Carbon attrition during the circulating fluidized bed combustion of a packaging-derived fuel

Cylindrical pellets of a market-available packaging-derived fuel, obtained from a mono-material collection of polyethylene terephthalate (PET) bottles , were batchwise fed to a laboratory scale circulating fluidized bed (CFB) combustor. The apparatus, whose riser was 41 mm ID and 4 m high, was operat ed under both inert and oxidizing conditions to establish the relative impo rtance of purely mechanical attrition and combustion-assisted attrition in generating carbon fines. Silica sand particles of two size distributions we re used as inert materials. For each run, carbon load and carbon particle s ize distribution in the riser and rates of attrited carbon fines escaping t he combustor were determined as a function of time. A parallel investigatio n was carried out with a bubbling fluidized bed (BFB) combustor to point ou t peculiarities of attrition in CFB combustors. after devolatilization, PET pellets generated fragile aggregates of char and sand, which easily crumbl ed, leading to single particles, partially covered by a carbon-rich layer. The injected fixed carbon was therefore present in the bed in three phases: an A-phase, made of aggregates of sand and char, an S-phase, made of indiv idual carbon-covered sand particles and an F-phase, made of carbon fines, a braded by the surfaces of the A- and S-phases. The effects of the size of i nert material on the different forms under which fixed carbon was present i n the bed and on the rate of escape of attrited carbon fines from the combu stor were investigated. Features of carbon attrition in CFB and BFB combust ors are discussed. (C) 1999 by The Combustion Institute.