During combustion or gasification of coals in power plants alkali metals are released to the hot flue gas, leading to high temperature corrosion of the gas turbine blades. Earlier investigations have shown the possibility of decreasing the alkali concentration in the hot flue gas by formation of aluminosilicate slags. Subject of this work is the investigation of the vaporisation and retention of alkalis in aluminosilicate slags depending on the parameters temperature and composition.
For the multicomponent system containing SiO2, Al2O3, M2O(M=Na, K) and additionally CaO, MgO, FeO, or TiO2 the partial pressures of the alkalis depending on the slag components were measured by Knudsen Effusion Mass Spectrometry. Assuming stoichiometric vaporisation of the M2O(gas) from slag the activities of the alkali oxides in the slag can be calculated. The measurements were carried out in the temperature range from 1550 to 1900 K.
For the four component oxide system SiO2-Al2O3-Na2O-K2O calculations of the alkali pressures and the activities of the slag components were carried out using different thermodynamic models and databases. Compared to experimental methods thermodynamic equilibrium modelling permits the prediction of oxide systems in a shorter period of time leading to a significant cost reduction. Allowing free variation of parameters such as temperature and chemical composition, thermodynamic modelling is particularly important for those systems that are difficult to realise in experiments.
Two basic approaches are used to describe and predict the thermodynamic properties in complex oxide systems such as aluminosilicates, where strong interactions between constituents, particularly with SiO2, are observed. The modified quasi-chemical model introduced by Pelton and Blander is used to describe the short-range ordering in slags. The second approach, the so called modified associate species model was first used for complex oxide solutions by Hastie and Bonnell. The basis for this approach is that the complex oxide solutions can be represented by an ideal solution of end-member species and intermediate associate species. A modification of the associate species model (Spear et al.) is the incorporation of positive solution model constants to represent any positive interaction energies in a solution. With these constants it is possible to accurately describe reported immiscibilities in solution phases (liquid-liquid immiscibilities in many SiO2-containing systems).
The primary data bases for calculation of thermodynamic properties (components activities in the slag) and phase relations are the FACT database (quasi-chemical model) and a database by Spear and Besman in which the associate species approach is applied. However, the field of application of both models is often restricted. In some cases, even binary systems can not be described accurately. For a better representation of phase equilibria in alkali containing binary and ternary systems an improved database had to be established. Therefore, the data base by Spear and Besman was modified with respect to the data of the associated (formation enthalpies and entropies) and augmented by assessment of new interaction parameters (Redlich-Kister polynomials) between the different constituents of the slag.
To obtain these parameters experimental data on phase boundaries, melting points and phase transition etc. and, as initial comparison, calculated data using the FACT database were used. The search for suitable parameters, which could accurately represent the phase relations in the considered systems, was carried out with the optimisation module of the program ChemSage. The result of this optimisation process are modified formation enthalpies and entropies for binary species, some ternary species and some interaction parameters between the constituents. This new set of parameters was then applied to the calculation of phase relations and activities of alkalis. For binary subsystems, such as M2O-SiO2, M2O-Al2O3, and Al2O3-SiO2 a good agreement with the experimental data has now been obtained.
In order to more accurately represent phase equilibria in ternary systems (M2O-Al2O3-SiO2, Na2O-K2O-Al2O3, Na2O-K2O-SiO2) and calculate alkali activities in slags, further improvements of the database are being carried out with regards to other ternary species and their interaction parameters.