Simultaneous sulfur dioxide and nitrogen dioxide removal by calcium hydroxide and calcium silicate solids.

At conditions typical of a bag filter exposed to a coal-fired flue gas that has been adiabatically cooled with water, calcium hydroxide and calcium silicate solids were exposed to a dilute, humidified gas stream of nitrogen dioxide (NO2) and sulfur dioxide (SO2) in a packed-bed reactor. A prior study found that NO2 reacted readily with surface water of alkaline and non-alkaline solids to produce nitrate, nitrite, and nitric oxide (NO). With SO2 present in the gas stream, NO2 also reacted with S(IV), a product of SO2 removal, on the exterior of an alkaline solid. The oxidation of S(IV) to S(VI) by oxygen reduced the availability of S(IV) and lowered removal of NO2. Subsequent acidification of the sorbent by the removal of NO2 and SO2 facilitated the production of NO. However, the conversion of nitrous acid to sulfur-nitrogen compounds reduced NO production and enhanced SO2 removal. A reactor model based on empirical and semi-empirical rate expressions predicted rates of SO2 removal, NO2 removal, and NO production by calcium silicate solids. Rate expressions from the reactor model were inserted into a second program, which predicted the removal of SO2 and NOx by a continuous process, such as the collection of alkaline solids in a baghouse. The continuous process model, depending upon inlet conditions, predicted 30-40% removal for NOx and 50-90% removal for SO2. These results are relevant to dry scrubbing technology for combined SO2 and NOx removal that first oxidizes NO to NO2 by the addition of methanol into the flue duct.