Experimental Investigation of Sewage Sludge Combustion in a Counter-Current Fixed Bed

This article presents the results of an experimental study into the energy recovery from sewage sludge by combustion. Combustion was carried out in a vertical counter-current fixed-bed boiler, using three separate normal air flows (1.5, 3 and 5 Nm3/h). Flame front propagation was monitored using thermocouples, connected to a data acquisition system, recording temperatures at four different positions along the bed. The experimental data reveal two clearly distinct phases of sewage sludge combustion: an ignition phase and a flame propagation phase within the lower layers of the bed. The latter phase is characterized by a significant increase in temperature profiles, attributable to oxidation in the lower parts of the bed, as well as to the possibility that coal has a higher calorific value than sludge. The study also examines the influence of variable air flow rates on temperature profiles, bed center temperature averages, ignition front velocity, ignition mass rate and conversion layer thickness.