Advanced exhaust-gas scrubbing for simultaneous de-SOx/NOx using a wet oxidative process with integrated washwater treatment

Abstract

Wet Oxidative Scrubbing (WOS) is gaining increasing attention due to its capacity to simultaneously remove SO2, NOx and other pollutants from exhaust gases. In this sense, WOS can be used for the process intensification of Exhaust Gas Cleaning Systems (EGCS) thanks to its high efficiency, compactness and reduced use of chemicals. However, as a wet process, the application of WOS also requires an adequate washwater cleaning treatment. In this work, we investigate the performances of a WOS process based on the use of sodium chlorite (NaClO2) for the simultaneous removal of SO2 and NOx from a simulated exhaust gas, together with an analysis of the main anionic pollutants in the scrubber effluents (NO3−, ClO2−, ClO2(aq) and Cl2(aq)), which must be removed before discharge. A washwater treatment process for residual chlorite and active chlorine contaminants is investigated using ferrous sulfate and caustic soda in a reduction, precipitation, coagulation and flocculation process (R-PCF). The experiments reveal that the R-PCF is a valid option for the treatment of the massive residual concentration of chlorite: a stoichiometric dosage of ferrous sulfate allows reducing the concentration of chlorine species below 1 mg/L even starting from solutions containing a few grams per liter of chlorine species, with overall removal efficiencies above 99.9% at pH around 7. The R-PCF process is completed in a few minutes and the residual solid contains mostly Fe(OH)3(s), which can find application in other water treatments. The longstanding experience in the design and operation of R-PCF equipment makes this process a valuable off-the-shelf technique for washwater cleaning with a simple implementation in the WOS processes.