Publications archive - Waste and recycling
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CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
Eco Logic International Inc. (Eco Logic), Canada has developed a hydrogenation process known as Eco Logic as an alternative to incineration for hazardous wastes. The process is based on gas-phase thermo-chemical reaction of hydrogen with organic compounds. At 850oC or higher, hydrogen combines with organic compounds in a reaction known as reduction to form smaller, lighter hydrocarbons, primarily methane. For chlorinated organic compounds, such as PCBs, the reduction products include methane and hydrogen chloride. This reaction is enhanced by the presence of water, which acts as a reducing agent and a hydrogen source.
The process is non-discriminatory; that is organic molecules such as PCBs, PAHs, chlorophenols, dioxins, chlorobenzenes, pesticides, herbicides and insecticides are quantitatively converted to methane. Approximately 40% of the methane produced can be subsequently converted to hydrogen via the water shift reaction and the remaining methane converted to hydrogen in the catalytic steam reformer. Thus, the process can operate without an external supply of hydrogen. For highly concentrated wastes (eg pure Askarel) the process produces an excess of methane.
A process flow diagram for a typical integrated plant, showing the four possible waste preparation and feed systems, is shown in Figure 9.1.
Organic liquid waste streams (OCPs, PCB fluids) are pumped from a feed tank via a positive displacement pump, through atomising nozzles, directly into the reactor. Recirculating product gas (PG) is used to atomise the liquid waste.
The mixture of gases and vaporised liquids are heated as they swirl downwards past electric heating elements situated around the central ceramic-coated steel tube of the reactor. Gases and any entrained fine particulates proceed up the central tube providing in excess of 2 seconds retention time at 9000C. The reactions come to completion before the gases reach the scrubber where the water, heat, acid and carbon dioxide are removed. A caustic scrubbing agent is added, if required, to maintain the scrubber water pH at between 6 and 9. The temperature of the exit gas is maintained near 350C by cooling the scrubber water using dual plate heat exchangers and cold water from an evaporative cooler.
The dirty scrubber water after filtration, returns to the scrubber through a drop-tube which extends well below the water surface. This acts as a seal against air infiltration and as an emergency pressure-relief mechanism. The system operating pressure is less than 10 kilopascals (gauge) (kPag).
The gas leaving the scrubber, comprising a clean dry mixture of hydrogen, methane, carbon monoxide and other light hydrocarbons, is drawn into a rotary lobe blower. In order to maintain the required concentration of hydrocarbons in the reactor, some of the gas may be recirculated to a catalytic steam reformer to convert methane (CH4), carbon monoxide (CO), and steam to hydrogen (H2) and carbon dioxide (CO2). Side-streams may also go to a Thermal Reduction Mill (TRM) and Sequencing Batch Vaporiser (SBV) as a sweep gas. Excess gas is directed to the compressor and product gas stage tank, where the gas is stored temporarily and analysed on a continuous basis. The compressed product gas may then be used to fuel the boiler, steam reformer, SBV or TRM.
Contaminated watery wastes may be pre-heated using steam prior to being pumped into the reactor via atomising nozzles.
Electrical equipment such as dismantled transformer cores and capacitors may be decontaminated in the SBV. Residual PCBs, oil, and solvent are thermally desorbed and swept into the reactor by the hydrogen-rich, hot recirculation gas. Solid materials such as personnel protective equipment and corroded drums and carcasses can also decontaminated in the SBV. Contaminated electrical equipment processed in the SBV constitutes a relatively small organic load to the reactor and high strength organic wastes such as Askarel fluids can be processed simultaneously.
The SBV is also suitable for processing high-strength organic wastes such as obsolete pesticides which are sufficiently volatile to evaporate directly from drums. The advantages of this approach are that handling is reduced, drums are cleaned in place and inorganic solids remain behind in the drums. Fugitive emissions are minimised due to the reduced handling requirements and the elimination of transfer operations.
Solid wastes such as soil or decanted sediment may be decontaminated using a thermal reduction mill (TRM) with the vaporised contaminants being directed to the reactor. The TRM is proprietary technology, however, it is designed to heat solids gradually while mechanically working them into a fine mixture. At this point in time a TRM has not been established at Kwinana; however, a unit has been commissioned by Eco Logic in Canada. Small quantities (eg 4 x 200 L drums per batch) of contaminated soil may be treated in the SBV.
A unique feature of the process is the continuous analysis of the product gas using a Chemical Ionisation Mass Spectrometer (CIMS), a bulk gas analyser (H2, CO, CO2, CH4), an oxygen analyser and a back-up gas chromatograph. The CIMS monitors up to 10 organic compounds continuously, with detection levels in the parts per billion range. These data are used for on-line process control of the plant. Likewise the bulk gas analyser results are also used for process control.
The CIMS and the gas chromatograph monitor the gas for potential products of incomplete destruction of waste. For example, while processing DDT wastes, potential byproducts include monochlorobenzene (MCB) and monochloromethane (MCM). Should levels of these compounds reach "trigger" levels as per licence conditions, the plant is automatically put into recycle mode with an immediate shutdown of waste feed. This ensures that no product gas is sent to the various burners on the plant, thus eliminating any potential fugitive emissions, or non-compliance with licensed emission limits.
The Eco Logic technology has been successfully demonstrated at pilot plant scale in Canada and the USA, processing PAH contaminated sediments, PCB oils, PCB/dioxin contaminated soil and PCB contaminated groundwater. These demonstrations, conducted under independent audit of Environment Canada and the USEPA, were completed in 1993. The test work demonstrated the capability of the Eco Logic technology to process soils contaminated with PCBs, hexachlorobenzene (HCB) and dioxins (USEPA, 1993).
In mid-1995, an Eco Logic plant was commissioned in Kwinana, Western Australia (Bridle, 1997). This plant is currently processing OCP and PCB wastes and an example of the performance of this plant is presented in Section 9.4.
In 1996, Eco Logic International commissioned a second plant on the General Motors site in St Catherine's, Ontario. This plant processed PCB contaminated concrete and soils and PCB capacitors. Large concrete slabs have been processed in the SBVs, reducing PCB contamination from up to 2790 ppm to less than 0.15 ppm.
In Ontario, Canada, clean up of contaminated soil and concrete is required if the levels of PCBs exceed 50 ppm. General Motors of Canada Ltd. have set an upper limit of PCBs in their treated concrete of 0.5 ppm (Bridle, 1996).
Since the Eco Logic technology is a hydrogenation process and will add hydrogen atoms to any incompletely hydrogenated organic molecule, it will dechlorinate molecules and break down aromatic rings, and is therefore indiscriminate in its treatment of organic substances. For this reason, it can be expected to treat PCP similarly to PCB, HCB and dioxins, and can also extend to other non-halogenated compounds such as PAHs. However, throughput will be determined by the total contamination of organics in the waste rather than just the scheduled components.
The Eco Logic process is likely to be preceded by a thermal desorption unit when treating solid wastes. There is potential for the removal of organic contaminants from the solid material to be improved in the Eco Logic process, as the thermal desorber will operate under a reducing hydrogen atmosphere, offering simultaneous destruction. Under a reducing atmosphere the formation of dioxins is less likely to occur, although partial hydrogenation of more chlorinated molecules (eg OCDD to TCDD) may still occur and will depend on the efficiency of the overall desorption and hydrogenation process.
The efficiency of the thermal desorption unit (SBV or TRM) and its ability to completely desorb organic contaminants in the desorption time available can be expected to be important in controlling the efficiency of waste treatment. This in turn can be expected to depend on both physical processes (heat, time, desegregation, and mixing) rather than just the chemical reaction process. The application of the thermal desorption unit to irregular solids such as concrete (containing reinforcing materials) is uncertain; however, the SBV has been used to treat concrete slabs. Material handling considerations for such wastes may restrict the application of the process.
In principle, the Eco Logic process can be applied to volatile solvent mixtures. However, care would be required to avoid high rates of gas generation which could over-pressurise the systems, the process has limited surge capacity; over pressurisation could result in a release of waste material (refer Section 9.5(g)).
The Eco Logic process is limited with respect to certain waste constituents (eg arsenic which passes through the process contaminating scrubber water, and iron) and a complete analysis of a prospective waste will require review by the operator prior to treatment. CMPS&F understands however, that initial limitations associated within the arsenic and iron content of OCP wastes have been largely overcome through process refinements, and more recently through the provision of an alternative hydrogen source (allowing the steam reformer to be taken out of service 2).
The Eco Logic process requires water in its operation and therefore can process wastes with a relatively high water content. This may provide an advantage over other thermally based processes, if aqueous sludges require treatment.
The operation of the Eco Logic process is relatively complicated although much of the technology is common to the process industries. As hydrogen gas under pressure is used, care must be taken to operate the system with suitable controls and safeguards to ensure that explosive air-hydrogen mixtures are not formed. The operating experience in Australia indicates that the process can be carried out safely and suggests that the current safeguards on the plant are satisfactory.
For most of the wastes treated, the process gas generated provides much of the process fuel needs. Electricity is required for all wastes, in the range of 96 kWh per tonne of soil treated, to around 900 kWh per tonne of pure organics treated (Bridle, 1995a).
The Eco Logic process is able to convert the chlorinated organics into fuel which are utilised in the treatment process. Chlorine is converted into a salt solution (scrubber blowdown) which will require disposal to a sewer. Desorbed solid waste can be disposed of to a landfill, if other waste constituents such as heavy metals are at acceptable levels. The typical composition of waste streams generated by the Eco Logic process summarised in Table 9.1.
|Parameter||Boiler Stack||Scrubber Blowdown|
|OCs||< 2 mg/Nm3||< 1 µg/L|
|Dioxins (TE)||< 0.1 ng/Nm3||< 1 ng/L|
Note: (1) Source: Bridle, 1994a
Preprocessing is required for large capacitors and building rubble. Large capacitors are punctured and drained of the PCB contaminated fluid. Building rubble and concrete must be reduced in size to less than one square metre (Bridle, 1995a).
A relocatable Eco Logic unit has been constructed in Western Australia and was commissioned in mid-1995. The facility was originally established by Environmental Solutions International (ESI), which entered into a joint venture with ELI Eco Logic (Canada) for development and operation of the plant. CMPS&F understands that all approvals and contracts were originally held by ESI. ESI have recently reached a commercial arrangement with ELI Eco Logic whereby ESI will cease to have an interest in the Eco Logic process in Australia. Instead, the plant established in Australia will be operated by Eco Logic (Australia) Pty Limited, a subsidiary of ELI Eco Logic (Canada). While agreement has been reached regarding the separation of ESI and Eco Logic in Australia, the specific timing for the financial settlement and issues associated with transfer of approvals and contracts are less clear.
During commissioning the following problems were encountered:
a) chemical attack of PVC piping on the scrubber circuit;
b) acid recovery not functioning; and
c) blockage of heat exchangers on the process water circuit.
To overcome these the PVC piping was changed to HDPE and the acid recovery system was converted into a caustic scrubber. The major problem of heat exchanger blockage was mitigated via installation of two disc centrifuges to remove some of the organic liquids and sludges from the process water.
By 1996, the plant was operational again and a small 9 drum SBV had been commissioned. In mid 1996 a larger 27 drum SBV was constructed and testing/trialing commenced in the latter part of 1996. Consideration is being given to construction of a second 27 drum SBV (Bridle, 1997). In addition, the gas scrubbing system is being refined in Canada which is expected to remove an excisting process bottleneck. An independent supply of hydrogen was secured early in 1997, allowing the steam reformer unit to be decommissioned. While the steam reformer was a key element of the original facility in Australia, allowing the process to be self-contained with respect to hydrogen, in practice it has been less reliable than other elements of the process. The Eco Logic plant developed in Canada operated without a steam reformer, instead relying on an independent supply of hydrogen. The difficulties associated with operation of the steam reformer and the availability of an alternative hydrogen source have led Eco Logic to decommission the reformer and simplify the overall process. This is a significant change of direction compared to the original operational philosophy.
Currently, the plant is operating intermittently albeit at reduced capacity, processing PCBs and OCPs (Bridle, 1997). For example, the plant was awaiting parts and was not operating in January and February 1997. However, the plant did achieve 80 to 90% operational time during March and April 1997 (Solomon, 1997).
The process has received licence approval in Western Australia, and generic approval of the technology has been granted in NSW.
Independent monitoring of the plant has been undertaken while processing organochlorine pesticides (DDT in toluene), PCB fluids and PCB capacitors. A summary of results are shown in Tables 10.2, 10.3 and 10.4.
|Date of Testing|
|Jul 95||Nov/Dec 95||Jan 96||Feb 96|
|DDT||µg/Nm3||< 0.7||-||-||< 0.8|
|PCBs||µg/Nm3||-||< 0.72||< 0.90|
|Polycyclic Aromatic Hydrocarbons||mg/Nm3||0.026||0.129||0.550||0.027|
|Total Suspended Particulates||mg/Nm3||10||-|
|HCl||mg/Nm3||-||-||< 0.9||< 1 µg/L|
|Date of Testing|
|Jul 95||Nov/Dec 95||Jan 96||Feb 96|
|DDT||µg/L||< 5||-||-/-||< 0.5/< 0.5|
|Total Dissolved Solids||mg/L||2390||2945||11000/20000||20000/19000|
|Date of Testing|
|Jul 95||Nov/Dec 95||Jan 96||Feb 96|
|Quantity in Stack||(mg)||< 2.5858||< 1.8662||< 2.4102||< 2.769*|
|Calculated DRE||(%)||> 99.999994||> 99.999999||NA||> 99.999993|
ESI advises that the plant has been in compliance with WA EPA licence limits. The PAH content of the scrubbed water would require review prior to discharging to a sewerage system, but is at levels which would normally be acceptable.
Ongoing monitoring of processed capacitors (both large and small) indicates PCB residue concentrations of less than 0.2 mg/kg, well below the management plan criterion of 2 mg/kg, and analyses of impermeable metal surfaces indicate values of less than 5 mg/m2 (Bridle, 1996).
The Eco Logic unit in Western Australia does not currently include a thermal reduction mill and therefore the system is limited in its ability to treat solid materials. While a TRM has been commissioned in Canada there are no fixed plans to install a TRM in Australia (Solomon, 1997). Only small quantities of more porous soils are currently able to be treated in the SBV.
After Western Australian stocks of pesticides and PCBs have been treated, the unit may be available for use elsewhere in Australia. Given its size, the plant is not readily transportable and a significant set up time would be required for its relocation. As such, it is likely the unit would be operated from a fixed location within a region and waste transported to the facility.
Although development of the Eco Logic unit has been slower than expected, the unit appears capable of servicing a proportion of Australia's chlorinated waste disposal requirements.
Eco Logic Australia Pty Ltd.
All waste types listed, with appropriate preprocessing. Note that special equipment available for the treatment of contaminated capacitor and transformer materials. Building rubble/concrete requires breaking up into slabs of less than 1 m2.
All scheduled compounds and other non-chlorinated compounds.
Commercial scale unit currently operating in Perth treating waste pesticide formulations and PCB wastes.
Currently available for liquids and PCB capacitors and small quantities of solid wastes. Treatment capability of large quantities of soil and sludge is dependent on construction of a thermal reduction mill in Australia.
$4000 - $6000/tonne for OCP Solids.
$4000 - $8000/tonne for PCB and OCP liquids.
$6000 - $11000/tonne for PCB contaminated capacitors.
There are a number of different emissions from the Eco Logic process when it is used for treatment of scheduled waste. Typical plant emissions when treating schedule wastes are detailed in Table 9.2.
In the early process development stage pipework associated with the SBV was subject to blockage due to insufficient heat tracing. This led to an over pressurisation of the SBV and minor release of vaporised PCBs via a door seal, however, negligible contamination of the environment is claimed by the proponents.
Use and storage of hydrogen within the process represents a potential safety hazard. The unit in Kwinana has been subjected to an internal HAZOP/HAZAN review and process control interlocks have been provided to prevent release of waste materials during a process upset. From the limited experience gained to date current safety and environmental risks from plant operation appear acceptable.
Relocatable unit. The size of the system suggests Eco Logic will be established at a central location within each region, requiring wastes only to be transported short distances for treatment.
Contaminants such as sulphur and arsenic were found to inhibit treatment in earlier plant development stages. Sulphur in combination with iron was found to form slimes which required additional centrifuge separation and low levels of arsenic were difficult to treat.
Materials handling of irregular solids can also limit waste treatment. The ball mill system is the likely constraint with difficulties in treating rubbery materials, mixed materials (such as metals and concrete) and tars. These materials can, however, be treated in the SBV. The Eco Logic process may in future need to be linked to special waste handling facilities in order to improve waste material handling options.
|Chapter 8 - Gasification||Chapter 10 - Molten Media Processes|