Effects of NOX and O2 on the Reaction of Ca(OH)2 with CO2 at Low Temperatures

The effects of NOX and O2 on the reaction of Ca(OH)2 with CO2 were studied under low-temperature and humid conditions prevailing in dry or semidry flue gas desulfurization processes. Without the simultaneous presence of NOX and O2, the extent of the carbonation of Ca(OH)2 was low and not affected by the concentration of CO2, NOX, or O2. With the simultaneous presence of NOX and O2, the carbonation of Ca(OH)2 was greatly enhanced, and the CO2 capture increased by more than 2 times at a relative humidity higher than 40%; in addition, the relative humidity and NOX concentration had positive effects, the temperature and O2 concentration had negligible effects, but the CO2 concentration had a negative effect. The simultaneous presence of NOX and O2 enhanced the formation of nitric and nitrous acids and salts of calcium nitrate and nitrite. The deliquescence of these salts collected a great quantity of water and thus enhanced the reactions of Ca(OH)2 with the reactive gases. Ca(OH)2 could be completely converted when reacted with the mixture of CO2, NOX, and O2 at a relative humidity ≥ 50% and a NOX concentration ≥ 300 ppm; CaCO3 formed would further react with NOX and O2 to form Ca(NO3)2 when Ca(OH)2 was depleted. The highest CO2 capture obtained was 0.49 g of CO2/g of sorbent (reacted at 60 °C, 70% relative humidity (RH), 12.6% CO2, 2400 ppm NOX, and 5% O2 for 1 h). The major products for the reactions of NO2 with Ca(OH)2 and CaCO3 were calcium nitrite and nitrate, respectively. The capture of CO2 in practice is preferably carried out before removing NOX and after removing SO2 from the flue gas. This work is useful to the development of technologies for CO2 capture using solid wastes containing CaO/Ca(OH)2.