Nitrogen chemistry in supercritical water.

Abstract

Supercritical water oxidation (SCWO) is a viable technology for hazardous organic waste destruction. Prior research has focused largely on reactions of model hydrocarbons. We have expanded the existing SCWO kinetics database to include nitrogen chemistry by investigating pyrolysis, hydrolysis, and oxidation of methylamine, a model nitrogen-containing compound, in supercritical water (SCW). We examined the reactivity of methylamine in SCW at temperatures between 386--500&deg;C. For water densities less than 0.28 g/cm<super>3</super>, methylamine disappearance is constant, and there is little evidence of hydrolysis. Under these conditions, the reaction seems to be governed by pyrolysis. At higher water densities, the rate of hydrolysis becomes more important and increases with water density. Methylamine was oxidized in SCW at 249 atm and temperatures between 390--500&deg;C. Ammonia appears to be the exclusive nitrogen-containing intermediate between methylamine and the final products, N₂O and N₂. A reaction model that involves initial cleavage of the C-N bond is quantitatively consistent with the experimental data. The oxidation of ammonia during methylamine SCWO is faster and produces more N₂O than N₂ compared to direct ammonia SCWO. These differences in the rate and selectivity of ammonia oxidation during methylamine SCWO are attributed to the presence of methylamine in the reaction environment and catalysis on the reactor walls. We constructed a detailed chemical kinetic model (DCKM) to describe methylamine oxidation and pyrolysis in SCW. The model is based on a free-radical reaction mechanism containing 72 species and 603 elementary reactions. Computer simulations reveal that the mechanism cannot fully describe either methylamine pyrolysis or oxidation in SCW. Reaction pathway and sensitivity analyses indicate that several of the most important elementary reactions are ones whose parameters were estimated. These modeling results highlight the need for improved free-radical reaction kinetic parameters for nitrogen chemistry. Additionally, they point to the possibility of a wall-catalyzed reaction mechanism during methylamine supercritical water oxidation and pyrolysis. Methylamine hydrothermolysis experiments with increased metal surface area indicate that methylamine disappearance in SCW is surface-sensitive. This finding, along with the modeling analysis, suggests that methylamine chemistry in SCW is controlled in part or whole by heterogeneous reactions with the reactor walls.