| Rapid cooling of char bed in recovery boilers following an emergency shutdown procedure (ESP) is of great importance for increasing the productivity of kraft pulp mills. Although sodium bicarbonate (NaHCO 3) has been widely used as a coolant to accelerate the char bed cooling, the fundamental principles underlying this technique are not well understood. This thesis project is the first to systematically study the mechanisms of char bed cooling with NaHCO3. Experimental studies and theoretical analyses were performed to examine the interaction between molten smelt and NaHCO3, the heat transfer processes involved, and the effect of NaHCO3 on the accelerated cooling of kraft smelt.; The results show that the injection of NaHCO3 into a molten smelt pool under conditions similar to those practiced commercially did not achieve the degree of accelerated cooling in the smelt interior, expected for the amount of NaHCO3 used. On the other hand, the slow addition of NaHCO3 into molten smelt was more effective. Near the smelt pool surface, the time required to cool the smelt from its initial temperature to the freezing temperature was shortened by 40%. At the location away from the surface, however, the smelt cooling rate was hindered due to the rapid formation of a frozen crust which has a lower thermal conductivity than molten smelt.; The theoretical analysis of heat transfer from molten smelt to NaHCO 3 particles suggests that lumps of NaHCO3 powder agglomerated during the addition are not likely to decompose in the smelt instantaneously. For a 20mm lump, it requires as long as 10 minutes for the lump to decompose completely. This is consistent with the observation that gases were continuously generated during the tests 8 minutes after the NaHCO3 addition. Furthermore, molten smelt was observed to ooze out of the crust after NaHCO 3 was applied, due probably to the accumulation of gases and pressure buildup under the crust. The slow generation and accumulation of gas bubbles increased the porosity of the newly formed crust, and lowered the thermal conductivity of the frozen smelt. As a result of the formation of a thick crust and subsequently lower heat transfer rate from the smelt, the effectiveness of NaHCO3 in accelerating smelt cooling was significantly reduced.; The knowledge obtained from this study can be used to develop a guideline for better implementation of the char bed cooling acceleration practices. |
