DESCRIPTION OF PROCESS
The dechlorination of insulating oils contaminated with low levels of PCBs depends on the reaction of active sodium with chlorine in the PCB molecule, under controlled conditions. This reaction forms sodium chloride and hydrocarbon residues. The principal reaction in the process is the direct removal of the chlorine atoms from the PCB:
RCl + Na° = NaCl + R°
where
RCl is a PCB molecule with up to 10 chlorine atoms
Na° is a reactive sodium atom
R° is a PCB molecule with 1 Cl atom removed
Now, R° is reactive and will combine with H° formed by the reaction of sodium with residual water in the oil. This will form a neutral RH molecule. If the RH still contains chlorine then it will react again with the sodium until all the chlorine atoms have been removed and replaced by hydrogen atoms. At this reaction endpoint the PCB has been converted to a biphenyl molecule and all the chlorine has combined with the sodium to form salt.
Comment: New or dechlorinated oils introduced into a transformer that was previously contaminated with PCBs will experience a leaching effect from the core and coil that was impregnated with contaminated oil. Consequently, the new or dechlorinated oil will take on PCBs and the contamination level will increase. It will not reach the level of PCB contamination that existed prior to the change out or dechlorination. Benchmarks vary globally but most country regulations imply that a transformer that has been retrofilled or dechlorinated must contain less than 50 PPM of PCB 90 days after treatment in order to be classified as successfully decontaminated. Some countries have lowered this limit to no more than 2 PPM, which is considered PCB free or non-detectable. Transformers that have been dechlorinated in situ must contain no more than 2 PPM at the time of treatment.
