Investigating improvements to potassium carbonate solvent absorption

© 2011 Dr. Kohei EndoSolvent absorption technology has been suggested as one method of capturing the carbon dioxide in post-combustion emissions of power plants. The work carried out in this thesis investigates the feasibility of two novel process improvements to potassium carbonate solvent. The first is a low temperature slurry process to capture carbon dioxide by forming solid and liquid potassium bicarbonate in the absorber to reduce material and energy costs. The second is the co-capture of sulfur and nitrogen oxidation gases to eliminate the capital cost of expensive flue gas pre-treatment stages. Experimental vapour-liquid equilibrium (VLE) studies of carbon dioxide over 30 weight percent potassium carbonate solution with and without boric acid were carried out at low temperature (50 and 70 °C) using an inert gas stripping method. The VLE was simulated using thermodynamic models from the literature. The effect of boric acid was simulated by including two chemical reactions that simplify the reaction of boric acid with potassium carbonate. The solid-liquid equilibrium (SLE) of potassium carbonate containing sulfate and nitrate formed by the reaction of SOx and NOx gases with the solvent was investigated using the Electrolyte-NRTL model and the Pitzer Ion-interaction model. The parameters to describe the potassium carbonate-bicarbonate-water system were obtained from the literature. The potassium sulfate-water and potassium nitrate-water parameters were regressed using data from the literature while mixing parameters were used to fit ternary solubility data. From the VLE and SLE models it was determined that a precipitating slurry can be formed when the absorber operates below 60 °C and contains potassium carbonate in overall concentrations of higher than 40 weight percent. SOx and NOx co-capture in a standard potassium carbonate solvent was analysed using the SLE model. It was determined that potassium sulfate could be removed by crystallization after three days of operation. In contrast, due to the low reactivity of NOx and the high solubility limit of nitrate, it was found that continuous operation for over three years would be required before nitrate could be removed by crystallization. The focus of the crystallization study was on potassium sulfate grown in 30 weight percent potassium carbonate solution. A mixed suspension mixed product removal (MSMPR) crystallizer was used to experimentally determine nucleation and growth rates. It was found that the growth rate was similar to that of potassium sulfate grown in aqueous solution, although minimal supersaturation dependence was observed. A noticeable change occurs to the crystal morphology with mostly octahedral crystals being formed across the entire size range beyond 50 microns. Due to the high nucleation rate the median crystal width is 400 microns while the coefficient of variation is 52%