# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 2 Fundamental Processes
### Ionic Reactions and Free Radical Reactions
The conversion rate of acetic acid hydrolysis is less than 30%, and acetic acid is completely degraded in the oxidation process at 575 \({}^{\circ}\)C, 246 bar and 8 s residence time in the experiment by [PERSON] et al. [64] [PERSON] et al. [65] investigated the hydrolysis and oxidation of ethanol in SCW and found the conversion rate of ethanol during the oxidation was much higher than that during the hydrolysis. The reaction order for oxygen in global rate expression for oxidation was \(0.55\pm 0.19\), which indicated oxidants played an important role in free radical reactions. Additionally, temperature also greatly affected the formation of free radicals. In the study on the degradation of organic compounds such as methanol [66], ethyl acetate [67] and quinazoline [68], some scholars found that temperature rise effectively shortened the induction time. [PERSON] et al. [69] believed that even at the same temperature, different water densities might accelerate or hinder the process of SCW reaction. One possible reason is that in the high water density range, water promotes reactions based on the ion reaction mechanism, while in the low water density range, reactions based on the free radical mechanism are strengthened. [PERSON] et al. [30] proposed the free radical reaction mechanism of SCWO based on wet oxidation and gas-phase oxidation, which included three stages: initiation of chain, development or transfer of chain, and termination of chain. Initiation of chain:  
\[\mathrm{RH}+\mathrm{O}_{2}\xrightarrow{}\mathrm{R}\bullet+\mathrm{HOO}\bullet \tag{1}\]  
\[\mathrm{RH}+\mathrm{HOO}\bullet\xrightarrow{}\mathrm{R}\bullet+\mathrm{H}_{2} \mathrm{O}_{2} \tag{2}\]  
\[\mathrm{H}_{2}\mathrm{O}_{2}\xrightarrow{}\mathrm{2 OH}\bullet \tag{3}\]  
Development or transfer of chain:  
\[\mathrm{RH}+\mathrm{HO}\bullet\xrightarrow{}\mathrm{R}\bullet+\mathrm{H}_{2} \mathrm{O} \tag{4}\]  
\[\mathrm{R}\bullet+\mathrm{O}_{2}\xrightarrow{}\mathrm{ROO}\bullet \tag{5}\]  
\[\mathrm{ROO}\bullet+\mathrm{RH}\xrightarrow{}\mathrm{ROOH}+\mathrm{R}\bullet \tag{6}\]  
\[\mathrm{ROOH}\xrightarrow{}\mathrm{HO}\bullet+\mathrm{RO}\bullet \tag{7}\]  
Termination of chain:  
\[\mathrm{R}\bullet+\mathrm{R}\bullet\xrightarrow{}\mathrm{R}\text{-}\mathrm{R} \tag{8}\]  
\[\mathrm{R}\bullet+\mathrm{RO}\bullet\xrightarrow{}\mathrm{R}\mathrm{OR} \tag{9}\]  
\[\mathrm{RH}+\mathrm{O}_{2}\xrightarrow{}\mathrm{R}\bullet+\mathrm{HOO}\bullet \tag{1}\]0  
\[\mathrm{RH}+\mathrm{O}_{2}\xrightarrow{}\mathrm{R}\bullet+\mathrm{HOO}\bullet \tag{1}\]1  
[PERSON] and [PERSON] [31] summarized eight key reactions for oxidation in SCW based on a wide range of verified gas-phase and liquid-phase oxidation reactions, which thus constructed a simplified model of the mechanism of [PERSON] et al. [30]. Table 1 shows elementary steps and reaction families for oxidation in SCW developed by [PERSON] and [PERSON] [31]. [PERSON] et al. [30] believed that HO\({}_{2}\bullet\) was an important free radical in the reaction process. Reactive oxygen in SCW interacted with the weakest C-H bond to produce the free radical HO\({}_{2}\bullet\) and then along with H in organic matters generate H\({}_{2}\)O\({}_{2}\), which could also further decompose and thus form hydroxyl radical (HO\(\bullet\)) that could react with almost any hydrogen-containing compound. When oxygen acted as an oxidant, it first grabbed the hydrogen atoms in the organic material to generate free radicals, which accelerated the subsequent reactions. [PERSON] and [PERSON] [70] pointed out that the reaction (1) was dominant in the chain initiation stage when developing the DCKM of methane's SCWO. This may be due to the high concentration of methane and oxygen and the low concentration of free radicals at the beginning of the reaction. However, using hydrogen peroxide as an oxidant can directly produce hydroxyl radicals, thus skipping the reactions (1) and (2) in the above chain initiation process. HO\({}_{2}\bullet\) is as important as HO\(\bullet\) free radical in the SCWO process.