Why should a supercritical pressure cooker use a through flow boiler?

The DC boiler has the following characteristics:

1. Theoretically, it is suitable for any pressure, but in the case of low boiler pressure, the working stability is poor; When the boiler pressure is high, such as exceeding the critical pressure, the natural circulation boiler is difficult to operate, and the through-flow boiler can only be used.

2. The through-flow boiler eliminates the need for heavy steam drums and the steel frame supporting the steam drums, so it can save 20 25% of steel. The higher the pressure in the boiler, the more steel is saved.

3. Easy to manufacture, install and transport. The through-flow boiler does not have a steam drum, which eliminates the need for a huge plate bending machine and a hydraulic press during manufacturing; It brings a lot of convenience to installation and transportation, which is more practical for ultra-high parameters and large-capacity boilers.

4. The heating surface can be flexibly arranged, which is easy to meet the requirements of the furnace structure.

5. It can be started and stopped quickly. The through-flow boiler does not have a thick-walled steam drum, so the bending and deformation of the steam drum metal caused by the temperature stress during start-stop can be ignored, so the start-stop time is short.

It can be seen that the supercritical pressure cooker should use a direct flow boiler.

A power plant boiler without a steam drum (boiler tube), the feed water is preheated, evaporated to superheated by the pressure of the feed pump, and flows through the heating surfaces at all levels to produce rated parameters and capacity steam. The circulation rate of the evaporation zone is 1.

The once-through boiler was invented in the early 20s of the 20th century and began to be applied in the 30s. Although it has a series of advantages: it does not use a gas drum; The pressure parameter range is wide, which can be used for both subcritical pressure boiler and supercritical pressure boiler; Convenient manufacturing and steel saving; Fast start-up and shutdown, etc.

However, due to its high requirements for water treatment and automatic control, and its advantages are not significant when the steam parameters and boiler capacity are not large, the development is not fast. It was not until the late 50s and early 60s that the direct flow boiler developed rapidly due to the development of power plant boilers in the direction of large capacity and high parameters, and the water treatment technology and automatic control technology have also made great progress. In addition to Britain, France and other foreign countries, many countries regard the direct flow boiler as the main type of boiler for large power plants.

In the 80s, the world's largest capacity of the DC boiler was the 4400-ton supercritical pressure DC boiler (with 1300 MW units) in the United States. In 1968, China built the first 220-ton-hour high-voltage once-through boiler, and later successively built a 400-ton-hour ultra-high-voltage first-hand boiler and a 1,000-ton hourly intermediate reheat first-flow boiler.

Steel for supercritical pressure boiler Because the steam pressure and steam temperature of supercritical pressure boiler, especially ultra-supercritical pressure boiler, are very high, the steel problem of high-temperature superheater and high-temperature reheater should be considered first when determining boiler parameters. When the steam temperature exceeds 540, the heating surface of the high temperature section of the supercritical pressure boiler needs to use austenitic steel, which not only has high temperature strength resistance, but also needs to have good performance of high temperature oxidation. Compared with subcritical boilers of the same capacity, the heating surfaces of supercritical boilers at all levels are usually made of smaller diameter and better grade tubes, which are designed to reduce the thickness of the pipe wall, meet the needs of peak regulation and extend the service life.

The supercritical pressure water wall wall temperature is much higher than the subcritical pressure, reaching more than 500'C or even 540'C, so it is necessary to use alloy steel, such as 10CrMo910 P22, P91 and other corresponding grades of materials. Supercritical pressure boiler transformer operation modern large-capacity supercritical boiler furnace all adopt transformer operation, supercritical transformer operation boiler design should pay special attention to the fatigue life calculation and analysis of pressure components, not only for high-temperature thick-walled parts, but also for medium-temperature parts such as water-cooled walls and their rigid beams and so on to make special calculations. In addition, it is also necessary to pay attention to the overall performance design of the boiler and the temperature regulation means to adapt to the large peak regulation range.

Supercritical boiler refers to the boiler where the pressure of the working fluid in the pot oak rock furnace is made on the mountain at the critical beam royal point.

Compared with natural circulation boilers, supercritical boilers have the following starting characteristics:

1. Set up a special start-up bypass system.

The start-up feature of the through-flow boiler is that before the boiler is ignited, water must be continuously fed to the boiler, and sufficient starting flow must be established to ensure that the feed water continues to flow through the heating surface and cool it.

Generally, high-parameter and large-capacity DC boilers all adopt unit system, in the start of unit system, the steam turbine requires that the steam warmed up and flushed has more than 50 heat under the corresponding inlet pressure, the purpose of which is to prevent the low-temperature steam from condensing after being sent to the steam turbine, resulting in the water impact of the steam turbine, so the DC furnace needs to set up a special starting bypass system to eliminate these unqualified working fluids.

2. Equipped with a separator and a hydrophobic system.

The supercritical unit runs within the normal range, the boiler feed water flows directly through the economizer, water wall and superheater by the feed pump head, and the load in the DC operation state is from the full load of the boiler to the minimum DC load, and the minimum DC load is generally 25% 45%.

Below this DC minimum load, the feed water flow should be kept constant. For example, at 20% load, the minimum flow rate of 30% means that there is 20% saturated steam and 10% saturated water at the outlet of the water wall, this steam-water mixture must be separated at the outlet of the water wall, and the dry saturated steam is fed into the superheater, so the supercritical boiler needs a steam-water separator and a hydrophobic system at low load, and the hydrophobic system is another system necessary for the supercritical boiler to work at low load, and its role is to make the boiler start safely and reliably and its heat loss is minimized.

3. Before starting, the boiler should establish the starting pressure and starting flow.

The starting pressure refers to the pressure of the working fluid in the water wall of the DC boiler during the start-up process, and the starting pressure increases. The difference between the volume and quality of the steam and water is reduced, the hydrodynamic characteristics of the boiler are stable, the expansion of the working fluid is small, and the expansion process is easy to control, but the higher the starting pressure, the more unfavorable the protection of the screen superheater and the reheat superheater. The start-up flow rate refers to the feedwater flow rate of the boiler during the start-up process of the through-flow boiler.

Start-up characteristics of supercritical DC boiler.

Package boilers include natural circulation boilers and forced circulation boilers. The flow of working fluid in the evaporation heating surface of a natural circulation boiler is carried out by the pressure difference caused by the density difference between the water in the descending pipe and the steam-water mixture in the rising pipe (water wall). In addition to relying on the density difference between water and steam-water mixture, the working fluid in the evaporation heating surface of the forced circulation boiler mainly relies on the pressure head of the furnace water circulation pump for steam-water circulation.

Both the natural circulation boiler and the forced circulation boiler have a large steam drum to separate the steam and water, and the steam drum is used as the demarcation point to divide the heating surface of the boiler into two parts: the heating and evaporation heating surface and the superheated heating surface.

Features of natural circulation boilers.

The object of natural circulation research is the behavior of working fluids in the evaporation system of natural circulation boilers. The evaporation system of the natural circulation boiler is composed of a steam drum, a descending pipe, a distribution pipe, a lower box, a rising pipe, an upper box, a steam-water outlet pipe, and a steam-water separator. This evaporation system is closed, and the flow of working fluid within the closed evaporation system is called circulation.

The study of natural circulation is the heating of the riser, the heat absorption and flow characteristics of the working fluid in the circulation circuit. Discipline-wise, natural circulation belongs to the category of hydrodynamics and boiling heat transfer, which is often referred to as natural circulation hydrodynamics.

The natural cycle works on the principle that the working fluid moves along a closed route depending on the density difference generated by the heating of the riser. It should be noted that:

The route taken by steam is not a closed route, in the circuit, only water circulates there, so it is also called the water cycle.

Answer]: B The test point involved in this question is the classification of boilers. According to the steam pressure generated by the boiler, it is divided into supercritical pressure sock power boiler, subcritical pressure boiler, ultra-high pressure old quiet boiler, high-pressure boiler, medium pressure boiler, and low-pressure wide slag boiler.

Answer] :(1) Composition: built-in separator, separator trap, water level control valve, water level bypass valve, hydrophobic wide vessel, 100% MCR high-pressure bypass system, 65% MCR low-pressure bypass system.

2) Working number respecting process: 0 37%MCR: wet operation of the separator; 37% 100% MCR: Separator dry operation.