Environ. Sci. Technol. 2007, 41, 2515-2522
PCDD/F Emissions and Distributions in Waelz Plant and Ambient Air during Different Operating Stages K A I H S I E N C H I , †,‡ SHU HAO CHANG,‡ AND M O O B E E N C H A N G * ,‡ Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan, and Graduate Institute of Environmental Engineering, National Central University, Chungli 320, Taiwan
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 plant to 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 °C, the PCDD/F concentration measured at the DSC outlet decreases from 1220 to 394 ng-I-TEQ/Nm3. By applying ACI, the PCDD/F concentrations of stack gas decrease from 139-194 to 3.38 ng-I-TEQ/ Nm3 (a reduction of 97.6-98.3%) while the PCDD/F concentration of reacted ash increases dramatically from 0.97 to 29.4 ng-I-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/m3 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-I-TEQ/kg EAF-dust treated are discharged via stack gas and reacted ash, respectively, in this facility.
Introduction Over 150 000 tons of electric arc furnace (EAF) dusts containing relatively high contents of heavy metals and PCDD/Fs (0.13-2.8 ng-I-TEQ/g EAF-dust) are generated each year in Taiwan. Relevant study (1) indicates that about half of the total PCDD/F mass flow in EAF is discharged via EAF dusts. Results of Buekens et al. (2) show that the polychlorinated biphenyl (PCB) concentration in EAF dusts is around 10.3-113 ng/g, respectively. The Waelz process has long been applied for treating EAF dusts to recover zinc and decon* Corresponding author e-mail:
[email protected]. † Research Center for Environmental Changes, Academia Sinica. ‡ Graduate Institute of Environmental Engineering, National Central University. 10.1021/es061665p CCC: $37.00 Published on Web 02/20/2007
2007 American Chemical Society
taminate the dust, considered as a hazardous waste. In most cases, the feeding material is heavily contaminated with chlorides and PCBs, resulting in potentially high PCDD/F formation. Previous study (3) indicates the PCDD/F concentration measured in stack gas of the Waelz plant investigated reached 145 ng-I-TEQ/Nm3 due to the relatively low PCDD/F removal efficiency (
