Replacement of a Multiple Hearth by a Fluid Bed Incinerator - The Greensboro Experience (PDF - File size 50K)
The incinerator at the T.Z. Osborne Plant in Greensboro, North Carolina burns sludge from its own Waste Water Treatment Plant and sludge pumped from the nearby North Buffalo plant. The two plants have a combined capacity of 36 million gallons per day of wastewater. In 1992, the Osborne plant concluded a study evaluating its options for future municipal sewage sludge disposal. The study included the options of rehabilitation of an existing eight year old multiple hearth incinerator, addition of a new multiple hearth, addition of a new fluid bed system, drying, composting and land application. The choice was a new fluid bed system based on both economic and environmental reasons.
Design of the new fluid bed system began in December 1994 and equipment delivery for the incineration system was begun in April 1995. Initial operation occurred in August 1996. Primary and secondary sludge, dewatered to 28% dry solids by centrifuge, is delivered by piston pumps to the 20’ freeboard ID incinerator. A shell and tube heat exchanger recuperates heat from the exhaust gas and preheats the combustion air to 1250 F, resulting in minimal auxiliary fuel use. The air pollution control device is a high energy dual tandem nozzle scrubber. Greensboro was the initial installation of this scrubber design on a fluid bed incinerator and its characteristics and performance are discussed. Ash is dewatered in an ash thickener/belt press system prior to disposal to landfill. The system includes a state of the art PLC system for computer control of the operation.
The unit was commissioned in August 1996 and has been in continuous operation since that time, except for a one week inspection and maintenance shutdown in February 1999. The plant operates 24 hours per day, seven days per week. The initial performance test showed the system to readily meet federal and state air emission standards. Particulate released was 0.002 grains per dry standard cubic foot, carbon monoxide was 22.5 ppmv and opacity was 0.4%. Comparison of results shows a significant emission reduction with the fluid bed when compared to the multiple hearth. Annual tests conducted since then and continuous emission monitoring have shown the unit to continue to be in compliance.
Since the fluid bed system became operational, the old multiple hearth system has been maintained on standby as a backup, but its use has not been required. Operational experience is discussed, the most interesting of which is the relatively trouble free operation. The minor problems which occurred and their solutions are detailed. Also included is a comparison of operation and maintenance experience of the fluid bed and the multiple hearth. Current sludge disposal actual cost data is also provided, including the average cost per ton of dry solids treated. The almost three years of operational experience to date has shown that the decision to install a new fluid bed system was the correct one on both an environmental and economic basis. It has provided benefits to all interested parties – the WasteWater Treatment Plant, the regulators, the taxpayers, and the surrounding community.