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Overview of Supercritical Water Oxidation (SCWO)

Supercritical Water Oxidation (SCWO), sometimes referred to as Hydrothermal Oxidation (HTO), is a high-efficiency, thermal oxidation process capable of treating a wide variety of hazardous and non-hazardous wastes. The SCWO reaction takes place at elevated temperatures and pressures above the critical point of water (Pc= 220.55 bar, Tc=373.976 C). SCWO is ideally suited for treating waste streams containing high concentrations of water. SCWO processing systems are fully enclosed and do not produce hazardous air pollutants (HAPS) or NOx.

In a traditional SCWO waste treatment system (Figure 1), dilute aqueous organic waste is combined with oxidizer at elevated pressure and temperature (P>221 bar, T>550 C) in a reactor for residence times on the order of 10 to 15 seconds. At supercritical conditions, the reactants comprise a homogeneous single-phase mixture facilitating complete reaction. For most wastes, these conditions are sufficient to achieve 99.99% destruction and removal efficiencies (DRE) and better.

Typical SCWO Process Flow Diagram
supercritical, oxidation, SCWO, reactor, transpiring-wall
- Figure 1 -

Once the reaction has achieved the desired DRE, the reactor effluent is cooled, depressurized and separated into gaseous and liquid streams. The process is totally enclosed up to the point of final discharge to the environment, facilitating post-processing and monitoring prior to release. This feature is especially useful when treating highly toxic or radioactive wastes. In many cases, high-quality excess heat of reaction can be recovered for use within the process, or for external purposes. The SCWO process is self-sustaining for organic concentrations in excess of approximately 1% concentration. Concentrated wastes can be diluted with water prior to injection into the process. Water-soluble fuels, such as ethanol, can be added to the waste to help support the oxidation reaction. SCWO processing is not limited to disposal of organic compounds and complex organic wastes. Inorganic waste constituents, such as nitrates or ammonia, can be destroyed with like efficiency by the addition of suitable reducing or oxidizing agents. Because of SCWO's unique ability to oxidize high-moisture organic materials with high combustion efficiency, there is also potential for developing SCWO power generation plants fuelled by raw biomass slurries.

Chemistry of SCWO:

Chemical reactions in SCWO processing systems obey the laws of mass, charge, energy and elemental conservation. At supercritical conditions, the reactions proceed as gas-phase free-radical reactions involving the formation of a variety of intermediary species and related subreactions as the reaction proceeds to completion. The following global stoichiometric relationships apply to complete oxidation of the listed waste compounds.
supercritical, oxidation, SCWO, reactor, transpiring-wall
Oxidation of halogenated or sulfur-bearing compounds results in the formation of hydrochloric acid and sulfuric acid, respectively, which can cause corrosion of the reactor and processing system unless steps are taken to mitigate these effects. Base neutralization of these acids produces salts which can form solid precipitates under supercritical conditions.

Special Engineering Requirements of SCWO Processing Systems:

Unless catered for by careful engineering design, the high-temperature environment within SCWO reactors and processing systems can present significant reliability and performance problems, as discussed below.

Experience has shown that corrosion rates can be rapid when treating wastes containing halogens, such as chlorine. Corrosion-resistant alloys such as Hastelloy C-276 and Inconel 625 do not provide adequate protection against chloride attack under the oxidizing conditions found in SCWO systems. In recent years, SCWO reactors have been built using liners fabricated from titanium alloys. These have shown increased resistance to chloride attack. However, these reactors are limited to approximately 650 C maximum reaction temperature due to mechanical strength limitations of the pressure vessel wall.

The aqueous solubility of salt decreases sharply at supercritical pressures when the temperature rises above the critical temperature. If salts are present in the waste feed, or formed during processing, they will precipitate from solution wherever local temperatures exceed the critical temperature. Other relatively insoluble solid compounds, such as carbonates and metal oxides, are also commonly formed during SCWO processing. Undissolved solids are often present in the waste stream. Unless these solids are effectively transported through the supercritical regions or otherwise removed from the process, accumulations will form and total plugging of the reactor can occur. Furthermore, significantly higher corrosion rates have been observed beneath deposited solids. It is essential to control any tendency for solids to accumulate.

Wastes containing halogenated species and solids are practically a "fact-of-life" for SCWO waste treatment systems and most other waste treatment technologies. Traditional tubular or vat-type SCWO reactors have been incapable of addressing these problems in realistic plant applications. Summit Research has developed a novel SCWO reactor designed to handle corrosive species and solids in a straight-forward manner utilizing proven engineering principles. (Transpiring-Wall SCWO Reactor)

In traditional SCWO processing systems the entire water effluent stream is depressurized and subsequently disposed of or treated for re-use by the process at ambient pressure. We have developed a proprietary closed-cycle process that separates and recovers water for the process at full system pressure. (Closed-Cycle SCWO Process )

Traditional SCWO processing systems utilize shell & tube type heat exchangers which are prone to plugging and corrosion. Our system utilizes an open quench-cooled heat exchanger where cooling is achieved by directly mixing cooled liquid effluent with the hot reactor byproducts. This ensures that solids are heavily diluted and flushed from the system. It is also a more compact heat exchanger for high latent heat load applications. The cooled liquid effluent can be treated with caustic and other additives to control corrosion, effluent pH, solids, and composition of all effluent streams. (Closed-Cycle SCWO Process)

Operating Pressures for SCWO Processing

Traditional SCWO processing systems are designed to operate at pressures in excess of the critical pressure of water (P>220.55 bar). Traversing a supercritical isotherm from, say, 250 bar to lower pressure, the corresponding changes in physical properties of the reactants and byproducts are only gradual in nature. This suggests that a continuum of reactor operating points may exist along any supercritical isotherm. Recent investigations at Sandia National Laboratory [1] indicate that methane oxidation rates at 135 bar are significantly faster than at 270 bar, as follows;

supercritical, oxidation, SCWO, reactor, transpiring-wall

As for sustainability of the fundamental reaction mechanism of the Transpiring-Wall SCWO Reactor to lower pressures, Sandia National Laboratory [1] has observed diffusion-type hydrothermal oxidation reactions to pressures as low as 15 bar.

The Transpiring-wall SCWO reactor and Closed-Cycle Processing System is designed to operate and handle solids in a straightforward manner at all pressures, down to densities far below the critical pressure of water. This capability allows exploitation of improved reaction kinetics that may exist for some materials at subcritical pressures.

Obviously, at lower pressures the costs for process equipment and compression is significantly less than at supercritical pressures.

Advantages of Turbosystems Engineering's SCWO Technology

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