Effect of Swelling on Droplet Trajectories, Carbon Burn-Out, and Entrainment in Black Liquor Combustion, 1991 Forest Products Symposium Proceedings

The swelling characteristics of black liquor are known to be liquor-specific and to have a strong influence on the rate of combustion of black liquor droplets. A key question is how important the swelling characteristics are on entrainment of the burning droplets along with the flue gases to the upper furnace and carbon burn-out within the lower furnace, factors which can have an impact on superheater and generator bank deposits, plugging of gas passages, and recovery boiler capacity.

The effect of differences in swelling behavior on carbon burn-out and entrainment was investigated using a numerical model for the combustion of single black liquor droplets. The model is based on heat and mass transfer principles and experimentally developed correlations for key black liquor combustion parameters. Trajectories and carbon bum-out times were calculated for droplets in simple gas flow fields.

The results show that the effect of swelling during devolatilization on entrainment and droplet burning rate are offsetting with respect to the path of the particle during char combustion - more highly swelling particles burn faster but also are transported more rapidly with the gases. This indicates that swelling is a less important parameter with respect to carbon burn-out and the carry-over of either burning char particles or smelt beads than might be expected. By comparison, the dry solids content of black liquor has a much greater impact on droplet trajectories and the location of droplet bum-out. Differences in swelling during drying would also affect trajectories and location of burn-out, but swelling during drying does not vary from liquor to liquor or with firing conditions.