversion was observed during the O2 part of the cycle, in which 1.5 sccm O2 was bubbled through LiI:LiOH for 1 h. Initially, molten LiI−LiOH was visually clear, but the melt became dark red after oxygen was introduced. We hypothesize that the red color is coming from dissolved iodine, which has been reported to result in I4.2−14 A separate experiment in which a pure oxygen bubble was held under the surface resulted in the bubble immediately becoming purple (the color of iodine gas) and then shrinking in size until it was too small to see. This is likely from a fast surface reaction of oxygen and LiI to form iodine gas, followed by the dissolution of iodine into the melt. The salt gradually became clear over the 6 h in which methane was introduced each time. No carbon oxides were observed at any point in the experiment. These results suggest that oxygen and methane do not have to be cofed, and that iodine dissolved in the salt can be used as an oxidant. The amount of iodine required to stoichiometrically convert the amount of methane converted in Figure 10 is 5 mmol, which would require at least 1.4 mol % iodine dissolved in the melt at t = 0 in Figure 10. Other reactor configurations that lead to

Figure 9. Iodine-mediated methane combustion with 0.2 bar CH4. When present, the oxygen pressure was 0.05 bar, and the iodine pressure was 0.1 bar. Oxygen selectivity is reported for the experiment in which CH4, O2, and I2 were present. The temperature was 650 °C, and the space time was 15 s in all cases. No solid or liquid catalyst was present, and the reactor was made of quartz. F


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ACS Sustainable Chemistry & Engineering high selectivity are therefore possible, for example, with oxygen and methane fed in separate locations. Br2-Mediated Partial Combustion of Methane. Bromine-mediated partial combustion was also investigated using bromide salts. A lithium halide salt analogous to the iodinemediated process was not studied because LiBr does not react with oxygen at an appreciable rate. Another chemical looping salt, NiBr2, was previously reported to have the lowest activation energy of investigated salts with appropriate thermodynamic properties for cycling between oxide and bromide.17 However, the rate of reaction is still not fast enough to generate bromine before hydrocarbon oxidation occurs. In addition, the oxidation product, NiO, suspended in a NiBr2− KBr salt reacts with methyl bromide, resulting in high selectivities to carbon oxides (Figure S3). For this reason, we propose a process making use of three separate reactors (Table 2 and Figure S4), in which methane and bromine react to form

Figure 11. Methane conversion to carbon and hydrogen bromide, and hydrogen atom-based selectivity as a function of temperature. 0.22:0.11 bar Br2:CH4 was bubbled through a 12 cm bubble column with 0.1:0.45:0.45 mol NiBr2:KBr:LiBr. 100% Br2 conversion was observed in each case. The major products were HBr and carbon, but some polybromides were also observed.

Table 2. Simplified Reaction Network for Iodine- and Bromine-Mediated Partial Combustion of Methanea

a column of LiI:LiOH to collect a sufficient amount of carbon for analysis. The carbon formed was light enough to be entrained in the gas flow. Much of it formed at the surface of the bubble column, which was then frozen, and the carbon was poured off for SEM (Figure 12) and Raman (Figure 13). Significant amounts of the carbon were also entrained in the gas flow and carried downstream, through the cool effluent lines, and to the aqueous trap, where it was observed to float on the liquid surface. The carbon powder was removed from the frozen salt surface after reaction, and SEM imaging (Figure 12a) indicated it was composed of a network of interconnected chains of spherical carbon structures (Figure 12d). Raman spectroscopy indicated that both graphitic (G) and disordered (D) modes were present, and the ID/IG intensity ratio of the two peaks observed was 0.88, indicating the carbon does not have significant graphitic character. In addition, there were no other peaks observed between 3664 and 119 cm−1 (Figure 13). The carbon was removed from the salt surface and analyzed by EDX. Salt from the frozen bubble column was present with the carbon (Table 3). Some types of carbon have been reported elsewhere to react with steam only at temperatures of 1000 °C and higher,24 and so the oxygen present is likely from the lithium hydroxide in the salt and from surface contamination during transfer. Both the SEM and the Raman spectroscopy are consistent with carbon black observed elsewhere.22,25,26

a

See Figure 1 and Figure S4 for simplified schematics of the reactors.

separable solid carbon and hydrogen bromide in one reactor. In a separate reactor, hydrogen bromide reacts with NiO form steam and NiBr2, which is then cycled to another reactor where it reacts with oxygen to form dry bromine and NiO. To investigate the novel part of the proposed brominemediated process (reactor 2, Table 2 and Figure S4), a stoichiometric ratio of 2:1 Br2:CH4 was bubbled through 0.1:0.45:0.45 mol NiBr2:KBr:LiBr. Conversion and selectivity observed are shown in Figure 11. Complete bromine conversion and nearly complete methane conversion were observed at 600 °C in a 12 cm bubble column (Figure 11). Hydrogen bromide was the primary gaseous product, along with methylbromide and dibromomethane. It is possible that a longer bubble column would result in fewer brominated hydrocarbons, as they are unstable. Carbon was also observed floating on the melt surface, where it could be continuously removed. NiBr2−KBr−LiBr was initially selected because it is known to be catalytically active for the other reactions in the process, but other molten media might achieve equivalent performance at lower cost. Characterization of Carbon Produced. Scanning electron microscopy and Raman spectroscopy were used to characterize the carbon formed in the LiI−LiOH bubble column. Methyl iodide, an intermediate, was bubbled through

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CONCLUSIONS A CO2-free process for producing heat, carbon, and steam from methane and oxygen was proposed and experimentally investigated in molten salts. Lithium iodide and hydroxide chemical looping salt mixtures were used to catalyze the reaction. The reaction generates heat and steam that could be used for power production. Selectivity to carbon oxides was observed to be less than 10%. Furthermore, the carbon formed in the molten salt was observed to float to the surface. Scanning electron microscopy and Raman spectroscopy of the carbon are consistent with carbon black. The mechanism was studied through kinetic modeling, feeding of reaction intermediates, and varying the gas and salt compositions. The activation energy was measured to be 156 ± 6 kJ/mol, and the rate-determining step was proposed to be G


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Figure 12. SEM images at four different magnifications of carbon from the surface of a 1:1 mole LiI:LiOH bubble column after cooling. 0.61 bar methyl iodide was bubbled through the melt and pyrolyzed at 600 °C. Carbon formed a separable layer at the surface. All images were taken with a 5 keV accelerating voltage and a 5 mm working distance.

A bromine-mediated process alternative was also investigated. The relatively slow reaction between oxygen and bromide salts requires that bromine be produced in a separate reactor and combined with methane to form carbon and hydrogen bromide. When nickel bromide was used as the active species in the molten salt, nickel oxide reacted with hydrocarbon intermediates to form carbon oxides. To prevent carbon oxide formation, a separate hydrogen bromide recovery step was proposed, resulting in a configuration of three reactors. This would likely increase the cost beyond that of the single vessel iodine-mediated process.

Figure 13. Raman spectra of carbon collected at the surface of a bubble column of 1:1 LiI:LiOH after cooling. The bubble column operated at 650 °C for 3 h with 0.61 bar CH3I bubbled through 22.14 g resulting in an 11 cm salt height.

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S Supporting Information *

Table 3. EDX Results from the Surface of a Bubble Column of 1:1 mol LiI:LiOH after CH3I Was Bubbled through and Pyrolyzed at 600 °Ca element

wt %

at. %

carbon (K) oxygen (K) iodine (L)

65.11 06.53 28.36

89.56 06.74 03.69

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04168. Four figures: stability over 3 h, equilibrium methane conversion using halogens as oxidants, methyl bromide conversion and selectivity data, and bromine-based process schematic (PDF)

a

Carbon formed on the surface was removed after cooling along with some surface salt.

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the gas-phase radical reaction between iodine radical and methane, which has an activation energy of 146 ± 5 kJ/mol. Lithium hydroxide was found to react rapidly with hydrogen iodide, and a kinetic model supports the experimental finding that removing hydrogen iodide rapidly prevents hydrogenation of methyl intermediates to methane. This increases conversion and selectivity to the desired products. In addition, the lack of carbon oxide formation was attributed to the reaction between lithium iodide and oxygen, which occurs more rapidly than the direct oxidation of hydrocarbons by oxygen.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael J. Gordon: 0000-0003-0123-9649 Horia Metiu: 0000-0002-3134-4493 Eric W. McFarland: 0000-0001-7242-509X Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This work was primarily supported by the U.S. Department of Energy, Office of Science Basic Energy Sciences Grant number DE-FG03-89ER14048.

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Halogen-Mediated Partial Combustion of Methane in Molten Salts To

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