Experimental Study of the Mechanisms of Fine Particle Deposition in Kraft Recovery Boilers, 1998 International Chemical Recovery Conference Proceedings

The major implications from experiments on the deposition of fume (submicron) and intermediate-sized (1-100 micron) particles produced during combustion of black liquor in a pilot-scale combustor are reviewed. Temperature-controlled, cylindrical probes in cross flow simulated heat transfer tubes in commericial boilers during these tests. Both sizes of particles form highly porous deposits. Fume deposits appear very smooth to the eye, but under closer examination are seen to comprise thin beads of particles emanating from the probe surface in an approximately radial direction. Intermediate particles generate much more irregular deposits and grow in columns much larger in diameter than the individual particles. The columns grow into the flow. Fume particle deposition rates are essentially independent of thermophoretic driving force and probe Reynolds number. Rates of deposition were very slow in all cases. Intermediate particles deposited by inertial impaction at rates far higher than fume particles. Deposit strength is believed to depend on deposit structure, with sintering rates between particles far exceeding sintering rates between columns or beads of particles.