The flue gas temperature decreases after FGD desulfurization, but why is the corrosivity higher?

Based on the mechanisms of corrosion, it can be mainly divided into the following five types of corrosion: 1) Acidic corrosion. Acidic gases such as SO2, HCl, and HF in flue gas form corresponding acids upon contact with liquids. The ions SO32‑, Cl‑, and SO42‑ are highly corrosive to metals, and they also cause significant diffusion and penetration damage to anti-corrosion linings. 2) Crystal corrosion. The sulfates and sulfites in the solution penetrate into the anti-corrosion lining and its capillaries as the solution moves inward. When the system is shut down, the absorption tower gradually dries out, causing the sulfates and sulfites in the solution to precipitate and crystallize. This leads to an increase in volume, which puts stress on the anti-corrosion lining. In particular, salts that contain crystal water can experience a volume expansion of several dozen times due to the alternating wet and dry conditions, resulting in even greater stress and severe delamination damage. 3) Electrochemical corrosion: Electrochemical corrosion occurs between the metal surface and water as well as electrolytes, and it is more evident at welds. 4) Low-temperature corrosion and thermal stress corrosion, influence of ambient temperature. Due to GGH failures or faults in the circulating liquid system, the flue gas temperature inside the tower rises, and the allowable stress of the anti-corrosion materials decreases sharply as the temperature increases. Sudden temperature changes cause asynchronous expansion due to the different expansion coefficients of the anti-corrosion lining and the substrate, and stress reduces the bonding strength of the lining. As the temperature rises, the corrosion resistance and impermeability of the lining material are reduced, accelerating its aging. Defects such as bubbles and cracks in the anti-corrosion lining cause these issues to develop rapidly under the effect of thermal stress, and the penetration of fluids further accelerates this process. 5) Erosion and wear corrosion. Due to the presence of solid particles in the slurry, as it flows through pipes or is sprayed downward, it exerts a certain scouring effect on the materials inside the tower; such corrosion is particularly likely to occur on the pipe walls or in areas with protrusions inside the tower.