Novel promoters for carbon dioxide absorption in potassium carbonate solutions

© 2018 Dr. Guoping HuCarbon dioxide is a major driver for climate change and carbon capture and storage (CCS) is widely recognized as an effective way to reduce CO2 emissions to mitigate climate change. However, managing the cost of carbon capture to an acceptable level is of vital importance to deploy it at an industrial scale. Potassium carbonate solvent (K2CO3) shows promise as a solvent for carbon capture due to its low cost, low corrosivity, low degradation rates and low environmental impact. However, as the absorption rate of CO2 using K2CO3 is relatively slow, improving its absorption kinetics via adding rate promoters is crucial for reducing the capital cost of absorption equipment required to build the carbon dioxide capture plant. In this study, a series of promoters were investigated to improve the absorption kinetics. Different promoters including organic promoters, inorganic promoters and enzymatic promoters have been reported in the literature. From the literature review, a good promoter should be economically acceptable, stable, non-toxic, non-corrosive, highly efficient, environmentally benign, recyclable, and have a low vapour pressure. It was recommended that more efforts should be focused on carbonic anhydrase enzyme and different amino acids, which is the focus of the present study. A carbonic anhydrase (NZCA) was first examined as a promoter in potassium carbonate solutions. The catalysis kinetics of this promoter were tested via the stopped flow technique and a wetted wall column (WWC). The Michaelis-Menten catalysis parameter (kcat/Km) was determined to be 2.7×107 M-1s-1 at 298 K, resulting an activation energy of 51±1 kJ/mol at 298‒328 K. The promoting coefficient of the NZCA was determined to be 5.3×107 M-1s-1 using a WWC in 30 wt. % potassium carbonate solutions (pH ~ 11‒12) at 323 K. Furthermore, the NZCA kept more than 70% of its initial catalysis efficiency after continuously running for 8 hours in 30 wt. % K2CO3 solutions at pH of 10.6‒10.8 and temperature of 323 K. Then, histidine was investigated as a promoter for CO2 absorption as this is an important component in carbonic anhydrase. Results showed that histidine anion ions (His‒) are the main species reacting with CO2 in basic conditions (pH>9) with a reaction order of 1.18±0.08 across the temperature range of 298–313 K. The zwitterion mechanism was used to fit the kinetic data and it showed that both protonation and deprotonation reactions contributed to the overall reaction rate. Ionic strength was also shown to have a significant influence on the reaction kinetics when the histidine concentration is high (≥0.2 M). The reaction rate between histidine and CO2 was shown to be slower than that of glycine and proline and slightly faster than that of taurine at low concentrations (<0.1 M). A range of different amino acids were next investigated as promoters. The amino acids investigated in this study were 2-piperazinecarboxylic acid, asparagine, aspartic acid, glycine, leucine, lysine, proline, sarcosine, serine and valine. Furthermore, proline, sarcosine, glycine, leucine and lysine were tested as rate promoters in potassium carbonate solvent for carbon dioxide absorption using a wetted wall column. Results showed that the anions of the amino acid salts are the major species reacting with carbon dioxide. Therefore, the promoting effect of amino acid salts is sensitive to changes in pH due to changes in species distribution. Sarcosine and proline were the most effective promoters among the amino acid salts tested in this study with comparable promoting performance at higher pH values (≥12.5) but with sarcosine more effective at lower pH values (12.0), while the promoting performance of MEA was comparable with that of proline and slightly poorer than that of sarcosine at low pH (