Biotechnology for the acceleration of carbon dioxide capture and sequestration

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Biotechnology for the acceleration of carbon dioxide capture and sequestration
Christopher K Savile and James J Lalonde

The potential for enzymatic acceleration of carbon dioxide capture from combustion products of fossil fuels has been demonstrated. Carbonic anhydrase (CA) accelerates post combustion CO2 capture, but available CAs are woefullyinadequate for the harsh conditions employed in most of these processes. In this review, we summarize recent approaches to improve CA, and processes employing this enzyme, to maximize the benefit from this extremely fast biocatalyst. Approaches to overcoming limitations include sourcing CAs from thermophilic organisms, using protein engineering to evolve thermo-tolerant enzymes, immobilizing the enzymefor stabilization and confinement to cooler regions and process modifications that minimize the (thermo-, solvent) stress on the enzyme.
Address Codexis, Incorporated, 200 Penobscot Drive, Redwood City, CA 94063, USA Corresponding author: Lalonde, James J (james.lalonde@codexis.com)

effects. While energy production technologies exist to limit or even eliminate carbon dioxide emission, fully halfof the electricity in the United States is generated from the combustion of cheap and abundant coal, and coal combustion is responsible for 40% of fossil fuel derived CO2 emissions worldwide. Thus, CO2 capture and sequestration (CCS) has been identified as the critical enabling technology to reduce CO2 emissions significantly, while allowing the continued use of coal [6]. The position ofequilibrium of reaction (1) is far to the left, so CCS processes employing CA, must do so in either an alkaline capture solvent approach (e.g. aqueous monoethanolamine (MEA), potassium carbonate or methyldiethanolamine (MDEA)) to neutralize the proton released or must use a biomineralization approach in which high levels of divalent calcium or magnesium precipitate CO2 as solid carbonate.

Current Opinionin Biotechnology 2011, 22:818–823 This review comes from a themed issue on Chemical biotechnology Edited by Guo-Qiang Chen and Romas Kazlauskas Available online 5th July 2011 0958-1669/$ – see front matter Published by Elsevier Ltd. DOI 10.1016/j.copbio.2011.06.006

Use of CA in solvent-based PCCC
Chemical absorption with regenerable alkaline aqueous solvents is considered the nearest termoption for post combustion CCS [7]. In this process (Figure 2), CO2 is removed from the flue gas stream in the absorber column and then desorbed in a heated stripper column to give relatively pure CO2 for compression and storage. The challenge facing these capture processes is in energy loss in desorption. Solvents such as MEA tend to bind CO2 tightly such that the parasitic energy loss in desorbingthe CO2 would result in a near doubling of the cost of electricity [8]. CA has been shown to facilitate the use of aqueous solvents with a far lower heat of desorption (e.g. hindered and tertiary amines), thus enabling a far lower energy penalty on CCS [9]. Absorption of CO2 tends to be slower in these solvents and thus requires the use of an accelerant such as CA. The poor stability and activityof naturally derived CA under the harsh conditions of these processes i.e. temperatures from 50 to over 1258C, high concentrations of organic amine, trace contaminants such as heavy metals, and sulfur and nitrogen oxides, have limited their use. Approaches to overcoming these limitations have included sourcing CAs from thermophilic organisms, using protein engineering techniques to createthermo-tolerant enzymes [10,11], immobilizing the enzyme (for both stabilization and restriction to the cooler process zones) or process modifications such as cooling of the flue gas. Small molecule analogs of CA have been reported with potentially higher stability than proteins,
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Introduction
Carbonic anhydrase (CA, E.C. 4.2.1.1) is among the fastest known enzymes, catalyzing...