Carbon Neutral Solar Fuels

The hydrogen economy is a typical ‘‘chicken and egg problem’’ (Figure 3.16). Until a hydrogen infrastructure is built, hydrogen production will not reward investment. Hence, (solar) hydrogen production plants, as well as vehicles, are not yet being manufactured on the large scale required by the urgency of the anthropogenic climate change problem caused by three centuries of burning fossil fuels to power the increasing energy needs of humankind.

Hydrocarbon compounds, however, are very attractive energy car­riers. Reducing our dependence on fossil hydrocarbon fuels as our primary energy source should not therefore prevent us from using (carbon neutral) hydrocarbons as energy carriers. Chemicals offer the advantages of being transportable as well as being able to be stored for extended periods of time. This point is important because energy demand is rarely synchronous with or geographically matched to inci­dent solar radiation.

In general, solar thermochemical processes for the production of synthetic fuels using concentrated solar radiation are thermodynamically favored because they inherently operate at high temperatures and utilize the entire solar spectrum. Recently, therefore, Konstandopoulos and his colleagues have modified the Hydrosol technology successfully and have managed, with a similar reactor technology, to produce carbon monoxide by splitting of carbon dioxide (CO2).23

Indeed, when CO2 is passed through the Hydrosol reactor, the coating material splits the molecules by adsorbing and incorporating oxygen to form a higher oxide. The effluent gas stream then consists of pure CO. The temperature in the reactor is increased subsequently by focusing more

Which one comes first?

infrastructure

Figure 3.16 Which will come first: Hydrogen infrastructure or hydrogen production?

mirrors onto the aperture of the reactor and the feed gas stream is cut off, which releases the trapped oxygen and regenerates the active coating.

If the operation of the CO2 splitting reactor is ‘‘combined’’ with the operation of the Hydrosol reactor, the carbon monoxide and hydrogen produced simultaneously will react to give synthetic fuel, produced either by the well-known Sabatier or by the Fischer-Tropsch process to convert H2 and CO into liquid hydrocarbons for the transportation sector, or into polymers.

In the Sabatier process the two gases (CO and H2, known as ‘‘synthesis gas’’) are heated at high pressure in the presence of a nickel catalyst to produce methane or methanol; in the Fischer-Tropsch pro­cess an iron-based catalyst is used to generate liquid hydrocarbon fuels. In addition to solar fuels, the solar ‘‘synthesis gas’’ can be effectively employed to synthesize a wide variety of hydrocarbon polymers (‘‘solar plastics’’), contributing further to a sustainable future (Figure 3.17).

These processes offer a very good alternative for dealing with the problem of carbon storage. The CO2 captured from power plants could constitute an ideal raw material for the production of synthetic fuels, rather than being buried in underground storage sinks. Such a devel­opment permits the continued use of the existing hydrocarbon fuel infrastructure to distribute carbon neutral solar fuels (hydrogen and hydrocarbons). In this way a viable and sustainable solution to pro­blems of both hydrogen storage and carbon storage can be provided.

A vision of a biomimetic carbon neutral fuel grid for Europe is given in Figure 3.18, where CO2 from the north is conveyed by pipelines

H. O

Solar Synthesis Gas co

Нг + CO^CjH, ! Liquid Fuels/Fist he r’Tropsth process) 4Нг + C02 —+ 2H20 / Gas fuels, methane/Saha tier process)

Нг + CO-»-CKHJ Plastics)

Figure 3.17 Coupling solar hydrogen generation with CO2 splitting could produce green energy and polymers from sunlight, H2O and CO2, similar to what Nature does to synthesize organic matter.

image126Chapter 3

CO2 producing Plants @ North

VEINS:

CO2 pipelines from Carbon Capture

ARTERIES: Carbon Neutral Solar Fuels Pipelines

HYDROSOL Plants @ South

Figure 3.18 A biomimetic system for sustainable energy as conceived by Hydrosol project researchers.

(‘‘veins’’) to the sunny south, where, through Hydrosol plants, it is converted into solar hydrocarbons and these are distributed to the areas of demand by solar fuel pipelines (‘‘arteries’’).24

A similar approach is being pursued by Aldo Steinfeld and his team at the Swiss Federal Institute of Technology, ETH Zurich. A research team consisting of ETH, the Paul Scherrer Institute (PSI), and the California Institute of Technology has recently developed a laboratory reactor for the dissociation of CO2 and H2O.25 The reactor consists of a thermally insulated cavity, in which a porous monolithic cylinder containing the ceria (CeO2) catalytic redox material is enclosed. The selected dimensions of the reactor ensure multiple internal reflections and efficient capture of incoming solar energy so that the apparent absorptivity, exceeding 0.94, approaches the ideal blackbody limit.

Concentrated solar radiation enters the reactor (Figure 3.19), is intensified by a compound parabolic concentrator, and is focused on a cerium oxide cylinder. The H2O and CO2 enter side inlets, and O2, H2, and CO exit via an outlet at the bottom.

The reactor’s solar concentrator, which is basically a set of giant curved mirrors that gather sunlight from a wide area, is the most difficult part to build. The redox pair CeO2-Ce2O3 is the other main redox system for thermochemical water splitting. Ceria has the advantage that

А со*

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Concentrated Solar Radiation

the melting point is higher than the temperature required for the thermal reduction step. Moreover, ceria is a highly attractive choice for two- step thermochemical cycling because it displays rapid fuel production kinetics and high selectivity, owing to the absence of distinct oxidized and reduced phases.

The two-step H2O/CO2 splitting solar thermochemical cycle based on oxygen-deficient ceria is represented by Equations (3.5 to 3.7):

High-T reduction: CeO2 CeO2_8 + 8/2O2 (3.5)

Low-T oxidation with H2O: CeO2_8 + 8H2O! CeO2 + 8H2 (3.6)

High-T oxidation with CO2: CeO2_8 + 8CO2 ! CeO2 + 8CO (3.7)

In the first high-temperature step, the ceria is thermally reduced to a non-stoichiometric state (T>1673K) and oxygen is released

(Equation 3.5). In the lower temperature steps, ceria is re-oxidized with H2O and/or CO2 to produce H2 and/or CO. In detail, non­stoichiometric CeO2 takes up oxygen from carbon dioxide or from water and produces either CO (Equation 3.6) or H2 (Equation 3.7), at tem­perature ranges of 700 and 500 °C, respectively. With further increase of the temperature (at approximately 1500 °C), ceria is thermally reduced again and the captured oxygen is released, closing the cycle.

In the experiment of Steinfeld and co-workers, using a solar cavity- receiver containing porous monolithic ceria, which aimed (in separate experimental solar runs) to produce H2 from H2O and CO from CO2, solar rays are concentrated to a strength of 1500 suns and directed into a reactor. The result is solar-thermochemical fuel production from the cycling process shown in Figure 3.20.

Although the behavior is generally reproducible between cycles, some run-to-run variations are evident. A much faster rate of fuel production than that of O2 release is clearly observed. Oxygen evolution reaches a peak value between 17 and 34 mL min-1, whereas the total amount evolved ranges from 0.54 to 0.94 L for 325 g of ceria, which is correlated with the peak reactor temperature obtained.

Beyond efficiency, material stability is an essential criterion for a viable thermochemical process. With use of the differential reactor system, which enables rapid access to multiple cycles, 500 cycles of water dissociation were performed without interruption.

image128
The results indicate that, after an initial stabilization period of about 100 cycles, both the oxygen and hydrogen evolution rates remained essentially constant for a subsequent 400 cycles (Figure 3.21).

image129

Figure 3.21 The O2 (black) and H2 (red) evolution rates for 500 water-splitting cycles. CeO2 was cycled between 1500 °C and 800 °C.

(Reproduced from Ref. 25, with kind permission.)

Scanning electron microscopy examination of samples of porous ceria that underwent heat treatment under similar conditions revealed that the decrease in reaction rate was accompanied by an increase in particle size. The morphology stabilized after 24 hours of heat treatment at 1500 °C, much as the fuel production rate stabilized after an initial period.

The originally reported solar-to-fuel efficiencies of 0.7 to 0.8% are largely limited by the system scale and design (Figure 3.22) rather than by chemistry. For comparison only, the solar-to-fuel energy conversion efficiency obtained in this work for CO2 dissociation is about two orders of magnitude greater than that observed with state-of-the-art (in 2010) photocatalytic approaches.27

A thermodynamic analysis28 indicates that efficiencies of 16% or more are achievable with the new reactor. Hence, the team optimized the solar reactor prototypes (at the 10 kW power level) for maximum solar-to-fuel energy conversion efficiency, and is currently scaling-up the system for industrial applications (at the MW power level) using concentrating solar tower technology.

Most recently, Steinfeld has demonstrated that the same geometrical cavity-type configuration reactor, this time packed with porous ceria felt, can co-produce H2 and CO (syngas) by simultaneously splitting a

Chapter 3

image130

Figure 3.22 The reaction chamber in which sunlight becomes chemical energy. This picture shows the reaction chamber of the new solar collector illuminated by light coming from a solar simulator. A quartz window at the top allows both infrared and ultraviolet radiation to enter the chamber in which the cerium oxide is deposited.

(Reproduced from Ref. 26, with kind permission.)

mixture of H2O and CO2.29 In detail, ten consecutive H2O/CO2 gas splitting cycles have been performed over eight hours with a 3 kW solar cavity receiver-reactor containing porous ceria felt exposed directly to high-flux (> 2800 suns) thermal radiation.

A constant and stable syngas composition (Figure 3.23), showing stable fuel production, was observed, which demonstrates the feasibility of using ceria-based redox cycles to produce repetitive and controlled amounts of syngas in a solar reactor that closely replicates the condi­tions expected in practical solar fuel applications.

Indeed, the solar reactor design is simple and robust, affording clear benefits – simplicity, robustness, stability, and use of earth-abundant elements – that render this thermochemical approach feasible for large – scale implementation. The material stability over 500 thermochemical cycles observed with monolithic ceria in the separate generation of hydrogen and carbon monoxide is already suitable for realistic appli­cations. Furthermore, the abundance of cerium, which is comparable to

image131

105

Figure 3.23 Temperature of the ceria felt, gas production rates, total amount of evolved gases, and H2: CO molar ratios during ten consecutive splitting cycles.

(Reproduced from Ref. 29, with kind permission.)

that of copper,30 is such that the approach is applicable at scales relevant to global energy consumption.31

The decision was taken by ETH not to patent the new discovery to enable further research.32 Research is now focused on finding proper dopants for ceria-based materials. The already investigated dopants (e. g. Gd and Sm) affect the thermodynamics of the reduction of ceria and as a result are expected to increase its reduction degree at lower tempera­tures. In that way the overall lifetime of the materials and the reactor will be extended.

Carbon dioxide can of course be accumulated from the atmosphere. Steinfeld and co-workers have developed another reactor made of a transparent tube filled with pellets of CaO (calcium oxide). As the light heats the tube and brings its contents to 400 °C, air mixed with a small amount of steam is pumped in at the bottom and up through the pellets. At this temperature, CaO reacts with CO2 to form calcium carbonate (CaCO3) and in less than 15 minutes removes all carbon dioxide in the air, decreasing it from 385 parts per million to practically zero.33 Sub­sequently, the intake valve is closed and the temperature in the reactor is raised to 800 °C by intensifying the light, which causes the CaCO3 to release the CO2 as a stream of pure gas and converts the calcium car­bonate back into calcium oxide. The reactor was taken through five cycles of absorption and release with no decline in performance.

Steinfeld and his team have thus developed a system that uses atmospheric CO2 to feed the solar fuel process (Figure 3.24). A parabolic

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image142which drives the CO, horn the pellets

Figure 3.24 The O2 (black) and H2 (red) evolution rates for 500 water-splitting cycles.

CeO2 was cycled between 1500 °C (pO2 = 10—5 atm, flow rate = 3.2 Lmin—1g—1 of ceria, 10min, ramp rate = 100°C min—1) and 800°C (pH2O = 0.13-0.15atm, flow rate = 0.75-0.76Lmin—1g—1 of ceria, 10 min).

(Reproduced from http://nextbigfuture. com/2009/01/co2-capture-from- air-for-fuel-or. html, with kind permission.)

mirror concentrates solar light onto a chamber containing calcium oxide (CaO). When calcium carbonate is heated to 800 °C it releases a pure stream of CO2 that is fed into a second reactor, in which a solar concentrator heats zinc oxide to 1700 °C, causing it to release oxygen molecules, leaving metallic zinc. The temperature is then lowered and CO2 and steam are pumped in, which react with the pure Zn to form syngas.

Finally, a team led by James Miller at the US Sandia National Laboratories have also built a solar reactor based on a counter-rotating­ring receiver/reactor/recuperator concept (termed the Counter Rotating Ring Receiver Reactor Recuperator, CR5), for the production of solar fuels from hydrogen (obtained through water splitting) and carbon monoxide (obtained by CO2 splitting), by making use of the two-step ferrite or ceria cycles.34 The most recent CR5 consists of two chambers separated by rotating rings of cerium oxide (Figure 3.25). As the rings spin, a large parabolic mirror concentrates solar energy onto one side, heating it to 1500 °C and causing the ceria there to release oxygen gas into one of the chambers, from which it is pumped away.

As the ring rotates further it cools before it swings round to the other chamber where CO2 is pumped, causing the cooled, non-stoichiometric ceria to split carbon dioxide and produce carbon monoxide. The process also works with water instead of CO2, with the reaction this time producing hydrogen. Initial test results for the CR5 prototype, in the 16 kW National Solar Thermal Test Facility (NSTTF) solar furnace in Albuquerque, demonstrated that the process can produce carbon monoxide, although the failure of certain parts meant that the device did not operate continuously for more than a few seconds at a time.36

The team is now working to improve reliability while building a bigger reactor with 28 rotating rings to process more CO2 and water. The short-term goal for the CR5 prototype is to demonstrate a solar to chemical conversion efficiency of at least 2%. To achieve the overall

image143

Figure 3.25 The Sandia Counter Rotating Ring Receiver Reactor Recuperator, CR5. (Reproduced from Ref. 35, with kind permission.)

long-term goal of 10% efficient conversion of sunlight to petroleum, the thermochemical solar conversion of sunlight to CO needs to be at least 20% efficient.

In conclusion, the results obtained by these teams in the last decade provide evidence for the viability of CSP-based thermochemical approaches to solar hydrogen and solar fuel generation. We believe that this technology will soon evolve into a central energy technology in our common, sustainable future. Eventually, solar H2 is considered to be a feasible (and ultimate) emissions-free solution. However, carbon neutral solar fuels, obtained from the solar conversion of water and recycled CO2, could cover the intermediate period during the passage from conventional to renewable hydrogen energy. This will give the time required to solve the issues related to hydrogen handling, storage, transport and distribution infrastructure that are challenging for its implementation in the near future.37

Updated: August 19, 2015 — 12:54 pm