PCDD/F emissions and distributions in Waelz plant and ambient air during different operating stages

Significant formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) has been observed in a typical Waelz process plant. In 2005, the Waelz plant investigated was equipped with a dust settling chamber (DSC), a venturi cooling tower, a cyclone (CY), and baghouse filter (BF). In early 2006 activated carbon injection (ACI) was adopted to reduce PCDD/F emissions from the plant investigated. Samplings of flue gases and ash were simultaneously conducted at different sampling points in the Waelz plantto evaluate removal efficiency and partitioning of PCDD/Fs between the gas phase and particulates. As the operating temperature of the dust settling chamber (DSC) is increased from 480 to 580 degrees C, the PCDD/F concentration measured at the DSC outlet decreases from 1220 to 394 ng-l-TEQ/Nm3. By applying ACI, the PCDD/F concentrations of stack gas decrease from 139-194 to 3.38 ng-l-TEQ/ Nm(3) (a reduction of 97.6-98.3%) while the PCDD/F concentration of reacted ash increases dramatically from 0.97 to 29.4 ng-l-TEQ/g, as the activated carbon injection rate is controlled at 40 kg/h. Additionally, ambient air PCDD/F concentrations were measured in the vicinity of this facility during different operating stages (shutdown, and operation with and without ACI). The ambient PCDD/F concentration measured downwind and 2.5 km from the Waelz plant decreases from 568 to 206 fg-I-TEQ/m(3) after ACI has been applied to collect the dioxins. Due to the high PCDD/F removal efficiency achieved with ACI + BF, about 24.3 and 3980 ng-l-TEQ/kg EAF-dust treated are discharged via stack gas and reacted ash, respectively, in this facility.