LEVELS AND CONGENER PATTERNS OF POLYCHLORINATED DIBENZO-P-DIOXINS AND DIBENZOFURANS IN SOLID RESIDUES FROM WOOD-FIRED BOILERS - INFLUENCE OF COMBUSTION CONDITIONS AND FUEL TYPE

The levels of polychlorinated dibenzo-p-dioxins and dibenzofurans have been determined in bottom ash and fly ash samples from different wood combustion systems for heat production (effect between 6-850 kW). Both untreated wood and waste wood were burned. The concentrations :expressed as 2,3,7,8-tetrachloro dibenzo-p-dioxin equivalents (2,3,7,8-TE, international model) in bottom ash were in the order of 1 pg/g when natural wood was incinerated. However, in filter ash the levels were about 2 orders of magnitude higher. Burning of waste wood resulted in concentrations in the range of 1-10 ng/g 2,3,7,8-TE. The reason for the high filter ash burden seems to be secondary formation on the ceramic filter surface at temperatures around 350 degrees C. Incineration of the combustible fraction of household waste in a small stove increased 2,3,7,8-TE levels in the bottom ash by three orders of magnitude compared to charcoal (0,5 pg/g). The PCDD/PCDF congener patterns found were influenced by both the type of wood burned and the construction of the combustion system.