time at the incineration temperature. To achieve efficient combustion, every part of the gas stream must reach an adequately high temperature for a sufficient period of time, and there must be adequate mixture of fuel and oxygen.
The temperature achieved is the result of heat released by the oxidation process, and has to be maintained high enough to ensure that combustion goes to completion, but not so high as to damage equipment or generate excessive nitrogen oxides. Typically, temperatures are controlled by limiting the amount of material charged to the furnace to ensure that the heat-release rate is in the desired range, and then tempering the resulting conditions by varying the amount of excess air.
Turbulence is needed to provide adequate contact between the combustible gases and oxygen across the combustion chamber (macroscale mixing) and at the molecular level (microscale mixing). Proper operation is indicated when there is sufficient oxygen present in the furnace, and the gases are highly mixed. Cool spots can occur next to the furnace's walls; where heat is first extracted from the combustion process. Such cool spots on walls are more substantial in waterwall furnaces than in refractory-lined furnaces.
A number of new design features and operating techniques have been adopted to increase temperature, extend residence time, and increase turbulence in waste incinerators in order to improve combustion efficiency and provide other benefits like improved ash quality. They include high-efficiency burner systems, waste-pretreatment practices such as shredding and blending, and oxygen enrichment in addition to the features and methods discussed below. Considerable attention has also been given to measurement and control of key process operating conditions to allow better control of the whole combustion process.
Table 3-2 lists the types of furnaces used for municipal solid-waste, hazardous-waste, and medical-waste incineration. Municipal solid-waste furnace designs have evolved over the years from simple batch-fed, stationary refractory hearth designs to continuous feed, reciprocating (or other moving, air-cooled) grate designs with waterwall furnaces for energy recovery. The newer municipal solid-waste incinerators are waste-to-energy plants that produce steam for electric power generation.
The predominant hazardous-waste incinerator designs are liquid-injection furnaces and rotary kilns. Hazardous wastes are also burned in cement kilns, light-weight aggregate kilns, industrial boilers, halogen-acid recovery furnaces, and sulfuric-acid regeneration furnaces.
Medical wastes are burned in fixed-hearth incinerators, with the primary chamber operated in the starved-air mode (newer “controlled air” designs) or excess air mode (older Incinerator Institute of America (IIA) design). Both designs incorporate secondary, afterburner chambers. The smallest medical-waste incinerators are single-chamber, batch-operated devices.