SMELT DROPLET-WATER INTERACTION IN THE KRAFT RECOVERY BOILER DISSOLVING TANK

In the dissolving tank of a recovery boiler, violent interaction between smelt droplets and water can cause the tank to rumble and tremor, and in severe cases, a dissolving tank explosion accident. A laboratory apparatus was constructed to visualize smelt droplet-water interaction with high speed imaging, and to examine the effects of various dissolving tank operating parameters on the explosion characteristics of synthetic smelt droplets composed of sodium carbonate and sodium chloride. The results show that smelt droplets are more likely to explode when 1) the water temperature is low, 2) the smelt composition approaches the eutectic composition, 3) the droplet size is large, and 4) there is an external disturbance. Experiments were also conducted using real kraft smelt and green liquor. The results show that real smelt behaves in a similar manner as synthetic smelt, and that replacing water with green liquor promotes droplet explosions. A 1-D heat transfer model of smelt droplet-water interaction was also developed to explain the explosion mechanism. The modeling results suggest that whether a droplet explodes or not depends on the thickness of the solidified layer formed on the smelt droplet surface at the moment the vapor film around the droplet collapses. Experimental results from this study also show that explosions that occur on the water surface generate less sound and tank vibration than explosions that occur beneath the water surface, and that droplet explosions promote smelt dissolution. To improve dissolving tank safety and efficiency, it is important to avoid accumulation of a large amount of smelt in the dissolving tank.Ph.D