The Rise and Combustion of Particles of Sewage Sludge and Petroleum Coke in a Slugging Fluidized Bed

The rise-times, from just above the base to the top of the bed, of single particles of dried sewage sludge and of two types of petroleum coke were measured in a slugging bed of sand fluidized by air. The sewage sludge and the petroleum cokes contained, on an as received basis, 49.5, 9.0 and 0.5 wt.% volatile matter, respectively. The bed (i.d. 25.5 mm; height 1300 mm) was electrically heated to a fixed temperature between 700 and 900°C. Two sizes of sand were fluidized: +212, −300 μm and +710, −850 μm. The expansion of the bed was held constant by keeping the excess gas velocity at (U − U_mf) = 190 mm/s. In a bed of the coarser sand, it was found that the time of rise of single particles of sewage sludge (< 4 mm diam.) was large enough to allow devolatilization and char burn-out to be complete before the particle reached the upper surface of the bed. The measured times of rise are significantly longer than those found in bubbling beds [12, 13]. Thus, a slugging bed, of say 2 m in depth, would have significantly better heat retention and contact of the volatile material with the particulate phase than a bubbling bed of the same depth. With the finer sand, particles of sewage sludge and of petroleum coke containing volatile material rose rapidly to the top of the bed, well before combustion was complete. Published theory [6] was used to predict the rise-times of inert particles in a hot slugging bed. The predictions compare favourably with measurements using petroleum coke containing negligible amounts of volatile material and moisture. The use of a modified form of Stokes’s law to predict the velocity of rise of inert particles is shown to compare well with experiments. The experimental results suggest that once a minimum rate of gas evolution (from volatiles or moisture) is achieved a bubble forms which is large enough to lift a fuel particle to the surface. For particles meeting this criterion, the rise-time appears to be independent of the size of the fuel particle.