Nitrogen oxide absorption and nitrite/nitrate formation in limestone slurry for WFGD system

Promoting the nitrogen oxide (NOx) absorption step is the major challenge for simultaneous removal of sulfur dioxide (SO2) and NOx in wet flue gas desulfurization. This paper studied NOx absorption and absorption product formation in limestone slurry. The effect of CaCO3 concentration, NOx composition, and SO2 and O2 concentration was investigated. A proposed reaction pathway of NOx absorption and conversion was developed based on the experimental results. NOx absorption is thought to involve two types of absorption pathways, namely, pure NO2 absorption and simultaneous NO and NO2 absorption. A positive correlation was found between CaCO3 concentration and NO2 absorption rate, and the best molar ratio of NO2/NOx in gas stream for NO absorption was 0.5. The main NOx absorption products were NO2− and NO3−. NO2− inhibits further NOx absorption and could be partially decomposed with release of NO, while NO3− has no obvious effect on NOx absorption. Nitrite can also be converted into nitrate through a reaction with dissolved oxygen. Addition of SO2 provides another NOx absorption pathway due to the reaction between S(IV) and NOx, which indicates the feasibility of simultaneously removing SO2 and NOx by wet scrubbing.