Cogen Plant Derives Multiple Uses for Paper Mill Sludge

1999 Power Plant Award

Several waste streams-paper-mill and municipal sludge and coal-fired powerplant flyash-are combined at this unique facility to cogenerate steam and electricity and produce high-quality Glass Aggregates for recycle. How established processes are configured to do the job is the innovation.

By CarolAnn Giovando, Associate Editor

Pulp and paper companies are always searching for ways to dispose of sludge, a clay-like substance with 60% moisture content. Much of the sludge is landfilled, a long-term liability because of potential groundwater and/or soil contamination. Over the years, landfills have become increasingly difficult to license because of concerns about aesthetics, environmental impact, and large amounts of space required for operation. Alternatives to landfilling have emerged, but technologies many times created significant quantities of ash and waste streams that posed their own disposal problems.

Minergy Corp, Milwaukee, Wis., a wholly owned subsidiary of Wisconsin Energy, pioneered technologies to recycle paper sludge, as well as coal flyash and municipal sludge, into saleable products with minimal waste. A process to create lightweight aggregate was proved at Wisconsin Energy’s Oak Creek powerplant in 1994, and has facilitated recycling of more than 500,000 tons of flyash and municipal and paper sludge into 100,000 tons of light-weight aggregate for use in the construction industry (POWER, August 1994, p 37).

Drawing on that success, Minergy designed a second facility to recycle pulp and paper sludge from mills in Neenah, Wis. The site was selected based on proximity to 41 paper mills located in the Neenah area. According to Terry Carroll, regional manager, “There is nothing new here. All of our systems use established technology put in a new sequence to solve an old problem.”

ESI Inc., Kennesaw, Ga., provided design and construction services for the first-of-its-kind, $45-million Neenah Glass Aggregate Plant (Fig 1). The plant was completed in January 1998 and commercial operation commenced on May 21, just 18 months after groundbreaking. Capacity was sized to process the entire sludge output in all of Winnebago County.

Ten area paper mills supply about 400,000 tons/yr. Sludge contracts collectively account for about two thirds of the county’s waste stream representing more recycled material than handled by the combined municipal programs in the entire state.

Commercially proven technologies, applied in new ways, comprise the main plant systems. Patented processes convey sludge through storage silos, to dryers, and into one of two Cyclone furnaces as fuel. The 7 ft diameter, 350 million Btu/hr Cyclones are adapted versions of older designs by Babcock & Wilcox Co (B&W), Barberton, Ohio. Major rework included installation of four tangential secant ports on each Cyclone to introduce the sludge fuel.

The resultant products: 80,000 tons/yr of Glass Aggregate, marketed as construction products, and 350 psig/600F steam for host facility and neighbor P H Glatfelter Co. Addition of a 6.5-MW steam turbine/generator will convert 20,000 lb/hr of surplus steam into electricity. At press time, the turbine/generator was in the final stages of commissioning. Electricity is considered renewable biomass energy, which will be sold to Alliant Energy, Madison, Wis.

For putting high-volume waste streams to beneficial use in generating energy and recyclable materials, Minergy Corp’s Neenah facility earns POWER’S 1999 Power plant Award.