Combustion of Dried Sewage Sludge in a Fluidized-Bed Reactor

Fluidized-bed combustors are capable of destroying effectively a variety of organic wastes such as sewage sludge. Because of its very high content of water, mechanically dewatered sewage sludge cannot be incinerated on its own. Peculiar characteristics of dried sewage sludge include its very high proportion of volatile matter and its high contents of the fuelbound nitrogen and ash. The experimental apparatus consisted of three fundamental parts: an electrically heated reactor, 0.98-m height and 0.0936-m diameter; a facility for the continuous withdrawal and analysis of gas samples, and a feeder of solids. Predried sewage sludge particles were injected near the base of a shallow, air-fluidized, bubbling bed of ceramsite under different operating conditions. The study comprised an experimental program of steady-state tests for providing a full picture concerning the influence of the main operating variables (i.e., bed temperature, freeboard temperature, and excess air) on the most significant gaseous emissions (i.e., CO, NOx, and N2O). Further, this work also explores the significant influence of the freeboard (i.e., the region above a dense, bubbling bed) on the overall combustor performance. Of concern are also elements such as As, Cd, Hg, Cr, Ni, P, Pb, and Zn and their partitioning between the bottom (bed) ash and the fly (cyclone) ash.