Physical property behaviour of North Dakota lignite in an oxygen/steam blown moving bed gasifier

Abstract

In this study lignite originating from North Dakota (USA) was thermally treated in an oxygen/steam blown commercial-scale moving bed gasifier operating on lump coal at the Dakota Gasification Company (DGC) in order to identify the physical property changes that occur during heating. After reaction, the solid particulate remnants were extracted from the reactor and characterised using standard techniques. Thermal fragmentation was found to be severe with the coal tested, i.e. the lump coal was found to decrease in size to about 90% <6.3mm particles in the drying and de-volatilisation zones of the reactor. The particle size (mean Sauter diameter) however increased slightly in the reduction and combustion zones due to ash melting and subsequent agglomeration. The particle as well as the bulk density residual profiles of the fuel bed samples in the reactor was found to be in alignment with the residual ash content profile. The ash fusion temperature (AFT) of the feed coal to the commercial-scale moving-bed gasifier was found to be higher as compared to the ash bed. Fluxing elements in the dominating glass phase determined in the reduction and combustion zones are most probably the reason behind this phenomenon.