Article Cite This: Energy Fuels 2017, 31, 11509-11514
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Combined Chemical Looping: New Possibilities for Energy Storage and Conversion Vladimir V. Galvita,* Hilde Poelman, and Guy B. Marin Laboratory for Chemical Technology, Ghent University, Technologiepark 914, B-9052 Ghent, Belgium S Supporting Information *
ABSTRACT: A novel concept of energy storage and conversion was demonstrated in a laboratory-scale test. The proposed combined chemical looping is able to store and release energy from chemical looping combustion for heat generation and hydrogen production by steam−iron processes integrated in one reactor. The reactor contains two concentric chambers, which are both filled with iron-based material. In a first step, all material is reduced to the metallic form, thus “charging” the reactor. For the second step or “discharging”, steam is fed to the inner chamber and air is fed to the outer chamber. The inner chamber is used for hydrogen production, and the external chamber is used for heat generation. In addition to iron, the external chamber contains a highly pyrophoric Ni-based layer at the air entry point to enable the startup of heat generation at room temperature. This concept of combined chemical looping was successfully tested: heat was generated by metal oxidation in air, and H2 was produced following contact of the reduced sample with H2O, with an average space time yield of 0.2 molH2 kgFe−1 s−1.
from clean combustion, chemical looping also holds promise as a novel approach for energy storage. The coupling of two such processes into combined chemical looping was demonstrated as a novel concept of energy storage in a laboratory-scale test.24 The proposed technology is able to store and release energy from redox chemical looping reactions combined with calcium looping. This process uses Fe3O4 and CaO, two low-cost and environmentally friendly materials, while CH4 + CO2 serve as feed. During the reduction of Fe3O4 by CH4, the so-called “charging step”, both carbon and metallic iron are formed. CO2 acts as mediation gas to facilitate the metal/metal oxide redox reaction and carbon gasification into CO. CaO, on the other hand, is used for storage of CO2. During the so-called “discharging step” temperature rise, CaCO3 releases CO2, which reoxidizes the carbon deposits and reduced Fe, thus producing carbon monoxide. After each redox cycle, the material is fully regenerated, so that it can be used repeatedly, providing a stable process. CO thus generated could be used in a solid oxide fuel cell (SOFC). We propose a simple, safe, and environmentally benign technology for the storage, transport, and supply of fuel or energy to any device. The objective of the present study is the experimental validation of a novel concept of energy storage and conversion based on an combination of two chemical looping processes in one unit. The first process, metal oxidation by oxygen (CLC) is applied for heat generation. This heat is used in the second process, chemical looping steam reforming (CLSR), for H2 production. The working principle of the “combined chemical looping” system is schematically shown in Figure 1. The unit contains two concentric reaction chambers based on two metal tubes with different diameter. The smaller tube is placed inside the larger tube, and both are filled with iron-
A reliable supply of energy is a basic requirement for sustainable development and economic growth of our society.1 Currently, energy can be stored in different forms: as chemical energy of reactants and fuels, as mechanical energy in an electric or magnetic field, or as nuclear fuel.2 In a typical energy storage process, one type of energy is converted to another form that can be used at will. Developing new materials and processes that provide high-performance energy storage combined with flexibility toward a wide range of technological applications is one of the most important topics in the 21st century.3−11 At present, the world’s energy needs are met predominantly through the combustion of fossil fuels.1 Chemical looping combustion (CLC) is emerging as a particularly promising technology that offers an elegant and efficient route toward clean combustion of fossil fuels.12,13 In CLC, the combustion is broken down into two separate half steps: (i) the oxidation of an oxygen carrier with air and (ii) the subsequent reduction of this carrier via reaction with a fuel. The overall reaction is equivalent to a conventional combustion. While initial interest in chemical looping focused only on combustion,13 current research demonstrates that it can provide a highly flexible technology platform for fuel conversion.14−16 For metals that are oxidizable by H2O or CO2 (Fe, Sn, Ce, In, and W), substituting air with these oxidizers yields the chemical looping equivalent to steam or dry reforming, resulting in either the production of high-purity hydrogen streams without the need for further cleanup steps or a novel route for efficient CO2 activation via reduction to CO.17−22 Recently, a “super-dry” CH4 reforming23 reaction was developed for enhanced CO production from CH4 and CO2. Ni/MgAl2O4 was used as a CH4 reforming catalyst; Fe2O3/MgAl2O4 was used as a solid oxygen carrier; and CaO/Al2O3 was used as a CO2 sorbent. The isothermal coupling of these three different processes resulted in higher CO production compared to conventional dry reforming by avoiding the water-gas shift reactions. Apart © 2017 American Chemical Society
Received: August 25, 2017 Published: August 26, 2017 11509
DOI: 10.1021/acs.energyfuels.7b02490 Energy Fuels 2017, 31, 11509−11514
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Energy & Fuels
Figure 1. H2 production unit combining two fixed-bed reactors into one reactor with two chambers. (A) Air feed provides heat for steam conversion into hydrogen by interaction with iron. Schematic working principle of combined chemical looping for energy storage and conversion: half crosssectional reactor view during (B) materials charge step and (C) materials discharge step.
at 110 °C. Finally, the materials were calcined at 750 °C for 6 h. The 50 wt % NiO−Al2O3 was prepared by co-precipitation. Precipitation was carried out from aqueous solutions of metal nitrates: Ni(NO3)2·6H2O and Al(NO3)3·9H2O. Ammonium bicarbonate was used as a precipitation agent. Co-precipitated samples were thoroughly washed with distilled water, dried at 110 °C in air, and then calcined in air at 600 °C for 4 h. Crystallographic analyses of the tested catalysts were performed by means of in situ X-ray diffraction (XRD) measurements in θ−2θ mode using a Bruker-AXS D8 Discover apparatus with Cu Kα radiation of wavelength 0.154 nm and a linear detector covering a range of 20° in 2θ with an angular resolution of approximately 0.1° 2θ. While the minimal capturing time is 0.1 s, a collection time of typically 10 s was used during these experiments. The evolution of the catalyst structure during temperature-programmed oxidation (TPO) was investigated in a flowing gas stream of O2 from room temperature to 800 °C. The in situ experiments were carried out using a home-built reactor chamber with a Kapton foil window for X-ray transmission. A 10 mg sample was evenly spread in a shallow groove of a single crystal Si wafer. The interaction of the catalyst material with the Si holder was never observed. The chamber atmosphere was pumped and flushed with a rotation pump (base pressure of ∼4 × 10−2 mbar) before introducing the reducing gas flow. The sample was heated from room temperature to 800 °C at a heating rate of 20 °C/min. In situ XRD during O2-TPO immediately followed upon temperature-programmed reduction (H2-TPR). Activity measurements were carried out at atmospheric pressure in a quartz tube microreactor (inner diameter of 10 mm) and a metal reactor tube (inner diameter of 2 mm) placed inside the quartz tube, positioned in an electric furnace (Figure S4 of the Supporting Information). Typically, 1 g of 100Fe or 80Fe−Ce sample was packed between quartz wool plugs and used for heat generation. For a typical experiment of H2 production, 250 mg of 80Fe−Ce was packed inside the metal reactor. The temperature of the material zone inside of metal tube was measured with K-type thermocouples inside of the reactor. In all experiments, the material was reduced by hydrogen at 700 °C for 20 min. The total flow rate of the gas feed into the reactor was maintained constant by means of Brooks mass flow controllers into each chamber independently. The space time yield (STY, mol s−1 kg−1) was calculated from the difference between the inlet and outlet molar flow rates, as measured relative to an internal standard (Ar) using an
oxide-based materials. The inner tube is used for hydrogen production via the steam−iron process CLSR, while the outer tube provides heat generation by CLC. Metals react easily with oxygen in air to form stable, non-toxic solid oxides upon combustion, and they have higher volumetric energy density than gasoline or other fossil fuels when burned with air.4,7,25,26 Given adequate insulation of the outside reactor wall, the efficiency of heat transfer toward the inner chamber will be strongly enhanced. The operation of the “chemical charge” and “discharge” cycles can be described as follows. During the “charge process” (material reduction step), fuel is fed into both chambers with iron oxides. Interaction of fuel with iron oxide leads to the formation of metallic iron. Reduced iron oxide can be stored in this “charged” condition at room temperature. For the “discharge” (material oxidation step), the temperature of the sample is increased by feeding air or oxygen into the outer CLC chamber to oxidize the reduced metal (eq 1). The heat thus generated is transferred through the metallic wall, which separates the CLC and CLSR chambers. At the same time H2O(l) is injected into the inner chamber. The interaction between H2O and hot metallic iron leads to the production of steam and further to hydrogen (eq 2). 2Fe + 3/2O2 → Fe2O3 + heat ° K = −405 kJ/molFe ΔH298
(1)
4H 2O + 3Fe → Fe3O4 + 4H 2 ° K = −100 kJ/molFe ΔH298
(2)
Generated H2 could then be fed, for example, into a fuel cell, where it is electrochemically oxidized, producing electricity and H2O, or into a hydrogen internal combustion engine.27 Once the iron material has been oxidized in the reactor to Fe3O4 and Fe2O3, it can be again reduced by any fuel: gaseous (solar hydrogen, CH4, syngas, ...), liquid (ethanol, methanol, gasoline, ...) or solid (coal, carbon, wood, ...) fuel. Samples of 100 wt % Fe2O3 and 80 wt % Fe2O3−CeO2 were investigated. The following chemicals were used in the preparation of the mixed oxides: Fe(NO3)3·9H2O and Ce(NO3)3·6H2O (99.99%, Sigma-Aldrich). Samples were prepared by co-precipitation by adding an excess of ammonium hydroxide. This mixture was kept at room temperature for 24 h. Hereafter, the sample was separated as a precipitate from the solution, washed with ethanol, and dried overnight in an oven 11510
DOI: 10.1021/acs.energyfuels.7b02490 Energy Fuels 2017, 31, 11509−11514
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Energy & Fuels online quadrupole mass spectrometer; i.e., STY = (F0 − Fi)/m, where Fi (mol/s) is the molar flow rate of component i and m is the amount of Fe in the sample. To simulate steady-state chemical looping operation based on a single reactor H2 space time yield, the space time yield for a multi-tubular reactor concept was calculated. Such a multitubular reactor is perceived as a bundle of fixed-bed reactors, operated in parallel. The core of the simulation concept is that all single reactors are operated in chemical looping regime but one after the other, i.e., with a delay relative to the previous one. A redox cycle is established in each single reactor by switching the feed valves at discrete times, leading to an oscillating H2 space time yield and eventually generating a permanent periodic regime for the multi-tubular reactor concept. Two samples of Fe2O3 (100Fe) and 80Fe2O3−CeO2 (80Fe− Ce) were investigated in this study. 80Fe−Ce preciously showed improved performance and high stability in chemical looping processes for H2 and CO production.28,29 The 100Fe sample was used as a reference material. In situ XRD measurements were performed during O2-TPO to understand the transformation of the sample during the reduction process. According to the literature, the oxidation reaction of Fe and Fe−CeO2 can run through several stages: Fe → FeO → Fe3O4 → Fe2O3.15 O2-TPO was performed immediately after cooling in Ar following H2 reduction at 700 °C for 15 min (reduction of the 80Fe−Ce sample presented in Figure S1 of the Supporting Information). The in situ time-resolved XRD patterns for both samples are presented in panels a and b of Figure 2, showing the characteristic Fe diffraction of ∼45°, which gives way to diffraction peaks of Fe2O3. The temperature at which the Fe phase transition occurs increases from 225 °C for 80Fe−Ce up to 425 °C for Fe. No intermediate FeO phase is apparent for both samples, and Fe3O4 was observed only for the monometallic sample.
The test results for the heat generation with these samples are shown in Figure 3. At room temperature, no ignition was observed for both materials. The 100Fe sample produced heat only at an initial sample temperature of 400 °C. The 80Fe−Ce sample produced a significant amount of heat from an initial temperature of ∼175 °C. When the same sample was reduced and reoxidized, the amount of produced heat decreased. Figure 3 shows that, after the third cycle, 100Fe only generated a maximal temperature of 425 °C. On the other hand, 80Fe−Ce still yielded a maximum temperature of 450 °C after the third, fourth, and fifth cycles. When the sample was removed from the reactor, strong sintering of the 100Fe sample into agglomerates was observed (see the inset in Figure 3A). It is well-known that pure iron oxide is prone to fast sintering during reduction−oxidation cycles.28 In contrast, 80Fe−Ce did not exhibit strong agglomeration and came out of the reactor as a powder (inset in Figure 3B). Scanning transmission electron microscopy (STEM) did not show drastic changes in this sample morphology (Figure S2 of the Supporting Information). However, the average crystalline size did increase from 50 to 200 nm. As confirmed by other researchers, the size of reacting particles has a profound effect on heat generation.30 For a fixed total amount of material, sintering leads to a decrease of the surface area. A lower surface area requires more initial heat to bring the reacting system up to its activation state, leading to a lower amount of net heat being released by the exothermic reaction. The experimental results demonstrate that iron particles in 80Fe−Ce react exothermically with oxygen in air and produce iron oxide with ignition at ∼175 °C and a maximum temperature of ∼730 °C. Hence, sintering needs to be limited to preserve the heat generation capacity. Then again, the same relation between the particle size and temperature can be used to bring down the ignition temperature to room temperature. The research involving ignition of powders with different size distributions and, in particular, nanopowders has shown that the ignition temperature is a strong function of the particle size.26,30 Reduced supported metals, such as Ni, Co, and Fe, become extremely pyrophoric when the metal crystallites are nanosized. If these catalysts are exposed to air, the metal particles instantaneously oxidize, generating large amounts of heat, such that the particles glow red. This heat may be directly used for ignition of 80Fe−Ce. In the present study, a reduced Ni−Al2O3 (Ni−Al) sample was applied as highly pyrophoric material, burning spontaneously in air without application of heat (eq 3). Experiments with a 50Ni−Al sample showed that the temperature rose from 30 up to 250 °C in 15 s. This temperature is sufficient to initiate the iron oxidation. 2Ni + O2 = 2NiO
° K = −469 kJ/mol ΔH298
(3)
This pyrophoric property can be used in the new reactor concept, as illustrated in Figure 4A. The chamber for heat generation now contains two consecutive fixed zones. Within the first, holding the 50Ni−Al catalyst, air oxidizes Ni, so that the temperature of this zone increases. Preheated air then reaches the second 80Fe−Ce catalyst zone, starting the metal iron oxidation, which will generate even more heat. An experimental test of low-temperature iron oxide initiation using this segmented chamber was performed with 50Ni−Al and 80Fe−Ce. After reduction by hydrogen at 700 °C for 30 min, the flow was switched to Ar and the temperature of the reactor decreased. Figure 4B present the temperature of the
Figure 2. Two-dimensional (2D) XRD pattern recorded during O2TPO for (A) 100Fe and (B) 80Fe−Ce. TPO measuring conditions: 20 °C/min and 20% O2 in He. 11511
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Figure 3. Comparison of heat generation versus time on stream during air feeding to pre-reduced (A) 100Fe and (B) 80Fe−Ce for different redox cycles. The insets present the photographs of the used samples. Black squares, first redox cycle; red circles, second redox cycle; blue triangles, third redox cycle; purple triangles, fourth redox cycle; and green diamonds, fifth redox cycle. Reduction: 100% H2 at 700 °C, time = 20 min, air = 100 N mL/min, and W = 1 g.
and exhibits a maximum as high as 650 °C after 2 min and then slowly decreases toward 450 °C after another 2 min (also see the video in the Supporting Information). The test results indicate that the use of pyrophoric 50Ni−Al and 80Fe−Ce in a two-zone configuration provides a higher heating value than that of a single Fe−Ce zone (Figure 3B) and is therefore more suitable for use as a heat source. Finally, the concept of combined chemical looping with two concentric chambers was tested for actual hydrogen generation. The inner chamber for hydrogen production contained only 80Fe−Ce. The outer chamber had two consecutive zones, with 50Ni−Al at the inlet and 80Fe−Ce beyond. Hydrogen was fed into these reactors at 700 °C for 30 min for material reduction. When the temperature of the reactor had dropped to 30 °C, air was fed into the outer chamber of the reactor. As soon as the temperature of the reactor reached 300 °C, the H2O feed to the inner chamber was started. H2 was produced following contact of the reduced 80Fe−Ce sample and H2O. Figure 4c presents the evolution of H2 production: the space time yield for H2 increases, passes through a maximum at 490 °C, and then steadily decreases toward zero at 600 °C as a result of complete oxidation of Fe into Fe3O4 by steam. Continuous production of H2 using such a process can be achieved if multiple reactors will operate in parallel.23 The H2 space time yield for a multi-tubular reactor concept was calculated by a dynamic simulation of redox cycles based on steady-state chemical looping operation of a single reactor. The space time yield was determined on the basis of the concept that all single reactors are operated in the chemical looping regime but one after the other, i.e., with a delay relative to the previous one.23,31 The time delay and number of reactors in operation are based on the experimental results obtained for a single reactor. The average space time yield for such a multireactor configuration reaches ∼0.2 molH2 kgFe−1 s−1. To assess the process viability, the stability of the materials and their activity was examined during repeated charge and discharge cycles. The space time yield of hydrogen remained close to constant after the fifth cycle, which confirmed the stability of the activity of 80Fe−Ce materials. In addition, this 80Fe−Ce sample was tested at 600 °C during 100 redox cycles, presenting high stability after the first 25 cycles (Figures S3 and S5 of the Supporting Information). However, after the first oxidation procedure, the temperature of the Ni−Al zone had to be increased from 30 to 160 °C to guarantee enough heat production for the 80Fe−Ce oxidation. Sintering lead to an increase of the crystallite size of Ni and, hence, a smaller surface
Figure 4. (A) H2 production unit using concentric chambers with air feed at room temperature into the outer chamber, segmented into two zones for metal oxidation, and providing heat to the inner chamber for steam conversion into hydrogen. (B) Heat generation versus time on stream during the air feed into the two-zone reactor with Ni−Al as a first zone and 80Fe−Ce as a second zone. (C) Space time yield of H2 during the feed of air into the heat generation chamber and H2O/Ar for hydrogen production over pre-reduced materials. WFe‑Ce = 1 g, WNi−Al = 0.25 g, T = 40 °C, H2O = 30 N mL/min + Ar = 30 N mL/ min, and air = 100 N mL/min. The error bar indicates twice the standard deviation.
Fe−Ce zone as a function of time on air stream. Shortly after air is fed to the reactor, the temperature of the zone increases 11512
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Energy & Fuels area, which required a higher ignition temperature. The latter illustrates that the future of this process depends greatly upon material advancement. Innovation can be achieved through adopting different synthesis strategies, such as core−shell preparation and stabilization of the active metal for heat generation in the pores of high-surface-area materials. The proposed combined chemical looping is able to store and release energy from chemical looping combustion for heat generation and hydrogen production by steam−iron processes integrated in one reactor. The reactor contains two chambers, which are both filled with iron-based material. The reactor can be “charged” at a high temperature (700 °C) by the material reduction to the metallic form. In addition to iron, the chamber for heat generation contains a highly pyrophoric Ni-based layer at the air entry point to enable the startup of heat generation at room temperature during “discharging”. A novel concept in a laboratory-scale test demonstrates H2 production with an average space time yield of 0.2 molH2 kgFe−1 s−1. This technology should certainly be applicable well beyond the steam−iron process. Extension can be made to, for example, catalytic hydrogen production by dimethyl ether (DME) or methanol steam reforming, which operate at moderate temperature, generation of CO from CO2 for solid oxide fuel cell (SOFC),24 or generation of thermoelectric power.30 As an extended proof of concept, methane was used as a fuel, instead of hydrogen, in the proposed process. It is wellknown that the interaction of methane with Ni leads to H2 and carbon formation and, hence, catalyst degradation. During the oxidation with H2O, produced H2 contains carbon monoxide as a result of gasification of surface carbon. To overcome this problem, a mixture of CH4 and CO2 was applied for material reduction (Figure S6 of the Supporting Information). As shown in this paper, the primary candidate for use in this process that can be oxidized in the chemical looping process and subsequently used for hydrogen production by the interaction with H2O is iron. Millions of tons of iron are currently produced worldwide for the powder metallurgy, chemical, and electronic industries. Iron−air batteries are being developed for economic reasons as a result of the abundance and low cost of iron resources.32,33 An important property of iron is that it is readily recyclable with well-established technologies. Iron can be obtained from iron oxide powders by the reaction with hydrogen or syngas at moderate temperatures below 800 °C. All of these make iron an ideal energy carrier, which is why more and more scientists are rediscovering iron.
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combined chemical looping (Figure S4), XRD patterns of 80Fe−CeO2 as prepared (A) and after 100 redox cycles (B) (Figure S5), and space time yield of H2 during the feed of H2O into over pre-reduced by CH4 and CO2 mixture materials in a two-zone (Ni−Al2O3 and 80Fe− Ce) reactor (Figure S6) (PDF) Fe−Fe2O3 video (MP4)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Vladimir V. Galvita: 0000-0001-9205-7917 Guy B. Marin: 0000-0002-6733-1213 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Long Term Structural Methusalem Funding of the Flemish Government and the Interuniversity Attraction Poles Programme, IAP7/5, of the Belgian State−Belgian Science Policy. The authors acknowledge support from Prof. C. Detavernier with the in situ XRD equipment (Department of Solid State Sciences, Ghent University) and from Dr. Vitaliy Bliznuk (Department of Materials Science and Engineering, Ghent University) for the high-resolution transmission electron microscopy (HRTEM) measurements. Additionally, the authors thank Daria Galvita for producing the video during her school project at the Laboratory for Chemical Technology.
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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.7b02490. Two-dimensional (2D) XRD pattern recorded during H2-TPR for 80Fe−Ce (Figure S1), STEM images of 80Fe−Ce sample (A) as prepared and (B) after redox cycles and (C) electron energy loss spectroscopy (EELS) map for Fe and Ce redox cycles (Figure S2), dynamic simulation of H2 space time yield from the multi-tubular reactor configuration based on each of 100 isothermal redox cycles (24 h time on stream) of 80Fe−Ce in a single reactor experiment (Figure S3), schematic representation of the concentric reactor setup for 11513
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