Mechanistic Investigation into Bed Agglomeration during Biomass Fast Pyrolysis in a Fluidized-Bed Reactor

This paper demonstrates that during the pyrolysis of mallee leaf (355–500 μm) in a fluidized-bed reactor (bed materials: silica sand, 125–355 μm) at 300–700 °C, bed agglomeration takes place due to the formation of char–char and/or char–sand agglomerates connected by carbon-enriched necks. There are two types of bed agglomeration: one formed due to solvent-soluble organic matter which dissembles upon solvent washing and the other due to solvent-insoluble organic matter produced from biomass pyrolysis. The yield of each type of bed agglomeration is broadly proportional to the yield of the corresponding type of organic matter in the bed samples. The total yield of bed agglomeration decreases with increasing pyrolysis temperature, from 16.5% at 300 °C to 9.5% at 500 °C and 1.8% at 700 °C. The distribution of the two types of bed agglomeration is also strongly temperature dependent. At low temperatures (e.g., 300 °C), bed agglomeration is dominantly contributed by those formed by solvent-insoluble organic matter. As pyrolysis temperature increases, bed agglomeration due to solvent-soluble organic matter becomes increasingly important and reaches a maximum at 500 °C. At pyrolysis temperatures above 600 °C, there is a drastic reduction in the bed agglomeration formed by solvent-soluble organic matter due to thermal cracking so that bed agglomeration is again dominantly formed by solvent-insoluble organic matter. Overall, bed agglomeration during biomass pyrolysis in a fluidized-bed reactor is due to the production of sticky agents, including both partially molten pyrolyzing biomass particles and the organic matter (both solvent- soluble and -insoluble) produced from biomass pyrolysis reactions.