(123c) Bench- and Pilot-Scale Studies on Mercury Removal by Potassium Iodide in Coal-Fired Flue Gas

Addition of halogens or halides has been reported to promote mercury removal in coal-fired power plants. In this study, bench- and pilot-scale experiments were conducted using various forms of potassium iodide (KI) as mercury sorbents. The factors that affect Hg removal by KI were examined and possible Hg?KI interaction mechanisms were proposed.

In the bench-scale studies, experiments were carried out in a packed-bed reactor (PBR) and an aerosol flow reactor (AFR) heated by a tubular furnace. Temperature was found to be important to Hg removal by KI. In the PBR system using either 2g granular KI or 0.5g powder KI, no Hg removal was observed at room temperature. As the temperature increased, Hg removal efficiency increased and reached 100% as the temperature was higher than approximately 300 °C. In the AFR system, KI aerosols were introduced through atomization of KI solution followed by a diffusion dryer. Increased Hg removal was observed when the temperature was above approximately 400 °C. Higher KI concentration and shorter gas residence time also improved Hg removal efficiency. Hg removal reached 100% at a molar ratio of KI/Hg = 600 with 5.8 s residence time for temperature above 500 °C.

Pilot-scale experiments were conducted in a 160 kW pulverized coal combustor. Hg speciation and concentration were measured at the ESP outlet using a continuous mercury monitor. KI was fed to the system by two ways: KI powder mixed with coal and KI solution sprayed into flue gas. Hg removal efficiency increased with the increasing mixing ratio of KI powder with coal, and 61% of Hg removal was achieved at a mixing ratio of KI/coal = 777 ppm_w. Mixing KI powder with coal was found to be more effective than spraying KI solution to the flue gas, possibly due to a longer residence time in the high temperature region.

Hg speciation measurements in both bench- and pilot-scale experiments showed that no oxidized Hg is formed in the gas-phase upon introducing KI. This, together with the positive temperature dependence of Hg removal by KI, indicates that Hg is chemically adsorbed on KI and an activation energy is required for the Hg?KI reaction. The species of the reaction products are not clear so far. A lower activation temperature for Hg removal was found in air than in N₂, indicating that O₂ promotes Hg removal by KI. Oxidation of KI to elemental I₂ may have also contributed to Hg removal.