Note: Descriptions are shown in the official language in which they were submitted.
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CONTROLLING GLAS SMELTING FURNACE OPERATION
Field of the Invention
The present invention relates to operation of glassmelting furnaces, in
which glassmaking ingredients are melted to produce a bath of molten
glassmaking material from which solid glass can be produced.
Background of the Invention
In the manufacture of glass, glassmaking materials are melted in a
glassmelting furnace by heat provided from burners which combust fuel with
oxygen. The fuel can be combusted with air as the source of the oxygen, or
with a
stream containing a higher oxygen content than that of air. The furnace must
be
manufactured of material that can withstand the very high temperatures that
prevail within the furnace. The materials of construction often employed,
which
typically include AZS and silica refractory and related materials, are well
known.
However, the conditions within the glassmelting furnace have been known
to cause corrosion of the inner surfaces of the furnace, especially of the
roof
("crown") over the glassmaking materials. The most widely used material for
the
crown is silica brick for soda-lime-silicate glass furnaces. Alkali vapors
(mostly
NaOH and KOH) generated from the glass batch material and molten glass in the
glassmelting furnace react with silica refractory brick and form over time a
glassy
silicate material on the inner surface of the crown. When a sufficient
concentration of alkali oxides (mainly Na20 and K20) accumulates in the glassy
silicate layer, the glassy material can become fluid enough to drip directly
into the
molten glass in the furnace or to run along the silica refractory surface and
over
other refractory surfaces in the furnace and dissolve or dislodge some of the
refractory particles which fall into the molten glass. This corrosion is
undesirable
as it leads to a loss of material in the crown, which eventually leads to
expensive
repairs or replacement of the crown, and because the corrosion products have
been known to fall into the pool of molten glass materials in the furnace and
to
cause defects in the glass product.
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The present invention provides methodology for controlling the furnace
atmosphere to reduce corrosion of refractory materials and to improve the
quality
of glass, in particular, to increase the oxidation state of glass, i.e., to
reduce the
redox ratio, which is the molar ratio of ferrous iron to ferric iron, to
produce glass
characterized by high transmission of light for uses such as clear flat glass
and
glass tablewares. Preferably the redox ratio is reduced by 0.01 to 0.20.
Brief Summary of the Invention
One aspect of the invention is a method of operating a glassmelting
furnace, the furnace including a glassmelting chamber defined by opposed side
walls, a back wall, a roof, and a front wall, the method comprising:
(A) melting glassmaking material in a melting zone of said glassmelting
chamber to establish a bath of molten glassmaking material, by heat provided
to
the melting zone over said bath by combustion of fuel and preheated oxidant
from
two or more pairs of opposed regenerator ports in said side walls of said
melting
zone, wherein said combustion forms an atmosphere comprising combustion
products over said bath in said melting zone, wherein a spring zone is present
in
said bath,
(B) passing molten glassmaking material from the melting zone into and
through a refining zone of the glassmelting chamber, and then out of said
glassmelting chamber through a port in said front wall,
(C) injecting at least one gaseous stream or atomized fluid stream of fuel
and at least one oxidant stream into the refining zone above the molten
glassmaking material and combusting said fuel and oxidant in said refining
zone
to increase the average oxygen concentration in the atmosphere near said bath
surface in said refining zone by 1 to 60 vol.%, and
(D) adjusting the fuel and combustion air flow rates of each of said
regenerator ports to make the oxygen concentration in the flue gas exiting
each of
said regenerator ports located between the spring zone and the refining zone
between 2 to 10 vol. %, preferably 2 to 6 vol.%.
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In preferred aspects of the invention, said at least one oxidant stream
injected in step (C) comprises 35 vol. % to 100 vol. % oxygen and said fuel is
injected in step (C) at a stoichiometric ratio relative to the oxidant that is
injected
in step (C) that is 110% to 2000%.
As used herein, "glassmaking materials" comprise any of the following
materials, and mixtures thereof: sand (mostly Si02), soda ash (mostly Na2CO3),
limestone (mostly CaCO3 and MgCO3), feldspar, borax (hydrated sodium borate),
other oxides, hydroxides and/or silicates of sodium and potassium, and glass
(such
as recycled solid pieces of glass) previously produced by melting and
solidifying
any of the foregoing. Glassmaking materials may also include functional
additives
such as batch oxidizers such as salt cake ( sodium sulfate, Na2SO4) and/or
niter
(sodium nitrate, NaNO3, and/or potassium nitrate, KNO3), and fining agents
such
as antimony oxides (Sb203).
As used herein, "alkali species" means chemical compounds containing
sodium, potassium and/or lithium atoms, including but not limited to sodium
hydroxide, potassium hydroxide, products formed by decomposition of sodium
hydroxide or potassium hydroxide at temperatures greater than 1200 C, and
mixtures thereof
As used herein, "oxy-fuel burner" means a burner through which are fed
fuel and oxidant having an oxygen content greater than the oxygen content of
air,
and preferably having an oxygen content of at least 50 volume percent and more
preferably more than 90 volume percent.
As used herein, "oxy-fuel combustion" means combustion of fuel with
oxidant having an oxygen content greater than the oxygen content of air, and
preferably having an oxygen content of at least 50 volume percent and more
preferably more than 90 volume percent.
As used herein, "atmosphere near said bath surface" means the gaseous
layer extending from the bath surface to one foot above the bath surface.
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Brief Description of the Drawings
Figure 1 is a top plan view of a glassmelting furnace in which the present
invention can be practiced.
Figure 2 is a graphical representation of gas flows in the furnace of Figure
1 when operated without the present invention.
Figure 3 is a graphical representation of gas flows in the furnace of Figure
1 when operated with one embodiment of the present invention.
Figure 4 is a graphical representation of the oxygen concentration profile
of the furnace atmosphere (in vol.% wet) near the glassmelt surface in the
furnace
of Figure 1 when operated without the present invention in the manner
represented by Figure 2.
Figure 5 is a graphical representation of the oxygen concentration profile
of the furnace atmosphere (in vol.% wet) near the glassmelt surface in the
furnace
of Figure 1 when operated with the embodiment of the present invention
represented by Figure 3.
Figure 6 is a top plan view of a glassmelting furnace depicting alternative
arrangements of the injection of gas into the furnace of Figure 1 in
accordance
with another embodiment of the present invention.
Figure 7 is a side cross-sectional view of a glassmelting furnace depicting
operation with the optional feeding of bubbles of gas into the molten glass.
Figure 8 is a side cross-sectional view of a glassmelting furnace depicting
flows of molten glass and the spring zone.
Detailed Description of the Invention
Turning first to the glassmaking furnace itself, Figure 1 shows a top plan
view of a typical cross fired float glass furnace 100 with regenerators, with
which
the present invention can be practiced. The present invention is not limited
to
float glass furnaces and can be practiced in other types of glass melting
furnaces
manufacturing, for example, tableware glasses, sheet glasses, display glasses,
and
container glasses. The furnace 100 includes melting zone 11 and refining zone
12.
Melting zone 11 and refining zone 12 are enclosed within back wall 21, front
wall
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23, and side walls 22. A crown or roof (not depicted) connects to side walls
22,
back wall 21, and front wall 23. The furnace 100 also has a bottom which
together
with back wall 21, side walls 22 and front wall 23 and the crown or roof, form
the
enclosure that holds the molten glassmaking materials.
5 Conditioning zone 13 is enclosed by side walls 24, front wall 25,
end wall
26, and a crown or roof (not depicted) that connects to side walls 24, front
wall
25, and end wall 26, as well as a bottom and a crown or roof Conditioning zone
13 (when present) is located with respect to refining zone 12 to receive
flowing
molten glassmaking material from refining zone 12 for further conditioning of
the
molten material in the manner already familiar in this field. Waist zone 14 is
a
narrow passage connecting refining zone 12 and conditioning zone 13.
The particular shape of the bottom is not critical, although in general
practice it is preferred that at least a portion of the bottom is planar and
is either
horizontal or sloped in the direction of the flow of the molten glass through
the
furnace. All or a portion of the bottom can instead be curved. The particular
shape of the furnace as defined by its walls is also not critical, so long as
the walls
are high enough to hold the desired amount of molten glass and to provide
(under
the crown) space above the molten glass in which the combustion can occur that
melts the glassmaking materials and keeps them molten.
The furnace 100 also has at least one material charging entrance (not
shown), typically along the inner surface of back wall 21 or in side walls 22
near
back wall 21 for other types of glass furnaces, through which glassmaking
material can be fed into the melting zone 11. There can also be one or more
flues
through which products of the combustion of fuel and oxygen (within melting
zone 11) can flow out of the interior of the furnace. The flue or flues are
typically
located in back wall 21, or in one or more side walls.
The bottom, sides and crown of the furnace should be made from
refractory material that can retain its solid structural integrity at the
temperatures
to which it will be exposed, i.e. typically 1300 C to 1700 C. Such materials
are
widely known in the field of construction of high-temperature apparatus.
Examples include silica, fused alumina, and AZS.
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The inner surface of the crown, i.e. the surface that is in contact with the
furnace atmosphere, may be comprised of the original material of construction
of
the crown, and in some places may instead comprise a layer of slag that has
formed on what was the uncorroded surface of the crown. Such a slag layer is
typically formed due to reactions of volatile vapors and dust from glassmaking
materials and molten glass and may often be found in furnaces that have
already
been in use. Typically, the slag layer contains silica, alkali oxide, alkaline
earth
oxide, and compounds thereof, such as contain calcium oxide and/or compounds
of calcium oxide with silica and/or alkali oxide. Thus, the present invention
can
be carried out in furnaces in which the inner surface of the crown comprises
corrosion product formed by reaction of the surface with alkali hydroxide, and
in
furnaces in which the inner surface of the crown does not comprise corrosion
product formed by reaction of the surface with alkali hydroxide.
Melting zone 11 includes two or more pairs of opposed regenerator ports
in side walls 22. By "opposed" is meant that in a given pair of regenerator
ports,
there is one port in each side wall 22, facing each other and both facing the
interior of melting zone 11. The opposed ports are preferably essentially
coaxial,
that is they face directly across from each other; ports that are offset,
wherein each
port's axis is not coaxial with the other's, can be used but are not
preferred.
Combustion occurs in melting zone 11 as natural gas or fuel oil, injected at
or near
the locations where these ports open into melting zone 11, mixes with hot
combustion air from regenerators 41 and 42, to form a flame and to generate
heat
in the melting zone to melt glassmaking material and maintain the glassmaking
material in the molten state. The regenerator ports communicate with
regenerators 41 and 42 as described further below. Figure 1 shows six pairs of
ports, with each pair of ports facing each other, the ports on one side of the
melting zone being numbered from 1L to 6L and the ports on the other side of
the
melting zone being numbered 1R through 6R. Any number of ports can be
employed, from 2 to 10 or even up to 20 or more, depending on the desired
glassmelting capacity of the furnace. At or near the exit of each port one or
more
fuel injectors (not shown) are placed to inject fuel to form a flame (not
shown)
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and generate heat in melting zone 11. Melting zone 11 is defined as the zone
between back wall 21 and either the last pair of regenerator ports closest to
the
front wall 23, or the fuel injectors for the last pair of regenerator ports
that are
closest to front wall 23 if the fuel injectors are located closer to the front
wall 23
than the port itself.
Optionally one or more flue gas ports (not depicted) not connected to
regenerators 41 and 42 may be placed in one or more walls in melting zone 11
or
in refining zone 12 to exhaust a portion of flue gas for additional heat
recovery
and other purposes.
Arrows 30 and 31 between back wall 21 and the ports 1L and 1R represent
optional oxy-fuel burners often used to increase production and/or glass
quality in
the glass furnace.
Referring to Figure 7, melting zone 11 optionally has gas bubblers
installed through the bottom of the furnace to enhance the circulation of
molten
glass. Air or oxygen from a source 74 (such as a storage tank or cylinder) is
typically injected through each bubbler 72 to produce large bubbles 71 of 3 to
8
inches in diameter as they burst in the surface of molten glass. Preferably
oxygen
is the gas that is injected through the bubblers. The flow of gas through the
bubblers is controlled by controls 73 which permit the operator to regulate
the
flow of the gas, such as a rate of 1-10 SCFH.
Refining zone 12 is characterized in that it may optionally have apparatus
for combusting additional fuel and oxidant over the molten glassmaking
materials.
Preferably, however, no regenerator ports are present in the side walls and
end
wall that contain the refining zone.
The molten glassmaking material in melting zone 11 and refining zone 12
experiences complex recirculating flow patterns within the furnace and has a
net
flow gradually in a direction from the melting zone 11 through refining zone
12
toward and through port 28 in front wall 23, preferably into a conditioning
zone
13. Referring to Figure 8, in a typical glass furnace two large recirculation
flows
82 and 83 of molten glass are formed in the longitudinal direction of the
furnace,
divided by the so called spring zone 81 which is typically found near the
hottest
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zone of the furnace. The first circulation loop 82 is formed between the
spring
zone 81 and back wall 21. The molten glass near the top surface flows backward
from the spring zone 81 toward back wall 21, then flows downward near back
wall 21 and then moves forward toward the spring zone 81. Near the spring zone
81, the molten glass flows upward and most of the glass circulates backward
toward back wall 21. Many gas bubbles are floated to the glass bath top
surface at
the spring zone and removed, i.e., the glass is fined. A portion 84 of the
fined
glass move forward from the spring zone 81 toward front wall 23, passes
through
waist zone 14 into conditioning zone 13. In the second glass circulation loop
83,
some of the glass from conditioning zone 13 flows backward near the bottom of
waist zone 14 into refining zone 12 toward the spring zone 81. Near the spring
zone 81, the glass flows upward, merging with the glass flowing from back wall
21, and some of the glass circulates forward toward front wall 23. Thus, the
"spring zone" is a region in the molten glass in the glassmeltiing furnace,
between
the circulating flow 82 of molten glass that passes adjacent to the back wall
of the
furnace, and the circulating flow 83 of molten glass that passes adjacent to
the
front wall 23 of the furnace. While the molten glass is in melting zone 11 and
refining zone 12, dissolved gases are able to rise to the bath surface and
leave the
bath, and less volatile materials can become more uniformly distributed within
the
bath.
In operation, glassmaking material is fed into melting zone 11.
Combustion in melting zone 11 provides heat that melts glassmaking material in
the melting zone, and maintains the resulting bath of molten glassmaking
material
in the molten state. This combustion is carried out by combusting fuel,
preferably
natural gas or oil, with oxygen that is typically provided as air, or
optionally as
oxygen-enriched air or a stream comprising 50 vol.% up to 99 vol.% oxygen. The
amount of fuel and oxygen fed and combusted must be sufficient to provide
enough heat to melt the glassmaking materials that are fed to melting zone 11.
When combustion is carried out in melting zone 11 using regenerators,
fuel (not shown in figure 1) is typically injected from below or from a side
of each
port at or near the port exit to the furnace toward the opposing port.
Combustion
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air is preheated in the regenerator in the same side of the melting zone 11
(such as
regenerator 41) and flows into melting zone 11, mixes with the injected fuel
and
forms a flame while gaseous products of the combustion, which are very hot,
are
withdrawn from melting zone 11 through the ports in the other side wall 22 of
melting zone 11 and through the other regenerator (in this illustration,
regenerator
42). The gaseous oxidant (i.e. air, oxygen-enriched air, or higher purity
oxygen)
represented by stream 43 passes through the regenerator and is heated by
transfer
of heat previously absorbed from hot gaseous products of combustion that were
withdrawn through that regenerator in a previous cycle, before the oxidant is
combusted with fuel in melting zone 11. While combustion is occurring in
melting zone 11 with fuel and oxidant that are fed at or through the ports
which
communicate with regenerator 41, the hot gaseous products withdrawn through
the ports that communicate with regenerator 42 heat the other regenerator 42.
The
regenerators are typically made of refractory brick or other material that can
absorb heat at the high temperatures that are present (optionally, the
regenerator
may also contain additional objects such as balls or blocks of refractory
material
to absorb heat from the hot combustion gases.
After a period of time which is typically every 10 to 30 minutes, operation
is reversed so that gaseous oxidant for combustion (e.g. air) from the other
regenerator (i.e. regenerator 42) flows into melting zone 11 and combustion
occurs with fuel injected from the same side as regenerator 42, and the
resulting
hot gaseous combustion products are withdrawn through the ports that are
connected to regenerator 41. The oxidant that participates at this point in
the
combustion in melting zone 11 passes through regenerator 42 and is heated by
heat transfer from heat stored regenerator 42 in the previous cycle. After
another
period of time, the direction of combustion air flow and fuel injection is
reversed
again. The regenerators represented by figures 41 and 42 may be one common
chamber on each side of melting zone 41, or may be a number of separate and
distinct chambers each communicating with but one port connected to melting
zone 11 of the furnace..
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In some types of glassmelting furnaces, a stream 50 of gas (typically, air)
flows into refining zone 12 through port 28 in front wall 23, in a direction
toward
melting zone 11. This stream 50 is typically a portion of air that cools the
bath of
molten glass in conditioning zone 13. In conventional practice not employing
the
5 present invention, stream 50 flows through refining zone 12 into melting
zone 11.
Conditioning zone 13 while preferred is not necessary in the present
invention.
When a conditioning zone 13 is employed, stream 52 of cooling gas is fed or
injected into conditioning zone 13, for instance through four openings in wall
24
as shown by four arrows, and then a portion of cooling gas 52 flows through
10 conditioning zone 13 into refining zone 12 through port 28 in waist zone
14 as gas
stream 50. The remainder of cooling gas 52 is exhausted through exhaust ports
(not shown) located in conditioning zone 13 or in waist zone 14.
In other types of glassmelting furnaces, no gas flows into refining zone 12
through port 28, as port 28 is submerged below the molten glass so that only
molten glass flows through port 28. In these types of furnaces, some air may
enter
the refining zone through other openings.
Arrows 32 and 33 in refining zone 12 indicate locations at which at least
one gaseous stream is injected in accordance with the present invention. These
locations are in refining zone 12. A preferred location is in one or both side
walls,
between the front wall 23 and the regenerator port that is closest to the
front wall
23 (or between the front wall 23 and the fuel injection port that is closest
to the
front wall 23, if such fuel injection port is closer to front wall 23 than the
associated regenerator port is). A more preferred location is near that
regenerator
port or fuel injection port. While continuous gas injection from both
injectors of
an opposing pair of injectors 32 and 33 constitutes a preferred embodiment of
this
invention, the present invention can also be practiced with cyclic injection
from
only one injector at a time, preferably the injector that is on the side wall
opposite
to the side wall in which is located the regenerator that is firing at any
given time.
That is, gas would be injected from injector 32 when regenerator 42 is in the
firing
cycle, followed cyclically by injection from injector 33 when regenerator 41
is in
the firing cycle. Each injector 32 or 33 can be an oxy-fuel burner to which
fuel
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(such as natural gas) and oxygen are fed which combust in refining zone 12 to
form a flame within the furnace. Each injector may comprise a single injector,
or
may comprise multiple injection nozzles or ports placed on side walls 22 from
which different gases or atomized oil can be injected. A preferred injector
has two
injection ports mounted one over the other vertically (as depicted and
described in
U.S. Patent No. 5,924,848). Alternatively, each injector 32 and 33 can inject
(uncombusted) oxygen alone, air alone, oxygen-enriched air, or a gas mixture
of
any suitable composition. When gas is injected from more than one injector,
such
as injectors 32 and 33, the gases that are injected from any injector can have
a
composition different from or the same as the gases injected from any other
injector. Optionally one or more streams of purge gas 55 through 58 is flowed
into refining zone 12 through openings placed in front wall 23 and/or side
walls
22. This purge gas stream, which is preferably oxygen, oxygen enriched air, or
air
when oxidized glass is produced, increases the oxygen concentration of the
atmosphere in refining zone 12.
In a cross-fired regenerative glassmelting furnace such as depicted in
Figure 1, the furnace gas circulation pattern in melting zone 11 is driven
principally by the momentum of combustion oxidant (air) and fuel injected into
the melting zone 11. When the present invention is not being implemented, the
combustion of oxidant and fuel in the melting zone (and the influence of the
gaseous stream 50 or other gas stream that, if present, flows into the
refining zone
12), have the effect of establishing a large recirculation gas flow pattern
between
the last pair of regenerator ports, i.e., ports 6L and 6R in Figure 1, and the
front
wall 23, circulating in a region of the melting zone and out of the melting
zone 11
into refining zone 12 and back into melting zone 11. When regenerator 41 is in
the
firing cycle the direction of the recirculation flow (shown as circle 61 in
Figure 2)
in the refining zone 12 is in the counter-clockwise direction, and the pattern
is
reversed and the direction of the recirculation flow becomes clockwise when
the
other regenerator is instead in the firing cycle. When no other gases are
injected
in the refining zone 12 the composition of the gas in this recirculation gas
flow
pattern becomes very close to that of the gaseous combustion products (i.e.
that
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are withdrawn through regenerator ports as described above) which typically
contains 1-3% 02 by volume. When cooling gas 50 flows into the refining zone
as
described herein, the composition of the atmosphere in the refining zone 12 is
determined by the mixing pattern of the cooling air flowing into the refining
zone
12 and the furnace gas circulating into the refining zone.
Figure 3 depicts the gas flow pattern when the present invention is
implemented with an opposing pair of oxy-oil burners placed on side walls 22.
Atomized fuel oil and oxygen are injected as two opposing jets at the same
time.
Instead of the flow of gases circulating throughout refining zone 12, as
depicted as
61 in Figure 2, there is very little flow of gases from melting zone 11
circulating
into refining zone 12. The flow of gases from the melting zone into the
refining
zone can be reduced by at least 10%, preferably by at least 20 or 25%, and
more
preferably by at least 40 or 50%. The amount of reduction can be determined by
comparing the oxygen content of the atmosphere in the refining zone before and
after implementation of the present invention. Implementation of the present
invention increases the oxygen content of the refining zone atmosphere,
proportionally to the degree to which the melting zone atmosphere has not been
able to flow into the refining zone and cause dilution (relative to the oxygen
content) of the refining zone atmosphere.
Application of computational fluid dynamic analysis to a typical 600
metric tpd float glass furnace (12.2 m wide x 38.2 m long in the main furnace)
of
the type depicted in Figure 1 when operated without the present invention
predicted the oxygen concentration profile of the furnace atmosphere (in vol.%
wet) near the glassmelt surface as shown in Figure 4. The local 02
concentration
in the refining zone 12 was reduced to as low as 4% in a corner formed by side
wall 22 and front wall 23 when 1,719 Nm3/hr of stream 50 (air) was flowing
into
the refining zone 12, which had about 21% 02 at the port 28 in wall 23.
Optional
purge gas streams 55-58 were not injected in this example. The low local 02
concentration in the refining zone 12 was caused by mixing with the
circulating
furnace gas which contained about 2% 02. Except for the small areas near the
port 28 in wall 23, the oxygen concentration in most of refining zone 12 was
less
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than 10%. The average oxygen concentration in the refining zone was estimated
to be about 5%. The furnace gas circulation pattern in refining zone 12 was
driven
primarily by the momentum of combustion oxidant (air) and fuel injected into
the
melting zone 11 from port 6 and port 5. The total momentum of the combustion
oxidant and fuel fired in port 6 was 5.58 kg m/s2.
Figure 5 is a graphical representation of the oxygen concentration profile
of the furnace atmosphere (in vol. % wet) near the glassmelt surface in the
furnace
of Figure 1 when operated with the embodiment of the present invention shown
in
Figure 3. An opposing pair of oxy-fuel burners of the type described in US
Patent
No. 5,601,425 were placed as injectors 32 and 33 in side walls 22 at 2.475m
from
the axis of port 6 (by which is meant the axis of ports 6L and 6R) to the axis
of the
injector in the refining zone. The firing rate of port 6 was reduced, which
reduced
the total momentum of port 6 to 3.4 kg m/s2. The total momentum of the
combustion oxidant and fuel oil and atomizing air fired from each of injectors
32
and 33 was 8.3 kg m/s2. The combustion stoichiometric ratio of fuel oil to
oxidant
plus atomizing air was set to produce combustion products with 2% excess 02 by
volume on a wet basis. The momentum ratio of (port 6 + injector 32)/(injector
33)
was 1.4 in this example.
The computational fluid dynamics model of the glass furnace found that
the lowest local 02 concentration was about 10 vol.% near a corner formed by
side wall 22 and front wall 23 of the refining zone. Except for small areas
near
the port 28 in wall 23, the oxygen concentration in most of the refining zone
is
between 10 vol.% and 16 vol.%. The average oxygen concentration in the
refining
zone was estimated to be about 14%, a surprising large increase compared to
the
average concentration of about 5% estimated for the condition depicted in
Figure
1 when operated without the present invention. Since the combustion
stoichiometric ratio of the oxy-fuel burners was set to produce excess 02 in
the
combustion product of 2% on a wet basis, simple mixing of the combustion
products from oxy-fuel burners would have reduced the average oxygen
concentration in the refining zone. Without being bound by any particular
theory,
these observations are consistent with the proposition that the jet momentum
of
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two opposing jets or flames from injectors 32 and 33 was sufficiently large
relative to that of the flame from ports 6L and 6R and, hence, reduced the
normal
circulation pattern of the gaseous combustion products from melting zone 11
into
refining zone 12, and increased the average oxygen concentration of the
atmosphere in the refining zone.
The location and momentum of each gas stream from injectors 32 and 33
are selected such that the circulation of the gaseous combustion products from
melting zone 11 into refining zone 12 is lessened and preferably minimized.
Preferably the ratio of the sum of the total momentum of port 6 and the total
momentum of injector 32 to the total momentum of injector 33 is between 0.25
and 3.0, more preferably between 0.5 and 2Ø
Since said gaseous combustion products contain a significant
concentration of alkali vapors (mostly NaOH and KOH), reduction of the
circulation of these products from the melting zone 11 into the refining zone
12
reduces the concentration of the alkali vapor in the refining zone 12 as long
as the
conditions of the refining zone is set to minimize the volatilization of
alkali
vapors. In this way the invention helps to reduce glass defects caused by
alkali
corrosion of silica-based materials of construction of the crown. It also
improves
the oxidation state of the glass by a higher average oxygen concentration in
the
refining zone and reduces glass color defects caused by a low 02 concentration
in
the refining zone. Since glass becomes more oxidized and the redox ratio is
reduced with the present invention, the invention is advantageous for the
production of highly oxidized glass such as flat glass useful e.g. for solar
panel
applications and for glass tablewares.
The present invention lessens or minimizes the mixing of the furnace
gases from melting zone 11 into the refining zone 12 and increases the purging
effect of the gas stream 50 (e.g. air) (when present, i.e. from conditioning
zone
13) and optional purge gas streams 55-58 into refining zone 12.
Instead of using two continuously flowing injectors 32 and 33 such as an
opposing pair of oxy-fuel burners, the flows from injectors 32 and 33 can be
alternated so that gas flows from only one of them at a time, with flow from
the
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single jet that is on the side of the furnace opposite to the side from which
a flame
is issuing from a port 6. The momentum of the single jet is preferably within
25 to
300%, more preferably within 50 to 200% of the momentum of the flame from
port 6. The angle of the single jet is preferably set toward the firing side
of port 6
5 or parallel to the front wall 23.
A preferred embodiment of the invention, whether injectors 32 and 33 are
injecting together or alternating, is to inject air or oxidant containing 21
to 100%
02 by volume. More preferably the oxygen concentration of the oxidant is 33 to
100 vol.% and most preferably the oxygen concentration of the oxidant is 85 to
10 100 vol.%. The gas compositions injected from injectors 32 and 33 and/or
the
stoichiometric ratios of the flames injected from injectors 32 and 33 can be
different from each other, to affect the temperature and the 02 concentration
profiles in refining zone 12. By injecting oxidant containing 02 at a
concentration
higher than the average 02 concentration in the refining zone, without
injecting
15 fuel which consumes oxygen by combustion reactions, the oxygen
concentration
in the refining zone is increased significantly by the present invention. For
example, typical average oxygen concentration of oxygen in the refining zone
of a
glass furnace making flat glass is in a range of 1% to 6% 02 by volume on a
wet
basis. A preferred embodiment of the invention, whether injectors 32 and 33
are
injecting together or alternating, is to inject oxidant to increase the
average
concentration of oxygen in the refining zone by 1 to 60% 02 by volume to
create
an atmosphere containing 2% to 60% 02 by volume on a wet basis. More
preferably air or oxidant containing 21 to 100% 02 by volume, optionally
preheated, is injected to increase the average concentration of oxygen in the
refining zone by 1 to 40% 02 by volume to create an atmosphere containing 2%
to
40% 02 by volume on a wet basis. Most preferably air or oxidant containing 21
to
100% 02 by volume, optionally preheated, is injected to increase the average
concentration of oxygen in the refining zone by 2 to 20% 02 by volume to
create
an atmosphere containing 3% to 20% 02 by volume on a wet basis. Average
concentration of oxygen in any given region, such as near the bath surface, is
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16
determined by measuring the oxygen concentration values at two or more
locations in the given region and averaging the measured values.
When a large amount of oxidant is injected, it has a cooling effect in the
refining zone. Cooling of the refining zone could accelerate the condensation
of
volatile alkali species on the furnace walls and roof in the refining zone and
potentially cause glass defects by run-down of the condensed materials into
the
glassmelt. Therefore, it is desirable to preheat the oxidant prior to
injection,
preferably within +/- 500 F of the refining zone temperature. Since the
typical
temperature of the refining zone is 2500-2900 F, it is difficult to preheat
the
oxidant up to or exceeding the temperature of the refining zone due to the
temperature limitation of conventional gas preheating systems. A preferred
method of producing a hot stream containing a high concentration of oxygen
without using a conventional indirect heat exchanger is described in U.S.
Patent
No. 5,266,024 which discloses that a small amount of fuel is combusted in-line
in
a flowing oxygen stream to produce a hot gaseous oxygen stream that can be
injected into the furnace.
In order to prevent the cooling effect, another preferred method is to inject
a small amount of fuel with a large amount of oxidant from a burner, i.e.,
under an
elevated stoichiometric condition, to produce heat and also produce an
atmosphere of high 02 concentration. For example when natural gas and oxygen
are injected at a 500% (i.e. 5:1, 02 to fuel on a molar basis) stoichiometric
ratio,
i.e., approximately at the ratio of 1 volume of natural gas to 10 volume of
pure 02,
the adiabatic flame temperature is about 3400 F and the 02 concentration in
the
combustion products becomes about 72% on a wet basis. Since the temperature
of the refining zone is typically less than 2800 F, such an example of fuel
and
oxidant gas injection under elevated stoichiometric ratio combustion
conditions
would cause a mild heating effect. By adjusting the stoichiometric ratio the
heating and cooling effects of the gas injection can be controlled to maintain
the
optimum glass quality while substantially increasing the 02 concentration of
the
atmosphere in the refining zone. A preferred range of the elevated
stoichiomeric
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ratio is 110% to 2000%. A more preferred range of the elevated stoichiomeric
ratio is 150% to 1500%. A most preferred range of the elevated stoichiomeric
ratio is 200% to 1000%.
The atmospheric conditions in refining zone 12 can be further enhanced by
optionally injecting an additional purge gas into refining zone 12 in such a
way
not to increase the furnace gas circulation from melting zone 11 to refining
zone
12. For example, additional oxygen can be injected from one or more purge gas
injectors 55-58 located in front wall 23 or in side walls 22 near front wall
23. A
preferred embodiment is to inject purge gas from injectors 55 and 56 from
front
wall 23 at proper momentums so as to reduce the furnace gas circulation from
melting zone 11, whether purge gas injectors 55 and 56 are injecting together
or
alternating. Preferably the total momentum of purge gas injected from each
injector 55 and 56 is less than that of fuel and air injected from port 6. The
purge
gas is preferably air or oxidant containing 21 to 100% 02 by volume. More
preferably the oxygen concentration of the oxidant is 33 to 100 vol. % and
most
preferably the oxygen concentration of the oxidant is 85 to 100 vol. %. The
gas
flow rates and compositions injected from purge gas injectors 55 and 56 can be
different from each other, to affect the temperature and the 02 concentration
profiles in refining zone 12.
When practicing the present invention with the optional purge gas or with
oxidant injection from injectors 32 and 33, the average excess oxygen in flue
gas
exiting the regenerator ports would increase. Injection of oxidant without
preheating, especially air, increases the furnace heat load. In order to
maintain or
improve the energy efficiency of the furnace and to minimize the emission of
NOx the fuel and combustion air flow rates of each regenerator port are
preferably
adjusted to make the oxygen concentration in the flue gas exiting each
regenerator
port at an optimum value, typically about 1 to 6 vol. %, more typically about
1 to
3vol. %. Since most of the gases injected into the refining zone exit from the
regenerator ports close to the refining zone, the fuel and combustion air flow
rates
of two to three regenerator ports are preferably adjusted to make the oxygen
concentration in the flue gas exiting each regenerator port at an optimum
value.
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When producing highly oxidized glass such as flat glass for solar panel
applications, however, it is considered advantageous to have an atmosphere of
high 02 concentration not only in the refining zone, but also in the melting
zone,
especially over the spring zone and over the area above the bubblers, if the
furnace is equipped with bubblers. In a typical six-port float glass furnace
the
spring zone is located between port 3 and port 6 in the melting zone. In such
a
case it is preferred to maintain the stoichiometric ratio of ports 4-6 at the
normal
excess air level or even increase to 110% to 120 % in order to increase the 02
concentration over ports 4-6 area. Further increases in the 02 concentration
near
the spring zone is achieved by closing port 6 firing and allowing the high 02
atmosphere of refining zone to expand to the port 5 line. Optionally both port
6
and port 5, and even including port 4 can be closed. When these ports are
closed
in order to increase the 02 concentration in the atmosphere near the spring
zone,
fuel inputs for the remaining ports and optionally the amount of fuel injected
from
for injectors 32 and 33 are increased to maintain a proper furnace temperature
profile.
