Formation mechanisms of “ jellyroll” smelt in kraft recovery boilers

Molten smelt normally flows smoothly down the smelt spout of a recovery boiler like water, but at times it suddenly becomes sluggish and forms a viscous blob on the spout trough that partially or completely blocks the smelt flow. This form of smelt is commonly referred to as “jellyroll” smelt. How such smelt forms has been a puzzle to boiler operators and mill personnel for years. Numerous mill observations and the results of a recent study performed on both smoothly flowing smelt and jellyroll smelt collected from a recovery boiler suggest that that jellyroll smelt can form through three main mechanisms: i) the freezing of the molten smelt, ii) the melting of fallen deposits, and iii) the inclusion of a large amount of unburned char in the molten smelt. These mechanisms are consistent with mill experience that jellyroll smelt tends to form in older recovery boilers burning liquor with low solids and low sulfidity.