Combustion characteristics of wet sludge in a fluidized bed: Release and combustion of the volatiles
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Combustion characteristics of wet (i.e. mechanically dewatered) sewage sludge were investigated in both laboratory and semi-pilot-scale fluidized bed combustors. The aims were to determine the influence of the high water content of sludge on its combustion process, the distribution of sludge carbon between the volatiles and char, the location of the release and combustion of the volatiles and the concentrations of char carbon in the bed. The results showed that there is a significant overlap of the drying, devolatilization and char combustion processes, so that the three processes may be considered to run in parallel. Both drying and devolatilization take place at very low particle temperature. Up to 80% of the sludge carbon is volatile carbon and sludge combustion is characterized by very low char carbon concentrations in the bed. Therefore sludge combustion is dominated by the gas-phase combustion of the volatiles. The release of the volatiles takes place mainly in the bed. Freeboard combustion is dominated by CO oxidation and takes place mainly within 2.5 m above the bed.