Changes in trace element contents in ashes of oil shale fueled PF and CFB boilers during operation

Abstract

Two oil shale combustion technologies, pulverized firing (PF) and circulated fluidized bed (CFB) were compared with respect to partitioning of selected elements (Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Rb, Sb, Sn, Sr, Th, Tl, U, V, and Zn) in the ashes along the flue gas ducts. The ash samples were characterized by high-resolution ICP-MS. The average contents of toxic heavy metals in fly ash samples from the CFB boiler are lower compared to the PF boiler. Main differences in trace element contents between combustion technologies were as follows: Cd content in the fly ash samples of PF boiler was up to 0.9mg/kg while in CFB boiler it remained below 0.1mg/kg in all analyzed ash samples; Hg was observed in the ashes of electrostatic precipitator (ESP) of CFB boiler while in the PF boiler it was close to or below detection limit. In the PF boiler content of Sn was detected only in the ashes of ESPs, while in CFB boiler it was evenly distributed between bottom and fly ash samples. Highest content among heavy metals in ash samples was observed for Pb in the last field of ESP of the PF boiler (142mg/kg).