Towards controlling PCDD/F production in a multi-fuel fired BFB boiler using two sulfur addition strategies. Part III: Cu speciation in the fly ash

PCDD/F abatement strategies – sulfur pellet addition and peat co-combustion – were tested for a BFB boiler facility utilizing SRF-bark-sludge as fuel. In this paper chemical and physical analyses of electrostatic precipitator (ESP) fly ashes were used to explain the differences in the performance of these strategies. These analyses revealed a difference between the coarse and fine fly ashes collected in the ESP. Chemical analysis of the fine fly ashes revealed high concentration of easily volatilized elements while the SEM micrographs showed that fine ash are composed of clusters of spherical particles, thereby leading to a conclusion that fine ashes were originally in a gas phase in the high temperature zones of the boiler. Variation in the distribution of active and passive forms of Cu in fly ashes was revealed using X-ray absorption spectroscopy (XANES mode). It was also found that peat co-combustion led to increased formation of Cu oxides that may act as active catalysts in de novo synthesis. Furthermore, XANES revealed the formation of CuSO4 for all the test cases. By applying the empirical ratio between mole fractions of the active and passive species of Cu, the role of Cu speciation to PCDD/F production was emphasized. It is concluded that sulfur pellet addition is more effective than peat addition as a PCDD/F abatement strategy for the BFB facility understudy.