Experimental and Modelling Evaluation of Slagging Behaviour of German Lignite

The behaviour of mineral matter is important for the characterisation of coal for use in entrained-flow gasification technologies. We investigated four behavioural characteristics—ash fusion temperatures (AFT), slag viscosity versus temperature profiles, temperatures of critical viscosity (TCV), and slag phase compositions—of Rhenish lignite coal ash using laboratory and modelling tools. Our modelling of AFT using coal ash composition provided a wide range of data, with only one (modified) model demonstrating agreement with experimental data. Experimental slag viscosity data were obtained over a temperature range from 1200uC to 1500uC. Where relevant, TCV was also measured. Comparison of these data to the calculated viscosities using several models from the literature revealed variation in the accuracy of the models due to the different databases used. Models that calculate TCV on the basis of ash composition fail to predict the data due to very high silica:alumina ratio. However, models based on AFT data match well with the experimental TCV values. The formation of solids in the slags in the temperature range below liquidus was calculated using thermodynamic modelling tools and compared with the microstructure of laboratory quenched samples. This revealed a strong relationship between solids formation and increasing viscosity. We also examined a modification of slagging behaviour by blending the Rhenish ash with ash of an Australian coal in different ratios. We show that greater viscosity and a wider operational temperature can be achieved by using an appropriate blending ratio.