# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 3 Kinetics Models and Their Development
### Semi-Empirical Model
#### 3.2.4 Heterocyclic Compounds  
[PERSON] et al. [111] studied reaction pathways, transition products and the kinetics of pyridine oxidation in SCW, and constructed a simplified reaction network. On the assumption that the initial reaction involved hydrogen abstraction followed by hydroxylation, all ring cleavage reactions were related to the formation of 2-, 3-, or 4-hydroxypyridine and its resonance isomer. The different product distribution in liquid effluent indicated that the reaction sites of hydroxyl substituted pyridine ring-opening and chain fracture after ring-opening were not unique. (i.e., the presence of maleic acid suggested at lease one nitrogen-carbon bond was broken and only the rupture of two carbon-carbon bonds could produce dimethylamine). It suggested a variety of products of 2-, 3-, or 4-hydroxypyridine ring-opening and carbon chain rupture, including glutaconic acid + NH\({}_{3}\), maleic acid + methyl amine, malonic acid + ethyl amine, and dimethyl amine + acetone. The above-mentioned products degraded into different carboxylic acids, alcohol molecules and amino organic compounds, and eventually into inorganic substances such as NH\({}_{3}\), CO\({}_{2}\), and H\({}_{2}\)O. Figure 9 summarizes partial simplified reaction pathways of pyridine oxidation in SCW. Figure 8: Reaction mechanism for SCWO of fluorine [108]. [PERSON] et al. [68; 112] investigated the reaction pathways of quinazoline oxidation in SCW, and found the oxidation of quinazoline included two ring-opening reactions: the ring-opening reaction on the benzene ring and the ring-opening reaction on the pyrimidine ring. Since the two N atoms in the pyrimidine ring, whose presence was attributed to the low electron cloud density in the pyrimidine ring [113], had a stronger ability to attract electrons, The benzene ring would be vulnerable to oxidation. The simplified reaction model based on the measured intermediate is shown in Figure 10 [68], and a quantitative kinetic model with 11 reaction pathways involved was proposed to describe the reaction pathways, which well matched most of the experimental concentration changes of major compounds (quinazoline, pyrimidine, napthalene, phenol, NH\({}_{3}\), CO\({}_{2}\), etc) at test temperatures between 673-873K. However, the model overpredicted NH\({}_{3}\) concentration at 773K and underestimated NH\({}_{3}\) concentration at 873K. The sensitivity analysis determined that the reaction was dominated by the selective ring-opening of quinazoline for pyrimidine yielding. When the temperature was  
Figure 9: Partial simplified reaction mechanism for pyridine oxidation in SCW (*, compounds identified in their study) [111]. above 823K, the reaction rate of pyrimidine degradation was significantly accelerated. Additionally, [PERSON] et al. [114] studied the degradation of quinolone in the SCWO process and proved that quinoline had comparable \"ignition temperature\" as methanol and isopropanol. The ring-opening reaction was a rate-limiting step. The ring-opening reaction on the pyridine ring was more difficult than that on benzene, which was consistent with studies on quinazoline degradation [68, 112]. The reaction pathway can be proposed as quinoline \(\rightarrow\) secondary components \(\rightarrow\) volatile species \(\rightarrow\) gaseous products. [PERSON] and [PERSON] [115] analyzed the migration of N element during the oxidation of quinazoline in SCW using ReaxFF reactive force field combined with the DFT method. A long carbon chain with N atoms was formed by the ring-opening reaction on the pyrimidine ring, and subsequently broke into several small fragments, of which C\(\equiv\)N fragments were vital for the formation of NH\({}_{3}\). C\(\equiv\)N fragments fractured into CO and NH\({}_{2}\) after being attacked by OH\(\bullet\), O\({}_{2}\), and H\({}_{2}\)O in turn, and the highly unstable NH\({}_{2}\) extracted H atoms from the SCW to form NH\({}_{3}\).