LEVELS AND CONGENER PATTERNS OF POLYCHLORINATED DIBENZO-P-DIOXINS ANDDIBENZOFURANS 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). Bot h untreated wood and waste wood were burned. The concentrations :expre ssed 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 na tural wood was incinerated. However, in filter ash the levels were abo ut 2 orders of magnitude higher. Burning of waste wood resulted in con centrations in the range of 1-10 ng/g 2,3,7,8-TE. The reason for the h igh filter ash burden seems to be secondary formation on the ceramic f ilter surface at temperatures around 350 degrees C. Incineration of th e 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 compar ed 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.