Coarse ash particle characteristics in a pulp and paper industry chemical recovery boiler

To understand recovery boiler ash deposit formation, it is necessary to have detailed information on the ash particle properties in the flue gas. Since coarse particle deposition is orders of magnitude more efficient than fine particle deposition, the coarse particles were studied in detail. In an industrial recovery boiler, ash particles were extracted directly from the flue gases with a settling probe, a double cyclone–filter system and a thermophoretic sampler in the superheater area. The particles were analysed for their bulk chemical composition with ion chromatography. The individual particles were studied for their morphology and elemental composition with a high-resolution scanning electron microscope connected to an energy dispersive X-ray analyser. Measured coarse particle mass fraction of the total particle concentration was about 40%, which is significantly larger than proposed 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 sintered irregular particles that had entrained from the surfaces by soot blowing, (iii) spherical particles that appeared highly porous inside, (iv) dense spherical particles, and (v) intermediate irregular particles of non-process mineral matter. At the boiler exit, the coarse particles were mainly partially sintered large agglomerates.