Thermal power generation has the advantages of abundant fuel resources, low cost, mature technology, small initial investment, short construction period, small site requirements, and not affected by seasons and climate. Therefore, whether it is in the past, present or a long period of time in the future All of them are the leading forces in my country’s power supply.
Thermal power generation will develop towards large-capacity, high-parameter units. my country’s thermal power generation uses coal as the main fuel. Improving the efficiency of coal-fired units and reducing pollutant emissions are the eternal theme of coal-fired power generation, and it is also an important task for my country’s current thermal power structure adjustment. my country’s thermal power units have small main capacity, low parameters, low thermal efficiency, and high average coal consumption for power generation; based on the strategic idea of sustainable development, by improving thermal efficiency and saving fuel, resource consumption can be reduced, while CO2, SO2, and NOx emissions can be reduced. To reduce the greenhouse effect and air pollution, and protect the environment. Throughout the development history of ultra (super) critical generating units in developed countries, the main measure to improve thermal efficiency of thermal power plants is to increase the parameters of steam, that is, to increase the steam pressure and temperature parameters of steam turbines. Large-capacity, high-parameter units, especially ultra- (super) critical thermal power units, have the characteristics of low coal consumption and low pollutant emissions, which are the leading direction of my country’s future thermal power construction and development.
The increase in the temperature and pressure of the steam of thermal power generating units, while improving the thermal efficiency of the units, puts forward higher and newer requirements on the materials of key components of the power station, especially the thermal performance of the materials, the ability to resist high temperature corrosion and oxidation, cold processing and heat Processing performance and so on. Therefore, in order to ensure the stable operation of the unit at higher temperatures and pressures, materials are one of the key factors. That is to say, the choice of materials and the corresponding manufacturing technology will become the technological core of the development of advanced units in the future.
The service conditions of ultra (super) critical units are very harsh, especially structures such as boiler drums, steam turbine casings and steam piping will withstand higher steam pressure and temperature. Among them, the materials of each part of the boiler should have the following characteristics;
①High temperature resistance strength. The main steam pipe, superheater/reheater pipe, header and water wall materials must have the performance characteristics of long-lasting high temperature strength compatible with high steam parameters.
② High temperature corrosion resistance. Corrosion on the flue gas side of the boiler is an important factor affecting the life of the superheater, reheater, and water wall. When the temperature of the metal increases, the corrosion rate will increase significantly, so the flue gas side corrosion problem in the ultra-supercritical unit becomes more prominent.
③ Anti-steam oxidation. The increase in operating temperature has aggravated the steam side oxidation of superheaters, reheaters, and even steam flow components such as headers and pipes. This has caused the following consequences: First, the adiabatic effect of the oxide layer makes the metal in an over-temperature state; It is the exfoliated oxide layer blocking the elbows and other places that cause over-temperature bursting and valve leakage; third, the exfoliated oxide particles have an erosion effect on the front stage blades of the steam turbine. Therefore, in the selection of materials for superheaters, reheaters, etc., the steam oxidation resistance and oxide layer peeling performance of the materials should be fully considered.
④ Thermal fatigue performance. Thermal stress can be generated when the unit starts and stops, changes in load, and coal quality fluctuates. For thick-walled components such as main steam pipes, headers, and valves, the thermal fatigue resistance of the material is an important index as high as the high temperature strength, and the strength should be ensured. Under the premise of choosing ferritic heat-resistant steel with high thermal conductivity and low thermal expansion coefficient as much as possible.
The main steam and high-temperature reheat steam pipelines of large-capacity ultra-supercritical units will face the test of higher pressure and higher temperature than conventional supercritical units. First of all, the increase of main steam temperature and pressure puts forward more stringent requirements on the performance of key components such as auxiliary change resistance, fatigue, high temperature oxidation and corrosion, and the high temperature creep strength must meet the requirements of thermal stress caused by pipeline thermal expansion. . Generally speaking, it is suitable as a material for high-temperature steam pipelines, and its 105h creep stress value at working temperature should reach 90~100MPa. At the same time, the thermal expansion coefficient of the pipeline material is required to be relatively small and the thermal conductivity is relatively large, which can reduce The level of thermal stress in the pipeline.
All in all, the metal materials for boilers must have the characteristics of good high temperature resistance, steam oxidation resistance, good processing performance and low price.
As for the steam turbine, when its rotor, blades and other rotating parts are working, they are subjected to the combined action of complex stresses such as centrifugal force, torque, bending moment, and thermal stress. At the same time, due to the improvement of operating parameters, higher requirements will be put on the thermal performance of heat-resistant steel; the materials of cylinders, valves and other components also need to have better thermal performance due to the increase in temperature and pressure; high temperature fastening The parts need to have higher tensile yield strength and creep relaxation strength, resistance to stress corrosion in a steam environment, and sufficient toughness and plasticity to avoid the formation of end deformation cracks; start and stop of the unit, variable load and coal quality Fluctuations, thick-walled parts such as rotors, cylinder blocks, and valve materials are required to have low thermal fatigue and creep fatigue sensitivity; for low-pressure rotors with reheated steam temperature higher than 593℃, tempering of materials within this temperature range must also be considered brittleness.
