## 2 Characteristics and Improvements of Experimental Apparatus
### Continuous Reactors
#### 2.2.2. University of Valladolid  
As one of the main institutes, the High Pressure Process Group in the University of Valladolid (UVa), Spanish, has been carrying out investigations from the effects of SCWO on recalcitrant organic matters to the enhancement of degradation efficiency with hydrothermal flame even to the performance of new cooled wall reactor (CWR, which resembles the WCHB of ETH, so the WCHB is used to represent CWR) in energy production. According to the corresponding research content, it has designed various types of reactors, including tubular reactors (Figure 4e-g) [30, 32], TWR [33, 73, 74] and WCHB, with the scale of experimental system expanded from the laboratory level to the pilot level. In terms of the hydrothermal combustor, its new structure upgraded based on the SCWO reactor consisted of the inlet, outlet, and heat-resistant walls (water-cooled wall and transpiring wall) among other parts. In 2006, [PERSON] et.al tested the performance and the reliability of three kinds of transpiring walls (Figure 4a) [73]. Wall 1 was completely made by porous materials; Wall 2 was made of nonporous materials for the top and porous materials for the left; Wall 3 was made out of nonporous materials for its upper and lower sections and porous materials for the left. The results showed that Wall 3 was the best. This was because (1) the upper section not only kept the wall from high-temperature damage for a long time, but prolonged the residence time before the mixture with the transpiring flow, and alleviated excessive cooling of the reaction chamber under a high reaction flow rate and a high cooling water rate; (2) the lower section could remain even under subcritical conditions, and the nonporous materials were safe and economical. Therefore, Wall 3 was adopted in subsequent TWRs [33, 74]. On the basis of the previous TWR scheme, the innovative WCHB with excellent cooling performance and durability for hydrothermal combustion has been proposed (Figure 4b) [31, 75, 76]. In addition, its simplicity made it  
Figure 3: TWR and WCHB from ETH (**a**) The first version of TWR (adapted from [72]); (**b**) The modified version of TWR (adapted from [72]); (**c**) The final version of TWR (adapted from [72]); (**d**) WCHB-1 (adapted from [27]); (**e**) WCHB-2 (adapted from [27]); (**f**) WCHB-3 (adapted from [27]); (**g**) WCHB-4 (adapted from [22, 23]).  
easy to instruct the design of the reactor and the study of hydrothermal flame by comparison between experimental results and numerical simulation results. In order to explore the possibility of using the hydrothermal flame as the way of energy production, the structure and the whole operation system of the reactor were perfected (Figure 4c) [37; 77]. The first modification was its inlet and outlet. The reactants flowed into the chamber from the bottom, and the cooling water flowed from the top to the bottom through the annulus between the outer wall and the reaction chamber, dissolving the deposited salt, and thus overcoming the blockage caused by salt-deposition. Furthermore, the other great change was its upper section. Some of the products could be collected from the upper outlet to avoid mixing with cooling water, which was beneficial for the subsequent product analysis and energy generation.