Study on competitive absorption of SO3 and SO2 by calcium hydroxide

Abstract Alkaline injection is an effective way to control the emission of high concentration SO 3 caused by the use of high-sulfur coal and selective catalytic reduction (SCR), but its performance is affected by the competitive absorption of SO 2 . A fixed-bed reactor was used to study the competitive absorption of SO 3 and SO 2 by Ca(OH) 2 . The dynamic absorption curves in different atmospheres were obtained, the intermediate products were characterized, and the competitive absorption mechanism was discussed. The absorption of SO 3 and SO 2 consists of the chemical kinetics-controlled and product layer diffusion-controlled stages. In the chemical kinetics-controlled stage, the SO 3 and SO 2 removal efficiencies were around 90% and 55% at first and then decreased; the SO 3 selectivity was around 0.1 at first and then increased. In the product layer diffusion-controlled stage, the SO 3 removal efficiency gradually decreased but kept greater than 10%, while the SO 2 removal efficiency deceased to zero; the SO 3 selectivity continued to increase and reached 0.31 at most when SO 2 absorption ceased. There existed significant competition between SO 3 and SO 2 absorption. The increasing SO 3 concentration promoted the SO 3 absorption and the increasing SO 2 concentration decreased it. The absorption of SO 3 increased the intensity of the SO 4 2 - band, decrease the intensity of the SO 3 2 - band, and made the product layer more compact. The possibility of SO 3 reacting with the components (CaSO 3 , CaSO 4 and CaCO 3 ) on the reacted Ca(OH) 2 surface was analyzed. Ionic diffusion was proposed as the main mechanism of the continuous SO 3 absorption after the formation of a dense product layer.