Experimental and theoretical study of carbon dioxide absorption into potassium carbonate solution promoted with enzyme

According to the Intergovernmental Panel on Climate Change (IPCC), carbon dioxide (CO2) concentration in the atmosphere has increased from its pre-industrial value of 280 parts per million by volume (ppmv) to 384 ppmv. IPCC predicts that CO2 concentration in the atmosphere will rise to 550 ppmv by the year 2100, if anthropogenic emissions continue to increase. The average temperature at the Earth's surface could increase 1.8-4.0K above the 1990 levels by the end of this century. Such warming is anticipated to cause sea level rise, increased intensity and frequency of extreme weather events, ice shelf disruption, and changes in rainfall patterns. As a result, reducing CO2 emissions from anthropogenic sources is a high priority. Combustion of fossil fuels for power generation is the major contributors of CO2 emission into the environment. Currently, CO2 chemical absorption using monoethanolamine (MEA) as a solvent is the best available option for CO2 capture from flue gas streams. The issue with this technology is the high capture cost which ranges from $50/metric ton to $70/metric ton CO2 avoided. Energy consumption by the process contributes to 60% of the cost. Thus, use of solvents with lower heats of absorption is preferable. A novel process called Integrated Vacuum Carbonate Absorption Process (IVCAP), which employs potassium carbonate (PC) as a solvent, has been proposed. Since chemical affinity of CO2 to K2CO3 is weak compared to MEA, the regeneration of CO2-rich solution can be operated under vacuum at a lower temperature. Hence, a low quality steam from the power plant steam cycle can be used as the heat source for the regeneration. IVCAP process is expected to have 25-30% lower energy requirements as compared to an MEA-based process. However, compared with the MEA solution, PC solutions with low heats of absorption generally exhibit much slower CO2 absorption rates. Hence, a biological catalyst, carbonic anhydrase (CA) was investigated to promote the rate of CO2 absorption into select PC solutions. Experiments were performed in a stirred-tank reactor to evaluate the activity of the CA enzyme under IVCAP conditions. Results revealed that addition of up to 300 mg/l CA enzyme to the PC solutions at 25oC increases the absorption rate by a factor of 6-20 when compared with the same solution without the CA. It was also observed that, the CO2 absorption rates into the aqueous PC solutions with different initial conversion levels of PC to potassium bicarbonate are similar, with differences no larger than 20%, when the concentration of CA enzyme is 300 mg/l. It was also observed that, at the 300 mg/l CA concentration, increasing the temperature from 25oC to 50oC reduces the rate of CO2 absorption, by up to 20%. A mathematical model based on Higbie's penetration theory was developed to simulate the absorption of CO2 into the PC solutions. A comparison of modeled to experimental absorption rates of CO2 provided agreement within 30%. The modeling results revealed that at CA concentrations > 3,000 mg/l, the absorption rate of CO2 is independent of CA concentration. Compared to the enzyme concentration (300 mg/l) used in this study, a further increase of enzyme concentration to a level not larger than 3,000 mg/l could further increase the absorption rate of CO2. Based on the experimental and modeling results obtained in this research, it is recommended that the CO2 absorption rate into PC-CA be further enhanced by improving other parameters such as the activity of CA enzyme and design optimization of the absorption column including the type of packing material. Further work is required to investigate the stability of the CA enzyme at longer test duration and use of immobilized CA enzyme. Effectiveness of the regeneration cycle also needs to be investigated. Further work should also include the test of an integrated absorption/ regeneration system for CO2 capture at a real flue gas condition. ??