Page 95 - Wastewater Solids Incineration Systems

64 Wastewater Solids Incineration Systems

Compressed Air
Exhaust
Gas Duct Continuous Emissions
High-Pressure Sand Service Water Monitoring System
Water Pump Storage
Stack
Sand
Caustic
Sludge Heat Exchangers
Feed PrimarySecondary
Purge Blower
Sludge
Dewatering Service Water
Feed Pump Circulating
Pump
Purge Air Blower Fluidizing
Air Blower Venturi Tray Caustic
Auxiliary Scrubber Scrubber Wet
Fuel Preheat Burner
Oil Combustion Air Reheat Air ESP
Preheat Preheat Blower Ash Treatment
Settling Lagoon
Gas
Feed
Auxiliary
FIGURE 5.8 A typical process flow diagram for a wet-ash system.

includes a Venturi scrubber followed by a tray tower. A wet electrostatic precipitator
could be used to eliminate submicrometer particulates. Hot air at 260°C (500°F) is
optionally added to the stack gas for plume suppression. Plume suppression air is
preheated in a secondary heat exchanger using the exhaust flue gas from the primary
heat exchanger. Ash and fine sand particles carried in the flue gas are removed in the
high-pressure drop Venturi scrubber where an ash-slurry is produced. The slurry is
either pumped or flows by gravity to an outdoor ash settling lagoon system for
dewatering. Dry ash at approximately 50% total solids is removed from the drying
lagoon approximately once per month, depending on the size of the lagoon.
In the wet-ash system, acid gases such as SO and HCl are removed by water in
2
the Venturi scrubber and cooling tray. These gases are soluble in water, which means
that up to 95% of the acids can be removed by effluent plant water alone. With stricter
regulation, a solution of caustic can be added to the cooling tray to increase removal
further. Mercury and dioxins can be removed from the flue gas in an activated carbon
adsorber, which is installed before the stack.
Because of its simplicity and the availability of effluent water and space at most
wastewater treatment plants, the wet-ash system is mostly used in North America
(more than 90% of existing plants).