Investigation of the chemical pathway of gaseous nitrogen dioxide formation during flue gas desulfurization with dry sodium bicarbonate injection

The chemical reaction pathway for the viable flue gas desulfurization process, dry sodium bicarbonate injection, was investigated to mitigate undesirable plume discoloration. Based on a foundation of past findings, a simplified three-step reaction pathway was hypothesized for the formation of the plume-discoloring constituent, NO₂. As the first step, it was hypothesized that sodium sulfite formed by sodium bicarbonate reaction with flue gas SO ₂. As the second step, it was hypothesized that sodium nitrate formed by sodium sulfite reaction with flue gas NO. And as the third step, it was hypothesized that NO₂ and sodium sulfate formed by sodium nitrate reaction with SO₂.; The second and third hypothesized steps were experimentally investigated using an isothermal fixed bed reactor. As reported in the past, technical grade sodium sulfite was found to be un-reactive with NO and O₂. Freshly prepared sodium sulfite, maintained unexposed to moist air, was shown to react with NO and O₂ resulting in a mixture of sodium nitrite and sodium nitrate together with a significant temperature rise. This reaction was found to proceed only when oxygen was present in the flue gas.; As reported in the past, technical grade sodium nitrate was shown to be un-reactive with SO₂. But freshly formed sodium nitrate kept unexposed to humidity was found to be reactive with SO₂ and O ₂ resulting in the formation of NO₂ and sodium sulfate polymorphic Form I. The NO₂ formation by this reaction was shown to be temperature dependent with maximum formation at 175°C.; Plume mitigation methods were studied based on the validated three-step reaction pathway. Mitigation of NO₂ was exhibited by limiting oxygen concentration in the flue gas to a level below 5%. It was also shown that significant NO₂ mitigation was achieved by operating below 110°C or above 250°C. An innovative NO₂ mitigation method was patented as a result of the findings of this study. The patented process incorporated a process step of sodium sulfite injection to remove flue gas NO prior to sodium bicarbonate injection.