The importance of the location of sodium chlorite application in a multipollutant flue gas cleaning system

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In this study, removing sulfur dioxide (SO2), nitrogen oxides (NOx), and mercury (Hg) from simulated flue gas was investigated in two laboratory-sized bubbling reactors that simulated an oxidizing reactor (where the NO and Hg0 oxidation reactions are expected to occur) and a wet limestone scrubber, respectively. A sodium chlorite solution was used as the oxidizing agent. The sodium chlorite solution was an effective additive that enhanced the NOx, Hg, and SO2 capture from the flue gas. Furthermore, it was discovered that the location of the sodium chlorite application (before, in, or after the wet scrubber) greatly influences which pollutants are removed and the amount removed. This effect is related to the chemical conditions (pH, absence/presence of particular gases) that are present at different positions throughout the flue gas cleaning system profile. The research results indicated that there is a potential to achieve nearly zero SO2, NOx, and Hg emissions (complete SO2, NO, and Hg removals and ∼90% of NOx absorption from initial values of 1500 ppmv of SO2, 200 ppmv of NOx, and 206 μg/m3 of Hg0) from the flue gas when sodium chlorite was applied before the wet limestone scrubber. However, applying the oxidizer after the wet limestone scrubber was the most effective configuration for Hg and NOx control for extremely low chlorite concentrations (below 0.002 M) and therefore appears to be the best configuration for Hg control or as an additional step in NOx recleaning (after other NOx control facilities). The multipollutant scrubber, into which the chlorite was injected simultaneously with the calcium carbonate slurry, appeared to be the least expensive solution (when consider only capital cost), but exhibited the lowest NOx absorption at ∼50%. The bench-scale test results presented can be used to develop performance predictions for a full- or pilot-scale multipollutant flue gas cleaning system equipped with wet flue gas desulfurization scrubber. Implications The control of NOx, Hg, and SO2 emissions from coal-fired combustors is an important, worldwide concern affecting environmental pollution. In both the European Union and the United States, NOx and SO2 emissions are subject to newly tightened emission limits. This paper investigated the NOx, Hg, and SO2 removals in a flue gas cleaning system equipped with a classical wet limestone scrubber, enhanced by a solution of sodium chlorite. This paper demonstrated that the location at which the sodium chlorite is applied in a flue gas cleaning system affects the control of these pollutants and discusses how possible solutions could be practically implemented.