An Overview of Solar Decarbonization Processes, Reacting Oxide Materials

An overview of solar decarbonization processes, reacting oxide materials, and thermochemical reactors for hydrogen and syngas production Srirat Chuayboon, Stéphane Abanades

To cite this version:

Srirat Chuayboon, Stéphane Abanades. An overview of solar decarbonization processes, reacting oxide materials, and thermochemical reactors for hydrogen and syngas production. International Journal of Hydrogen Energy, Elsevier, 2020, 45 (48), pp.25783-25810. 10.1016/j.ijhydene.2020.04.098. hal- 02990443

HAL Id: hal-02990443 https://hal.archives-ouvertes.fr/hal-02990443 Submitted on 5 Nov 2020

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. An overview of solar decarbonization processes, reacting oxide materials, and thermochemical reactors for hydrogen and syngas

production

Srirat Chuayboonb, Stéphane Abanadesa,*

a Processes, Materials and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-

Romeu, France

b Department of Mechanical Engineering, King Mongkut’s Institute of Technology Ladkrabang, Prince of

Chumphon Campus, Chumphon 86160, Thailand

*Corresponding author: Tel +33 (0)4 68 30 77 30

E-mail address: [email protected]

Declarations of interest: none

Abstract: Solar decarbonization processes are related to the different thermochemical conversion pathways of hydrocarbon feedstocks for solar fuels production using concentrated solar energy as the external source of high-temperature process heat. The main investigated routes aim to convert gaseous and solid feedstocks (methane, coal, biomass…) into hydrogen and syngas via solar cracking/pyrolysis, reforming/gasification, and two-step chemical looping processes using metal oxides as oxygen carriers, further associated with thermochemical

H2O/CO2 splitting cycles. They can also be combined with metallurgical processes for production of energy-intensive metals via solar carbothermal reduction of metal oxides. Syngas can be further converted to liquid fuels while the produced metals can be used as energy storage media or commodities. Overall, such solar-driven processes allow for

1 improvements of conversion yields, elimination of fossil fuel or partial feedstock combustion as heat source and associated CO2 emissions, and storage of intermittent solar energy in storable and dispatchable chemical fuels, thereby outperforming the conventional processes. The different solar thermochemical pathways for hydrogen and syngas production from gaseous and solid carbonaceous feedstocks are presented, along with their possible combination with chemical looping or metallurgical processes. The considered routes encompass the cracking/pyrolysis (producing solid carbon and hydrogen) and the reforming/gasification (producing syngas). They are further extended to chemical looping processes involving redox materials as well as metallurgical processes when metal production is targeted. This review provides a broad overview of the solar decarbonization pathways based on solid or gaseous hydrocarbons for their conversion into clean hydrogen, syngas or metals. The involved metal oxides and oxygen carrier materials as well as the solar reactors developed to operate each decarbonization route are further described.

Keywords: solar fuel, pyrolysis, reforming, gasification, chemical looping, metallurgy, oxygen carrier

1. Introduction

One of the most serious concerns in the near future is the depletion of fossil fuels associated with increasing greenhouse gas emissions. The continually growing worldwide energy demand led to an unprecedented increase in the global energy-related CO2 emissions by 1.7% in 2018, thereby contributing to hastened global climate change. Consequently, it is urgent to develop environmentally friendly routes for alternative fuel production such as hydrogen and synthesis gas (syngas), supplied by sustainable renewable energy sources such as biomass, biogas, and solar energy. Syngas can be used as an interesting energy carrier for a multitude of chemical synthesis processes. Moreover, it can be burned directly in internal combustion engines, furnaces, boilers and stoves, utilized to produce methanol and hydrogen, or further converted into synthetic liquid fuels via the Fisher-Tropsch method [1].

Solar thermochemical fuel production processes include the thermochemical conversion of solid and gaseous carbonaceous feedstocks as well as metal oxides (MxOy) into hydrogen/syngas and metals utilizing concentrated solar energy to drive endothermic

2 chemical reactions. The conventional processes (reforming, gasification, metallurgy…) show several major drawbacks regarding the need for fossil fuels or partial feedstocks combustion as heat source, related CO2 emissions, products contamination by combustion by-products, and catalysts requirements. The use of solar energy as the external source of process heat for supplying such solar thermochemical processes offers several advantages and permits to: (i) eliminate the need for fossil fuels combustion as heat source, which in turn reduces the total world fossil fuels energy consumption, (ii) avoid producing CO2, which seriously induces climate change and global warming, (iii) save feedstock resources as partial combustion (autothermal) for process heat is avoided, (iv) store intermittent solar energy within the chemical products through the endothermic reactions, (v) produce clean, storable and transportable chemical fuels and materials commodities, (vi) provide very high temperatures, depending on the magnitude of solar concentration, which may exceed those supplied by combustion-based heat sources.

Solar energy includes radiant light and derived heat from the sun. In fact, the total amount of solar energy incident on Earth is largely in excess of the anticipated energy requirements; however, its intensity is quite low, dilute and non-equally distributed. If harnessed, this energy source has potential to meet global energy demands without the concomitant production of greenhouse gases [2]. The areas that exhibit high solar irradiance are potential regions capable for producing heat by solar concentrating systems, in order to supply chemical reactions or electricity generation processes. Among the different evolving technologies for harnessing solar energy, concentrating solar power systems utilize lens or mirrors and tracking systems to focus a large area of sunlight into a small concentrated beam. These technologies mainly consist of parabolic troughs and concentrating linear Fresnel reflectors, solar dishes, solar power towers, double reflection solar furnaces, and solar simulators. Parabolic troughs are the oldest solar thermal technology for concentrated solar power (CSP) plants [3]. The concentrating systems utilize parabolic-shaped collectors made of reflecting mirrors, according to Fig. 1a. The mirrors reflect the incident solar radiation onto a focal line, where tubular receivers are placed to be heated. Parabolic trough reflectors can concentrate sunlight between 60 and 100 times [4], which is sufficient to raise the temperature of the heat transfer fluid to as much as 550 °C. Another system, which is similar to the parabolic troughs, is the linear Fresnel reflector system (Fig. 1b). The flat plane mirrors are split into multiple parts to track sunlight reflected onto the secondary reflector and

3 absorber tube [5]. Regarding point focusing systems, Fig. 1c shows a parabolic-dish solar concentrator. Sunlight is concentrated and reflected by a parabolic dish toward the thermal receiver positioned on the focal point of the dish collector. The maximum operating temperature is above 1000 °C with a concentration ratio in the range of 1000-5000 [3]. This technology is limited by the size of the dish determining the available solar thermal power at the focal point and can be equipped with “dish-Stirling” engines. The solar tower system includes a massive heliostat field focusing on a single solar receiver mounted on a tall tower positioned at its center, according to Fig 1d. Each mirror tracks the sun independently, making the solar collection system relatively more expensive than for a solar trough plant. The benefit is that a higher temperature can be achieved with this plant type while enabling high solar thermal powers depending on the heliostat field area. A concentration factor of between 600 to 1000 times is possible, reaching temperature in the range 800-1000 °C [6].

(a) (b)

Figure 1. Schematic of (a) solar trough, (b) linear Fresnel reflector system, (c) parabolic dish solar concentrator, and (d) solar tower system (adapted from [3] and [5]).

For double reflection solar furnace, the concentrating system uses one or several heliostats like solar towers and, instead of a tower with a central receiver, a parabolic mirror is employed as a secondary optical component to concentrate sunlight at the focal point with a concentration ratio up to 20000 times. Several parabolic mirrors can be joined because a single mirror is restricted in size. The largest system is the solar furnace in Odeillo, which is 54 m high and 48 m wide, including 63 heliostats with thermal power capacity of 1 MW and maximum temperature above 3500 °C [7]. Finally, solar simulator devices are developed for research purpose only and deliver illumination approximating natural sunlight [8]. They can be generally categorized into two types: those utilizing a pointing source of simulated solar radiation positioned away from the collector and those having a large area of multiple lamps positioned close to the collector. The objective of the solar simulator is to provide a controllable indoor test facility under laboratory conditions without the issues of unstable natural solar irradiation. Solar thermochemical processes use the concept of the conversion of solar energy to chemical energy carriers. For solar thermochemical processes, the theoretical total required energy to transform reactants to products is equal to the enthalpy change of reactions (ΔH), while the amount of energy supplied by solar energy as process heat to complete reversible process is equivalent to TΔS. Therefore, Gibbs free energy (ΔG) decreases with increasing temperature. To estimate the feasibility of chemical reactions, ΔG of system needs to be negative, so that the reaction is thermodynamically favorable and can proceed spontaneously. The key performance indicator of CSP plants is the solar energy absorption efficiency of the receiver (ηabsorption). It represents the ratio of the net rate of absorbed energy to the solar power input from the concentrator. Unless accounting for the conduction and convection losses, the absorption efficiency is given by [9]:

4 (훼푒푓푓 ∙푄푎푝푒푟푡푢푟푒)−(휀푒푓푓∙퐴푎푝푒푟푡푢푟푒∙휎∙푇 ) 휂푎푏푠표푟푝푡푖표푛 = (1) 푄푠표푙푎푟

where 푄푠표푙푎푟 is the total solar power input,∙ 푄푎푝푒푟푡푢푟푒 is the amount of solar power captured by the aperture area (퐴푎푝푒푟푡푢푟푒), 훼푒푓푓 and 휀푒푓푓 are the absorptance and emittance of the solar cavity receiver, respectively, T is the nominal cavity receiver temperature, 휎 is the Stefan- Boltzmann constant. The ability of the collector to concentrate solar energy is represented by the mean concentration ratio (퐶̃) over the aperture as follows: 5

푄 퐶̃ = 푎푝푒푟푡푢푟푒 (2) 퐼퐴푎푝푒푟푡푢푟푒 where I is the intensity of solar radiation (direct normal irradiation, DNI).

Assuming that all the incoming solar energy can be ideally captured, i.e. 푄푎푝푒푟푡푢푟푒=푄푠표푙푎푟, and the cavity receiver is a perfectly insulated blackbody (no conductive/convective heat losses), with 훼푒푓푓 = 휀푒푓푓=1, η푎푏푠표푟푝푡푖표푛 can be simplified as:

휎푇4 η = 1 − ( ) (3) 푎푏푠표푟푝푡푖표푛 퐼퐶̃

The absorbed solar power drives the endothermic chemical reaction; therefore, the conversion of solar energy to chemical form is quantified as the exergy efficiency:

−푛̇ ∆퐺푟푥푛 휂푒푥푒푟푔푦 = (4) 푄푠표푙푎푟

where 푛̇ is the molar flow rate of products, and ∆퐺푟푥푛is the maximum amount of work that may be obtained from the products when converted back to the reactants at 298K. Thus, the nd 2 law is employed to calculate the maximum exergy efficiency (휂푒푥푒푟푔푦,푖푑푒푎푙). Because the conversion of the solar process heat to chemical energy is limited by both solar absorption and Carnot efficiency, the 휂푒푥푒푟푔푦,푖푑푒푎푙 is given by:

휎푇4 푇 휂 = 휂 ∙ 휂 = (1 − ) ∙ (1 − 퐿) (5) 푒푥푒푟푔푦,푖푑푒푎푙 푎푏푠표푟푝푡푖표푛 퐶푎푟푛표푡 퐼퐶̃ 푇 where TL is ambient temperature (298K).

Fig. 2 shows the ideal exergy efficiency (휂푒푥푒푟푔푦,푖푑푒푎푙) as a function of temperature (T) for different solar concentration ratios. An increase in concentration ratio increases the maximum achievable 휂푒푥푒푟푔푦,푖푑푒푎푙. For example, at 1100 K the maximum 휂푒푥푒푟푔푦,푖푑푒푎푙 of 65–72% can be obtained for concentration ratios of 1000–10000, respectively. In addition, an optimum temperature exists for a given concentration ratio and increasing the temperature above this optimum downgrades the exergy efficiency because of prevailing radiative heat losses. The optimal operating temperatures that maximize 휂푒푥푒푟푔푦,푖푑푒푎푙 can be calculated for each

6 concentration ratio [10]. Therefore, maximizing the actual solar-to-chemical energy efficiency as close as possible to the ideal efficiency in solar thermochemical endothermic processes operated at 1100-1800K is a key challenge for solar thermochemical research.

Figure 2. Exergy efficiency 휂푒푥푒푟푔푦 푖푑푒푎푙 as a function of operating temperature at different solar concentration ratios for a blackbody cavity receiver (adapted from [10]).

2. Solar thermochemical fuel production processes

Solar thermochemical processes use solar energy to drive endothermic chemical reactions for the conversion of either gaseous/solid feedstocks or metal oxides to storable and transportable fuels or chemical commodities [11]. They can be divided into two groups based on the feedstocks (Fig. 3) [12,13]. On the one hand, H2O/CO2 splitting, which consists of thermolysis (or electrolysis) and thermochemical cycles, represents a long-term ultimate goal for sustainable fuel production. On the other hand, decarbonization routes, which consist of cracking/pyrolysis, reforming/chemical looping reforming (CLR), gasification/chemical looping gasification (CLG), and carbothermal/methanothermal reduction, represent various promising approaches for the short-term solar process implementation. They consist of the

7 conversion of solid/gaseous carbonaceous feedstocks to hydrogen or syngas that can be combined with the conversion of metal oxides to metals.

Figure 3. Thermochemical routes for solar fuel production using concentrated solar energy (adapted from [14]).

The physical state of carbonaceous feedstocks can also be used to categorize the solar thermochemical decarbonization processes into two main groups (Fig. 4). Regarding gaseous feedstocks, natural gas, methane (CH4), and biogas can be converted to syngas via cracking, steam/dry reforming, or chemical looping reforming (CLR) or to both syngas and metals via methano-thermal reduction (MTR). In addition, the gaseous H2O/CO2 feedstock can also be converted to H2/CO via direct thermolysis or two-step H2O/CO2 splitting, but this route requires higher temperatures, thus representing an attractive option for the long-term sustainable fuel production. Concerning solid feedstocks, pet coke, coal/char, biomass, or wastes can be converted to syngas via pyrolysis/gasification or chemical looping gasification (CLG). In addition, they can be used as a chemical reducing agent to separate oxygen from metals oxides, thereby generating elemental metals concomitantly with carbon monoxide (CO) or syngas. This process is known as carbothermal reduction (CTR) and is considered as an attractive avenue to produce metals. As shown in Fig. 4, the solar thermochemical processes offer different decarbonization pathways to produce solar fuels. The details of each solar thermochemical process are reviewed and discussed thereafter.

Figure 4. Diagram of the conversion routes of carbonaceous feedstocks for solar thermochemical processes.

2.1. Thermochemical H2O/CO2 splitting

The H2O/CO2 splitting processes use only H2O or CO2 as feedstocks to produce H2/CO. They consist of thermolysis (or electrolysis if electricity is supplied instead of thermal energy) and two-step splitting cycles [15]. Direct thermal water splitting is the simplest pathway for solar thermochemical hydrogen production from H2O, according to Eq. 6. 1 퐻 푂 → 퐻 + 푂 (6) 2 2 2 2

Nevertheless, this process is hardly practical due to both very high temperatures needed as evidenced by the ΔG° equal to zero at 4330 K for 1 bar (Fig. 5) [16], and difficulty in effectively separating/quenching H2 and O2 to avoid recombination and explosive mixtures.

High solar flux concentration is thus required to reach the reduction temperature of H2O, thereby resulting in adverse issues regarding solar reactor materials thermal stability, costs, and heat losses.

Figure 5. ∆퐻°, 푇∆푆°, and ∆퐺° as a function of temperature for direct thermal water splitting at 1 bar [16].

In order to decrease significantly the operating temperature, thermochemical cycles consisting of a series of consecutive chemical reactions can be employed. Two-step redox cycles are the simplest for solar process implementation [15]. In this pathway, metal oxides (either non-volatile or volatile metal oxides) are employed as oxygen carrier materials that need to be reduced to release oxygen in the first solar step (endothermic) and recycled via oxidation with H2O or CO2 in the second step at a lower temperature (exothermic). Therefore, temperature-swing cycle generally needs to be applied to increase the redox thermodynamic driving force, and the cycling process temperatures strongly depend on the thermodynamic properties of the applied metal oxides. Moreover, the reduction step requires a reaction temperature much lower than that of the thermolysis of water (single step), thereby allowing operation at temperatures compatible with solar concentrating systems (towers and dishes). Moreover, the separation issue can be avoided as two-step thermochemical cycles produce

H2/CO and O2 separately, similarly to the electrolytic process. The two-step thermochemical cycles using metal oxide redox pairs are shown in Eqs. 7 and 8: 푦 1st step endothermic reaction (solar): 푀 푂 → 푥푀 + 푂 (7) 푥 푦 2 2 nd 2 step exothermic reaction (non-solar): 푥푀 + 푦퐻2푂(퐶푂2) → 푀푥푂푦 + 푦퐻2(퐶푂) (8)

Various metal oxide redox pairs have been proposed and studied as promising candidates for two-step splitting of either H2O or CO2 into gaseous fuels. The most developed include chiefly ZnO/Zn, SnO2/SnO, Fe3O4/FeO (and iron-based (ferrite) systems), CeO2/Ce2O3, and non-stoichiometric oxides such as ceria (CeO2/CeO2-, MxCe1-xO2/MxCe1-xO2-) and 10 perovskites (ABO3/ABO3-) [17–24]. These selected materials show a trade-off between reduction and oxidation capabilities and are thus the preferred choice for redox cycling. According to their thermodynamic properties, the oxides that can be straightly reduced at moderate temperatures (e.g., Mn3O4, and Co3O4) usually show poor H2O (or CO2) splitting capabilities (MnO and CoO oxidation with water to produce hydrogen is not thermodynamically favorable [25]). In contrast, favored thermodynamic driving force for oxidation usually implies unfavored reduction and very high temperatures required, as in the case of MgO/Mg, MoO2/Mo, SiO2/Si, WO3/W… For such oxides, the solar step can be realized more favorably with reducing carbonaceous compounds (solid carbon, methane) to decrease the reaction temperature and alternative cycles can be proposed (e.g., carbothermal reduction of metal oxides to produce metals), as discussed in the following.

In short, an attractive pathway to produce H2/CO from H2O/CO2 conversion relies on the use of metal oxides as oxygen carrier materials, possibly combined with the use of reducing agents to lower the endothermic reduction temperature, which is considerably dependent on the thermodynamic properties of the applied metal oxides. Therefore, choosing those metal oxides that both match the process temperature and exhibit suitable redox capabilities is very important for H2O/CO2 splitting cycles.

2.2. Decarbonization processes Decarbonization of hydrocarbon species consists of the conversion of solid/gaseous carbonaceous feedstocks (methane, natural gas, coal, biomass…) to hydrogen and syngas [26], which can be combined with the conversion of metal oxides to metals using such feedstocks as reducing agents. The solar conversion of hydrocarbons or biomass consists in substituting the fossil fuel combustion for process heat supply (endothermic reactions) by concentrated solar energy. The considered routes for H2-rich fuel synthesis are the cracking, reforming, and pyrolysis/gasification. The thermal cracking produces hydrogen and solid carbon, whereas both reforming and gasification produce syngas. These solar processes for hydrocarbon conversion show the following advantages: (1) fossil fuel saving and chemical storage of solar energy, (2) reduction or suppression of greenhouse gas emissions (CO2, SO2,

NOx) with respect to conventional processes, (3) absence of products contamination by the combustion gases.

2.2.1. Solar cracking/pyrolysis

The solar cracking involves the thermochemical decomposition of fossil fuels including natural gas, oil, and other hydrocarbons (biomass, wastes…) in the absence of oxygen (pyrolysis). The simplified net reaction is given in Eq. 9.

푦 퐶 퐻 → 푥퐶(푠) + 퐻 (9) 푥 푦 2 2

The products of the solar cracking process mainly consist of a carbon-rich condensed phase and hydrogen-rich gas phase. For example, thermal methane decomposition yields hydrogen and carbon black (with various applications in rubber reinforcement, polymers, and batteries) [27–37]. The investigated operating temperature range for such a process was quite large, generally within the range of 1300-2073K [33,35], and the conversion of methane increased with increasing methane flow rate or temperature [32]. Thermo-catalytic decomposition with metallic or carbon-based catalysts [38–55] can be used to lower the process temperature (900-1200 °C) compared to thermal decomposition (>1400°C), and to avoid formation of pyrolytic carbon deposited inside the reactor while producing carbon nanostructures such as nanotubes or nanofibers. However, other compounds may occur depending on the composition of starting materials and reaction kinetics such as secondary hydrocarbons (C2H2, C2H4…). Some examples regarding the utilization of various catalysts for thermal methane decomposition to increase CH4 conversion and yield and decrease temperature are reported hereafter. Shah et. al. [38] studied the catalytic decomposition of undiluted methane towards hydrogen production. Fe-M (M= Pd, Mo, or Ni) were employed as catalysts supported on alumina. As a result, methane decomposition temperature was lowered by 400-500 °C relative to noncatalytic thermal decomposition. Pinilla et. al. [49] conducted thermo-catalytic decomposition of methane utilizing Ni-Cu-Al catalyst in a pilot scale fluidized bed reactor. The reaction proceeded at around 700 °C and the performance of the fluidized bed reactor operated with different methane velocities was highlighted. The same research team [48] also employed four catalysts consisting of Ni, Ni:Cu, Fe or Fe:Mo for the catalytic decomposition of methane in a rotary bed reactor and the influence of catalysts on methane conversion was emphasized.

This CO2-free process requires the development of high-temperature solar reactors (Fig. 6) enabling high methane conversion and hydrogen yield [32–36,45,46]. Abanades et. al. [46] studied thermo-catalytic decomposition of methane using carbon black catalysts in an indirect heating tubular packed-bed solar rector, and reported that almost complete CH4 conversion to

H2 was accomplished in the temperature range of 1050-1250 °C. An analogous pyrolysis process related to solid carbonaceous feedstocks (biomass, wastes) can also be applied and usually yields syngas, tars (bio-oil) and chars with proportions and properties depending on various operating parameters [56]. Solar pyrolysis uses highly concentrated solar radiation as external process heat source to drive reactions in an inert atmosphere. Thus, solar energy is chemically stored in an amount equal to the enthalpy change of reactions, which upgrades the feedstock energy. The products yields, composition and properties depend on pyrolysis parameters (type of feedstock, temperature, heating rate, pressure…). The solid carbonaceous products from pyrolysis can be used as reducing agents for carbothermal reduction processes or valorized as high-value by-products.

Figure 6. Solar reactor developed at CNRS-PROMES for thermo-catalytic methane decomposition with carbon black catalysts [45].

2.2.2. Solar reforming and solar chemical looping reforming (CLR) The conventional steam/dry reforming method utilized in the chemical industry to produce syngas uses both fossil fuels as the source of process heat and metal-based catalysts to catalyze the endothermic chemical reactions. This results in both CO2 emissions contributing to global warming and catalysts deactivation. Gaseous carbonaceous feedstocks such as methane (CH4) are oxidized to syngas by utilizing H2O/CO2 as oxidizing agents, and the reaction temperature is usually 1000 K at 1 atm. The simplified net reaction of steam reforming is given by Eq. 10. Alternatively, the required heat for such endothermic reaction can be supplied by concentrating solar technologies, thereby storing solar energy into transportable and storable chemical fuels [57]. 13

푦 퐶 퐻 + 푥퐻 푂 → ( + 푥) 퐻 + 푥퐶푂 (10) 푥 푦 2 2 2

In addition, bio-ethanol was employed as feedstock to carry out the steam reforming reaction process in a Pd–Ag dense membrane reactor in order to produce a CO-free hydrogen stream. The experimental tests were performed between 250 and 400 °C, at 1.5 bar of reaction pressure without using solar energy [58,59]. After that, a steam reformer integrated with a catalytic membrane system for pure hydrogen production using molten salts as heat transfer fluid and concentrating solar power as heat source to drive endothermic reaction [60] was developed and tested by the CoMETHy project team [61,62], demonstrating an innovative reformer system. The reformer was operated at temperatures of 400-550 °C, lower than conventional steam reforming processes (850-900°C), due to the limitation of molten salts maximum operating temperature (550°C). Besides, selective membranes allowed the recovery of high-grade hydrogen and increased CH4 conversion at low temperatures. In contrast to the conventional method, solar chemical-looping reforming of methane (CLRM) employs solid metal oxides as oxygen carriers in place of pure oxygen as the oxidant. For the chemical looping scheme, in the endothermic step, gaseous CH4 is partially oxidized with the metal oxides to produce syngas while the metal oxide is reduced. The reduced metal oxide is subsequently oxidized in the exothermic step with H2O/CO2 to generate H2/CO. The solid metal oxide oxygen carrier is then circulated between these two steps (Eqs. 11 and 12):

1 1 Endothermic partial oxidation of methane: 퐶퐻 + 푀푂 → 푀푂 + 2퐻 + 퐶푂 4 ∆훿 푥−훿표푥 ∆훿 푥−훿푟푒푑 2 (11) 1 1 Exothermic step: 훼퐻 푂 + (1 − 훼)퐶푂 + 푀푂 → 푀푂 + 훼퐻 + (1 − 2 2 ∆훿 푥−훿푟푒푑 ∆훿 푥−훿표푥 2 훼)퐶푂 (12)

Compared to the first step in two-step redox cycles, the methane-driven reduction of metal oxides significantly lowers the reduction temperature due to the CH4 reducing agent [63–71].

Since both CH4-driven metal oxide reduction and H2O/CO2 splitting steps can proceed at similar temperatures, isothermal operation of the cycle is possible, thereby reducing the constraints imposed to reactor materials and thermal radiative losses. The advantages of the

CLRM are: (i) the CH4 utilization during the reduction step makes possible isothermal operation between first and second steps (sensible heat losses occurring in temperature-swing cycles are thus avoided and the need for heat recovery is bypassed), (ii) solid oxide is used instead of gaseous O2 (oxygen production from air is thus not needed), (iii) reduced metal oxide can be oxidized with H2O or CO2 in the subsequent oxidation step to generate additional syngas and close the cycle, (iv) carbon deposited on metal oxide structures can be removed by concomitant gasification during the oxidation step, thus eliminating deactivation of oxide material and avoiding the need for costly catalysts. The feasibility of using metal oxide materials (either non-volatile or volatile) as oxygen carriers for CH4 partial oxidation was experimentally studied e.g. for ceria (CeO2) [64], cerium-based mixed oxides [72], iron oxide (Fe2O3) [73], tungsten oxide (WO3) [74], and zinc oxide (ZnO) [75].

2.2.3. Solar gasification and solar chemical looping gasification (CLG) Conventional technologies (autothermal) for gasification of solid carbonaceous feedstocks require a significant portion of feedstocks (up to 30-45%) being combusted with air or oxygen to drive endothermic gasification reactions [76], in turn discharging a large amount of

CO2 [77]. Moreover, the produced syngas purification may be needed, thereby consuming additional energy for gas separation requirement. Either solid fossil fuels (coal and pet coke) or renewable fuels (lignocellulosic biomass) can be employed as feedstocks for the gasification. Additionally, when biomass is used as the feedstock, the process can be considered to be carbon neutral. A novel approach for converting such solid carbonaceous feedstocks to syngas without CO2 emissions is solar thermochemical gasification (allothermal) [12,13]. In this approach, two sustainable energy sources regarding solar energy and biomass can be coupled in a single process for converting both biomass and intermittent solar energy into high-quality syngas [78–80]. The ideal stoichiometric steam gasification of solid carbonaceous materials to syngas is represented by the simplified overall reaction as:

퐶 퐻 푂 + (푥 − 푧)퐻 푂 → (푦 + 푥 − 푧) 퐻 + 푥퐶푂 (13) 푥 푦 푧 2 2 2

The solar steam gasification of carbonaceous materials with respect to coal and petroleum coke has been widely studied, dealing with thermodynamic and kinetic [81–83], experimental [78,84–86], and modeling [87,88] studies. The development of a continuously reactant-fed

15 vortex flow reactor for the solar steam-gasification of coal and petroleum coke was performed, consisting of reactor design [89], modeling [89–91], testing [84,85], and scale up [92]. Maximum petcoke conversion of 87% at 1500K and solar energy conversion efficiency of 17% were reported; however, the feedstock particle size was limited in the range 8.5–200 μm [84]. When employing biomass as feedstock, the thermochemical gasification reactions are very complex due to the variability of starting biomass compositions in accordance with the carbonaceous feedstock type, location, age, and period [93-–95]. Nevertheless, the global mechanism can be mainly divided into two sequential reactions. First, a pyrolysis step takes place consisting of the decomposition of solid hydrocarbon compounds at temperatures from 300 ºC to 1000 ºC. The biomass is mainly decomposed into char, liquid tars, and incondensable gases, as shown in Eq. 14. During this step, both high temperatures and heating rates favor the production of gaseous species over char and tars [13].

Biomass → char (carbon) + CO + CO2 + H2 + CH4 + Tars (14)

In a second step, the produced solid char is gasified by reacting with an oxidant (such as water steam or CO2), thereby leading to several possible reactions (Eqs. 15 to 19):

C+H2O→CO+H2 H° = 131.3 kJ/mol (15)

C+2H2↔CH4 H° = -74.6 kJ/mol (16)

C+CO2↔2CO H° = 172.4 kJ/mol (17)

CO+H2O↔CO2+H2 H° = -42 kJ/mol (18)

CH4+H2O↔CO+3H2 H° = 206 kJ/mol (19)

Gas species equilibrium compositions for carbon/steam and carbon/CO2 systems show that the main produced gas are H2 and CO when temperature is above 1000°C [13]. In contrast,

CH4 markedly decreases when temperature increases because the formation of CH4 is not thermodynamically favored at higher temperatures (CH4 is not stable over 1000°C and the methane formation kinetics is too slow). Compared with the conventional process, the advantages of the solar biomass gasification are: (i) the partial combustion of biomass feedstock for supplying process heat (in the range

30-45% of feedstock) is eliminated [96], thus avoiding CO2 emissions, saving resource and enhancing syngas output per unit feedstock [97]; (ii) an energy-rich and high-quality syngas

16 is produced (no contamination by the combustion products); (iii) the feedstock calorific value is upgraded by solar energy, enabling conversion of intermittent solar energy into storable and dispatchable chemical fuels; (iv) the need for additional energy consumption for downstream gases separation is avoided [98,99]; (v) the pollutants emission is circumvented [100]; (vi) the high-temperature solar reactor operation (possibly above 1200°C) results in enhanced reaction kinetics, improved syngas quality, and avoids the presence of tars in the formed syngas [90,101]. Besides, crop residues, which are widely available particularly in developing countries, can be employed as feedstock for solar-driven thermochemical gasification, thereby offering a promising pathway to convert them into high-quality syngas [102,103]. To deal with the issues of intermittent or fluctuating solar energy input, the concept of solar allothermal/autothermal hybrid gasification was considered [104,105], highlighting the importance of around-the-clock systems operation.

Alternatively, another novel approach is solar chemical looping gasification (CLG). The operating principles for CLG are similar to CLRM. The difference is just the use of solid carbonaceous feedstocks in place of gaseous feedstocks. In this pathway, the solid carbonaceous feedstocks, which can be fossil fuels or biomass, are partially oxidized to generate syngas utilizing metal oxides as oxygen carriers, while the metal oxide is simultaneously reduced, as given by:

1−푏 푎 1−푏 퐶퐻 푂 + 푀푒푂 → 퐶푂 + 퐻 + 푀푒푂 (20) 푎 푏 훿 푥 2 2 훿 푥−훿

When the reduced metal oxide is re-oxidized with H2O or CO2, the overall reaction is given by Eq. 12. The CLG benefit lies in the advantages of outperforming the solar steam gasification in both being able to avoid the use of gaseous oxidant in the reaction (thereby alleviating corrosion issues), and to operate as chemical looping cycles. Recently, the feasibility of the combination of biomass gasification and ZnO reduction for syngas and Zn production was experimentally investigated in a particle-fed reactor using wood biomass feedstock mixed with solid ZnO oxidant [106]. The goal was thus to prove the concept of continuously producing both syngas and metal Zn in a single process. Experiments were performed in spouted bed with biomass/ZnO molar ratios of 0.5-1 at 1050-1300 °C, using beech wood biomass in continuous feeding mode. The influence of both temperature and reactant molar ratio on ZnO conversion as well as syngas production was studied and

17 additionally compared to pyrolysis tests (without any oxidant). As a result, the feasibility of continuous biomass gasification combined with pure Zn production was experimentally proved. Increasing the temperature significantly increased H2 and slightly increased CO, while CO2 and CH4 decreased. The optimal biomass/ZnO molar ratio was determined to be

0.75 (Fig. 7). For this ratio, maximum syngas production up to ~8 molsyngas/molbiomass was reached at 1250 °C. The syngas yield of the combined gasification/carbothermal reduction process was much higher than pyrolysis because of higher feedstock conversion thanks to ZnO. The energy upgrade factor of the feedstock through the solar power input was 1.17 and the solar-to-fuel energy conversion efficiency was 19.8%.

Figure 7. Syngas yield from combined gasification/carbothermal reduction at different biomass/ZnO molar ratios (1100°C) [106].

2.2.4. Solar carbothermal and methano-thermal reduction Solar carbothermal reduction (CTR) and methano-thermal reduction (MTR) relate to the reduction of metal oxides using solid or gaseous feedstocks as reducing agents, respectively and concentrated solar energy as the process heat source. Such reductants can be either fossil sources such as coke [107,108], coal [109], methane [75] or renewable sources like biomass or biogas [106,110]. When employing biomass feedstock, the chemical reactions become the combination of CTR and gasification. The objective of the CTR and MTR is basically to produce metals (metallurgical processes) by removing oxygen from metal oxides utilizing carbonaceous feedstocks, generating both metals and CO/syngas. The energy-intensive metals can be used as energy storage media (solid fuels), chemical commodities or further 18 converted to pure H2 (or CO) by oxidation with water (or CO2). The CTR and MTR overall reactions can be written as Eqs.21 and 22 respectively:

푀푥푂푦 + 푦퐶 → 푥푀 + 푦퐶푂 (21)

푀푥푂푦 + 푦퐶퐻4 → 푥푀 + 푦퐶푂 + 푦2퐻2 (22)

Compared to direct thermochemical dissociation of metal oxides without any reducer, the carbothermal/methanothermal chemical reactions can be conducted at much lower temperatures thanks to the reducing agent. For example, the solar direct ZnO dissociation

(ZnO(s) + solar heat → Zn(g) + ½ O2) requires temperatures up to ~1975 °C at atmospheric pressure [15,111–116], while the CTR of ZnO can be performed at a reduction temperature as low as 950 °C, resulting in 1025 °C lower compared to direct dissociation of ZnO. The production of Mg and Zn via the solar CTR of MgO and ZnO using solid carbonaceous feedstocks such as solid carbon or biomass as reducing agents was studied in a ceramic cavity-type solar reactor [117]. The reactions also produce CO or syngas, as valuable co- products. The solar CTR of ZnO and MgO was studied as a function of the different reducing agents (activated charcoal, carbon black, graphite, and beech wood biomass as particles or pellets), carbon/metal oxide molar ratios (1.5-2), pressures (vacuum and atmospheric), and temperatures (950-1650 °C) in batch and continuous operation, thus demonstrating flexibility, reliability, and robustness of this scalable metallurgical process for Zn and Mg production. The equilibrium thermodynamics of the CTR of ZnO and MgO were also analyzed. ZnO and MgO conversion, reduction extent, and CO yield rose with decreasing pressure and increasing temperature, in agreement with thermodynamics. High-purity Zn and Mg production was achieved with net conversion above 78% and 99%, respectively. Using activated charcoal as reducing agent led to the maximal MgO and ZnO conversion and CO yield. Mg recovery in the outlet products was found to be challenging because the produced nanopowder is pyrophoric and is promptly oxidized with air. Alternatively, utilizing wood biomass as a sustainable reducer was proved to be an attractive choice to produce both metallic Zn and high-quality syngas in a single process, demonstrating the feasibility of combined biomass gasification with the ZnO/Zn redox system in continuous operation for the first time [106]. High conversion efficiency and solar energy storage were demonstrated with the maximum energy upgrade factor up to 1.2 and 1.9 and the maximum solar-to-fuel energy conversion efficiency up to 6.0 % and 7.8 % for the CTR of ZnO and MgO, respectively. The

19 produced Zn and Mg as nanopowders were further proved to be reactive in a second step to produce fuel via CO2-splitting in a complete and fast reaction.

In summary, there are several approaches for solar decarbonization processes. Solar cracking/pyrolysis aims at decomposing carbonaceous feedstocks into both CO2-free hydrogen and solid carbon in a single process without oxygen requirement. In this pathway, the utilization of catalysts may be necessary for lowering the process temperature and promoting hydrogen yield. Solar reforming/gasification and associated chemical looping processes aim at converting gaseous (natural gas, methane) and solid (coal, biomass) carbonaceous feedstocks toward syngas. In comparison, chemical looping reforming process outperforms the conventional one by eliminating the issue of catalysts deactivation, producing syngas with a H2:CO ratio of 2:1 (suitable for methanol synthesis), and avoiding an excess in oxidizer (the conventional process needs to be operated with excess oxidizer

(H2O:CH4≥3), which rises energy requirements and reduces process efficiency). Similarly, solar gasification outperforms the conventional process by saving the available feedstock and storing/upgrading solar energy into syngas. Finally, solar carbothermal and methano-thermal reduction aims at producing metals (metallurgical processes) and CO/syngas in a single process by utilizing carbonaceous feedstocks as reducing agents.

3. Metal oxides redox pairs for solar thermochemical processes

Several metal oxide redox pairs have been studied extensively for solar thermochemical processes (two-step splitting cycles, CLR, CLG, CTR, and MTR). They can be mainly classified into two groups based on their phase change regarding volatile oxides such as ZnO

[118], SnO2 [119], and MgO [120] and non-volatile oxides such as iron oxides (Fe2O3,

Fe3O4, ferrites [121]), ceria (CeO2-) [69,70,122] and perovskites (such as La1−xSrxMnO3−δ, however never used in solar reactors and processes to date) [24,123]. In this study, some of the most attractive metal oxides candidates regarding non-volatile oxides (ceria and iron oxides) and volatile oxides (ZnO and MgO) involved in solar thermochemical processes are especially considered as promising oxygen carriers or oxidants for decarbonization processes due to their interesting chemical and physical properties.

3.1. Volatile oxides When employing volatile metal oxides (stoichiometric materials), a solid-to-gas phase transition of the products (ZnO/Zn, MgO/Mg, SnO2/SnO) occurs in the reduction step [124,125]. The reduced product species are first melted/vaporized and then condensed in the form of fine solid particles when temperature decreases. Volatile metal oxides usually exhibit high oxygen release capacity and high entropy creation since they can be completely reduced to their metallic elements via stoichiometric reactions, thus enhancing fuel production capacity. However, they come at the expense of a recombination issue with oxygen (from O2 or CO) during (carbo)thermal reduction. Two main attractive volatile oxides candidates (ZnO and MgO) are reviewed in different thermochemical processes.

3.1.1. ZnO ZnO is an attractive candidate for various thermochemical processes because its decomposition temperature at atmospheric pressure is not too high (1975 °C) compared to other volatile candidates [126]. In addition, metal zinc is highly reactive for oxidation with

H2O/CO2 to generate high-purity H2/CO [127] and is needed for the applications of both corrosion-resistant zinc plating of iron and electrical batteries [128]. This system has been extensively investigated both experimentally and numerically by research teams (e.g. at PROMES/CNRS-Odeillo in France and PSI in Switzerland). Abanades et al. [111] experimented and simulated a rotary kiln particle-fed solar reactor for the direct thermal dissociation of ZnO involved in water-splitting thermochemical cycles for hydrogen production. They found that reaction completion was achieved for reactant temperature exceeding 2200 K for a 1 mm initial particle diameter, and the higher the particle surface area, the higher the conversion rate. Koepf et al. [116] tested a 100 kWth scale reactor for ZnO dissociation. The solar reactor was operated for over 97 h and yielded ZnO dissociation rates as high as 28 g/min totaling over 28 kg of processed reactant during 13 full days of experimentation. However, the major drawback of the thermal dissociation of ZnO is attributed to its high reduction temperature and partial recombination with O2 that can be tackled by fast products quenching and dilution with inert gas (to lower the O2 partial pressure) [129,130]. Alternatively, utilizing gaseous [131–137] or solid carbon [131,138] species as reducing agents for the reduction of ZnO regarding a CTR/MTR approach can tremendously lower the reduction temperature to below 900 °C (Fig. 8a), generating Zn and CO/syngas according to Eqs 23 and 24.

Carbothermal reduction of ZnO: ZnO+C→Zn+CO (23)

Methano-thermal reduction of ZnO: ZnO+CH4→Zn+2H2+CO (24)

Prior thermodynamic and experimental studies on ZnO reduction in different solar thermochemical processes dealing with the utilization of solid/gaseous carbonaceous feedstocks have been conducted. Osinga et al. [107] experimentally investigated the CTR of ZnO using a two-cavity packed-bed reactor. Thermal efficiency up to 20% was achieved for batch tests but 20% of non-reacted ZnO remained and was assumed to diffuse into the insulation material. The influence of temperature, carbon source, and carrier gas composition on CTR of ZnO was later examined utilizing the same reactor to gain data for designing a scaled up reactor [108]. Wieckert et al. [139] tested a 300 kW packed-bed batch reactor for the CTR of ZnO in the temperature range 1300-1500 K, yielding 50 kg/h of 95%-purity Zn and 30% of thermal efficiency, and they confirmed that ZnO condensation and rock crystal generally grew on cooled surface. Various works on thermodynamic and experimental analysis of MTR of ZnO were also proposed [131–137] . For example, the combined solar thermal reduction of ZnO and reforming of CH4 was examined in a fluidized-bed tubular quartz reactor at 1200 K and 1 atm [131]. It was reported that the combined process offered the simultaneous production of Zn and syngas from ZnO and CH4 without discharging greenhouse gases. The same process was also tested in a gas-particle vortex flow under continuous operation in the temperature range 1000-1600 K, yielding up to 90% of Zn conversion [135]. Koepf et al. [125] studied the CTR of ZnO with beech charcoal in a continuous beam down, gravity-fed solar reactor, yielding 12.4% of thermal efficiency, 75% of Zn content, and 14% of reactant conversion; however, critical issues related to clogging and significant unreacted reactant were encountered. Recently, Brkic et al. [140] tested CTR of ZnO in a drop tube reactor at pressure between 1 and 960 hPa and found that the Zn production rate is maximal at ~100 hPa and significantly drops under vacuum because of insufficient residence time and limited particles heat up in the reaction zone. Besides, vacuum CTR of ZnO was proposed in order to enhance the reduction rate [124,141] and decrease products recombination reaction, according to Le Chatelier’s principle. Levêque and Abanades [124] studied the influence of the total pressure and the oxygen partial pressure

(dilution) for the CTR of different volatile oxides (ZnO, SnO2, GeO2, and MgO) via solar- driven vacuum thermogravimetry [142]. They reported that when metal oxides were reduced by lowering pressure, the reaction rate was greatly enhanced, and the required temperature to

22 achieve a given reduction rate was significantly lowered. A decrease in total pressure also lowered the need for a diluent gas.

3.1.2. MgO MgO is also considered as an attractive candidate for solar thermochemical processes [124] towards Mg commodity. Mg product is commonly used as structural material involving magnesium-based alloys [143,144] and power generation in magnesium-based combustion engines, and it also shows high reactivity and stability for H2O/CO2 splitting [117,119]. The melting and boiling points of MgO are extremely high (2852 °C and 3600 °C), while those of Mg are 650 °C and 1091 °C, respectively. Mg is produced conventionally by “Pidgeon”, “Magnetherm”, and electrolytic methods. The “Pidgeon” and “Magnetherm” techniques involve calcined dolomite ore reduction with ferrosilicon at high temperatures (1700 °C), whereas electrolysis involves molten magnesium chloride reduction in the range 680–720 °C [145]. They also proceed with additional compounds (silicothermic process) and consume high energy amounts (either electrical or fossil fuels) to drive the endothermic reactions, thereby raising serious environmental concerns. For these reasons, solar thermochemical reduction of MgO becomes an alternative attractive approach towards CO2-free industrial Mg production. Nevertheless, solar direct reduction of MgO (MgO(s) +solar heat → Mg(g) +

1/2O2) is not practical because of its extremely high dissociation temperature (3600 °C).

Hence, carbothermal (MgO + C → Mg(g) + CO) and methano-thermal (MgO + CH4 →

Mg(g) + CO + 2H2) reduction can be alternatively considered to be a better option for producing Mg. Given the Mg boiling point, the produced Mg is gaseous at the reaction temperature and it is recovered as fine nanopowder (like Zn). Previous works mainly investigated the kinetics of CTR of MgO using different solid carbonaceous materials (graphite and charcoal) [146–149] via thermogravimetry analysis (TGA). The two-step thermochemical MgO/Mg cycle for syngas production using charcoal and petcoke was examined via thermodynamics and TGA under atmospheric pressure [120], and the steam hydrolysis of Mg was also studied in the temperature range 350–550 °C. The methano- thermal reduction of MgO was barely studied before [117]. However, since methane decomposition prevails at the reaction temperature, the formed carbon is the main specie that reacts during MgO reduction. Although carbonaceous materials are employed for lowering the reduction temperature, high temperatures above 1600 °C at atmospheric pressure are still required for the CTR of MgO to reach complete conversion (Fig. 8b). Two different approaches can be applied to limit the 23 recombination of gaseous products and recover metal products: fast vapor quenching or direct magnesium dissolving in a suitable metal solvent before reverse reaction can proceed [150]. Alternatively, Mg yield can also be improved by decreasing the CO partial pressure [151]. Vacuum MgO carbothermal reduction has been studied theoretically and experimentally to lower the temperature [117,124,152–158]. Thermodynamics also confirms that the required temperature for complete MgO carbothermal reduction decreases when decreasing pressure (Fig. 8b). This approach leads to reduced heat losses and alleviates reactor materials issues while increasing reduction rate and conversion of MgO, however at the expense of additional energy required for pumping.

Figure 8. Products composition at thermodynamic equilibrium for ZnO and MgO carbothermal reduction as a function of temperature.

3.2. Non-volatile oxides In contrast to volatile oxides, non-volatile oxides remain in the solid state over the thermochemical process; therefore, only oxygen is released from their structure, thereby bypassing the recombination issue and suitably matching with CLRM and CLG pathways in terms of process operation in a single reactor. In this case, they act as oxygen carriers for the chemical looping process. However, non-volatile oxides exhibit issues of both nonstoichiometric reaction (case of ceria), which results in lower oxygen storage/release capacity leading to low fuel productivity, and sintering/densification (case of iron oxides). For these reasons, physicochemical characteristics regarding morphological aspects (specific surface area, porosity, particle size) and thermal stability become important factors for non-volatile oxides. Ferrites and ceria materials are reviewed for different solar thermochemical processes.

3.2.1. Ferrites Ferrites are usually ferromagnetic ceramic compounds developed from iron oxides, e.g. magnetite (Fe3O4), which is a promising candidate due to abundant source, low cost, and easy handling. They exhibit large oxygen release capability in comparison with other candidates 25 thanks to stoichiometric materials but may be subjected to sintering issues, inducing low cycling stability [73,159]. Nakamura [19] first assessed the two-step iron oxide cycle

(Fe3O4/FeO). The reduction reaction of Fe3O4 proceeds at temperature of 2500 K, and the subsequent hydrolysis reaction proceeds at 450 K. The Fe3O4 and FeO melting points are

1870 K and 1650 K, respectively. Thus, Fe3O4 is thermally reduced into liquid FeO in the first step. In the second step, solid FeO (after milling into a fine powder) reacts with water steam to produce H2 and Fe3O4. Steinfeld and Fletcher [160] conducted the CTR of Fe2O3 with graphite in the temperature range 1300-2390 K using solar energy. They found that the iron yields were extremely low when the reaction temperature was below its melting point due to slow reaction kinetics and diffusion limitation; moreover, the highest iron yields were obtained in a higher temperature range 1850-2390 K, demonstrating a relatively strong effect of the reactor temperature and of the cooling and/or heating rate. The solar Fe3O4+4CH4 system was also proposed [121,161] and thermodynamic equilibrium species consisting of solid metallic iron and a gaseous mixture of 2/3 H2 and 1/3 CO were reported (at 1 bar and

1027 °C), while it was shown experimentally that Fe3O4 reduction with CH4 depends strongly on both temperature and residence time [71]. The reactivity of magnetite (Fe3O4) through

CH4 reforming and H2O splitting was investigated in a continuous reactor and the reduction kinetics was also studied [73]. Besides, Bleeker et al. [159] investigated iron oxide deactivation in the steam-iron process to produce H2 and they argued that the main drawback of utilizing iron oxide is the inherent structural changes occurring during oxygen absorption and release, resulting in severe material deactivation because of loss of specific surface area. In addition, doped iron oxides with spinel structure were examined thermodynamically and experimentally to improve their reactivity and stability [162–168], e.g. Ni0.39Fe2.61O4/ZrO2.

Kodama et al. [162] evaluated M0.39Fe2.61O4 with M = Co, Ni, Zn. The Ni ferrite achieved the highest conversion and selectivity (XCH4 = 31%, SH2 = 75%, SCO = 72%), and supporting on

ZrO2 reduced sintering and thus enhanced reactivity.

3.2.2. Ceria Currently, ceria is recognized as an attractive non-volatile redox candidate for both two-step

H2O/CO2 splitting and CLRM. It shows crystallographic stability during extensive thermochemical cycling, reproducible oxygen storage and release capacities, rapid oxygen exchange rates, and reversible shift between Ce(IV) and Ce(III) oxidation states in nonstoichiometric reactions [18,23,169–177]. Ceria keeps a stable cubic structure (fluorite) during large oxidation state variations and exhibits fast kinetics during thermochemical 26 reactions in comparison with other non-volatile metal oxides including ferrites [162,169– 177]. Ceria has been widely investigated as a non-stoichiometric redox material for two-step thermochemical H2O/CO2 splitting cycles in which reduction is carried out in inert sweep gas or at sub-atmospheric pressures. The two-step solar-driven water-splitting cycle based on cerium oxides was initially proposed by Abanades and Flamant [170]. Ceria is first reduced partially to non-stoichiometric state (at above 1400 °C). Then, the reduced oxygen-deficient ceria (CeO2-) is completely re-oxidized with H2O/CO2 to produce H2/CO with fast reaction kinetics at lower temperature. Alternatively, utilizing ceria-based materials as oxygen carriers for CLRM is also particularly attractive. Previous experimental works considering CH4 partial oxidation utilizing the ceria redox properties were first reported by Otsuka et al. [178], without using solar energy. They demonstrated that the CH4 conversion to syngas with a H2/CO ratio of two (suitable for methanol synthesis) was possible, and the reduced ceria was re-oxidized with CO2 to form CO. Then, both thermodynamic and experimental studies were conducted with combination of concentrated solar energy [65,66]. Coupling the ceria reduction with the CH4 partial oxidation allows for isothermal operation at 950 °C with production of high-quality syngas during the reduction step (maximum solar-to-fuel efficiency of 40% is predicted). The CH4 reforming over ceria in a particle transport reactor was experimentally investigated [65]. This reactor yielded an averaged oxygen non-stoichiometry (δ) of 0.25 but unreacted ceria particles were entrained by the produced syngas. Otsuka et al. [179,180] reported that carbon accumulation tends to form when over than 80% of the oxygen available from the oxygen carrier material has been consumed. The ceria macrostructure has a significant impact on the combined two-step process performance with respect to material reactivity and conductive/radiative heat transfer across the structure. Various metal oxide structures including porous foams [69,70,181], textured plates [182], vertical pins [182], powder or bio- templated granules [70,183], multi-channeled honeycombs [184], felts [176] and three- dimensionally ordered macroporous (3DOM) ceramics [183,185] have been investigated for

H2O/CO2 splitting redox cycles in order to offer an improved interface for uniform volumetric concentrated solar radiation absorption and suitable surface area for achieving rapid solid/gas reactions. The structures consisting of packed-bed powder or powder mixed with inert and stable promoter [63,70] showed rapid oxidation rates; but at the expense of radiative opacity leading to temperature gradients through the bed. Such an issue can be overcome by utilizing porous foam structures enabling high geometrical surface area [69,70],

27 although heat transfer limitation may still occur due to their high optical thickness (foams with large open cells should therefore be preferred). Such reactive porous structures can thus be beneficially applied in the solar isothermal CLRM process by alternately switching between CH4 and oxidant gas (H2O or CO2) flows through the oxide structure (Fig. 9).

Figure 9. Syngas production rates and reactor temperature during isothermal redox cycling of

ceria foam with CH4 at 1000 °C: (a) reduction with CH4 and (b) oxidation with H2O.

4. Reactors for solar thermochemical processes

The design of scalable and reliable solar reactors for solar thermochemical processes remains one of the most significant challenges to improve process efficiency. Concentrated solar power input, chemical reactants, heat and mass transfer, chemical thermodynamics, residence time, reaction kinetics as well as operating temperature are important parameters that must be considered for reactor design with respect to reactor type, geometry/size, materials of construction, and modes of operation. The reactors may be classified according to either reactor operation mode (batch and continuous reactors) or solar irradiation pathway

(indirectly and directly irradiated reactors). For directly-irradiated reactors, the chemical reactants are directly exposed to concentrated solar radiation, thereby providing efficient radiative heat transfer directly to the reaction site, at the expense of the need for a transparent window, which remains a critical and troublesome component at large scale and, particularly, under industrially-preferred high pressures. The indirectly-irradiated reactors offer radiative heat transfer to the reaction site via an opaque absorbing wall, eliminating the need for a transparent window at the expense of limitations and requirements imposed by the absorbing materials such as maximum operating temperature, resistance to thermal shocks, thermal conductivity, radiative absorptance, and chemical inertness. The different solar reactors are reviewed according to their operation with volatile or non-volatile oxides, and application to gasification of carbonaceous feedstocks.

4.1. Solar reactors for volatile metal oxide processes A large number of solar reactors have been designed, developed, and tested for direct dissociation, CTR, and MTR of ZnO. In this case, the solar reactors can be classified according to the processing mode regarding batch and continuous operation. The most significant and representative concepts are described thereafter. On the one hand, packed-bed reactors, whether directly or indirectly irradiated are usually operated in a batch mode [107,108,139,186]. These reactor concepts demonstrate high extent of reaction as a result of the relatively large exposed reactant surface area able to absorb upcoming solar radiation and long residence time [139]; however, they exhibit temperature gradient issues [108], and unreacted reactant remaining after solar processing [186]. For example, packed-beds at 5 kWth lab-scale [107] and 300 kWth pilot-scale [139] (Fig. 10) were developed featuring two- cavity indirectly-irradiated solar reactors for the CTR of ZnO. They consist of two cavities in which the upper one acts as the solar absorber and the lower one acts as the reaction chamber. With this concept, the upper cavity can protect the window against products deposition and condensable gases coming from the reaction chamber.

Figure 10. Two-cavity packed-bed solar reactor for batch ZnO carbothermal reduction [139].

On the other hand, entrained particle-laden flow [111,113,116,128], moving-front [115], gravity-fed [125,187], drop tube [140], vortex flow [75], and spouted-bed [188] solar reactors were operated in a continuous mode. These reactors achieve rapid conversion rates, but the reactant feeding rate must match well the reaction rate to avoid a reactant accumulation issue [189,190], and heat and mass transfer limitations must be taken into account [114]. The continuous system may also require large amounts of carrier gas for maintaining particle entrainment. When compared to batch operation, reactants conversion efficiencies may be lower, as residence time is shorter, and solid reactant can escape from the reactor with carrier gases. For example, continuous particle-fed [191] and moving-front pellet-fed solar reactors [115] for the direct dissociation of ZnO under controlled atmosphere are presented in Figs. 11 and 12, respectively. The former reactor was mounted on a horizontal-axis solar furnace, and ZnO particles were fed by a screw feeder in a rotary cavity, while the latter one was settled at the focus of a vertical-axis solar concentrator and ZnO pellets were injected by a pushing rod. Both reactors were heated by real concentrated sunlight delivered by a sun-tracking heliostat and a 2 m-diameter parabolic concentrator.

Figure 11. Particle-fed rotating solar reactor for continuous ZnO thermal dissociation [191].

Figure 12. Pellet-fed moving-front solar reactor for continuous ZnO thermal dissociation [115].

Other directly-irradiated reactors designed for the solar thermal ZnO dissociation were operated in a semi-continuous mode in the temperature range of 2000-2300 K and were all based on the same concept featuring a windowed rotating cavity-receiver lined with ZnO particles and directly exposed to high-flux solar irradiation [113,116,128]. A vortex-flow solar reactor concept [75] for the continuous combined ZnO reduction and

CH4 reforming at 1300 K is shown in Fig. 13. It includes a cylindrical cavity containing a windowed aperture to capture concentrated solar energy. A mixture of ZnO particles and CH4

31 was continuously fed into the cavity receiver via a tangential inlet port while the chemical products regarding Zn vapor and syngas exited the cavity via a tangential outlet port.

Figure 13. Vortex-flow solar reactor concept for the combined ZnO reduction and CH4 reforming [75].

A high-temperature ceramic cavity-type thermochemical reactor was designed for experimental investigation of CTR of MgO and ZnO [117]. The cavity receiver is a vertical cylinder made of alumina with a volume of 121.4 cm3 and directly exposed to solar radiation (Fig. 14). This reactor is also flexible for operation in batch and continuous modes under vacuum and atmospheric pressures, and allows reaching targeted temperatures above 1600 °C under 1.5 kWth solar power input. On-sun experiments were carried out by varying different operating parameters such as the type of solid reducing agents (activated charcoal, carbon black, and beech wood biomass) and reactant molar ratio in batch or continuous modes under rough vacuum and atmospheric pressures. High conversion of ZnO and MgO to Zn and Mg was highlighted, thus demonstrating the feasibility of the novel solar-aided metallurgical process.

Figure 14. Schematic of the 1.5 kWth high-temperature ceramic cavity solar reactor for CTR of MgO and ZnO [117].

4.2. Solar reactors for non-volatile metal oxide processes Similar to the solar reactors developed for volatile metal oxides, the most representative solar reactor concepts are generally fixed bed/structured and fluidized/particle bed reactors. Regarding fixed bed/structured reactors, Marxer et al. [175] studied the two-step solar thermochemical CO2 splitting cycle using a reticulated porous structure made of ceria. The directly-irradiated solar reactor configuration consists of a windowed cavity-receiver including a reticulated porous ceramic (RPC) foam-type structure directly exposed to high- flux solar irradiation provided by a solar simulator. The maximum CO2 conversion of 83% and solar-to-fuel energy conversion efficiency of 5.25% were achieved. Another example of solar fixed bed concept is an indirectly-irradiated tubular reactor (Fig. 15) [63,183]. This reactor was tested for both two-step ceria redox cycle and CLRM using ceria particles (as well as ceria porous foam and biomorphic ceria made from cork template) as oxygen carrier and H2O/CO2 as oxidants in a second exothermic oxidation step. It comprises a cavity receiver with the aperture (15 mm diameter) placed at the focal point of a

33 horizontal axis solar furnace. The cavity made of graphite is lined with a surrounding insulation layer and separated from the atmosphere by a glass window. High reduction rate and bed-averaged oxygen non-stoichiometry (δ) up to 0.431 during reduction with CH4 at 1000 °C were achieved.

Figure 15. Schematic of the indirectly-irradiated tubular reactor for solar-driven ceria redox cycles [183].

The solar CLRM through non-volatile oxides including ceria (CeO2) and iron (Fe2O3) oxides was studied in a volumetric solar reactor (Fig. 16). Forty-four on-sun cycling conditions were carried out via two-step redox cycling encompassing endothermic CeO2 or Fe2O3 reduction with CH4 (partial oxidation of methane) and exothermic oxidation of reduced metal oxides with H2O/CO2 at the same operating temperature, confirming the possibility to operate the cycle isothermally [71]. The influence of metal oxides macrostructure (packed-bed powder, packed-bed powder mixed with inert alumina particles, and reticulated porous foam), oxidants (H2O and CO2), CH4 flow-rates (0.1-0.4 NL/min), reduction temperatures (900-1150 °C), sintering temperatures of metal oxide structure (1000-1400 °C) on the syngas production rate, syngas yield, bed-averaged oxygen non-stoichiometry (δ), CH4 conversion (XCH4), and solar reactor performance outputs (energy efficiencies) was evidenced. The metal oxides cycling stability was also demonstrated. Iron oxide reduction with CH4 is strongly dependent on temperature and exhibits relatively lower reaction rate than CeO2. The reduced metallic iron (Fe) cannot be completely re-oxidized to Fe3O4 after water-splitting step because of its

34 strong sintering and agglomeration (low thermal stability), leading to material deactivation.

Employing Fe2O3 as oxygen carrier is thus not appropriate for solar CLRM, but reduction of iron oxide with CH4 could be an alternative option for solar-driven metallurgical process targeting both syngas and metallic iron production. In contrast, cerium oxide exhibits fast reactions and stable syngas yields while achieving H2/CO molar ratios approaching 2 over several repeated cycles, demonstrating complete ceria oxidation with either H2O or CO2. Successive redox cycles with stable patterns in δ, syngas production yield, and reactor performance confirmed ceria material stability (Fig. 17). A decrease of the sintering temperature of the ceria foam results in an increase in syngas selectivity, CH4 conversion, and reactor performance, at the expense of structure embrittlement. The increase of both the CH4 flowrate and reduction temperature promotes δ up to 0.41, in turn resulting in a substantial improvement of the syngas yields (up to 8.08 mmol/gCeO2), but concomitantly favoring CH4 cracking reaction [70]. The ceria reticulated porous foam structure shows improved heat transfer performance thanks to effective volumetric solar radiation absorption and uniform heating with lower consumption of solar power input [69]. Using CO2 as oxidant results in longer oxidation duration compared to H2O, and slower oxidation kinetics. Solar energy storage is confirmed by the maximum solar-to-fuel energy conversion efficiency of 5.60% and the energy upgrade factor up to 1.19.

Figure 16. Cross-section scheme and working principle of the volumetric solar reactor for CLRM with ceria and iron oxides [69].

Figure. 17. Cycling stability of ceria regarding evolutions of energy upgrade factor and total

syngas yields produced from both CeO2+CH4 and CeO2-δ+H2O [69].

To enhance heat and mass transfer, fluidized-bed solar reactors were proposed in which reactants are continuously mixed and stirred with inert particles by flowing a neutral gas and/or oxidizing agent. Gokon et al. [192] tested the two-step water splitting redox cycle for hydrogen production by using a single reactor consisting of an internally-circulating fluidized bed of NiFe2O4 or ceria particles at a temperature up to 1600 °C (Fig. 18a). A metal foam distributor made of porous stainless-steel was placed at the bottom of the fluidized-bed region of the reactor. The circulating fluidized bed was subjected to simulated solar light from Xe lamps with an input power in the range 1.6–2.6 kWth. As a result, hydrogen productivity reached about 1000 Ncm3 for successive reactions of thermal-reduction and water- decomposition, while ferrite conversion was in the range 5–35%. However, the major drawbacks of fluidized bed reactors are the reactive particle size limitation, attrition and abrasion, and the awkward operation in continuous mode. In addition, Welte et al. [65] examined the continuous combined ceria reduction and methane reforming in a 2 kWth particle-transport solar reactor utilizing a vertical Al2O3 tube receiver including a downward gravity-driven particle flow of ceria (Fig. 18b). Ceria particles were delivered to the alumina tube in either co-current or counter-current to a CH4 flow. Methane conversion was up to 89% at 1300 °C for residence times below 1 s and the maximum reduction extent (δ) was 0.25. 36

(a) (b)

Figure 18. Schematic of (a) fluidized bed solar reactor for two-step H2O splitting using metal oxide particles [192] and (b) solar particle-transport reactor for continuous combined ceria reduction and methane reforming [65].

4.3. Solar reactors for gasification

The solar reactor concepts for solar gasification of solid carbonaceous materials are similar to those for volatile and non-volatile oxides. For example, steam gasification of coal, biomass, and carbonaceous waste feedstocks for syngas production was conducted in 5 kW and 150 kW indirectly irradiated packed-bed reactors operated in a batch mode [193–195]. Such reactors feature two cavity receivers separated by an emitter plate between them (working principle shown in Fig. 10). They achieved high solar-to-chemical energy conversion efficiencies varying between 17.3% and 29.0% but exhibited issues with liquid tars in the product gas stream and incomplete conversion because of non-homogeneous heating. The gasification of carbonaceous materials with fluidized-bed reactors was also investigated [196–198]. The reactor uses oxidative gas to stir solid feedstocks within the cavity receiver (Fig. 19a). This reactor can especially overcome the limitations of heat and mass transfer,

37 enhance solid residence times, and reduce ash buildup, however at the expense of limitation of maximum allowable feedstock size (<1 mm) and operation usually in batch mode.

(a) (b)

Figure 19. Schematic of (a) internally circulating fluidized bed reactor for coal gasification [198] and (b) molten-salt solar gasification reactor [199].

Additional designs and studies of solar-driven gasification reactors relate to the utilization of molten salts, which offers the advantages of enhanced heat transfer, gasification catalysis, lowered tars production and thermal stability (thermal inertia) for sustaining fluctuating (transient) solar power inputs [200]. A 2.2 kW prototype molten salt solar gasification reactor was demonstrated in continuous process operation [199] (Fig. 19b). A solar-to-fuel thermochemical efficiency up to 30% and carbon conversion of 47% were achieved at 945 °C. However, the problems of unreacted char release and salt leakage were pointed out. In addition, entrained-flow reactors were developed for improvement of heat and mass transfer rates and continuous process operation; however, the limitation of such reactors is the excessively short residence time, and the requirement for small particle sizes (<1 mm) [201]. In view of enhancing the residence time notably affecting the reaction extent, vortex-flow and drop-tube reactors were considered; nevertheless, the feedstocks particle size is still quite restrained for such reactors. Bellouard et al. [202] proposed and tested a high-temperature solar tubular reactor (1.5 kWth) combining both drop tube and packed-bed features for continuous solar biomass gasification, according to the concept in Fig. 15. This reactor reached an energy upgrade factor of 1.21 and yielded 28% solar-to-fuel thermochemical efficiency at 1400 °C. A 3 kW indirectly-irradiated (windowless, Fig. 20a) vortex-flow solar reactor was also studied for thermochemical gasification of carbonaceous materials at high 38 pressures (1–6 bars) [203]. The reactor performance was determined and compared to a directly-irradiated (windowed, Fig. 20b) vortex-flow reactor. The feedstock calorific value was solar upgraded from 16% to 35% and the solar-to-fuel energy conversion efficiency peaked at 20%. Higher gasification extents, solar-to-fuel energy conversion efficiencies and energy upgrade factors were obtained when using the indirectly-irradiated configuration.

Figure 20. Scheme of (a) indirectly-irradiated and (b) directly-irradiated vortex flow solar reactor[203].

A 1.5 kWth prototype spouted bed solar reactor (Fig. 21) was designed, simulated and experimentally tested [80,204,205]. This reactor was utilized for the continuous solar gasification of wood biomass with different types and sizes. The cavity receiver features a directly-irradiated vertical cylinder made of high-temperature resistant metal alloy with a conical part (60° angle) at its bottom (volume: 0.299 L). It is insulated with an alumino- silicate porous insulation (28 mm thick) and placed vertically in the reactor stainless steel shell cooled by water circulation. This reactor is flexible and displays high solid and gas residence times and enhanced heat and mass transfer via particle-gas circulation [80]. The experimental assessment of continuous solar biomass gasification was performed via sixty- four on-sun runs with continuous feedstock injection [188]. A comprehensive parametric

39 study that considered different lignocellulosic biomass feedstocks (five wood biomass types), steam/biomass molar ratios (1.6-2.8), biomass feeding rates (0.6-2.7 g/min), carrier gas flow rates (2-3.3 NL/min), and reaction temperatures (1100-1300 °C) was performed for determining the syngas production capacity, energy upgrade factor, and gasifier performance.

Different wood feedstocks were continuously gasified with H2O during on-sun testing and syngas was continuously produced, thereby successfully demonstrating the reliable reactor operation with different biomass particle types, sizes, and shapes. A slight water excess with respect to stoichiometry is beneficial to biomass gasification as it increases H2 and CO and decreases CH4, CO2 and C2Hm production. A gas residence time increase through the decrease of total carrier gas flow results in improving both the syngas yield and quality. A noticeable rise of syngas yields and production rates by increasing the operating temperature is observed (activation energy in the range of 24-29 kJ/mol). The increase of both temperature and biomass feeding rate improves the syngas yields, gasification rates, and syngas quality because both biomass consumption rates and reaction kinetics are enhanced, in turn improving reactor performance [190]. However, the performance outputs are reduced, and reactants accumulation within the cavity receiver occurs when biomass feeding rate exceeds its optimal feeding point. The optimal biomass feeding rate regarding maximum syngas yield exits at 2.2 g/min at 1200 °C and 2.5 g/min at 1300 °C. The solar reactor temperature of 1300°C is recommended to operate reliably the biomass gasifier with the considered biomass particle size range (0.3-4 mm) in a continuous feeding mode enabling complete biomass conversion, as verified by the carbon consumption rate closely matching the carbon feeding rate. By optimizing biomass feeding rate consistently with operating temperature, the calorific value of the biomass feedstock is solar upgraded by 24% with carbon conversion extent above 90%, solar-to-fuel energy conversion efficiency up to 29%, and thermochemical reactor efficiency above 27%.

. Figure 21. Spouted-bed solar reactor for continuous solar biomass gasification [189].

To conclude, various types of solar reactors have been developed to fit the needs of each solar thermochemical process. Directly irradiated reactors are more efficient than indirectly irradiated ones thanks to better solar radiative absorption onto the reaction sites consisting of reacting particles. However, this comes at the expense of transparent windows requirement considered as critical and troublesome components under high-pressure environments and large-scale designs. Packed-bed reactors exhibit high extent of reaction at the expense of temperature gradients. Entrained particle-laden flow, vortex flow, spouted-bed, and fluidized bed reactors were developed to overcome the temperature gradients issue, at the expense of limitations regarding reacting particle size or residence time. These reactor concepts were applied to various solid/gas reactions encompassing the processing of different metal oxides (volatile or non-volatile in the form of either particles or macroporous structures) and the solar gasification of carbonaceous materials such as coal, coke, or biomass. Solar-driven reactors have been developed and tested at laboratory-scale mainly, and an essential future step would be to initiate demonstration of the solar process technology at pre-commercial scale to transform results obtained at lab-scale into industrial applications. Research on scaling up the reactors towards the commercial level needs to be conducted. This is very

41 important for a future scale-up and implementation of the solar decarbonization processes as part of a sustainable energy economy based on solar energy.

5. Conclusions Solar thermochemical processes offer promising pathways to convert gaseous and solid carbonaceous feedstocks to solar fuels (hydrogen and syngas) and further allow production of metals from their corresponding metal oxides. These energy-demanding processes use solar energy to drive endothermic chemical reactions, concomitantly storing intermittent solar energy into transportable and storable energy carriers. Cracking, reforming, gasification, chemical-looping, and carbothermal/methanothermal reduction represent attractive approaches for short-term solar process implementation, while two-step H2O/CO2 splitting redox cycles represent a promising option for long-term sustainable fuel production. Future research on such processes should focus on new reactor designs, modeling, testing and optimization, as well as reactor scale up. Regarding chemical-looping processes using solid oxygen carriers, the optimization of the morphological characteristics of redox materials and their integration in solar reactors should be conducted to enhance incident solar radiative flux absorption (volumetric heat transfer) and reactive gas flow through pore cell channels, in turn improving reaction kinetics and reactor efficiency. In addition, the on-sun long-term testing of materials over a large number of repeated cycles should be achieved to ensure their performance stability. Regarding the carbonaceous feedstocks, the use of biomethane, biogas, or biohythane as gaseous feedstocks instead of fossil methane (for pyrolysis/reforming) is an alternative to produce renewable fuel. The solar gasification can be applied to various types of solid biomass (lignocellulosic) or waste feedstocks (municipal solid wastes, solid recovered fuels) and represents another attractive carbon-neutral path toward green biofuels. The combination of biomass gasification with metal oxide reduction is also a promising new approach for solar-aided clean metallurgical processes. In the field of solar reactors for the above solid-gas thermochemical processes, numerical reactor modeling and simulation coupling the multiphase fluid/solid flow, heat and mass transfer, and chemical reactions are recommended for reactors design optimization and scale- up. New reactor concepts based on direct or indirect heating can be devised and developed aiming to increase residence time of reactants and solar radiative absorption. Upscaling the reactors is advised to improve the solid and gas residence time and reduce heat losses, which should enhance chemical conversion, products yields, and solar-to-fuel efficiency. Finally, 42 continuous solar/autothermal hybrid processes should be investigated to operate the process around-the-clock under intermittent or fluctuating solar power input.

Acknowledgements The authors would like to gratefully acknowledge the King Mongkut’s Institute of Technology Ladkrabang (KMITL), Thailand and PROMES-CNRS, France for all supports.

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