(including fuel availability and cost) and environmental goals, as well as the specific conditions at the candidate plant. Natural-gas repowering often has an initial economic advantage over alternatives such as CFBC and integrated gasification combined cycles, as the gas turbine entails a relatively low capital investment. It also has the benefit that it can be integrated with the existing coal-fired steam cycle, thereby retaining the maximum productivity from the existing plant capital. However, if the coal-fired plant does not have access to a gas-distribution pipeline or have oil-storage facilities on site, then it may be uneconomic to repower the plant in this way. This section reviews some of the benefits gained by repowering with natural-gas turbines before outlining some of the different approaches to achieve a combined cycle. 13.9.4 Why add gas turbines? Repowering with natural-gas turbines can be a cost-effective solution compared to building a separate combined-cycle unit with the same capacity. It benefits from the combination of both lower coal-fuel costs and higher efficiency combined-cycle technology. The main driving forces behind repowering coal-fired facilities by adding gas turbines are: l improved plant efficiency, thus making the plant more competitive. The extent to which the efficiency is increased depends on the individual plant, the size and type of gas turbine, and the repowering mode (see Section 13.9.5 below); l increased generating capacity. The gain in capacity partly depends on the individual plant, the size of the gas turbine installed and the capacity of the steam turbine, as well as the repowering mode; l plant-life extension; l flexibility of operation. The ability to start and stop the gas turbines several times a day builds a high level of flexibility into the system. This capability can be used strategically in connection with load dispatch. In some repowering modes, it is possible to operate the steam turbine or gas turbine on their own, as well as in combination. Thus the system can take advantage of fuel-price changes. Coal can be bought when it is cheap and can be stockpiled. Gas, on the other hand, must be bought for immediate use, and there are often seasonal-based changes in the cost. These changes can be accommodated and offset by the way the repowered plant is fuelled and the load dispatched (Noppenau and Watson, 1999). However, this flexibility has a capital and efficiency cost; l reduced emissions. Although flue-gas cleaning equipment will still be needed to meet environmental regulations, adding gas turbines to a steam cycle can reduce particulate, SO₂, NOx and CO emissions. CO₂ emissions are also drastically reduced (Termuehlen, 1998). Repowering with gas turbines can therefore help in complying with greenhouse-gas emission-reduction regulations; and l reduced ash. This can be an advantage at plants where the cost of ash storage and disposal is high. More details on repowering with natural-gas turbines can be found in Doig and Morrison (1997) and in Fernando (2000). 13.9.5 Modes of repowering coal-fired boilers There are four main approaches to repowering coal-fired plants with gas turbines to form a combined cycle (Bauer and Joyce, 1997; Joyce, 1996): l full repowering of steam cycles; l topping or hot-windbox repowering; l parallel or compound repowering; or l boosting steam turbines. The first option is termed ‘full’ repowering because the coal-fired plant is replaced completely by a natural-gas combined-cycle plant. This is sometimes termed ‘unfired’ combined cycle. The other three options are ‘partial’ repowering, with up to one-third of the total heat input coming from natural gas. Partial repowering is known as ‘fired’ combined-cycle because the coal- fired boiler is retained. Full repowering The steam-turbine component of a coal-fired plant will often outlast the coal-fired boiler. It is therefore feasible to retain the steam turbine, a major capital plant item, but to replace the boiler. For full repowering, the boiler is replaced by one or more new gas turbines with heat- recovery steam generators (HRSGs) at the gas-turbine exhaust to raise steam for the steam turbine. This is shown schematically in Figure 59. To operate the IEA Clean Coal Centre 176 stack gas turbine heat recovery steam generator (triple pressure) reheat steam turbine new equipment Figure 59    Required new equipment to fully repower a reheat steam turbine (Bauer and Joyce, 1997)