Ash–Bed Material Interaction during the 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 O 2 ) 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 KCaPO 4 ; 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.

Impact of residual fuel ash layers on the catalytic activation of K-feldspar regarding the water–gas shift reaction

“…While studies referring to industrial-scale plants mainly focused on olivine as a bed material [26], several alternative bed materials have been studied in lab-to pilot-scale experiments, where an activation could be observed [16,17,27,28]. Studies have shown that the layer formation during (steam) gasification is comparable to layer formation during combustion [29,30]. Only fuels rich in sulfur lead to different layers and agglomerates.…”

“…Only fuels rich in sulfur lead to different layers and agglomerates. During combustion, sulfur is retained in the ash to form various sulfates, while it is released in the gas phase during gasification [29]. More potassium and sodium are then available to react with the bed material to form, e.g., low-melting silicates in the case of a quartz bed.…”

Layer formation mechanism of K-feldspar in bubbling fluidized bed combustion of phosphorus-lean and phosphorus-rich residual biomass

“…Mechanism (c) describes the direct adhesion of partly molten ash-derived potassium silicate particles on bed particles. These mechanisms have been found to be suitable to describe the occurring phenomena [10,13,15] and were further developed for various biomass fuels [7,16].…”

“…Due to the observed problems of agglomeration and deposition formation with quartz beds [7,16,17], several alternative bed materials, like olivine [6], K-feldspar [6], magnesite [17][18][19], calcite [18,19], dolomite [17], sillimanite [18], bauxite [18], and plagioclase [6], were tested for their suitability in fluidized bed applications. Further studies on K-feldspar focused on the layer formation in combustion [14] and gasification atmosphere [12], or a direct comparison of both [20].…”

Layer formation on K-feldspar in fluidized bed combustion and gasification of bark and chicken manure

“…First insights into ash layer formation in both combustion and gasification [15,16] have been reported, indicating that the operational mode has no influence on the layer formation, apart from S-rich feedstocks, where the formation of a melt only occurring during combustion enhances the agglomeration tendency of S-rich feedstocks [15]. The influence of P on the layer formation has been studied in combustion [2] and gasification [17] atmosphere, but to the authors' knowledge no study focused on a comparison of P-rich feedstocks in both combustion and gasification atmosphere.…”