Modification of activated carbon to enhance the absorption of PCDD/F and dl-PCBs emissions in flue gas in South Africa

Abstract

Activated carbon (AC) adsorption is commonly used for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) removal from flue gas. However, the AC available in the market predominantly possesses a substantial specific surface area and numerous micropores leading to significant variations in their adsorption characteristics. In this study, three types of activated carbon (AC) impregnated with different activating agents, namely K2CO3, KOH, and H3PO4 were evaluated for their effectiveness in removing PCDD/Fs and dl-PCBs compounds. For dl-PCBs, AC impregnated with H3PO4 resulted in a substantial reduction of PCB concentrations with an impressive 98% reduction achieved. Impregnation with KOH proved even more effective attaining a remarkable 99% reduction. In contrast, impregnation with K2CO3 while still effective achieved a slightly lower reduction rate of 95%. For PCDD/F, LACH3PO4 showed selectivity in achieving high reductions across both CDD and CDF congeners however proved effective in reducing the ∑PCCDF from 403.94 ng/Nm3 to 3.463 ng/Nm3 and ∑TEQ from 41.34 ng I-TEQ/Nm3 to 0.35 ng I-TEQ Nm3. LACKOH proved effective in reducing the ∑PCCDF from 540.45 ng/Nm3 to 0.25 ng/Nm3 and ∑TEQ from 57.42 ng I-TEQ/Nm3 to 0.12 ng I-TEQ Nm3 obtaining the overall removal efficiency of 99.5%. While the LACKOH injections were successful in the absorption of PCDD/Fs from the gas phase, it was observed that certain PCDD/F congeners became desorbed and entrapped within the ash necessitating further treatment of the ash residue. The ash underwent a thermal treatment first at 300 °C and 500 °C. The thermal treatment at 500 °C achieved a remarkable degradation of 99.7% removal efficiency rendering the ash and residue safe for disposal in a landfill site. Graphical Abstract