In a traditional thermal power station, pulverized coal (or fuel oil or gas) is burned in a boiler to generate steam at high temperature and pressure, which is then expanded through a steam turbine to generate electricity. The efficiencies of modern power stations can exceed 40% (lower heating value (LHV)), although the average efficiency, worldwide, of the installed stock is about 30% (Ishitani and Johansson, 1996). The typical cost of a modern coal- fired power station, with SO2 and NOx controls, is US$1,300/kW (Ishitani and Johansson, 1996). These costs vary considerably and can be more than 50% higher depending on location. Less efficient designs with fewer environmental controls are cheaper.
The development of new materials allows higher steam temperatures and pressures to be used in supercritical designs. Efficiencies of 45% are quoted in the Second Assessment Report, although capital costs are significantly higher at around US$1,740/kW (Ishitani and Johansson, 1996). More recently, efficiencies of 48.5% have been reported (OECD, 1998b) and with further development, efficiencies could reach 55% by 2020 (UK DTI, 1999) at costs only slightly higher than current technology (Smith, 2000).
Developments in gas turbine technology allow for higher temperatures which lead to higher thermodynamic efficiencies. The overall fuel effectiveness can be improved by capturing the waste heat from the turbine exhaust in a boiler to raise steam to generate electricity through a steam turbine. Thus in such a CCGT plant, electricity is generated by both the gas and steam turbines driving generators. The efficiency of the best available natural gas fired CCGTs currently being installed is now around 60% (LHV) (Goldemberg, 2000) and has been improving at 1% per year in the past decade. Typical capital costs for a power station of 60% efficiency are around US$450-500/kW, including selective catalytic reduction (for NOx), dry cooling, switchyard, and a set of spares. Costs can be higher in some regions, especially if new infrastructure is required. These costs have been falling as efficiencies improve (IIASA-WEC, 1998). Together with high availability and short construction times, this makes CCGTs highly favoured by power station developers where gas is available at reasonable prices. Developments in the liquefied natural gas markets could further expand the use of CCGTs. Further improvements might allow electricity generating efficiencies of over 70% to be achievable for CCGTs within a reasonable period (Gregory and Rogner, 1998).
IGCC systems utilize the efficiency and low capital cost advantages of a CCGT by first gasifying coal or other fuel. Gasifiers are usually oxygen blown and are at the early commercial stage (Goldemberg, 2000). Coal and difficult liquid fuels such as bitumens and tar can be used as feedstocks. Biomass fuels are easier to gasify (Section 126.96.36.199.2), which may reduce the cost and possibly the efficiency penalty as an oxygen plant is not required (Lurgi GmbH, 1989). Gas clean-up prior to combustion in the gas turbine, which is sensitive to contaminants, is one of the current areas of development. The potential efficiency of IGCCs is around 51%, based on the latest CCGTs of 60% efficiency (Willerboer, 1997). Vattenfall, using a GE Frame 6 gas turbine, indicated a net efficiency of 48% in trials (Karlsson et al., 1998), and an efficiency of 50%-55% was claimed to be achievable by using the latest gas turbine design. With continuing development in hot gas cleaning and better heat recovery as well as the continuing development of CCGTs, commercially available coal- or wood-fired IGCC power stations with efficiencies over 60% may be feasible by 2020.
In addition to the potential high efficiencies, IGCC offers one of the more promising routes to CO2 capture and disposal by converting the gas from the gasifier into a stream of H2 and CO2 via a shift reaction. The CO2 can then be removed for disposal before entering the gas turbine (see Section 188.8.131.52). The resultant stream of H2 could be used in fuel cells and not just in a gas turbine.
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