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Shandong Huaxing Group Wangcun Alumina Flue Gas Desulfurization Case Study
1. Sodium-Calcium Dual-Alkali Desulfurization Process
The Sodium-Calcium Dual-Alkali Desulfurization Process (Na2CO3/Ca(OH)2) is a technology developed based on the combination of limestone/gypsum method and sodium alkali method. It overcomes the disadvantages of scaling in the limestone/gypsum method and high operating costs in the sodium alkali method. By utilizing the high solubility of sodium salts in water, the process employs sodium alkali to absorb SO2 within the absorption tower. The desulfurization solution, after absorption, is regenerated in a regeneration tank using inexpensive lime, allowing for the recycling and utilization of sodium ions.
This process combines the characteristics of the lime method and the sodium alkali method, addressing the issue of scaling within the lime method towers while not possessing the high absorption efficiency of the sodium alkali method.
2. Process Characteristics
Compared to limestone or lime wet desulfurization processes, the dual-alkali process has the following advantages in principle:
The use of sodium alkali for desulfurization results in circulating water that is primarily a sodium ion (Na+) solution. This reduces the risk of corrosion and blockage in water pumps, pipelines and equipment, making it easier to operate and maintain the equipment.
The regeneration of the absorbent and precipitation of desulfurization residues occur outside the tower, thereby avoiding blockage and abrasion within the tower. This improves operational reliability and reduces operating costs.
The sodium-based absorbent has a fast absorption rate for SO2, allowing for a higher desulfurization efficiency with a relatively small liquid-to-gas ratio, typically exceeding 90%.
In terms of integrated desulfurization and dust removal technology, it can also enhance the utilization of lime.
3. Process Flow Introduction
The desulfurization system consists of the SO2 absorption system, flue gas system, desulfurizer feeding system, byproduct treatment system, process water system, and electrical control system.
Flue Gas Desulfurization Process for Furnace Emission
Furnace Flue Gas → Desulfurization Absorption Tower → Chimney
After removing most of the particulate matter through the dust collector, the flue gas from the furnace is drawn out by the induced draft fan and blown into the high-efficiency atomizing spray desulfurization tower under positive pressure (the flue gas inlet is located in the lower part of the desulfurization tower). A pre-cooling and pre-desulfurization system is installed at the inlet of the desulfurization tower. The flue gas, after being cooled, humidified, and partially desulfurized, enters the desulfurization tower. Inside the tower, the flue gas flows upward and comes into contact with the spray slurry in a countercurrent manner, thoroughly mixing the two. The tower is equipped with four layers of high-velocity swirl plates and four layers of spray systems. In this section, the space is filled with atomized liquid droplets with particle sizes of 100-300 μm. The SO2 in the flue gas reacts with the absorbing alkaline solution again, removing more than 90% of sulfur dioxide. The rational selection and scientific arrangement of the spray system create a misting zone with no dead zones and minimal overlap, ensuring the uniform distribution of misty liquid droplets. The flue gas travels through the misting zone for an extended period, providing ample opportunities for SO2 in the flue gas to come into contact with the desulfurization solution and continuously collide with the mist droplets. The SO2 reacts with the absorbing solution, thus being removed, while the remaining smoke particles are carried along with the "water droplets," increasing their mass. The desulfurized liquid falls into the bottom of the desulfurization tower and is periodically discharged into the collection system installed after the desulfurization tower. After replenishing a certain amount of alkaline solution, it is pumped back into the atomization and liquid supply system for reuse. The desulfurizing agent remains in a circulating state throughout the process.
4. Principle of the Process
The sodium-calcium double alkali process [Na2CO3/Ca(OH)2] utilizes pure alkali initiation and employs the sodium-calcium method for SO2 absorption and lime regeneration. Its basic chemical principles consist of the desulfurization process and the regeneration process:
Desulphurisation Process
Na2CO3 + SO2 → Na2SO3 + CO2
(1) The equation represents the absorption initiation reaction.
2NaOH + SO2 → Na2SO3 + H2O
(2) The equation represents the main reaction, occurring at pH > 9 (higher alkalinity).
Na2CO3 + SO2 + H2O → NaHSO3
(3) The equation represents the reaction that takes place when the alkalinity decreases to neutral or even acidic conditions (5 < pH < 9).
Regeneration Process
2NaHSO3 + Ca(OH)2 → Na2SO3 + +CaSO3↓ + 2H2O
Na2SO3 + Ca(OH)2 → 2NaOH + CaSO3↓
In the lime slurry (lime reaches saturation), the neutral (amphoteric) NaHSO3 quickly reacts with lime, releasing [Na+]. Subsequently, the generated [SO32-] continues to react with lime, and the resulting calcium bisulfite slowly precipitates as a semi-hydrate compound, thereby regenerating [Na+]. The absorbent solution recovers its ability to absorb SO2 and can be recycled.
The byproducts of desulfurization are calcium sulfite or calcium sulfate (after oxidation). Users can choose different methods to treat the byproducts according to their needs.
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