High temperature solar thermal fuel production

December 10th, 2014, Published in Articles: Energize

 

Thermal processes for the production of fuels such as liquid hydrocarbons and hydrogen from hydrocarbon gas and carbon dioxide are well established processes, but all use fossil fuels as an energy source. The development of high temperature solar thermal systems offers an energy savings alternative to the use of fossil fuels, and opens opportunities to the use of other carbon-free or carbon-neutral processes to produce fuels.

The use of concentrated solar power (CSP) to produce electricity is well established. The development of high temperature CSP plants and the availability of materials capable of withstanding these high temperatures has generated interest in the use of CSP for other high temperature processes, which usually require fossil fuels or electrical energy to operate.

High temperature solar fuel production could lead to a new paradigm where carbon dioxide (CO2) is seen as a valuable commodity that can be used in solving other shortage problems rather than being seen as a pollutant that must be buried or sequestrated. There are already a number of projects running which do convert recovered carbon dioxide to liquid fuels for instance, but these all use other sources of energy such as electricity, primarily “surplus” renewable electricity.

Fig. 1: Solar reactor used in HTS processes [1]).

Fig. 1: Solar reactor used in HTS processes [1]).

Solar thermal reactors

The core of the solar thermal (ST) process is the solar reactor, which is the equivalent of the receiver of CSP system. ST systems operate at higher concentration ratio than CSP, and thus generate higher temperatures. Current pilot systems under development, or in use, have power levels up to 1 MW, although higher ratings are planned. The smaller system designs use parabolic mirrors, fixed or adjustable in a variety of configurations, including tower based reactors and ground based reactors, while larger systems using arrays of individual mirrors in a CSP-type of configuration are in existence. Reactions take place within the reactor, which an most cases has to be designed specifically for the process taking place, but some applications use collectors and heat transfer systems such as molten metal, which allows a variety of reactors to be coupled to the same source. Fig. 1 shows a reactor used in a specific ST process [1]. Fig. 2 shows the mirror used to focus sunlight on the reactor.

Fig. 2: Solar mirror used in to focus  sunlight on the reactor [1].)

Fig. 2: Solar mirror used in to focus
sunlight on the reactor [1].)

Solar thermal fuel processes

The carbon-free processes broadly encompass electrolysis and thermochemical processes. Two processes are being investigated :

  • Solar splitting of water and carbon dioxide to produce hydrogen and carbon monoxide, which includes
    –    High temperature splitting of water to produce hydrogen
    –    High temperature steam electrolysis
    –    High temperature production of CO from CO2

Hydrogen and carbon monoxide can be combined in further solar driven processes to produce methanol or other hydrocarbon fuels.

  • Solar decarbonisation of hydrocarbons, which includes:
    –    Solar steam reforming of natural gas to produce syngas consisting of CO and H, which could be subject to further processing to separate the two components to produce pure hydrogen.  The syngas could be used to create liquid fuel by the Fischer-Tropsch process.
    –    The solar CO2 (dry) reforming of methane to produce syngas consisting of CO and H can serve as a near-term means of recycling emitted CO2 as fuel.
    –    Solar cracking of fossil fuels to produce hydrogen, carbon black and carbon nanotubes.
    –    Solar steam gasification of carbonaceous materials to produce syngas, which can be applied to many forms of waste product including biomass, industrial waste, waste coal and petroleum coke.

Solar splitting of water

Two methods are being developed: high temperature hydrolysis, and high-temperature reduction and water re-oxidation of metal oxides.

High temperature electrolysis

High temperature splitting of water uses the established high temperature electrolysis process to produce hydrogen. The electricity required to electrolyse steam reduces as the steam temperature increases, while the thermal requirement for the electrolysis increases with temperature. Energy from the high temperature steam electrolysis (HTSE) of water decreases the amount of electrical energy required (Fig 3).
Problems encountered include a decreasein the life of the electrode as the temperature increases, and the current research is focused on intermediate temperature electrolysis (ITSE) in the range 600 – 700°C which falls within the range for solar thermal operation.

Metal oxide reduction/oxidation method

Solar thermal water-splitting (STWS) cycles have long been recognized as a means of generating hydrogen gas from water. Two-step, metal oxide–based STWS cycles generate H2 by sequential high-temperature reduction and water re-oxidation of a metal oxide [2]. The general equations involved are the following:

Table 1: Metal oxides used in the solar thermal redox process (Furler [4])
Volatile metal oxides (Undergo gas-solid phase transition) Non volatile metal oxides (Remain in the solid state during reduction)
Zinc oxide: ZnO(s) → Zn(g) Iron oxide: Fe3O4
Tin oxide: SnO2(g) → SnO(g) Ferrites
Hercynite FeAl2O4
Perovskite CaTi O3
Cerium oxide CeO2

Step 1: Reduction of metal oxides to metal and oxygen

Metal oxide → Metal + ½ O2    (1)

This takes place typically at a temperature in the region of 900°C

Step 2: Re-oxidation of metal in the presence of steam

Metal + H2O → Metal oxide + H2    (2)

Which takes place typically at a temperature in the region of 1300°C

This process normally involves two sequential stages at the different temperatures. Researchers at the university of Colorado have developed an isothermal redox process which allows both stages to be implemented at the same temperature, using a complex combination of metal oxides [2].

Solar splitting of CO2

High temperature CO2 splitting perhaps offers the greatest longer-term potential for the re-use of captured CO2. Current projects are based on a multi stage process using metal oxides to produce hydrogen and syngas from water and CO2. Most processes are still at research stage, but the ferrites and zinc oxide processes have been demonstrated at power levels of 100 kW [3]. The process in involves the reduction of a metal oxide to produce the metal and carbon monoxide.

Fig. 3: Intermediate temperature steam electrolysis zone.

Fig. 3: Intermediate temperature steam electrolysis zone.

MO +CO2 → M + CO    (3)

Where M is the metal used.

This reaction takes place at temperatures in the region of 1700°C. Pilot plants based on the use of Zn/ZnO are in operation [5].

Syngas production

Advanced processes have been developed that combine the hydrogen and carbon monoxide production in the same cycle [4]. The use of Zn/ZnO has been superceded by the use of more complex metal oxides which have superior properties such as porosity.

Table 1 lists some of these [4].

The reaction shown in block form in Fig. 4.

Methanol is produced from the syngas (CO/H2) using the Fisher-Tropf process. A further development has been suggested where methanol could be produced directly in the same cycle with the use of a catalyst. This would allow production of methanol in one reactor, and in a contiguous process.

Fig. 4: Combined  CO2/H2O dissociation process.

Fig. 4: Combined CO2/H2O dissociation process.

Solar decarbonisation

The processes listed all make use of hydrocarbon feedstocks, and are in current use in the petrochemical and other industries. Existing processes use fossil fuels to provide the heat necessary for the reactions to take place. In the solar equivalent, solar heat replaces the fossil fuel. All processes have been demonstrated to be possible using solar reactors, and several, particularly gasification of carbonaceous materials, are approaching industrialisation. The problem with these processes is that they all produce CO2 , and are therefore not strictly renewable, carbon free or carbon neutral, and therefore may not play a long term role in the future production of fuels. The prime position of solar decarbonisation is as an interim step to reduce both the energy usage and fossil fuel consumption of the existing processes.

References

[1]    C Burroughs: “Sandia’s sunshine to petrol project seeks fuel from thin air”, Sandia National Laboratories, https://share.sandia.gov/news/resources/releases/2007/sunshine.html
[2]    C Muchich, et al: “Efficient generation of hydrogen by splitting water with an isothermal redox cycle” Science magazine, 2 August 2013.
[3]    A Meier: “Solar thermal electricity and solar thermal chemical fuels”, FFF hybrid conference on solar fuels and high temperature solar applications, August 2014.
[4]    P Furler, et al: “ Solar reactors for thermochemical CO2 and H2O splitting via metal oxide redox reactions”, ETH Zurich, http://sfera2.sollab.eu/uploads/images/networking/SFERA%20SUMMER%20SCHOOL%202014%20-%20PRESENTATIONS/Solar%20Reactor%20Reduction%20-%20Philipp%20FURLER.pdf
[5]    C Hutter, et al: “Operational experience with a 100 kW solar pilot plant for thermal dissociation of zinc oxide”, Solar technology laboratory, Paul Scherrer institute.

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