ARE SALT-LADEN RECOVERY BOILERS A SIGNIFICANT SOURCE OF DIOXINS

Stack gases from recovery boilers at two western coastal kraft mills w ere sampled and analyzed for dioxins. Dioxins emissions, expressed in terms of toxicity equivalents, ranged from 0.38 to 5.2 pg/m(3), with c hloride concentrations as high as 5.4% on black liquor solids. In the three 1995 trials, the 2378-TCDD congener was non-detectable; the 2378 -TCDF congener was nondetectable in two of these trials and present at close to the detection limit in the third trial. These stack emission s are far below the guideline for dioxins emissions from municipal was te incinerators (500 pg TEQ/m(3)) recommended by the Canadian Council of Ministers of the Environment (CCME) and are very similar to the emi ssion levels observed for recovery boilers at interior mills with no s alt load (1.2-2.5 pg TEQ/m(3)). The attenuated dioxins formation in sa lt-laden recovery boilers, relative to that from salt-laden power boil ers, is attributed to a combination of three factors: greater uniformi ty in the fuel, longer furnace residence times at higher temperatures, and relatively low atomic chlorine-to-sulphur ratios.