Particle Development in a Fluidized Bed Black Liquor Steam Reformer

Low-temperature gasification of black liquor is a new alternative to the conventional Tomlinson recovery boiler for recovering pulping chemicals and energy from black liquor in pulp and paper mills. This process can be carried out in a fluidized bed based on steam reforming at 600 C using indirect heat supplied via heat exchangers built into the fluid bed. As black liquor is injected into the bed, bed particles evolve through different mechanisms, exhibiting variations in size and characteristics. Coating, agglomeration, attrition and fragmentation are mechanisms that have been identified as contributing to bed particle evolution. Four experimental approaches were used to develop fundamental understanding of these mechanisms, and to qualitatively address particle development in this unit. Visual analysis of cross-sectional areas of bed solids from two commercial steam reforming units showed that bed particles grow by deposition of one to two superimposed layers of residue rich in Na and K, which are more distinctive on bed particles larger than 250 μm. Lab-scale fluidized bed reactor experiments showed important effects of adding large liquor droplets on bed particle characteristics, particularly on particle size distribution and shape, which promotes the formation of droplet-induced agglomerates. Cold fluidization of bed solids in a Plexiglas column indicated that considerable mechanical attrition initially occurs with bed particles larger than 300 μm, followed by a rapid attrition decay as particles are rounded off. Compression, glass cyclone and tube furnace experiments showed that chemical consumption of the organic matter remaining in the char residue combined with vigorous solids fluidization presumably leads to significant fragmentation of agglomerates, which leads to the formation of indirectly coated particles. It was concluded from this study that particle development in a fluidized bed black liquor steam reformer is a complex combination of mechanisms and submechanisms that simultaneously participate in bed particle evolution. Coating and attrition seem to be the most significant mechanisms, followed by agglomeration which results from poorly controlled conditions. Once agglomeration has occurred, fragmentation then controls the excessive particle growth caused by agglomeration.