A Comparison of Fluid Bed and Multiple Hearth Biosolids Incineration

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Abstract

Incineration has been used as a disposal method for wastewater treatment biosolids for over sixty years. The first multiple hearth furnace for biosolids incineration was built in 1935 in Dearborn, MI. From that time through the late sixties, the multiple hearth was the thermal technique of choice for biosolids disposal. Today there are still some 150-175 aging multiple hearths in operation in North America.

In the seventies, the fluid bed furnace became the preferred thermal technique for biosolids disposal, primarily due to tighter emission regulation and to increasing cost of auxiliary fuel. The first municipal fluid bed was installed in the USA at the Lynnwood, WA wastewater treatment plant in 1962. There have been over 125 fluid bed incinerators installed since then in North America.

Since 1988 there have been 43 new municipal fluid bed systems installed. Among these new fluid bed installations, 11 replaced existing multiple hearths.

The dominance of fluid bed over multiple hearth for the last three decades can be explained by the advantages of the fluid bed, which derive from the basic design differences between the two technologies. These fundamental differences which result in lower emission, reduced auxiliary fuel use and reduced operating and maintenance costs are developed in this paper.

In recent years, to deal with tighter emission regulations, some multiple hearths have been retrofitted with afterburners to increase their exhaust gas temperatures. Different alternatives of refurbishment are discussed in this paper. A graphical method is developed and presented to determine the fuel requirement in the afterburner. This is a useful tool in the evaluation of the rehabilitation of a multiple hearth. While the organic and carbon monoxide releases from the multiple hearth can be reduced with the use of an afterburner, the penalty is higher fuel usage and higher nitrogen oxides generation.

When life cycle costs are considered, including capital, labor, fuel and maintenance, it is often more economical to install a new fluid bed system than to rehabilitate an existing multiple hearth.

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