Study of elemental mercury re-emission in a simulated wet scrubber

Mercury is a toxic pollutant that has motivated environmental regulations for emissions reduction from coal-fired power plants. Acid gas wet scrubbers are known to provide the co-benefit of SO2 control and mercury removal, when mercury is found in the flue gas in the oxidized form. The aqueous ionic chemistry in the scrubbing solution can lead to transformation of the absorbed ionic mercury to the insoluble elemental mercury form, resulting in mercury re-emission and a reduction of the scrubber mercury capture efficiency. Laboratory-scale experiments were performed in a simulated batch scrubber. The experiments were carried out to simulate a forced oxidation limestone reactor. The effect of scrubber temperature and pH, ionic mercury concentration in the liquor, total sulfite, and chloride and bromide ion concentration in solution, and O2 and CO2 concentration in the gas on mercury re-emission was investigated. Of particular interest was the investigation of the impact of slurry temperature and CO2 concentration in the gas, under conditions typical of oxy-fuel combustion condition, on mercury re-emission. The results confirm that oxidized mercury is reduced by aqueous S(IV). Higher concentrations of sulfites, chloride and bromide ions inhibit oxidized mercury transformation to elemental mercury. Higher concentrations of ionic mercury in the liquor and increased scrubber temperature and pH value results in higher re-emission levels of elemental mercury. Additionally, on the impact of oxy-fuel conditions on mercury, it was found that high availability of excess oxygen in the flue gas in contact with the scrubbing solution was found to result in lower conversion of oxidized to elemental mercury. No impact from CO2 in the flue gas was found on elemental mercury re-emission. However, higher slurry temperatures under oxy-fuel combustion conditions would lead to increased elemental mercury re-emission. For completeness, the effectiveness of sulfide-based additives for abatement of elemental mercury re-emission was also demonstrated in this study. The suppression effectiveness of sodium hydrogen sulfide, sodium sulfide, TMT15® and the Nalco-8034 reagent was found at 99%, 97%, 93% and 99%, respectively. These additives promote precipitation of ionic mercury as mercury sulfide.