Mineralogical behaviour of North Dakota lignite in an oxygen/steam blown moving bed reactor

Abstract

In this study, lignite from North Dakota (USA) was thermally treated in a commercial-scale oxygen/steam blown moving bed reactor at the Dakota Gasification Conpany (DGC) in order to identify mineralogical changes that occur during the conversion process. After reaction, the solid particulate remnants were extracted from the reactor reaction zones and characterised using XRD, CCSEM and SEMPC techniques. It was found that the feed coal mineralization was mainly dominated by organically bound calcium. On the other hand, the crystalline phases present in the hottest regions of the reactor were governed by gehlenite and bredigite which may have formed from the transformation, (at higher temperatures), of the organically bound Ca and Mg to form CaO and MgO and subsequent interaction with the reactive silica and transformation products of the clays. A significant amount of calcite was found to develop at the start of the reduction zone; it is suggested that the calcite was synthesised from the reaction of CaO (formed from the transformation of the organically bound Ca) with the CO2 from the product gas in the reactor, in agreement with the literature. The glass phase was found to constitute the major part of the ash mineral assemblage in the reduction and combustion zones of the reactor. This phase was composed mainly of the Ca, Mg, Na aluminosilicates with some Fe. There was therefore a significant amount of melting that occurred in the hotter reaction zones (i.e. reduction and combustion zones) of the reactor. The organically bound Ca, Mg and Na seemed to have played a significant role in the formation of this glass phase.