How boiler works

The work of the boiler includes three processes, the combustion of fuel, the heat transfer process of flue gas to water and the vaporization process of water, and these three processes are carried out simultaneously in the boiler.

1-The combustion process of fuel

Different combustion methods have different combustion conditions. Taking a chain grate boiler as an example, its combustion equipment is a chain grate. The fuel falls on the grate surface by its own weight in the coal hopper, and the grate is driven by the sprocket after being decelerated by the motor through the variable speed gearbox. The chain grate is like a transporter, which continuously brings the fuel into the furnace. The fuel burns on the grate on one side and moves backward on the other side; the air required for fuel combustion is passed through the air duct and the air silo at the lower part of the grate by the blower, and passes upward through the grate to the fuel layer for combustion to form high-temperature flue gas. The fuel is finally burned into ash, which is discharged after turning over the slag removal board (commonly known as eagle iron) at the end of the grate. This whole process is called the combustion process. Of course, for the continuous combustion of the boiler, it is necessary to continuously supply fuel, air, and discharge flue gas and ash. For the requirements of environmental protection, the flue gas must also be dedusted. Coal slag removal equipment and smoke and dust removal equipment.

2-The heat transfer process of flue gas to working fluid (water, steam, heat transfer oil, etc.)

The temperature in the furnace is very high due to the exothermic combustion of the fuel. On the surrounding walls of the furnace, a drain pipe, commonly known as a water wall pipe, is arranged. The high-temperature flue gas and the water-cooled wall conduct strong radiative heat exchange, and transfer the heat to the working medium in the tube. Then the flue gas flows to the top of the furnace by the gravitational force of the induced draft fan and the chimney. After the flue gas exits the smoke window (furnace outlet) and passes over the anti-slag pipe, it flushes the steam superheater—a set of vertically placed serpentine heating surfaces to saturate the steam generated in the pot (steam drum and water wall). The steam is superheated by the flue gas in it. Economizers and air preheaters are arranged in sequence in the tail flue. The flue gas after multi-stage heat transfer is finally discharged from the boiler.

3-Water vaporization process

The vaporization process of water is also the generation process of steam, which mainly includes water circulation and steam-water separation process. The boiler feed water that has undergone water treatment is pressurized by the feed pump, first passed through the economizer to be preheated, and then enters the steam drum. When the boiler is working, the working medium in the steam drum is a mixture of steam and water in a saturated state. The convection tube bundle located in the lower flue gas temperature section has a relatively large proportion of soda-water mixture due to weak heating; while the water wall and convection tube bundle located in the flue gas high-temperature area have a correspondingly smaller proportion of the soda-water mixture due to strong heating; thus The soda-water mixture with a large specific gravity flows down into the lower drum, and the soda-water mixture with a small specific gravity flows up into the upper drum, which forms a natural circulation in the pot. In addition, in order to more effectively organize the water circulation and the need for flow distribution, an unheated descending pipe is generally provided outside the furnace wall, so as to lead the working medium to the lower header of the water wall, and pass through the upper header of the upper header. The soda-water outlet pipe leads the soda-water mixture into the upper drum. With the help of the steam-water separation device installed in the upper drum and the gravity separation in the space of the drum itself, the steam-water mixture is separated; if there is a superheater, the steam is drawn out from the top of the upper drum and enters the steam superheater, while The separated saturated water still falls back to the water space in the lower half of the upper drum. The water circulation in the steam drum also ensures that the metal heating surface in contact with the high temperature flue gas phase can be cooled without burning out, which is a necessary condition for the long-term safe and reliable operation of the boiler. The separation equipment of the steam-water mixture is the necessary equipment to ensure the quality of the steam and the reliable operation of the steam superheater.