Combined SO2 and NOx removal at moderate temperature by a dual bed of potassium-containing coal-pellets and calcium-containing pellets

The combined removal of both SO2 and NOx from a gas mixture that contains NOx, SO2, O2, and N2 was carried out in a dual bed of calcium-containing pellets and potassium-containing coal-pellets. The calcium-containing pellets retain SO2 and nitrogen oxides are reduced to N2 by the potassium-containing coal-pellets. The NOx-carbon reaction is catalysed by the potassium species on pellets, which also avoid the massive consumption of carbon by O2. The optimum temperature for the combined removal of SO2 and NOx is around 450 °C. This temperature is high enough to ensure the decomposition of Ca(OH)2, used as CaO precursor [CaO is much more effective for SO2 retention than Ca(OH)2], and low enough to avoid an important combustion of carbon by O2. The ratio between massCa-pellets/massK/carbon-pellets is a key factor in this process in order to avoid the potassium catalyst poisoning by SO2 chemisorption. Under the experimental conditions of this study, an appropriate massCa-pellets/massK/carbon-pellets ratio of 10/1 was found. With this ratio, the efficiency of the potassium-containing coal-pellets for NOx reduction is similar to that observed in the absence of the calcium-containing pellets for a SO2-free gas mixture.