Effect of chloride ion concentration on equipment in flue gas wet desulfurization

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The main sources of chloride ions in the desulfurization system consist of two parts: process water and flue gas. The first source is the process water used in the desulfurization system, which is usually ordinary industrial water containing a certain amount of chloride ions. If the chlorine ion content in this water is already high, it further increases the chloride ion level in the desulfurization system ； The second source is flue gas; the HCl gas in the flue gas and the chlorides in the dust cause a significant amount of chlorides to enter the slurry when the flue gas undergoes desulfurization, where they dissolve to form chloride ions. Even the sources of these two types of Cl ions are not sufficient to reach the 20,000 ppm limit; sometimes the chlorine concentration can reach as high as 40,000 ppm or even higher. This is mainly due to the recycling of wastewater, which keeps chloride ions in the solution, allowing them to accumulate over time and result in such high concentrations. I. Hazards of high chloride ion levels 1. An excessive amount of chloride ions in the slurry of the absorption tower can cause severe corrosion in the pipes, towers, and equipment that come into contact with this slurry within the desulfurization system. This leads to serious corrosion of devices such as the slurry circulation pumps and mixers, thereby reducing their service life. 2. In the slurry of the wet flue gas desulfurization system, chlorides mostly exist in the form of calcium chloride. An increase in calcium ion concentration, under the influence of the isoelectronic effect, will inhibit the dissolution of limestone, thereby affecting the physical and chemical absorption of SO2 and hindering the proper progress of the desulfurization reaction, resulting in a decrease in desulfurization efficiency. Additionally, to offset the effects of a decrease in desulfurization efficiency, it is inevitable that the supply volume of limestone slurry will be increased. This leads to a higher content of residual limestone in the absorber tower slurry, an increase in impurities, a sharp deterioration in the dehydration performance of gypsum, and poor quality of the dehydrated gypsum. At the same time, as the chlorine content in the slurry in the absorption tower increases, the properties of the slurry may change; a large number of bubbles will form in the slurry within the tower, leading to overflow in the absorption tower and the creation of an artificial liquid level. This interferes with the operators’ ability to make judgments and make adjustments, can cause cavitation or tripping in the slurry circulation pump, and may even result in the slurry entering the main flue. Additionally, due to the strong coordinating ability of chloride ions, at high concentrations they rapidly coordinate with metal ions such as Al, Fe, and Zn in the flue dust to form complexes. These complexes encapsulate Ca or CaCO3 particles, significantly reducing their chemical reactivity. As a result, the utilization rate of the slurry decreases, ultimately leading to an excess of CaCO3 in the absorber tower slurry; meanwhile, the pH value fails to rise, and the desulfurization efficiency is reduced. If it is necessary to increase the liquid-to-gas ratio to ensure desulfurization efficiency and achieve compliant emissions, this will result in higher power consumption of the slurry recirculation system. It affects the quality of gypsum. An increase in the chloride concentration in the slurry of the absorption tower inhibits the dissolution of sulfur dioxide to produce bisulfate ions, resulting in an increased calcium carbonate content and a higher chloride content in the gypsum. This leads to reduced dehydration properties of the gypsum and a decline in its quality. To obtain gypsum of higher quality, it is necessary to significantly increase the amount of flushing water. This creates a vicious cycle within the entire system; meanwhile, the chloride ion content in the desulfurization wastewater increases substantially, making wastewater treatment more difficult. II. Measures Among the various factors affecting chloride ions entering and leaving the absorption tower, increasing the wastewater discharge volume and thereby raising the amount of chloride ions carried away by the absorption tower is the main method for reducing the chloride ion concentration in it. Reduce the amount of water used for gypsum flushing, strictly control the chloride content in the gypsum to keep it within a reasonable range, and increase the amount of chlorine removed during the dehydration process ； Regularly monitor the chloride content in the absorbent tower slurry, and strictly keep the chloride level in this slurry below 10,000 mg/L; the chloride content in the gypsum slurry should also be kept within 10,000 mg/L. If an increase in chloride levels is detected, it is necessary to increase the amount of wastewater discharged and to dehydrate the gypsum slurry, thereby replacing it with fresh gypsum slurry ； Operate the desulfurization wastewater treatment system to ensure that the desulfurization wastewater is discharged in sufficient quantities and in compliance with standards. At the same time, enhanced maintenance of the filtrate pump and filtrate pipelines should be carried out to prevent failures, which is also an important means of reducing the harm caused by chloride ions. A failure in the filtrate pump, along with pipe ruptures, can result in an inability to discharge wastewater for extended periods, causing the chloride ion level in the slurry in the absorption tower to soar.