2. Thermochemical cycle

The existing thermochemical cycle for hydrogen production mainly includes metal oxide system thermochemical cycle, sulfur-containing system, sulfuric acid decomposition method, metal-halide system, and reformed methane hydrogen production. All of the thermochemical cycles could be classified as multi-step thermochemical cycles and two-step thermochemical cycles.

### 2.1 Multi-step thermochemical cycles

#### 2.1.1 Hydrogen generation system containing sulfur

There are four main types of hydrogen production in sulfur-containing systems: iodine-sulfur cycle, H2SO4-H2S cycle, sulfuric acid-methanol cycle, and sulfate cycle. Among them, the iodine-sulfur cycle is the most famous. It was invented by the United States GA company in the 1970s, so it is also called the GA cycle. The process is shown in Figure 8. The main reaction process is as follows:

$$\text{2SO}\_2 + \text{I}\_2 + 2\text{H}\_2\text{O} \rightarrow 2\text{HI} + \text{H}\_2\text{SO}\_4 \tag{10}$$

$$\text{H}\_2\text{SO}\_4 \rightarrow \text{H}\_2\text{O} + \text{SO}\_2 + \frac{1}{2}\text{O}\_2\tag{11}$$

$$\text{2HI} \rightarrow \text{H}\_2 + \text{I}\_2 \tag{12}$$

$$\text{H}\_2\text{O} \rightarrow \text{H}\_2 + \frac{1}{2}\text{O}\_2\tag{13}$$

GA company found [19] that the excess I2 exists, and HI and H2SO4 can be separated into two liquid phases, which is the basis for the development of the IS cycle. The advantages of the IS cycle are using of thermal energy below 1000°C for hydrogen generation, closed circuit, only water being needed to be added in the circulation process, and the expected efficiency which can reach 52%. The disadvantages are concentrated sulfuric acid being highly corrosive when heated at high temperature; the equilibrium decomposition ratio of HI being low (20%); and the reaction intermediate products sulfur dioxide and iodine being easy to cause pollution and liable to have side reactions.

#### 2.1.2 Sulfuric acid decomposition method

This type of method is best known as the Westinghouse cycle [20], and its main process is shown in Figure 9. The highest temperature in the process needs to be above 800°C, and the efficiency of the cycle can reach 40%. If multi-stage electrolysis is used, it can reach 46%. However, the disadvantage is that concentrated

sulfuric acid is highly corrosive at high temperatures and has high requirements for material selection.
