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Kiln shell corrosion
 

Kiln shell corrosion

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    Kiln shell corrosion Kiln shell corrosion Presentation Transcript

    • Kiln shell corrosionPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Kiln shell corrosionPDF created with pdfFactory Pro trial version www.pdffactory.com
    • PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Causes of kiln shell corrosion • Composition of the kiln atmosphere • Content of volatile components • Alkalis ,chloidic,sulfur and vanadic compounds, water vapour etc are the main volatile components predominating in the kiln gas atmosphere of cement kiln systems. • Alkalis and chloridic compounds are mainly taken into the kiln by the kiln feed and the sulfur and vanadic compounds by the fuel • The combustion of alternate fuels and especially waste fuels Increases the concentrations of volatile and further harmful compounds within the kiln system accelerating shell corrosion. • Attack by the back fly Ferrum Pedis.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Schematic mechanism of shell corrosionPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Different shell corrosions Three different kinds of kiln shell corrosion are observed in Cement kiln systems • Corrosion during kiln operation resulting in scaling of the kiln shell by oxidation of the metallic iron at elevated temperature ( so-called high temperature corrosion • Rusting during longer kiln shut downs caused by condensation or absorption of humid ( moist air , water) • Wear of oxidised kiln shell during longer kiln shut downs caused by small blackflies eating iron compounds ( so-called Ferrum Pedis syndrome)PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corroded kiln shell samples ( high temperature corrosion)PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corroded kiln shell samples( rusting during longer kiln shut downsPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion of corroded kiln shell by Ferrum PedisPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corroded kiln shellPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Kiln shell corrosion • migration and efforescences of salts between brick work and kiln shell • chemical attack of salts under kiln operating conditions (high thermal corrosion) • depending on the alkali-sulfur ratio and oxygen partial pressure bi- and trivalent iron oxides and / or iron sulfides are formed.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Pictures of corroded shell by Optical microscopePDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion 1. Scaling : Oxidation by dry gases at elevated temperature ( S , Cl ) 2. Rusting : Oxidation by moist air or water of condensation at low temperatures ( KCl )PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion ( Study ) • 12 Kilns at four plants located relatively close to one another have been studied. • Clear indications were found of a relationship between shell corrosion and the cement-making process, • Corrosion was found only in the eight precalciner kilns studied. • Suspension preheater kilns , showed no signs of corrosion.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Macroscopic description of the corrosion • The fastest corrosion rates are found from end of the coating over a distance of 5 * the kiln dia towards the kiln inlet. • The rate of corrosion then gradually decreases towards the kiln inlet. • There is hardly any corrosion on the burner side of the coating. • Fig. 1 illustrates the usual pattern of the coating.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion scale morphology and composition Brick side : Brownish Kiln shell side : Shiny dark brown or shiny blackPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion scale morphology and composition Three types of corrosion scales were observed • Chloride-free, alkali-free scales • Scales containing chloride without alkalis • Scales containing both chloride and alkalisPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion scale morphology and composition Chloride-free, alkali-free scales • Alternating layers of iron oxides and sulphides • The minerals found by X-ray diffraction were 1. Fe2O3 ( haematite ) 2. Fe3O4 ( magnetite ) 3. FeS2 ( pyrite ) 4. FeS ( pyrrhotite )PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion scale morphology and composition Scales containing chloride without alkalis • High sulphur layers alternate with high oxygen layers • With presence of chloride • Quantitative microanalysis of corrosion powder ( % w/w ) : Iron : 74 , Sulphur : 15 , Chloride : 2.5, Potassium : 0.5PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Corrosion scale morphology and composition Scales containing both chloride and alkalis •Quantitative microanalysis of corrosion powder ( % w/w ) : Iron : 64 , Sulphur : 14 , Chloride : 12 Potassium : 6.5PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion 1. Oxidation 2. Sulphidization 3. Hot corrosionPDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion Oxidation In an oxidising atmosphere the iron from the steel shell will react with oxygen to form an oxide scale. Compound with highest oxygen content , Fe2O3 being found at the scale-brick interface , and the compound with highest iron content , FeO at the metal-scale interface .PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion Sulphidization Where no oxygen is present , SO2 takes over as the oxygen donor and a different reaction occurs as follows : 4 Fe + 2 SO2 ( g ) = Fe3O4 + FeS2 Suphidization is enhanced by the presence of chlorides , mainly because they affect the morphology of corrosion scale ,hindering the formation of a strong , protective oxide layer.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion Sulphidization 1. High temperature hydrolysis of the thermally unstable alkali chlorides to form the more stable sulphates. 2. Followed by re-oxidation of hydrogen chloride gas ( by oxygen or SO2 ) at lower temperatures to elemental chlorine which attacks the metal. 2 KCl ( g) + H2O ( g ) + SO2 ( g ) + ½ O2 (g ) = K2SO4 + 2 HCl ( g ) ( T < 900 0C ) 2 HCl ( g ) + O2 ( g ) = Cl2 ( g ) + H2O ( T< 400 0C )PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion Sulphidization 1. In preheater kilns , chlorides evaporates during or shortly after calcination without substantial formation of hydrogen chloride gas. 2. Kilns with tertiary air duct, show delayed alkali chloride evaporation and, consequently, evaporation will be followed by more extensive hydrolysis of chlorides.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion Sulphidization • Once Cl2 ( g ) is formed, it can reach the kiln shell through the protective refractory bricks or through cracks and fissures in the lining, and will react with either the oxide-sulphide layers or, most likely, directly with the kiln shell according to the following reactions : - reaction with the oxide-sulphide layers FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 + O2 - reaction with the kiln shell Cl2 + Fe = FeCl2PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Mechanisms important to kiln shell corrosion Hot corrosion • Alkalis can probably only penetrate the lining as part of a liquid potassium and/or sodium salt melt. • If corrosion products therefore contain substantial quantities of potassium or sodium the form of corrosion is termed as Hot corrosion, indicating that a liquid phase takes part in the corrosion reactions.PDF created with pdfFactory Pro trial version www.pdffactory.com
    • Measures to prevent kiln shell corrosion 1. Apply gas-tight coatings on the kiln shell. 2. Maintain a full size dense lining. 3. Keep a high oxygen surplus in the kiln. 4. To cut down on chloride and sulphur input to the kiln. 5. To increase kiln chloride valve : to reduce burning zone temperature.PDF created with pdfFactory Pro trial version www.pdffactory.com