CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
The Steam Detoxification process involves very high temperature steam reforming (ie. 1100 to 15000C) to destroy hazardous wastes. Vent gases are carbon dioxide and water. The process has been developed in the USA by Synthetica, and the proponent in Australia is Environautics Pty Ltd.
Information for this review has been provided by Environautics. Independent information on the process was not available for this review. Steam Detoxification consists of a two step process, and is carried out in a Pyrolysis Detoxifier. The hydrocarbon component of the waste is first evaporated in a first-stage waste feed evaporator unit and the vaporised gases are then mixed with superheated steam and fed into a "pyrolysis reactor" where they are further electrically heated under a slight vacuum. A carbon monoxide converter oxidises the detoxified gases and an activated carbon adsorber removes the last of the trace organics and metals.
The waste feed evaporator can take a variety of forms depending on the type of hazardous waste. Vapours are first exposed to superheated steam within the evaporator unit. In this unit, steam reforming chemistry is initiated at temperatures of 300 to 6000C. Steam reforming of the hydrocarbons produces carbon monoxide, water vapour, hydrogen and a small amount of methane. After the gases are mixed with excess superheated steam they are pulled into the main reactor, where the detoxification reaction proceeds to completion.
The vapour stream from the evaporator unit is pulled into the steam detoxifier unit under a slight vacuum. Vaporisation of the hazardous waste can occur from drums placed in the evaporator. After the steam pyrolysis detoxifier unit removes the toxic organics, hot steam reformed gas is generated and recirculated to the evaporator acting as a carrier for more hazardous wastes. A small vent stream of treated gas is slowly released to maintain the pressure balance in the system. The system is designed to be fully automatic, with the waste handler placing the waste in the evaporator unit, selecting the waste type, and initiating the waste destruction operation.
After the carbon monoxide, carbon dioxide, water vapour and hydrogen gases are formed in the main pyrolysis reactor, a small proportion of the gases is split off from the main flow and either fed to the catalytic carbon monoxide converter, fuel cell, synthesiser, or the reclaimer. In the carbon monoxide converter excess air is added to convert the detoxified gases to carbon dioxide and water vapour. The heat of reaction released in the converter is recovered and reused in the process. The carbon monoxide converter catalyst also destroys any residual contaminants that may be present. The fuel cell can generate electric power, the synthesiser can produce organic by-products, and the hydrogen reclaimer can recover hydrogen for fuel.
Although the steam reforming process is initiated in the evaporator unit, the reaction continues as the gases are raised in temperature by the superheated steam and is completed in the pyrolysis reactor where a residence time of more than one second at elevated temperatures in the nearly isothermal reactor ensures essentially complete reaction of the waste materials. This conversion process is claimed to be significantly more effective than in high temperature incinerators where the gases are only briefly exposed to the high temperature incinerator flame. As the reactor is heated electrically the gases are free of the fuel combustion particulates common to incinerator systems.
The demonstration unit evaluated was 120 cm x 180 cm x 230 cm in size, and was transported to various treatment sites. A schematic detailing the operational features is shown in Figure 14.1.
The demonstration unit is capable of treating up to 5 drums or one ton per day, depending on the type of waste being processed.
A range of organic tracer compounds have been evaluated, with DREs of 99.99% claimed for even the most refractory organics.
With no significant oxygen but excess steam, typical aromatics such as benzene, toluene and naphthalene were reduced from percent levels to ppm levels. The compound 2,3,7,8 TCDD was not detected at the detection limit of 0.2 picogram/L. Detailed GC/MS analysis of the gas stream indicates that the aromatic emissions are above that expected based on equilibrium thermodynamic relationships, but are well below levels that would pose a significant risk.
The following is a summary of the high temperature reactor emissions. These results were obtained from a halocarbon solvent mixture of 66% acetone, 32% xylene, 1% 1,1,1-TCA and 1,2-dichlorobenzene, operating without any air emission abatement devices.
|Emission Gas||Level||Emission Gas||Level|
|Carbon monoxide||2,000 ppm||NOx||0.3 ppm|
|Particulate mat PM10||< background||Hydrogen chloride||4 ppm|
|1,2 Dichlorobenzene||4 ppm||1,1,1 TCA||3 ppm|
|2,3,7,8 TCDD||< 0.3 ppt||1,1,2 TCA||< 0.3 ppm|
|Acetone||30 ppm||Xylene||30 ppm|
|Benzene||9,870 ppm||Toluene||300 ppm|
|Ethylbenzene||< 40 ppm||Styrene||40 ppm|
|Benzyl chloride||40 ppm||Naphthalene||40 ppm|
Various abatement devices have been tested including the carbon monoxide catalytic converter which removes > 97% of the organics, and an activated carbon bed which can remove > 95% of the organics and volatile inorganics, and a halogen adsorber which can remove 95% of halogenated products. Application of these devices to emissions from the pyrolysis reactor typically results in a reduction in organic emissions by a factor of more than 500. The detection limits reported in Table 14.1 for dioxins are not useful, as they are higher than typical regulatory levels (0.1 ng/m3).
The steam reforming pyrolysis reactor can destruct hazardous wastes sourced from the electronics, petrochemical, petroleum and general manufacturing industries. The demonstration unit is mobile and the promoters claim the unit can:
Liquid organic hazardous wastes can be fed directly into the evaporator, with little or no pre-processing. The unit can process low concentration wastes such as aqueous waste or groundwater.
Heavy solid/liquid slurries or thick liquids that are difficult to vaporise can be fed into a moving bed evaporator that vaporises volatile constituents and produces a powdered residue at the outlet. The vapour stream leaves at the top of the moving bed evaporator and can be drawn directly into the steam reforming pyrolysis reactor unit.
For wastes that are integrally mixed with municipal, hospital, industrial or other refuse a heated shredder evaporator can be used. This unit shreds and grinds the waste and simultaneously heats and volatilises the organic contaminants. The solid residue, stripped of its volatile components, is then disinfected and ready for disposal.
Solvent or organic hazardous waste vapour or contaminated ground water can be processed by the reactor reprocessing scheme via the activated carbon adsorption bed which concentrates the hazardous waste. After concentration the carbon canister is reactivated by the reactor unit either in situ with hot gas from the reactor unit or from small canisters in the drum feed evaporator.
The process requires potentially elaborate gas treatment systems, and the overall system can be expected to be of similar complexity to the Eco Logic system.
The technology has not yet been introduced into Australia.
(a) Proponents (in Australia)
Environautics Pty. Ltd., Queensland
(b) Wastes Applicable
Waste types include waste solvents, aqueous wastes, paint sludges, laboratory wastes, degreasing sludges, waste organic products, mixed organics, PCB contaminated wastes, pharmaceutical wastes, and biomedical wastes.
(c) Contaminants Applicable
A wide range of organic compounds. Those on which the process has been trialed include acetone, carbon tetrachloride, chloroform, dichlorobenzene, isopropanol, methanol, methylene chloride, methyl isobutyl ketone, 1,1,1trichloroethane, and xylene.
The technology has reached the commercialisation phase in the US and is represented locally by Environautics in Queensland.
(e) Timing for Commercialisation in Australia
Unknown. Dependent on commercial viability and regulatory approvals. There is the potential for a facility to be established within the next few years depending on the experience gained with commercial scale units in the US.
Costs are estimated by the proponent at approximately half the average cost of incineration or landfill in the US.
(g) Safety/Environmental Risk
The process operates at high temperatures and low pressure in a closed treatment facility. End products are mainly carbon dioxide and water vapour which are vented from the unit during operation. Some of the solid residues left over after the evaporation stage may require solidification and fixation before landfill disposal.
The high temperatures used in the process require specialised equipment and reactor materials. The process requires containment of potentially hazardous gases at high temperatures and therefore will require careful design and operation.
(h) Non-Technical Impediments
None known at this stage.
(i) Preferred Mode of Implementation
Mobile or permanent installation units.
The solid waste residue products may require suitable treatment before disposal. This technology is mainly limited to the treatment of liquid or aqueous wastes.
|Chapter 13 - Plasma Arc Systems||Chapter 15 - Supercritical Water Oxidation|