Characteristics of slurry pipeline medium
Wet flue gas desulfurization slurry pipeline has almost all the characteristics of ordinary fluid pipelines. At the same time, due to the medium inside the slurry pipeline being a mixture of small particles such as limestone powder or gypsum powder mixed with water, and containing some chloride ions (within 20000ppm) and heavy metal ions, the slurry pipeline is prone to wear, corrosion, and blockage.
2.1 Wear resistance
The wear resistance of slurry refers to the impact and damage of solid particles (especially silicates) in the slurry on the worn material. The wet flue gas desulfurization slurry medium is mainly composed of limestone (CaCO3) particles (containing a small amount of SiO2), gypsum (CaSO4 • 2H2O) particles, and water. The solid content of the slurry is generally 4.0% to 50%. The diameter of limestone slurry particles depends on the mesh size of limestone powder. According to the low standard requirement of 250 mesh, the diameter of limestone slurry particles is generally less than 60 μ m, while the particle size of gypsum particles is mostly less than 100 μ m. At higher flow velocities (above 3m/s), these particles will cause severe wear or erosion on the inner wall of the pipeline.
2.2 Corrosivity
Due to the weak acidity of the slurry and the presence of some chloride and fluoride ions, these substances can react chemically with the carbon steel pipe wall, causing corrosion and eventually leading to wear and tear, which affects the service life of the desulfurization device. The main reaction equation is: 4Fe+SO42-+4H2O=FeS+3Fe (OH) 2+2OH-
In addition, Cl - is more likely to adsorb on metal surfaces than oxygen and displace oxygen, causing local damage to the passivation state of the metal and resulting in pitting corrosion, which is also difficult to avoid in some stainless steel materials. The corrosion forms of slurry on metal pipes include pitting corrosion, crevice corrosion, stress corrosion, fatigue corrosion, electrochemical corrosion, and so on. A good method to prevent corrosion in the selection of lime slurry regulating valves is to prevent the slurry from coming into contact with the metal surface, such as lining rubber (currently widely used as butyl rubber) or lining plastic.
2.3 Blockability
The wet flue gas desulfurization slurry pipeline is a two-phase flow. The characteristic of two-phase flow is that the flow velocity must be controlled within an appropriate range. High flow velocity can easily cause wear and greatly increase pipeline resistance, while low flow velocity can lead to sedimentation, reducing the flow surface of the pipeline and eventually blocking the entire pipeline. The easy blockage of slurry pipelines is also manifested in the long-term uncleaning of sediments, which can lead to hardening and clumping, resulting in the entire pipeline being scrapped later.