THE ASH FORMATION DURING COCOMBUSTION OF WOOD AND SLUDGE IN INDUSTRIAL FLUIDIZED-BED BOILERS

Ashes from an industrial bubbling fluidized bed (BFB) and a circulatin g fluidized bed (CFB) boilers burning biomass and paper mill sludges a re characterized. Bulk sampling was carried out with a simultaneous lo w pressure impactor sampling (BLPI) and with on-line monitoring of sub micron ash gas phase size distribution. The principal ash forming mech anism was sintering of 1-10 mu m inorganic paper filler particles, int o 5-200 mu m ash agglomerates. Despite few spherical particles no ash melting was detected. The fraction of vaporized and condensed ash was below 0.1 mass% in both units as determined with BLPI and with on-line monitoring. According to elemental mass size distributions no signifi cant vaporization of any main element occurred including sodium and po tassium. Wall deposit observed on refractory lining in BFB furnace was composed of sintered ash binding large ash and sand particles in the structure. No molten phases were detected to participate in the layer growth. Most quartz sand grains in the bed were covered by a thin laye r of small ash particles, below 5 mu m in size. (C) 1998 Elsevier Scie nce B.V.