Residue Handling

In a continuously fed incinerator, the ash or residue is discharged continuously through a chute into a conveyor trough, which is filled with water to cool the residue before it is hauled away for final disposal. The chute is submerged under quench water to seal the furnace outlet and prevent entry of atmospheric air. In newer, mass-burning facilities, full-size discharge chutes minimize hangups with large pieces of residue.

The residue conveyor pulls the settled residue from the bottom of the trough and transports it to an ash hopper, storage bin, roll-off carrier, or dump truck. The trough is constructed of steel or concrete, and the residue-discharge system usually has two conveyor troughs so that a full standby is available. Having a full standby permits switching between systems for even wear and scheduled maintenance.

As the conveyor carries the residue, most of the quench water runs off and returns to the trough. The conveyor should run at velocities not exceeding 5 to 10 ft/min (1.5 to 3 m/min) for good dewatering and minimal wear (Stelian and Greene 1986). The moisture content of the ash is usually 25 to 40% or more by weight. Reducing the water content of the ash minimizes transportation costs and water pollution. By reducing the speed of variable speed conveyors, operators can achieve this reduction by maximizing the residence time of residue on the wet-drag conveyor. Wet-drag conveyors can operate at slopes up to 45°, but some operators prefer lower slopes to protect bulky items from rolling back.

The design of a residue-handling system should minimize the discharge of water pollutants. Ash can be acid or alkaline; therefore, the water pH must be controlled in the range of 6 to 9 pH. The water can also contain high concentrations of BOD, dioxins, heavy metals, and other suspended or dissolved toxic or polluting constituents. For this reason, the ash-handling system must operate in the zero discharge mode (Stelian and Greene 1986). A water circulation and clarification system, including properly designed basins, sumps, and an easily maintained pumping station, is required. To capture the water that might drain off in the ash-transfer process, the system should have catch troughs where the conveyor transfers the ash into the receiver.

In cold regions with freezing winter temperatures, the ash-handling system must be protected against freezing. In cold areas, heated trucks transport the ash, and the fly-ash conveyors are insulated for protection against corrosion and caking. The ash conveyor can unload wet residue into a temporary container or directly into a transport vehicle for removal from the site. In mass-burn systems, directly discharging into dump trucks is best and simplest.

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