Mathematical Modelling for Carbon Dioxide Removal from Flue Gas Using Micron-size Water Droplets
Numerous technologies have been developed to reduce the amount of CO2 released to the environment via combustion processes. A promising process named the C-3 process uses micron size water droplets to remove contaminants (e.g., CO2, NOx, SOx, and Hg) from flue gas. However, the mechanisms for capturing CO2 using this relatively simple water-droplet-based process are not fully explained or understood. This work uses mathematical models to explore possible mechanisms for capturing CO2 by high-velocity water droplets with large surface-area-to-volume ratio. Several capture mechanisms are identified based on a literature review including: i) dissolution of CO2 in liquid water, ii) formation of H2CO3 and its ions, iii) adsorption of CO2 on the droplet surface, and iv) congregation of H2CO3 inside the droplet surface. A preliminary mathematical model is developed to study the importance of the proposed mechanisms, with the droplet surface treated as a region where CO2 and related species can accumulate. In addition, a more complex model that includes two additional phenomena: i) heat transfer from flue gas to water droplet and ii) shrinkage of the droplet due to water evaporation are also developed to explore the influences of these phenomena on the CO2 capture process. Simulation results reveal that equilibrium absorption following Henry’s law is insufficient to account for the relatively large quantities of CO2 (~27 g CO2/kg water) that have been absorbed in a demonstration unit, suggesting that surface adsorption/absorption effects are important. Adsorption/absorption of CO2 and H2CO3 at the droplet interface could explain the high observed levels of CO2 removal, if unknown partition coefficients K_(IL,CO2 )and K_(IL,H2CO3) are as large as 0.01 m and 0.1 m, respectively, or if the mean droplet diameter is smaller than 5 µm. Simulation results predict that the amount of CO2 captured increases as temperature and water droplet size decrease. Velocity of the water droplet is important when heat effects are considered in the simulations because high-velocity water droplets are predicted to shrink faster than stationary droplets. Droplets can either shrink or grow depending on the temperature and composition of the surrounding flue gas, which will change with position in the C-3 process.
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