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coal-fired plants aged more than 25 years and having capacity below 300 MW withlarger and significantly more efficient plants and, where technically and economicallyappropriate, replacing or re-powering larger inefficient plants with high-efficiency (morethan 40 per cent) plants. Technologies that help improve the efficiency of coal-firedpower plants are discussed below.(a) Fluidized Bed Combustion (FBC)Fluidized bed combustion (FBC) is a very flexible method of electricity production –most combustible material including coal, biomass and general waste can be burnt.FBC systems improve the environmental impact of coal-based electricity, reducingSOx and NOx emissions by 90 per cent. In fluidized bed combustion, coal is burned ina reactor that consists of a bed through which gas is fed to keep the fuel in a turbulentstate. This improves combustion, heat transfer and recovery of waste products. Thehigher heat exchanger efficiencies and better mixing of FBC systems allows them tooperate at lower temperatures than conventional pulverized coal combustion (PCC)systems. By elevating pressures within a bed, a high-pressure gas stream can beused to drive a gas turbine, generating electricity.FBC systems fit into two groups, non-pressurized and pressurized systems, and twosub-groups, circulating or bubbling fluidized bed. Non-pressurized FBC systems operateat atmospheric pressure and are the most widely applied type of FBC. They have 30-40 per cent efficiencies, similar to PCC. Pressurized FBC systems operate at elevatedpressures and produce a high-pressure gas stream that can drive a gas turbine, creatinga combined cycle system that is more than 40 per cent efficient. Bubbling FCB usesa low fluidizing velocity so that the particles are held mainly in a bed, and is generallyused with small plants offering a non-pressurized efficiency of around 30 per cent.Circulating FCB uses a higher fluidizing velocity so that the particles are constantlyheld in the flue gases, and are used for much larger plants offering efficiencies above40 per cent. The flexibility of FBC systems allows them to utilize abandoned coalwaste that previously could not be used due to its poor quality.(b) Supercritical and ultra-supercritical technologiesCurrently used sub-critical steam cycles operate well below the steam pressure of 221bar. New PCC systems that utilize supercritical and ultra-supercritical technologiesoperate at increasingly higher temperatures (600 ° C) and pressures (300 bar), andtherefore achieve higher efficiencies (45 per cent) than conventional PCC units andsignificant CO 2reductions. Supercritical steam cycle technology has been used fordecades and is becoming the system of choice for new commercial coal-fired plants inmany countries.At present R&D is under way for ultra-supercritical units that operate at even higherefficiencies, potentially up to around 50 per cent. The introduction of ultra-supercriticaltechnology has been driven over recent years in countries such as Denmark, Germanyand Japan, by the need to achieve improved plant efficiencies and reduce fuel costs.Research is focusing on the development of new steels for boiler tubes and on highalloy steels that minimize corrosion. These developments are expected to result in adramatic increase in the installation of ultra-supercritical units in the coming years.50