Grate Incinerators

About 90% of incinerators treating MSW use grate incinerators because of their simplicity and ability to handle a wide range of waste particle sizes. Grate incinerators are also used for commercial and industrial nonhazard­ous waste. This type of incinerator contains (a) a waste feeder unit, (b) incin­erator grate on which waste materials are placed, (c) incinerator chamber, (d) incinerator air duct system, (e) auxiliary burner, and (f) bottom ash dis­charger [13].

6.5.1.1 Rotary Kilns

The rotary kiln consists of a cylindrical vessel slightly inclined on its hori­zontal axis. The vessel is usually located on rollers, allowing the kiln to rotate or oscillate around its axis. Solid, liquid, gaseous, or sludge waste is conveyed through the kiln as it rotates. For most fluids, direct injection is preferred. Rotary kilns are used more for hazardous and clinical wastes and less for MSW. Although the rotary kiln is normally operated at 850°C, the temperature can vary from 500°C (as a gasifier) to 1,450°C for an ash-melting kiln [7]. For hazardous waste, the kiln is operated between 900-1,200°C. The residence time in the kiln normally varies between 30 to 90 minutes. For complete destruction of toxic compounds, a postcombustion chamber may be required [7, 11].

6.5.1.2 Fluidized Beds

For RDF, sewage sludge, and finely divided waste, fluidized bed incin­erators are widely used. These types of incinerators are normally used for large-scale operations. Various types of fluidized bed—stationary, bubbling (atmospheric or pressurized), rotating, or circulating—have been used in industrial practice. These fluidized beds differ in gas velocities, the design of the nozzle plate, and their internal design. The waste is injected from the top in an inert sand or ash fluidized bed (by air) which is preheated at the desired level [15]. Sometimes the waste is fluidized by simple injection of air (no inert solids) through holes in the bed plate. The inert solids provide a better heat transfer to the waste. For most waste, the space above the bed is kept at 850-950°C and the temperature of the bed is kept around 650°C [7]. A fluidized bed combustor generally gives uniform temperature and oxygen concentration distributions as well as more stable operation due to a high level of mixing within the bed.

A fluidized bed generally operates with waste particle size of 50 mm. For heterogeneous waste, this may require a pretreatment and sizing step that may add to the overall cost. For a rotating fluidized bed, a particle size of 200-300 mm [16] is possible due to additional mixing caused by the bed rota­tion. The unburned waste and ash in a fluidized bed incinerator are removed at the bottom of the reactor [11, 13]. The combustion heat can be either cap­tured by an internal device, at the exit, or both together.