Hydrosol: Thermochemical Water Splitting

Very high temperatures are required to dissociate water into hydrogen and oxygen. Given the thermodynamic restrictions, sufficient yields from the direct thermal splitting of water can only be achieved at tem­peratures above 2500 K. Temperatures this high impose extraordinary demands on materials and reactor design. Over the past 30 years numerous thermochemical cycles for hydrogen production through water splitting have been proposed and studied to a varying extent. Several cycles have been demonstrated at the laboratory scale, a couple have reached the pilot scale, but none has yet matured to production.

An interesting concept is that of oxide-based thermochemical cycles, during which a simple oxide (such as iron, zinc or cerium oxide) or a mixed oxide (such as a ferrite) cycles between a lower and a higher valence state, participating in an oxidation-reduction process that produces H2 and O2 in separate steps.13 The concept has been proven experimentally for pairs of oxides of multivalent metals or metal-metal oxide systems (for example the ZnO-Zn system studied by Weimer, Steinfeld and co-workers, Figure 3.9).14

However, even though water splitting is taking place at temperatures lower than 700 °C, material regeneration (i. e. reduction) takes place at much higher temperatures (>1600 °C). In addition, despite basic research into active redox pairs, the solar reactors reported in the lit­erature are based on particles fed into rotating cavity reactors, which are complicated and costly to operate. Hence, on the larger scale required to make solar-based H2O splitting a practical technology in terms of quantity and cost using only the energy of the sun, an efficient and robust redox material is required to make the process operate at feasible temperatures.

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Figure 3.9 The ZnO-Zn thermochemical cycle. Primary issues revolve around both i) material development for operation day in and day out at 1800 °C in the presence of air and for rapid heating/cooling (i. e. thermal shock resis­tance) and ii) the development of methods to recover heat from the solar reactor while at the same time preventing recombination.

(Reproduced from Wikipedia. org, with kind permission.)

Using a CSP plant it is easy to achieve temperatures in excess of 1200 °C through proper sizing and control of the heliostat field (a field of sun-tracking mirrors), hence a solar tower facility is the natural choice to host a solar thermochemical reactor.

Funded by the EU and coordinated by the Greek Aerosol and Particle Technology Laboratory of CPERI/CERTH (with partners the German Aerospace Center, DLR, the British company Johnson Matthey, the Spanish research center CIEMAT, and the Danish company Stobbe Tech), in 2008 the Hydrosol-II project established a 100 kW pilot plant at the Plataforma Solar de Almeria in Spain (Figure 3.10).15

The thermochemical process – which involves an endothermic reac­tion that requires a significant energy input – employs a multichannel ceramic honeycomb reactor resembling that of the familiar catalytic converter of automobiles.

The reactor displayed in Figure 3.11 is coated with active mixed iron oxides (the redox material) with a high activity in the water splitting reaction. The reactor is thus heated by concentrated solar radiation using a set of heliostats that concentrate the solar energy into a certain area and lead to an increase in the reactor temperature (Figure 3.12).

Inaugurated on 31 March 2008, this solar reactor for the continuous production of solar hydrogen was the first ever closed solar-only,

Chapter 3

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Figure 3.10 In March 2008, a 100 kW reactor for hydrogen production through water splitting using solar energy was put into commission at the Plataforma Solar in Almeria as part of the Hydrosol project. The reactor is located inside the tower on the right.

(Reproduced from hydrosol-project. org, with kind permission.)

thermochemical reactor for hydrogen production. Recently, the same research team has presented the results from the operation of the plant for 40 consecutive cycles of constant and continuous H2 production in a two-day period.16

Significant concentrations of hydrogen were produced, with a con­version of steam of up to 30%. Operation has demonstrated that the combination of CSP facilities with high temperature processes will be a viable way to produce hydrogen at a reasonable cost without any greenhouse gas emissions, paving the way for a purely renewable solar hydrogen economy. Further scale-up of the technology and its effective coupling with CSP are in progress to demonstrate the large-scale feasibility of a solar hydrogen production plant.17

The design of this 100 kW pilot plant is based on a modular concept, and its scaling up to the megawatt range could follow both the tradi­tional tower CSP approach (taller tower/larger heliostat field) or a parallel deployment of multiple units.

The scheme in Figure 3.12 shows that in the first step of water­splitting the activated redox reagent (usually the reduced state of a metal oxide) is oxidized by taking oxygen from water and producing hydrogen, according to the reaction in Equation (3.3).18

Подпись:MOx_1 + H2O(g) ! MO0x + H2

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During the second step the oxidized state of the reagent is reduced, to be used again (regeneration), delivering some of the oxygen of its lattice according to Equation (3.4):

MOox! MOx_4 + 1O2 (3.4, endothermic)

The advantage is the production of pure hydrogen and the removal of oxygen in separate steps, avoiding the need for high-temperature separation and the chance of formation of explosive mixtures. The active redox material is capable of water-splitting and regeneration, so that the complete operation (water-splitting and redox material regen­eration) is achieved in a closed solar reactor.

In brief, the uniqueness of the Hydrosol approach is based on the combination of two novel concepts: nanoparticle materials with very high water-splitting activity and regenerative ability (synthesized by novel routes such as aerosol processes, combustion techniques and reactions under controlled oxygen pressure) and their incorporation as coatings on special refractory ceramic monolithic reactors whose geo­metry first emerged from traditional chemical engineering, with its most familiar application in automobile catalytic converters.19 Coated monolithic reactors are therefore one of the two enabling technologies for renewable solar hydrogen production.

The solar thermochemical reactor for the production of hydrogen from water-splitting is constructed from special refractory ceramic thin – walled, multi-channeled (honeycomb) monoliths (Figure 3.13) that absorb solar radiation.

The reactor contains no moving parts, and converts the solar radia­tion into hydrogen very efficiently.20 When steam passes through the

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Figure 3.13 The monolith channels are coated with active water-splitting materials capable of splitting the steam passing through the reactor by ‘trapping’ its oxygen and leaving, in the effluent gas stream, pure hydrogen as the product.

(Reproduced from hydrosol-project. org, with kind permission.)

reactor, the coating material splits the water molecules by adsorbing and incorporating oxygen to form a higher valence state oxide. The effluent gas stream then consists of pure H2.

The temperature in the reactor is then increased, for example by focusing more mirrors onto the aperture of this reactor. The feed gas stream is cut off, the trapped oxygen is released and the active coating is regenerated. Two reaction chambers (designated as Eastern and Western modules) are operating in parallel, one for water splitting and one for regeneration.

Accurate temperature control is necessary in particular for the high temperature reaction, the regeneration, on the one hand to avoid overheating and on the other hand to ensure sufficient reaction rates.

Figure 3.14 demonstrates the effect of varying the number of different heliostats focused on the two apertures and the feasibility of the described control concepts when using only the number of heliostats for tempera­ture control. For both temperatures, 800 °C and 1200 °C, the sufficient control can be applied using the heliostats to ensure steady states.

Chapter 3

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Figure 3.14 Effect of a varying number of heliostats on temperature in the Hydrosol reactor.

(Reproduced from Ref. 16, with kind permission.)

One of the prototype redox materials for this kind of reaction is Fe3O4. The redox pair in this case is FeO-Fe3O4. In practice, the pure oxide cannot be cycled because the temperature needed to reduce Fe3O4 thermally is higher than its melting point. However, replacing some of the iron in Fe3O4 with other metals, such as zinc, manganese, nickel or cobalt, can lower the reduction temperature while maintaining the spinel structure of these ferrite materials. Integrating the ferrites into a stabi­lizing matrix, such as yttrium-stabilized zirconia or cerium oxide (ceria), finally, slows down sintering and deactivation of the metal oxide.21

In the Hydrosol 2 pilot plant, the composition of the off-gas stream was detected by a gas chromatograph (GC). A plot of hydrogen con­centrations in the product stream of the plant is displayed in Figure 3.15. The first broad peak at t = 5000 s is attributed to the splitting of residual water in the apparatus.

Apparently the highest output of hydrogen was produced during the first cycle at about t = 7000 s. The measured concentration corresponds to a conversion of 30% of the steam fed in. After that, a reduction of hydrogen concentration, and therefore of the yield, by a factor of about two was observed.

This effect is similar to what has been observed earlier in smaller reactors in the laboratory and in the solar furnace, and is attributed mainly to deactivation of the particular redox system (and to a minor extent to inhomogeneous temperature distribution of the absorber).

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Figure 3.15 Concentration of hydrogen in product stream in the Hydrosol 2 pilot plant reactor.

(Reproduced from Ref. 16, with kind permission.)

There is evidence that some of the zinc in the particular ferrite for­mulation volatilizes during cyclic operation, resulting in a reduction of the activity of the redox material from its initial value. The strongly diminished hydrogen production indicated by the last peak was caused by the occurrence of a leakage and therefore by air infiltration into the reactor. Experiments with new, more robust ferrite compositions are expected to start in the spring of 2012.

Konstandopoulos and colleagues are now working to scale up their technology and build a 1 MW hydrogen-producing plant, in a project known as Hydrosol 3D, which involves the pre-design and design of the whole plant, including the solar hydrogen reactor and all necessary upstream and downstream units needed to feed in the reactants and separate and bottle the products.

The Hydrosol-3D consortium consists of the Aerosol & Particle Technology Laboratory of CPERI/CERTH, Germany’s DLR, Spain’s CIEMAT, and finally the French company Total and the Belgian company Hygear.22 Two alternative options are currently being ana­lyzed: adapting the hydrogen production plant to an already available solar facility or developing a new, completely optimized hydrogen production/solar plant.

These and related developments and large scale demonstration are now urgently needed because, as stated by Steinfeld,7 the weaknesses in the economic evaluations of thermochemical solar hydrogen production are related primarily to the uncertainties in the viable efficiencies and investment costs of the various components due to their early stage of development and their economy of scale.

Updated: August 17, 2015 — 6:39 am