(517f) Novel Process for Energy Storage and Conversion: Combined Chemical Looping

Novel process for energy storage and conversion: combined chemical looping

Vladimir V. Galvita, Hilde Poelman and Guy B. Marin

Laboratory for Chemical Technology, Ghent University, Technologiepark 914, B-9052 Ghent, Belgium, E-mail: Vladimir.Galvita@Ugent.be

The development of reliable and environmentally friendly approaches for energy conversion and storage is one of the key challenges that our society is facing [1, 2]. Energy can be stored in different forms: in an electric or magnetic field; as mechanical energy; or as chemical energy [3]. In a typical energy storage process, one type of energy is converted into another form which can easily be stored and converted for use when needed [4].

Chemical looping combustion (CLC) is an emerging combustion technology. In this process, fuel is oxidized by a reducible metal oxide and the reduced metal oxide is re-oxidized by air in a separate step [5, 6]. CLC hence produces a pure CO₂ stream, not diluted by N₂. By replacing air with H₂O or CO₂ as oxidizer, the chemical looping analogue to steam (CLSR) and dry reforming (CLDR) is obtained [6-8]. The latter converts CO₂ to high-purity CO, providing an efficient path for CO₂ conversion. The overall stoichiometry of this process indeed demonstrates that the oxidation step converts more CO₂than was produced in the fuel combustion step.

The calcium looping technology is a promising new technique for high-temperature scouring of CO₂ from flue gas and syngas. Calcium looping cycles have been intensively investigated in order to produce a concentrated CO₂ stream from the utilization of fossil fuels and biomass. Efficient CO₂ capture and storage using Ca-based sorbents can be achieved via the reversible reaction CaO + CO₂  CaCO₃ (ΔH_298 K = −178kJ mol^–1), the so-called Ca-looping cycle [9].

The present novel concept proposes energy storage and conversion based on a combination of chemical and calcium looping processes [10]. It uses a physical mixture of a CO₂ sorbent and an oxygen storage material. CO₂ serves as mediation gas to facilitate metal oxidation and carbon gasification into CO by means of chemical looping, while the calcium looping process ensures storage and release of CO₂.

Combination of Fe₃O₄/Fe redox cycles with CaO/CaCO₃ looping in this novel concept of “combined chemical looping” provides a number of important advantages. Where chemical looping dry reforming immediately oxidizes fuel to CO₂, which is further converted to CO, the CO₂ mediation gas is now stored to be used at will. The CaCO₃ solid material acts both as CO₂ reservoir and supplier, by cyclic CaO/CaCO₃ calcination-carbonation. In addition, the adsorption of CO₂gas from the stream during iron oxide reduction promotes faster material reduction and carbon formation.

The working principle of the “combined chemical looping” can be described as follows [10]. During the reduction step (“charge process”), CH₄ + CO₂ is fed into a mechanical mixture of Fe₃O₄ and CaO. Interaction of methane with Fe₃O₄ leads to formation of metallic iron and surface carbon as well as CO₂, H₂O and H₂ (Eq. 1 and 2). The H₂ (Eq. 2) obtained can be used for fuel cells. At the same time carbonation of calcium oxide occurs by interaction of CO₂ with CaO (Eq. 3). CaO particles react with CO₂ at typical high temperatures (600–700°C) to form CaCO₃.

CH₄ + Fe₃O₄ → 2H₂O + CO₂+ 3Fe                 (1)

CH₄ → C + 2H₂                                               (2)

CaO + CO₂ → CaCO₃                                     (3)

The material can be kept in this “charged” condition if storage is required or be put to immediate use. For the oxidation step (“discharge”), the temperature of the sample is increased by 50-150^oC which leads to decomposition of calcium carbonate into CaO and CO₂ (Eq. 4).

CaCO₃ → CaO + CO₂                                     (4)

At the same time, the interaction of CO₂with metallic iron as well as with carbon produces CO  via the following chemical reactions:

4CO₂ + 3Fe → Fe₃O₄ + 4CO                          (5)

C + CO₂ → 2CO                                             (6)

The thus generated CO could for example be used in a solid oxide fuel cell (SOFC) where it is electrochemically oxidized,

CO + O²⁻ = CO₂ + 2e⁻, producing electricity and CO₂.

The objective of the present study is the experimental validation of the proposed concept of combined chemical looping. The proof of concept was performed in two steps. In the first, a single NiO-Fe₂O₃/CeO₂ material was used and CH₄ was fed for iron oxide reduction and carbon formation. Interaction of this material with CO₂ led to CO formation by iron and carbon oxidation by CO₂. In the second step, the NiO-Fe₂O₃/CeO₂ material was mixed with CaO to perform a complete combined chemical looping test with a feed of CH₄+CO₂. The amount of CO produced is higher than the theoretical amount for Fe₃O₄, because carbon deposits from CH₄equally contribute to the CO yield. It was shown that this cycle of “charge-discharge” can be repeated with limited decrease  of the rate of the involved reduction/oxidation steps. The proposed methodology offers a wide flexibility both in energy storage/release and fuel type. For charging, diluted gas streams can be applied, making it very attractive for present-day applications.

Acknowledgments

This work was supported by the “Long Term Structural Methusalem Funding by the Flemish Government”, the Fund for Scientific Research Flanders (FWO, project number 3G004613), and the Interuniversity Attraction Poles Programme, IAP7/5, Belgian State – Belgian Science Policy.

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