Lab scale investigation on the formation of aerosol nuclei by a wet electrostatic precipitator in the presence of SO2 in a gas stream

Abstract

Aerosol based emissions have been reported from several Post-Combustion CO2 capture (PCCC) pilot plants. These are known to pose a significant challenge in large scale implementation of PCCC processes. The aerosol nuclei are in the form of H2SO4 droplets or Na2SO4 particles, typically less than 0.1 μm and 0.255 μm, respectively. A wet electrostatic precipitator (WESP) is considered to be a potential counter-measure for removing these ultrafine aerosol nuclei from the flue gas. However, it has been observed that at certain operating conditions of the WESP, additional particles are formed which can act as aerosol nuclei. This study was performed to find out the specific operating conditions in terms of feed SO2 concentration and applied WESP voltage, at which WESP results in generation of new particles. In the absence of SO2, no additional particles are formed for the entire range of applied WESP voltage. At the lowest SO2 concentration of 9 ppmv, additional particles were generated beyond 10 kV. The smallest size fractions contribute to more than 99% of the total number of particles, with 0.006 μm contributing to more than 80%. At the outlet of the absorber, although majority of the particles are of 0.006 μm (>60%) and 0.0158 μm (ca. 5%), larger particles of sizes 0.0945 to 0.945 μm, contributes to the remaining 35% of the total number. The shift in number distribution results in increase of aerosol mass from about 10−4 to 5 × 10-3 mg/m3 to 0.2 to 0.5 mg/m3, from the inlet to the outlet of the absorber, respectively. Aerosol based MEA emissions were observed for the lowest SO2 concentration of 9 ppmv and a voltage of 16 kV. Total MEA emissions were in the range of 600–1050 mg/Nm3. Based on these tests, industrial scale WESPs are likely to result in additional particles which can act as aerosol nuclei even at a SO2 concentration as low as 9 ppmv. Additional tests aimed at SO2 concentration lower than 9 ppmv in the feed gas, combinations of varying H2SO4 aerosol and SO2 in feed gas, and varying flue gas velocity needs immediate attention to define the operating limits for WESP.