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The requirements in terms of load-following capabilities for the remaining and also for the new power plants increased during the past years, driven by the incentives policy for the support of renewable energy sources in the European energy mix. Nevertheless, it is known that renewables are characterized by low capacity utilization and difficult to forecast.
In this context, a new regulation for the support of the network frequency through the available capacity margin was defined already in the year 2000 which says that 2% of the nominal capacity of additional capacity should be available within 30 s and must stay available for at least 15 min. These requirements will be fulfilled through the throttling of the live steam control valves, the condensate-stop-process, the condensate-retain-process, or the closure of the bled steam pipes. Adequate cycling ability, in terms of number of starts and their duration, hot starts, as well as the ability for rapid load change rates are expected from a modern coal-fired power plant. This requirement is challenged by practical design considerations that higher steam parameters increase the wall thickness of boiler components and thus increasing thermal-mechanical stress potential and therefore limit operational flexibility. Such a contradiction is addressed by balancing the number of start-ups and load changes, the associated ramp rates, and the operating temperatures.
Designing the boiler for low minimum once- through load affects the evaporator design and makes more sense if such low loads can still be achieved without the need for secondary fuel firing.
The limitation of the steam temperatures up to 550/570 ℃ allows a high number of hot starts and a daily start-up and shut-down of the unit. This flexibility is paid by a reduced efficiency of approximately 1%~2% points for the whole plant.
The limitation of the live steam pressure up to approximately 250 bar, while the steam temperatures are retained in the range of 600/620 ℃ reduces the efficiency by approximately 0.3% points but can reduce the wall thickness of the high pressure outlet header by approximately 10 mm against a 290 bar design.
Alternatively the outlet headers can be designed by using the 100 000 h creep values and therefore the selected wall thickness may be reduced. The unit can be operated as a cycling unit. However such measures must follow economic considerations and to be considered case by case depending on the particular boundary conditions on site.
New project inquiries are increasingly specifying ultra-supercritical technology with high steam temperatures up to 600/620 ℃ and it is anticipated that the share of ultra-supercritical cycle units will continue to increase. The challenge coming out of this requirement is to achieve the combination of high efficiency with high operational flexibility.
For the increase of the efficiency of coal-fired power plants the main development trend is the increase of steam parameters with supercritical pressures > 250 bar and higher temperatures at the boiler outlet > 600/620℃,called “ultra-supercritical” steam parameter (see Figure 5).
To meet these new steam cycles, attention must be paid on the proper material selection adapted to the new steam cycle requirements. Several boiler materials with improved mechanical properties have been developed in the last years. Some new materials are still in the stage of development. Besides the aspects of strength and workability of the materials, attention must be paid to the corrosion and oxidation behaviour at high temperatures.
3.1 Waterwall
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The waterwall is defined as the flue gas-tight enclosure of the furnace and the heat exchanger section of the boiler.
For supercritical boilers with moderate steam conditions of 545℃ (SH outlet) and 570℃ (RH outlet) the low-alloy CrMo steel 13CrMo4-5 (T12)is used as waterwall material. Depending on fuel characteristics steam conditions up to 270 bar and 580℃ to 600℃(operating conditions at SH outlet)can be realized with the material T12. This corresponds to a maximum waterwall outlet temperature of approximately 460℃.
For advanced supercritical boilers with steam conditions of 600℃ (SH outlet) and 620℃(RH outlet) the 2.0%~2.5% chromium steels HCM2S (T23) or 7CrMoVTiB10-10 (T24) are needed due to the requirements for higher creep strength. Both materials are developed based on the steel 10CrMo9-10 (T22) but have much higher creep strength values. By use of the T23 or T24 steels, the steam temperature limit at the waterwall outlet can be raised by approximately 50 °K in comparison with the conventional T12 steel.
However the experience with both T23 and T24 shows that special efforts are necessary at the fabrication of the panels in the workshop or at least with the welding on site. Alstom has spent a lot of efforts to develop a suitable fabrication technology for this material in order to offer ultra supercritical steam parameter units with high reliability.
3.2 Superheater and Reheater Tubes
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For superheater and reheater tubes, the creep strength of the used materials has to be high enough at the relevant pressure and temperature range. In addition to this requirement for higher strength, more attention has to be paid to the corrosion and oxidation behavior of the materials. The oxide layer on the steam side can become significant and leads to higher material temperatures, which could cause creep damage. External high-temperature corrosion on the flue gas side reduces the wall thickness of the tubes and can lead to pictures.
Due to the requirements for high creep strength and high corrosion resistance, the martensitic steels like X10CrMoVNb9-1 (T91) can only be used for sub-and supercritical boilers with steam temperatures up to approximately 550℃ (SH outlet)and 570℃ (RH outlet). Above this temperature, for advanced supercritical boilers with steam conditions up to approximately 600℃(SH outlet)and 620℃(RH outlet), austenitic materials are mandatory. The austenitic materials have much higher chromium content which is beneficial in terms of increased oxidation and corrosion resistance. Typical austenitic materials for superheater and reheater tubes, which are commercially available are TP347HFG (fine grained), Super304H, HR3C and TP310N. The fineness of the metallographic structure enhances the oxidation resistance properties. In order to further increase the oxidation resistance on the tube inside, shot blasting procedures can be applied for some materials.
Figure 6 shows the 100 000 h creep rupture values of the superheater and reheater materials previously discussed.
3.3 Thick-walled Components
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As candidate materials for thick-walled components such as high pressure outlet headers and steam piping, main attention has been paid up to now to the improvement of the modified 9%~12% chromium steels.
In Europe, the conventional steel for thick-walled components has historically been X20CrMoV121. Extensive operating experience is available in the steam temperature range up to 560℃. With an increase of the steam parameters, the limit of the steel X20CrMoV121 I will be reached shortly. In order to have higher strengths, new materials have been developed. The next development in this area led to the application of the material X10CrMoVNb9-1(P91), which was co-developed by ALSTOM and can be regarded meanwhile as well-proven material. With the material P91, the steam parameters can be increased to 270 bar and 580℃ (operating conditions at SH outlet).
In a further development, the creep rupture values of the new steels have been further improved through the addition of tungsten. The typical materials from this new development are the tungsten-alloyed chromium steels P92, E911 and P122 (HCM12A). With such materials the steam parameters can be extended to approximately 290 bar and 600℃ on live steam.