1. The source of chloride ions in the FGD system
The chloride ions in the FGD system mainly come from coal combustion, desulfurizer limestone, and process feed water. The chlorine content in coal combustion is generally about 0 1%, with a small amount of coal having a chlorine content of 0 2% ～ 0. 3%, limestone has a chlorine content of 0 0.1%, the chloride ion content in the process water is slightly lower, about (10-150) mg/L, and the chlorine content in the flue gas outlet (volume converted to standard state) is about 1 mg/m3. Due to the recycling of water in the FGD system, chloride ions are enriched in the absorption solution, with a mass fraction of up to 1%, sometimes even higher. At this point, high concentrations of chloride ions can pose a threat to pipes, equipment, and gypsum.
2. The impact on system materials
The equipment and pipelines that come into contact with the slurry and waste liquid in the FGD system are mostly made of stainless steel, including absorption tower agitators, oxidation air ducts, water replenishment pipelines, waste liquid storage tanks, and dosing agitators. Most of the chloride ions are brought in by coal-fired flue gas and are absorbed and enriched in the absorption tower. Coupled with acidic media, the equipment and pipeline environment become more harsh, leading to metal crevice corrosion, pitting corrosion, stress corrosion, bubble corrosion, and erosion corrosion.
Gap corrosion often occurs in areas with insufficient oxygen supply, such as welding, riveting, and bolted connections in desulfurization devices, appearing in the form of cracks. The electrolyte in the gap is more oxygen deficient than other parts due to slow diffusion, and the hydrolysis of chloride releases heat, causing an increase in electrolyte concentration in the gap and exacerbating electrochemical corrosion.
Electric corrosion often occurs in power equipment such as agitators or impellers, where the solid content of the slurry is between 10% and 30%. The impact of the slurry can damage the protective film on the surface of the material. The metal at the site of destruction becomes the anode, corroding to form pits. The oxygen inside the hole participates in the cathodic reaction and is quickly depleted. In order to maintain electrical neutrality, negatively charged chloride ions diffuse from the outside into the pores. Due to the hydrolysis of metal chlorides, hydrochloric acid is produced, creating an acidic environment. In acidic environments, when metals dissolve, more chloride ions migrate into the pores, accelerating metal corrosion. In severe cases, it can cause equipment perforation
3. The impact on desulfurization efficiency
Bubble corrosion and erosion corrosion occur in parts that are subjected to intense operation or high-speed liquid flow, such as pump casings, impellers, elbows, pipelines, etc. The reason for the formation of this type of corrosion is due to the damage of the passivation film and the high surface mechanical stress of the material. Stress corrosion occurs in environments with both tensile stress and corrosive media, such as elbows.
The impact on desulfurization efficiency
In the FGD system, calcium chloride ionizes in the medium, increasing the concentration of Ca2+and causing the reaction to shift to the left, resulting in a decrease in the decomposition rate of CaCO3 and affecting the absorption of sulfur dioxide. In addition, chloride ions have strong coordination ability and can form complexes with metal ions such as FeCl4-, AlCl2+, ZnCl42-, etc. This complex can encapsulate Ca2+or CaCO3 particles, increase inert substances, reduce the amount of Ca2+or CaCO3 involved in the reaction, and increase the consumption of limestone. The increase of inert substances will increase the density of the slurry and increase power consumption. Moreover, chloride ions are higher than HSO3- or SO2-3 has stronger erosive power and can repel the action of HSO3- or SO2-3, affecting the dissolution and reaction of sulfur dioxide, thereby reducing desulfurization efficiency.
4. Impact on Gypsum Quality
4. 1. Increasing the moisture content of gypsum slurry, due to supersaturation, gradually crystallizes from small particles into larger gypsum particles. During the crystallization process, chloride ions are encapsulated inside the crystal and combine with calcium ions to form stable calcium chloride with four crystal waters, leaving a certain amount of water in the gypsum crystal and causing an increase in the gypsum moisture content. The moisture content of gypsum is generally required to be below 10% [11].
4. 2. Increase the difficulty of dehydration
During the dehydration process of gypsum, a large amount of water is removed, but there are still a small amount of chloride ions and calcium ions remaining between the gypsum grains, blocking the free water channels and making dehydration difficult. The chloride ion content in gypsum will also exceed the normal standard.
4. 3. Change the crystal structure of gypsum
Chloride ions can cause lattice distortion in gypsum, resulting in more crystal nuclei. The diversification of crystals will reduce the compactness of gypsum particles, which is not conducive to further dehydration of gypsum.
