Ash and slag qualities of Australian bituminous coals for use in slagging gasifiers

The suitability of Australian bituminous coals from an ash and slagging viewpoint, has been examined for use in entrained-flow slagging gasifiers which form the basis for more efficient power generation technologies based on integrated gasification-combined cycle (IGCC). Several coal deposits in both NSW and Queensland appear suitable for slag tapping without the addition of any flux. Many more thermal coals are shown to require a limestone flux addition of <3% CaCO3 by weight of coal and there could well be opportunities to reduce costs of flux addition by blending with other coals with lower ash fusion temperatures. Coal blending to yield a SiO2/Al2O3 ratio of 1.6–2.0 can minimise limestone flux requirements and avoid some limitations which arise from slag crystallisation. Limestone flux requirements can also be reduced by slag tapping at 1500°C rather than 1400°C, but this should be balanced against increased operating costs and losses in cold gas efficiency at higher gasification temperatures. Australian export coals with very low iron contents, after limestone flux addition, appear to have the potential advantage that slag viscosities (and hence slag tapping performance) are essentially independent of variability in coal ash composition. This should lower gasifier operating and maintenance costs, offsetting the costs of flux addition. Ash and slag characteristics and possible strategies for optimum coal use in entrained-flow slagging gasifiers are discussed.