Characterization of sulphuric acid and ammonium sulphate aerosols in wet flue gas cleaning processes

Sulphuric acid aerosols can be formed in wet flue gas cleaning processes by spontaneous condensation initiated mainly by homogeneous nucleation. Even at low gas inlet concentrations of SO3 (2 mg/m3 (STP)) aerosol formation can be observed. For the design of absorption processes and sulphuric acid mist precipitators the aerosol characteristic data like mean diameter and number concentration are required for different process conditions and raw gas concentrations. In the present contribution, a combination of an experimental method with the simulation tool AerCode is described, which permits the determination of the diameter and the number concentration of sulphuric acid aerosols formed in wet flue gas cleaning processes. As a result of the extreme azeotropic phase behaviour of the H2SO4–H2O system the mechanism of homogeneous nucleation is predominant for aerosol formation in absorption processes. Furthermore, the influence of soluble and insoluble particles on homogeneous nucleation in the system H2SO4–H2O is shown. The investigation of the influence of insoluble foreign nuclei shows that at high SO3 raw gas concentrations >30 mg/m3 (STP), the mechanism of homogeneous nucleation is predominant. At lower raw gas concentrations between 2 and 10 mg/m3 (STP) SO3 heterogeneous nucleation is the major reason for aerosol formation. Solid (NH4)2SO4 particles that are formed by the reaction between H2SO4 and NH3 in the gas phase are used as soluble foreign nuclei. The studies show that aerosol formation is determined by the reaction component whose concentration is below the stoichiometric ratio. It was observed that in the case of an excess of sulphuric acid a decrease of the amount of ammonia leads to an increase of the particle size combined with a decrease in the number concentration. If ammonia is in excess, the decrease in the concentration of ammonia changes neither the particle size nor the number concentration of the aerosol significantly.