# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 2 Fundamental Processes
### Roles of Oxygen and Hydrogen Peroxide
[76] found that the reverse reaction of Equations (14)-(16) existed:  
\[\mathrm{H_{2}O+O_{2}}\xrightarrow{\mathrm{k_{3}}}\mathrm{HO_{2}\bullet+HO\bullet} \tag{17}\]  
\[\mathrm{H_{2}O+O_{2}}\xrightarrow{\mathrm{k_{3}}}\mathrm{HO_{2}\bullet+HO\bullet} \tag{18}\]  
\[\mathrm{H_{2}O_{2}+O_{2}}\xrightarrow{\mathrm{k_{4}}}\mathrm{HO_{2}\bullet+HO _{2}\bullet} \tag{19}\]  
Thus, oxygen and water can also generate hydrogen peroxide through the reactions (17)-(19), that is, in the pure SCW environment without organics, no matter whether the initial state of the system is O\({}_{2}\)-H\({}_{2}\)O or H\({}_{2}\)O\({}_{2}\)-H\({}_{2}\)O, it will be transformed into a mixed system. Under the condition that the transformation is not in equilibrium, the total reaction rate of O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) is different since they have different oxidation rates with organics. [PERSON] et al. [77] studied the oxidation of ethanol using a SCWO device with a preheater and obtained the same results when O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) were separately used as oxidants and preheated to supercritical condition. Based on these results, they speculated whether hydrogen peroxide or oxygen was used as an oxidant, they both could be quickly balanced through the above free radical reaction. At equilibrium, the concentration distribution of each species was independent in the initial state, and the subsequent major oxidation process was carried out under such an equilibrium distribution. On the contrary, under the subcritical conditions at 350 \({}^{\circ}\)C and 25 MPa, the conversion rate using hydrogen peroxide as oxidant was significantly higher than that of oxygen. This phenomenon may be caused by the mass transfer resistance between oxygen and water and the low decomposition rate of hydrogen peroxide under subcritical conditions. [PERSON] et al. [73] explored the degradation of acetic acid and 2_A_-dichlorophenol using an intermittent SCWO device without preheater. The conversion efficiency of acetic acid when using hydrogen peroxide as oxidant was 8 times higher than that when using oxygen (at 400 \({}^{\circ}\)C, a water density of 0.15 g/mL, and a reaction time of 5 min). As the reaction temperature increased, the conversion efficiency difference between the two oxidants decreased. The conversion efficiency ratios of H\({}_{2}\)O\({}_{2}\) to O\({}_{2}\) were larger at shorter reaction times, indicating the rapid initial reaction by hydrogen peroxide. However, since hydrogen peroxide can fully mix with the reactants before the start of reaction and oxygen is hard to do so before the supercritical point due to the interphase resistance, it is impossible to determine whether the better oxidation effect of hydrogen peroxide depends on the better oxidation capacity or on the premature reaction before the supercritical point. [PERSON] et al. [78] verified the effect of hydrogen peroxide by a continuous SCWO device. The unheated waste feeds and oxidants were pre-mixed outside the reactor and then mixed with the SCW/air feeds at the reactor inlet to achieve supercritical conditions. The temperature distribution in the reactor was measured by 16 thermocouples, and the degree of reactions was determined by temperature changes at different locations. When O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) were injected together, through analyzing the temperature data curve of isopropanol oxidation, it was found that H\({}_{2}\)O\({}_{2}\) mainly played an enhanced role in the initial stage of the reaction, while oxygen mainly played a role in the later stage of the reaction. The above research indicated that the way oxygen and hydrogen peroxide acted was greatly affected by the heating mode. In a SCWO unit with a preheater, using O\({}_{2}\)-H\({}_{2}\)O as the oxidant and preheating to supercritical condition had the same effect as using H\({}_{2}\)O\({}_{2}\) as the oxidant, and its operating cost was lower.