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91 <br />Comprehensive Review of <br />Solid Waste Collection and Disposal Options <br />12.3.1.1 Stages of Combustion <br />Solid waste normally has a )moisture content of 20 to 25% by weight. In order to successfully <br />burn solid waste in a furnace, this moisture must be evaporated. Generally, most solid waste <br />combustion units have three stages of reaction: <br />a Drying. Moisture driven off. <br />• Ignition. Solid waste ignited. <br />• Burnout. Solid waste is gradually moved through the furnace by the grate subsystem <br />where the combustible organic fraction of the solid waste is burned out. <br />Successful combustion of solid waste is accomplished by controlling the "3 Ts of Combustion% <br />Time, Temperature and Turbulence. <br />• Time. The period taken for solid waste to pass from the charging hopper until the <br />bottom ash is discharged at the end of the grate subsystem (usually 45 to 60 minutes). <br />Temperature. Usually exceeds 1,800 °F (980 °C) within the furnace and is directly <br />proportional to the residence time. If there is insufficient time in the furnace, the <br />combustion reaction cannot proceed to completion and temperature declines. <br />Turbulence. Provided by the grate subsystem moving the solid waste downward <br />through the furnace to expose it to and mix it with air. <br />Normally, solid waste combustors reduce the original weight of the solid waste by 75 + % and the <br />volume by 85 to 90 %. <br />Combustion is aided by the introduction of air at two locations in the furnace. Air is introduced <br />underneath the grates (underfire air) to increase the agitation and turbulence within the furnace <br />and help cool the grates. Air is also introduced above the burning solid waste (overfire air). <br />Overfire air ensures that there is adequate oxygen available to completely oxidize and burn the <br />entire combustible fraction of the solid waste. Overfire air also aids mixing of the combustion <br />gases thereby ensuring complete oxidation and destruction. Combustion gases (also called flue <br />gases) move from the furnace through the flues and the air pollution control systems and are <br />eventually discharged out the stack into the atmosphere. <br />12.3.1.2 Waste -to- Energy Solid Waste Combustors <br />In a WTE solid waste combustor, the energy released from combustion in the form of heat is <br />used to generate steam in a boiler. The common method of capturing this released energy is <br />either through refractory or waterwall furnace systems. The major difference between these two <br />designs is the location of the boiler. <br />Refractory Units. This design consists of boilers located downstream of the <br />combustion (furnace) chamber. The hot combustion gases pass through the boiler <br />tubes to create steam. <br />v2.1 157 10/22/1 2 <br />