Carbon Capture

The capture of CO₂ is motivated by the forecasted change in climate as a result of the world’s dependence on fossil fuels for energy generation. Mitigation of CO₂ emissions is the challenge of the future for stabilizing global warming. The separation of CO₂ from gas mixtures is a commercial activity today in hydrogen, ammonia, and natural gas purification plants. Typically, the CO₂ is vented to the atmosphere, but in some cases, it is captured and used. The current primary uses of CO₂ include enhanced oil recovery (EOR) and the food industry (carbonated beverages). The traditional approach for CO₂ capture for these uses is solvent-based absorption. It is unclear whether this technology will be the optimal choice to tackle the scale of CO₂ emitted on an annual basis (~ 30 Gt worldwide). A new global interest in extending CO₂ capture to power plants is producing a dramatic expansion in R&D and many new concepts associated with clean energy conversion processes. The application of CO₂ capture technologies beyond concentrated sources is in view, but less tractable. The first and second laws of thermodynamics set boundaries on the minimum work required for CO₂ separation. Real separation processes will come with irreversibilities and subsequent inefficiencies taking us further from best-case scenarios. The inefficiency of a given process reveals itself in the form of operating and maintenance, and capital costs.

Following capture and separation of CO₂ from a gas mixture, the CO₂ must be compressed for transport, regardless of whether it is destined for storage in underground geologic reservoirs or used as a feedstock in chemical processing, aggregate formation, or EOR. There are various modes of transport such as rail, ship, truck, and pipeline. On average, a single 500-MW coal-fired power plant would need to transport on the order of 2–3 Mt per year of compressed CO₂, which makes pipeline transport a feasible option due to the potential large-scale application (Energy Conv Manag 45(15–16):2343–2353, 2004). For instance, the installed capacity in the U.S. is approximately 315 gigawatts (GW) (EIA Existing Electric Generating Units by Energy Source; Energy Information Administration (EIA), Department of Energy (DOE): Washington, D.C., 2008), China’s installed capacity of coal-fired power is approximately 600 GW and India’s approximately 100 GW. The installed capacity in China and India will undoubtedly grow since as of 2009 these regions had populations of 186 million (China and East Asia) and 612 million (South Asia) without electricity (World Energy Outlook. Accessed August 13, 2011). China, U.S., and India are the top three coal producers, with production of 2716, 993, and 484 million tons produced in 2008, respectively (World Energy Outlook. Accessed August 13, 2011). With this heavy reliance on coal for advancing electrification in these regions, there will be an inevitable increase in CO₂ emissions. Mitigation of CO₂ via carbon capture will require CO₂ transport. This chapter focuses on the compression and transport of CO₂ after capture.

In absorption and stripping processes mass transfer takes place between gas and liquid phases at each stage throughout a column. In the absorption process the solute, or component to be absorbed (e.g., CO₂), is transferred from the gas phase to the liquid phase. In the stripping process, the opposite occurs, i.e., mass transfer occurs from the liquid to the gas phase. These two units are traditionally coupled, as shown in Fig. 3.1, for the solvent to be recovered and recycled, and for an effective separation of CO₂ from a gas mixture to produce a somewhat pure stream of CO₂.

In an adsorption process a gas mixture contacts small porous particles, which can selectively adsorb or complex with CO₂ for its effective removal from the gas mixture. Sorbent technologies may also be developed to capture CO₂ indirectly by focusing on the selective adsorption of other gases in a given gas mixture, e.g., N₂, O₂, CH₄, H₂, etc. Adsorption is particularly known for its effectiveness in the separation of dilute mixtures. Molecules of CO₂ may be held loosely by weak intermolecular forces, termed physisorption or strongly via covalent bonding, termed chemisorption. Generally, physisorption occurs when the heat of adsorption is less than approximately 10–15 kcal/mol, while chemisorption occurs with heats of adsorption greater than 15 kcal/mol. These are rules of thumb, however, and exceptions do exist. For instance, the heat of physisorption of CO₂ in some zeolites has been reported to be as high as 50 kcal/mol, with heats of chemisorption known to extend from as low as 15 kcal/mol to over 100 kcal/mol. The heat of adsorption is a direct measure of the binding strength between a fluid molecule and the surface.

Membrane separation processes have many advantages over absorption and adsorption processes, some of which include the following: no regeneration, ease of integration into a power plant, process continuity, space efficiency, and absence of a phase change, which can lead to increases in efficiency. Membrane applications, however, require a sufficient driving force for effective separation of a more permeable species. In postcombustion capture of CO₂ for a traditional coal-fired or natural gas-fired power plant this is a challenge due to the somewhat somewhat low concentration of CO₂ in the flue gases of these processes. This is in the case that CO₂ is the selective component for separation from the gas mixture. For membrane technology to be applicable for these somewhat dilute systems, either the CO₂ concentration in the flue gas would have to be increased or the selective component would have to be the dominant species (i.e., N₂) in the gas mixture.

Various advanced coal conversion-to-electricity processes are discussed in Chap. 1 that depend on the use of a gas stream comprised primarily of oxygen; therefore, air separation into its primary components, i.e., nitrogen (N₂), oxygen (O₂), and argon (Ar) are discussed within the context to CO₂ capture. One of the dominant processes used for air distillation is cryogenic distillation. Cryogenic separation may also be used as a polishing step to enhance the purity of a gas stream predominantly comprised of CO₂.

An even stronger policy driver than climate change today is the expansion of alternatives to crude oil for transportation fuels. The U.S. currently imports more than 60% of its petroleum, of which two-thirds are used for the production of transportation fuels. These two drivers are likely to remain present for a long time, with indeterminate relative weights. The reduction of CO₂ via photosynthesis is a route to alternative fuels. The purpose of including a chapter on the algae route to biofuels, but not other routes, is that this process specifically involves the use of a CO₂ stream to enhance the value of the primary biological feedstock.

In addition to the algae-mediated process discussed in Chap. 7, to generate hydrocarbon-based fuels and useful chemicals from CO₂, it is also possible to use electrochemical and photocatalytic processes to carry out CO₂ reduction. As previously mentioned in Chap. 7, today, an even stronger policy driver than climate change is the expansion of alternatives to crude oil for transportation fuels. Another driver for advancing electrochemical and photocatalytic reduction of CO₂ is that it may allow for the storage of stranded energy from resources such as wind, solar, tidal, and geothermal in the form of chemical energy within the bonds of hydrocarbons.

Mineral carbonation takes place through the reaction of CO₂ with an alkalinity source that includes divalent cations such as calcium (Ca^2 +) andmagnesium (Mg^2 +) as well as hydroxyl anions to form stable carbonate minerals. Similar to algae-based (Chap. 7) and electrochemical CO₂ reduction (Chap. 8) processes, mineral carbonation has the potential to couple the capture with the long-term storage of CO₂. Although most studies of mineral carbonation to date have focused on the carbonation of a pure CO₂ gas (i.e., assuming a previous capture step), CCS in a single-step mineral carbonation process may one day be possible. Therefore, it is important to consider this concept in the broader portfolio of CO₂ capture technologies. Figure 9.1 shows a possible mineral carbonation stream in which an alkalinity source reacts with CO₂ to form mineral carbonate. Energy requirements include thermal or mechanical treatment of the alkalinity source to improve its reactivity toward mineral carbonation. Determining the end use of the mineral carbonate may provide an upper limit on how much energy one is willing to spend on the carbonation process.