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Formation of PBCDD and PBCDF during flue gas cooling.

The presence of bromine and chlorine in combustion, particularly in waste combustion, results in the formation of PBCDD and PBCDF. Formation pathways of PCDD/F are not fully understood, although much is known about them, and they are currently being further investigated. PBCDD/F is likely to be formed in reactions similar to those leading to PCDD/F, but the processes involved have been less intensively studied. In this study a fuel with bromine and chlorine in equal molar amounts was combusted in a pilot-scale fluid bed reactor and the cooling flue gas was sampled at 800, 350, and 250 degrees C. Analysis revealed that levels of PBCDD and PBCDF increased between all sampling ports. The chlorine/bromine ratio also increased with reductions in temperature and increases in residence time for both PBCDD and PBCDF. However, the formation of PBCDD and PBCDF seems to follow different routes with respect to the level and pattern of halogenation.