Method of melting raw materials for glass or the like with staged combustion and preheating

Raw materials such as glass batch are preheated by combustion of liquid or solid fuel such as fuel oil or coal mixed with the batch material. In one embodiment the material being heated and the gaseous products of combustion are passed cocurrently through the preheating stage to avoid carbonaceous effluents. Preheated materials and any ash from the fuel are transferred to a second stage where they are liquefied. Carbon monoxide from the preheating stage may serve as fuel for the liquefying stage.

BACKGROUND OF THE INVENTION 
This invention relates to the use of solid carbonaceous fuels such as coal 
as a fuel source in a process for making glass or similar fusion processes 
and to the use of mixtures of solid or liquid fuels with the raw 
materials. 
It is well known that in regions where coal is available it is usually the 
cheapest source of energy relative to other traditional energy sources 
such as natural gas, fuel oil, and electricity. Therefore, it has been 
suggested that coal be used as a fuel source for melting glass and the 
like. Examples of such proposals may be seen in U.S. Pat. Nos. 3,969,068 
and 4,006,003. However, the use of coal to fuel direct fired process 
furnaces has been found to have certain drawbacks that have discouraged 
its widespread use. A major drawback is the ash content of coal. When coal 
is combusted with an overhead burner in an open hearth type furnace 
conventionally employed to melt glass, substantial amounts of ash are 
entrained in the exhaust gas stream which can cause the regenerators to 
become plugged and which necessitates removal of the ash from the exhaust 
gas before it can be discharged to the atmosphere. Some of the ash becomes 
deposited on the walls of the melting chamber where it melts to a liquid 
slag that runs down the walls of the vessel into the melt. The runnage of 
molten slag has a deleterious affect on the refractories of the furnace, 
and the molten slag entering the melt introduces unwanted compositional 
variations and inhomogeneities into the product material. The slag often 
has a high iron content relative to glass, and runnage of the slag into 
the melt can cause undesirable streaks of coloration. These problems have 
discouraged the use of coal as a direct fuel for melting products for 
which uniformity of composition is an important consideration. This is 
particularly the case with flat glass, where compositional variations 
cause optical distortion in the product glass. 
A drawback to the use of coal or other carbonaceous fuel in admixture with 
the raw materials, particularly when melting clear glass, is that carbon 
in amounts sufficient to provide significant energy to the melting process 
also has a reducing effect on the melt, and iron and sulfur present in a 
reduced glass cause brown coloration. Moreover, coal itself contributes 
iron and sulfur to the melt. Small amounts of powdered coal (typically 
less than 0.1% by weight) have been included in clear glass batch to aid 
the melting process, but such amounts are not significant energy sources, 
and larger amounts were considered detrimental. Even when brown glass is 
being produced, the amount of carbon employed would not be considered a 
significant fuel contribution. 
U.S. Pat. No. 3,294,505 discloses melting glass in a bed of batch 
briquettes and coke. The process is restricted to a relatively narrow 
group of low viscosity glass compositions for low quality applications. 
Additionally, it would be desirable to avoid the cost of agglomerating the 
batch. 
In commonly assigned, copending U.S. application Ser. No. 624,879 filed 
June 27, 1984, now U.S. Pat. No. 4,551,161, there is disclosed a technique 
of wetting glass batch with fuel oil. Only a minor portion of the energy 
requirement of the melting process is supplied by the fuel oil. 
Another problem with using coal and some other carbonaceous fuels is that 
such fuels contain relatively volatile hydrocarbon fractions that are 
driven off and escape with the exhaust gas if heated before ignition. This 
is a problem particularly if it is desired to preheat raw materials in 
admixture with carbonaceous fuel. Also, feeding carbonaceous fuels in a 
non-atomized form to a combustion zone generally produces smoke-laden 
exhaust that is environmentally undesirable. After-burning or otherwise 
treating the exhaust gas or carbonizing the fuel in a preliminary 
operation are costly options that are preferably avoided. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention fuel having an ash content (e.g., 
coal) is employed as a substantial energy source for a melting process 
while avoiding the problems usually associated with the ash. The 
ash-producing fuel is combusted in a discrete batch preheating stage of 
the melting process where the ash becomes incorporated into the batch 
material. Preferably the batch material and the fuel are fed to the 
preheating stage in admixture with each other to establish distribution of 
the ash throughout the batch and to provide intimate contact between the 
batch and the fuel during combustion. Combustion is sustained by feeding 
an oxidant (preferably substantially pure oxygen) to a zone of combustion 
in the preheating stage. Agitation of the material in the preheating stage 
may be provided to enhance contact between the batch and the combusting 
fuel and to assist in mixing the ash into the batch. The heated batch and 
ash mixture, preferably still in a pulverulent state, is passed to a 
subsequent stage where the mixture is liquefied. 
A significant portion of the overall energy required for melting can be 
provided by means of an economical fuel such as coal by thus preheating 
the batch material to a temperature just below the temperature at which 
significant fusing occurs. By feeding a nearly homogeneous mixture of 
batch and ash to the liquefying stage, the melt that is subsequently 
formed can be essentially uniform in composition even though substantial 
quantities of ash have been introduced. Therefore, one of the problems 
associated with the use of an ash-producing fuel such as coal is 
substantially alleviated. 
By combusting the fuel while in contact with the batch material and thus 
avoiding ash entrainment in the exhaust and slag formation on the interior 
surfaces of the vessel, environmental problems and deterioration of the 
furnace are avoided, which is desirable for any melting process. But the 
avoidance of slag runnage into the melt makes the present invention 
particularly attractive for the melting of glass and the like where 
compositional homogeneity is important. Even relatively viscous, difficult 
to homogenize glass such as soda-lime-silica flat glass, for which there 
are very high standards of optical quality, can be produced by the present 
invention. It is also an advantage that no agglomeration of the batch is 
required. 
In yet another aspect of the invention, the oxygen supply and temperature 
at the preheating stage may be controlled so as to produce substantial 
amounts of carbon monoxide as a product of the combustion of the fuel. The 
carbon monoxide is passed to a subsequent stage such as the liquefaction 
stage where it serves as at least part of the fuel for combustion in that 
stage. In another alternative, the first stage combustion may be 
incomplete, whereby some of the fuel may be permitted to remain 
uncombusted so that it may be passed along with the batch to the second 
stage to serve as at least part of the fuel there. 
To permit the use of higher temperatures in the preheating stage and to 
increase the amount of carbon monoxide produced, the constituents of glass 
batch that fuse at relatively low temperatures may be omitted from the 
first stage and introduced at the second stage or a subsequent stage. 
Excluding the sodium source (e.g. soda ash) from a soda-lime-silica glass 
batch permits the use of temperatures in the first stage that are 
sufficiently high to calcine carbonate source materials such as limestone 
and dolomite as well as to produce carbon monoxide. Treating the silica 
source material (sand) alone in the first stage enables the first stage to 
operate at very high temperatures that can yield large amounts of carbon 
monoxide exhaust. 
Following the liquefaction stage, a subsequent third stage may be provided 
in which the melting process may be carried further. When fuel is mixed 
with the batch, there may be incomplete contact between the fuel and the 
oxidant in the liquefaction stage, or there may be excess fuel present, 
and thus the liquefied material may exit the second stage in a reduced 
state. In that case, the third stage may also include means to re-oxidize 
the melt, for example, by means of submerged combustion with an 
oxygen-rich flame and/or by bubbling an oxidizing agent (preferably 
oxygen) through the melt. Re-oxidizing is particularly useful for avoiding 
discoloration of clear glass. Undesired coloration of clear glass by iron 
and sulfide ions can be avoided by re-oxidizing the melt in the third 
stage. This aspect of the invention relates to commonly assigned U.S. 
patent application Ser. No. 748,639 filed on even date herewith by Gerald 
E. Kunkle, Henry M. Demarest, Jr., and Larry J. Shelastak entitled 
"Melting Raw Materials for Glass or the Like Using Solid Fuels or 
Fuel-Batch Mixtures." 
The chemical constituents of coal ash are generally compatible with those 
for most glasses, and therefore the glasses can incorporate some of the 
ash with little or no detrimental affect on the glass product, provided 
that the ash can be thoroughly homogenized in the melt. However, the 
amount of ash produced when coal constitutes the major fuel source for a 
conventional melting process is difficult to adequately homogenize for 
some types of glass for which optical requirements are critical. 
Therefore, it is an advantage of the present invention that coal is 
employed as the fuel in a discrete stage of the overall melting process so 
that mixing of the batch with the ash is provided prior to liquefying. 
Also, in the discrete preheating stage less than the total energy 
requirement need be provided so that less coal is required and less ash is 
produced. Moreover, the overall efficiency of the staged melting process 
has been found to reduce the overall energy requirements for melting 
glass, further reducing fuel requirements. As a result, coal may 
constitute a major portion or the entire energy source for preheating even 
flat glass batch up to the fusion temperature. In some modes of operation 
the coal may constitute the major or entire energy source for the entire 
liquefying operation. 
The novel fuel arrangements of the present invention may constitute the 
entire fuel source or may supplement conventional heat sources. The 
portion of the total thermal energy requirement of the preheating stage 
contributed by the novel arrangements is substantial; that is, greater 
than that provided by prior art inclusion of carbonaceous material as a 
melting aid, coloring agent, or binder. It is believed that contributing 
as much as 5 percent of the energy is uncharacteristic of these prior art 
uses of carbonaceous materials in a melter. For economic reasons, it is 
preferred that the novel fuel usage of the present invention be maximized 
so that it supplies a majority of the energy through the preheating stage, 
and optimally all of the energy. 
Another feature of preferred embodiments of the present invention is the 
suppression of exhaust emissions of products of incomplete combustion such 
as smoke, soot, or substances volatilized from the fuel. As batch material 
in contact with solid or liquid fuel is conveyed toward a heated zone, the 
temperature of the fuel gradually increases and the fuel begins to give 
off volatiles and to smolder before being fully ignited. The resulting 
objectionable emissions are substantially eliminated by this aspect of the 
present invention by maintaining the flow of gas in the preheating stage 
in generally the same direction as the conveyance of the batch-fuel 
mixture through the preheating stage. This co-current flow pattern carries 
the emissions from the early stages of heating into the combustion zone, 
where the combustible emissions are incinerated. Not only are the 
objectionable emissions eliminated, but also their combustion contributes 
to the heating of the batch materials. The exhaust gas from the preheating 
stage may be subjected to further combustion by passing it to a subsequent 
combustion zone such as in the liquefying stage. 
Other environmental advantages also result from the invention. The 
stagewise approach lends itself to the use of oxygen instead of air to 
support combustion. The elimination or reduction of the amount of nitrogen 
in the combustion gases reduces the amount of nitric oxides (NO.sub.x) 
produced. Exhaust gas volumes are considerably reduced when using oxygen 
firing, thereby reducing gas velocity, which in turn yields less 
entrainment of particulate batch material. The absence of nitrogen also 
produces a higher flame temperature. The use of essentially pure oxygen 
and the exclusion of all air maximizes these advantages, but the 
advantages can be partially realized in accordance with the degree to 
which the oxygen concentration exceeds that of air. 
Another environmental advantage is that some of the sulfurous emissions 
usually associated with the combustion of sulfur-containing fuels such as 
coal may be suppressed. Contact between the combustion gases and the batch 
material (particularly glass batch containing limestone or the like) may 
remove sulfur oxides from the gas stream. 
The invention will be more fully understood from the drawings and the 
description which follows.

DETAILED DESCRIPTION 
The detailed description of the invention is made with reference to an 
example of a glass melting operation for which the invention has been 
found to be particularly useful. The invention is useful with all types of 
glass, including flat glass, container glass, fiber glass, and sodium 
silicate glass. 
However, it should be understood that the invention is applicable to the 
melting of other, similar materials and in particular to the conversion of 
mineral-type materials to a molten state. Other examples include: fusing 
of glassy and ceramic materials, melting of frits, and smelting of ores. 
The first stage may take the form of a variety of gas/solid contact 
devices, but the preferred embodiment is a rotary kiln 1 as shown in FIG. 
1. Alternative devices include a fluidized bed and a cyclonic 
separator/contactor as are known in the art. The rotary kiln comprises a 
cylindrical shell 2 rotatably supported on rolls 3 at a slight angle from 
horizontal. A single-walled metal shell as shown in the drawing may be 
adequate, or better thermal efficiency may be attained by means of a 
refractory lining or a double walled metal shell with insulation between 
the walls. 
A stationary inlet housing 4 closes the inlet end of the rotary kiln. Feed 
duct 5 extends through the housing wall for directing pulverulent batch 
material into the rotary kiln from a feed rate control device 6. The batch 
B may be mixed with fuel prior to being fed to the rotary kiln, or fuel 
and batch may be fed separately to the rotary kiln where they become 
mixed. Oxidizing gas (e.g., air, but preferably oxygen) may be fed to the 
rotary kiln by a conduit 7 extending through the wall of inlet housing 4. 
The conduit 7 may project into the rotary kiln a sufficient distance to 
establish the combustion zone some distance downstream from the batch feed 
area. Batch materials that include oxygen-containing compounds such as 
carbonates may contribute some of the oxygen for supporting the 
combustion. This is advantageous because carbon dioxide is removed before 
the batch is liquefied. After liquefying, release of the carbon dioxide 
would have produced bubbles in the melt that are difficult to eliminate. 
In the preferred embodiment the products of combustion flow cocurrently 
with the batch through the rotary kiln into the second stage liquefying 
means 10 by way of an outlet enclosure 36 that joins the two stages. 
Ignition may be initiated in the combustion zone by auxiliary heating means 
such as a burner temporarily inserted into the kiln. Once ignition of the 
fuel in contact with the batch is established, the combustion zone can be 
maintained at a substantially fixed region of the rotary kiln by balancing 
the oxygen feed rate and the rate at which the batch and fuel are conveyed 
along the rotary kiln. The latter rate is essentially controlled by the 
speed with which the inclined kiln is rotated. The solid materials and the 
gas streams move cocurrently through the rotary kiln so that volatile 
materials initially driven from the fuel are carried into the combustion 
zone where they are incinerated. 
Although not preferred, gas and batch could flow countercurrent to each 
other in the rotary kiln or other preheater. In that case, it may be 
necessary to provide means to render the exhaust environmentally 
acceptable, such as a bag collector for particulates. A portion of the 
exhaust may be recycled to a combustion zone in either the preheater or 
the liquefaction stage so as to eliminate combustibles. Another technique 
to treat the exhaust gas and to recover waste heat is to pass the exhaust 
into contact with the batch material in an additional, preliminary 
preheating stage. A batch mixture that includes carbonates (e.g., 
limestone) is also useful in stripping sulfur oxides from the exhaust. 
The specific preferred embodiment of the second stage 10 is shown in FIG. 2 
and is in accordance with the teachings of U.S. Pat. No. 4,381,934 of 
Kunkle et al. and of U.S. patent application Ser. No. 661,267 filed Oct. 
16, 1984 also of Kunkle et al., the teachings of which are hereby 
incorporated by reference. The second stage is adapted to liquefy the 
batch and is characterized by a sloped melting surface to receive batch 
materials that melt as a thin layer on the sloped surface and quickly 
drain therefrom when liquefied. The liquefaction stage 10 shown here is a 
preferred embodiment of the Kunkle et al. teachings wherein the sloped 
surface substantially encircles a central cavity and the vessel rotates 
about a substantially vertical axis. The circular arrangement offers 
distinct advantages for the present invention and for the efficiency of 
the melting process in general, but it should be understood that the 
present invention in its broader aspects is not limited to the circular 
liquefaction arrangement. 
By separating the liquefaction step from the remainder of the melting 
process, energy is employed more efficiently in each stage of the process 
by optimizing the conditions in each stage to meet the particular needs of 
the step being performed there. Additional efficiencies are achieved by 
encircling the heated zone with the batch material and by employing an 
insulating layer of the batch material or a compatible substance to 
thermally insulate the liquefaction zone. Because of the overall energy 
efficiency of the stagewise process, and because only a portion of the 
overall energy requirement for melting is consumed in the liquefaction 
zone, it has been found that the amount of energy consumed in the 
liquefaction stage is relatively low and a variety of heat sources can be 
used efficiently. Combustion of fuel, particularly with oxygen firing, is 
preferred, and electrical sources such as electric arc or plasma torch may 
be used. Coal or other solid fuel may constitute a portion or all of the 
fuel in the second stage, some of which may be unburned fuel from the 
first stage. When carbon monoxide is produced in the first stage, the 
exhaust from the first stage may be passed to the second stage where it 
may supply a substantial portion of the energy requirement there. 
With reference to FIG. 2, the liquefaction stage 10 includes a generally 
cylindrical vessel 12 which may consist of a steel drum. The vessel 12 is 
supported on a circular frame 14 which is, in turn, mounted for rotation 
about a generally vertical axis corresponding to the center line, or axis 
of symmetry, of the vessel on a plurality of support rollers 16 and 
aligning rollers 18. A bottom section 20 of the vessel holds an axially 
aligned annular bushing 22 defining a central drain opening 24. The 
bushing 22 may be comprised of a plurality of ceramic pieces, and the 
bottom section 20 may be detachably secured to the remainder of the vessel 
12 so as to facilitate changing the bushing 22. 
A refractory lid 26, preferably in the configuration of an upward dome, is 
provided with stationary support by way of a surrounding frame member 28. 
The lid 26 may include at least one opening through which may be extended 
at least one cooled gas supply conduit 30. The supply conduit 30 may 
constitute a burner or merely a supply conduit for oxygen or other 
oxidizing agent to support combustion of the fuel being supplied to the 
liquefaction chamber. If fuel is being supplied from the first stage, the 
conduit 30 may be used to supply oxygen or the like to the vessel after 
the ignition temperature has been achieved. Optionally, a portion of the 
heat for the liquefaction stage may be supplied by a conventional burner 
or other heat source in addition to the energy being provided by fuel from 
the first stage. The conduit 30 may be centrally located as shown to flood 
the entire cavity with oxygen, or it may be angled or located off-center 
to direct the oxygen and/or fuel onto the melting layer. 
An opening 32 through the lid 26 may be provided for feeding the batch to 
the liquefaction stage, and, as shown in FIG. 2, an outlet enclosure 36 at 
the end of the rotary kiln 1 may be provided with a chute portion adapted 
to direct material into the liquefaction stage. An adjustable baffle 38 
may be provided at the end of the chute to direct the flow of batch onto 
the sidewalls of the vessel 12. 
Preferably, a stable layer of pulverulent material 40 lines the interior of 
the vessel 12. This layer acts as the insulating lining to protect the 
vessel 12 from the heat within the vessel. In those applications where it 
is desired to avoid contamination of the product material, the layer 40 is 
preferably of substantially the same composition as the batch material. 
Before the melting process is started, the stable lining 40 is provided in 
the melter by feeding loose pulverulent material such as the batch 
material into the vessel 12 while the vessel is rotated. The loose 
material assumes a generally parabolic contour as shown in FIG. 2. The 
pulverulent material may be wetted, for example, with water during the 
initial stage of forming the stable lining to facilitate cohesion of the 
layer along the sidewalls. When the lining 40 is comprised of batch 
material, it need not include the fuel component that may be mixed with 
the batch during operation. Other minor differences between the lining 
material and the throughput material may be acceptable, depending upon the 
requirements of the particular process. 
During the melting process, continuous feeding of batch to the liquefaction 
stage 10 results in a falling stream of batch that becomes distributed 
over the face of the stable lining 40, and by the action of the heat from 
combustion within the vessel 12 becomes liquefied in a transient layer 42 
that runs to the bottom of the vessel and passes through the open center 
24 in the bushing 22. The liquefied material 44 falls from the first stage 
10 into the second stage 11 for further processing. In this manner, the 
initial step of liquefying the batch can be efficiently carried out 
because the material, once it has become liquefied, immediately is removed 
from the vicinity of the heat source and is continuously replenished with 
fresh batch material, thereby maintaining a large temperature difference 
and therefore a high rate of heat transfer in the liquefaction vessel. The 
constant replenishment with relatively cool, fresh batch in cooperation 
with the insulating lining serves to preserve the structural integrity of 
the liquefaction vessel without the use of forced cooling of the vessel. 
The material for the lining 40 provides thermal insulation and preferably 
also serves as a non-contaminating contact surface for the transient 
melting layer 42 and, most preferably, the stable lining includes one or 
more constituents of the batch material. It is desirable for the thermal 
conductivity of the material employed as the lining to be relatively low 
so that practical thicknesses of the layer may be employed while avoiding 
the need for wasteful forced cooling of the vessel exterior. In general, 
granular or pulverulent mineral source raw materials provide good thermal 
insulation, but in some cases it may be possible to use an intermediate or 
product of the melting process as a non-contaminating, stable layer. For 
example, in a glassmaking process pulverized cullet (scrap glass) could 
constitute the stable layer, although a thicker layer may be required due 
to the higher thermal conductivity of glass as compared to glass batch. In 
metallurgical processes, on the other hand, using a metallic product as 
the stable layer would entail unduly large thicknesses to provide thermal 
protection to the vessel, but some ore materials may be satisfactory as 
insulating layers. 
The preferred embodiment of the liquefaction stage described above entails 
rotating the lining about the central cavity, but it should be understood 
that the present invention is applicable to embodiments in which the 
lining encircles the heated cavity but is not rotated. Additionally, the 
invention is applicable to embodiments in which the lining is a sloped 
surface, but does not encircle the heat source (e.g., melting takes place 
on a ramp). Examples of such variations are described in the aforesaid 
Kunkle et al. patent and application. 
Air could be used as the oxidant, but it is preferred to use oxygen (i.e., 
a higher concentration of oxygen than in air) so as to reduce the volume 
of gaseous throughput. As a result, the equipment in both first and second 
stages may be made compact since the exhaust gas stream is relatively low 
in volume and high in temperature. Also, eliminating nitrogen from the 
system increases the emissivity of the flame and therefore increases heat 
transfer. The intense heat of combustion supported by oxygen firing is 
compatible with the preferred embodiments of the second stage because of 
the thermal protection and efficient heat transfer afforded by the 
encircling lining. 
The temperature attained in the preheating stage depends upon the amount of 
combustion, which in turn depends upon the amounts of fuel and oxygen 
provided. Even a small amount of combustion is useful for the sake of the 
heat it transfers to the batch materials. Preferably, the amount of heat 
generated by the combustion in the first stage is enough to produce a 
maximum temperature increase of the batch without fusing batch ingredients 
to the extent that the batch is no longer free-flowing. For example, a 
typical flat glass batch mixture containing substantial amounts of soda 
ash would be essentially restricted to temperatures below the melting 
point of soda ash (851.degree. C.), preferably lower, to avoid clogging of 
the rotary kiln. In one option, the relatively low temperature melting 
ingredients of the batch may be omitted from the batch being fed to the 
first stage, but may be fed directly to the second stage, thereby 
permitting higher temperatures to be attained in the first stage. 
Advantageously, at temperatures above about 870.degree. C. calcium 
carbonate and magnesium carbonate, typical ingredients of a glass batch, 
are calcined, i.e., they decompose and release carbon dioxide. Eliminating 
the carbon dioxide while the materials are still in the solid state is 
advantageous because it avoids production of carbon dioxide bubbles in the 
melt. Even higher temperatures can be permitted in the preheater if the 
material being heated in that stage is restricted to the highest 
temperature melting portions of the batch and the remainder of the 
ingredients are fed directly to a downstream stage. For example, heating 
only sand in the rotary kiln would permit preheat temperatures in excess 
of 1000.degree. C. to be attained. Separate preheating facilities may be 
provided for any of the materials that bypass the first stage. Some of the 
glass batch ingredients such as soda ash or caustic soda may be fed to the 
second stage in molten form. It may also be preferred to feed cullet 
directly to the second stage when the first stage is operated at 
relatively high temperatures, in which case the cullet may be preheated by 
contact with exhaust gas. 
At most operating temperatures the first stage combustion can produce some 
carbon monoxide if insufficient oxygen is provided to complete combustion 
of the fuel. Thus, the exhaust from the first stage can be passed to the 
second stage where the carbon monoxide content serves as part or all of 
the fuel for the second stage when combusted with additional oxygen. The 
proportion of carbon monoxide in the products of combustion increases and 
the proportion of carbon dioxide decreases at higher temperatures. 
Therefore, to produce predominating proportions of carbon monoxide to fuel 
the second stage, it is preferred that the first stage be operated at a 
peak temperature greater than about 900.degree. C. When supplied with 
sufficient fuel and a deficiency of oxygen, the entire fuel requirement 
for the second stage can be supplied by carbon monoxide from the first 
stage. Combustion of fuel to carbon monoxide releases approximately one 
third of the heat content of the fuel, the remainder being released upon 
combustion of the carbon monoxide to carbon dioxide. Therefore, the 
relative energy requirements of the first and second stages should be 
taken into account when selecting the amount of carbon monoxide to be 
produced in the first stage. For example, glass batch is capable of 
utilizing twice as much energy in the preheating stage as in the 
liquefying stage, so that producing only carbon dioxide in the first stage 
may not be the most efficient use of the energy. When preheating a 
complete flat glass batch mixture, a preferred distribution of the heat 
content of the fuel can be achieved at an output from the first stage of 
approximately 50% carbon monoxide and 50% carbon dioxide (molar basis). 
The ability to employ coal is an advantage of the present invention because 
of the abundant supply and relative low cost of coal in some regions. But 
other solid or liquid carbonaceous fuel materials may be used to advantage 
in the present invention, for example, fuel oil, coke fines, petroleum 
coke, peat, lignite, oil shale, sawdust, bagasse, and paper waste. Liquid 
petroleum products such as fuel oil also have the advantage of wetting the 
batch so as to suppress dust formation and entrainment in the exhaust gas 
stream. 
For economic reasons, coal is the preferred fuel, in particular, bituminous 
coal. The heating value of a typical Pennsylvania bituminous coal is 
generally in the range of 11,000 to 15,000 BTU per pound (25.5 million to 
34.8 million joules per kilogram) with an ash content ranging from about 3 
percent to 9 percent by weight depending upon the source. To melt glass in 
a conventional, efficiently operated, overhead fired regenerative furnace 
burning natural gas or fuel oil is generally considered to consume at 
least about 6 million to 7 million BTU per ton (7 million to 8 million 
joules per kilogram) of glass produced. Taking a typical Pennsylvania coal 
as an example, with a heat value of about 13,800 BTU per pound (32 million 
joules per kilogram) and an ash content of about 7 percent by weight, 
combustion of such a coal in a conventional glass melting furnace to meet 
the entire energy requirements of melting would yield an unacceptably 
large amount of ash. The liquefaction process described above has been 
found to consume from about 2 million to about 3 million BTU's per ton 
(2.3 million to 3.5 million joules per kilogram) of throughput. At that 
level of energy consumption, much less coal is required to supply the 
energy needs, and therefore the ash introduced into the melt from the coal 
is at acceptable levels even for producing glass of the high quality level 
required for flat glass. 
The amount of coal to be utilized will depend upon the temperature to be 
achieved in the preheating stage and the heat content of the particular 
coal, which in turn is a function of its fixed carbon content. Because 
combustion may not be complete due to inaccessibility of oxygen to all 
parts of the coal, adding slightly more coal than is theoretically 
required may be preferred. By way of example, about 2% to 3% by weight of 
the Pennsylvania coal described above mixed with a flat glass batch 
mixture has been found to preheat the batch to about 550.degree. C. to 
650.degree. C. when combusted with excess oxygen. The amount of carbon 
monoxide produced in such a case is small. In another example, a flat 
glass batch with the soda source (e.g., soda ash) omitted from the first 
stage (thus comprised chiefly of sand, limestone, and dolomite), having 
about 6% to 10% by weight of coal mixed therewith, is preheated to about 
1100.degree. C. to 1300.degree. C. when combusted. A substantial amount of 
the limestone and dolomite are calcined, and if a limited amount of oxygen 
is supplied to the combustion zone, carbon monoxide predominates over 
carbon dioxide in the combustion product stream. Other carbonaceous fuel 
materials may be substituted for coal in amounts determined by their 
respective heat contents. 
It should be understood that while the fuel in contact with the batch 
advantageously provides at least the majority, and preferably all, of the 
energy required for the preheating stage, the advantages of the present 
invention can be obtained by degrees with lesser amounts of fuel fed with 
the batch. In such a case, some of the energy may be provided by 
conventional burner means heating the preheating stage. In embodiments 
where the gas flow is countercurrent to batch flow in the preheater, 
exhaust gas passed from the liquefaction stage to the preheater may 
provide some energy for preheating. 
The solid fuels such as coal to be mixed with the batch are preferably 
finely divided. The coal for example, is preferably no coarser than 60 
mesh (U.S. standard sieve size) and 200 mesh coal has been found to be 
particularly satisfactory. The ignition point of coal varies somewhat, but 
oxidation of a typical bituminous coal may begin at about 170.degree. C., 
and combustion is generally self-sustaining at temperatures above 
250.degree. C. when supplied with pure oxygen. 
The following is a typical ash content from 25 parts by weight of coal: 
______________________________________ 
SiO.sub.2 1.2 parts by weight 
Al.sub.2 O.sub.3 0.6 
Fe.sub.2 O.sub.3 0.27 
CaO 0.1 
Na and K 0.5 
______________________________________ 
It can be seen that these ash constituents are compatible with the 
composition of soda-lime-silica flat glass which may have the following 
composition: 
______________________________________ 
SiO.sub.2 72-74% by weight 
Al.sub.2 O.sub.3 0-2 
Na.sub.2 O 12-15 
K.sub.2 O 0-1 
MgO 3-5 
CaO 8-10 
Fe.sub.2 O.sub.3 0-0.2 
SO.sub.3 0-0.5 
______________________________________ 
Soda-lime-silica glass of the above type usually has a viscosity of at 
least 100 poises at a temperature of 1425.degree. C. 
The temparature at which the batch liquefies in the second stage will 
depend upon the particular batch materials, especially the amount and 
melting temperature of its lowest melting temperature ingredients. With 
glass batch, the most common low temperature melting ingredient is soda 
ash which melts at 1564.degree. F. (851.degree. C.). In practice, it has 
been found that commercial flat glass batch formulas liquefy at a somewhat 
higher temperature, about 2,000.degree. F. (1090.degree. C.) to about 
2100.degree. F. (1150.degree. C.). Heat within the liquefaction stage may 
raise the temperature of the liquefied material slightly higher before it 
drains from the stage, and thus liquefied glass batch flowing from the 
liquefaction stage 10 may typically have a temperature on the order of 
about 2300.degree. F. (1260.degree. C.) but usually no higher than 
2400.degree. F. (1320.degree. C.). Such a temperature and the short 
residence time in the liquefaction vessel are seldom adequate to fully 
complete the complex chemical and physical reactions involved in the 
melting process. Accordingly, the liquefied material is transferred to a 
third or "refining" stage 11 in which the melting process is furthered. 
For glass, treatment in the refining zone typically entails raising the 
temperature of the liquefied material to facilitate melting of residual 
sand grains and to drive gaseous inclusions from the melt. A peak 
temperature of about 2500.degree. F. (1370.degree. C.) to about 
2800.degree. F. (1510.degree. C.) is considered desirable for refining 
flat glass. Another desirable operation that may be carried out in this 
stage is to homogenize the molten material by agitation. Also, when the 
batch is liquefied under reducing conditions, resulting in the molten 
material entering the refining stage in a reduced condition, re-oxidation 
of the melt may be required for some end uses. Therefore, a function of 
the refining stage in the present invention can be the introduction of an 
oxidizing agent into the melt. All of these objectives are achieved by the 
preferred embodiment shown in FIG. 2. The vigorously stirred refining 
stage is well adapted not only for adjusting the oxidation state of the 
melt, but also for adding colorants, cullet, or compositional modifiers 
that are relatively easily melted. Great flexibility for making a wide 
variety of products is thus provided. 
The preferred embodiment of the refining stage as shown in FIG. 2 employs 
submerged combustion in two chambers. A single-chambered refining stage 
may suffice for some applications, but for flat glass the preferred 
embodiment entails two submerged combustion chambers 50 and 52, each 
retaining a pool 53 and 54, respectively, of the molten material. The 
chambers may be provided with oxygen bubbler tubes 55 and 56 and 
water-cooled burners 57 and 58 below the level of the molten material. A 
submerged throat 59 permits material to flow from chamber 50 into chamber 
52. An opening 60 at the top of chamber 50 permits the molten material 44 
to fall from the liquefaction stage 10 into the chamber 50. Exhaust from 
the liquefier 10 and the first stage 1 may enter the refiner through an 
opening 60. An exhaust opening (not shown) may be provided in the upper 
portion of chamber 50. In chamber 52 an opening 62 is provided in the 
upper portion thereof for the escape of exhaust gases. 
Fuel such as natural gas and an oxidant, preferably oxygen, are fed to the 
burners 57 and 58 and combustion occurs as the gas streams enter the 
molten pools 53 and 54. Another fuel which may be used to advantage in the 
submerged combustion burners is hydrogen because its product of combustion 
is water, which is highly soluble in molten glass. Employing oxygen as the 
oxidant is advantageous because it avoids introducing into the melt the 
major nitrogen constituent of air, which has poor solubility in molten 
glass. Using undiluted oxygen also improves contact between the oxygen and 
the reduced species in the melt. An excess of the oxidant may be provided 
to the burners beyond that required for combustion of the fuel so as to 
correct the reduced condition of the liquefied material entering the 
refining stage. Alternatively, if the liquefied material entering the 
refining stage includes a sufficient amount of uncombusted carbon, or if 
the temperature of the melt need not be increased, the oxidant alone may 
be injected into the molten pools 53 and 54 so as to provide the 
re-oxidizing function only. The oxidant may be introduced separately from 
the submerged combustion burners, such as through bubbler tubes 55 and 56. 
It has been found advantageous to use bubblers in combination with 
submerged combustion. The bubblers can be adapted to inject a stream of 
small bubbles of oxidant into the melt, which enhances the surface area of 
contact between the melt and the oxidant gas, and the submerged combustion 
provides vigorous agitation to mix the oxidant bubbles throughout the 
molten mass. The submerged combustion also provides very effective 
homogenization of the melt. 
The amount of excess oxidant to be supplied to the refining stage will vary 
depending upon the particular conditions encountered and will depend upon 
the degree of reduction of the material entering the stage and the 
oxidation state desired for the final product. The degree of agitation, 
the vessel size and configuration, the effectiveness of the gas-liquid 
contact, and the residence time within the refining stage are factors in 
achieving re-oxidation. In order to achieve homogeneous re-oxidation to 
meet the standards for flat glass, it has been found preferable to carry 
out the re-oxidation in two sequential chambers as shown in the drawing, 
thereby providing greater assurance that each portion of the throughput is 
subject to oxidizing conditions during an adequate residence time. In 
glass, a reduced condition yields a brown colored glass due to the 
presence of sulfur in the sulfide state and iron. If clear glass is 
desired, re-oxidation is carried out to sufficiently raise the oxidation 
state of the coloring ions, typically expressed in terms of the Fe.sup.+3 
/Fe.sup.+2 ratio. For a standard commercial grade of clear float glass the 
Fe.sup.+3 /Fe.sup.+2 ratio is in the range of about 1.5 to 3.0, with a 
transmittance of at least 70% (preferably at least 80%) to light having a 
wavelength of 380 nanometers at a thickness of 6 millimeters. Clear float 
glass may sometimes also be characterized by at least 60 percent 
transmittance at 1000 nanometers (6 millimeter thickness). Fe.sup.+3 
/Fe.sup.+2 ratios considerably greater than the above have been achieved 
by bubbling oxygen into molten glass that was initially dark brown. The 
change in coloration from brown to clear upon oxidation is readily 
observable, so that the appropriate degree of oxidation can be easily 
estimated by visual observation. Although coal may contribute excess iron 
to the melt, a clear glass can be obtained by re-oxidizing. But precise 
spectral matching of standard float glass transmittance may require 
reducing the amount of iron that is usually deliberately included in the 
batch (usually as rouge) for coloration. 
Downstream from the re-oxidizing chambers, there may be provided a 
conditioning chamber 64 as shown in the drawing in which additional 
residence time may be provided for the escape of gaseous inclusions from 
the melt and for the melt to cool to a temperature suited for subsequent 
processing. The molten material may enter the conditioning chamber 64 
through a submerged throat 66. In the arrangement shown, residence time 
within the chamber 64 is extended by means of a submerged dam 67 and a 
skim barrier 68 which establish a tortuous path for the melt stream. The 
processed molten material may be drawn from the refining stage 11 through 
a canal 70 which may lead to a forming process or the like, which, in the 
case of glass, may form the glass into a sheet, fibers, bottles or the 
like by known means. 
The detailed description of this invention has been set forth in connection 
with a best mode, but it should be understood that other variations and 
modifications that would be evident to those of skill in the art may be 
employed within the spirit and scope of the invention as defined by the 
claims which follow.