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However, the egg shell diagram also indicates that the optimum is relatively flat (as long as the reheat pressure moves up and down in a coordinated way) leaving the plant designer with a certain freedom to consider the need also for other systems. In particular, the MC group has focused on:
- Concerns about boiler cost means that the first section of reheat 1 should be located in the flue gas stream just upstream of the economizer.
- Steam temperature at the inlet of reheat 1 should be kept as low as possible and approximately 80-100K below the temperature of cold reheat 2.
- Keep the second hot reheat pressure high and the dimensions of the second hot reheat steam lines and the IP2 turbine inlet valves as low as possible, guaranteeing a sufficient number of manufacturers.
- Keep steam temperature in the crossover lines from IP2 to LP turbines below 660°F.
- Temperature rise of the HP feedwater heaters should have a reasonable split in order to achieve reasonable and balanced weights of the HP heaters.
Final feedwater temperature
Increasing the final feedwater temperature by the regenerative heaters is a well-proven and effective way to increase plant efficiency. However, concerns about maximum allowable wetness of the exhaust steam from the LP turbines and the erosion by water droplets of the tips of the last stage running blades still impose limitations on selection of the optimum FFWT and reheat pressure for single reheat cycles. On the other hand, the nature of double reheat cycles, including MC, means that the last stage of wetness of the LP turbines is much lower than for single reheat cycles and does not impose any limitations on FFWT optimization.
Significant heat rate improvements by higher FFWT are illustrated in Figure 5 where heat rate improvements of MC600 and DRH600 are shown on the left coordinate axis, and heat rate improvements of single heat cycles are shown on the right coordinate axis. Figure 5 indicates optimum heat rate in the range of 680°F-689°F for the double reheat cycles and 626°F for the single reheat cycles.
The figure also indicates that if FFWT is increased from 590°F to 625°F, MC600 net heat rate will be improved by ~0.6 percent, which is similar to what can be achieved through enhanced main and reheat steam temperatures of ~12K.
The investigations also have shown that for the MC, an FFWT of 626°F is economically viable and complies well with the performance of a very large, state-of-the-art single-line rotating air preheater, and it will still be able to cool the boiler flue gas outlet temperature to its minimum value of 239°F.
For the single reheat cycle, optimum FFWT is 626°F, but the optimum is flat and the heat rate improvement by increasing FFWT 20K from 590°F to 626°F is small. Furthermore, concerns about a temperature rise of the topping HP heater – which should not be more than 35K for 800 MW+ plants – combined with concerns about the moisture of the LP turbine exhaust steam, mean that FFWT of the single reheat cycles should not exceed 590°F.
Figure 6 shows a side view of an MC boiler, and it will be explained shortly. The furnace screen is the first heating surface the flue gas meets after having left the furnace. The furnace screen collects all supporting tubes to a radiant superheater, protecting the final superheater. After the screen, the flue gas meets the final superheater (SH3), the final reheater 1 (RH1.2), the final reheater 2 (RH2.2), superheater 2 (SH2), the first section of reheater 2 (RH2.1), and the first section of the first reheater 1 (RH1.1). Finally, the flue gas meets the second section of the economizer, the deNOX catalyst, the first section of the economizer and it leaves the boiler through the rotating air preheater.