# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 3 Kinetics Models and Their Development
### Semi-Empirical Model
#### 3.2.3 Polycyclic Aromatic Hydrocarbons  
Polycyclic aromatic hydrocarbons (PAHs), the largest, ubiquitous and carcinogenic environmental chemical groups, have a wide range of sources like oil extraction, petroleum refining, coal mining, bush fires and coal burning [108]. Some studies have investigated the degradation pathways of biphenyl [108,109], fluorine [108], and naphthalene [98,110] in high-pressure hot water.  
[PERSON] et al. [109] developed principal reaction pathways in the biphenyl SCWO reaction network, as shown in Figure 6. In their research, more than 50 intermediates detected under various SCWO conditions were mainly benzene derivatives such as acetophenone and dibenzofuran. The degradation of biphenyl was initiated by OH\(\bullet\) to hydroxylbiphenyl, in which _ortho_-hydroxylbiphenyl and _para_-hydroxylbiphenyl are more reactive than _meta_-hydroxylbiphenyl. The hydroxylbiphenyl formed open-ring carbonylic compounds through keto-enol tautomerism in the subsequent oxidation process and eventually into mineral products. However, [PERSON] and [PERSON] [108] found different ring-opening products, such as phenylethene, benzaldehyde, phenylethen-2-ol, and acetophenone, when investigating the degradation of biphenyl. This was probably caused by lower temperatures (<380 \({}^{\circ}\)C) and pressure (<22.5 MPa) while [PERSON]'s study [109] was carried out in higher temperatures (673-823 K) with methanol as the auxiliary fuel.  
[PERSON] et al. [110] summarized a four-step oxidation process for naphthalene degradation in SCW: (1) One benzene ring breaks and forms light hydrocarbons such as formic acid, acetic acid and other aromatic hydrocarbons including aromatic carboxylic acid, ketone and aromatic ester. (2) The polymerization of some intermediates forms dibenzofuran, fluorenone and 2'-hydroxyacetophenone. (3) Light hydrocarbons and polymers oxidize and produce acetic acid at the same time during the polymerization. (4) Acetic acid and other intermediates decompose to CO\({}_{2}\) and H\({}_{2}\)O. Figure 7 displays the reaction network for naphthalene degradation in their subsequent studies, including five main ways (R01-R05) for oxygen to \"invade\" naphthalene rings [98].  
Figure 5: Reaction network for SCWO of monosubstituted phenols [34].  
As expected, the oxidation of fluorine started with OH\(\bullet\) attacking the saturated carbon on the fluorene molecule, which is similar to most other organic compounds [108]. The intermediates found included biphenyl, benzophenone, dibenzofuran, benzofuran, hydroxybenzofuran, xanthone and fluorenone, of which fluorenone is the key intermediate and very stubborn at a lower temperature. The formation of 9-fluorenone, which led to the formation of xanthone or decomposed to benzaldehyde, phenol, acetophenone and other single-ringed aromatic compounds subsequently, may be the major pathway for fluorene oxidation under hydrothermal conditions. Furthermore, the oxidation of fluorene can also be carried out by another possible route, by which the -CH\({}_{2}\)O group was removed from hydroxylfluorene and 9-fluorenone decarbonylate, leading to the significant production of biphenyl observed. Hence, after the formation of biphenyl, the process assumed the pattern earlier described for  
Figure 6: Simplified reaction network for SCWO of monosubstituted phenols [109].  
Figure 7: Reaction network for SCWO of naphthalene in batch reactors [98].  
biphenyl oxidation in their early study [108]. The further decomposition products included benzofuran, isobenzofuranone, acetophenone and benzoic acid, and the detailed mechanism for fluorene oxidation is presented in Figure 8.