Material: Recycled
Glass
Issue: Glass container manufacturers
are using increasing amounts of recycled glass cullet.
While reducing the demand for natural resources,
the use of cullet is also cost-effective from a
manufacturing standpoint. Cullet melts at a lower temperature than raw materials, reducing
the demand for energy and lowering production costs. Further energy savings can be achieved by using furnace exhaust
gases to preheat the cullet.
One method of preheating involves direct
transmission of furnace exhaust through a cullet
medium. Another
approach involves passing the cullet through a vertical
funnel that is heated externally by the furnace
exhaust. Preheating
cullet also helps to promote a longer furnace life. This best practice
explains the benefits of using preheated cullet
in the manufacturing of glass containers.
Preheater designs using direct and indirect
heat transmission are also discussed.
Best Practice: Typical glass container manufacturing furnaces are
capable of producing from 100 to 400 tons of glass
per day. Because recycled glass melts at lower temperatures
than raw batch materials, it requires less furnace
energy during production.
Studies show that every 10% increase in the
amount of cullet used reduces melting energy by
about 2˝%. Reducing the energy input reduces fuel consumption,
and ultimately helps reduce the level of nitrogen
emissions produced by the furnace.
Preheating cullet with the
furnace exhaust gas allows further reduction in
the melting energy. The exact energy saved depends on the amount
of cullet and the preheat temperature used.
Studies show that to achieve notable savings,
the cullet must be preheated to at least 650° F.
However, if the cullet temperature exceeds
about 1025° F it will begin to soften and become
difficult to transport.
Thus exhaust gas hotter than 1025° F should
not be used for preheating.
Cullet preheating can be achieved
by direct transmission of furnace exhaust through
a cullet layer. This cullet layer is usually contained in a
preheating unit connected to the furnace emission
system. One advantage of direct heat transmission is
that the cullet also filters dust from the furnace
exhaust. During exhaust flow, up to 40% of the dust
particles are retained in the cullet and eventually
return to the furnace. The resulting gasses require less treatment
prior to release into the atmosphere.
However, the re-circulated dust may contain
alkali sulfates, bisulfates, or pyrosulfates, which
should be considered when the sulfur content of
the glass composition is evaluated.
Air jets may also be necessary in the preheating
unit to prevent dust from clogging the cullet.
Although heat transfer to the cullet is most
efficient at higher exhaust speeds, the dust filtration
capacity will be lower.
The efficiency of heat transfer also depends
on the thickness of the cullet layer. Thicker cullet layers take longer to preheat,
as the exhaust cools during transfer.
The ideal cullet size for preheating systems
of this type is between 1/8- and 1.0-inch.
If the amount of material finer than 1/8-inch
exceeds 10%, the porosity of the cullet becomes
so small that it may cause a reduction in the exhaust
velocity, increasing the gas pressure in the filter
and furnace. After
leaving the preheating unit, the furnace exhaust
passes through a conventional filter system and
is released to the atmosphere through a stack.
The preheated cullet is then transported
to the batch charger.
Preheating systems of this type reduce furnace
energy by up to 12% for cullet contents of 50% or
greater.
Cullet can also be preheated
with an indirect heat transfer system.
These systems generally involve a vertical
funnel surrounded by hollow chambers.
During operation, cullet is deposited on
the top of the unit and funneled downward at a speed
of about 10 to 16 feet per hour.
Furnace exhaust simultaneously travels up
through the adjacent chambers, raising the ambient
temperature in the funnel.
Once preheated, the cullet is released from
the base of the funnel for transport to the batch
charger. Systems
of this type normally employ an ultrasonic cleaning
device to remove furnace dust that accumulates in
the exhaust chambers. After leaving the hollow chambers,
the furnace exhaust passes through a conventional
filter system and is released to the atmosphere.
Research suggests that indirect heat transfer
systems can reduce furnace energy by up to 20%.
Preheated cullet requires a lower melting temperature
than raw material, and requires less variation in
furnace temperature during melting.
These factors help reduce wear and tear to
the furnace, and ultimately lower maintenance costs
and prolong furnace life.
Implementation: The design and implementation of the preheating unit should
be evaluated with the over-all system configuration. Many technical issues, such as monitoring of the preheating temperature,
should be carefully reviewed prior to the implementation.
Benefits:
Using preheated cullet
in conjunction with raw materials helps container
manufacturers lower production costs by reducing
energy consumption. This also prolongs the life of the glass furnace
and lowers nitrogen emissions.
In addition, a direct heat transfer preheating
system helps to reduce exhaust treatment by filtering
dust from the furnace waste gasses.
Application
Sites: Glass
container manufacturing facilities.
Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org.
References: Glass Recycling Source Book, Glass Packaging Institute, Washington, D.C., 1996. Herzog,
J. and Settimo, R., “An Energy Saving Cullet Preheater,” Glass Industry,
September 1992. Hess,
M., “How Increased Cullet Levels Affect Energy Usage,” Glass Industry, January 1993. Moser,
H., Economics
of Batch and Cullet Preheating, Ceramic Engineering
Science Processes, 1995.
Issue
Date / Update: November 1996
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