An efficient calcium-based sorbent for flue gas dry-desulfurization: promotion roles of nitrogen oxide and oxygen

The development of sorbents for flue gas desulfurization in a dry mode is essential to control emission of sulfur dioxide. Based on the novel concept of "treating waste with waste", a low-cost and highly activated calcium-based sorbent (ACS) was prepared using coal fly ash, CaO and waste gypsum as the raw materials via the one-step incipient wetness impregnation method. Based on characterization using scanning electron microscopy and nitrogen adsorption-desorption, the ACS possessed a fibrous and netted structure with high porosity, which improved SO2 adsorption greatly. The SO2 adsorption capacity of ACS with coal fly ash/CaO/CaSO4 = 1/2/1 was high, up to 44.26 mg g-1, with 100% removal efficiency at 150 °C. In the absence of O2, SO2 was rapidly adsorbed on the sorbent to form CaSO3 according to in situ DRIFTS analysis, while when O2 was present in the flue gas, SO2/SO32- tended to be oxidized into SO42- species. Moreover, the presence of NO can further enhance the SO2 adsorption capacity of the ACS due to the formation of adsorbed NO2 or nitrate species with strong oxidizing properties. Therefore, the ACS can be considered as a sustainable sorbent with the advantage of employing fly ash for the removal of sulfur dioxide.