The relevance of attrition to the fate of ashes during fluidized bed combustion of a biomass

The fate of ashes during the fluidized-bed combustion of a biomass fuel (Robinia pseudoacacia) has been investigated both experimentally and theoretically. Fluidized-bed combustion experiments with Robinia have been carried out with a bed of pure quartz at temperatures ranging from 700 C to 850 C at different oxygen concentrations in order to investigate the tendency of the biomass ashes to deposit on the bed particles and to give rise to bed agglomeration. SEM/EDX analysis of the silica bed particles after the tests was carried out to determine the formation of alkali-rich layers on inert bed particles and possibly of silicate melts. Results indicated that large quantities of biomass ashes are retained on the bed particles under all operating conditions, but only at the higher temperatures could molten surface layers and agglomerated bed particles be noticed. Experimental results have been interpreted on the basis of a single-particle combustion model applied to both fine and coarse char particles' burnout in a fluidized bed. Calculations show that extremely high temperatures are rapidly reached by fine particles even at very low oxygen concentrations. These temperatures are well beyond typical potassium silicate melting temperatures. On the other hand, coarse particles burn at temperatures only slightly above bed temperature. Experimental and theoretical results indicate that the following mechanism is relevant to the fate of ashes during fluidized-bed combustion of biomass: ash is mostly detached from the coarse char as attrited fines whose temperature is raised significantly by carbon afterburning. Fines can further adhere onto inert bed particles, with formation of alkali-rich surface layers. If the bed temperature is higher than the alkalisilicate eutectic, a melt forms that enhances bed particle stickiness and may ultimately lead to bed agglomeration.