The Fusibility of Blended Coal Ash G. W. Bryant,* G. J. Browning, H. Emanuel, S. K. Gupta, R. P. Gupta, J. A. Lucas, and T. F. Wall Cooperative Research Centre For Black Coal Utilisation, Department Of Chemical Engineering, University Of Newcastle, Callaghan, NSW, 2308, Australia Received May 20, 1999. Revised Manuscript Received November 3, 1999 The fusibility of blended coal ash was studied by comparing the standard ash fusibility temperature test, temperatures at particular penetration levels measured by thermomechanical analysis (TMA), and predictions of liquid proportion with temperature thermodynamic equilib- rium. Ash fusibility temperatures (AFT) of coal ash are found at temperatures below the predicted liquidus temperature and, for ashes from blended coals, are generally nonlinear with respect to the blend proportion. The conclusion that trends in AFTs with blend proportions are mirrored by changes in the liquidus temperature from ternary systems, as was found by previous investigators, is not supported. This study provides support for the use of TMA to characterize ash fusibility. That is, TMA temperatures change with blend proportions when AFTs do not, and also mirror temperature changes at defined liquid contents predicted at thermodynamic equilibrium. Introduction Coal blending is used to provide a consistent feedstock of fuel for the power generation industry. Blending may also be used to reduce costs and compensate for unde- sirable characteristics of a coal, such as sulfur content. Several studies have indicated greater fouling and slagging upon coal blending or switching than would be expected upon combustion of either of the parent coals. 1 Huggins et al. 2 have concluded that the liquidus surfaces of the SiO 2 -Al 2 O 3 -XO (where X ) Fe, Ca, K 2 ) ternary systems correlate with trends obtained for ash fusibility temperature (AFT) measurements, in particu- lar the flow/fluid temperature, for coal ash-additive mixtures (using Fe 2 O 3 , CaO, K 2 CO 3 ). This work leads to the conclusion that the change in liquidus tempera- ture with coal ash blend ratio may correlate with changes in AFTs. Further, this study also indicated that the principal factors associated with coal ash melting and subsequent slag flow properties were the SiO 2 :Al 2 O 3 ratio and basic oxide levels. The operational tempera- tures of pulverized fuel plants are in the range of 900 to 1500 °C, 3,4 which is below the liquidus temperature of most ashes, and as such, the liquidus temperatures alone may not be capable of providing the correlation suggested. The AFT test and thermomechanical analy- sis (TMA) is used to examine this correlation. The AFT test provides four temperatures that characterize the fusibility behavior of laboratory ash (prepared at 815 °C) as it is heated at approximately 5 °C/min from 1000 to 1600 °C. 5 The deformation tem- perature/initial deformation temperature (DT/IDT) is the temperature at which a standard pyramid of ash begins to fuse or show initial evidence of deformation as it is heated. The sphere/softening temperature (ST) is the temperature at which the height of the standard pyramid is equal to the width, and hemispherical temperature (HT) is the temperature at which the height is equal to half the width. The flow temperature (FT) is the temperature at which the height of the molten sample becomes 1/16th of the width. 5 One of the principal limitations in the interpretation of AFT data is that for samples of varying mineralogy the AFT temperatures do not represent the same physical state in the sample. 6 A new technique based on TMA has been developed to characterize the various stages of coal ash fusion * Corresponding author. (1) Bogomolov, V. V.; Artemjeva, N. V.; Aleknovich, A. N.; Gladkov, V. E. The Slagging Behaviour Of Coal Blends In The Pilot Scale Combustion Test Facility. Proceedings EF Conference, Impact Of Mineral Impurities During Solid Fuel Combustion, 1997; Wall, T. F., Baxter, L. L., Eds.; Kona, Hawaii, 2-7 Nov. (2) Huggins, F. E.; Kosmak, D. A.; Huffman, G. P. Fuel 1981, 60, 577-584. (3) Benson, S. A.; Jones, M. L.; Harb, J. N. Ash Formation and Deposition, Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; 1993; Chapter 4, pp 299-373. (4) Singer, S. Pulverized coal combustion: recent developments, Energy Technol. Rev. 1984, 90, xxx. (5) Australian Standard AS1038: 15, 1995, Coal and Coke Analysis and Testing, Part 15: Higher Rank Coal Ash and Coke Ash-Ash Fusibility. Table 1. Ash Analysis of Coal Ash Samples as Oxide Weight Percent ash oxide analysis (wt %) A B C D E F SiO2 42.2 49.4 60.8 85.8 26.0 62.3 Al2O3 32.0 31.6 22.3 12.2 11.4 26.4 CaO 1.8 1.8 3.2 0.1 29.6 0.4 Fe2O3 18.3 11.8 5.6 0.9 12.2 4.4 MgO 1.0 1.0 1.2 <0.05 5.0 0.8 Na2O 0.2 0.1 1.0 0.1 5.2 0.5 K2O 0.1 0.2 2.1 0.4 0.3 3.5 TiO2 2.1 1.8 0.8 0.6 0.8 1.2 Mn3O4 0.4 0.2 0.09 <0.05 0.2 <0.05 SO3 1.7 1.9 2.2 0.1 9.2 0.3 P2O5 0.1 0.1 0.13 <0.05 <0.05 0.2 316 Energy & Fuels 2000, 14, 316-325 10.1021/ef990093+ CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000