CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
Thermal desorption has been employed widely as a means of removing contaminants from solid waste streams. In some cases, treatment may be effected within the thermal desorption unit, whereas in other cases, the off gases are subject to separate treatment.
Thermal desorption has been used or is proposed to be used as a pretreatment step in conjunction with the following technologies:
For further discussion of pretreatment processes, refer to Section 19.
Thermal desorption has also been employed or is proposed to be used in a number of processes where some contaminant degradation occurs within the thermal desorption unit, followed by combustion of the off gases. Examples include:
These processes are discussed in greater detail in the following sections.
16.2.1 Technology Description
The PCS Technology (also named Product Control Soméus Environment Control Technology) is a technology developed by Soméus & Partners. The process is based on thermal desorption and uses a flash pyrolysis technique to decompose organic and/or inorganic solid hazardous and solid waste, followed by combustion.
Soméus & Partners advises that the main components of the indirect thermal treatment method and apparatus include:
The rotary reactor operates on a continuous basis. Heat is transferred indirectly, supplied from hot flue gases through the mantle of the reactor.
Following pyrolysis, the vapour phase is combusted and then rapidly cooled. The gas stream is cleaned in a wet gas multi venturi scrubber prior to discharge. Although dioxin and furan gases are not generally formed in a reductive environment, it is possible that they could be formed following the combustion step. Therefore, following combustion, the resulting gases are treated by scrubbing. The scrubber process water is cleaned, neutralised and water recirculated. Heavy metals dissolved in the scrubber water are neutralised, precipitated and separated as hydroxides.
The solid phase is cooled indirectly and is discharged for later use.
The proponent advises that the process applications include:
In the context of this report the pretreatment of PCB contaminated soils and related wastes is of primary interest.
Soméus & Partners states that the technology can treat the following waste streams:
The standard arrangement for a PCS treatment system is for a fixed treatment unit, however, a mobile treatment unit is available as an optional alternative.
For a solid hazardous waste treatment PCS System with a 14000 m3/year input capacity, the basic cost of machinery is estimated at approximately $7.5 million US (ex works) and the treatment cost for the first year of operation is estimated to be in the order of $470/m3 (US$350/m3) (when amortised over a 5 year payback period).
The anticipated PCS thermal treatment process emissions have been compared to European Commission Draft legislation. It has been indicated that the anticipated emissions are below the draft criteria and that PCS is likely to meet the criteria at a reduced cost and with improved technical safety compared to other thermal treatment methods. The proponent anticipates destruction and removal efficiencies (DREs) greater than:
No information was provided regarding PCB concentrations in the treated material that are likely to be achieved. A summary of process emissions from the PCS system, based on 11% oxygen in exhaust gas as advised by Soméus and Partners (1997) is given in Table 16.1.
|Emission Type||Flue Gas Analysis (mg/Nm3)|
|Total Organic Carbon||1|
|Chlorine (as HCl)||2|
|Fluorine (as HF)||0.1|
|Other heavy metals||0.007|
With regard to safety and environmental risk, it should be noted that the PCS is operated under a reduced pressure and does not use large amounts of excess air or water during treatment. Also, the post treatment of the gas and solid phases are separate from the main process treatment.
16.2.3 Considerations in the Application of the Technology
The PCS process has been designed for the treatment of mixed solid hazardous waste. Size reduction (<5 mm) may be required and drying to a moisture content below 15%.
Metal hydroxides from treatment of the waste water require disposal (this can be expected to involve dewatering, immobilisation if necessary, and landfill disposal).
The PCS technology is not considered suitable for:
16.2.4 Experience and Availability in Australia
Pilot plant tests were undertaken in Europe during 1990-94 using a rotary kiln of 0.5 m3/hr capacity in continuous operation. The process is being promoted for use at a commercial scale in Australia by ITS Engineering Services. However, the process is yet to be applied at full scale.
Detailed manufacturing designs have been completed for full scale units, including a unit with a processing capacity of 5 tons/hr of PCB contaminated soil. Soméus & Partners expect to offer PCS technology on a commercial basis after 1997.
(a) Proponents (in Australia)
ITS Engineering Services (Cooma, NSW) are acting as agents for Soméus and Partners in Australia.
(b) Wastes Applicable
The PCS technology is suitable for a range of solid materials including contaminated soil and sludges. The PCS technology is expected to be suitable for treatment of solid OCP materials.
The PCS technology is not suitable for liquids (water, flammable liquids and solvents), explosive or highly oxidising materials.
(c) Contaminants Applicable
Proponents claim full range of chlorinated hydrocarbons, PCBs, organochlorine pesticides and other organic and inorganic materials that can be decomposed by reductive thermal treatment below 600 0C.
The technology has been demonstrated on a pilot scale in Europe.
(e) Timing for Commercialisation in Australia
Uncertain. Being considered for commercialisation internationally in 1997.
(f) Cost (Example only)
Information on treatment costs provided by proponents indicates a cost of approximately $470/m3 for contaminated soils and similar wastes.
(g) Safety/Environmental Risk
The PCS technology operates at a slight negative pressure reducing the potential for fugitive emissions. Off-gas volumes produced by the PCS process are lower than for many other thermal processes. Combustion of off-gases requires careful control and emissions treatment to minimise dioxin formation.
(h) Non-technical Impediments
No information provided at this stage.
(i) Preferred Mode of Implementation
Relocatable unit or centralised treatment facility.
Solid wastes. Contaminants able to be decomposed reductively at 6000C. Careful off-gas treatment required to ensure dioxin formation is minimised following combustion of gases in the pyrolysation chamber.
16.3.1 Technology Description
The TFS Thermal Desorption Retort Technology for the treatment of contaminated soils has been developed in Australia by Tox Free Systems (TFS). TFS and related companies developed retorting systems in the US during the late 1970's, and these systems have been adapted for a number of uses including the treatment of contaminated soils containing volatile or some semi-volatile contaminants (such as OCPs). In the context of scheduled wastes, the TFS process has been configured for the treatment of pesticide contaminated soils, particularly from dip sites. While contaminated soil is not strictly a scheduled waste, the process may be applicable to other OCP contaminated solid and semi-solid wastes (Stone, 1997).
The system involves an indirectly fired retort that is used to remove the volatile materials by way of an off gas-vent, leaving the treated soil for return to its original site. The retort operates on a continuous basis under negative pressure, and under neutral conditions (ie neither oxidising nor reducing) resulting in some leakage of air into the system.
Contaminated soil is fed to the retort via a hopper and an auger. The treated soil leaves the retort via an overflow launder from where it is transferred to a stockpile. The soil is quarantined in the stock pile pending analysis and validation of remediation.
The retort contents are indirectly heated. A combustion chamber surrounds the retort. The retort contents are initially brought up to operating temperature by heating a batch charge of inert material. When this mass is at opening temperature, feed will be commenced. Bed temperatures are monitored to ensure that conditions are maintained by varying either the feed rate or the firing rate. Temperatures will normally be in the range of 400o - 700o C depending on the residence time required, the contaminant to be desorbed and the soil properties. Typically in treating organochlorine pesticide contaminated soils the retort operates with a bed temperature of around 450oC to 500oC.
In the retort the OCPs are volatilised and/or decomposed and leave as part of the off-gas. Approximately 20% of any arsenic present is also volatilised as arsenic fume.
The off-gases are then drawn by a fan through a hot gas filtration system that removes particulates allowing the cleaned gases to go to an afterburner for destruction of organics. The high temperature filter temperature is maintained slightly above that of the retort temperature to prevent condensation. The afterburner is designed to operate at 1100oC with a two second residence time.
From the afterburner, the gases are quenched to minimise dioxin and/or furan formation. After the gases are quenched, solid calcium hydroxide or calcium oxide is injected into the gas stream in excess of the stoichiometric rate to neutralise the HCl formed. Residual organics are also absorbed on the lime. Any solids, including condensed arsenic fume, are collected in a gas filter. The gases are then discharged to atmosphere.
In an alternative treatment system, the off gases may be condensed after leaving the high temperature filter. Solids are filtered from the condensate which then passes through an activated carbon filter after which it may be discharged for further treatment or added back to treated soil (assuming the contaminants of interest have been removed by the carbon filter). Those components that are not readily condensed are further treated as necessary before discharge.
Process flow diagram options are attached as Figures 17.1 and 17.2 respectively.
Two units are available in Australia both of which are transportable.
Thermal Desorption Retort Technology
Flow Sheet Option 2
Trials have been conducted in a nominal 50 kg/hr pilot plant using OCP contaminated soil from a number of dip sites. These results are summarised in Table 16.1.
|Dip Sites||Contaminant||Contaminant Concentration (mg/kg)|
|Cobaki - Run 1||DDE
|Cobaki - Run 2||DDE
The trials showing low residual OCP concentrations were conducted at low retort temperatures but, even so, gave residuals below the prescribed levels for disposal. In subsequent trials at higher temperatures the OCPs were removed to below the level of reporting.
Trials on other materials have demonstrated the ability of the retort to remove a range of volatile and semi-volatile contaminants from soil.
16.3.3 Experience and Availability in Australia
The technology has been developed and used in OCP application on a pilot plant scale, and in hydrocarbon application on a commercial scale.
Two retorts have been built with nominal capabilities of 2 tonnes/h and 50 kg/hr respectively.
A licence to trial the treatment of OCP contaminated soil in the nominal 2 tonnes/h plant was granted under the NSW Environmentally Hazardous Chemicals Act. This licence has expired without trials having been carried out as the equipment was fully utilised on other projects. The NSW EPA has indicated it would re-issue the licence when the company was in a position to proceed with trial work.
The plants are licensed in Queensland under the Environmental Protection Act 1994 to treat contaminated soils, sludges and other materials where the contaminants are products and residues from the processing of fossil fuels. They are operating commercially on hydrocarbon contaminated soil at oil refineries and in the near future at a gasworks site, however, this work has not extended to OCP contaminated soil.
16.3.4 Consideration in the Application of the Technology
The technology consists of two major operations; the thermal desorption of the contaminant from the soil, and the treatment of the resulting off gases.
Soils to be treated must have oversize material screened out prior to processing. If required this can be milled or washed for processing, or treated as clean.
The unit is mobile, offering the advantage of treating contaminated soils on site. The associated benefits include the elimination of transport risk, and the minimisation of operational risk in a fixed location.
A number of options are available for treatment of the retort off gases including condensation. A range of gas treatment systems can also be used alone or in conjunction with condensation.
Arsenic content of the soil is also partially reduced (usually less than 50% removal), and collaborative research is to be undertaken with NSW Agriculture aimed at immobilisation of the residual arsenic in the soil.
When treating OCP contaminated waste, careful operation of the offgas treatment system (filtration, afterburner and scrubber) is required to minimise the formation of dioxins and furans.
The system has been developed in Australia by Tox Free Systems Limited.
(b) Wastes Applicable
The system is applicable to waste soils and sludges including soils from former cattle dips.
(c) Contaminants Applicable
A range of volatile and semi volatile contaminants can be treated including OCPs.
A trial licence under the National Protocol for Treatment/Disposal of Schedule X wastes has been approved by NSW EPA and is awaiting activation. The process is being implemented commercially for non-scheduled wastes and bench-scale trials have been conducted for OCP contaminated soils.
(e) Timing for Commercialisation in Australia
TFS plans to apply for a commercial licence for the treatment of solid and semi-solid materials under the National Protocol for Treatment/Disposal of Schedule X wastes in soil within the next six months.
As the plant is a mobile unit costs vary considerably depending on the location, operating hours allowed on a particular site, and the quantity of soil to be treated. While specific information was not provided by TFS, costs similar to those for other thermal desorption based systems may be expected (say, $150 to $500 tonne depending on the scale of the project).
(g) Safety/Environmental Risk
There is a gaseous emission from the plant which is subject to stringent treatment prior to discharge. Process conditions have been selected to minimise the risk of dioxin and furan formation, and pollution control equipment selected to treat these in the event that small quantities are formed.
The plant is operated under a slight vacuum to minimise fugitive emissions.
Normal safety and regulating controls are installed on the LPG fuel system.
(h) Non Technical Impediments
Although some elements of combustion are involved, the quantities are small. The process can be differentiated from incineration and should be more readily accepted by the public.
(i) Preferred Mode of Implementation
The system is mobile although a centralised system could be built.
The process is only able to treat solids and sludges, although liquids (eg OCP formulations) could be treated by first producing a slurry.
Only volatile or semi-volatile contaminants (such as OCPs) can be handled. Treatment of less volatile compounds such as PCBs is not proposed at this stage.
ADI has developed both direct and indirect fired thermal desorption units which have varying applications in the treatment of scheduled wastes. The Indirect Thermal Desorption (ITD) unit has been used largely in conjunction with BCD process and its use in this context has been described in Section 5.
The Thermal Soil Remediation Unit (TSRU) developed by ADI is a direct fired thermal desorber filter fitted within an afterburner, designed primarily for the treatment of hydrocarbon contaminated soils and related wastes (Truong, 1997). The unit is relatively large, with a capacity in the order of 20 tonnes per hour. The TSRU has been used successfully over an extended period of time for the treatment of soil contaminated with explosive organic compounds at ADI's St Marys site.
The TSRU has a capacity to treat wastes contaminated with low levels of chlorinated organics 2; particularly the semi-volatile compounds such as some organochlorine pesticides. The TSRU has been licensed by the NSW EPA to treat wastes where chlorinated hydrocarbons represent less than 1% of the total hydrocarbon content. This represents a significant limitation of the technology in the treatment of scheduled wastes.
Given that treatment of scheduled wastes is not the primary objective for the TSRU, a full review of the technology has not been conducted.
|Chapter 15 - Supercritical Water Oxidation||Chapter 17 - In Situ Vitrification|