Study of elemental mercury removal from flue gases using Tetravalent manganese Feroxyhyte

- The high oxidation potential of TMFx ensures Hg(0) oxidation.
- Adsorption capacity Q50 = 13.9 mg Hg/g at 120 °C.
- Exothermic behavior of the process.
- EBCT strongly influences Q50 value.
- Effective TMFx regeneration by KI at room temperature.

An innovative material, namely Tetravalent Manganese Feroxyhyte (TMFx), was synthesized and tested for its capacity to capture elemental mercury, Hg(0), from boiler flue gases. TMFx presents characteristics well-suited for this purpose, since it incorporates high oxidation capacity, high surface area and high negative surface charge density. Results show that the Hg(0) uptake capacity (Q50) at break through concentration equal to the EU emission limit (50 μg/m3) was essentially affected by synthesis pH, temperature and Empty Bed Contact Time (EBCT). The significantly high Hg(0) uptake capacity of 13.9 mg Hg/g, was determined for the material prepared at pH 9, and tested in a fixed bed reactor at T = 120 °C and EBCT = 40 ms. Uptake capacity was reduced at higher temperatures of 150-180 °C, due to the exothermic nature of the process and degradation of the TMFx's surface properties. An XPS study and regeneration tests indicated oxidation of Hg(0) in its bivalent form that allows for regeneration of spent TMFx samples under mild chemical conditions (KI solution) at room temperature. These mild regeneration conditions result in preserving TMFx uptake capacity. The presence of flue gas constituents affected positively the TMFx's uptake capacity for Hg(0). Oxygen alone increased adsorption yield by 180%, NO by 80%, and HCl by 64%. In contrast, SO2 had a strong negative effect, almost zeroing the Hg(0) uptake. The effect became positive when oxygen was added and yield increased by 25%. Contact time significantly influenced mercury uptake capacity, since by increasing the EBCT from 40 ms to 90 ms at 150 °C, the Q50 value almost tripled.

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