Working Group III: Mitigation

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Substantial reductions in emissions of CO2 from fossil fuel combustion for power generation could be achieved by use of technologies for capture and storage of CO2. These technologies have become much better understood during the past few years, so they can now be seriously considered as mitigation options alongside the more well established options, such as the improvements in fossil fuel systems described in Section, and the substitutes for fossil fuels discussed in Sections and Strategies for achieving deep reductions in CO2 emissions will be most robust if they involve all three types of mitigation option.

The potential for generation of electricity with capture and storage of CO2 is determined by the availability of resources of fossil fuels plus the capacity for storage of CO2. Fossil fuel resources are described in Section 3.8.2 and published estimates of CO2 storage capacity are discussed below. These show that capacity is not likely to be a major constraint on the application of this technology for reducing CO2 emissions from fossil fuel combustion.

The technology is available now for CO2 separation, for piping CO2 over large distances, and for underground storage. This technology is best suited to dealing with the emissions of large point sources of CO2, such as power plant and energy-intensive industry, rather than small, dispersed sources such as transport and heating. Nevertheless, as is shown below, it could have an important role to play in reducing emissions from all of these sources. Technologies for Capture of CO2

CO2 can be captured in power stations, either from the flue gas stream (post-combustion capture) or from the fuel gas in, for example, an integrated gasification combined cycle process (pre-combustion capture). At present, the capture of CO2 from flue gases is done using regenerable amine solvents (Audus, 2000; Williams, 2000). In such processes, the flue gas is scrubbed with the solvent to collect CO2. The solvent is then regenerated by heating it, driving off the CO2, which is then compressed and sent to storage. This technology is already in use for removing CO2 from natural gas, and for separating CO2 from flue gases for use in the food industry.

The concentration of CO2 in power station flue gas is between about 4% (for gas turbines) and 14% (for pulverized-coal-fired plant). These low concentrations mean that large volumes of gas have to be handled and powerful solvents have to be used, resulting in high energy consumption for solvent regeneration. Research and development is needed to reduce the energy consumption for solvent regeneration, solvent degradation rates, and costs. Nevertheless, 80%-90% of the CO2 in a flue gas stream could be captured by use of such techniques.

In pre-combustion capture processes, coal or oil is reacted with oxygen, and in some cases steam, to give a fuel gas consisting mainly of carbon monoxide and hydrogen. The carbon monoxide is reacted with steam in a catalytic shift converter to give CO2 and more hydrogen. Similar processes can be used with natural gas but then air may be preferred as the oxidant (Audus et al., 1999). The fuel gas produced contains a high concentration of CO2, making separation easier, so a physical solvent may be better suited for this separation; the hydrogen can be used in a gas turbine or a fuel cell. Similar technology is already in use industrially for producing hydrogen from natural gas (e.g., for ammonia production). The integrated operation of these technologies for generating electricity whilst capturing CO2 has no major technical barriers but does need to be demonstrated (Audus et al., 1999).

The concentration of CO2 in a power station flue gas stream can be increased substantially (to more than 90%) by using oxygen for combustion instead of air (Croiset and Thambimuthu, 1999). Then post-combustion capture of CO2 is a very easy step, but the temperature of combustion must be moderated by recycling CO2 from the exhaust, something which has been demonstrated for use with boilers but would require major development for use with gas turbines. Currently, the normal method of oxygen production is by cryogenic air separation, which is an energy intensive process. Development of low-energy oxygen separation processes, using membranes, would be very beneficial.

Other CO2 capture techniques available or under development include cryogenics, membranes, and adsorption (IEA Greenhouse Gas R&D Programme, 1993).

After the CO2 is captured, it would be pressurized for transportation to storage, typically to a pressure of 100 bar. CO2 capture and compression imposes a penalty on thermal efficiency of power generation, which is estimated to be between 8 and 13 percentage points (Audus, 2000). Because of the energy required to capture and compress CO2, the amount of emissions avoided is less than the amount captured. The cost of CO2 capture in power stations is estimated to be approximately US$30-50/t CO2 emissions avoided (US$110-180/tC), equivalent to an increase of about 50% in the cost of electricity generation.

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