DEVELOPMENT AND APPLICATION OF A NEW STRUCTURE BASED VISCOSITY MODEL FOR OXIDE MELTS RELEVANT TO FUEL SLAGS

摘要

IGCC power plants have high efficiency and represent a good opportunity to control the CO_2 emissions produced from the use of fossil fuels such as coal. The core of an IGCC power plant is the gasifier, in which slag viscosity as a function of temperature and composition plays a significant role in determining the optimum operating conditions. Many processes during gasification are related to the slag viscosity, such as particle sticking (or droplet sticking), slag flow and slag tapping. However, most of the previous viscosity models are only valid in a limited range of temperatures and compositions resulting from the lack of an effective description of the structural dependence of the viscosity. In this study, a structure based model has been developed for the fully liquid system SiO_2–Al_2O_3–CaO–MgO–Na_2O–K2O–FeO–Fe_2O_3 and its subsystems in the Newtonian range, based on the thermodynamic modified associate species model. To obtain an effective structural dependence of the viscosity, it is linked to the associate species distribution and the connectivity of associate species. Using this principle, both the temperature- and composition-induced structural changes of oxide melts can be described by a set of monomeric associate species in combination with the critical clusters induced by self- and inter-polymerization. With the new model, one of the challenges of the viscosity behavior in SiO_2-based binary systems, the so-called lubricant effect, is well described. The viscosity behavior when substituting one network modifier for another at constant SiO_2 content is also well described. The Al_2O_3-induced viscosity maximum is also described, in which the position and magnitude of the viscosity maximum as a function of temperature and composition (charge compensation effect) are properly predicted. The new model is self-consistent and gives a reliable prediction over the whole range of compositions and a broad range of temperatures using only one set of model parameters, which all have a clear physico-chemical meaning. In addition, the iso-viscosity lines and 3-dimensional viscosity surfaces are generated and further applied to determine the effects of coal ash fluxing and blending.