Economizer ash presents material handling challenges do to a combination of factors.
The ash collected in economizer hoppers typically contains coarse particles and sintering may occur in the elevated gas temperatures present in the hoppers.
Some installations may have wet collection tanks for economizer ashes but most have been converted to dry systems.
Vacuum conveying from economizer hoppers was a common arrangement when hydraulic conveying of bottom ash is used. If sluice systems are converted to closed-loop operation or mechanical conveying, conversion of the hydraulic vacuum conveying systems to dry operation is a common approach to address issues associated with economizer or other boiler hopper ashes.
A vacuum system is often the most economical choice for transporting economizer ash especially if an existing dry pneumatic system is installed at the plant. These systems can typically be extended to convey ash from the collection hoppers or dry storage tank to the existing storage silo or even to a separate silo should the plant have a market for their fly ash that cannot accept boiler hopper ashes (economizer, SCR or air heater).
The preferred arrangement for handling economizer ashes to continuously remove ash from the high temperature collection hoppers. Where possible, a storage tank is located below the hopper outlets and the ash is stored in the tank for intermittent conveying. Alternatively, downspouts are sometimes used from the individual hoppers to a storage tank(s) at a lower elevation.
Positive pressure conveying of boiler hopper ashes is also used where longer distance conveying (>1,000 ft./ 300 m) is needed.
New boiler installations are normally designed for continuous ash removal from the boiler hoppers using a Dry Flight Conveyor (DFC). DFCs use either a single or double chain arrangement with connecting flights to move ash along the bottom of an abrasion resistant housing. Flight conveyors can be all horizontal, may include an inclined section or may even be designed with a vertical section.
DFCs can be installed in series to allow boiler ashes to discharge directly into the bottom ash conveyor or can discharge into a storage tank from which ash is pneumatically transported to a remote silo or the storage tank may be designed for direct discharge of ash into transport vehicles.
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