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The double-alkali flue gas desulfurization technology was developed to overcome the problem of scaling that occurs in the limestone-lime method. The traditional limestone/lime-gypsum flue gas desulfurization process uses calcium-based desulfurizers to absorb calcium sulfite and calcium sulfate formed from sulfur dioxide; due to their low solubility, these substances easily cause scaling and blockages within the desulfurization towers and pipelines. Scaling and blockage issues severely affect the proper operation of the desulfurization system, and even more so, they have a significant impact on the proper operation of the boiler system. In order to minimize the disadvantages associated with calcium-based desulfurization agents, calcium-based desulfurization processes generally require a corresponding forced oxidation system (aeration system), which increases the initial investment and operating costs. The use of cheaper desulfurization agents can lead to scaling and blockages. Relying solely on sodium-based desulfurization agents results in high operating costs, and the desulfurization by-products are difficult to handle. These contradictions have given rise to the dual-alkali flue gas desulfurization process, which effectively addresses the aforementioned issues. 1. Basic principle of the process: The double-alkali method uses sodium-based desulfurizers for desulfurization within the tower. Due to the strong alkalinity of these sodium-based desulfurizers, the reaction products formed after absorbing sulfur dioxide have high solubility, which prevents supersaturation and crystallization, thus avoiding problems such as scaling and blockages. On the other hand, the desulfurization products are discharged into a regeneration tank where they are reduced and regenerated using calcium hydroxide; the thus regenerated sodium-based desulfurizer is then sent back to the desulfurization tower for reuse. The double-alkali desulfurization process reduces investment and operating costs, making it suitable for desulfurization upgrades of small and medium-sized boilers. The double-alkali flue gas desulfurization technique uses sodium hydroxide solution as the desulfurization agent; this prepared sodium hydroxide solution is directly injected into the desulfurization tower to wash away SO2 from the flue gas, thereby achieving flue gas desulfurization. The desulfurization products are then reduced back to sodium hydroxide in a desulfurization agent regeneration tank and sent back into the desulfurization tower for reuse. The desulfurization process mainly consists of 5 parts: (1) Absorbent preparation and replenishment ； (2) Absorbent slurry spraying ； (3) Contact and mixing of mist droplets with flue gas inside the tower ； (4) Sodium-based alkali reduction in the regeneration tank slurry ； (5) Gypsum dehydration treatment. The double-alkali flue gas desulfurization process shares a similar reaction mechanism to other wet desulfurization methods such as limestone/lime; the main reaction involves SO2 in the flue gas first dissolving in the absorbent solution, and then dissociating into H+ and HSO3 — ； SO2(g) = SO2(aq) (1) SO2(aq) + H2O(l) = H+ + HSO3- = 2H+ + SO32- — ； (2) Equation (1) represents a slow reaction, which is one of the rate-control processes. Then H+ reacts with OH— in the solution to form salts and water, thereby facilitating the continuous absorption and dissolution of SO2. The specific reaction equations are as follows: 2NaOH + SO2 → Na2SO3 + H2O; Na2SO3 + SO2 + H2O → 2NaHSO3. The products of desulfurization are sent to a regeneration tank where they are regenerated using another base, usually Ca(OH)2. The regeneration reactions are as follows: Ca(OH)2 + Na2SO3 → 2NaOH + CaSO3; Ca(OH)2 + 2NaHSO3 → Na2SO3 + CaSO3·1/2H2O + 1/2H2O. In the presence of oxygen, the following reaction also occurs: Ca(OH)2 + Na2SO3 + 1/2O2 + 2H2O → 2NaOH + CaSO4·H2O. The sulfur removed is precipitated in the form of calcium sulfite and calcium sulfate, after which it is pumped into a gypsum dehydration system or simply piled up and discarded. The regenerated NaOH can be reused. 2. Process flow description: The flue gas from the boiler first passes through a dust collector to remove dust, and then enters the desulfurization tower via flue ducts at the bottom of the tower. By arranging several layers of swirl plates inside the desulfurization tower (the number of layers depends on specific conditions), the swirl plate tower provides excellent gas-liquid contact conditions. The alkaline solution sprayed from the top of the tower is atomized on these swirl plates, allowing SO2 in the flue gas to be fully absorbed and react with the sprayed alkaline solution. The flue gas after desulfurization and washing is dehumidified in a demister before entering the heat exchanger; the heated flue gas is then discharged into the atmosphere through a chimney via an exhaust fan. The original double-alkali process typically had only one circulation tank, in which NaOH, lime, and the fly ash captured during desulfurization were mixed together in that same tank. When removing the ash from the circulation pool, soot, the reaction products calcium sulfite and calcium sulfate, as well as lime sludge and unreacted lime are all removed at the same time. The resulting mixture is difficult to utilize comprehensively and thus becomes waste slag. To overcome the disadvantages of the traditional double-alkali method, it was improved. The main process involves adding sodium hydroxide to the clear water tank in one go to create a desulfurization solution, which is then pumped into the absorption tower for desulfurization. All three types of products are soluble in water. During the desulfurization process, the fly ash contained in the flue gas is simultaneously wetted by the circulating liquid and captured, while the circulating slurry discharged from the absorption tower flows into the sedimentation tank. The ash slag is removed regularly after sedimentation and can be recycled, for example in brick manufacturing. The supernatant flows into the reaction tank where it reacts with the lime added there; the sodium hydroxide generated as a result dissolves in the circulating water, while insoluble substances such as calcium sulfite, calcium sulfate, and calcium carbonate are formed, which can be removed through precipitation. 3. Process flow description: The double-alkali flue gas desulfurization process mainly consists of five components: the absorbent preparation and replenishment system, the flue gas system, the SO2 absorption system, the desulfurized gypsum dewatering system, and the electrical and control system. A. Absorbent preparation and replenishment system: When the desulfurization unit is started, sodium hydroxide is used as the absorbent. Dry sodium hydroxide powder is added to the alkali tank, where water is added to prepare a sodium hydroxide solution. This solution is then pumped into the return water tank and from there into the desulfurization tower for desulfurization. To regenerate and reduce the desulfurization products resulting from the use of this sodium-based desulfurizing agent, a slurry preparation tank is required. Lime powder is added to the pulping tank; after adding water, it is converted into a lime slurry, which is then poured into the regeneration tank where it reacts with sodium sulfite and sodium sulfate. Throughout the operation, many solid residues and other particulate materials generated by desulfurization are pumped into the gypsum dewatering system using slurry pumps. Since some sodium hydroxide is lost in the waste residues discharged, sodium hydroxide can be replenished regularly in the alkali tank to ensure the proper operation of the entire desulfurization system and the compliant emission of flue gas. To prevent the regenerated calcium sulfite and calcium sulfate from being pumped into the desulfurization tower, which could lead to scaling and blockages in the pipes and within the tower, a sparging device can be installed for forced oxidation, or the tank size can be increased. The regenerated desulfurizing agent solution is then subjected to thorough sedimentation in three-stage sedimentation tanks to ensure that larger particles do not end up back in the tower. Additionally, a filter can be installed in front of the circulation pump to remove large particles and impurities in the liquid. B. Flue gas system: The boiler’s flue gas enters the dust collector through the flue ducts to have dust removed, and then proceeds to the desulfurization tower. The flue gas, now cooled after desulfurization, passes through two stages of mist eliminators to have any remaining mist droplets removed before entering the main flue duct. After being reheated, it is released into the atmosphere through the chimney. When the desulfurization system fails or is shut down for maintenance, the system closes the inlet and outlet dampers, and the flue gas enters the chimney for emission via a bypass around the boiler’s main flue. C. SO2 absorption system: The flue gas enters the absorption tower and flows upward, where it is washed in counter-current manner with the limestone slurry sprayed downward, ensuring thorough contact between the gas and the liquid. The desulfurization tower is equipped with several layers of swirl plates inside, and a nozzle is installed on the topmost desulfurization swirl plate within the tower. The sprayed sodium hydroxide solution is injected through the spraying layer onto the water distributor at the axis of the swirl plate; thereafter, the alkaline solution spreads evenly. Under the guidance of the swirl plate, the flue gas rotates upward and comes into contact with the evenly distributed alkaline solution on the swirl plate, further atomizing the solution and enabling effective absorption of acidic gases such as SO2, SO3, HCl, and HF, resulting in the formation of NaSO3 and NaHSO3, while simultaneously consuming the sodium hydroxide used as an absorbent. The sodium hydroxide solution added as a supplement enters the return water tank, where it, together with the sodium hydroxide solution regenerated by lime, is pumped into the absorption tower via a circulation pump to cyclically absorb SO2. At the outlet of the absorption tower, a two-stage swirl plate (or baffle) mist eliminator is installed to remove the water mist carried by the flue gas during the washing process. During this process, the dust and other solid particles carried in the flue gas are also captured by the mist eliminators. Both stages of mist eliminators are equipped with water spray nozzles that are flushed at regular intervals to prevent clogging of the mist eliminators. D. Desulfurization product treatment system: The final desulfurization product of the desulfurization system is still gypsum slurry (with a solid content of about 20%), whose main components are CaSO3 and CaSO4; there is also some sodium sulfate NaSO4 formed as a result of oxidation. It is discharged from the slurry outlet pipe at the bottom of the sedimentation tank and sent to the hydrocyclone by a slurry pump. Since the solid product is contaminated with various ash components and NaSO4, which severely affects the quality of gypsum, it is generally discarded. Inside the hydrocyclone, the gypsum slurry is concentrated (with a solid content of about 40%) and then pumped to the slag treatment site, while the overflow fluid is returned to the regeneration tank. E. Electrical and Control Systems: The power supply for the desulfurization unit is drawn from the power panel in the power plant, and is connected to the power panel in the desulfurization electrical control room via high-voltage power cables. In the desulfurization electrical control room, the power supply is divided into two circuits; one circuit connects directly to the high-voltage motor (slurry circulation pump) via the distribution panel and control switchgear. In another case, a desulfurization transformer is used, whose output terminal is connected to low-voltage electrical equipment through a distribution panel and control switchgear; the low-voltage power supply is provided by motors located in the power center. The system is equipped with a low-voltage DC power supply to power the electric control section. The desulfurizer feeding equipment and the cyclone separator in the desulfurization system are under on-site control, while the rest are centrally controlled via the desulfurization control panel in the control room; local manual operation is also possible. During normal operation, the vertical control panel automatically controls various control valves to regulate the supply of lime and sodium hydroxide in the desulfurization system, enabling automatic adjustment in response to changes in boiler load. The control of flue gas volume is based on the boiler’s exhaust volume; it is achieved through the damper at the inlet of the exhaust fan, with the boiler load signal being used to convert this into a feedback signal that controls the flue gas volume and thus the amount of flue gas that actually enters the desulfurization unit. The control of the absorbent slurry flow rate is achieved through the amount of SO2 entering the desulfurization unit and the pH value of the slurry in the circulation tank. The supply amount of the by-product slurry is controlled by the flow rate of the absorbent slurry. The flow rate of the cleaning water for the demisting device, the pressure of the flushing water at the inlet of the absorption chamber, and the flow rate of the discharge liquid from the dehydrator are controlled separately. The liquid level at the bottom of the desulfurization tower is also controlled separately, that is, through the amount of water supplied. The control of the absorbent slurry concentration is achieved by adjusting the speed of the feeder based on the amount of water supplied, thereby controlling the amount of lime added and thus regulating the concentration. The cleaning of the demister at the outlet of the absorption chamber is carried out by turning on and off the water spray valve at regular intervals to flush it with make-up water. 4. Solution to the problem of secondary pollution: Sodium hydroxide is used as a desulfurization agent; it absorbs sulfur dioxide in the desulfurization tower at a rapid rate, resulting in high desulfurization efficiency. However, the desulfurization product, Na2SO4, is difficult to handle, and this can easily lead to serious problems of secondary pollution. The dual-alkali flue gas desulfurization process is employed: sodium hydroxide is used to absorb sulfur dioxide, and the resulting substance is regenerated using lime. Only a small amount of Na2SO4 is introduced into the gypsum slurry. This gypsum slurry, containing a slight amount of Na2SO4, is pumped into a cyclone separator for solid-liquid separation. The large volume of solid residue with a low moisture content resulting from this separation is sent to a waste dump for storage, while the solution flows back to the regeneration tank for further use, thus preventing secondary pollution. 5. Process Characteristics: Compared with the wet flue gas desulfurization process using limestone or lime, the double-alkali method has the following advantages in principle: (1) Since NaOH is used for desulfurization, the circulating water is essentially an aqueous solution of NaOH; therefore, there is no corrosion or clogging of pumps, pipelines, and equipment during circulation, which facilitates the operation and maintenance of these devices ； (2) The regeneration of the absorbent and the precipitation of desulfurization slag occur outside the tower, which prevents blockages and wear inside the tower, improves operational reliability, and reduces operating costs ； At the same time, efficient plate towers or packed towers can be used in place of empty towers, making the system more compact and improving the desulfurization efficiency ； (3) Sodium-based absorption solutions can absorb SO2 rapidly; therefore, a lower liquid-to-gas ratio can be used to achieve a high desulfurization efficiency, generally above 90% ； (4) For the integrated desulfurization and dust removal technology, it can improve the utilization rate of lime. The disadvantage is that Na2SO4, the oxidation by-product of NaSO3, is difficult to regenerate, requiring continuous addition of NaOH or Na2CO3 and thus increasing alkali consumption. Furthermore, the presence of Na2SO4 will also reduce the quality of gypsum. The double-alkali desulfurization technology is a mature technique used both domestically and internationally. It is particularly suitable for flue gas desulfurization in small and medium-sized boilers, and holds broad market prospects.