4.3.1.2 Chemical Plasma Process
SJT Consultants
2697 Steeles Avenue West
Toronto, Ontario
Contact: Dr. Stanley Townsend
(416) 661-2685
This process involves the controlled burning of fuel and liquid waste in pure oxygen in such a manner as to maximize the temperature achieved in the combustion zone. The process has grown out of magneto-hydrodynamic (MHD) process development. In the MHD processes developed by SJT, plasmas were created from the chemical energy available from combustion. Temperatures in excess of 5000°C are achieved. Their MHD reactors have been modified with gas scrubbing equipment. Hydrochloric neutralization is achieved by caustic injection along with the waste feed or in the scrubber.
Limited information is available on this process or on the status of their testing program.
The future development plan for this process is to make the unit mobile.
References: (Townsend, 1982; Johnston, 1982)
4.3.2 Molten Salt Processes
Molten salt processes involve oxidation of waste PCBs with the same chemical result as the foregoing combustion processes. These processes are different because they take place in a condensed phase while still operating at relatively high temperature. The effects are twofold:
- wastes are contacted with a more concentrated oxidant, i.e. oxygen dissolved in the molten phase; and
- wastes are preheated more rapidly up to reaction temperature since the molten medium has greater heat capacity per unit volume.
As a result lower temperatures and shorter residence time accomplish similar destruction efficiencies. Another potential advantage is the ease with which neutralizing chemicals can be used directly in the combustion zone, simplifying the process by avoiding the acid gas scrubbing after the destruction reaction.
4.3.2.1 Rockwell's Molten Salt Destruction Process
Head Office: Rockwell International Energy Systems Group
8900 DeSoto Avenue
Canoga Park, California 91304
Contact: J.G. Johansen
(213) 700-3508
The Rockwell Molten Salt Destruction Process and technology achieves essentially complete decomposition and destruction of hazardous chemical wastes by exposing these materials to a pool of molten sodium carbonate at temperatures ranging from 840 to 982°C. The process is highly efficient and has been effectively demonstrated in both deficient and excess oxygen modes of operation.
In the process, waste and air are introduced beneath the surface of a pool of molten salt in a vertical, cylindrical, atmospheric pressure vessel. Both solid and liquid wastes can be handled.
If the chemical waste is combustible (minimum of 10 467 J/g), exothermic reaction takes place rapidly, and the waste material breaks down into its constituents which are realeased as gases: carbon dioxide from the carbon element in the waste and steam from the hydrogen component. If sulphur, phosphorus, chlorine, or other halogens are constituents of the waste, they are retained in the melt as the corresponding sodium salt. Uncombustible portions of the wastes are retained as ash in the melt.
In cases where wastes to be disposed of have a Btu value lower than 10 467 J/g, a combustible material must be co-burned with the waste to increase its heating value. This additional material could be another waste or some inexpensive, combustible material such as coke, waste oil, scrap solvent, or coal. This supplemental heat is required only when low-Btu value wastes are destroyed. In some cases, combustion of the waste may be enhanced by way of oxygen-enriched air, in which event no other combustible material may be necessary.
Since the reaction of the waste in the melt is rapid and complete, the effluents require little or no further cleanup and present no undesirable elements to the atmosphere. This environmental acceptability may lend itself to readily approved siting and permitting. MSD plants require minimal size sites, are simple to operate, and are not prone to upset.
The Rockwell molten salt technology also lends itself to recovery of energy or material resources from the waste. The gases are produced at temperatures ranging up to 982°C and can be conducted through waste heat recovery systems to produce steam or power. Valuable metals (silver, vanadium, tin, and chromium) which may occur in some wastes may be recovered as either pure metal or a metallic salt from which the metal may be recovered by standard chemical procedures.
The spent salt is withdrawn and allowed to solidify, following which it can be disposed of as non-hazardous ash.
Rockwell has engineered and designed two basic combustor systems. The Santa Susana Test Facility has a multi-purpose ceramic-lined vessel with appropriate equipment to feed the waste and withdraw the molten salt, either continuously or on a batch basis. Instrumentation continuously monitors stack gases during operation.
The Molten Salt Combustor is an all metal combustor vessel providing for continuous waste feed and a spent melt overflow discharge which takes place as wastes are combusted and ash builds up in the melt (see Figure 6).
A simplified diagram and flow chart of a commercial system using the combustor design of Figure 6 is shown in Figure 7. The system incorporates three all-metal combustor modules with a capacity of up to 1 tonne/h of waste. This equipment will handle liquid or solid wastes such as PCBs with destruction efficiency of 99.9999% or more.
A schematic of a transportable system, available in 1983, using the combustor in Figure 6 which will handle up to 102.1 kg/h of halogenated-type waste, such as liquid PCB, or other combustible chemical wastes is shown in Figure 8. This unit could be installed onsite to handle small quantities of waste as produced. Alternately, it can be moved from place to place for destruction of larger quantities of accumulated waste.
Rockwell has conducted test burns on many typical varieties of hazardous wastes with destruction values of at least 99.9999%. Extremely sensitive gas analytical equipment and sampling devices have been involved in these tests. In all cases, destruction efficiency exceeds minimum EPA requirements and off-gas quality is far below EPA thresholds.
4.3.3 Fluid Bed Processes
Fluid bed processes operate on the same general principle as molten beds except that the bed material is actually a solid. The solid, in a fine granular state is fluidized by blowing a gas through it. The simplest applications use air as the fluidizing gas. The bed is heated by combustion of supplementary fuel and/or waste. The reaction of material fed to the bed is facilitated as before by the heat content of the bed. The reactions will take place in the gaseous phase rather that in the molten phase as before.
Figure 6 Schematic of Molten Salt Combustor - Commercial (Rockwell International)
Click image to view a full size version.
Figure 7 MSD - Commercial Stationary Unit (Rockwell International)
Click image to view a full size version.
Figure 8 Portable MSD Unit (Rockwell International)
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4.3.3.1 Rockwell's Fluidized Bed Incinerator
Head Office: Rockwell International
Rocky Flats Plant, Energy Systems Group
P.O. Box 464
Golden, Colorado 80401
Contact: Jerry Langheim,
Manager, External Communications
(303) 497-4255
The Rockwell fluidized bed incinerator uses two fluidized bed reactors arranged in series to carry out the destruction of PCBs. The pilot testing facility, has been operated for testing of PCB destruction (see Figure 9).
The bed material in the primary reactor was sodium carbonate with a CR203 catalyst (20% Cr203 and A1203). Hydrochloric acid produced by combusting PCBs reacts with the sodium carbonate portion of the bed material to form sodium chloride. This is a surface reaction on the sodium carbonate particles. Sodium chloride is removed from the surface of the sodium carbonate particles by attrition caused by the fluidized condition of the bed. The sodium chloride and some sodium carbonate is elutriated from the bed by the fluidizing gas, preventing excessive buildup of sodium chloride in the bed.
The beds are preheated by burning kerosene in air. The primary fluid bed reactor is operated at 590-600°C. When waste or fuel/waste mixtures are being fed the temperature is controlled by diluting combustion air with nitrogen. This pilot unit was tested with 13% PCB in kerosene fed at a rate of 3 kg/h. Off gas from this fluid bed reactor is cycloned to remove sodium chloride and ash, and fed to a second fluid bed reactor loaded with catalyst, operating at approximately 700°C. This second reactor acts as a catalytic after-burner. Gas from the second reactor is cycloned and passed through sintered metal filters prior to sampling. In this test rig an air ejector provides the motive force for the gases and is backed up by high efficiency particulate air filters. The unit was designed originally to burn transuranic waste so a high degree of emission protection was used. For PCB destruction this particular configuration may not be needed.
A significant feature of this process is the all metal construction. Process temperatures are low enough to avoid the use of refractories.
At this point only brief tests on PCBs have been completed so that details of waste types that can be fed and residue removal practices have not been tested. Destruction efficiencies observed in the PCB test burn exceeded 99.9999%. The observed combustion efficiency was 99.89% (CO2/CO2+CO) with 9.3% excess oxygen. NO x emissions were low and HCI removal efficiency was 98% |
