BACKGROUND AND PURPOSEThe golden age of the multiple hearth furnace (MHF) for burning sewage solids appears to have been the latter part of the 20 th century when most of the units were put into service. Can a 20 th century technology be viable for the 21 st century? The Green Bay Metropolitan Sewerage District (District) has made that commitment with the renewal of their MHFs which originally went into service in the early 1970s. The MHFs were upgraded in 2006 for improved emissions control, enhanced processing capabilities, better operations, and better fuel efficiency.The purposes of this paper are to describe the modifications completed for the District's two 10.7 m (22 ft. 9 in.) diameter 7-hearth MHFs and the results in positioning the MHFs for the 21 st century, including enhanced emissions control, increased incinerator capacity, improved operations, and fuel cost savings.
WHAT WAS DONE TO UPGRADE THE MHFSThe Green Bay incinerators have been in continuous use (one unit operated at all times) for over 30 years. Solids processing has also changed from burning dewatered thermally conditioned solids at about 40 percent dry solids to burning belt press dewatered solids at 21 to 28 percent dry solids.When the project was initiated, the MHFs were in good operating condition but the District staff was asking the question: How long will the incinerators last? The District contracted Black & Veatch to answer that question. Initially, physical inspections and "desktop" analyses were completed. The following summarizes the approach used to determine rehabilitation needs for the incinerators.• The incinerators and auxiliaries were thoroughly inspected to determine repairs, with particular respect to refractory and exhaust ductwork as a result of wear and tear for 30 years. • Based on previous testing, the emissions control scrubbers -primarily impingement tray scrubbers -were determined to be marginal for particulate emissions control. In addition, the scrubber performance was constraining
The Green Bay Metropolitan Sewerage District (GBMSD) will replace their existing solids handling facilities at their Green Bay Facility (GBF). The existing solids handling facilities are challenged by the following:• Aging facilities at the end of their useful life, increasing O&M and decreasing reliability • Pending Federal Clean Air Act Maximum Achievable Control Technology (MACT) air pollution regulations for sewage sludge incinerators that the existing multiple hearth incineration system cannot meet• A system now at full capacity because of the addition of waste activated solids from GBMSD's De Pere FacilityBetween 2008 and 2011, GBMSD and CH2M HILL developed a Solids Management Facility Plan that evaluated numerous solids processing technologies and process trains to respond to these issues. Seventy-three solids unit processes were considered, some were eliminated and the remaining 52 unit processes were used to develop 17 process configurations. Of these seventeen configurations, six alternative configurations were selected and evaluated in detail. The Digestion with Thermal Processing and Electrical Generation alternative was selected and later named the Resource Recovery and Electrical Energy Project, or R2E2.The paper evaluates energy recovery options from the anaerobic digestion and biosolids incineration system and uses for the recovered energy. Recovered energy from anaerobic digestion is converted to electricity and heat, while energy recovered from biosolids incineration is used for partial drying of the biosolids feed and other heating uses. The paper demonstrates the effective use of recovering and utilizing energy to replace a significant portion of purchased power and natural gas.
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