The Effect of Flashing and Spraying Conditions on Black Liquor Droplet Formation

Pulping processes produce excessive quantities of black liquor, more than 200 Mtn/year as dry solids. It is produced during the cooking process of wood in the pulp and paper industry and is burned in recovery boilers for the recovery of chemicals and to produce energy. All new boilers use high dry solids content (DSC) black liquor for better energy efficiency, decreased SO2 emissions, and higher capacities. The high DSC black liquor spraying demands temperatures above its atmospheric boiling point. Therefore, high DSC black liquor behaves very differently compared to low DSC black liquor; the spray behavior and droplet formation are different. Understanding of black liquor spray properties is still inadequate for both liquor types. This research experimentally examines the spray properties of low and high DSC black liquors. The experiments were implemented by spraying with industrial scale splashplate nozzles in test chambers and in operating furnaces. The droplet size, size distribution, and spray velocity were measured for two of the most used nozzle types. The temperature and mass flow rate were varied in the same ranges used by pulp mills. In addition, spray velocities and black liquor sheet break-up mechanisms were studied in operating furnaces using a furnace endoscope.Mean droplet size was found to decrease when the temperature was raised at temperatures more than 10 deg C above the boiling point at a constant mass flow rate. Black liquor sheet break-up mechanisms and flashing were found to affect droplet size. In test chambers, the median droplet size was found to be between 2 mm and 5 mm for low DSC (70%) black liquor, and between 4.5 mm and 15 mm for high DSC (75–78%) black liquor. The shapes of the droplets were spherical in the case of the low DSC black liquor and mostly non-spherical in the case of the high DSC liquor. The best fit for experimental data was achieved by square-root normal and log-normal droplet size distribution functions. In furnace conditions, the median droplet size was found to be between 8 mm and 11 mm at the spray center line, but variation was larger in spray edges. The spray velocity was measured in horizontal and vertical spraying chambers, in six furnaces with two nozzle types, and through a liquor gun port when spraying along the wall. The velocity of the spray was increased due to flashing. The measured velocities were then converted to dimensionless velocities, whereas mass flow rates were converted to mass fluxes. These dimensionless velocities and mass fluxes formed a diagram called a flashing map, which is useful in velocity predictions for different nozzle sizes in varying spraying conditions. The measured values of droplet size, shape, size distribution, as well as spray velocity are important for furnace modeling. They allow for the design of effective low-emission boilers with high capacity. Boiler operators can utilize results for enhanced control of the char bed and fouling.