Experimental studies on pulp and paper mill sludge ash behavior in flulidized bed combustors: Dissertation

Ash formation during the fluidized bed combustion (FBC) of pulp and paper mill sludges has been experimentally studied on an industrial and bench scale. The methods included aerosol measurements, chemical and crystalline composition analyses, thermogravimetry and electron microscopy. Fly ash mass and number size distributions and elemental enrichment in submicron particles and bottom ash were measured. Fly ash, bottom ash and ash deposits were characterized and their formation mechanisms are discussed. Fly ash included over 90 % of the ash-forming species, the rest remaining in the bed. The ash-forming minerals were fine paper-making additives, clay minerals and calcite, located within the sludge fibers. During combustion the minerals sintered, forming porous agglomerates. The mineral structure was transformed into amorphous phases, calcium silicates and alkali silicates in the fly ash. Increased ash residence time in the furnace enhanced the silicate formation. The fly ash mass mean size was 7.5-15 µm and the ash did not fragment much into submicron particles. The supermicron particles included 93.6-97.3 % of the fly ash. Condensation of the volatilized inorganic species formed spherical submicron particles in the fly ash. Their mass concentration was almost negligible when co-firing paper mill sludges and wood. Correspondingly, no significant enrichment of the volatile inorganic species or trace elements in the ultrafine ash was detected during paper mill sludge and wood firing suggesting that the fraction of the volatilized inorganic species in the paper mill sludges was low. Results from pulp mill sludge and bark co-firing were different. This fuel mixture produced a clear mass mode below 0.3 µm, presenting 2.2-5.0 weight-% of the fly ash. The condensed species included K, Na, S and Cl. Their mass fraction was higher in the pulp mill sludge than in the paper mill sludge and evidently this resulted in increased volatilization and formation of condensed particles. The following trace elements were enriched in the submicron ash during pulp mill sludge and wood co-firing: As, Cd, Rb and Pb. The main part of the volatile species was not captured in the condensed ultrafine particles, but in the bulk ash. Presumably, this was due to the high surface area concentration in the bulk ash, which enhanced the gaseous species condensation on the bulk ash and chemical reactions with solid particles. Sludge moisture reduced the inorganic species volatilization. Probably this was due to steam vaporization from the wet particles through the burning layer resulting in decreased combustion temperatures on char surface and a lower fraction of formed char. During sludge combustion the largest ash particles, over 200-300 µm, were accumulated in the bed due to their size. Their mass fraction in the bed was in the order of a few percent in the industrial units and they did not cause bed agglomeration. Lower fluidization velocities were used on a bench scale, where the large particle fraction grew in an unlimited manner. Sintering of these particles resulted in gradual bed defluidization. Ash was also accumulated in the bed as micron-size particles adhered to the sand surface forming a sintered layer. Bench-scale tests showed that this type of layer, formed during bark combustion, initiated bed agglomeration and resulted in defluidization without combustion at 988 °C. With pulp and paper mill sludges the layer formation was not connected to bed agglomeration at the studied temperatures up to 1,000 °C. Presumably, the bark-derived K species in the layer decreased the ash viscosity and triggered bed agglomeration. Trace elements were not accumulated excessively in the bed. Ash deposits were formed on the furnace wall above the bed. Ash and sand particles were deposited on the surface roughness probably from the down-flowing particle flux near the wall. Ash sintering densified the structure. Condensed or molten ash species were not detected in the structure. The ash viscosity was lower in the pulp mill sludge resulting in larger deposits than during paper mill sludge firing. Calculations indicate that when the ash viscosity is less than 1,000-3,000 Pa s, large ash and sand particles are incorporated in the deposit.