Off-gas treatment systems

A Biocube off-gas treatment system installed at a domestic wastewater pumping facility in June 1995 cost 15,000. The pumping facility treats approximately 200 m of water per day (D13550C, pp. 1-4). At a sewage lift station in Miami, Florida, a Biocube unit was installed at a cost of 90,000 (D221465, p. 1). 

According to researchers, limitations of DC arc systems include the corrosive nature of the vitrified material, limitations on salt and water content, and uncertain performance in the destruction of organic wastes. The addition of flux materials may be required to allow the vitrified material to be poured and to allow the final waste form to meet performance goals. Volatile radionuclides and metals may accumulate in the off-gas treatment system. 

In 1998, the DOE prepared a cost estimate of a 10-ton/day DC arc system. The system includes a furnace, waste feed system, off-gas treatment system, secondary combustion chamber, power supplies (arc power, glass overflow heating system, and metals drain), instrumentation, control systems, and product removal and handling systems. Site permitting costs and site preparation costs were also estimated (D207307). These estimates are summarized in Tables 1 and 2. 

Geotech Development Corporation offers a proprietary Cold Top ex situ vitrification process for the treatment of contaminated soil. The system melts the soil using an electric resistance furnace that can operate at temperatures of up to 5200°F. The vendor claims that wastes are transformed into an essentially monolithic, vitrified mass. The process is termed cold top vitrification because soil is added to the top of the melt to act as an insulator and to minimize the loss of volatile metals into the off-gas treatment system. The technology has been evaluated in a pilot-scale facility and is commercially available. 

A continuous circulation of groundwater is generated in the area surrounding the remediation well, as aquifer waters replace the annulus water. The circulation thus delivers new contaminants to the stripping zone. Volatile contaminants dissolved in the groundwater are transferred from the liquid to the gas phase and are extracted from the groundwater surface via a double-cased screen. Soil air from the unsaturated zone is also extracted and transported to the off-gas treatment system. 

Different waste streams require different feeder and off-gas treatment systems. 

Mercury or cadmium in the contaminated material may be volatilized during treatment and therefore would have to be captured by the off-gas treatment system. If mercury or cadmium is present in the off-gas waste, it will be captured in a scrubber solution that would then require drying and stabilization prior to disposal at a landfill licensed under Resource Conservation and Recovery Act (RCRA) criteria. 

In a high containment plant, the release of fuel solution would probably be below current detection limits. However, if minimal off-gas treatment is applied (i.e. caustic scrubbing and filtration is omitted) then there would be potential for the release of fine droplets of fuel solution. The release fractions of volatile radionuclides will depend on the type of off-gas treatment process used. An indication of the activity release for a 50 kg dissolution of 1000 MWd/t fuel, involving the use of different off-gas treatment system (consider, scrubber), is presented in Table 12.7 (after McMahon et al., 1993). 

Other configurations of treatment processes using thermal desorption as the primary separation technique can be applied to organically contaminated soils. Alternative physical/chemical processes can be used to treat the desorber off-gas and the contaminants. To achieve complete contaminant destruction, the off-gas can be treated by using conventional fume incineration or other thermal treatment technology. The choice of the type of desorber and off-gas treatment system depends on the concentration and properties of the chemical contaminants, soil characteristics, quantity of contaminated material, site characteristics, availability of off-site disposal, and regulatory and related requirements. 

The reactor is maintained at 90°C (194°F) during hydrolysis and blanketed with nitrogen. The pressure is maintained at 3 psig, and the reactor is vented to the MPT off-gas treatment system except during agent filling, when the vent is closed. 

ICBs to a clarifier for separation into sludge and overflow streams (Parsons, 2000e). The sludge was to be dewatered in filter presses and sent off site to a landfill. The filtrate from the filtration step was to be combined with the clarifier overflow, and the eombined stream (about 100 gallons/min) was to be sent to a brine evaporator. The distillate, about 90 percent of the feed, was to be recycled as process water. The bottoms were to be sent to an evaporator/crystallizer for additional water recovery and the crystallized salts sent off site for disposal the distillate was to be added to the recycled process-water sheam. However, EDS test results showed that (1) the clarifier is not needed, (2) the bioreactor effluent can be recycled without clarification, and (3) a slipstream can be sent to the evaporator for removal of salts and sludge. Vented air from the ICBs can be sent to the off-gas treatment system (Parsons, 2000e). 

Superheated steam at nominally atmospheric pressure is used as a sweep gas. The amount of steam is 50 percent in excess of the amount needed to destroy the maximum amount of agent preliminarily estimated to be present. The steam is supplied at nominally atmospheric pressure and 538°C (1,000°F) from a superheater, which also supplies steam to the batch MPT (Parsons, 2000a). An off-gas effluent heater heats the vent gas to 649°C (1,200°F) with a residence time of 0.5 seconds to ensure the destruction of any organics present. These gases are then sent to a quench tower, where they are contacted with a recirculating alkaUne brine solution. Vent gases from the quench tower are sent to the CATOX unit of the CST off-gas treatment system. A liquid purge from the quench tower is fed to the ICBs. 

The off-gas treatment system has six trains, each with its own CATOX unit (Parsons, 2000a). The monolithic catalyst beds, heaters, reactors, and control systems for each train are of conventional design (Parsons, 2000d). Four trains, one for each ICB module, serve only air vented from the ICB modules. The other two trains serve the MPTs and the CST. Figure 4-5 is a flow diagram of the off-gas treatment system. 

FIGURE 4-5 Off-gas treatment systems. Source Adapted from Parsons, 2000a. 

Process gases vented from the rotary MPT, batch MPT, ERD, and various process tanks are sent to the MPT quench tower, from which they are passed in series through a flame arrestor, a preheater, a CATOX unit, and a water-cooled heat exchanger. The cooled gases are then sent to the MDB ventilation system, which contains activated carbon adsorbers. The CST off-gas treatment system has the same design and capacity as the MPT off-gas system. 

Serious and damaging consequences may result from accidents caused by fires and explosions. Therefore, the incineration system and its off-gas treatment system should be designed to withstand the effects of the overpressure caused by an explosion, and provided with a suitably located pressure relief mechanism. Furthermore, the following measures shall be instituted to minimize the potential for explosions or fires ... 

During the conceptual design it is necessary to establish what constitutes waste feed. The waste feed characteristics and the relative volumes of the different waste categories to be incinerated will probably affect the selection of the combustion technique and also the basic concept of the off-gas treatment system. The distinct categories of wastes that may affect the design or selection of the incineration system are as follows ... 

Depending on the particular design of the overall incineration system (including the off-gas treatment system), some of the items or batches of waste having any of the following characteristics may have to be excluded from or minimized in the waste feed ... 

The combustion technique should be selected according to the waste feed characteristics, with the objective of achieving complete combustion and of meeting the primary goal of selecting incineration as a treatment method. An off-gas treatment system can subsequently be chosen on the basis of the particular combustion technique selected and the specific environmental and occupational requirements. 

The primary objective of off-gas treatment is to ensure that radiological and chemical releases to the environment are minimal and in compliance with the regulatory standards for the specific pollutants. Analysis of waste feed characteristics and the combustion conditions should determine the expected presence of objectionable materials in the off-gas, and therefore the type of treatment necessary. The off-gas treatment systems may consist of dry (non-aqueous) or wet (aqueous) components or process steps. Further information on the treatment concepts may be found in Annex in of this guide and in Ref. [9]. 

The off-gas treatment systems using dry (non-aqueous) cooling and treatment steps are commonly referred to as dry systems, as compared with wet systems that incorporate any of the wet (aqueous) steps. Systems using water or steam injection for partial temperature reduction without the scrubbing function, but otherwise containing dry process steps, are considered as dty systems. The wet... 

The wet off-gas treatment systems containing wet scrubbers have the capability to remove corrosive gases, and exhibit the highest decontamination factors for the total radionuclides. In particular, such a system would be used for processing wastes containing substantial amounts of corrosive acid gas forming materials. 

The dry off-gas treatment systems are simpler and more economical to operate. These systems may preferably be used if there are no specific reasons for the incorporation of wet scrubbers. The dry systems keep the off-gas temperature well above the dewpoint so that corrosion by acid gas condensation can be minimized. A dry system may be selected when processing tritiated wastes (subject to regulatory conditions and to tritium concentrations) since it will release tritium, together with the dry-filtered off-gas, into the envirotunent. This practice may present a lesser radiological hazard when compared with the aqueous scrubbing of tritium and the consequent need for the handling and conditioning of the tritiated scrub solutions. 

The operation of both the dry and wet off-gas treatment systems generates secondary radioactive wastes. These typically include filters and filtrafitm materials, adsorption materials, and in the case of wet scrubbers, liquid scrub solutions and blowdowns. Dry scrubbers would produce secondary wastes in the form of solid neutralizing agents and their residues, probably mixed with fly ash. Some of these secondary wastes (both solid and liquid) may be processed by incineration or drying in the same incineration system. Others need to be handled by a separate volume reduction system or disposed of with or without immobilization. The ash removed from the various collection points in the off-gas treatment system also needs to be treated and disposed of with the bulk of the ash collected from the combustion system. 

Although the use of a plasma torch increases the global electticity cost, it reduces the electrical power required by the off-gas treatment system (exhaust fans, scrubbers, particulate filters) because the off-gas flow rate is much lower than with a burner. Indeed, there is about an 80% off-gas flow rate reduction when using a plasma torch instead of a burner. Not only does this reduce operating costs of the off-gas treatment system in existing plants, it also reduces the capital cost for future plants because a smaller and less complex off-gas treatment system can be purchased. 

The replacement of fuel oil burners by plasma torches provides considerable operating costs reduction for existing plants as well as a capital cost reduction for future plants. The off-gas flow rate generated by a plasma torch being much less than that generated by a burner, the off-gas treatment system can be downsized significantly. In addition, plasma torches allow a major GHG reduction by avoiding the combustion of substantial amounts of fossil fuel in burners. 

An important feature in the ABWR is that the condenser plays a role of a deaerator. The system consists of the main deaerating condenser connected with air ejector and off-gas treatment system. 

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