Towards controlling PCDD/F production in a multi-fuel fired BFB boiler using two sulfur addition strategies. Part I: Experimental campaign and results

Levels of PCDD/F production in a 140 MWth bubbling fluidized bed boiler were measured. The boiler uses solid recovered fuel, bark and sludge. Homologue distribution patterns suggest the de novo mechanism is the main pathway for the generation of dioxin and furans in the post combustion zones of the boiler. Two modes of sulfur addition were tested to induce the deactivation of Cu which has been identified as the prime catalyst of this mechanism. First, S-pellet promoted Cu sulfation as supported by aerosol sampling data and resulted in a decrease in PCDD/F levels. The second approach was adding sulfur through peat; this resulted in an increase in PCDD/F concentration. Factors such as high Cu content in the SRF-peat-sludge fuel mixture and reduced volatilization of Cu may have contributed to the said increase. For all test cases, phase redistribution of PCDD/F was observed in the electrostatic precipitator favoring more gaseous PCDD/F at the outlet. The homologue distribution pattern did not change in the flue gas path, suggesting that further synthesis and/or chlorination in the stream were minimized. There is however evidence for subsequent reactions happening in the ESP fly ash. The homologue distribution pattern in the latter was different from that of the flue gas, and more highly chlorinated PCDD/Fs were present. Furthermore, the ratio of PCDD and PCDF was different from that of the samples in the flue gas path.