Thread Content
This post was last edited by Cuaili Yisu on 2018-5-11 at 16:14. According to reports from Polaris Environmental Protection Network, taking a 660MW power unit as an example, the reasons for the excessive scaling in the area at the dry-wet interface at the inlet of the flue gas desulfurization tower in limestone-gypsum wet desulfurization systems were investigated. The composition of the scale in that area was analyzed, the process by which scaling occurs in the flue gas duct at the inlet of the desulfurization tower was preliminarily summarized, and solutions to this problem were proposed. Properly installing guide vanes to improve the airflow distribution in the inlet flue and optimize the system’s operation can effectively solve this problem. 1 General situation: Since most thermal power plants in our country have eliminated desulfurization bypasses, the operation of the desulfurization system directly affects the normal operation of the units. The flue at the inlet of the desulfurization tower is a typical dry-wet interface, where scaling easily occurs, and it can even lead to blockages. The occurrence of scaling in this area is directly related to the dust concentration in the raw flue gas, the layout of the flues, and the uniformity of airflow. Meanwhile, the inlet flue gas velocity also has a significant impact on the flow field distribution inside the absorption tower. This paper conducts an in-depth investigation into the causes of fouling and blockage at the dry-wet interface of the flue gas inlet in the desulfurization tower of a 660MW power generation unit, and proposes a series of solutions in the hope of providing guidance and assistance for similar issues in future units of this type. A power generation company operates a 660MW supercritical once-through boiler, equipped with a limestone-gypsum wet flue gas desulfurization system. The flue gas flow rate at the inlet of the desulfurization system is 2,206,020 m3/h, the inlet flue gas temperature is 120°C, the SO2 concentration in the inlet flue gas is 6,400 mg/m3, and the dust concentration in the inlet flue gas is 30 mg/m3. The parameters of the main equipment in the desulfurization system are shown in Table 1. Table 1: Selection parameters for key FGD equipment. 2. Existing problems: During various shutdowns for maintenance, it was found that there was a slight amount of scaling at the dry-wet interface of the inlet flue in the desulfurization system of this power plant’s 660MW units. However, after the GGH was removed, the unit began to show obvious abnormalities just 3 months after operation; the pressure at the inlet of the booster fan increased from -800 to -400 Pa to a positive value of +400 to +700 Pa. Subsequently, when the system was operating under high load, the booster fan experienced significant surging. To reduce the surge in the booster fan, the unit can only operate at a reduced load, yet the current drawn by the booster fan remains roughly the same as that at full load. During the shutdown for maintenance of the unit, a large accumulation of scale was found at the manway, which significantly reduced the smoke flow area in that area and caused a substantial increase in system resistance. The demister was also inspected at the same time, and no scaling or blockage was found; therefore, it can be determined that the cause of surging in the booster fan was the blockage resulting from excessive scaling in the flue at the inlet of the absorption tower. The accumulation of large amounts of scale in the inlet flue not only creates system resistance and affects the proper operation of the booster fan, but it also alters the residence time and distribution characteristics of the flue gases, posing safety risks to the proper functioning of equipment such as the oxidation air ducts and agitators inside the tower. When the scale accumulation is high, due to the higher density of the scale, it can severely affect the normal load-bearing capacity of the flue, causing the flue to deform and damaging the anti-corrosion coating of the inlet flue. 3 Analysis of scale samples at the dry-wet interface: To determine the source of the scale, the crystal phase composition of the scale samples from the area at the dry-wet interface in the flue gas duct at the inlet of the desulfurization tower was analyzed and tested, with the results shown in Table 2. A comparative analysis was conducted on the components of solid substances in the gypsum samples and desulfurization slurry of this system; the specific results are shown in Table 3. Table 2 shows the analysis results of scale samples in %. Table 3 presents the analysis results of solids contained in gypsum and slurry in %. Gypsum is produced by dehydrating the desulfurization cycle slurry; analysis indicates that the composition of solids in gypsum is essentially the same as that in the desulfurization slurry. The gypsum crystals are distinctly short columnar in shape, with particle sizes mainly ranging from 40 to 60 μm. The main components of the scale in the inlet flue are anhydrite CaSO4, gypsum CaSO4·1/2H2O, and limestone CaCO3. Its elemental composition is similar to that of desulfurization gypsum, but its crystal structure differs significantly from that of gypsum; it appears as a porous, mixed structure containing numerous flaky crystal formations and amorphous substances, with relatively high levels of Mg and Si. Preliminary analysis shows that the scale in the flue at the inlet of the desulfurization tower mainly originates from the circulating slurry. Since this area represents a dry-wet interface, the high-temperature flue gas continuously washes the surface of the scale, causing the loss of its crystalline water. Additionally, as the inlet flue gas contains a certain concentration of dust, this dust reacts with calcium sulfate and calcium sulfite present in the desulfurization slurry under the influence of the high-temperature flue gas, resulting in the formation of complex substances and large amounts of hard scale. 4 Analysis of the reasons for excessive fouling at the dry-wet interface 4.1 Improper arrangement of flue ducts To reduce the outlet SO2 emission concentration, this power unit’s desulfurization system was upgraded. The first step in this upgrade involved removing the GGH, along with making corresponding modifications to the flue ducts at the inlet of the desulfurization tower. Due to the limited space at the site, after removing the GGH, a 1200° elbow was installed directly after the booster fan outlet, and then the flow passed through several more elbows before reaching the absorption tower. There are numerous elbows in the flue gas duct from the outlet of the booster fan to the inlet of the absorption tower; as a result, the flue gas changes direction multiple times. Moreover, no flow guiding devices are installed inside the flue duct, which causes the flue gas flow pattern in this section to enter the absorption tower without being organized first. The flue gas flow field is uneven, with severe turbulence; in particular, vortices are generated at the elbow before entering the absorption tower. Some of the flue gas circulates back into the flue, causing the droplets in the spray layer to be drawn into the inlet flue by this circulating gas. Meanwhile, these droplets can also capture dust particles carried by the inlet gas, which then deposit on the inner walls and internal support rods of the inlet flue, resulting in the formation of large amounts of scale. 4.2 Unreasonable operation mode: The desulfurization absorption tower of this unit is equipped with four spray layers, corresponding to the 4 slurry circulation pumps outside the tower. By reviewing the operational data of the system from two months prior to the accident-induced shutdown, it was found that during this period the circulation pump associated with the lowest spray layer was operating continuously. Moreover, the load of this unit was adjusted frequently during operation. The centerline of the lowest spray layer was only 2.10 meters away from the upper edge of the flue at the inlet of the absorption tower; this short distance meant that, during low-load operation, due to the lower flow velocity of the smoke gas at the inlet of the desulfurization tower, the slurry sprayed by this spray layer could easily enter the inlet flue. In particular, the operation at low load for an extended period later on to mitigate surging in the booster fan led to rapid scaling in that area. On the other hand, when starting up units without a desulfurization bypass, in order to protect the anti-corrosion coating in the absorption tower and the demister, it is necessary to start the slurry circulation pump first before introducing flue gas. Similarly, when shutting down the unit, the circulation pump must be stopped only after the flue gas flow to the tower has ceased and the outlet temperature drops below 60°C. When the circulation pump is operating and no flue gas enters the tower, lacking the impact of flue gas, the sprayed slurry droplets will directly reach the inlet of the absorption tower. Once high-temperature flue gas is introduced, hard deposits will form as the water evaporates. This phenomenon becomes more apparent when there is a long interval between the start-up times or shutdown times of the circulation pump and the booster fan. Based on preliminary analysis, the formation of excessive scale at the dry-wet interface in the flue gas duct at the inlet of the desulfurization tower occurs through the following processes: (1) Small slurry droplets are directly sprayed or carried by the flue gas to the wall of the flue gas duct at the inlet of the desulfurization tower, where they collide and deposit; (2) The flue gas at the inlet contains certain amounts of dust, which reacts with calcium sulfate and calcium sulfite present in the droplets to form hard scale; (3) Under the continuous action of high-temperature flue gas, the water contained in this scale is lost, resulting in the formation of large amounts of dry, hard scale. 5 Solutions to the Problem of Excessive Deposits at the Dry-Wet Interface 5.1 Optimizing the Layout of the Flue Gas Path from the Booster Fan Outlet to the Desulfurization Tower Inlet – The layout of the flue gas path carrying the raw flue gas from the booster fan outlet to the desulfurization tower inlet is unreasonable and requires design optimization. Through flow field simulation of the flue gas distribution in this flue, an optimization scheme for the flue was proposed using computational fluid dynamics methods, involving the installation of 4 flue gas guide vanes at the bottom of the elbow at the outlet of the booster fan and at the upper part of the inclined flue. By installing guide vanes, the flow field within the flue can be improved; the optimized flue flow field becomes relatively uniform, which effectively eliminates local vortices in the flue gas. This prevents the formation of airflow that returns to the flue at the inlet of the absorption tower, while also reducing the resistance in the flue gas system. Furthermore, improving the flow field distribution within the flue ducts and absorption towers by installing flue gas deflector plates can also enhance the desulfurization efficiency. 5.2 Optimization of the system operation mode: During operation, the flue gas flow rate in the desulfurization system is directly related to the unit load; it is recommended that operators optimize the configuration of the circulation pumps based on actual conditions. At low load, the inlet flue gas velocity is low; to ensure that the SO2 level at the outlet of the desulfurization system meets the specified standards, the circulation pump corresponding to the lowest spray layer is shut down, thereby preventing the slurry from being sprayed directly into the inlet flue. When the unit starts up, the time interval between the startup of the circulation pump and that of the booster fan should be minimized; similarly, when the unit shuts down, the time interval between the shutdown of the booster fan and that of the circulation pump should also be minimized. During shutdown, after the booster fan stops operating, the demister flushing water or the emergency cooling water at the inlet of the absorption tower can be used to rapidly reduce the temperature inside the absorption tower, thereby shortening the downtime of the booster fan and the circulation pump. Efforts should be made to prevent the slurry from being directly sprayed into the inlet flue when no flue gas is flowing. 6. Implementation results: After cleaning the scale, repairing the flue ducts, installing guide vanes, and applying anti-corrosion treatment, the unit was put back into operation. The smoke pressure at the inlet of the booster fan returned to normal, the fan’s surge phenomenon disappeared, and the vibration levels were within acceptable ranges. The unit was operated in an optimized manner; after six months of continuous operation, the pressure values throughout the desulfurization system remained normal and stable, and the booster fans were functioning properly. The most recent inspection revealed only a small amount of scale near the edge at the inlet of the desulfurization tower, which demonstrates that by installing guide vanes to improve the airflow distribution in the inlet flue and optimizing operations, it is possible to effectively prevent the recurrence of severe scaling at the dry-wet interface of the inlet flue of the desulfurization tower. 7 Conclusion: The large amount of scale that forms in the flue at the inlet of the desulfurization tower is difficult to remove. This not only results in long downtime and high cleaning costs, but it also easily damages the anti-corrosion coating in that area, severely affecting the normal operation of the unit. During subsequent operations, it is recommended to indirectly assess the degree of fouling in the dry-wet interface area of the flue gas inlet of the desulfurization tower by monitoring changes in parameters such as the flue gas pressure throughout the system, the current of the booster fans, and the differential pressure across the demister. In the event of any abnormalities, the cause should be identified promptly to prevent excessive fouling from occurring there again and leading to serious consequences such as flue blockage. Original title: [Technology Focus] Analysis of the causes of fouling at the dry-wet interface of the flue gas duct at the inlet of the desulfurization tower and corresponding countermeasures