Green Bay Retools MHFS for the 21st Century

Abstract

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