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<br /> Comprehensive Review of <br /> S olid W aste Collection and Disposal Options <br /> <br /> <br />v2.1 159 10/22/12 <br />incoming waste so that materials with different moisture contents are gradually intermixed to try <br />to get uniform moisture content. <br />Exhibit 12 -1. Cross -Section of Typical Mass -Fired Waterwall <br />Facility <br /> <br /> <br />The refuse is then discharged into refuse feed hoppers, which meter out the refuse into the <br />combustion chamber, either by gravity feeding or by a hydraulic feeding device. In a majority of <br />systems, the waste is then pushed onto an inclined, step-like, mechanical grate system which <br />continuously rocks, tumbles, and agitates the refuse bed by forcing burning refuse underneath <br />newly fed refuse. Generally, most systems have three zones of activity along the grates: drying, <br />ignition, and burnout. Holes in each grate bar allow underfire air to pass through the grates <br />resulting in cooling and, thus, preventing thermal damage to the grate system. The width of the <br />grate and the number of grate steps is dependent not only upon the manufacturer's specifications, <br />but also on the overall size of the WTE system. There are five basic moving grate designs: <br /> Reciprocating Grate. This grate resembles stairs with alternating fixed or moving <br />grate sections. The pushing action may be in the direction of waste flow or in an <br />upward motion against the waste flow. <br /> Rocking Grate. Pivoted or rocked grate sections produce an upward or forward <br />motion, advancing the waste down the grate. <br /> Roller Grate. A series of rotating stepped drums or rollers agitate the waste and move <br />it down the grate. <br /> Circular Grate. A rotating annular hearth or cone agitates the waste.