Combustion and Attrition of Biomass Chars in a Fluidized Bed

Abstract

The fluidized bed combustion of char from three different biomass fuels, pine seed shells, olive husk, and wood chips, was investigated in a bench scale combustor. A combination of experimental techniques was used to characterize the relevance of attrition phenomena during the combustion of the chars and their impact on the fuel particle size distribution and overall carbon conversion. Results showed that, depending upon the biomass, extensive primary and secondary fragmentation could be experienced by the char particles, significantly influencing the particle size distribution of the fuel in the bed. This is the result of the mechanical properties of the raw fuel particles and the large porosity of the char after devolatilization. Char conversion closely followed the shrinking-particle constant-density model and occurred to a large extent via the generation of carbon fines by percolative fragmentation followed by postcombustion during their residence time in the bed. Approximately 25-45% of the initial fixed carbon followed this pathway, with the remainder being directly burnt as coarse char. This resulted in an enhancement of the combustion rate of the char particles. The significance of the particle shape was also highlighted in the evaluation of the particle-exposed surface. Finally, indications were given on how to account for attrition phenomena in a simplified population balance on biomass char.

Modeling of Biomass Char Gasification, Combustion, and Attrition Kinetics in Fluidized Beds

“…This two-class description was originally proposed by models for fluidized bed combustion of coal 16,30 and can be assumed when the number of intermediate-sized particles is small or zero 20,22,31 . Secondary fragmentation of biomass chars derived from spruce pellets and wood chips under combustion conditions generates relatively few (~2.5-3) fragments per original char particle, and appears to be largely independent of the oxygen concentration 32,33 . On the other hand, fluidized bed gasification of lignite char appears to generate a large number of fragments (>70) and depends on the operating conditions 15 .…”

“…Scala et al 33 give a simplified model to predict an initial average char diameter ,0 from an initial raw particle diameter ,0 in m,…”

“…The primary fragmentation factor, 1 refers to the total number of fragments generated by a single biomass particle. For a given feedstock, it has shown dependence on the initial particle size, with larger particles generating more fragments than smaller ones 33 . Because the penetration of reactants into the particle during the devolatilization process is limited, these factors are not strongly dependent on the gas phase environment of the bed 36 .…”

Mass Transfer around Active Particles in Fluidized Beds

“…Sh can be overestimated up to a factor of two (for porous and fragile chars, like those resulting from biomass devolatilization) if these phenomena are not taken into account. In a later paper, Scala et al (2006) highlighted the significance of the shape of non-spherical particles on the correct evaluation of the exposed particle surface and of the mass transfer coefficient. Paterson (2000) and Hayhurst (2000) noted that the implicit assumption of equimolar counter-diffusion of gaseous reactants and products around an active particle (which is typically made for the calculation of the mass transfer coefficient from experimental data) might not always be valid.…”

Effects of Mixing Using Side Port Air Injection on a Biomass Fluidized Bed

“…The large differences in size and density between the biomass and inert particles lead to non-uniform distribution of the biomass within the fluidized bed, and particle interactions and mixing become major issues. Therefore, the fluidization characteristics of biomass particles are of critical importance because of known problems such as particle agglomeration, defluidization, elutriation, and segregation [3][4][5][6][7][8].…”