Combustibility of dried sewage sludge and its mineral transformation at different oxygen content in drop tube furnace

Combustion of two kinds of sludge was studied in a laboratory-scaled drop tube furnace to understand their combustibility and evolution of mineral matters in the process. Experimental conditions were selected as: reaction temperature was 1200 °C; oxygen content was 10%, 30% and 50%, and the residence time varied from 0.6 to 2.4 s. The experimental results show that increasing the oxygen content significantly improved the combustibility of sludge. Sludge with high volatile matter, e.g., 70.8% in sludge B, could not combust completely even at 50% O 2, which is due to the fact that the formed ash surrounded the combusting char, it acted as a resistance for the diffusion of oxygen to unburnt carbon, about 2–6% unburnt carbon, having particle size of about 1.5 μm, was kept in the ash. In addition, at low oxygen content, the major mineral compounds in sludge, including calcium oxide, phosphorus oxide and aluminosilicate, mainly underwent coalescence within char to form the large molten agglomerates. Increasing oxygen content resulted in the rapid decomposition of sludge, and accordingly, many droplets were formed and condensed into fine particles. Finally, the vaporization model for both sodium and zinc was built considering the internal diffusion of both two elements with char and their reaction with phosphates. The simulation results had a fairly good consistence with experiments. Sodium was mainly released under the controlling of its internal mass-transfer diffusion within ash layer. Conversely, zinc was released under the controlling of both internal mass-transfer diffusion and its reaction with molten phosphate salts. Improving oxygen content mitigated the diffusion resistance in ash layer, and hence, more the sodium and zinc were vaporized.