## 3 Key Component Update
### Three-Stage H\({}_{2}\)SO\({}_{4}\) Decomposition Reactor  
To ensure sufficient heat exchange, the H\({}_{2}\)SO\({}_{4}\) catalytic decomposer is designed with dimensions of \(\Phi\)108 \(\times\) 1500 mm. Prior to using the H\({}_{2}\)SO\({}_{4}\) decomposer, an HC276 alloy superheater is used as a preheating device to ensure that the preheating section outlet temperature reaches 850 \({}^{\circ}\)C, enabling efficient H\({}_{2}\)SO\({}_{4}\) decomposition. The superheater dimensions are \(\Phi\)98 \(\times\) 1600 mm. Previous experiments revealed severe corrosion in the HC276 alloy during phase changing of the H\({}_{2}\)SO\({}_{4}\) stream. To address this problem, a two-step strategy was utilized here. Firstly, an enamel-lined air heat exchanger was used to evaporate liquid-phase H\({}_{2}\)SO\({}_{4}\) into a gas phase, followed by HC276 superheating. The enamel-lined air heat exchanger is shown in Figure 3. The evaporator has a shell-and-tube heat exchanger structure with dimensions of \(\Phi\)400 \(\times\) 2100 mm and a heat exchange tube diameter of 42 mm. Electrically heated air was used as a heat source for evaporation. As shown in Figure 4, H\({}_{2}\)SO\({}_{4}\) is pumped from the bottom of the air heat exchanger into the H\({}_{2}\)SO\({}_{4}\) evaporator for evaporation. The gaseous H\({}_{2}\)SO\({}_{4}\) then enters the top inlet of the H\({}_{2}\)SO\({}_{4}\) superheater from the top. Heat exchange is enhanced using small silicon carbide balls in a cylindrical reactor chamber, where the first step of decomposition occurs, decomposing H\({}_{2}\)SO\({}_{4}\) into SO\({}_{3}\) and H\({}_{2}\)O, with heat supplied by external electric heating. Finally, gaseous SO\({}_{3}\) enters the H\({}_{2}\)SO\({}_{4}\) catalytic decomposition reactor from the superheater's base. The reactor cavity is filled with iron oxide-based catalysts, where catalytic decomposition occurs, producing SO\({}_{2}\) and O\({}_{2}\). Before conducting the experiments, a Computational Fluid Dynamics (CFD) simulation was performed for the H\({}_{2}\)SO\({}_{4}\) evaporator to ensure that H\({}_{2}\)SO\({}_{4}\) would be evaporated properly and prevent corrosion in the subsequent HC276 reactor. Due to the complexity of the H\({}_{2}\)SO\({}_{4}\) evaporator structure, this simulation mainly focused on the primary heat exchange region, ignoring the upper and lower heads of the evaporator. It was assumed that the mass flow rate of H\({}_{2}\)SO\({}_{4}\) was uniform across all 14 heat exchange tubes, the external surface of the evaporator had good insulation with no heat loss, and the heat transfer through the baffles and shell was neglected. A geometric model of the H\({}_{2}\)SO\({}_{4}\) evaporator is shown in Figure 5.  
The heat exchange tubes of the H\({}_{2}\)SO\({}_{4}\) evaporator are made of enamel-lined carbon steel. Using parameters from the network database, the thermal resistance of the heat exchange tube walls was calculated. The specific parameter settings are shown in Table 4.  
Figure 3: \(\rm H_{2}SO_{4}\) evaporator: (**a**) conceptual images; (**b**) physical images. Due to the complexity of simulating H\({}_{2}\)SO\({}_{4}\) boiling and the primary goal of evaluating heat exchanger performance, an equivalent heat capacity method was employed. The latent heat of H\({}_{2}\)SO\({}_{4}\) vaporization was converted into the heat capacity of sulfuric acid. This simplification avoids the complex calculations of the boiling process. Additionally, since the heat transfer coefficient during boiling is much higher than that of normal convective heat conduction, using the equivalent heat capacity method to calculate the heat exchange power will yield better results in practice than using the calculated values. The relevant boundary conditions are shown in Table 5.