Optimized Steam Cycle
There are rules of thumb we use to quickly determine the benefits of USC operating conditions versus subcritical steam turbine conditions that we would like to share:
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Raising the main pressure by 100 psia improves the plant net efficiency by about 0.16%.
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Increasing the main steam temperature by 10F improves plant efficiency by 0.16%.
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Increasing reheat steam temperature by 10F improves plant efficiency by approximately 0.13%.
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A 10F increase of the final feedwater temperature improves plant net efficiency by about 0.1%.
These rules do have application limitations. For example, increasing steam conditions to improve efficiency is limited by available metallurgy and cost. Nevertheless, the key to improved cycle efficiency is to raise steam temperatures as high as possible. The final optimized steam conditions selected for TPP are shown in Table 2.
Table 2. Steam conditions for AEP’s John W. Turk, Jr. Power Plant. Source: Alstom
TPP was designed with eight heaters to raise the final feedwater temperature to improve efficiency as compared with a traditional subcritical unit utilizing six to seven heaters (Figure 3). Also, a heater above the reheat pressure (HARP cycle) is used. Downstream, four stages of low-pressure condensate heaters, one deaerator, and three stages of high-pressure feedwater heaters are used. An extraction from the HP turbine steam path feeds the top heater.
3. Eight is enough. The TPP steam cycle uses eight feedwater heaters in a HARP configuration. The optimized steam cycle is shown with data taken from the average conditions for the heat rate guarantee. Source: Alstom
The HARP cycle has one big advantage: Its design allows optimization of the final feedwater temperature independent of the reheater pressure while reducing moisture at the LP exhaust. At TPP, a final feedwater temperature of 570F was chosen to optimize performance while maintaining boiler operating constraints.
TPP’s optimized steam cycle heat rate guarantee is based on 20% summer, 20% winter, and 60% annual average operating conditions. The condenser pressure on the cycle diagram (Figure 2) represents the average condition for the heat rate guarantee. In addition, the TPP cycle uses a 100% single-flow boiler feed pump turbine that is fully integrated into the main steam turbine systems. The entire steam turbine system is controlled by an Alstom digital control system.
Due to the elevated steam parameters and increased final feedwater temperature, the 672-MW gross TPP plant will be about 6.2% higher in efficiency than a new-build subcritical unit of comparable power rating. This increased efficiency equates to a reduction of more than 300,000 metric tons of CO 2 per year and about 10 million metric tons of CO 2 over a 30-year lifetime compared to a new-build subcritical steam turbine unit. A comparison of various cycle parameters based on Alstom cycle calculations is illustrated in Figure 4.
4. Reducing CO2 emissions. Ultrasupercritical steam conditions will increase the TPP plant’s efficiency by approximately 6.2% and reduce CO2 emissions by more than 300,000 metric tons per year over a conventional subcritical steam plant design. Source: Alstom
Alstom has been in the business of supplying supercritical steam turbines since 1957. Alstom’s supercritical fleet now numbers 66 units with a total capacity of 44 GW. Included in that total are AEP and Tennessee Valley Authority cross-compound units rated at 1,300 MW each. The two units in the Lippendorf power plant in Germany (930 MW each) are Alstom’s largest single-shaft units. They have been in operation since 1999. An 1,100-MW single-shaft unit is under construction today with main steam temperature of 1,112F and reheat up to 1,148F.
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