Effect of pH on Absorption Efficiency

From the equation above, it is clear that a higher pH value indicates a higher concentration of limestone in the slurry, which enhances the absorption of SOâ‚‚. To achieve a high SOâ‚‚ absorption rate, maintaining a suitable pH level is essential. The pH in the absorption tower is typically maintained by continuously adding limestone slurry. However, it's not always true that a higher pH is better. While a high pH favors the absorption of sulfur dioxide, a low pH promotes the precipitation of reaction products. These two effects are somewhat contradictory.

Within a certain range, increasing the pH of the slurry tends to improve desulfurization efficiency. This is because a higher pH means more CaCO₃ is available in the slurry, which is beneficial for the desulfurization process. However, once the pH exceeds 5.8, the desulfurization efficiency may start to decrease. This happens because as H⁺ concentration drops, the precipitation of calcium sulfate becomes more difficult, leading to incomplete reactions.

For example, when the pH reaches 5.9, the CaCO₃ content in the slurry is about 2.98%, while the CaSO₄·2H₂O content remains below 90%. This suggests that the reaction between SO₂ and the desulfurization agent is not complete at this point, resulting in both wasted limestone and lower quality gypsum. As the pH decreases again, the CaSO₄·2H₂O content increases, while the CaCO₃ content decreases.

Therefore, the pH of the slurry must be carefully controlled—it should neither be too high nor too low. Under normal operating conditions, keeping the pH of the absorption tower between 5.4 and 5.5 helps maintain a Ca/S ratio of around 1.02, which leads to efficient desulfurization and good product quality.

In conclusion, pH control plays a crucial role in wet flue gas desulfurization. It directly affects both the desulfurization efficiency and the purity of the final product. Choosing the right pH electrode is key to ensuring accurate and reliable monitoring of the process. Proper pH management can significantly enhance the performance and economic viability of the desulfurization system.