Charging hoppers hold up some volume of refuse to guarantee a reasonably uniform waste flow into the incinerator. MSW enters charging hoppers in the following ways:
In larger plants where the hoppers are located above the storage pits, MSW is lifted by cranes. In larger plants where the hoppers and storage area are at the same elevation, MSW is transferred into storage hoppers by ram feeders or by front-end loaders. In plants under 100 tpd capacity, MSW is loaded directly from the trucks into the charging hoppers. In multicell furnaces, each furnace cell usually has one charge hopper.
In a continuous-charging hopper, the outlet gate is kept open, and the air seal is maintained by the MSW and the movement of the mechanical grate charging the furnace. Most hoppers have an angle-of-slide surface of 30 to 60° from the vertical to prevent bridging. The feed chute is normally 4 ft (1.2 m) wide, to pass large objects with minimum bridging, and 12 to 14 ft (3.6 to 4.2 m) long from the hopper to the front end of the furnace. Because of its proximity to the combustion zone, the continuous-charging hopper is usually water cooled.
The continuous-charging hopper allows better furnace temperature control and thereby reduces the need for refractory maintenance. It also spreads the MSW more evenly across the grate, in a relatively uniform and thin layer, while sealing the furnace from cold air.
Lessons learned on RFD facilities suggest (1) using simple RDF floor storage, not bins that can become plugged; (2) using simple RDF transfer via a conveyor rather than pneumatic systems; (3) maintaining a uniform flow of RDF to boiler feeders and avoiding slug feeding, which results in unstable boiler control; and (4) using a proven RDF feeder, which maintains even grate distribution and is responsive to load change (Gibbs and Kreidler 1989).
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