"To operate such a plant, the annual cost would be $8 700 000. This amount would be increased to $14 075 000 when considering the cost of capital, plant depreciation, taxes and insurance. A breakdown of the figure $14 075 00 is as follows:
Figure 1 Schematic of Rollins Envrionmental Services' Incinerator (EPRI, 1979)
Click image to view a full size version.
| Operating and Administrative Expense |
$ 8 700 000 |
| Depreciation (20 yrs, straight line) |
1 250 000 |
| Interest charge at 16.5% |
4 125 000 |
| |
$14 075000 |
Based on a feed rate of 4 260 kg/h, 1 kg would cost $0.44 to destroy.
Assumptions made during the calculation of these costs included a plant utilization of 310 days on a four shift, seven-day week with a standard 40 hour week for receiving and storage.
The components of the Rollins package are available, some more readily than others. Certain items such as the kiln, after-burner and parts of the scrubbing system would have to be fabricated (in accordance with Rollins design) and are not necessarily off-the-shelf units.
The impact of this technology has been summarized by the US EPA based on the observation of small amounts of dioxins and dibenzofurans in incineration products. An upper limit risk estimate was produced on the basis of series worst case assumptions. Approximately 10 improbable assumptions went into the estimate that produced a result of less than one additional cancer per 50 000 people exposed. This represents a public impact and is rated extremely low.
The presence of PCBs in these incinerators does not pose any additional worker impact provided the usual precautions for handling hazardous materials are observed. The Rollins incineration system has received intense scrutiny from regulators in the US and has met and exceeded all their requirements.
References: (Ackerman et al., 1981; Gregory, 1981; Rollins Environmental Services, 1981; EPRI, 1979)
4.2.1.2 ENSCO Incorporated
Head Office: 1015 Louisiana St.
Little Rock, Arkansas, 72202
Plant Location: Eldorado, Arkansas
Contact: Charles Robertson (501) 375-8444
The ENSCO incinerator is built at the site of an abandoned oil refinery and makes use of the steam plant boilers as its combustion and after-burning chambers. The incinerator configuration is shown in Figure 2. Drummed wastes (capacitors) are fed to a totally enclosed shredder. Shredded solids drop into a hopper that feeds an auger that conveys solids into a rotary kiln. PCB liquid wastes are mixed in with the shredded solids or injected directly into the kiln or combustion chamber. The air in the shredder enclosure is drawn through a fan into the rotary kiln.
The rotary kiln is 2.1 m in diameter by 10.4 m long and is slightly inclined down to an ash drop where most of the solid residue drops out. The kiln is double walled with air being drawn through the outer annular space and the canopy covering the interface between the rotating kiln and the feed system. Flue gases from the kiln are ducted to a combustion chamber (85 m3) where fuel (usually an organic waste) is burnt with air to create the higher temperature zone (outlet temperature 1250°C). A second combustion chamber of similar size acts as an after-burner (outlet temperature 1040°C). The combustion products are scrubbed in a spray tower (combined caustic and lime slurry) prior to demisting and discharge to the atmosphere.
The ENSCO system is designed to operate on solid and liquid wastes; the kiln section can be by-passed for liquid waste applications. PCB feed rates typically are 180 kg/h for liquid PCB (as 81% PCB in oil) and 110 kg/h for solid PCB (as 25% PCB in capacitors). Supplementary fuel amounted to 3000 kg/h averaging 16 400 kW heat input. The PCB destruction efficiency observed was 99.99998%. Based on additional testing done in December 1981, the ENSCO incinerator PCB feed rate will be increased to 8 140 kg/h in mid 1982.
The ENSCO experience (while less extensive than Rollins overall) parallels that of Rollins in PCB incineration. Their interest in PCB destruction has spanned at least six years and they are now licenced to incinerate PCBs.
Similar to the Rollins system, the technology involved is a straightforward application of a rotary kiln with an after-burner. Process instrumentation provides interlocks to shut down the PCB rate for non-compliance with PCB regulations specific-ally for kiln temperature, after-burner temperature, kiln and after-burner drafts, CO and C02 concentration.
Capital costs would be $5 340 000 as of January 1982, the main components being the capacitor hammermills and feed system, rotary kiln, after-burner, scrubbing system and effluent water treatment facilities. Approximately 30 hectares of land would have to be purchased as an additional expenditure. This cost figure, in addition to these modules, also includes costs for receiving and storage, lime storage and slaking, test burn and funding during construction.
Figure 2 Schematic of Ensco Incineration System (ENSCO, 1981)
Click image to view a full size version.
The annual operating cost would be $3 920 000 with labour, utility and maintenance allowances composing the major portion. Allowances for insurance, taxes, capital cost and plant depreciation would require a further $1 560 000. The cost to destroy 1 kg of capacitor or liquid is about $0.07. Basic assumptions made for this estimate are:
- a four shift, seven-day week;
- standard 40 hour week for receiving and storage; and
- 85% plant utilization.
As in the Rollins unit, the ENSCO system design is readily available but time would be required for fabrication of the kiln, after-burner and other components in accordance with ENSCO specifications.
The impact of this technology was summarized by the US EPA. For ENSCO, based on observed dioxin and dibenzofuran emission, the additional cancer risk to the public was one cancer for every 2 500 000 people exposed in the worst case fashion. This risk is very low; worker impact is unaffected by the technology over and above that associated with the correct handling of hazardous materials.
The ENSCO system has been approved by the US EPA after scrutiny similar and parallel to the Rollins system.
References: (Ackerman et al., 1981; ENSCO, 1981; EPRI, 1979 |
