Attrition phenomena during fluidized bed combustion of granulated and mechanically dewatered sewage sludges

Abstract

Fluidized bed combustion of two types of sewage sludge was investigated in a bench scale reactor. The sludges differed as regards their preparation: one sludge was mechanically dewatered, the other was dried and granulated. The single particle conversion pattern and rate, and the nature and extent of attrition phenomena occurring during burn-off were quantitatively assessed. Primary fragmentation and conversion patterns of the residual char were characterized by periodic retrieval of fuel particles from the bed using a basket. Attrition by abrasion was assessed by collection of elutriated fines at the exhaust during batch fluidized bed combustion of chars of the two sludges. The size distribution of primary ash particles (PAPSD), relevant to bed inventory, particle size distribution, and balance between fly- and bottom-ash during steady fluidized bed combustion, was characterized using a purposely designed experimental procedure. Primary fragmentation was negligible for the granulated sludge, moderate for the mechanically dewatered one. Both sludges are characterized by the formation of coherent ash skeletons, responsible for a shrinking unreacted core conversion pattern of the char particles. Carbon attrition was very limited, as the coherent ash skeleton acted as a mechanical shield with respect to the unreacted carbon-rich core. The PAPSD closely followed the size distribution of the parent fuel particles for the granulated sludge, while it was slightly shifted to smaller sizes for the mechanically dewatered one. A simple combustion model, based on the shrinking core conversion model, has been developed to analyse the combustion behaviour of sludge char particles. The model is based on the consideration of resistances to mass transfer both in the boundary layer around the particle and in the carbon-depleted ash shell. The model satisfactorily reproduces the main features of the fluidized bed combustion of both sludges under the experimental conditions tested.