To understand recovery boiler ash deposit formation, it is necessary to hav
e detailed information on the ash particle properties in the flue gas. Sinc
e coarse particle deposition is orders of magnitude more efficient than fin
e particle deposition, the coarse particles were studied in detail. In an i
ndustrial recovery boiler, ash particles were extracted directly from the h
ue gases with a settling probe. a double cyclone-filter system and a thermo
phoretic sampler in the superheater area. The particles were analysed for t
heir bulk chemical composition with ion chromatography. The individual part
icles were studied for their morphology and elemental composition with a hi
gh-resolution scanning electron microscope connected to an energy dispersiv
e X-ray analyser. Measured coarse particle mass fraction of the total parti
cle concentration was about 408, which is significantly larger than propose
d earlier. Five coarse particle types were observed at the superheater area
: (i) partially sintered large agglomerates formed from fine fume particles
that had entrained from the heat exchanger surfaces, (ii) extensively sint
ered irregular particles that had entrained from the surfaces by soot blowi
ng, (iii) spherical particles that appeared highly porous inside, (iv) dens
e spherical particles, and (v) intermediate irregular particles of non-proc
ess mineral matter. At the boiler exit, the coarse particles were mainly pa
rtially sintered large agglomerates. (C) 2001 Elsevier Science Ltd. All rig
hts reserved.