Initial Stages and Overlapping Processes of a Black Liquor Droplet in a Kraft Recovery Boiler, 2011 PEERS Conference

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Black liquor is sprayed into the recovery boiler at a temperature above boiling point. Furnace operations are greatly affected by spraying and droplet formation stages before and during drying and devolatilization. Essential dynamics are already defined inside the nozzle, inside which small bubbles have formed. These bubbles grow and increase black liquor velocity in the nozzle pipe. Black liquor hits the splash plate resulting in ligaments and droplets. The radiation and convective heat transfer affect the surface of black liquor particles, so overlapping drying, devolatilization and even combustion start almost immediately in the furnace. In this study spray formation was examined at different black liquor mass flow rates and at different temperatures. The effect of flash boiling on spray formation was studied experimentally, based on images taken at varying distances from the nozzle. The dimensions of the spray were found to be affected by flashing. Moreover, the velocity of different droplet sizes was connected to spraying practice and the droplet formation process. Theoretical estimations of flashing in the nozzle and the consequent overlapping of flashing and spray formation were confirmed by spraying tests. The results were validated by measurements inside an operating recovery boiler furnace.