Temperature effects on riser pressure drop in a circulating fluidized bed

摘要

Effects of temperature on pressure drop across a riser due to solids holdup in the riser of a circulating fluidized bed (CFB) were investigated at the atmospheric pressure condition. The bed material was a group of sorbent particles and the experimental variables included temperature, gas velocity, and solids flux in the riser. With gas velocity and solids flux held constant, the riser pressure drop decreased as temperature increased. However, temperature effects decreased with increase in gas velocity. The effects of temperature on riser pressure drop were confirmed qualitatively by the same effects on the average ratio of gravity to drag force on a single spherical particle while the particle was accelerated. The pressure drop across the riser increased almost linearly with the ratio of solids flux to gas flux at the given gas velocity. Because gas momentum per unit mass of gas transferred to solids increased, the slope of the linear relationship decreased as temperature increased. This result confirmed the validity of the concept of momentum transfer from gas to particles at high temperature, proposed at ambient temperature in the prior study. The amount of gas momentum per unit mass of gas available to carry over the solid particles was finite; thus as the solids flux increased at the given gas velocity, the gas momentum shared to the unit mass of solids decreased and the mean residence time of solids in the riser, i.e., the pressure drop across the riser increased linearly. The slope of the linear relationship was proportional to the ratio of momentum flux by gravity and buoyancy forces on solids to gas momentum per unit mass of gas by drag force transferred to solids. Correlations were proposed to predict effects of temperature on the pressure drop across the riser and the solids flux in the riser within the range of experimental conditions.