ARTICLE pubs.acs.org/EF
Mesoporous Alumina-Supported Amines as Potential Steam-Stable Adsorbents for Capturing CO2 from Simulated Flue Gas and Ambient Air Watcharop Chaikittisilp, Hyung-Ju Kim, and Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100, United States
bS Supporting Information ABSTRACT: Carbon management by a means of CO2 capture from large stationary sources such as coal-fired power plants or from ambient air is a significant global issue. In the context of steam-stripping as a regeneration process for solid CO2 adsorbents, new adsorbent materials robust enough for direct contact with low temperature steam are needed. Here, mesoporous γ-aluminasupported poly(ethyleneimine) composite materials are prepared and evaluated as effective CO2 adsorbents, using dilute CO2 streams such as simulated flue gas (10% CO2) and ultradilute streams such as simulated ambient air (400 ppm CO2). In comparison to the silica-supported amine adsorbents typically utilized for CO2 capture applications, the alumina-supported amine adsorbents give better performance in terms of both capture capacity and amine efficiency, in particular, at ambient air conditions. In addition, the alumina-supported amines are stable over short multicycle temperature swing tests and, more importantly, appear to be more robust than the silica-based counterparts upon direct contact with steam. Thus, the resulting alumina-supported amines are suggested to be promising new materials for CO2 capture processes equipped with steam-stripping regeneration, especially from ultradilute gas streams.
’ INTRODUCTION Many studies have suggested that anthropogenic CO2 emissions are, in part, responsible for the climate change witnessed over the last century.1 3 Concern about the growing atmospheric CO2 concentration has led to the exploration of technologies capable of stemming anthropogenic CO2 emissions and, more preferably, reducing the atmospheric CO2 concentration.4 7 A carbon capture and sequestration (CCS) process is conventionally comprised of three key steps: (i) extraction and concentration of CO2 from a gas stream; (ii) pressurization, pipelining, and subsequent transportation to a storage site; and (iii) sequestration of compressed CO2 into permanent or semipermanent storage location.5,6 Large stationary sources such as power and industrial plants are responsible for approximately 60% of global CO2 emissions.8,9 Moreover, coal-fired electricitygenerating power plants account for about three-quarters of power-sector emissions, that is, about 30% of global emissions.9 Thus, the postcombustion capture of CO2 emitted from such large point sources, especially coal-fired power plants, has been considered as a first step toward the stabilization of the atmospheric CO2 concentration because such capture units can be installed into existing plants worldwide in the short term. In the long term, other alternatives are being considered and developed, including precombustion capture of CO2 from synthesis gas products after a water gas shift reaction,10 carbon capture via oxyfuel combustion,11 and chemical looping combustion.12 Fossil fuels will continue to be the dominant energy source in the near future.9 However, the aforementioned processes can, at best, decrease the rate of increase of the atmospheric CO2 level. r 2011 American Chemical Society
To address the CO2 emitted from distributed sources such as the transportation sector, the direct capture of CO2 from ambient air, the so-called “air capture approach”, was proposed by Lackner as an alternative carbon mitigation technology.13 Air capture has a potential advantage over conventional CCS (capture from stationary sources) because it can, in principle, be installed anywhere and can capture CO2 released from all sources, including mobile sources. In addition, air capture is a potentially “negative” carbon technology while conventional CCS can provide only “avoided” carbon (avoided carbon is defined as carbon that would be released to the atmosphere but is not).6 Since Lackner first proposed the concept, the technological, economical, and environmental feasibility has been evaluated.14 19 Although air capture is proposed to be technologically feasible, its economical feasibility is still uncertain, mainly depending on the adsorption process, the desorption method, and the design of structured contactors. Along this avenue, we first reported the application of supported amine adsorbents for the direct extraction of CO2 from ambient air in 2009.20 23 In the case of supported amine adsorbents, the CO2 capture capacities under simulated ambient air conditions were about 50% of those observed under simulated flue gas conditions when the CO2 partial pressure was decreased by a factor of 250 (from 10% to 400 ppm), suggesting that supported amine adsorbents are promising materials for the air capture applications.20,21,23 25
Received: August 11, 2011 Revised: October 12, 2011 Published: October 13, 2011 5528
dx.doi.org/10.1021/ef201224v | Energy Fuels 2011, 25, 5528–5537
Energy & Fuels In the capture of CO2 from large stationary sources (e.g., the traditional postcombustion capture from flue gas), absorption of CO2 by amine-based aqueous solutions is the benchmark technology.26 This amine scrubbing technology has been employed to separate CO2 from natural gas and is being extended to trap CO2 from flue gas. Although it is a robust process, it suffers from an energy-intensive regeneration step, the corrosion of process equipment, and the volatilization and oxidative degradation of the active amine species. To overcome such problems, the adsorption of CO2 on several classes of solid adsorbents has been investigated.6,27 29 Among other adsorbents, solid-supported amines are a promising class of adsorbents because they can be operated at low temperature (ambient to ∼120 °C) with excellent capture performance.27 Being chemisorbants with a substantial heat of reaction with CO2, they have been examined for CO2 capture from ambient air, as well as from flue gas.20 25,27,30 48 Such supported amine adsorbents have been defined by us into three classes:49 (i) class 1 amine adsorbents, on the basis of monomeric or polymeric active amines that are physically impregnated into/onto porous supports, conventionally silica;22,30 38 (ii) class 2 materials, consisting of monomeric amines that are covalently bound to supports, most often aminosilanes linked to a silica surface;24,25,39 44 and (iii) class 3 materials, constructed by in situ polymerization of amine-containing monomers in the pores of supports, thereby resulting in aminopolymers covalently tethered to supports, chiefly silica.20,21,23,45 48 For the captured CO2 to be stored underground or used in beneficial applications, such as feeding algae for biofuel production or use as a carbon source for conventional fuel or chemical synthesis,50,51 it must be isolated from emission sources and subsequently purified into a concentrated CO2 stream. In most of the current literature, unfortunately, the design of adsorbents with high uptake capacities has been a singular focus of the academic community to date.27 As a result, the regeneration of the adsorbents by heating under a flow of inert gas for laboratory scale studies, as done here, has been a routine procedure. It must be recognized that this regeneration method is absolutely impractical, as it does not offer effective CO2 concentration; both inlet and regenerated streams are diluted CO 2 gas mixtures. Moreover, regeneration by conventional temperature or vacuum swing desorption has been considered by some to be too costly.52 Thus far, only a few studies have investigated more practical approaches for regeneration of adsorbents to potentially produce concentrated CO 2 streams via (i) temperature vacuum swing desorption,24,53 57 (ii) heating the samples in a pure CO2 stream,34,58,59 and (iii) steam stripping.49 Combined temperature vacuum swing operation is considered to be more viable than conventional temperature swing or vacuum swing processes but may be still expensive in a large-scale operation. Although adsorbents can be regenerated and a concentrated CO2 stream can be produced by the second method above, a report showed that significant deactivation of the amines via urea formation is pronounced when the supported amines are heated in the presence of CO2.59 Nonetheless, this approach could be more useful if the amines can be stabilized, for example by humidity.41 We reported the first demonstration of steamstripping regeneration of supported amine-based CO2 adsorbents in 2010.49 In this method, the adsorbents are regenerated by direct contact with low-grade steam (e.g., saturated
