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The Transpiring-Wall Reactor ("TWR")

A key feature of the TWR reactor is that all supercritical regions of the process are fully enclosed within a permeable, or "transpiring" wall.

Principles of Design & Operation:

At the top end of the reactor, supercritical water and oxidizer, typically compressed air or oxygen, is injected into the reactor vessel annulus, and into the fuel injector assembly. Waste is also injected into the fuel injector assembly. Supercritical water/oxidizer mixture to the injector is mixed at an approximately 100 to 150% stoichiometric ratio with the waste.

The modular design shown allows the swirl chamber to be fabricated from ceramic and/or high temperature alloy. The waste injector nozzle is designed to allow easy removal for cleaning, inspection or replacement.

The supercritical water/oxidizer mixture to the annulus is distributed evenly around the annulus. The flow pattern through the transpiring wall is designed to provide cooling to the transpiring wall, continuously divert and flush solid particles away from the wall, dilute the concentration of corrosive species along the wall, and provide a source of oxidizer for any unreacted species near the wall. The fluid flowing through the transpiring wall is largely non-reacting since the oxidizer was injected in super-stoichiometric proportions via the fuel injector. A key feature of the combustion mechanism is that "over-ventilated" conditions exist at all locations downstream of the injector nozzle, thereby preventing reactions from occurring at the wall...this is a routine feature of gas turbine combustors and is nothing new. The transpiring-wall liner is designed and installed such that differential thermal expansion and contraction is unhindered, allowing use of ceramic materials for this component, if so desired.

Central reaction zone is sized to provide a residence time sufficient to achieve the final design DRE. Upon exiting reaction zone, the hot reaction by-products enter a transpiring-wall quench cooler located at the bottom end of the reactor where they are thoroughly mixed and scrubbed by cooled liquid brine recirculated from the bottom of the brine separator, or by direct injection of coolant. Again, similar benefits are derived from utilizing transpiring-wall technology in the quench cooler. For example, all free solids passing theough the main reaction chamber will be continuously flushed from the quench cooler, and/or possibly dissolved in the quenching fluid. Caustic or other additives may be added to the cooled brine for beneficial effect, such as control of effluent composition, prior to re-injection of the brine into the quench cooler. The quench cooler is designed such that the lower seal between pressure vessel and transpiring-wall component is free-floating, allowing unhindered differential expansion and contraction and the use of corrosion-resistant ceramic materials, if so desired. The resulting mixture of cooled reaction products and brine then leave the reactor vessel and enter a vapor/liquid separator for further treatment. (Closed-Cycle SCWO System)

Advantages of the Transpiring-Wall SCWO Reactor & Closed-Cycle SCWO Processing System:

Cost-Effectiveness
The Turbosystems TWR can be built for a fraction of the cost of a platelet-type TWR.

For a 2" diameter transpiring-wall combustion liner, platelet prices have been quoted between $10,000 to $20,000 PER INCH of liner length. For a 36" long liner, this translates to US$ 360,000-720,000 for the platelet liner alone. A 36" long porous metallic liner for the Turbosystems TWR costs less than US$ 5,000.

Clearly, the patented Turbosystems porous-wall TWR commands an insurmountable cost advantage over the platelet-type TWR, and is the clear choice for commercial applications where cost matters.

Experimental results at FZK Karlsruhe demonsrate that the TWR can destroy organic compounds at inlet concentrations up to 30%, versus 2-5% for a pipe reactor. The vertical TWR has a much lower surface to volume ratio than a pipe reactor. Another way to look at this is that the TWR can treat substantially more waste "per pound of metal" than a pipe reactor, and therefore should cost less as well as being more compact.

Resistance to Corrosion
An important advantage of the design is that all high temperature reaction-containing zones and quench-cooler chamber can be fabricated from materials chosen primarily for their resistance to high temperature corrosion and oxidation. Material tensile properties are not the primary consideration in these critical areas. Therefore, advanced corrosion-resistant materials, including ceramics and ceramic composites, may be chosen for these components. Furthermore, the quench cooler concept allows neutralization of the reaction products for pH control prior to entering the brine separator, thereby mitigating corrosion attack of the brine separator internals. Finally, total segregation of oxidizer and waste stream prior to injection into the waste injector avoids high temperature oxidizing acidic conditions within feed stream pre-heaters, thereby mitigating corrosion attack.

Resistance to Solids Deposition
An important advantage of the design is that the flow of supercritical water and oxidizer through the transpiring-wall creates a protective boundary layer, which serves primarily to resist deposition of solid particles, as may be present within the reaction zone. In the event of salt build-up, the transpiration flow temperature can be temporarily reduced to the vicinity of the critical point, where salt solubilities increase sharply. This "trans-critical" cleaning cycle can be performed on a regular or as-needed basis.

Furthermore, the quench cooler concept defers base neutralization of acid gases produced in the reaction zone until the cooling operation, whereby any produced salts are immediately dissolved and flushed away by the re-injected chemically-treated brine.

Emissions Control
An important advantage of the design is that the recirculated brine injected into the quench cooler can be dosed with a variety of chemical additives for controlling effluent characteristics and composition. For example, one may inject caustic such as NaOH to neutralize HCL. Thus, the quench cooler also operates as an effective effluent scrubbing system, as well as cooling and solids transport system. When the transpiring-wall SCWO reactor is incorporated into the Closed-Cycle SCWO Processing System, the gaseous effluent stream is scrubbed TWICE prior to de-pressurization and release to the vent stack; (1) first in the reactor quench cooler, and (2) again in the water recovery quench cooler (re: Figure 3 ). Thus, there are two opportunities for post-oxidation treatment of gaseous effluent inherent to the design!

Another important advantage of the design is that all excess water is removed from the system either; (a) as water vapor along with the non-condensable combustion gases, or (b) as water vapor flashed off during depressurization of hot brine to atmospheric pressure. Isothermal depressurization of hot brine to atmospheric pressure is expected to produce a dry solid particulate waste material.

Variable Pressure Operation
An important advantage of the design is that the entire process can be operated over a wide range of pressures spanning the supercritical and subcritical range. This is possible because water, organics and commonly-used oxidizers are entirely miscible so long as the water is maintained in a high temperature vapor state. As mentioned elsewhere, Sandia National Laboratory has recently observed that reaction kinetics for SCWO destruction of methane peak at pressures below the critical pressure of water. Thus, the design allows exploitation of possible improved reaction kinetics at lower pressures. Furthermore, at lower pressures, plant capital costs, operating costs and overall reliability are all appreciably enhanced.

Recycling and Reutilization of Process Water
An important advantage of the design is that all water required by the process is continuously recovered and recirculated within the closed-cycle system. (Ref: Figure 3) The quench cooler operates by re-injecting cooled liquid brine recovered from the brine separator. The transpiring-wall reactor operates using water recovered from the hot vapor stream leaving the brine separator. To maintain mass balance, only that water produced by hydrogen oxidation or free-water introduced with the waste/fuel stream is ultimately ejected from the system, as water vapor.

Pressure Vessel Isolation
An important advantage of the design is that the pressure vessel walls are isolated from the hot, corrosive reaction products. The pressure vessel does not have to be manufactured to resist both corrosive attack and thermal degradation at the highest temperatures within the main reaction chamber. Furthermore, the transpiring wall is subjected to relatively low mechanical stresses, albeit at high temperature. There exists a wide variety of engineering alloys, ceramics and ceramic composites with combined resistance to both high-temperature and corrosion.The lower pressure vessel wall temperature results in exended reactor vessel life and reliability, while at the same time minimizing vessel weight and cost.


Increased Operating Temperatures
An important advantage of the design is thermal isolation of the pressure vessel, as described above. This thermal isolation should enable operation of the main reaction chamber at temperatures up to 1,000ºC and higher, if desirable. Maximum operating temperature of tubular or vat-type SCWO reactors is limited to approximately 650ºC because of material allowable stress limitations. High temperature operation should result in higher DRE and/or reductions in required reactor residence time, volume and cost.

Increased Safety
An important advantage of the design is increased operating safety, as the reactants remain fully segregated until mixed in the main reaction chamber.

It is important to note that due to the structure of the diffusion-type hydrothermal reaction zone, conventional reactor designs where oxidizer is injected into the reaction zone may experience localized "hot spots" in the vicinity of these oxidizer injection points where temperatures far exceed allowable metal temperatures. The Transpiring-Wall SCWO Reactor design inherently avoids such hazardous situations.

Utilization of Proven Technology
An important advantage of the design is the utilization of proven aerodynamic design and fabrication technologies for building the transpiring wall components. Transpiring wall devices have been used for decades to contain high temperature combustion reactions in gas turbines engines and rocket engines where peak reaction temperatures can exceed 3,500 K. The transpiring wall may be constructed from materials such as porous ceramics, or from a variety of semi-porous "quasi-transpiration" techniques as used for film-cooling and effusion-cooling of gas turbine and rocket engine combustion chambers.

Modularized Reactor Design
An important advantage of the design is that all reactor vessel internals can be easily replaced in the event of changing operating conditions, waste stream characteristics or wear. This allows such things as (a) optimization of reactor operations at off-design condition by installing suitably optimized internal components, (b) oxidation of more (or less) corrosive wastes/fuels by installing internals fabricated from more (or less) expensive or application-focused materials of construction, (c) oxidation of wastes/fuels containing higher (or lower) quantities of solids, by installing internals optimized for solids content of the particular waste/fuel stream, or (d) rapid inspection and/or replacement of internals during plant turnaround or waste/fuel stream changeover.


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