Agglomeration of Particles and Defluidization Phenomena in the Fluid Bed

Abstrakt

Fluidized-bed reactors can be employed in various thermal processes like combustion, gasification or pyrolysis aimed at gaining energy or material recovery. Stable long-term operations can be plagued with particle agglomeration which can dramatically degrade the quality of fluidization. The stickiness of particles mainly depends on chemical interactions between the bed material and the fuel. Although the agglomeration and defluidization of beds of sticky particles are well-reproducible phenomena, their mechanisms are not fully understood yet. It appears that processes such as plastic deformation, viscous flow, molecular diffusion, etc., develop adhesive forces between particles bringing about their agglomeration that can eventually lead to defluidization. The agglomeration and defluidization processes are very sensitive to temperature. At usual combustion temperatures, the mineral residue of a fuel (e.g., alkalis-containing ash) reacts with inert bed particles (e.g., silica sand) forming mixed oxides and corresponding low-melting-point eutectics on the surface of particles. The factors that enhance the formation of agglomerates include local reducing conditions and high temperature on the surface of the burning fuel particles. When planning a high-temperature operation with materials prone to agglomeration, the phenomena of cohesion and defluidization should carefully be observed in small-scale tests. The tendency of a bed to agglomerate can be reduced or minimized by additives such as dolomite, limestone, kaolin and other clays or using low-silica bed materials.