Supercritical Water Oxidation (SCWO) is an emerging technology under development for the treatment of hazardous wastes such as obsolete chemical munitions, mixed wastes, and excess naval shipboard hazardous materials. Understanding of the rates and mechanisms of reactions in supercritical water was limited to a handful of empirical rate expressions for very simple chemicals. These expressions were of limited use in the formulation of predictive models of SCWO for the design and operation of large-scale waste processing equipment. To be applicable as design tools, the models needed to be based on elementary reaction steps or, at minimum, a detailed quantitative mechanistic description incorporating all of the key fundamental reactions.

The objective of this project was to develop a SCWO technology to treat aqueous hazardous wastes. The project was designed to result in a user-friendly, computer-based model that could predict reaction rates and conversion efficiency for a range of waste feeds and reactor conditions.

Schematic of the Supercritical Water Oxidation (SCWO) process.

This project produced a good understanding of the reactivity of aliphatic and aromatics containing carbon, hydrogen, and oxygen and has resulted in verified model predictions for the reactivity of feeds of this type. In addition, the behavior of chlorinated systems was completed. The work has shown the direct application of combustion-based modeling as being the quantitatively correct approach for SCWO as opposed to liquid-phase oxidation as a starting point. The development of the computer model and the user interface were critical to the final success of the project. This required 1) developing an integrated new mechanism for low-temperature, highpressure “combustion” containing specific and lumped reactions as a Chemkin input file and 2) developing a user-friendly interface for Chemkin data input and output. The final steps in this project took the kinetic model and applied it in a code that represents the performance of an actual SCWO reactor. The final phase used Sandia National Laboratory’s computational fluid dynamic code CURRENT to simulate the performance of the Army’s SCWO reactor at Pine Bluff Arsenal, AR.

This project developed oxidation rates for common organic compounds in supercritical water. These data provide the basis to develop a model (combustion-based as opposed to liquid-phase oxidation) to be used as a design engineer’s tool for testing the effects of reactor design changes and producing advanced strategies for large-scale system optimization. This project focused on the development of a detailed chemical kinetic description of SCWO within the context of practical applications for the Department of Defense and Department of Energy. The oxidation rates of a variety of prototypical hazardous organic compounds have been studied and quantitative elementary reaction models have been developed that can reproduce the experimental results accurately. Significant progress was made in accounting for the effects of mixtures of different types of organic species as well, setting the stage for implementation of these kinetic models into actual engineering design computational tools. This project was completed in FY 1999.

The SCWO process, operating at two orders of magnitude greater density than atmospheric gaseous combustion, provides high reaction rates at moderate temperatures. The improved understanding of reaction rates and the kinetic models developed by this project produced advanced strategies for reactor design and improved methods for commercial system optimization.