Dissolution of lime into synthetic coal ash slags

One of the alternate processes presently being investigated to produce electrical power from coal is Integrated Gasification Combined Cycle (IGCC). The ash, which remains when the coal is gasified in this process, is removed by tapping the molten ash at 1400-1500 degrees C. To ensure that the coal ash is molten at 1400-1500 degrees C, the melting temperature of the coal ash may need to be reduced by addition of a flux, usually limestone, which is added with the coal to the gasifier. The rate of dissolution of the flux is uncertain. This paper reports the investigation of the rate of lime dissolution into synthetic coal ashes, consisting of SiO2, Al2O3 and CaO. Results previously reported have shown that the free dissolution of fine particles (50-200 mu m) is mass-transfer controlled [S.M. Wang, T.F. Wall, J.A. Lucas, L. Elliott, A.C. Beath, Experimental studies and computer simulation of dissolution of lime particles into coal ash slags. Australian Symposium on Combustion and the Fourth Flame Days, Univ. of Adelaide, South Australia, November 9-10, 1995]. To investigate forced dissolution, a high-temperature viscometer was used to rotate a cylinder of lime in the molten slag for a given period. At temperatures between 1450 degrees C and 1650 degrees C, reaction products of 3CaO . SiO2/3CaO . Al2O3, 2CaO . SiO2/3CaO . Al2O3 form around the lime cylinder. The concentration gradient involved in the mass transfer and the diffusion coefficient are currently being investigated. (C) 1998 Elsevier Science B.V. All rights reserved.