Boiler: Silica Carryover
What is SiO2?
Silicon dioxide (SiO2) is not a molecule, at least when it is found in nature. It is the repeating unit of a giant 3-D covalent structure called silica. SiO2?is an empirical way of expressing the presence of silicon (Si) in SiO2?in which every Si atom is surrounded by 4 oxygen (O) atoms by covalent bonds and every O atom is surrounded by two Si atoms by covalent bonds. Thus 4 O atoms are shared by 2 Si atoms in a large crystal structure of silica with the ratio of silicon to oxygens?being 1:2. This gives the?ratio of Si : O = 1:2 and a molecular formula SiO2.??In steam power generation, silica can form deposits on heat-exchange surfaces which reduces thermal efficiency. In high-temperature boilers at 600 to 900°K and 1-100 atmosphere (atm) steam pressure, Si(OH)4 is the most likely silicon-containing gaseous species. This form of silica can volatilize volatilize and carryover with steam where it can form deposits on turbine blades and lower aerodynamic efficiency. In cooling water.
Silica carryover is unstoppable
Silica carryover can happen even when there is no mechanical carryover. At operating, drum pressure > 41 bar, the silica has appreciable vaporous solubility. The only solution lies in limiting its concentration in boiler-feed water.
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Attemperator spray water major source of contaminants.
The attemperator controls the maximum temperature of superheated steam by water injection. Often feed water is used for spray in an attemperator because it is high-pressure water that is already available, however, If the feedwater does not meet required purity standards then that puts the equipment at great risk.
The highest potential contaminant is silica.
Silica scaling preferentially takes place at the first opportunity on the first-stage blades of geothermal turbines.?In the case of saturated steam, this happens due to the evaporation of liquid films on turbine surfaces leading to supersaturation and then deposition. In the case of superheated heated steam, the deposition of solids takes place due to the pressure difference of superheated steam leading to a reduction of silica solubility. Once the silica gets supersaturated it deposits. Typically, the second-stage turbine blades have more moisture therefore, generally silica deposition over second-stage blades is near zero.
Online steam purity analysis can assist operators in avoiding steam turbine damage.
The Nuanced Art of Power Plant Operation
8 个月Elevated boiler water pH may slightly reduce concentration of (volatile) silicic acid in steam. This is based on the idea that steam solubility favors "neutral" or "acidid" molecules rather than "salt like" ones. Silicic acid pKa at 300 C saturated water is 9.3 - J. Solution Chem. 17 (1988) - For HCl the driving force of association reactions at elevated temperature seems to be larger entropy change in Gibbs energy dG = dH - tdS than the positive change in enthalpy. Association of HCl "frees" more hydrated water molecules than in case of NaCl, thus the larger entropy change - Chem. Rev. 94 (1994), J. Solution Chem. 17 (1988). Steam chooses H+Cl- rather than Na+Cl-. It can be same effect for Si(OH)4 and SiO(OH)3-.
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