Absorption of HCl on sodium-based absorbents at medium and high temperatures: The effect of competing absorption of SO2 and H2O

Abstract

The utilization of sodium bicarbonate (NaHCO3) as a solid reactant for the mitigation of acidic pollutants from industrial flue gas streams represents a straightforward and efficient process solution. The paper addresses the problem of medium- to high-temperature flue gas dechlorination by investigating the effects of base particle size, temperature, water vapour and different flue gas fractions on the removal of HCl from simulated flue gases by NaHCO3 in a fixed-bed reactor, and the effects of the presence of SO2 and H2O on the chlorination reactivity of NaHCO3 are also investigated by density functional theory (DFT) calculations. The adsorption results implied that 300 °C was the optimum temperature for HCl removal by NaHCO3 in a reaction system not affected by SO2 or H2O; The higher the HCl concentration, the greater the HCl capture, but when the concentration is too high, due to the dense product layer makes the HCl gas molecules to the internal diffusion is blocked, the dechlorination effect becomes poor. The smaller the particle size of NaHCO3, the better the removal of HCl. In the system containing SO2, theoretical calculations based on density functional theory show that the presence of SO2 affects the exchange of electrons between the Cl atoms in HCl and Na2CO3, and the bonding ability of the Na-Cl bond is weakened, thus weakening the dechlorination ability of Na2CO3; However, SO2 has little effect on HCl removal at the same concentration, due to the fact that HCl rapidly consumes the easily accessible surface of the absorber and SO2 must diffuse deeper into the particles for the reaction to occur, suggesting a preferred reaction between HCl and NaHCO3. After the addition of H2O, the adsorption energy of HCl on the surface of Na2CO3(200) was greater than the adsorption energy of SO2 on the surface of Na2CO3(200), and HCl was preferentially adsorbed on the surface of Na2CO3(200), which preferentially reacted with the absorber. These findings help to elucidate the principle of HCl absorption by NaHCO3 and the competition mechanism between HCl and SO2, and improve the theoretical knowledge of acid gas absorption by NaHCO3.