Kinetics study on hydrothermal combustion of methanol in supercritical water

• A mechanism-based model for hydrothermal combustion of methanol was validated.
• Ignition and extinction temperature were explained from kinetics perspective.
• The influences of operational parameters on combustion reaction were studied.
• Methanol could provide high oxidation heat and co-oxidize refractory compounds.
• The existence of refractory species can suppress the combustion of auxiliary fuel.

Supercritical hydrothermal combustion has received a great deal of attention as an innovative and potential treatment technology wherein the corrosion and salt deposition problems during supercritical water oxidation processes can be avoided effectively. A detailed chemical kinetics model for methanol was employed and validated to understand the hydrothermal combustion process mechanism in supercritical water. Based on this elementary reaction model, how the two key indicators (ignition and extinction temperatures) worked during combustion reaction was studied. Moreover, the influences of operational parameters (fuel concentration, oxidation coefficient and reactor type) and corresponding reaction mechanism were investigated. It reveals that H2O2 was identified as one key intermediate product in the combustion kinetics of methanol. Initial concentration and injection flow rate of aqueous fuel were two significant factors that determined the extinction temperature, which decreased as the concentration increased or the injection flow rate reduced. Moreover, a minimum limit for the initial fuel concentration existed above which a stable hydrothermal flame could form. The oxidation coefficient affected combustion temperature in a manner that depended on the coefficient range. The combustion temperature elevated with the oxidation coefficient in the fuel-rich area while dropped in the oxidant-rich area. This phenomenon was interpreted from the point of view of thermodynamics and mechanism. In order to maintain stable flames at as a low injection temperature as possible, vessel reactors were more desirable to be applied for hydrothermal combustion reaction, deriving from the discrepancy in order of magnitude for flow velocity between tubular and vessel reactors. Finally, it is found that the enhancement of auxiliary fuel methanol in decomposition of organic pollutants stemmed from two reasons: high reaction heat release and co-oxidative effect, whereas the refractory compounds could suppress the ignition of methanol in supercritical water.