The setting of bypass flue is currently one of the hot issues in the field of flue gas desulfurization in China. Feasibility studies have been conducted to eliminate bypass flue. The "Measures for the Management of Electricity Prices and Operation of Desulfurization Facilities for Coal fired Power Generation Units (Trial)" issued by the National Development and Reform Commission stipulates that it is encouraged not to set up bypass flues when constructing desulfurization facilities for new (expanded) coal-fired power units. However, most of the desulfurization units currently in operation and under construction in our country are equipped with bypass flues, and a considerable number of the operating units also adopt the operation mode of opening the bypass. Considering the actual situation of the desulfurization project, the conditions for canceling the bypass system of the desulfurization units that have been put into operation and are under construction are not yet mature. Therefore, it is necessary to study the operation mode of the bypass system to ensure the environmental protection effect of the desulfurization device and the safe and stable operation of the desulfurization device and the generator set.

For desulfurization devices with bypass flues and baffles, the general design baffle state after the FGD is put into operation is: fully open inlet and outlet baffles, and closed bypass baffles. But for systems equipped with booster fans, the operation mode of opening the bypass baffle can also be adopted, that is, the inlet and outlet of the system and the bypass baffle are all open. For some desulfurization units, this operation mode can also achieve ideal desulfurization effect by appropriately adjusting the booster fan. For example, in a 300MW unit, the desulfurization rate of the flue gas system of the desulfurization device can still reach 95.47% with the bypass baffle open, and there is no situation of clean flue gas reflux.

1.1 Advantages of operating mode with bypass baffle open

(1) Improved the safety of the operation of the generator set and desulfurization device. Although the bypass baffle undergoes multiple quick opening tests during the commissioning of the desulfurization system, accidents occur due to various factors that prevent the bypass baffle from functioning properly during long-term operation of the desulfurization system. If the booster fan of the desulfurization system trips and the bypass baffle cannot be opened, it will inevitably lead to boiler MFT; If all circulating pumps trip and the bypass baffle cannot be opened, it will cause damage to the desulfurization equipment. A certain power plant once suffered damage to its defogger due to the bypass baffle not being opened after the circulating pump of the desulfurization device tripped, causing hot flue gas to enter the FGD system. If the operation mode of opening the bypass baffle is adopted, the flue gas channel can always remain unobstructed, and there is no possibility of the above-mentioned accidents occurring, greatly improving the operational safety of the unit and desulfurization device
(2) Improved the adaptability and adjustability of the system. At present, the stability of fuel in thermal power plants in China is generally poor, and the disadvantage of the dominant spray tower desulfurization device in China is its poor adaptability to fuel. When the SO2 content in the flue gas is significantly higher than the design value, the desulfurization rate of the spray tower FGD system decreases, and the pH value of the absorption tower often decreases significantly and is difficult to maintain stability; When the SO2 content in the flue gas is significantly lower than the design value, the power consumption of the spray tower FGD system remains high. If the bypass baffle is opened, the exhaust gas volume can be adjusted to adapt to the fuel by adjusting the output of the booster fan

1.2 Disadvantages and changes in the operation mode of opening the bypass baffle.

(1) Not all desulfurization devices are suitable for the operation mode of opening the bypass baffle. The prerequisite for using the bypass baffle opening operation mode is that the original flue gas can be completely introduced into the desulfurization system through the booster fan and there will be basically no clean flue gas reflux. From operational practice, not all desulfurization devices meet this condition. Some desulfurization devices may experience partial clean flue gas reflux but still have some raw flue gas discharged from the bypass. Taking the operation of the FGD system in a 600MW unit as an example, when the bypass baffle is opened and the opening of the booster fan remains unchanged, the inlet flue gas temperature drops from 126.9 degree to 122.2 degree , and the return flow of clean flue gas reaches about 8%. At the same time, the mass concentration of SO2 in the chimney is increased from 13mg/m3 increased to 64.2mg/m3, which means about 8% of the raw flue gas directly enters the chimney. At this point, if the original flue gas needs to be completely introduced into the desulfurization system, the only solution is to further increase the output of the booster fan, which will exacerbate the backflow of flue gas; To completely avoid the backflow of clean flue gas, it only depends on the output of the small booster fan, which will further increase the leakage of raw flue gas. In a 300MW desulfurization unit, there was a situation where less than 50% of the flue gas entering the desulfurization system had already flowed back with clean flue gas. Whether there will be a situation where clean flue gas reflux and raw flue gas direct discharge coexist, as well as the amount of reflux and direct discharge, depends entirely on the layout of the unit and desulfurization device flue gas system and needs to be determined based on actual calculations.
(2) Higher requirements have been put forward for the operation and regulation of the booster fan. The operation mode of the desulfurization device with bypass opening must adjust the output of the booster fan reasonably. If the fan output is too low, a portion of the flue gas will directly enter the chimney through the bypass flue, thereby reducing the desulfurization rate of the system. But the output of the booster fan cannot be too large, otherwise the clean flue gas will flow back. Unlike the operation with the bypass closed, at this time, not only does the power consumption of the booster fan increase, but also the output of the boiler induced draft fan needs to be increased to maintain the negative pressure balance of the furnace. Therefore, the power consumption of the induced draft fan will also increase. At the same time, if the amount of flue gas flowing back is too large, it may also cause corrosion of the booster and the original flue gas duct.
(3) Some power plants have reduced the output of their booster fans, resulting in a significant decrease in the amount of desulfurization in the system. Due to insufficient monitoring of pollutant emissions from thermal power plants in some regions, some power plants have significantly reduced the output of booster fans while operating with bypass baffles open, resulting in a decrease in system desulfurization capacity. This is also the main reason why relevant parties are considering canceling the bypass flue of the desulfurization device.