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Chapter 3: Capture of CO2 121 follow the scheme shown in Figure 3.2a, where the combustion flue gas is put in contact with the sorbent in a suitable reactor to allow the gas-solid reaction of CO2 with the sorbent (usually the carbonation of a metal oxide). The solid can be easily separated from the gas stream and sent for regeneration in a different reactor. Instead of moving the solids, the reactor can also be switched between sorption and regeneration modes of operation in a batch wise, cyclic operation. One key component for the development of these systems is obviously the sorbent itself, that has to have good CO2 absorption capacity and chemical and mechanical stability for long periods of operation in repeated cycles. In general, sorbent performance and cost are critical issues in all post-combustion systems, and more elaborate sorbent materials are usually more expensive and will have to demonstrate outstanding performance compared with existing commercial alternatives such as those described in 3.3.2. being pursued by several groups around the world. 3.3.4 Status and outlook Virtually all the energy we use today from carbon-containing fuels is obtained by directly burning fuels in air. This is despite many decades of exploring promising and more efficient alternative energy conversion cycles that rely on other fuel processing steps prior to fuel combustion or avoiding direct fuel combustion (see pre-combustion capture – Section 3.5). In particular, combustion-based systems are still the competitive choice for operators aiming at large-scale production of electricity and heat from fossil fuels, even under more demanding environmental regulations, because these processes are reliable and well proven in delivering electricity and heat at prices that often set a benchmark for these services. In addition, there is a continued effort to raise the energy conversion efficiencies of these systems through advanced materials and component development. This will allow these systems to operate at higher temperature and higher efficiency. As was noted in Section 3.1, the main systems of reference for post-combustion capture are the present installed capacity of coal and natural gas power plants, with a total of 970 GWe subcritical steam and 155 GWe of supercritical/ultra-supercritical steam-based pulverized coal fired plants, 339 GWe of natural gas combined cycle, 333 GWe natural gas steam-electric power plants and 17 GWe of coal-fired, circulating, fluidized-bed combustion (CFBC) power plants. An additional capacity of 454 GWe of oil-based power plant, with a significant proportion of these operating in an air-firing mode is also noted (IEA WEO, 2004 and IEA CCC, 2005). Current projections indicate that the generation efficiency of commercial, pulverized coal fired power plants based on ultra-supercritical steam cycles would exceed 50% lower heating value (LHV) over the next decade (IEA, 2004), which will be higher than efficiencies of between 36 and 45% reported for current subcritical and supercritical steam-based plants without capture (see Section 3.7). Similarly, natural gas fired combined cycles are expected to have efficiencies of 65% by 2020 (IEA GHG, 2002b) and up from current efficiencies between 55 and 58% (see Section 3.7). In a future carbon-constrained world, these independent and ongoing developments in power cycle efficiencies will result in lower CO2-emissions per kWh produced and hence a lower loss in overall cycle efficiency when post-combustion capture is applied. There are proven post-combustion CO2 capture technologies based on absorption processes that are commercially available at present . They produce CO2 from flue gases in coal and gas- fired installations for food/beverage applications and chemicals production in capacity ranges between 6 and 800 tCO2 d-1. They require scale up to 20-50 times that of current unit capacities for deployment in large-scale power plants in the 500 MWe capacity range (see Section 3.3.2). The inherent limitations of currently available absorption technologies when applied to post-combustion capture systems are well known and their impact on system cost can be estimated relatively accurately for Solid sorbents being investigated for large-scale CO2 capture purposes are sodium and potassium oxides and carbonates (to produce bicarbonate), usually supported on a solid substrate (Hoffman et al., 2002; Green et al., 2002). Also, high temperature Li-based and CaO-based sorbents are suitable candidates. The use of lithium-containing compounds (lithium, lithium-zirconia and lithium-silica oxides) in a carbonation-calcination cycle, was first investigated in Japan (Nakagawa and Ohashi, 1998). The reported performance of these sorbents is very good, with very high reactivity in a wide range of temperatures below 700oC, rapid regeneration at higher temperatures and durability in repeated capture-regeneration cycles. This is essential because lithium is an intrinsically expensive material. The use of CaO as a regenerable CO2 sorbent has been proposed in several processes dating back to the 19th century. The carbonation reaction of CaO to separate CO2 from hot gases (T > 600oC) is very fast and the regeneration of the sorbent by calcining the CaCO3 into CaO and pure CO2 is favoured at T > 900oC (at a partial pressure of CO2 of 0.1 MPa). The basic separation principle using this carbonation-calcination cycle was successfully tested in a pilot plant (40 tonne d-1) for the development of the Acceptor Coal Gasification Process (Curran et al., 1967) using two interconnected fluidized beds. The use of the above cycle for a post-combustion system was first proposed by Shimizu et al. (1999) and involved the regeneration of the sorbent in a fluidized bed, firing part of the fuel with O2/CO2 mixtures (see also Section 3.4.2). The effective capture of CO2 by CaO has been demonstrated in a small pilot fluidized bed (Abanades et al., 2004a). Other combustion cycles incorporating capture of CO2 with CaO that might not need O2 are being developed, including one that works at high pressures with simultaneous capture of CO2 and SO2 (Wang et al., 2004). One weak point in all these processes is that natural sorbents (limestones and dolomites) deactivate rapidly, and a large make-up flow of sorbent (of the order of the mass flow of fuel entering the plant) is required to maintain the activity in the capture-regeneration loop (Abanades et al., 2004b). Although the deactivated sorbent may find application in the cement industry and the sorbent cost is low, a range of methods to enhance the activity of Ca-based CO2 sorbents arePDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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