# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 3 Kinetics Models and Their Development
### Dckm
#### 3.3.1 Hydrogen  
[PERSON] and [PERSON] [119] developed a DCKM for the SCWO of hydrogen based on a validated gas-phase mechanism and corrected for the high-pressure environment by techniques such as RRKM theory. The major free-radical reaction pathways in DCKM for hydrogen oxidation in SCW are shown in Figure 11. The 19-reaction mechanism in model YDR91] can be approximated by the seven dominating fast reactions (see Table 3), which are several orders of magnitude faster than the others. The model reproduced nearly all of the global characteristics of the experimental data, such as hydrogen conversion, hydrogen concentration and oxygen concentration. Additionally, the fast rate of H+O\({}_{2}\)+M\(\rightarrow\)HO\({}_{2}\)+M may lead to the reaction's global independence of oxygen concentration.  
Figure 11: DCKM for hydrogen oxidation in SCW [119].  
Moreover, because the oxidation was greatly inhibited by surface reactions, [PERSON] and [PERSON] [119] incorporated the surface reactions into model YDR91A to moderate the model's overpredicted oxidation rates and seek better consistency with experimental data. Unexpectedly, models with (YDR91S) and without (YDR91J) surface reactions gave equally good agreement with the experimental data, and the sensitivity analysis of model YDR91S indicated that the oxidation process is highly sensitive to the surface rate constant, which suggested that experiments might be irreproducible, yet the data in their study showed good reproducibility. In their subsequent studies, they investigated the role of water in SCWO by describing the effects of pressure or water density in the quantitative analysis [118]. The high pressure can affect the rate constant of the pressure-dependent reactions, such as the dissociation of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)\(\rightarrow\)OH\(\bullet\)+OH\(\bullet\)) and the dissociation (recombination) of hydroperoxyl radical (H\(\bullet\)+O\({}_{2}\)\(\rightarrow\)HO\({}_{2}\)\(\bullet\)), which are at or near its high-pressure limit. Additionally, water concentration greatly varies with the change of operating pressure in the unique SCW environment, which led to the greatly accelerated rate of the HO\({}_{2}\)\(\bullet\)+H\({}_{2}\)O\(\rightarrow\)H\({}_{2}\)O\({}_{2}\)+OH\(\bullet\) branching reaction and promoted the H\({}_{2}\) formation via H\({}_{2}\)O+H\(\bullet\)\(\rightarrow\)H\({}_{2}\)+OH\(\bullet\).  
[PERSON] and [PERSON] [120] constructed a DCKM to describe the SCWO of CH\({}_{4}\), CH\({}_{3}\)OH, CO, and H\({}_{2}\) for the first time, and used the Lindemann model to explain the pressure dependence for unimolecular reactions in their model, improving on previous models [119] that only considered the pressure dependence of a few reactions. The Arrhenius plot indicated that the model shows good agreement for experimental results at high temperatures. However, in all cases, the H\({}_{2}\) concentrations predicted by the model were much lower than the experimental ones while the curve shapes are very similar. Compared to the good prediction of [PERSON] and [PERSON]'s model [119], which may be caused by the fortuitous combination of inaccuracies in the predicted induction times and kinetic decay constants, the model developed by [PERSON] and [PERSON] [120] confirmed the error is caused by the short induction time of the model while the kinetic decay constants maintain a high level of consistency with experimental results.