This process provides for disposal of sludge from a municipal sewage treatment plant as well as grease, grit and screenings. All organics are incinerated with the resulting heat recovered by preheating fluidizing air and/or generating steam. Ash/fines are recovered using a flue gas cyclone with water quench/slurrying conveying system at it's bottom and a two stage flue gas scrubber. Ash slurry is conveyed from the cyclone and scrubber to ponds outside using non-potable water.

The fluid bed is a bed of particles, usually sand or slag in the range of 10 to 50 mesh (Tyler Sieve) with air flowing through the bed at uniform velocity, high enough to cause the bed to boil and expand about 30 to 50% in volume. Bed particles are consequently set in fluid motion.

Combustion air from the fluidizing air compressor (multi-stage centrifugal blower) enters the windbox of the fluid bed reactor (via the preheater, if preheating the air) and is evenly distributed across the bed by the orifice plate developing a slight pressure drop. The orifice plate separates the fluidized bed area from the windbox zone by permitting air to flow upward yet preventing any bed particles to flow downward into the windbox even when the system is not operating.

An over the bed burner (natural gas or fuel oil) is used to bring fluidized bed temperature to near desired operating temperature (usually about 1500 degrees F) during the start-up period. At 1500^oF sludge is fed to the reactor with sufficient concentration and heating value to maintain the bed temperature while the over the bed burner is shut down. Sludge can be fed at the top of the freeboard, just above the bed or sometimes injected directly into the bed. Grit, screenings, grease and #6 fuel oil can also be injected directly into the bed.

Hot combustion gas from the fluidized bed enters the freeboard where the fine bed particles fall back to the bed and the hot gas, along with ash, discharges the reactor and into the cyclone. Quench water is sprayed into the freeboard to control freeboard temperature.

Ash discharges the bottom of the cyclone and is quenched with non-potable water. The resulting ash slurry is mixed with an ash slurry stream from the first stage scrubber in the ash tank. The mixture is then pumped to settling ponds outside the facility.

Hot gas with 95% or better of the ash removed passes through the fluidizing air preheater and flows to the boiler. There are two high temperature valves in the inlet and outlet duct work, which allow the operator to by-pass the heat recovery boiler, with the hot gas from the fluidizing air preheater, if no steam is to be generated.

Consequently, hot gas from either the fluidizing air preheater or from the boiler (depending upon whether or not the boiler is on stream) flows to a two stage scrubber. The first stage is a venturi scrubber with a vapor/liquid separator. The second stage is usually a packed tower scrubber which removes all remaining particulates, and is mounted on top of the venturi vapor/liquid separator. In some cases, when burning at low temperatures is required, the secondary scrubber can be used to remove odors by scrubbing with a dilute caustic solution.

The fluidizing air blower inlet is often used to ventilate (suck air from) an area where a specific odor emission source is a problem. Prefiltering the fluidizing air is required to protect the blower.

Good combustion efficiency exists even at low temperatures since the fluidized bed intimately mixes combustion air and combustible material by virtue of violent turbulence within the fluidized bed of particles. In fact, very little excess air is required for complete combustion in a fluidized bed reactor for this reason. Of course minimizing excess air minimizes the heat required to heat it. In addition, the high heat capacity of the bed resists temperature changes and provides for very stable combustion. In summary:

• Low operating temperature minimizes heat losses and emissions of oxides of nitrogen (NOx).
• The fluidized bed is a large heat sink and as such provides very stable combustion.
• Low excess air requirement can provide lower operating cost.
• Space requirement is low.
• Ash slurry is pumpable and easy to handle.
• Incineration can take place in the presence of a sorbent which will remove up to 95% of
the sulfur fed.

Heat transfer surfaces can be installed directly in the fluidized bed for heat recovery where high heating value fuel is burned (i.e. coal, fuel oil, wood, etc.) and where heat transfer rates are high.