Activity and stability of immobilized carbonic anhydrase for promoting CO 2 absorption into a carbonate solution for post-combustion CO 2 capture Shihan Zhang a , Zhaohui Zhang a,1 , Yongqi Lu a,⇑ , Massoud Rostam-Abadi a,b , Andrew Jones c a Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, United States b Department of Civil & Environmental Engineering, University of Illinois at Urbana-Champaign, United States c US Department of Energy, National Energy Technology Laboratory, United States article info Article history: Received 30 July 2011 Received in revised form 12 September 2011 Accepted 12 September 2011 Available online 17 September 2011 Keywords: CO 2 capture Enzyme immobilization Stability Carbonic anhydrase Carbonate abstract An Integrated Vacuum Carbonate Absorption Process (IVCAP) currently under development could signif- icantly reduce the energy consumed when capturing CO 2 from the flue gases of coal-fired power plants. The biocatalyst carbonic anhydrase (CA) has been found to effectively promote the absorption of CO 2 into the potassium carbonate solution that would be used in the IVCAP. Two CA enzymes were immobilized onto three selected support materials having different pore structures. The thermal stability of the immo- bilized CA enzymes was significantly greater than their free counterparts. For example, the immobilized enzymes retained at least 60% of their initial activities after 90 days at 50 °C compared to about 30% for their free counterparts under the same conditions. The immobilized CA also had significantly improved resistance to concentrations of sulfate (0.4 M), nitrate (0.05 M) and chloride (0.3 M) typically found in flue gas scrubbing liquids than their free counterparts. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Carbon dioxide (CO 2 ) is a greenhouse gas that has been shown to be a major contributor to global warming. In the US, fossil fuel- fired power plants emit about 2.4 billion metric tons of CO 2 each year, or about 40% of total US emissions. Various technologies, including absorption, adsorption and membranes, have been pro- posed for capturing the CO 2 emitted from fossil fuel-fired power plants. However, only mono-ethanol-amine (MEA)-based absorp- tion processes are currently available for post-combustion CO 2 capture, but they are expensive. A major part of the capture cost using MEA processes, amounting to about 60%, is the parasitic power loss due to the extraction of power plant steam for strip- ping the CO 2 from the MEA (Singh et al., 2003). Recently, a novel Integrated Vacuum Carbonate Absorption Process (IVCAP) has been proposed to reduce the energy usage for CO 2 stripping (Chen et al., 2007). The process employs a potassium carbonate (PC, K 2 CO 3 ) aqueous solution as a solvent for CO 2 absorption. The weak affinity of CO 2 for K 2 CO 3 allows the CO 2 to be stripped from the CO 2 -rich solution at a low temperature (50–70 °C) and vacuum condition (2–8 psia). These temperate/pressure condi- tions enable the use of either waste steam or low-quality steam from the power plant’s steam cycle system to provide the heat re- quired for CO 2 stripping and this significantly reduces the para- sitic power loss. However, K 2 CO 3 is only weakly alkaline and thus a K 2 CO 3 -based system has a much slower CO 2 absorption rate than MEA-based systems. A conventional approach for increasing the rate of CO 2 absorption into a weak solvent is to mix one or more solvents with stronger CO 2 affinities into the prime solvent (Cullinane and Rochelle, 2004, 2005). This ap- proach, however, will increase the heat of absorption and change the phase equilibrium of the system. As a result, a higher temper- ature, or a higher level of vacuum, will be required for CO 2 strip- ping. This study suggests that the carbonic anhydrase (CA) enzyme can be employed as a catalyst to accelerate the absorption rate in the IVCAP without changing the heat of absorption and phase equilibrium of the system. Carbonic anhydrase is a zinc metalloenzyme that can efficiently catalyze the hydration of CO 2 to form bicarbonate. The turnover rate of the CA enzyme can be as great as 10 6 s 1 (Kernohan, 1965). A free CA enzyme homogeneously dissolved in a solution may offer a greater specific activity compared to an immobilized enzyme. However, immobilization can improve the stability of the enzyme, which is important for the CO 2 capture application in a power plant. The enzyme can either be immobilized onto pack- ing materials, or onto fine particle carriers that are suspended in 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.09.043 ⇑ Corresponding author. Tel.: +1 217 244 4985. E-mail address: lu@isgs.illinois.edu (Y. Lu). 1 Visiting scholar from the College of Biological & Environmental Engineering, Zhejiang University of Technology, China. Bioresource Technology 102 (2011) 10194–10201 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech