Thermochemical Production of Fuels with Concentrated Solar Energy

Aldo Steinfeld; Alan W. Weimer

doi:10.1364/OE.18.00A100

1. Thermodynamics of Solar Thermochemical Conversion

Solar thermochemical processes make use of concentrated solar radiation as the energy source of high-temperature process heat to drive endothermic reactions aimed at the production of chemical fuels [1]. Solar reactors for highly concentrated solar systems usually feature the use of a cavity-receiver type configuration, i.e. a well-insulated enclosure with a small opening – the aperture – to let in concentrated solar radiation. Because of multiple internal reflections, the fraction of the incoming energy absorbed by the cavity greatly exceeds the surface absorptance of the inner walls. As the ratio of the cavity’s characteristic length to the aperture diameter increases, the cavity-receiver approaches a blackbody absorber. The solar energy absorption efficiency of a solar reactor, η_absorption, is defined as the net rate at which energy is being absorbed divided by the solar radiative power coming from the solar concentrator. For a perfectly insulated cavity-receiver (no convection or conduction heat losses), it is given by [2]:

\tilde{C} = Q_{a p e r t u r e} / I A_{a p e r t u r e}

where Q_solar is the solar power coming from the solar concentrator, Q_aperture is the amount intercepted by the aperture of area A_aperture, α_eff and ε_eff are the effective absorptance and emittance of the solar cavity-receiver, respectively, T is the nominal reactor temperature, and σ is the Stefan-Boltzmann constant. The numerator denotes the difference between the power absorbed and reradiated, which should match the enthalpy change of the chemical reaction. The incoming solar power is determined by the normal beam insolation I, by the collector area, and by taking into account for the optical imperfections of the collection system (e.g., reflectivity, specularity, tracking imperfections). The capability of the solar concentrator system to concentrate solar energy is often expressed in terms of its mean flux concentration ratio

\tilde{C}

over the cavity’s aperture, normalized to the incident normal beam insolation:

\tilde{C} = Q_{a p e r t u r e} / I A_{a p e r t u r e} .

For simplification, we assume an aperture size that captures all incoming solar power so that Q_aperture = Q_solar. With this assumption, and for a perfectly insulated isothermal blackbody cavity-receiver ( $α_{e f f} = ε_{e f f} = 1$ ), Eqs. (1) and (2) are combined to yield:

η_{a b s o r p t i o n} = 1 - (σ T_{}^{4} / I \tilde{C}) .

The absorbed concentrated solar radiation drives an endothermic chemical reaction. The measure of how well solar energy was converted into chemical energy for a given process is the solar-to-fuel energy conversion efficiency, η_{solar-to-fuel}, defined as

η_{s o l a r - t o - f u e l} = \frac{- \dot{n} {Δ G |}_{298 K}}{Q_{s o l a r}}

where ΔG is the maximum possible amount of work that may be extracted from the products as they are transformed back to reactants at 298 K. The Second Law is now applied to calculate the maximum η_{solar-to-fuel} for an ideal cyclic process, limited by both the solar absorption and Carnot efficiencies,

η_{s olar-to-fuel,
ideal} = η_{a b s o r p t i o n} \cdot η_{C a r n o t} = [1 - (\frac{σ T_{H}^{4}}{I C})] \times [1 - (\frac{T_{L}}{T_{H}})]

where T_H and T_L are the upper and lower operating temperatures of the equivalent Carnot heat engine. η_{solar-to-fuel,ideal} is plotted in Fig. 1 as a function of T_H for T_L = 298 K and for various solar flux concentrations. Because of the Carnot limitation, one should try to operate thermochemical processes at the highest upper temperature possible; however, from a heat-transfer perspective, higher T_H implies higher re-radiation losses. The highest temperature an ideal solar cavity-receiver is capable of achieving, defined as the stagnation temperature T_stagnation, is calculated by setting Eq. (5) equal to zero, to yield

Fig. 1 Variation of the ideal solar-to-fuel efficiency as a function of the operating temperature T_H, for a blackbody cavity-receiver converting concentrated solar energy into chemical energy. The mean solar flux concentration is the parameter: 1000,...40000 [1].

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T_{s t a g n a t i o n} = {(I \tilde{C} / σ)}^{0.25} .

At this temperature, η_{solar-to-fuel,ideal} = 0 because energy is being re-radiated as fast as it is absorbed. Stagnation temperatures exceeding 3000 K are attainable with solar concentration ratios above 5,000. However, an energy-efficient process must run at temperatures substantially below T_stagnation. There is an optimum temperature T_optimum for maximum efficiency, obtained by setting

\partial η_{o v e r a l l, i d e a l} / \partial T = 0.

Assuming uniform power-flux distribution, Eq. (7) yields:

T_{o p t}^{5} - (0.75 T_{L}) T_{o p t}^{4} - (α_{e f f} T_{L} I \tilde{C} / 4 ε_{e f f} σ) = 0.

The locus of T_optimum is shown in Fig. 1, and varies between 1100 and 1800 K for uniform power-flux distributions with concentrations between 1000 and 13,000. For example, for $\tilde{C}$ = 5000, the maximum η_{solar-to-fuel,ideal} of 75% is achieved at T_optimum = 1500 K. For a Gaussian incident power-flux distribution having peak concentration ratios between 1000 and 12,000 suns, the optimal temperature varies from 800 to 1300 K. In practice, when considering convection and conduction losses in addition to radiation losses, the efficiency will peak at a somewhat lower temperature.

In order to illustrate the use of these equations, we consider as an example a 2-step solar thermochemical process for splitting H₂O using ZnO/Zn redox reaction, comprising: (1) the solar endothermal dissociation of ZnO(s) into its elements; and (2) the non-solar exothermal steam-hydrolysis of Zn into H₂ and ZnO(s), and represented by [1]

1st step (solar ZnO - decomposition) : Z n O \to Z n + 0.5 O_{2}

2nd step (non - solar Zn - hydrolysis) : Z n + H_{2} O \to Z n O + H_{2}

A model flow diagram for the proposed 2-step solar thermochemical cycle is shown schematically in Fig. 2 . It uses a solar reactor, a quenching device, a hydrolyser reactor, and a H₂/O₂ fuel cell. All materials are recycled. The complete process is carried out at a constant pressure of 1 bar. In practice, pressure drops will occur throughout the system and pumping work will be required. The solar reactor is assumed to be a cavity-receiver operating at 2000 K. The molar feed rate of ZnO to the reactor, $\dot{n}$ , is set to 1 mol/s, and is equal to that of H₂O fed to the hydrolyser reactor. Chemical equilibrium is assumed inside the reactor. The net power absorbed in the solar reactor should match the enthalpy change per unit time of the reaction,

Fig. 2 Schematic of an ideal cyclic process for calculating the maximum solar-to-fuel energy conversion efficiency of the 2-step water-splitting cycle using ZnO/Zn redox reactions [1].

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Q_{r e a c t o r, n e t} = {\dot{n} Δ H |}_{Z n O (s) @ 298 K \to Zn(g) + 0.5 O_{2} @ 2000 K} = 557 kJ/mol

For $\tilde{C}$ = 5000, η_absorption = 82% and Q_solar = 680 kJ/mol. Products Zn(g) and O₂ exit the solar reactor at 2000 K and are cooled rapidly to 298 K. It is assumed that the chemical composition of the products remains unchanged upon cooling in the quencher. Since the quench step is required for avoiding recombination of products, no heat exchanger is used for recovering their sensible and latent heat. Thus, the amount of power lost during quenching is:

Q_{q u e n c h} = {- \dot{n} Δ H |}_{Zn(g) + 0.5 O_{2} @ 2000 K \to Zn(s) + 0.5 O_{2} @ 298 K} . = - 20 9 kJ / mol

After quenching, the products separate naturally (without expending work) into gaseous O₂ and condensed phase zinc. Zinc is sent to the hydrolyser reactor to react exothermally with water and form hydrogen, according to reaction (10). The heat liberated is assumed lost to the surroundings, as given by:

Q_{h y d r o l y s e r} = {- \dot{n} Δ H |}_{Z n + H_{2} O @ 298 K \to Z n O + H_{2} @ 298 K} . = - 62 kJ / mol

The cycle is closed by introducing an ideal H₂/O₂ fuel cell, in which the products recombine to form H₂O and thereby generate electrical power. W_F.C. and Q_F.C. are the work output and heat rejected, respectively given by:

W_{F . C .} = - {\dot{n} Δ G |}_{H_{2} + 0.5 O_{2} @ 298 K \to H_{2} O @ 298 K} = 237 kJ / mol

Q_{F . C .} = - T_{L} {\times \dot{n} Δ S |}_{H_{2} + 0.5 O_{2} @ 298 K \to H_{2} O @ 298 K} = - 49 kJ / mol

Finally, assuming no heat recovery during quenching and hydrolysis,

η_{solar-to-fuel} = \frac{W_{F . C .}}{Q_{s o l a r}} = 35 %

The major sources of irreversibility are associated with the re-radiation losses from the solar reactor and the heat lost during quenching and hydrolysis. To some extent, the sensible heat of the hot products exiting the reactor may be recovered to pre-heat the reactants, increasing the efficiency up to 50%. η_{solar-to-fuel} can be further increased with higher $\tilde{C}$ , e.g. by incorporating a CPC at the aperture, which results in a smaller aperture to intercept the same amount of solar power, and, consequently, lower re-radiation losses. Note that, for a given $\tilde{C}$ , smaller apertures intercept a reduced fraction of the incoming solar power. Thus, the optimum aperture size of the solar cavity-receiver becomes a compromise between maximizing solar radiation capture and minimizing re-radiation losses [3]. Re-radiation losses can also be diminished by implementing selective windows with high transmissivity in the solar spectrum around 0.5 μm where the solar irradiation peaks, and high reflectivity in the infrared range around 1.45 μm where the Plank’s spectral emissive power for a 2000 K blackbody peaks.

This kind of process modeling establishes a base for evaluating and comparing different solar thermochemical processes for ideal, closed cyclic systems that recycle all materials. For open materials cycles, in which fuels are the reactants being solar-upgraded (see next section: cracking, reforming, gasification), the solar-to-fuel energy conversion efficiency is calculated as:

η_{solar-to-fuel} = \frac{W_{F . C .}}{Q_{s o l a r} + H H V_{reactants}}

where HHV_reactant is the high heating value of the fuel being processed, e.g. about 890 kJ/mol for natural gas and 35700 kJ/kg for anthracite coal. The higher η_{solar-to-fuel}, the lower is the required solar collection area for producing a given amount of solar fuel and, consequently, the lower are the costs incurred for the solar concentrating system, which usually correspond to half of the total investments for the entire solar chemical plant. Thus, high η_{solar-to-fuel} implies favorable competitiveness.

2. Solar Thermochemical Processes and Reactors

Five thermochemical routes for solar fuels production are depicted in Fig. 3 . Indicated is the chemical feedstock: H₂O and/or carbonaceous feedstock (e.g. natural gas, oil, coal, biomass). All of these routes are highly endothermic processes that proceed at high temperatures and make use of concentrated solar radiation as the energy source of process heat [3].

Fig. 3 Five thermochemical routes for solar fuels production using concentrated solar radiation as the energy source of high-temperature process heat [4].

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The single-step thermal dissociation of water, known as water thermolysis, although conceptually simple, has been impeded by the need of a high-temperature heat source at above 2500 K for achieving a reasonable degree of dissociation, and by the need of an effective technique for separating H₂ and O₂ to avoid recombination or ending up with an explosive mixture. Among the ideas proposed for separating H₂ from the products are effusion and electrolytic separation [2].

Water-splitting thermochemical cycles bypass the H₂/O₂ separation problem and further allow operating at relatively moderate upper temperatures. Previous studies performed on H₂O-splitting thermochemical cycles were mostly characterized by the use of process heat at temperatures below about 1200 K, available from nuclear and other thermal sources. These cycles required multiple steps (>2) and were suffering from inherent inefficiencies associated with heat transfer and product separation at each step. In recent years, significant progress has been accomplished in the development of optical systems for large-scale collection and concentration of solar energy capable of achieving solar concentration ratios of 5,000 suns and higher. Such high solar radiation fluxes allow the conversion of solar energy to thermal reservoirs at 1500 K and above which are needed for the more efficient two-step thermochemical cycles using metal oxide redox reactions [4–17]. The cycle is depicted in Fig. 4 .

Fig. 4 Scheme of a 2-step solar thermochemical cycle based on metal oxide redox reactions. Here, M_xO_y denotes a metal oxide, and M the corresponding metal or lower-valence metal oxide. In the first, endothermic, solar step, M_xO_y is thermally dissociated into the metal or lower-valence metal oxide M and oxygen. Concentrated solar radiation is the energy source for the required high-temperature process heat. In the second, exothermic, non-solar step, M reacts with water to produce hydrogen. The resulting metal oxide is then recycled back to the first step.

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The first, endothermic step is the solar thermal dissociation of the metal oxide to the metal or the lower-valence metal oxide. The second, non-solar, exothermic step is the hydrolysis of the metal to form H₂ and the corresponding metal oxide. The net reaction is H₂O = H₂ + 0.5O₂, but since H₂ and O₂ are formed in different steps, the need for high-temperature gas separation is thereby eliminated. The second hydrolysis step can be accomplished on demand at the H₂ consumer site, as it is decoupled from the availability of solar energy. Alternatively, CO₂ can be co-fed with H₂O to react with the metal and produce syngas, which can be further processed to liquid fuels [15–17]. One promising redox system is ZnO/Zn (see Eqs. (9-10). Figure 5 shows a solar chemical reactor configuration for performing the thermal dissociation of ZnO that consists of a windowed rotating cavity-receiver lined with ZnO particles. With this arrangement, ZnO is directly exposed to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant [18,19].

Fig. 5 Scheme of the solar reactor configuration for the thermal dissociation of ZnO, as part of a 2-step water-splitting thermochemical cycle based on ZnO/Zn redox reactions. It consists of a windowed rotating cavity-receiver lined with ZnO particles. With this arrangement, ZnO is directly exposed to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant [18].

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Spinel ferrites of the form M_xFe_3-xO₄, where M generally represents Ni, Zn, Co, Mn, or other transition metals, have been shown to be capable of splitting water according to the two-step cycle shown below:

M_{x} F e_{3 - x} O_{4} + s o l a r t h e r m a l e n e r g y \to x M O + (3 - x) F e O + 0.5 O_{2}

x M O + (3 - x) F e O + H_{2} O \to M_{x} F e_{3 - x} O_{4} + 2 H_{2}

It has recently been shown that the reduction reaction (18) is thermodynamically limited to about 60% conversion and that metal oxide solutions play a critical role in the overall process [16]. Hence, for a nickel based ferrite, the best that one can hope for is:

N i F e_{2} O_{4} \to (N i O) (1.2 F e O) (0.4 F e_{2} O_{3}) + 0.3 O_{2}

where ΔH _1673K = 214.6 kJ/mol, followed by:

(N i O) (1.2 F e O) (0.4 F e_{2} O_{3}) + 0.6 H_{2} O \to N i F e_{2} O_{4} + 0.6 H_{2}

If one achieves 70% conversion for reduction reaction (20) and 100% conversion for oxidation reaction (21), the total solar energy required for producing 100,000 kg H₂/day annual average is 2821 GWhr/yr, 1799 GWhr/yr to drive the reaction and 1022 GWhr/yr for sensible heat [20]. The process is assumed to cycle between 1673 K for reduction and 1273 K for oxidation. The solid products of the reduction reaction are stored adiabatically at 1273K and then oxidized with steam to generate H₂ at 300 psig. The 100,000 kg H₂/yr can be achieved with five 265 m tall towers utilizing 2.46 x 10⁶ m² of heliostats. Each of the five receivers provides for 295 Mw_thermal energy to drive the reduction reaction. System design uses a secondary concentrator providing 3868 suns concentration. Overall solar efficiency for a plant in Daggett, CA is 0.414 and overall efficiency for the process to produce H₂ (no work) is 0.178 [20].

The solar cracking route refers to the thermal decomposition of natural gas (NG), oil, and other hydrocarbons, and can be represented by the simplified net reaction:

C_{x} H_{y} = x C (g r) + \frac{y}{2} H_{2}

Other compounds may also be formed, depending on the presence of impurities in the raw materials. The thermal decomposition yields a carbon-rich condensed phase and a hydrogen-rich gas phase. The carbonaceous solid product can either be sequestered without CO₂ release or used as material commodity under less severe CO₂ restraints. It can also be applied as reducing agent in metallurgical processes. The hydrogen-rich gas mixture can be further processed to high-purity hydrogen that is not contaminated with oxides of carbon and, thus, can be used in PEM fuel cells without inhibiting platinum-made electrodes. From the point of view of carbon sequestration, it is easier to separate, handle, transport, and store solid carbon than gaseous CO₂. Assuming carbon sequestration, Eq. (17) yields η_{solar-to-fuel} = 0.55 [21]. Figure 6 shows a scheme of a solar reactor that features a continuous flow of CH₄ laden with μm-sized carbon black particles, confined to a cavity receiver and directly exposed to concentrated solar irradiation [22]. The carbon particles fed serve the functions of radiant absorbers and nucleation sites for the heterogeneous reaction.

Fig. 6 Scheme of the solar chemical reactor for the co-production C and H₂ by thermal decomposition of CH₄. It consist of a continuous flow of CH₄ laden with μm-sized carbon black particles, confined to a cavity receiver and directly exposed to concentrated solar irradiation. The carbon particles fed serve the functions of radiant absorbers and nucleation sites for the heterogeneous reaction [22].

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The steam-reforming of hydrocarbons (e.g. NG, oil, and others), and the steam-gasification of carbonaceous materials (e.g. coal, coke, biomass, and other C-containing solids) can be represented by the simplified net reaction:

C_{x} H_{y} O_{z} + (x - z) H_{2} O = (\frac{y}{2} + x - z) \cdot H_{2} + x C O

Other compounds may also be formed (e.g. H₂S), depending on the reaction rate and on the impurities contained in the raw materials. The principal product is high-quality synthesis gas (syngas), the building block for a wide variety of synthetic fuels including Fischer-Tropsch type chemicals (liquid fuels), hydrogen, ammonia, and methanol. The syngas quality is determined mainly by the H₂:CO and CO₂:CO molar ratios. For example, the solar steam gasification of anthracite coal at above 1500 K yields syngas with a H₂:CO molar ratio of 1.2 and a CO₂:CO molar ratio of 0.01.

Some of these processes are practiced at an industrial scale, with the process heat supplied by burning a significant portion of the feedstock. Alternatively, using solar energy for process heat offers a three-fold advantage:

• higher energetic value of the syngas produced; i.e. higher syngas output per unit of feedstock, as the calorific value of the feedstock is upgraded by the solar energy input in an amount equal to the enthalpy change of the reaction.
• higher quality of the syngas produced, especially in terms of the low CO₂/CO molar ratio, as no syngas contamination by combustion byproducts occurs;
• elimination of the air-separation system needed for supplying a pure oxygen stream in the autothermal gasification.

For the solar steam-reforming of natural gas and the steam-gasification of coal followed by syngas processing to H₂ (assuming water-gas shift and H₂/CO₂ separation unit based on the pressure swing adsorption technique at 90% recovery rate and power consumption of 9.17 kJ/mol H₂), Eq. (17) yields η_{solar-to-fuel} = 0.71 [21]. A solar reactor for the solar reforming of NG that uses a reticulate porous ceramic foam coated with Rh-based catalyst has been scaled-up to power levels of 300 kW in a solar tower facility [23]. The solar steam-gasification of petroleum coke and coal were studied in fluidized-bed, vortex-flow, and aerosol-flow solar reactors [24–29]. Thermal irradiation of these particle suspensions was found to be an effective means of heat transfer directly to the reaction site, leading to extremely fast heating rates (~1000 K/s) and enhanced kinetics. A packed-bed solar reactor that can accommodate a wide range of carbonaceous feedstock is shown in Fig. 7 . It consists of two cavities separated by a radiant emitter plate, with the upper one serving as the solar absorber and the lower one containing the reacting packed bed that shrinks as the reaction progresses [30,31]. If biomass is used as feedstock, the syngas produced is CO₂-neutral. A 1 MW_thermal solar gasification pilot plant based on solar tower technology has recently been commissioned to convert wood waste and other forms of biomass into syngas that can be further processed into gasoline [32].

Fig. 7 Packed-bed solar reactor configuration for the steam-gasification of carbonaceous materials. It consists of two cavities separated by a radiant emitter plate, with the upper one serving as the solar absorber and the lower one containing the reacting packed bed that shrinks as the reaction progresses [30,31].

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3. Conclusions

Solar thermochemical processes offer an efficient path for storage and transportation of solar energy. The products are renewable chemical fuels for delivering clean and sustainable energy services. There is a pressing need to develop greenhouse gas mitigation options that can be applied in the mid-term. Solar cracking, reforming, and gasification processes for upgrading and converting fossil fuels to syngas conserve fossil fuels, reduce emissions, and could become an important transition path towards solar liquid fuels. The production of solar hydrogen via 2-step thermochemical cycles using metal oxides redox reactions has favorable long-term potential, warranting further development and large-scale demonstration.

Nomenclature

A_aperture area of aperture

C solar flux concentration ratio

I normal beam insolation

$\dot{n}$ molar flow rate

HHV high heating value

Q_aperture incoming solar energy intercepted by the aperture

Q_FC heat rejected to the surroundings by an ideal fuel cell

Q_quench heat rejected to the surroundings by the quenching process

Q_reactor,net net energy absorbed by the solar reactor

Q_solar total solar energy coming from the solar concentrator

T nominal solar reactor temperature

T_stagnation maximum temperature of a blackbody absorber

T_optimum optimal temperature of the solar reactor for maximum η_{solar-to-fuel}

W_FC work output by an ideal fuel

α_eff effective absorptance of the solar cavity-receiver

ε_eff effective emittance of the solar cavity-receiver

ΔG Gibbs free energy change

ΔH enthalpy change

η_absorption solar energy absorption efficiency

η_Carnot efficiency of a Carnot heat engine operating between T_H and T_L

η_{solar-to-fuel} solar-to-fuel energy conversion efficiency

σ Stefan-Boltzmann constant (5.6705 × 10⁻⁸ Wm⁻²K⁻⁴)

References and links

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Thermochemical Production of Fuels with Concentrated Solar Energy

Abstract

1. Thermodynamics of Solar Thermochemical Conversion

2. Solar Thermochemical Processes and Reactors

3. Conclusions

Nomenclature

References and links

Cited By

Figures (7)

Equations (23)

Optics Express