Search

Flexibility in feedstocks and products is one of the most important advantages of an IGCC power plant based on entrained-flow gasifiers. Moreover, it can be combined with the carbon capture and storage (CCS), which is a promising solution to control the CO2 emissions produced from the use of fossil fuels such as coal. The core of an IGCC power plant is the slagging gasifier, whose performance is significant for an IGCC power plant. In slagging gasifiers, the majority of the inorganic components are converted to a liquid slag flowing down the wall of the gasifier, whereas the remaining inorganic components entrained in the syngas form fly ash. Slag viscosity plays a very important role in determining the optimum operating conditions for slagging gasification, in which many processes are related to the viscosity, such as the particle sticking (or droplet sticking), slag flow, slag tapping, and refractory degradation that may lead to concerns about efficiency, process reliability, and safety. Slag viscosity is also identified as a critical property for simulations based on computational fluid dynamics (CFD). However, most of the early viscosity models are only capable of describing the viscosity over a limited range of temperatures and compositions, due to the lack of an effective description about the structural dependence of viscosity.This study aims at developing a new viscosity model for fully liquid slag systems in the Newtonian range. In the new viscosity model, the structure of slag is effectively taken into account, in which the viscosity is linked to the type and distribution of species, as well as the connectivity of species. The type of species is determined from the Gibbs energy, and thereby the distribution of species is calculated by using a completely self-consistent thermodynamic dataset, where the modified associate species model was applied for the slag. This dataset provides the phase diagram and other thermodynamic properties to be calculated in good agreement with the experimental data. In consequence, both the temperature- and composition-induced structural changes of molten fuel slags can be described with a set of monomeric associate species in combination with the critical clusters induced by the self- and inter-polymerizations.The new viscosity model has been developed for fully liquid slag system SiO2–Al2O3–CaO–MgO–Na2O–K2O–FeO–Fe2O3 and its subsystems in the Newtonian range, based on the thermodynamic modified associate species model. The model 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. With the new model, one of the challenges of the viscosity behavior in SiO2-based binary systems, the so called lubricant effect, can be excellently described. The viscosity behavior when substituting one network modifier for another at constant SiO2 contents is well described. The Al2O3-induced viscosity maximum is also well described, in which the position and magnitude of the viscosity maximum as a function of composition and temperature (charge compensation effect) are properly predicted. Another viscosity maximum when replacing Al2O3 with SiO2 for constant contents of the network modifiers is presented. The Fe2O3-induced charge compensation effect is less pronounced than that induced by Al2O3. It was found that the Fe2O3-induced viscosity maximum does not always occur depending on the charge compensators for Fe3+. The viscosity maximum when replacing Fe2O3 with SiO2 for constant contents of the other network modifiers might not occur.In addition, a rotational viscometer is employed to validate the current viscosity model. Viscosities of several real coal ashes were measured by this rotational method. Moreover, examples of the application of the current model in determining coal ash fluxing and blending are presented.