Alloy Steel Superheater And Reheater For Pulverized Coal Boilers With Natural Circulation

Summary

For power plant boiler, it is a crucial way to increase thermal economy of thermal power station by improving parameter of superheated steam. The increase of superheated steam parameters are limited by metal materials. The design of superheaters must make sure that the temperature of outer wall of heating surface pipes is lower than allowable temperature of oxidation resistance of steel, and meanwhile make sure its mechanical strength. with the development of metal material used in boilers, power station boilers in our country already generally apply high pressure, high temperature(9.8MPa, 540 degrees) and superhigh pressure parameters(13.7 MPa, 540 and 555 degrees), and have developed subcritical pressure parameters(16.7MPa, 540 and 555 degrees). Now lots of boilers apply supercritical pressure parameters(24.5MPa, 540-570 degrees), and even very few units apply higher pressure and temperature parameters.

With the increase of steam pressure, in order to reduce the steam humidity of steam turbine tail as well as further improve the thermal economy of power station, middle reheating system is widely used in high parameters power station, e.g. sending the exhaust of the high pressure cylinder in steam turbine again back to be re-heated in the boiler to high temperature, and then re-sending them back to the medium-pressure and low-pressure cylinder of the steam turbine, expanding and working. The element for reheating is called reheater.

In general, steam heated in high pressure superheater is named superheated vapour, the steam heated by reheater is called reheated vapour. The parameter of reheated steam is relating with economy of thermal circulation. Generally, the pressure of reheated steam is about one fifth of that of superheated steam, and the temperature of the former is almost the same as that of the latter. For example, in our nation for boiler with 125MW, 400t/h, the parameter for superheated steam is 133.7Mpa and 555 degrees; the inlet and outlet pressure of reheated steam is 2.5/2.35MPa, their temperature is 555 degrees, too. For boilers with 200MW, 670t/h, the parameter of superheated steam is 13.7MPa, 540 degrees; the inlet and outlet pressures are 2.7/2.5Mpa, and the temperature is also 540 degrees. For subcritical pressure control circulation boilers with 300MW, 600MW, superheated steam parameter is 18.27Mpa, 540 degrees; the inlet and outlet pressures are 3.83/3.63MPa, the temperature is also 540 degrees. Application of steam reheating system can increase the thermal economy for power stations by 4-5%. In our country, units with capacity over 125MW all apply one time middle reheating system, and overseas, two times middle reheating systems are applied for some units with higher parameters.

In modern boilers, the heat absorption consumption of the superheaters and reheaters is over half of total heat absorption for the working fluid, thus the heating surfaces of superheaters and reheaters account for a large proportion in total heating surfaces of boilers, and they must be placed in the areas where smoke temperature is higher, the working conditions are the worst among all the heating surfaces.Tube wall temperature of the heating surfaces is close to the max. allowable temperature of steel, so the reasonable arrangement and design of superheaters and reheaters matter so much for economy and reliability to the while boiler.  When it comes to their design, on the basis of assurance of safety and reliability of superheater and reheater, it is very necessary to save metal consumption at the best, especially to save usage of alloy steel.

For boilers in large power stations, superheaters and reheaters are necessary parts, to a large extend, they have influnced the safety and economy of boilers. Superheaters are the heating surfaces which heat the main steam from the surnatured temperature to the rated superheated temperature, and the reheaters are the heating surfaces which heat the exaust from the steam turbine high pressure cylinder to a certain temperature.When running conditions like boiler loads, coal types change, it is to adjust them to make sure the outlet temperature at the rated level of -10-+5 degrees.

For large boilers in power stations, superheaters and reheaters account for a big proportion in the total heating surfaces, they are to be arranged in the areas where smoke is with higher temperature. To improve the efficiency of thermal circulation in power stations, it is needed to improve the original parameters f steam gradually, the increase of steam pressure requires the corresponding increase of temperature for superheated steam, or the humidity of large stage exhaust of steam turbine will be too high, which will influence its safety. Thus, the steam flowing through superheaters and reheaters are high temperature steam, their heat transfer performance is quite bad, which determines that tube wall temperatures of superheaters and reheaters are high.

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