Supercritical Cycles
The biggest competition for gasification based power plants are advanced so-called supercritical steam cycles. These are called supercritical as both the maximum temperature and the maximum pressure of the steam are above the critical values of 374°C and 221 bar. By combining the benefits of the above features for improving the efficiency, the state-of-the-art steam cycles can now reach about 45% FHV basis for plants that are equipped with DeSOx and DeNOx facilities. The costs of these plants are very competitive, and gasification-based power stations must have even higher efficiencies and lower cost in order to be successful.
Examples of modern supercritical units are the pulverized coal (PC) coal-fired double-reheat unit at Aalborg in Denmark (285 bar/580°C/580°C/580°C) and the single-reheat unit at Matsuura in Japan (241 bar/593°C/593°C). In Germany and in Japan units are now under design or construction for single-reheat plants of 260 bar/580°C/600°C and 250 bar/600°C/610°C respectively. The next step will feature plants at steam conditions of 300 bar/600°C/620°C. A simplified flow scheme for such a plant is given in Figure 7-13. The new steels that are required are a major issue for all these developments (Viswanathan, Purgert, and Rao 2002).
As a result of all these improvements, it has been possible to increase the efficiency of steam cycles to 45% LHV basis, that is 67% of the maximum Carnot efficiency, as shown in Table 7-10.
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HP BFW HP BFW LP BFW CONDENSATE PREHEAT PUMP PREHEAT PUMP Figure 7-13. Ultra-Supercritical Cycle Flowsheet (Source: Welfordetal. 2002) |
This result is obtained with water as a working fluid, which is in some ways the worst choice for a Rankine cycle because of the very high heat of evaporation and the high specific heat of liquid water. Yantovski (1996) has studied extensively the merits of using cycles in which C02 is used as the main working fluid, which has the advantage that it has a low heat of evaporation. Such a cycle has advantages when CO2 has to be captured and sequestered. A major disadvantage of C02 is that it is more difficult to handle, as the vapor pressure of liquid C02 is already 40-50 bar at ambient temperatures.
The reason for discussing in some detail the steam cycle is twofold. On the one hand the steam cycle plays an important role in combined cycles (CC), and on the other hand much can be learned from its development that is also of importance for the Brayton or Joule cycle as used in gas turbines.