High-Temperature Interactions between Molten Miscanthus Ashes and Bed Materials in a Fluidized-Bed Gasifier

Abstract

One of the main concerns about biomass fluidized bed gasification and combustion is the risk of bed particle agglomeration due to ash melting. Although many studies have been conducted about the agglomeration mechanism using silica sand, olivine is mostly mentioned as an alternative bed material for tar decomposition, and its interaction with biomass ash has not been yet fully understood. The aim of this work is to investigate the agglomeration of miscanthus ashes, focusing on thermophysical and thermochemical aspects. Three different bed materials (silica sand (SiO 2 ), raw and calcined olivine ((Mg,Fe) 2 SiO 4 )) and an additive to prevent agglomeration (dolomite (CaMg(CO 3 ) 2 ) were tested. The effects of atmosphere and miscanthus harvest time were also investigated. It was found that the key parameter of agglomeration is the wettability of bed particles by molten ashes. In contact with ashes all three bed material showed good wetting tendencies, while dolomite had non-wetting properties. The adhesion between bed materials and molten ashes increases in the order of silica, olivine and calcined olivine. While in the case of silica sand only physical adhesion occurred, the diffusion of iron oxide into the molten ash was observed using olivine. Calcined olivine has a roughened surface which further increased the adhesion. The atmosphere did not influence the mechanism of ash/bed material interaction. On the other hand, miscanthus harvest time had a significant effect on ash reactivity and interaction with raw and calcined olivine.

Interactions of Olivine and Silica Sand with Potassium- or Silicon-Rich Agricultural Residues under Combustion, Steam Gasification, and CO2 Gasification

“…This observation is consistent with some previous chemical equilibrium calculations that suggested a low chemical driving force for K to be able to react with olivine . The reduced agglomeration potential with olivine sand instead of silica sand has also been reported in the literature for other materials. ,,− The distinct chemical reaction tendency of alkalis with olivine compared to that of silica has been previously correlated with the different degrees of polymerization of the silicate structure in the bed materials . For instance, some biomass elements have been found previously to integrate into silica by depolymerizing the tectosilicate structure via an inosilicate intermediate and eventually form a nesosilicate .…”

Ash–Bed Material Interaction during the Combustion and Steam Gasification of Australian Agricultural Residues

“…Most studies of agglomeration have been carried out in fluidized bed reactors, while other studies also used fixed-bed reactors to simulate the ash–bed material interaction in fluidized bed reactors. − Both the ash chemistry and the particle physics influence agglomeration in the fluidized bed reactor . The ash chemistry influences the formation of chemical bonding between particles, while the effect of particle physics may include particle size, particle velocity, and collision frequency .…”

Deposit build-up and ash behavior in dual fluid bed steam gasification of logging residues in an industrial power plant

“…Previous studies showed an interaction between bed particles and ash during combustion of biomass feedstock. As a consequence of this interference calcium-rich layers are built around bed particles [8][9][10]. Investigations of time dependence regarding layer formation on quartz sand particles from a 30 MW th bubbling fluidized bed biomass combustion plant showed several steps of layer growth.…”