Thermochemistry of the system K2O-CaO-SiO2

In their Energy 2020 agenda the EU defined several ambitious goals such as increasing the amount of energy produced from renewable sources by 20% and simultaneously reducing greenhouse gas emissions by 20%. Furthermore, the EU is committed that moving to a competitive low-carbon economy is completed by 2050. Solid biomass is considered to be an important sustainable energy source for meeting these targets. However, the combustion or gasification of biomass such as firewood, agricultural waste, crop stalks or straw for the production of heat and electricity results in enormous amounts of ashes and/or slags for which utilization options are eagerly required and studied. Just to give a number for Austria, a recent report on the biomass ash flows issued in 2016 by the federal environment agency for the base year 2013 evaluated a total volume of about 133.000 t biomass ash. On the other hand, the formation of slags and ashes has also implications for process control since they can cause severe problems in practical operation and may in the worst case result in an unscheduled shutdown of the whole biomass-fired power/heat generation plant. Within this context the system K2O-CaO-SiO2 comes into play because it is the main subsystem of silicate/oxides ashes in biomass and can be used as a model system for the interpretation of the relevant high-temperature processes and reactions. The last comprehensive and frequently used study of the phase equilibria in this system has been performed back in the 1930s. Recent investigations of our group show that considerable suspicion has to be attached not only what concerns the number of phases but also what concerns their chemical compositions and melting characteristics. Therefore, it is not that surprising that the prediction of ash and slag behavior during biomass combustion that was based on the previous study turned out to be imprecise and error- prone. The central goal of this project is a complete re-investigation of the ternary system K2O-CaO-SiO2. An important innovative aspect will be the combination of experimental equilibrium studies with measured thermodynamic properties and state-of-the art thermodynamic modeling within the same project. For this purpose the following key questions will be addressed: (1) How many solid compounds (phases) do exist and how is their melting behavior? (2) What are their melting points and heat capacities? (3) What are the solid state properties of the compounds including their crystal structures? (4) Is it possible to model thermodynamic properties of the system based on the novel experimental data? (5) How good is the agreement between the predicted and measured thermodynamic characteristics? Generally speaking, the proposal tries to bridge the gap between fundamental research on a ternary oxide system and applied science related to biomass combustion and technical mineralogy.

In the context of the increasing severity of the greenhouse impact, CO2 neutrality has become the major challenge for modern societies. Therefore, it is not surprising that renewable and carbon neutral energy sources such as biomass or agricultural waste have seen increased market shares during the past decade. However, the thermal utilization of biomass produces large amounts of slag and ashes, which are considered critical. In fact, fouling, slagging, and corrosion threaten the long-term operation of biomass power plants. In addition, biomass slags are generally landfilled, which is an expensive solution and can also create environmental problems. For a fundamental understanding of the slag-forming processes, the system K2O-CaO-SiO2 is of special importance. The last comprehensive investigation of this ternary system dates back to the 1930s and has been demonstrated to be inaccurate not only in terms of the number of existing crystalline phases but also in terms of the melting characteristics of the potassium calcium silicates. Within the research proposal we (i) re-examined the number of ternary phases that actually do exist. In fact, a total of nine thermodynamically stable phases and at least one metastable phase have been found. We (ii) developed new sol-gel-based synthesis approaches to produce potassium-calcium-silicates at much lower temperatures when compared to classic ceramic routes (avoiding substantial losses of the volatile K2O component during preparation), (iii) determined the crystal structures of previously unknown compounds of the system, and (iv) deciphered the stable phase assemblages at sub-solidus conditions. Furthermore, in-situ X-ray powder diffraction studies were used to get a deeper insight into the reaction pathways during the solid-state reactions. Solid-state properties of the materials, such as molar heat capacities, were also the focus of interest. It turned out that two of the compounds which are thermodynamically stable under ambient conditions undergo phase transformations upon heating, which involve challenging crystallographic phenomena involving aperiodic structures and diffuse scattering. In addition to interesting new insights into fundamental aspects related to the crystal chemistry of silicates and the influence of heating/cooling rates on the sequence of structural phase transitions, the heat capacities and entropies that we determined can be directly incorporated into thermodynamic databases that are used in industry to model the ternary system and to predict the parameters that control the slag-forming mechanisms. Our results clearly demonstrate that chemically not really exotic ternary oxide-based systems of main group elements are not as well understood as one would expect.