Understanding the influence of acidic components (Si, Al, and Ti) on ash flow temperature of South African coal sources

Abstract

Ash flow temperature is one property that specifically gives more information on the suitability of a coal source for combustion or gasification purposes. Therefore the chemistry and mineral interaction have to be understood in order to determine the suitability for fixed bed gasification purposes with regards to ash flow properties. Various authors ([Seggiani, M., 1999. Empirical correlations of the ash flow temperatures and temperature of critical viscosity for coal and biomass ashes. Fuel 78, 1121; Alpern, B., Nahuys, J., Martinez, L., 1984. Mineral matter in ashy and non-washable coals—its influence on chemical properties. Commun. Serv. Geol. Portugal 70 (2), 299]) have expressed the fusibility of coal ash as a function of the content of the principal oxides frequently found in coal ash, i.e. SiO 2, Al 2O 3, TiO 2, Fe 2O 3, CaO, MgO, Na 2O and K 2O. However, coal ash fusibility characteristics are difficult to determine precisely, partly because coal ash contains many components with different chemical behaviours, and may very from coal source to coal source. The purpose of this study is primarily to understand the effect and chemistry of the acidic components (Si, Al and Ti) of South African coal sources, as well as the manipulation or addition of these components to the coal sources, with the view to understand their effect on the ash flow properties. A representative coal blend as it is currently used for gasification purposes, as well as coal mixtures with the addition of pure SiO 2, Al 2O 3 and TiO 2, respectively, were prepared. The variation of the acidic components SiO 2, Al 2O 3 and TiO 2 varied from 1 mass% to 50 mass% to the coal blend. The particle size of the samples were representatively prepared to <1 mm and analyzed for ash flow temperature and ash composition as dependant variables according to ASTM D1857-87 and ASTM D2795-95, respectively. The raw data was then statistically evaluated by means of regression models. Results of the statistical evaluation of ash flow temperature and the ash elemental composition indicated that based on the 95% confidence interval Al 2O 3, Fe 2O 3, CaO, MgO, P 2O 5, as well as the SiO 2–Al 2O 3 ratio have a statistical significant effect on ash flow temperature. Regression trends of the coal and Al 2O 3, SiO 2 and TiO 2 mixtures indicated that Al 2O 3 has the biggest effect on the ash flow temperature. A shift towards increasing the Al 2O 3 concentration has a significant increasing effect on the ash flow temperature in the three-component system Al 2O 3–SiO 2–TiO 2 phase diagram. The ash flow temperature is the highest at the point where the Al 2O 3 concentration is maximized. The shifts in frequency of the absorption band associated with the 6s–6p electron transaction which relates to the basicity of a glass or slag also relates to the different effect of the individual acidic components on ash flow temperature. The shift can be considered as a measure of the electron donor power and is usually expressed in terms of the optical ( Λ) basicity. The ion–oxygen attraction and the stronger (more positive) value obtained for Al 2O 3 in comparison with TiO 2 and SiO 2 could be seen; and possibly explain why Al 2O 3 has a bigger influence and effect on increasing the ash flow temperature. It can be concluded from this experimental work with the emphasis on flow properties, that the acidic components Al 2O 3, SiO 2 and TiO 2 all have an increasing effect on ash flow temperature when added to the coal blends currently used for gasification. In addition to this general conclusion, it has also been confirmed that Al 2O 3 addition to the coal blend has the most significant effect towards increasing the ash flow temperature when compared to SiO 2 and TiO 2.