Behaviour of inorganic constituents and ash characteristics during fluidised-bed combustion of several Australian low-rank coals

Abstract

Behaviour of inorganic constituents during fluidised-bed combustion of several Australian low-rank coals was studied using a laboratory-scale spouted-bed combustion system. Coals from Victoria (Loy Yang and Morwell) and South Australia (Lochiel and Bowmans) were chosen for the present study. Characteristics of ash buildup on bed material and bed defluidisation were compared for the coals tested at temperatures between 800°C and 900°C. Samples of ash-coated bed particles and fly ash withdrawn from the system were subjected to various analytical techniques including chemical analysis, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Experimental results indicated that the coal type has a significant effect on the ash characteristics and buildup on bed material particles. Coals with high contents of sodium and sulphur have resulted in the formation of low-melting-point compounds (viz. alkali sulphates) in the ash coating on bed material particle surfaces, rendering them more sticky at fluidised-bed temperatures. This led to increased ash buildup on bed material particle surfaces with such low-rank coals. In contrast, for coals with low sodium and sulphur contents, the combustion lasted for longer periods without any particle agglomeration and defluidisation. The varying content of sodium and sulphur in these coals were identified to be responsible for the different ash buildup and defluidisation behaviour observed.