Influence of Coal Blending on Ash Fusibility in Reducing Atmosphere
Coal blending is an effective way to organize and control coal ash fusibility to meet different requirements of Coal-fired power plants. This study investigates three different eutectic processes and explains the mechanism of how coal blending affects ash fusibility. The blended ashes were prepared by hand-mixing two raw coal ashes at five blending ratios, G:D = 10:90 (G10D90), G:D= 20:80 (G20D80), G:D = 30:70 (G30D70), G:D = 40:60 (G40D60), and G:D = 50:50 (G50D50). The samples were heated at 900 °C, 1000 °C, 1100 °C, 1200 °C, and 1300 °C in reducing atmosphere. XRD and SEM/EDX were used to identify mineral transformations and eutectic processes. The eutectic processes were finally simulated with FactSage. Results show that the fusion temperatures of the blended ashes initially decrease and then increase with the blending ratio, a trend that is typical of eutectic melting. Eutectic phenomena are observed in D100, G10D90, and G30D70 in different degrees, which do not appear in G100 and G50D50 for the lack of eutectic reactants. The main eutectic reactants are gehlenite, magnetite, merwinite, and diopside. The FactSage simulation results show that the content discrepancy of merwinite and diopside in the ashes causes the inconsistent eutectic temperatures and eutectic degrees, in turn decrease the fusion temperature of the blended ash and then increase them with the blending ratio.
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 2 O 3 , TiO 2 , Fe 2 O 3 , CaO, MgO, Na 2 O and K 2 O. 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 2 O 3 and TiO 2 , respectively, were prepared. The variation of the acidic components SiO 2 , Al 2 O 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 Results of the statistical evaluation of ash flow temperature and the ash elemental composition indicated that based on the 95% confidence interval Al 2 O 3 , Fe 2 O 3 , CaO, MgO, P 2 O 5 , as well as the SiO 2 –Al 2 O 3 ratio have a statistical significant effect on ash flow temperature. Regression trends of the coal and Al 2 O 3 , SiO 2 and TiO 2 mixtures indicated that Al 2 O 3 has the biggest effect on the ash flow temperature. A shift towards increasing the Al 2 O 3 concentration has a significant increasing effect on the ash flow temperature in the three-component system Al 2 O 3 –SiO 2 –TiO 2 phase diagram. The ash flow temperature is the highest at the point where the Al 2 O 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 2 O 3 in comparison with TiO 2 and SiO 2 could be seen; and possibly explain why Al 2 O 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 2 O 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 2 O 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 .
Expanding the range of suitable gasification fuels
Abstract The efficient operation of entrained flow slagging gasifiers is dependent upon the mineral content of the coal supply that is used in the gasifier. When the gasifier is operated at the optimal temperature for gas production, the minerals must form a molten slag which has a viscosity low enough for the slag to flow down the lining of the gasifier without causing deposits that can plug the gasifier, yet not so low as to cause excessive erosion of the gasifier lining. This work details the use of new fluxing agents that can be used at low levels and are effective at lowering the viscosity of coal slags and expanding the range of coals that can effectively be used as gasifier fuels.