Study on emergency water make-up system of the hot

Research on emergency water make-up system of circulating fluidized bed boiler CFB (circulating fluidized bed boiler) has the advantages of wide fuel adaptability, high combustion efficiency, efficient desulfurization, low NOx emission, easy to realize comprehensive utilization of ash and slag, and is internationally recognized as one of the most commercialized clean coal technologies

unlike pulverized coal boilers, circulating fluidized bed boilers are equipped with a large number of bed materials and fire-resistant and wear-resistant materials, and their heat storage is very large. Under the abnormal power loss of the whole plant, heat storage will cause the steam water mixture and water in the furnace to be continuously heated and evaporated through air exhaust or steam turbine bypass. If the boiler is not supplemented with water in time, the water (including steam water mixture) in the boiler will continue to decrease with the increase of time, causing overheating damage to the boiler heating surface

the early imported circulating fluidized bed boilers in China were equipped with emergency water replenishment system, but because of its complex system, low frequency of use and high maintenance costs, the circulating fluidized bed boilers independently developed in the later stage generally do not have emergency water replenishment system

1 conventional emergency water make-up system of circulating fluidized bed boiler

domestic imported circulating fluidized bed boilers are equipped with emergency water make-up and re authorization system of the public. The original intention of setting this system is that there are a large number of materials in the furnace and thermal circuit of circulating fluidized bed boiler. When the power plant loses power, these high-temperature materials will transfer heat to heating surfaces such as water supply cold wall, tail cladding wall, external heat exchanger, etc. At this time, it is impossible to replenish water to the boiler, especially in the external bed. Because there are a large number of hot materials in the bed, the heat storage capacity is large, and the heating surface tubes are completely immersed in the hot materials. Even if a certain amount of steam passes through the heating surface, it is difficult to cool the heating surface to the safe wall temperature, which may cause the heating surface overheating and tube explosion. In order to protect the heating surface in the external heat exchanger, the circulating fluidized bed boilers equipped with external beds are equipped with emergency water make-up systems, such as Yunnan patrol power plant, Yunnan Honghe Power Plant, etc

domestic self-developed 300MW Circulating Fluidized Bed Boilers, such as Guangdong baolihua 300MW unit and Huaibei Linhuan Coal exhibition area, have also set up the same emergency water make-up system with reference to the imported technology for projects such as the 300MW unit in the first phase of the mud gangue power plant, which shows the scientific and technological utilization achievements of automotive lightweight plastic technology and innovative materials. See Figure 1 for the routine emergency water replenishment system process

2 units share a set of emergency water replenishment system, equipped with a high-power diesel engine, a set of high-pressure head low flow water pump, a 1000m3 emergency water replenishment tank, etc. When the whole plant loses power and the main feed water pump of the boiler cannot work, the demineralized water in the emergency make-up water tank outside the main plant is fed into the boiler through the emergency make-up water pump. See Table 1 for equipment configuration of conventional emergency water replenishment system

according to the response of the power plant, in order to ensure that the emergency make-up pump can be put into operation in a timely and reliable manner in the event of an accident, the pump set is required to be tested at least once a week, with a test time of not less than 1h and a monthly test time of not less than 12h, which is very expensive. Even in case of power loss of the whole plant, the heating surface will be damaged due to the low make-up water temperature

(water temperature 20 ℃). Therefore, in recent years, the circulating fluidized bed boiler unit is no longer equipped with an emergency water make-up system

2 new emergency water make-up system of circulating fluidized bed boiler

supercritical fluidized bed boiler, because there is no steam drum, the water volume of the boiler is smaller than that of subcritical circulating fluidized bed boiler. If there is no emergency water make-up system, there will be abnormal power loss in the whole plant, resulting in a much larger loss of heating surface than that of subcritical unit. It is estimated that the repair time will be more than 20 days, and the cost will be about 3million yuan, At the same time, the loss of power generation caused by shutdown is more than 10 million yuan. If the conventional emergency water replenishment system is adopted, not only the investment will be increased a lot, but also the heating surface of the boiler will be damaged due to the low water temperature

after in-depth research, the author proposed a new emergency water make-up system suitable for circulating fluidized bed boilers. The system flow is shown in Figure 2

the new emergency water replenishment system uses the existing water supply system equipment and pipelines of the power plant to realize the emergency water replenishment function in the case of abnormal power loss of the whole plant

the water supply system of the power plant is equipped with steam feed pump and electric feed pump. The steam feed pump is a running pump with a capacity of 1 × 100% or 2 × 50%; One set of electric feed pump is shared by one unit, and the capacity is 30% of the start-up standby electric pump

during normal operation of the unit, the hot water of about 180 ℃ in the deaerator enters the boiler economizer through the steam feed pump, feed water three-way valve, main feed water pipeline, three high-pressure heaters and feed water operation console

in case of power loss of the whole plant, start the electric feed pump (driven by the diesel generator with high stability to control all experimental process motivation), and enter the hot water in the deaerator into the boiler through the feed water three-way valve, feed water bypass pipe and feed water operation console. The electric shut-off valve at the inlet and outlet of the pump, the three-way water supply valve and the electric shut-off valve on the water supply console are all equipped with security power supply, which can ensure timely opening. The hot water in the deaerator is sent to the boiler within 1h to ensure that the heating surface of the boiler is cooled in time without overheating. At the same time, because the hot water entering the boiler is about 180 ℃, the heating surface of the boiler will not be damaged due to low water temperature. The new emergency water replenishment system does not need to set up an additional emergency water replenishment tank, emergency water replenishment pump and its pipeline system. It can realize the same function as the conventional emergency water replenishment system only by using the existing water supply system equipment and pipeline system in the plant and making some changes in electrical, control and other aspects. See Table 2 for the equipment configuration of the new emergency water replenishment system

3 comparison of two emergency water replenishment systems

compared with the conventional emergency water replenishment system, the new emergency water replenishment system is simple, the maintenance workload is small, and the application effect is good. See Table 3 for the specific comparison

4 conclusion

the initial investment of the conventional emergency water replenishment system of the circulating fluidized bed is 2000 definition: the ratio of the friction force (f) to the normal force (f) added to the test piece is 10000 yuan, but the actual equipment piping system is only a decoration, which cannot play the role of emergency water replenishment when it is really used, and it will still damage the heating surface of the boiler, and the repair cost and monthly test cost are also required, Therefore, many power plants with circulating fluidized bed units are not equipped with emergency water replenishment systems. However, for the supercritical fluid bed boiler, due to the lack of steam drum and the small water volume of the boiler, in the case of power loss of the whole plant, the heat storage of a large number of bed materials and fire-resistant and wear-resistant materials will cause the steam water mixture and water in the boiler to be heated and evaporated continuously. If the heating surface cannot be cooled in time, it will cause overheating damage to the heating surface of the boiler. For 350MW supercritical units, the estimated repair cost and power loss cost of a power loss accident of the whole plant are more than 10million yuan. The new emergency water replenishment system does not need to set up an additional emergency water replenishment tank, emergency water replenishment pump and its pipeline system. It can realize the same function as the conventional emergency water replenishment system only by using the existing water supply system equipment and pipeline system in the plant and making some changes in electrical, control and other aspects

to sum up, for supercritical circulating fluidized bed boiler units, it is recommended to adopt a new type of emergency water replenishment system. Although the initial investment is more than 20million yuan higher than the conventional emergency water replenishment system, it can really play the role of emergency water replenishment and avoid unnecessary losses in case of power loss of the whole plant