Development of a new database for thermodynamic modelling of the system Na 2 O-K 2 O-Al 2 O 3 -SiO 2

摘要

Complex oxide systems containing silica and alumina (so-called slag) are important in many fields of science and industry. One of the main problems in coal combustion processes is the alkali release. This alkali release leads to an alkali concentration in the flue gas significantly higher than the specifications of the gas turbine manufacture`s. Molten coal ash slag containing high concentration of SiO2, Al2O3, Fe2O3 and alkali and other alkaline-earth oxides has a high potential for alkali retention depending on the alkali oxide activity in the slag. Compared to experimental methods thermodynamic equilibrium modelling is able to generate results within less time at lower costs. In order to perform calculations concerning systems which have a complicated structure and strong interactions between the constituents an adequate thermodynamic model and assessed thermodynamic data are needed. The accuracy of the calculation depends on the reliability of the database and adaptability of the model.The goal of the present work is the development of a new database optimised for the basic quaternary oxide system containing the main slag components SiO2 and Al2O3 and the alkali oxides Na2O and K2O. This system can be considered as a basis for the future addition of further slag-relevant oxides such as Fe2O3 and the alkaline earth oxides CaO and MgO.This complex oxide system was investigated with respect to the thermodynamic description of the liquid solution (slag). The available thermodynamic data were collected, analysed and modified for the purpose of improving the solution database. The associate solution model was adapted to the accepted database FACT.The associate species approach was applied to represent the phase relations in the binary systems Me2O-SiO2, Me2O-Al2O3 (Me = Na, K) and SiO2-Al2O3 and in the ternary systems K2O Na2O-SiO2 and Na2O-Al2O3-SiO2. The phase equilibria calculated using the associate model with the new optimised slag solution data show good agreement with the experimental points. In the literature, the binary systems Me2O-SiO2 and Me2O-Al2O3 (Me = Na, K) were not investigated regarding their phase diagrams in the composition range near the pure alkali oxide. In contrast to the old database, the new dataset allows the description of the whole composition range of the binary systems. The investigations performed show that the associate species approach cannot only be used for the representation of liquid solutions but also for the representation of solid solutions (nepheline and carnegieite) without taking into account the structure of the solid solution.The comparison of measured partial pressures of potassium over the liquid phase and of potassium oxide activities in the quaternary Na2O-K2O-SiO2-Al2O3 melt with calculated values show relatively good agreement although these data have not yet been taken into account for the optimisation. The associate species model can therefore be applied to describe and predict the thermodynamic properties of the considered system (phase diagram and activity data) and extended to multicomponent systems taking into account “multicomponent” interactions as well.