Destruction Technologies for Polychlorinated Biphenyls

* current in December 1981

References:
(Oppelt, 1981; MM. Dillon, Ltd., 1981; EPRI, 1979; Gregory, 1981; EPA, 1981).

4.2.2 Co-Treatment Processes

The second major category of conventional incineration is incineration using a combustion process designed for some purpose other than incineration. In these processes combustion conditions are appropriate for PCB destruction during the routine operation of the process for its design purpose. These combustion processes have been labelled co-treatment processes, and are processes based on conventional combustion such as:

• high efficiency boilers fired with coal, oil or natural gas;
• kilns for the production of cement or lime; and
• aluminum melting furnaces.

The PCB waste is generally liquid waste that is fed only at a small fraction of the fuel feeding rate such that the PCB content of the fuel is very low. Within the US regulations there is a special, less restrictive, status for contaminated oil containing less than 500 ppm of PCB. This has led to considerable approval action for burning such low level contaminated material.

4.2.2.1 High Efficiency Boilers

High efficiency boilers burning coal, oil or natural gas are designed to make maximum use of fuel. These units are designed to produce steam for steam-generated electricity or process heating. Many applications have been made for approval in the US particularly for lightly contaminated PCB oils. In the US, regulations governing burning of oils up to 500 ppm PCB are subject to much less stringent rules. To gain approval an applicant need only demonstrate:

• boiler heat release in excess of 15 119 kW;
• stack CO < 50 ppm for oil or gas and CO < 100 ppm for coal;
• 3% excess oxygen; and
• routine stack gas, feed rate and performance monitoring.

These limitations should not limit the applicability of this process to burning only PCB-contaminated oil. Studies by Ontario Hydro and EPRI, suggest that PCBs can be burned at high concentration provided certain feed rate limitations are observed.

The concept of the incineration process is simple. PCB liquids as contaminated oils, pure PCB liquids, or PCB liquids diluted with a fuel oil are fed directly into the active combustion zone of the boiler. Suitable interlock systems connected with the boiler operation control the PCB feed. Generally, due to corrosion considerations inside the boiler, the chlorine content of the PCB is the controlling factor with respect to the feed rate. As a result, scrubbing of product HCI gas is not practiced. HCI emissions are usually within the local emission regulations by virtue of dilution. The type of boiler selected for PCB incineration depends on the following general factors:

• boiler firing configuration;
• types of fuel; and
• residence time.

Boilers, with horizontally opposed fuel guns, generally give a more thoroughly mixed turbulent flame than do tangentially fired boilers. Coal fired boilers are more likely to be constructed of HCI resistant materials because coal contains small amounts of chlorine (as chloride) not found in oil or natural gas. Residence times in high efficiency boilers are in the range of 1.0 to 3.7 seconds with combustion zone temperatures of 1430 to 1675°C. While these conditions closely match the recommended combustion conditions care must be exercised that "channelling" or "by-passing" does not take place.

Waste firing in surveyed applications extends only to liquid PCBs although the EPRI study discussed firing shredded solid material. Feed rates suggested are the range 150 kg/h (Ontario Hydro) to 300 kg/h (EPRI Study). The solids used in the EPRI system were fed through a rotary kiln with the off gases ducted into the boiler combustion zone. However, it seems unlikely that a high efficiency boiler operator would opt for burning shredded solids.

Destruction efficiencies are usually difficult to define because:

• most tests to date are on lightly contaminated oils with small amounts of PCB fed to the process initially so that it is difficult to determine the actual amount of PCB entering the system.
• sampling of hot, dust laden gas containing large quantities of gaseous diluent makes the detection of even smaller amounts of PCB difficult.

Despite the above restrictions, destruction efficiencies claimed are in the range of 99 to 99.99%.

The operation of high efficiency boilers is a highly developed technology. The introduction of PCB liquid wastes is a relatively simple operation. The monitoring of boiler operation and interface with the PCB feed systems for automatic feed shutdown is a straightforward step.

Compared to the operation of the high efficiency boiler, the operation of the PCB feed systems are not complex and are well within the skills found in boiler operators. The automatic feed shutdown system which interlocks with operational parameters is a well developed technology. Such a system shuts down the PCB feed during a process upset, i.e. furnace temperature too low or combustion efficiency too low. Such a system also controls the feed rate to minimize HCI emissions to within regulatory standards.

Capital costs, for a boiler preprocessing system, including construction funds over a one year period but excluding land costs would be $1 140 000. Such a facility would have the capacity for destroying 260 kg/h of liquid PCBs at 50% concentration in addition to 130 kg/h of shredded capacitors. Total heat release rate would be 2.4 x 10⁵ kcal/h for the preprocessing system. A rotary kiln would be used as the primary combustion chamber to separate PCBs from the shredded solids and to volatilize the liquid PCBs prior to introduction into the boiler which would act as an after-burner