Understanding recovery boiler smelt runoff phenomena

There is considerable interest in the nature and causes of heavy smelt runoff from recovery boilers because of the role it has played in numerous dissolving tank explosions and for personnel safety around the dissolving tank. Most mills have experienced runoff problems, which can be caused by cleaning plugged spouts, burning down a large bed, low sulfidity smelt, startup with a bed in the unit, and improper firing practices. The peak smelt flow during a runoff is often 3 to 5 times normal and may be much greater in severe cases. Heavy runoffs are self-limiting and typically last less than 30 min. The geometry of the lower furnace plays a significant role in runoff events. Sloped floor units are more vulnerable to smelt pool buildup and heavy runoff when released. Decanting bottom units are inherently more tolerant of smelt pool buildup. Low sulfidity results in smelt with a high melting temperature, making the smelt easy to freeze and difficult to flow. Sulfate-rich slag/deposits falling on the hearth from the upper furnace can contribute significantly to runoff problems by causing dams and spout plugging, increasing the load of smelt pool to be removed, decreasing bed temperatures, and lowering the sulfidity of the smelt leaving the furnace.