A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles
Introduction
Conventional, fossil, liquid hydrocarbon fuels will continue to cover a major portion of the ever-increasing world energy requirements for the foreseeable future, basically due to their demand in transportation and the existing relevant extensive infrastructure. In this perspective, the direct use of non-fossil, Synthetic Liquid Fuels (SLFs) is an intensively pursued alternative. Currently, the term “Synthetic Fuel (synfuel)” refers to a liquid fuel produced at commercial scale from low energy content carbonaceous sources, such as coal, natural gas, oil shale and other biomass, that are upgraded at the expense of additional energy, also obtained from the combustion of fossil fuels [1]. The terms Gas-To-Liquid (GTL) and Coal-To-Liquid (CTL) refer to processes converting natural gas (or other gaseous hydrocarbons) and coal, respectively, into longer-chain liquid hydrocarbons such as gasoline or diesel fuel. In the GTL route, methane-rich gases are converted into SLFs either via direct conversion of methane to methanol in one step, or via syngas as an intermediate.
Syngas is a gas mixture containing varying amounts of CO and H2 and whose exothermic conversion to fuels has been commercially practiced since a long time ago e.g. via the Fischer–Tropsch (FT) technology [2]. Syngas can be also used as a source of pure hydrogen and carbon monoxide [3], [4]. Thus, in fact hydrogen and syngas are the basic raw materials to produce SLFs and chemicals via industrially available processes. These procedures can be rendered more attractive and environmentally friendlier when combined with a renewable energy source, such as solar energy [5]. Indeed, it is generally accepted that only solar-driven technologies offer a permanent solution to both oil independence and climate change due to the unmatched magnitude and availability of solar resource [6]. When solar energy is employed for the production of the raw materials for the synthesis of such fuels, the latter are characterized with the term “solar fuels”. In the broad sense this term can contain in addition to “solar hydrogen”, synthetic liquid hydrocarbons and alcohols produced from reactions between H2 and CO that have originated from solar-aided dissociation processes as well as metal powders obtained by solar thermal reduction of metal oxides [7].
Fig. 1a (adapted from [6,8]) depicts a partial listing of the various feedstocks and solar energy variances that can be employed to produce such fuels. In this perspective, there are basically three pathways for producing syngas with the aid of solar energy: photochemical/photobiological, thermochemical and electrochemical [9], [10], [11]. The first, low-temperature route makes direct use of solar photon energy for photochemical and photobiological processes [11]. The thermochemical route uses solar heat at high temperatures supplied by Concentrated Solar Power (CSP) systems – i.e. special mirror assemblies that track the sun – for performing various high-temperature reactions that produce syngas from transformation of various fossil and non-fossil fuels (Fig. 1b). Such chemical reactions are natural gas steam reforming [12], [13], [14], gasification of solid carbonaceous materials like coal or biomass [15], [16], [17], or Water Splitting (WS) to hydrogen and oxygen. This last reaction can then be followed either by reaction of H2 with CO2 via the Reverse Water–Gas Shift (RWGS) reaction or by reaction of H2 with CO coming from Carbon Dioxide Splitting (CDS) to CO and O2, to produce syngas [18], [19]. In addition, CSP systems can be employed alternatively to photovoltaics for syngas synthesis indirectly, e.g. by supplying the (solar thermal) electricity for high-temperature electrolysis of steam or of steam/CO2 mixtures [20], [21] according to the third, electrochemical route in Fig. 1a. In the next step, the most promising liquid fuels to be generated from solar syngas are methanol, dimethyl ether (DME) and Fischer–Tropsch diesel [22], [23], [24].
Among the thermochemical routes to solar syngas shown in Fig. 1b, solar reforming requires lower temperatures compared to WS/CDS thermochemical processes. However, the latter route employs CO2 as a reactant. In this perspective lies within the broad, increasing R&D effort on developing effective solutions for reusing and “valorizing” atmospheric CO2 as a carbon-containing raw material for the production of fuels and chemicals rather than currently treating it as a waste [21]. A review on CSP-aided reforming (dashed line) has been recently published by the present authors [25] whereas CSP-aided coal/biomass steam gasification (dotted line) has been also covered in a series of publications [16], [17]. The present work is a review on the activities on CSP-aided syngas production via WS/CDS thermochemical processes (solid line). First, the evolution of thermochemical cycles from WS to CDS is presented. Then the key thermochemistries of the various redox pair materials tested for such applications are delineated, followed by their coupling to relevant solar reactors. In closing, particular issues for future work on the topic are identified and discussed.
Section snippets
Redox-pair thermochemical cycles for syngas synthesis: Chemistry issues
The so-called WS thermochemical cycles were proposed initially for the production of hydrogen from the dissociation of water to hydrogen and oxygen. The single-step thermal dissociation of water (thermolysis) is conceptually the simplest reaction to split water at a first glance. Unfortunately it is impractical due to the very high temperatures needed (above 2500 K) as well as the problem of effectively separating H2 and O2 to avoid explosive mixtures. Thermochemical cycles are a series of
Two-step water/carbon dioxide splitting thermochemical cycles
Various redox pairs have been initially explored extensively for WS/TR thermochemical cycles; the relevant issues can be found in several review articles [35], [56], [57], [58]. Investigations among the most promising of them extended recently to include CDS; these are described in more detail below. A classification of such cycles in the literature is to the so-called “non-volatile” and “volatile” cycles according to whether the metal-containing species remain in the condensed state during the
Solar concentration systems
Once a redox composition system has been selected, the next step is coupling it to solar energy. The first step of a solar-powered process requires the reflection and concentration of direct insolation using collectors/heliostats. Large-scale concentration of solar energy is accomplished at pilot and commercial Solar Thermal Power Plants (STPP) aimed at the production of electricity from the sun’s rays with four kinds of optical configuration systems using movable reflectors (mirrors) that
Solar reactors employed for thermochemical WS/CDS cycles
As mentioned in the Introduction, the initial interest on (water-splitting) thermochemical cycles stemmed from the nuclear energy sector, with the intention to diversify the use of thermal energy supplied by nuclear reactors. In such a case an indirectly heated TR/WS reactor would be heated “allothermally” i.e. via a de-coupled heated fluid, for example Helium coming from nuclear reactors. However, in the case of solar TR/WS–CDS reactors, the high temperatures required for the TR step
Current development status and future prospects
The acknowledgement of the fact that the experience and technology accumulated with solar-aided, two-step, redox-pair-based WS thermochemical cycles for hydrogen production could be “transferred” to CDS and essentially culminate to the synthesis of liquid hydrocarbons from solar energy, water and (waste) carbon dioxide, has created many aspirations and research efforts worldwide. Taking into account the facts that solar energy is currently unmatched in its magnitude and availability as well as
Conclusions
The work over the last 30 years on solar-aided, redox-pair-based thermochemical cycles exploited initially for hydrogen an currently for syngas and solar fuels production has been presented. In conclusion it can be said that even though thermodynamic and process efficiency studies have shown that such cycles meet the metric of potential viability compared to “benchmark” technologies like electrolysis powered by solar electricity, it is clear that further research efforts are needed for the
Acknowledgements
Dr. C. Agrafiotis would like to thank the European Commission for funding his staying at DLR during the time of this work within the Project “FP7-PEOPLE-2011-IEF, Grant Number 300194” under the Intra-European Fellowship Marie Curie Actions of the 7th Framework Programme. Part of the work was co-funded by the Initiative and Networking Fund of the Helmholtz Association of German Research Centres (HGF) within the Virtual Institute SolarSynGas (Contract Number VH-VI-509).
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