Slag Compositional Limits for Coal Use in Slagging Gasifiers

The suitability of Australian bituminous coals has been further confirmed for use in entrained-flow slagging gasifiers which form the basis for advanced power generation technologies based on integrated gasification - combined cycle (IGCC). The ash compositional limits for coal use in entrained- flow slagging gasifiers have been established by the definition of the 1400oC contours for the temperature of critical viscosity (Tcv) at iron levels up to 15% FeO. These contours allow the estimation of the CaO concentration necessary to avoid slag crystallisation and potential blockages in the taphole. A Tcv of <14000C reflects an optimum condition which enables continuous slag tapping in all the main gasifiers. A higher value for Tcv may well disadvantage a coal for use in several gasifiers. From these contours and earlier slag viscosity results, a series of empirical diagrams have been developed to provide a means of defining the compositional limits for optimum coal use in entrained- flow slagging gasifiers. The limits established are a guide rather than an accurate prediction but allow identification of coals that may face crystallisation problems and allow estimates of the limestone flux additions needed to overcome such problems. The empirical diagrams define the compositional limits in terms of the CaO concentration required to yield a tappable slag at 14000C as a function of the FeO content and the silica to alumina (S/A) ratio of the slag. The diagrams reveal how flux requirements can be minimised by coal blending to a S/A ratio of about 2. Using the results a survey has been conducted on coal suitability and blending opportunities for export thermal coals shipped out of the port of Gladstone. A number of "optimum" coals and coal blends have been identified, confirming that Australian export coals will be technically competitive in the IGCC market, as and when it develops. It remains to obtain some further slag viscosity data at high iron contents and low S/A ratios, in order to cover two important deficiencies that were found in the current procedures for IGCC coal evaluation. Initial results have shown that the minor component K2O has a significant effect on Tcv.