Reviving A 25 Year Old Fluid Bed Sewage Sludge Incinerator

In the early 1970's, The Gloucester County Utilities Authority installed a fluidized bed incinerator to burn sewage sludge produced at its sewage treatment facility. At the time of installation, fuel oil prices were below 20¢ per gallon. Capital cost was high, so the installation was designed without heat recovery. Wet sludge was pumped into the fluidized sand bed along with sufficient air and oil to complete the incineration process.By the mid 1980's sludge production had increased, and the cold windbox model was struggling to keep up. Preheating of the combustion air was used to improve operation and production rate. Unfortunately, the flat metal plate used for the cold fluidizing air distribution system was limited to a preheated air temperature of 700°F. A shell and tube air preheater using the hot flue gas from the incineration process to heat the combustion air was installed. To avoid fouling, the dirty gas passed downwardly through the tubes while the clean air was passed over the outside of the same tubes.In 1998, even greater capacity was needed from this old incinerator. Replacing the metal air distribution plate with a refractory dome or a high alloy metal plate was impractical since the windbox was too short and uninsulated. An air distribution system used for decades in the oil industry was the solution. The petroleum industry uses a set of parallel perforated pipes to introduce reactant gas into the fluid bed catalytic crackers. Adapting this technology to the cold windbox unit was quite successful. The 8'-6” diameter bed section was widened to 9'-0” to allow room for 8 parallel air pipes. Each pipe was perforated along two lines, 22½° from the bottom center line. The pipes were placed at an elevation such that their air discharge was approximately equal to bubble elevation from the original tuyeres. Preheated air is forced down each pipe-tuyere from a common manifold supported from the incinerator shell.The original metal tuyeres were cut from the plate and the resulting holes welded closed. A layer of insulating castable was applied to the top of the metal plate to protect it from the high temperature of the bed. Several openings were provided to allow air to freely circulate through the abandoned windbox below the metal plate. To increase the preheater air temperature, a taller heat exchanger was installed on the center line of the previous preheater. Minor duct work changes reconnected the flue gas to the venturi scrubber.After more than four years of operation, there are no signs of wear of the new pipe-tuyeres. The predicted capacity increase and fuel reduction were both achieved. As a means of demonstrating the improvement, we evaluated the performance based on the sludge from the present dewatering equipment (23% dry solids).With the cold and warm windbox limitations, much of the available fluidizing air was needed to burn oil just to maintain the minimum bed temperature. By preheating to a higher temperature, that air is now available for burning sludge dry solids.The paper presents detailed photographs of the installation and results of performance evaluations.