Ash-bed material interaction during combustion and steam gasification of Australian agricultural residues

Abstract

The time-dependent layer-formation process of the agglomerates for three common agricultural residues in Australia with different ash-forming elements, together with quartz sand as the bed material, were investigated in a lab-scale, fixed-bed reactor under combustion (5% v/v O₂) and steam-gasification (50% v/v steam) atmospheres at 900 °C. The impact of the atmosphere on the ash–bed material interaction was studied from the elemental composition and the morphology of the agglomerates, which were characterized with scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The ash–bed material interaction mechanisms for the three feedstock were identified as part of the alkali metals react to form ash particles, which, for wheat straw and cotton stalks, consist of Na, Mg, Si, P, K, and Ca and, for grape marc, is composed mostly of KCaPO₄; the remaining alkali metals react with either Si from the quartz sand (for grape marc and cotton stalk) or reactive Si from the fuel (for wheat straw) to form a low-melting-point alkali silicate coating layer; Ca dissolves or diffuses into the coating layer (for wheat straw and cotton stalk); and the ash particles formed in the first step then deposit on, and progressively embed in, the coating layer. The elemental composition of the coating layer is relatively independent of both the reaction time and the gas atmosphere. The coating layer increases in thickness with an increase in the reaction time. The addition of steam results in the production of more liquid alkali silicates, which augment the agglomeration. Any residual S may form sulfate particles with K, Ca, or Na in a combustion atmosphere, while in a steam-gasification atmosphere, the S is released to the gas phase so that more alkali metal may remain to form the low-melting-point alkali silicate.