Detailed Chemical Kinetic Modeling of Methylamine in Supercritical Water

We constructed a detailed chemical kinetics model (DCKM) for the oxidation and pyrolysis of methylamine in supercritical water (400−500°C). The model contains 72 species and 603 elementary free-radical and molecular reactions. This DCKM for methylamine oxidation is the first to include peroxy radical chemistry, which becomes important at these lower temperatures. The major products from oxidation in supercritical water are predicted to be formamide, ammonia, CO₂, and formic acid. The major products predicted for methylamine hydrothermolysis are ammonia, HCN, methane, water, CO₂, and H₂. The activation energies are predicted to be 47 and 51 kcal/mol for oxidation and thermolysis, respectively, in supercritical water. The main route for methylamine removal during oxidation is OH attack. The main route to ammonia is hydrolysis of formamide. Predictions from the DCKM, which is built upon homogeneous chemistry, are not consistent with published experimental results for either methylamine pyrolysis or oxidation in supercritical water. This inconsistency is due primarily to heterogeneous catalytic reactions occurring in the experimental system, but also to uncertainties in the homogeneous mechanism and its kinetics parameters. Reaction pathway and sensitivity analyses indicate that several of the most important elementary steps are ones with estimated kinetic parameters, due to a lack of information in the combustion literature. These modeling results highlight the need for an improved understanding of the mechanism and kinetics for organo-nitrogen chemistry during gas-phase oxidation in this low-temperature region. Additionally, they point to the need for experimental data for methylamine reactivity in supercritical water in the absence of heterogeneous reactions.