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Moving the IP turbine bleeds to the T-turbine means that the reheat steam flows of the MC boiler are smaller than for a conventional single- or double-reheat boiler, but main steam flow is of the same size as for the single reheat boiler. Typically the reheat steam flow is reduced from ~85 percent to ~70 percent, and it has a clear impact on boiler design without changing the basic well-proven concept of USC boilers. Calculations have demonstrated ~6 percent weight savings of the MC reheater sections with ferritic and martensitic tubes.
Optimization results
In order to make a fair comparison of the different water/steam cycle concepts, it is necessary to keep the boundary conditions constant, i.e. the same boiler materials for furnace, the same water wall outlet temperature (842°F), the same unit flexibility with respect to fuel and operational behavior. Furthermore, flue gas exit temperature should be kept constant at 239°F.
Table 1 shows that the Master Cycle improves the net unit heat rate by approximately 3 percent, based on a rather
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The water/steam cycle of the steam flowing through the T-turbine is a simple non-reheat cycle, having significant lower efficiency since the reheat benefits are missing. On the other hand, the high super heat of the IP turbines means large energy losses when the heat of the bleed steam is being transferred to the condensate and feed water heaters, and investigations have shown that the tuning turbine efficiency and its integration within the water/steam cycle can be optimized beyond the values of Table 1.
In other words: The thermodynamic optimal design of the T-turbine arises when the exergy loss of the highly superheated IP turbine bleed steam balances the exergy gain of reheating the same bleed steam.
The DONG Energy investigations have shown that by a perfect optimization of the seawater-cooled MC, a net efficiency of 49 percent can be achieved, and with the introduction of flue gas coolers after the rotating air heater, the barrier of 50 percent net efficiency can be broken with a steel based (and seawater cooled) power plant operating at maximum steam temperatures in the range of 1,112°F. This result demonstrates the effectiveness of developments along the Carnot track.
Investigation of the investment costs
The additional capital cost of the MC has been estimated by DONG Energy and compared with a single reheat cycle with two motor driven 50 percent feed pumps. The outcome is listed in Table 2.
Table 2 indicates additional investment costs in the range of 40 million euro for the MC concept.
The net present value of the reduced fuel savings and CO2 reductions is estimated at minimum 45 million euro for each percent reduction of the heat rate. With roughly 4 percent improvement of the heat rate, as demonstrated by the optimized 800 MW MC, the total value of the savings is worth a minimum of 180 million euro.
This means a very high award for the better efficiency of the double reheat cycles, and it may be concluded that with additional investment costs of 40 million euro, and a capitalized value of fuel and emission savings of minimum 180 million euro, the total economy is excellent and very profitable – much better than for any other ongoing developments of advanced power plants.
No contingencies are foreseen, but even if 20 percent is added, the very robust economic viability of the MC concept remains.
