Bildung und Kondensation alkali-, bor- und schwefelhaltiger Phasen in Abgasen aus Glasschmelzwannen- Thermodynamik, Kinetik und Technologie

Evaporation processes generally occurring in glass melting furnaces are undesirable phenomena. Volatile glass components like sodium or boron evaporate from batch and glass melt. Their loss does not only have an effect on glass composition, homogeneity and quality, but also makes flue gas filters behind glass melting tanks indispensable. Although the operation of these filters is often based on long experience and collected empirical data, filter corrosion or slip-through of components can still lead to enormous problems. In boron containing flue gases, the main difficulties during filter operation are the slip-through of the gaseous species (HBO2)n and H3BO3 and the back-reaction of already precipitated filter dust to gaseous species. This study focuses on the thermodynamic and kinetic conditions in flue gases consisting of combustion gases, sodium, boron and sulphur compounds. To identify the reaction mechanisms in flue gases, results from thermodynamic calculations, kinetic experiments and the analysis of industrial filter dust are combined. The obtained data may be used to adjust filter operation in such a way that the above mentioned problems are avoided.In sulphur free flue gases, the possible species are NaBO2, KBO2, Na2B4O7 and H3BO3. In sulphur containing gases Na2SO4, NaHSO4, Na3(HSO4)(SO4) and K3Na(SO4)2 will form. Sodium and potassium borates and sulfates form condensates that can easily be precipitated between 400 and 200 °C. Only if boron has no alkaline reaction partner, can it pass the filters as gaseous species. In this case, boron will condensate as H3BO3. The condensation temperatures of each species depend on the partial pressure of the compound in the flue gas. For a typical partial pressure of PH3BO3 = 1 ∙ 10-4 bar H3BO3 will condensate below 87 °C. The analysis of industrial filter dust in combination with thermodynamic calculations shows, that condensation of compounds from flue gas will occur in the following order:Na2SO4-NaF-CaSO4-NaxByOz-Na2CO3-CaF2-NaOH-CaxByOz-CaCO3-Ca(OH)2The formation of the salts of sulphuric or hydrofluoric acid is preferred. The back-reaction from filter dust results from the reaction of precipitated species with acid components. As the mixing energy of the phases in the row above increases from left to right, SO3, F, B2O3, CO3 and H2O can be disposed by any species positioned left in the row