2.2 Two-step thermochemical cycles

The common two-step thermochemical cycle hydrogen production process is mainly metal oxide thermochemical cycle, which has the following three forms: Oxide:

or

$$\text{XO} \rightarrow \text{X} + \text{0.5O}\_2 \tag{18a}$$

atoms from water molecules to generate hydrogen. During the thermochemical

ZnO ! Zn þ

Fe3O4 ! 3FeO þ

the metal surface to prevent the reaction from proceeding. Wegner et al. [23] designed a spray reactor for solving this problem. By increasing the specific surface area of metal Zn to increase the contact area in the reaction, the experiment proves that the chemical conversion of Zn can reach 83%. The disadvantage of this method is that the metal Zn needs to be gasified and atomized, which requires large energy consumption; Zn, Sn, and other metals are also easily oxidized again during the decomposition process, affecting the reaction efficiency. The oxidation rate of iron oxide is easily reduced due to sintering, and ferrite has strong reducing ability. It can reduce CO2 to C solid element and cover the surface of ferrite to prevent the reaction from proceeding. One of the materials currently considered to be the most suitable for the thermochemical cycle of metal oxides is cerium oxide (CeO2), because cerium oxide can efficiently reduce water or carbon dioxide to hydrogen or carbon monoxide [24], and cerium oxide also has good anti-coking properties. The

CeO2�δox ! <sup>1</sup>

δred � δox

δred � δox

CeO2�δred ! <sup>1</sup>

In the two-step thermochemical cycle hydrogen production process, because there is a large heat transfer temperature difference between the "oxidation step" and "reduction step" (e.g., the temperature difference between cerium oxide heat transfer is about 700°C), the thermal energy recovery of solid materials has always been a very difficult problem. Hao et al. [25] proposed an "isothermal" thermochemical cycle, that is, the "oxidation step" and "reduction step" reactions are performed at the same temperature. The "isothermal" thermochemical cycle effectively overcomes the defect that a large amount of solid sensible heat in the "dual-temperature" thermochemical cycle cannot be efficiently recovered and does not generate thermal stress, which can maintain high energy utilization efficiency at high temperatures. However, the isothermal thermochemical cycle also has certain limitations that need to be resolved, such as the requirement of maintaining a quite low oxygen partial pressure, less hydrogen production in a single cycle, etc. The thermochemical cycle with metal oxide can also be utilized for CO generation from CO2, and the thermodynamics is similar to that of H2 generation from H2O, which will not be discussed here.

CeO2�δred þ

1 2

O2 (28)

CeO2�δox þ H2 (29)

Metal oxides may also be reduced from higher valence to lower valence oxides,

Among them, metal Zn is easy to form a dense oxide film, which is wrapped on

1 2

> 1 2

Zn þ H2O ! ZnO þ H2 (25)

H2O þ 3FeO ! Fe3O4 þ H2 (27)

O2 (24)

O2 (26)

cycle, metal oxides can be reduced to simple metals, such as:

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

such as:

specific reaction equations are:

High temperature (reduction step):

Low temperature (oxidation step):

H2O þ

151

1 δred � δox

> 1 δred � δox

$$\rm{X} + \rm{H}\_{2}\rm{O} \rightarrow \rm{XO} + \rm{H}\_{2} \tag{19a}$$

$$\frac{1}{\delta} \mathbf{XO}\_2 \to \frac{1}{\delta} \mathbf{XO}\_{2-\delta} + \frac{1}{2} \mathbf{O}\_2 \tag{18b}$$

$$\frac{1}{\delta} \mathbf{XO}\_{2-\delta} + \mathbf{H}\_2 \mathbf{O} \to \frac{1}{\delta} \mathbf{XO}\_2 + \mathbf{H}\_2 \tag{19b}$$

Hydride:

$$\mathbf{X} \mathbf{H}\_2 \to \mathbf{X} + \mathbf{H}\_2 \tag{20}$$

$$\rm{X} + \rm{H}\_{2}\rm{O} \rightarrow \rm{XH}\_{2} + \rm{0.5O}\_{2} \tag{21}$$

Hydroxide:

$$\text{2XOH} \rightarrow \text{2X} + \text{H}\_2\text{O} + \text{0.5O}\_2\tag{22}$$

$$\text{2X} + \text{2H}\_2\text{O} \rightarrow \text{2XOH} + \text{H}\_2\tag{23}$$

Among them, metal oxide thermochemical hydrogen production is the most common. The process is shown in Figure 10.

As shown in Figure 10, metal oxides are reduced by releasing oxygen at high temperatures, and oxidized with water at low temperatures, taking away oxygen atoms from water molecules to generate hydrogen. During the thermochemical cycle, metal oxides can be reduced to simple metals, such as:

$$\text{ZnO} \rightarrow \text{Zn} + \frac{1}{2}\text{O}\_2 \tag{24}$$

$$\text{Zn} + \text{H}\_2\text{O} \rightarrow \text{ZnO} + \text{H}\_2\tag{25}$$

Metal oxides may also be reduced from higher valence to lower valence oxides, such as:

$$\text{Fe}\_3\text{O}\_4 \rightarrow \text{3FeO} + \frac{1}{2}\text{O}\_2\tag{26}$$

$$\text{H}\_2\text{O} + \text{3FeO} \rightarrow \text{Fe}\_3\text{O}\_4 + \text{H}\_2\tag{27}$$

Among them, metal Zn is easy to form a dense oxide film, which is wrapped on the metal surface to prevent the reaction from proceeding. Wegner et al. [23] designed a spray reactor for solving this problem. By increasing the specific surface area of metal Zn to increase the contact area in the reaction, the experiment proves that the chemical conversion of Zn can reach 83%. The disadvantage of this method is that the metal Zn needs to be gasified and atomized, which requires large energy consumption; Zn, Sn, and other metals are also easily oxidized again during the decomposition process, affecting the reaction efficiency. The oxidation rate of iron oxide is easily reduced due to sintering, and ferrite has strong reducing ability. It can reduce CO2 to C solid element and cover the surface of ferrite to prevent the reaction from proceeding. One of the materials currently considered to be the most suitable for the thermochemical cycle of metal oxides is cerium oxide (CeO2), because cerium oxide can efficiently reduce water or carbon dioxide to hydrogen or carbon monoxide [24], and cerium oxide also has good anti-coking properties. The specific reaction equations are:

High temperature (reduction step):

$$\frac{1}{\delta\_{red} - \delta\_{ox}} \mathbf{C} \mathbf{e} \mathbf{O}\_{2-\delta\_{ox}} \to \frac{1}{\delta\_{red} - \delta\_{ox}} \mathbf{C} \mathbf{e} \mathbf{O}\_{2-\delta\_{red}} + \frac{1}{2} \mathbf{O}\_2 \tag{28}$$

Low temperature (oxidation step):

$$\text{H}\_2\text{O} + \frac{1}{\delta\_{\text{red}} - \delta\_{\text{ox}}} \text{CeO}\_{2-\delta\_{\text{red}}} \to \frac{1}{\delta\_{\text{red}} - \delta\_{\text{ox}}} \text{CeO}\_{2-\delta\_{\text{ox}}} + \text{H}\_2 \tag{29}$$

In the two-step thermochemical cycle hydrogen production process, because there is a large heat transfer temperature difference between the "oxidation step" and "reduction step" (e.g., the temperature difference between cerium oxide heat transfer is about 700°C), the thermal energy recovery of solid materials has always been a very difficult problem. Hao et al. [25] proposed an "isothermal" thermochemical cycle, that is, the "oxidation step" and "reduction step" reactions are performed at the same temperature. The "isothermal" thermochemical cycle effectively overcomes the defect that a large amount of solid sensible heat in the "dual-temperature" thermochemical cycle cannot be efficiently recovered and does not generate thermal stress, which can maintain high energy utilization efficiency at high temperatures. However, the isothermal thermochemical cycle also has certain limitations that need to be resolved, such as the requirement of maintaining a quite low oxygen partial pressure, less hydrogen production in a single cycle, etc. The thermochemical cycle with metal oxide can also be utilized for CO generation from CO2, and the thermodynamics is similar to that of H2 generation from H2O, which will not be discussed here.

HBr by electrolytic method or "cold" plasma method. This reaction has the following advantages: its expected thermal efficiency is 35–40%, and if the power is generated at the same time, the overall efficiency can be improved by 10%; the two-step key reaction is a gas-solid reaction, which significantly simplifies the separation of products and reactants; the elements used are cheap and readily available; the process involves only solid and gaseous reactants and products. However, the separation of intermediate products in the reaction process is also a

The common two-step thermochemical cycle hydrogen production process is mainly metal oxide thermochemical cycle, which has the following three forms:

or

XO2�<sup>δ</sup> þ

δ

1 2

1 δ

1 δ

common. The process is shown in Figure 10.

XO2 ! <sup>1</sup> δ

XO2�<sup>δ</sup> <sup>þ</sup> H2O ! <sup>1</sup>

Among them, metal oxide thermochemical hydrogen production is the most

As shown in Figure 10, metal oxides are reduced by releasing oxygen at high temperatures, and oxidized with water at low temperatures, taking away oxygen

XO ! X þ 0:5O2 (18a) X þ H2O ! XO þ H2 (19a)

XH2 ! X þ H2 (20)

X þ H2O ! XH2 þ 0:5O2 (21)

2XOH ! 2X þ H2O þ 0:5O2 (22) 2X þ 2H2O ! 2XOH þ H2 (23)

O2 (18b)

XO2 þ H2 (19b)

problem and challenge in the process.

Metal oxide thermochemical cycle for hydrogen production.

Wind Solar Hybrid Renewable Energy System

2.2 Two-step thermochemical cycles

Oxide:

Figure 10.

Hydride:

Hydroxide:

150
