Impact of the Reactor Structure on Biomass Pyrolysis in Fluidized-Bed Reactors: A Coarse-Grained CFD-DEM Study

A coarse-grained computational fluid dynamics coupled with the discrete element method (CFD-DEM) model, combined with a smoothed voidage algorithm and a multicomponent and multistep pyrolysis scheme, has been developed, validated, and used to study biomass pyrolysis in different-shaped fluidized-bed reactors. Correspondingly, fluidized-bed reactors with three different section shapes (i.e., circular, square, and rectangular sections) and gas inlet areas (i.e., 25, 50, and 100% of the bottom section) were devised to explore the effect of the reactor structure on pyrolysis behavior. The simulation results have been extensively analyzed and discussed by various indicators, such as the gas-solid flow patterns, bubble behaviors, pyrolysis product species, biomass reaction rates, and reactor wall erosion depths. It is found that the circular reactor has the largest mean bed height but the lowest fluctuation frequency. Moreover, the circular reactor generally has the largest bubble equivalent diameter at the same height, followed by the square reactor, and the rectangular reactor provides the smallest bubbles. Regarding the effect of the gas inlet area, the fluctuation frequency monotonically increases with reducing the gas inlet area. The largest gas inlet gives the lowest biomass mass loss rate and the smallest bubble equivalent diameter, whereas the other two smaller gas inlets provide quite similar results. Moreover, decreasing the gas inlet area also generates large bubbles more frequently. Finally, the circular reactor suffers the severest wear, followed by the square and the rectangular reactors. These findings are helpful in optimizing the design and operation of biomass fluidized-bed pyrolysis processes.