Chapter 20 Carbon Capture and Storage for Mitigating Climate Changes
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| CHAPTER 20
Carbon Capture and Storage for Mitigating Climate Changes Tian C. Zhang and Rao Y. Surampalli 20.1 Introduction Currently, three options are being explored to stabilize atmospheric levels of greenhouse gases (GHGs) and global temperatures without severely and negatively impacting standards of living: a) increasing energy efficiency, b) switching to less carbon-intensive sources of energy, and c) carbon capture, storage and sequestration (CCS) (White et al. 2003). While all three options must be used in concert, this chapter focuses on CCS because the CCS option is very compatible with the large energy production and delivery infrastructure now in place (MIT 2012). For the foreseeable future, fossil fuels will continue to be the world’s most reliable and lowest-cost form of energy (USDOS 2010). Global energy models suggest that with current global coal use patterns, it will not be possible to stabilize atmospheric GHG concentrations at acceptable levels. Therefore, it is necessary to make major reductions in GHG emissions. The EU, for example, wants to set a target of halving carbon emissions by 2050 in order to keep the rise in global temperature below 2°C. If the temperature increases more than this, it may trigger runaway climate change, i.e., temperatures could easily rise another 2°C, which would be a catastrophe to all living beings. Therefore, to mitigate climate changes, we must aggressively pursue CCS from fossil fuel power plants, other industries, and places where CCS technology can be used. It should be noted that cutting emissions of other gases (e.g., methane, nitrous oxide, man-made chemicals) could slow changes in climate, leading to some rapid changes for the better. However, the climate-related benefits of reductions in non-CO2 GHGs have limits. Even if all human-related, non-CO2 GHG emissions could be eliminated today, it would not be enough to stabilize the warming influence from all GHGs over the next 40 years (Montzka et al. 2011). Therefore, in this chapter, we will focus on CCS. Here, we define the term CCS (= Carbon Capture, Storage, and Sequestration) as any technologies/methods that are to a) capture, transport, and store carbon (CO2), b) monitor, verify, and account the status/progress of the CCS technologies employed, and c) advance development/uptake of low-carbon technologies and/or promote beneficial reuse of CO2. Although CCS issues have been addressed/reviewed since the early 1990s (e.g., Riemer et al. 1993; USDOE 1999; Herzog 2001; Anderson and Newell 2003; IPCC 2005; IEA 2009; Lackner and Brennan 2009; CCCSRP 2010; ITF 2010), still there is a need to review CCS technologies because new information is now being generated at a faster pace. In this chapter, the concept of CCS is introduced with a focus on technologies for CO2 capture, long-term storage, monitoring, and beneficial reuse of CO2. Major issues (e.g., concerns, constrains, and major barriers) and future perspectives are discussed. Understanding the technologies and issues would better prepare us for future actions. 20.2 Background To provide the background for CCS, this section describes and discusses issues related to: a) carbon cycle, b) sources of CO2 and targeted CO2 sources for CCS and c) historical evaluation of CCS. Carbon Cycle. The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere (the soil-containing earth surface), geosphere, hydrosphere, and atmosphere of the Earth. Carbon moves from: a) atmosphere (as CO2) to plants (via photosynthesis) or the oceans/other water bodies (via absorption processes), b) plants (or other animals) to animals via food chains, c) died plants/animals to the ground (e.g., fossil fuels formed in millions and millions of years), and d) living things (via respiration) and/or fossil fuels (upon being burned) to the atmosphere. When humans burn fossil fuels for energy, most of the carbon quickly enters the atmosphere as CO2. Each year, 5.5 billion tons of carbon is released by burning fossil fuels; 3.3 billion tons enter the atmosphere and most of the rest is absorbed by the oceans. CO2 and other GHGs trap heat in the atmosphere to keep the Earth warm and livable for living beings. However, there is about 30% more carbon dioxide in the air today (due to human fossil burning activities) than there was about 150 years ago, which is causing our planet to become warmer (Johnson 2010). Sources of CO2 and Targeted CO2 Sources for CCS. CO2, CH4, nitrous oxide and three groups of fluorinated gases (sulfur hexafluoride, HFCs, and PFCs) are the major GHGs and the subject of the Kyoto Protocol. All GHGs are released from either point or non-point sources (IPCC 2005). Several points need to be discussed below:
Table 20.1. Sources of CO2 and targeted sources for CCSa
a References: Zero (2012); WRI (2012); USEPA (2012). b 43% = power (24%) + industry (14%) and other energy related (5%); 22 = transportation (14%) + buildings (8%); and 35% = Land use (18%) + agriculture (14%) + waste (3%) + others (3%). Details can be found in IPCC (2005). Historical Evolution of CCS Technologies. In general, CCS processes include three major steps: a) capturing carbon (or CO2) from different sources; b) transporting to a storage site; and c) injecting into a geological formation for CO2 storage (USDOS 2010). It should be noted that we define CCS in a much broader way (see above), including all technologies/methods that promote carbon sequestration in natural environments, monitor the status/progress of the CCS technologies employed, and advance development/uptake of lower carbon technologies and/or promote beneficial use of carbon (e.g., reuse). Therefore, the boundary of the CCS technologies addressed in this chapter is broader. CCS technologies themselves are not new. In the 1940s, chemical solvents (e.g., monoethanolamine (MEA)-based solvents) were developed to remove acid gases (e.g., CO2 and H2S) from impure natural gas to boost the heating value of natural gas. The same or similar solvents were used to recover CO2 from their flue gases for application in the foods-processing and chemicals industries by power plants. On the other hand, the feasibility of capturing CO2 from ambient air was evaluated in the 1940s (Tepe and Dodge 1943; Spector and Dodge 1946). The first patent for CO2EOR technology (CO2 enhanced oil recovery) was granted to Whorton, Brownscombe, and Dyes of the Atlantic Refining Company 4 in 1952. In 1964, a field test was conducted at the Mead Strawn Field to inject CO2 for oil recovery. In 1972, the first commercial CO2 EOR was initiated by Chevron at SACROC Unit, Texas. Today, petroleum industry operates CO2 EOR projects in 74 fields and produces 245,000 barrels of incremental oil a day (about 5% of the total US production), and injects over 2.14 BCF of CO2 per day (Meyer 2012). In the 1970s, two post-combustion commercial amine capture processes were developed, one by Kerr-McGee (using 20% MEA solution) and another by Dow Chemical (using 30% MEA). In 1989, the Massachusetts Institute of Technology (MIT) initiated the Carbon Capture and Sequestration Technologies Program. During that time, there were only a handful of research groups working in CCS (Herzog 2011). Also around that time, Zaslavsky (2006) proposed the concept of the “energy tower.” In 1991, the Norwegian government instituted a tax on CO2 emission, which triggered the start-up of the Sleipner Project. The First International Conference on Carbon Dioxide Removal (ICCDR-1) was held in Amsterdam with > 250 attendees from 23 countries. Since 1996, Statoil has been using CCS at the Sleipner oil and gas field (located in the North Sea) to compress and then pump CO2 into a 200-m-thick sandstone layer that lies ~1000 m below the seabed. The Sleipner Project demonstrates the commercial application of CCS, which can be viewed as one of the most significant milestones in evolution of CCS technology (Herzog 2003). In 1997, the Dakota Gasification Company (DGC) agreed to send all of the waste gas (96% CO2) from its Great Plains Synfuels Plant through a pipeline to the Weyburn oil field (330 km away), Canada; in 2000, EnCana began to inject CO2 into 37 injection wells to help the oil to flow toward 145 active producer wells. In 2000, Brown proposed replacing water with supercritical CO2, which is being evolved into CO2-based geothermal energy (CO2GE) technology (Randolph and Saar 2011). CO2GE is up to 5 times more efficient than water geothermal because CO2 is a more efficient working fluid in natural than hydrofractured reservoirs. Nowadays, a variety of alternative methods (e.g., amine solvents, physical solvents, cryogenic, oxygen methods) are used to separate CO2 from gas mixtures during the production of hydrogen for petroleum refining, ammonia production, and other industries. All of these capture technologies are considered relatively mature (Plasynski, et al. 2009). Other major milestones about CCS can be found in Wikipedia (2010). There is considerable interest in improving CCS technology, particularly CO2 capture processes. Currently, a wide range of options have been evaluated, such as aqueous ammonia, metal organic frameworks (MOFs), ionic liquids (ILs), membranes (e.g., polybenzimidiazole, ion transport membrane, oxygen transport, enzymatic mambranes), solid regenerable sorbents, ceramic auto-thermal recovery, hydrates, chemical-looping, mineral formation, and biological processes. As the knowledge and understanding of CCS technologies grow, some of these emerging CCS processes are anticipated to be commercialized in 5–10 years, others in > 20 years (Figueroa et al. 2008; Herzog et al. 2009). 20.3 CCS Technologies 20.3.1 Carbon Capture Technologies There are many carbon capture technologies. These technologies can be categorized as a) physical/chemical and biological technologies and b) technologies for carbon capture from concentrated point sources and mobile/distributed point- or non-point sources. In general, on-site capture is the most viable approach for large sources and initially offers the most cost-effective avenue to sequestration. For mobile/distributed sources like cars, on-board capture at affordable cost would not be feasible, but are still needed. On the basis of category b), we will present these technologies as follows. Concentrated Point-Source CO2 Capture. All the current commercially available processes for CO2 capture are similar in concept (i.e., a two-vessel process with liquid, solid or liquid-impregnated solid sorbents). In the first vessel, the CO2-containing gas contacts a lean solvent (e.g., MEA-based), and the CO2 is absorbed there. The CO2-rich solvent is regenerated in the second vessel and then returned to the first vessel. Solvents used in these processes can be broadly grouped into two categories: physical solvents and chemical solvents (Table 20.2). CO2 can also be captured with solid sorbents; detailed information about these sorbents can be found in different reports (e.g., Choi et al. 2009). Greenleaf and SenGupta (2009) proposed to use ion-exchange fibers (containing F–COOH) to remove water hardness (Ca and Ma) and sequester CO2 without requiring any chemicals for regeneration or without the production of residuals (sludge): 2F-COOH + 2Ca(HCO3) ⇔(F-COO)2Ca + 2H2CO3 (Eq. 20.1) 2F-COOH + 2Mg(HCO3) ⇔(F-COO)2Mg + 2H2CO3 (Eq. 20.2) (F-COO)2Ca + 2CO2(g) + 2H2O ⇔2F-COOH + Ca2+ + 2HCO3 (Eq. 20.3) Table 20.2. Physical/chemical technologies for CO2 capturea
aMajor references: Herzog (1999); VGB (2004); Choi et al. (2009); Greenleaf and SenGupta (2009); Lackner and Brennan (2009); and Plasynski et al. (2009). CO2 is permanently sequestered in the aqueous phase as calcium or magnesium alkalinity; no additional chemicals or salts are present in the regeneration solution. The process is not energy intensive, and CO2 does not need to be compressed to excessive pressures (150 psi) for efficient use. The use of raw flue gas (17% CO2) is feasible with the rate of sequestration governed only by the partial pressure of CO2. The energy balance for a typical electric utility shows that up to 1% of carbon dioxide emitted during combustion would be sequestered in the softening process. The major component of flue gas is nitrogen, which enters originally with the air feed. If there were no nitrogen, CO2 capture from flue gas would be greatly simplified. Therefore, for existing coal-fired combustion plants, there are two main options for CO2 capture: removal of nitrogen from a) flue gases or b) from air before combustion to obtain a gas stream ready for transport or geo-sequestration. Therefore, there are three major options in CO2 capture, i.e., post-, pro-, and oxy-combustion capture (Fig. 20.1). 
Figure 20.1. Overview of CO2 capture from power plants (VGB 2004) Currently, power plants burn their fuel and generate a flue gas at atmospheric pressure with a CO2 concentration < 15% and the CO2 partial pressure < 0.15 atm (Plasynski et al. 2009). Thus, the thermodynamic driving force for capturing CO2 from flue gas is low. Furthermore, the flue gas approaches in use today require clean-up of the NOx and SO2 prior to CO2 separation, which creates a significant technical challenge for the development of cost-effective post-combustion CO2 capture processes. The post-combustion CO2 capture system will reduce the plant’s overall thermal efficiency by 24%, about one-third (8%) is due to compression, with the rest (16%) attributable to separation (Herzog et al. 2009). Nevertheless, post-combustion technologies are widely used currently for CO2 capture in power plants because they can be retrofitted to the existing power plants and they constitute, by far, the largest source of CO2 emissions appropriate for CCS (Herzog et al. 2009). In the past, the amount of CO2 captured was between a few hundred tons and over a thousand tons of CO2 a day. However, for combating climate change, the scale of post-combustion CO2-capture facilities is significantly large since a 500MWcoal-fired plant produces about 10,000 tons/day of CO2. To improve CO2 capture, current research and development (R&D) focus on several pathways (Table 20.2). Details of post-combustion capture and their improvement can be found in Choi et al. (2009), Herzog et al. (2009), and Plasynski et al. (2009). On the other hand, pre- and oxy-combustion processes were developed with the thinking of obtaining much more concentrated CO2 before its capture or storage. The concept of the pre-combustion capture is to increase the concentration and pressure of the CO2 containing stream so that the size and cost of the capture facilities can be reduced. Integrated coal gasification combined cycle (IGCC) plants (i.e., the hydrogen route) are an example of the pre-combustion option. Coal is gasified to form syngas of CO and H2. The gas then undergoes the water-gas shift, where the CO is reacted with steam to form CO2 and H2. H2 is sent to a gas turbine combined cycle; and the CO2 is then removed (e.g., via a physical solvent process like Selexsol) with much less energy as capture takes place from the high pressure syngas as opposed to the atmospheric pressure flue gas. A similar process is available for natural gas, where the syngas is formed by steam reforming of methane. The hydrogen route opens up opportunities for “polygeneration” of products besides electricity and CO2. For example, instead of sending hydrogen to a turbine, it can be used to fuel a “hydrogen economy”. In addition, syngas is an excellent feedstock for many chemical processes (Herzog 1999). However, the pre-combustion capture is not an option at the pulverized coal (PC) power plants that comprise most of the existing capacity. An oxyfuel plant that is fed pure O2 and coal does not need a flue stack; it is sometimes referred to as "zero emission" cycles, because by design they have no gaseous exhaust. Zero emission plants can capture CO2 and all air pollutants (e.g., NOx, SO2, and particulate matter). The technique is promising, but the initial air separation step (such as a cryogenic air separation unit, ASU) demands a lot of energy. Various process options are being considered to make oxy-combustion more economically attractive, such as the use of a) a circulating fluidized bed (CFB) with a reduced requirement for recycled flue gas and easier temperature control, b) an oxygen ion transport membrane to replace the ASU, c) ceramic autothermal recovery (CAR) (Plasynski et al. 2009). Mobile/Distributed Point- or Non-Point-Source CO2 Capture. Capturing CO2 directly from the air could a) offset emissions from distributed sources; b) provide a constant supply of carbon-based liquid fuel; c) balance out a significant fraction of the world’s CO2 emissions; d) operate at the site of disposal and thus eliminating the need to transport CO2 over long distance; and e) help assure the long-term viability of storage sites (Lackner and Brennan 2009). In addition, it may have several advantages, such as freedom of location, economy of scale, and negative emissions even though it is more difficult and more expensive. In atmospheric air, CO2 concentration is ~ 0.04% (about 390 ppm), much smaller than the CO2 concentration (5–15%) coming out from flue stacks. This makes CO2 capture from mobile/distributed point-sources and non-point-sources costly and difficult. However, CO2 is still a reactive sour gas, and thus, can be absorbed by highly selective sorbents, which makes it possible to reduce the partial pressure of CO2 in air (~40 Pa) to a small fraction of one Pascal. Considering that we do not need to extract all the CO2 out of the air, it is possible to design sorbent-based air capture systems that scales its energy cost with the amount of CO2 capture, instead of with the volume of air processes (Stolarooff 2006; Lackner and Brennan 2009). Stolarooff (2006) reviewed the existing alternative routes to air capture, including: a) organic carbon production; b) metal-carbonate production; c) capture with a regenerated sorbent; and d) metal hydroxide sorbents. In this chapter, we classify the methods based on CO2 capture from mobile/distributed point sources and non-point sources. As indicated in Table 20.3, methods 1) to 4) are for CO2 capture from non-point sources; they are biologically related, and in many cases, are called biosequestration (i.e., the capture and storage of the atmospheric GHG CO2 by biological processes). Trees and other photosynthesizing organisms perform CO2 capture routinely. Cutting down forests may contribute ~20% of the overall GHGs entering the atmosphere (IPCC 2012). Therefore, the Kyoto Protocol requires mandatory land use, land use change and forestry (LULUCF) accounting for afforestation (no forest for last 50 years), reforestation (no forest on Dec. 31, 1989), and deforestation. While it looks like that method 1) in Table 20.3 has big potential to capture a large fraction of anthropogenic CO2 emissions, they all have limitations and constrains. For example, biomass is limited by the land available and by the secondary impacts of agriculture. As a base line, trees over an acre will only sequester between 2 to 8 tons of CO2 each year. Corn, on the other hand, at 150 bushels an acre is taking out more CO2 than trees do, plus that corn can be used to make biofuels. Therefore, one of the current research focuses is to genetically improve crops for CO2 capture and biofuel production. One example is to increase the Earth’s proportion of C4 carbon fixation photosynthetic plants because these plants account for ~30% of terrestrial carbon fixation even though they only represent about 5% of Earth’s plant biomass (Osborne and Beerling 2006). Wheat, barley, soybeans, potatoes and rice (all C3 staple food crops) can be genetically engineered with the photosynthetic apparatus of C4 plants, i.e., modifying C3 crops’ ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) genes (Beerling 2008). Algae are nature’s CO2 scrubbers and they fix > 65 Gt of carbon every year, which is equal to the output of about 65000, 500-MW power generation plants. Algae inhale CO2 at rates that a few months of algae growth can equal decades of tree growth. Algae such as Dunaliella salina are high value commercial cosmetics and food supplements. Recently, algae become more and more popular due to the fact that the yields of bio-diesel from algae are one order of magnitude higher than those for traditional oilseeds; algae can even produce bio-hydrogen. Everything can be used in the algae: after death it can be digested into bio-gas (methane, carbon dioxide, and the fertilizer needed to grow more algae). Algae can be cultivated in open ponds or closed bioreactor systems. It has been proposed to use watershed nutrients, animal waste, sewage waste, or flue gases to cultivate algae next to power-plant flue stacks for CO2 capture with marginal lands. However, the available marginal lands may not be enough to solve the problem. For example, a 50-MW 50% base-load natural gas-fired electrical generation plant operating 18 h/day over a 240-day season would produce 216 million kWh/season, releasing 30.3 million kg-C/season of fossil-fuel CO2. An algal process designed to capture 70% of the flue-gas CO2 would require an area of 880 ha of high-rate algal ponds operating at a productivity of 20 g VS/m2-day, which would produce 42.4 million kg algal dry wt/season (Brune et al. 2009). If 100% of the algal biomass were harvested and used for replacing biogas methane usage, soybean feed replacement, and biodiesel production, the gross GHG reduction would be about 36%, the net parasitic energy cost to harvest and process the algal biomass would be about 10% of plant total energy output, resulting in a new GHG reduction of 26% (Brune et al. 2009). Another suggestion is to utilize the “Jelly Pump” for CO2 storage. For example, thaliacean (appearing similar to jellyfish) are one-third carbon by weight, while jellyfish, by comparison, are 10% carbon, and single-celled algae around 20%. The high carbon content explains why thaliaceans are so dense and why they sink so quickly after they die. Thaliaceans gather around the world in feeding swarms, billions strong, feasting on algae, and can sink almost twice as much carbon as algae do (Hoffman 2009). However, no detailed information is available on how to use this mechanism to store more CO2. Accordingly, another proposal is to seed pulverized iron over the ocean to germinate a plankton bloom. While small-scale tests are not successful, large-scale tests may overshoot the goal and drive CO2 to levels too low. For each iron atom added to the water, between 10,000 and 100,000 carbon atoms are sunk (Wikipedia 2012). Therefore, further studies are needed to get accurate information on the biological activity that a bloom causes and to measure how much carbon will be displaced. Method 3) represents an emerging commercial sector that combines carbon sequestration and energy production. For biochar sequestration to work on a much larger scale, an important factor is combining low-temperature pyrolysis with simultaneous capture of the exhaust gases and converting them to energy as heat, electricity, biofuel or hydrogen; emissions reductions of using pyrolysis processes for bioenergy production can be 12–84% greater if biochar is placed into the soil instead of being burned to offset fossil-fuel use (Wikipedia 2011). Thus, biochar sequestration offers the chance to turn bioenergy into a carbon-negative industry. |