Measuring and modeling aerosols in carbon dioxide capture by aqueous amines
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Pilot scale CO2 capture plants have shown that amine condensation onto seed nuclei results in very high amine emissions which are very difficult to control using traditional aerosol removal techniques. Aerosol emissions can be suppressed by adjusting operating conditions such that drops evaporate, or, alternatively, grow to a size that can be efficiently captured by low cost methods. The effects of operating conditions on aerosol growth were investigated by experimental measurement and numerical modeling with sensitivity analyses. Total particle densities and particle size distributions (PSDs) were measured using a custom-built phase Doppler interferometer (PDI) on bench and pilot scale CO2 absorbers. Seed nuclei were generated using vaporized H2SO4, gaseous SO2, and flue gas from a coal-fired power plant. PSDs were used to calculate the aerosol amine concentration when compared to total phase (gas and aerosol) measurements collected by FTIR. The effects of operating conditions on aerosol growth were simulated in a combined heat and mass transfer model coded in MATLAB®. Aerosol transport equations included corrections for surface curvature and transport length scale regimes. Absorber and water wash models were simulated using Aspen Plus®. Inlet CO2 is crucial in creating supersaturation in the absorber; the loading difference between the aerosol and bulk solvent creates an amine driving force for condensation. Aerosols grow faster in non-intercooled columns due to differences in solvent composition (CO2 loading) and temperature. H2O condensation is the primary growth mechanism in the water wash. Reducing the water wash amine concentration and providing additional residence time leads to more aerosol growth. Doubling the water wash height results in a 13.7 % increase in the final aerosol diameter for a generic 8 m PZ absorber. Similar to some other volatile amines, PZ forms 1–5 μm aerosols because its amine volatility is a strong function of CO2 loading. The amine concentration in measured aerosol distributions, calculated by PDI/FTIR comparison, was one-to-two orders of magnitude lower than the bulk solvent. SO2 forms aerosol with PZ. 65 % of injected SO2 leaves in the aerosol phase. Therefore, SO2 polishing scrubbers are essential and systems should not be designed for simultaneous absorption of CO2 and SO2.