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At Fluent Asia Pacific's 2000 Users' Group Meeting in Tokyo, engineers
from Kubota Corporation of Japan described their work on the development
of a novel rotating surface melting furnace demonstration plant. The plant
has been designed to address an issue of major concern in today's municipal
and solid waste (MSW) incinerators, namely the minimization of highly
undesirable dioxin production. The pilot furnace that Kubota has been
investigating is designed to melt MSW incinerator residues, sewage sludge,
and high calorific value wastes, such as plastics, that are normally hard
to handle in a conventional MSW incinerator. The melted material is subsequently
processed to produce a glassy slag, and hazardous substances like heavy
metals are concentrated in the fly ash and separated in the flue gas treatment
plant. Because of the high temperatures inside the primary combustion
chamber, over 99% of dioxins can be decomposed in this process.
The furnace consists of two refractory lined concentric cylinders, oriented
with a vertical axis. The inner cylinder has a roof in the shape of a
truncated cone, with three burners positioned 120 degrees apart. Each
burner has a centered fuel feed pipe, surrounded by an annular pipe containing
swirling vanes, for air input. The outer cylinder, with a diameter about
50% larger than that of the inner cylinder, fits around the inner cylinder
and has a small circular opening centered in its base. Processed material
that is to be burned in the furnace is fed into the region between the
two cylinders. The material settles in this region in such a way that
its upper surface is exposed to the flames from the burner. The outer
cylinder rotates slowly to maintain a stable flow of processed material
into the primary combustion chamber. As a result of the slow rotation
and feed of the processed material, the material surface develops a shape
that is also conical, similar to the roof. This surface, when combined
with the roof surface, forms a primary combustion chamber that is elliptical
in shape. Only the roof of this combustion chamber is a solid wall. The
sides and base are the surface of the material that is being incinerated.
Surface grid in the CFD model of the incinerator. The
roof of the inner cylinder is shown in green, with exposed details of
the burners. The red surface below the roof is the surface of the processed
material, being fed into the primary combustion chamber by a slowly rotating
outer cylinder (not shown). The secondary combustion chamber is at the
bottom of the diagram.
The primary combustion chamber is kept between 1,300° C and 1,400°
C, and usually uses heavy oils, city gas, and/or combustible processed
materials as fuel. After being dried, burned, and melted in the primary
combustion chamber, the material falls into the secondary combustion chamber,
directly below, where an additional burner is located. Exhaust gases from
the secondary combustion chamber exit via a duct to a final combustion
chamber, where combustion is completed and the exhaust gases are sent
to a flue gas treatment plant. Solid material in the secondary combustion
chamber falls into a slag pit, which is filled with water so that the
melted slag can be rapidly quenched.
Kubota engineers modeled the 3 m diameter pilot furnace combustion chamber
with FLUENT, using the k-e turbulence model
and the eddy break-up and PDF combustion models for comparison. A modified
pulverized coal combustion model was used to represent the processed material
fed into the primary combustion chamber. Good agreement was found between
the CFD predictions and experimental measurements for most gas species
involved in the combustion process. Further CFD studies are currently
ongoing for this novel incinerator design.
Flow path lines from two of the three burners in the
primary combustion chamber
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