While other methods are no doubt possible, in practice most existing air capture development is focused on two broad separation methods, chemisorption in aqueous solutions and chemisorption on solid surfaces.
Separations that take advantage of CO2's acidity in solution are the current standard for industrial processing. Literally dozens of such processes and solvents have been developed for the removal of CO2 and H2S (collectively called acid gases) from natural gas, either separately or as a mixed gas stream. These processes have more recently been used for treating synthesis gas mixtures from gasification of coal, natural gas, or heavy petroleum fractions (IPCC, 2005). These processes differ functionally from each other in their selectivity for CO2 against H2S; their ability to remove these gases to very low (ppm or lower) levels; their sensitivity to other gases such as ammonia; their maintenance and operating costs; and their trade-off of capital and energy costs.
The primary barrier to using such process is that the kinetics of CO2 dissolution into water are limited by the initial reaction to form carbonic acid (CO2 + H2O ^ H2CO3). While this reaction is sufficiently fast to make aqueous systems cost-competitive for capture of CO2 from power plant exhaust streams, it is too slow at the much lower concentrations in ambient air. Two methods are being explored to get around the kinetic limitation in aqueous systems.
One option is to accelerate the reaction using a catalyst. The naturally occurring enzyme carbonic anhydrase can accelerate the CO2 + H2O reaction by a factor of ~109 and facilitates respiration in living cells by catalysing the reverse reaction (all catalysts speed their target reactions in both directions). Using an enzyme as a catalyst is challenging because, to name only a few issues, they only operate in a narrow pH and temperature range and as organic compounds they may be decomposed by micro-organisms (Bao and Trachtenberg, 2006). Roger Aines and collaborators at Lawrence Livermore National Laboratory are developing synthetic catalysts that would be somewhat less effective in accelerating the reaction than carbonic anhydrase but which could be tailored for the air-capture application (Aines and Friedmann, 2008).
An alterative to catalysis is to use aqueous solutions with very high pH. For example, our group has focused on using NaOH solutions with a concentration between 1 and 6 mol/L with pH near 13. In these solutions the kinetics are dominated by the direct reaction with the hydroxyl radical (CO2 + HO- ^ HCO3-), which enables mass fluxes of ~3 gCO2/hr-m2 for the applicable case in which mass transfer is liquid-side limited. The advantages of strong bases are that (a) they use simple inorganic chemistry which is insensitive to contamination, (b) vapour pressures are low so evaporative loss of the base to the atmosphere is minimal, (c) their high molarity enables low liquid-side fluid pumping work, (d) at sufficiently high molarity evaporative water loss can be eliminated, and (e) the technique does not depend on the development of novel solids or catalysts.
The primary disadvantage is the difficulty of regenerating the resulting carbonate solution back to hydroxide. Recovery of NaOH from Na2CO3 is closely related to 'caustic recovery', one of the oldest processes in industrial chemistry. In Kraft pulping for paper making, wood is digested using sodium hydroxide to liberate cellulose and produce pulp. The remained solution, so-called 'black liquor', consists of mainly other organic material originated from wood (e.g. lignin) along with sodium carbonate. The standard process for recovering NaOH from Na2CO3 depends on a calcium cycle, a process that has been used on a continuous basis for more than 80 years. Several studies have investigated adaption of this process to recovery of NaOH for air capture (Zeman and Lackner, 2004; Keith et al., 2005; Baciocchi et al., 2006), alternative caustic recovery methods include the titanate cycle (Mahmoudkhani and Keith, 2009).
Using an alkaline solvent, typically an organic amine compound, allows good selectivity and solvent loading without excessive regeneration costs. However, for air capture any evaporative loss of solvent to the air stream is a significant loss compared to the amount of CO2 captured and will make the overall economics untenable. The same type of chemistry used in aqueous absorption processes can be adapted to solid sorbent phases which will not evaporate. A high surface area material can be chemically modified so that it reacts with CO2 and can remove even low concentrations from air. The challenge then is to provide a large surface area for CO2 capture without having a large mass of solid support which must be heated to drive off the bound CO2.
Two solid sorbent systems are being actively developed for air capture. Both processes offer the advantage of low regeneration energy. Klaus Lackner and collaborators at Global Research Technologies (GRT) are developing an ion exchange membrane which captures CO2 using a carbonate to bicarbonate swing driven by changes in humidity. A significant challenge is that the partial pressure of CO2 achieved during the regeneration phase is only about 0.1 bar, so to obtain pure CO2 suitable for sequestration it is necessary either to purge all the air from the sorbent beds (including the internal pores of the sorbent material) before performing the regeneration step in a vacuum, or to regenerate in air and then capture the CO2 from air at a 10% concentration. Large-scale vacuum operations, especially repeated batch processes, present a variety of engineering issues with sealing and with the generally low energy efficiency of vacuum pumping systems.
An alternative solid sorbent system is being developed by Peter Eisenberger and Graciela Chichilnisky of Global Thermostat, using solid amines on a mesoporous silica substrate similar to those that are being developed for CO2 capture from power plants (Gray et al., 2007). Capture is accomplished using temperature-swing regeneration driven by low-grade heat (~100 °C or less).
In general, these solid sorption methods offer the potential to achieve low capture energies with minimal water loss. Perhaps the central challenge in commercializing them is the need to build a solid surface with very high surface area at low manufacturing cost while simultaneously achieving long service lifetimes when operating in the free atmosphere which will inevitably be contaminated with various trace chemicals and windblown dust.
In general, a direct air-capture process will require electrical power to drive systems such as fans and pumps as well as the thermal input to drive the regeneration process itself. For a stand-alone air-capture system, thermal power could in principle be supplied by a wide range of energy sources including solar thermal power, natural gas, coal or nuclear heat. Electrical power could be imported from the grid or (co-)generated on site depending on economics and the opportunities for heat integration.
Solar offers zero energy cost and no CO2 production but at the expense of relatively high capital costs and intermittent operation which does not integrate well with the desired steady operation of chemical processes. Because the plant would not operate at full capacity for a large fraction of the time, this raises the capital costs per ton of CO2 collected. While it is possible to reach high temperatures (>1000 °C) in solar furnaces, most of the commercial development of solar thermal electricity is now focused on parabolic troughs which produce lower grade heat. Nikulshina et al. (2009) have explored the use of solar furnaces for CaCO3 (limestone) calcination as part of an air-capture process, and such solar kilns could also be applied to the titanate process we describe below (Section 6.5).
In general, natural gas combustion offers the simplest and lowest capital cost plant design at the expense of relatively high energy costs. Combustion gas turbines offer the possibility of efficient cogeneration of power and process heat. Because our air-capture approach needs substantial quantities of high-grade heat and we wish to minimize initial technical risk, all of our current design efforts focus on natural-gas-driven systems using cogeneration so that the plant has no significant external electricity demand. We do note however that large air-capture plants, like any large chemical process, will operate for long periods at steady high rates; this situation is analogous to base-load electrical power production where coal and nuclear systems have proven to be the most economical technologies. Where cost-effective, CO2 emissions from the natural gas combustion in the process are captured using either oxy-fuel combustion or post-combustion capture.
Coal offers low energy cost but requires integrated carbon capture. The simplest technical approach for supplying high-grade heat to a calcination process would be direct combustion of coal with the material to be calcined (e.g. CaCO3 or titanates). This process is widely used for lime production. The disadvantage is the management of the coal ash and the possibility that the ash interferes with process chemistry once the lime or titanates are cycled into the capture process. Alternatively, coal could be used in a gasification system to supply syngas or hydrogen to heat the air-capture process, with CO2 capture and storage for the CO2 emissions from gasification.
Nuclear heat offers high capital and low operating costs without generating additional carbon dioxide, but it has some disadvantages beyond the well-known issue of local public acceptance. One is the requirement to manage design standards and safety engineering in the integrated plant, because different engineering standards apply to chemical/thermal and to nuclear systems. This primarily affects the documentation and engineering analyses needed to license the nuclear plant for this integrated operation. A second disadvantage is the potential operational difficulty of running a nuclear plant closely coupled to a chemical facility. This concerns economic performance rather than safety performance. Almost all existing commercial nuclear plants have been used for electrical power production, some with modest cogeneration of heat for local district heating. In operating the first of any new technology such as nuclear-powered air capture, there is always the possibility of unexpected maintenance needs or poor performance.
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