Controlling the Droplet Size and Initial Velocity of Flashing Black Liquor Sprays - Initial Results from a Modelling Approach

摘要

The current trend in modern Kraft Recovery boilers is that the dry solids content of the black liquor fired is very high, typically 75-85 %. In order to pump and spray black liquor with this high dryness and high resulting viscosity, its temperature needs to be high, typically 135-145 °C, which means that the spraying takes place significantly above the atmospheric boiling point. This means that flashing takes place in the spraying, already inside the nozzle tube and results in a high initial velocity and due to this, a smaller droplet size. Two phase flows are often quite unstable and therefore, the control of the spraying situation can be very challenging. The objective of this paper is to study how the initial velocity and the droplet size in the flashing case could be effectively controlled by adjusting the spraying parameters. As a tool to describe the multiphase flow inside the nozzle tube, we used our detailed two phase nozzle flow model developed and widely published earlier. These flow simulation results were combined with droplet sizes estimated using a correlation based on a broad data set of our different small and full scale spraying experiments. The paper will give initial results easily applicable to practical boiler cases on how to adjust spraying parameters such as temperature, pressure and nozzle geometry to maintain stable combustion and a constant droplet size if for example, the dry solids content suddenly changes.