Dual-alkali desulfurization process
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Principle of the double-alkali method for desulfurization and dust removal: Flue gas is washed with alkaline water (sodium hydroxide or sodium carbonate); the reaction products are sent to a displacement tank where lime (calcium hydroxide) is added for displacement. The resulting substances after displacement undergo oxidation, concentration, and drying, after which the water is reused. After years of use, the disadvantages of the double-alkali desulfurization process have gradually become apparent, mainly in the following aspects: 1. When sodium hydroxide or sodium carbonate reacts with sulfur dioxide, it also reacts with carbon dioxide as well. The concentration of carbon dioxide in the flue gas is much higher than that of sulfur dioxide; carbon dioxide reacts with sodium hydroxide to form sodium carbonate, while sulfur dioxide reacts with sodium hydroxide to form sodium sulfate and sodium sulfite. These two reactions occur simultaneously. Due to the high concentration of carbon dioxide, a significant amount of sodium carbonate is produced. Part of this sodium carbonate reacts again with sulfur dioxide that is dissolved in water. However, after the first spray washing, the amount of sulfur dioxide remaining is very low (as some of it has already reacted with sodium hydroxide to form sodium sulfate and sodium sulfite). As a result, a large amount of sodium carbonate is discharged into the replacement system after this second reaction. Therefore, the amount of calcium hydroxide consumed to deal with carbon dioxide (the actual amount consumed after replacement) is quite high (depending on factors such as desulfurization efficiency, washing efficiency, and residence time). 2. Sodium sulfite reacts rapidly with lime (calcium hydroxide) (within the displacement system), but sodium sulfate has difficulty reacting with calcium hydroxide. As the concentration of sodium sulfate in the desulfurization cycle water increases, the desulfurization efficiency declines, resulting in excessive flue gas emissions and an increased consumption of caustic soda. 3. There is no effective way to control the reaction efficiency in the section where sodium sulfite reacts with lime (calcium hydroxide); in other words, it is not possible to control the displacement efficiency. As a result, undisplaced sodium sulfite oxidizes to sodium sulfate, and since sodium sulfate reacts poorly with calcium hydroxide, this leads to waste and a reduction in desulfurization efficiency. 4. Since the displacement system cannot distinguish between sodium hydroxide and calcium hydroxide, the concentration of calcium hydroxide in the spray water becomes very high, leading to scaling.
The double-alkali method is only slightly more advanced than the calcium method, but it is not the best approach for desulfurization. The best approach depends on the company and local resources and conditions to optimize the solution.
The double-alkali method requires high investment, and there are issues with the market for sodium sulfate.
Using the double-alkali method, we produce calcium sulfate, but its purity is not high enough, and it contains a significant amount of heavy metals, which makes it difficult to sell and results in the formation of solid waste. Converting waste gas into solid waste is also a major challenge that plagues us
The operating control conditions for the double-alkali method are quite important; sodium carbonate has an alkaline nature, and the pH can be maintained around 7 during operation. Sodium carbonate can react with SO2. During actual operation, sodium sulfite is partially oxidized to sodium sulfate, resulting in a loss of sodium salts; therefore, a small amount of antioxidant needs to be added. The key to the double-alkali method lies in whether sodium hydroxide can be displaced, and the design of the entire displacement reaction tank is also crucial. However, the double-alkali method seems to be less used these days, and it is difficult to achieve theoretical performance levels when operating it.
Yes, pipe blockages and tower clogs occur frequently, and the consumption of sodium hydroxide is much higher than the theoretical value