This study deals with fuel injection in kraft recovery boilers used in pulp mills. Black liquor is sprayed into the furnace of a recovery boiler through large splashplate nozzles. The drop size and drop size distribution of black liquor sprays have a major impact on the operation of a recovery boiler. When they are entrained by upwards-flowing flue gas flow, small droplets form carry-over and cause the fouling of heat transfer surfaces. Large droplets hit the char bed and the walls of the furnace without being adequately dried.
In this study, the drop size of black liquor with a high solids content was measured for two types of industrial-scale splashplate nozzles. The mass flow rate was varied between 4.3 and 6.1 kg/s and the spraying temperature was varied between 130-135°C. The excess temperature above the elevated boiling point (EBP) varied between 14-18°C.
The drop size and drop size distribution of black liquor sprays were measured by an image-analysis-based method. Although the spray consisted of spherical and non-spherical particles, it was assumed that non-spherical particles would form spherical droplets. Four theoretical drop size distributions were compared by the method of least-square difference.
The square-root normal distribution was slightly better compared to the log-normal, Rosin-Rammler, and normal distributions. The median diameter of equivalent spheres varied from 5 to 11.6 mm, when the square-root normal distribution was adapted. The span of drop size distribution was roughly 1.2. The volume median diameters were remarkably higher than those detected earlier for black liquor with lower solids content.