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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
- 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.
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;
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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