Effect of solution pH on SO2, NOx, and Hg removal from simulated coal combustion flue gas in an oxidant-enhanced wet scrubber

This paper presents a study on the simultaneous removal of SO2, NOx and Hg (both Hg0 and Hg2+) from a simulated flue gas by oxidant injection in a bench-simulated wet limestone scrubber for a wide range of slurry pH. The slurry pH strongly influenced the chemical mechanism in the scrubber and, therefore, affected pollutant removal. This paper also examines the potential ClO2(gas) reemission from a developed multipollutant scrubber at different slurry pHs. To better understand the chemical mechanisms at each slurry pH and to apply a mass balance to the process, detailed product ion analyses were performed for all experiments. Ion analysis covered three different chlorine species (chlorite, chloride, chlorate), sulfate, nitrite and nitrate. Different NOx removal efficiencies and mechanisms were found in acidic and alkaline pHs in the multipollutant scrubber. The acidic solution was favorable for NO and Hg0 oxidation, but increasing the slurry pH above 7.0 was disadvantageous for NO and Hg oxidation/removal. However, the rate of NOx absorption (by percentage) was higher for the alkaline solution. Implications This paper demonstrated a method for controlling multipollutant (SO2, NOx, Hg) emissions from a gas stream of a stationary combustion source in a wet limestone wet-FGD scrubber enhanced by an oxidizing additive. This paper explores how the flue gas conditions, especially the slurry pH, affect the chemical mechanisms of the multipollutant process and the pollutant removal efficiencies. The pH may be important when considering the location of oxidant injection into the scrubber/channel, where local parameters such as pH strongly vary. The usage of chlorite additive is potentially threatens ClO2(gas) emission; therefore, research in this area was also presented in this paper.