U.S. patent application number 11/394229 was filed with the patent office on 2007-10-04 for method and apparatus for preheating glassmaking materials.
Invention is credited to Hisashi Kobayashi, Kuang Tsai Wu.
Application Number | 20070227191 11/394229 |
Document ID | / |
Family ID | 38441757 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070227191 |
Kind Code |
A1 |
Kobayashi; Hisashi ; et
al. |
October 4, 2007 |
Method and apparatus for preheating glassmaking materials
Abstract
Heat in a stream of combustion products obtained from a
glassmelting furnace heated by oxy-fuel combustion is passed to
incoming glassmaking materials in a heat exchanger without
requiring reduction of the temperature of the stream yet without
causing softening of the glassmaking material.
Inventors: |
Kobayashi; Hisashi; (Putnam
Valley, NY) ; Wu; Kuang Tsai; (Williamsville,
NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
38441757 |
Appl. No.: |
11/394229 |
Filed: |
March 31, 2006 |
Current U.S.
Class: |
65/134.4 |
Current CPC
Class: |
F28F 1/02 20130101; F28D
2021/0045 20130101; F28D 7/0008 20130101; Y02P 40/50 20151101; Y02P
40/55 20151101; F28D 7/00 20130101; C03B 3/023 20130101 |
Class at
Publication: |
065/134.4 |
International
Class: |
C03B 5/237 20060101
C03B005/237 |
Claims
1. A glassmelting method comprising (A) passing heated glassmaking
material into a glassmelting furnace; (B) combusting fuel with
oxidant having an overall average oxygen content of at least 35
vol. % oxygen to produce heat for melting said heated glassmaking
material in said glassmelting furnace and produce hot combustion
products having a temperature greater than 1800.degree. F.; (C)
withdrawing said hot combustion products from said glassmelting
furnace and feeding said hot combustion products into a first
passageway of a heat exchange unit, wherein the temperature of said
hot combustion products entering said first passageway is at least
1800.degree. F.; (D) flowing said hot combustion products through
and out of said first passageway; (E) feeding glassmaking material
into and through a second passageway of said heat exchange unit
that is separated from said first passageway by a barrier through
which said glassmaking material and said hot combustion products
cannot pass and through which heat from said hot combustion
products passes to said glassmaking material to form said heated
glassmaking material; and (F) maintaining the heat flux from hot
combustion products in said first passageway to said barrier
sufficient that the temperature of the surface of said barrier that
is in contact with said glassmaking material does not exceed
1600.degree. F. and that the temperature of said glassmaking
material does not reach or exceed the temperature at which the
glassmaking material becomes adherent.
2. A method according to claim 1 wherein in step (G) the heat flux
from hot combustion products in said first passageway to said
barrier sufficient that the temperature of the surface of said
barrier that is in contact with said glass forming material does
not exceed 1400.degree. F. and that the temperature of said
glassmaking material does not exceed 1200.degree. F.
3. A method according to claim 1 wherein in step (G) the heat flux
from hot combustion products in said first passageway to said
barrier sufficient that the temperature of the surface of said
barrier that is in contact with said glass forming material does
not exceed 1200.degree. F. and that the temperature of said
glassmaking material does not exceed 1000.degree. F.
4. A method according to claim 1 wherein the hot combustion
products fed in step (D) have a temperature of at least
2000.degree. F.
5. A method according to claim 1 wherein the hot combustion
products fed in step (D) have a temperature of at least
2200.degree. F.
6. A method according to claim 1 wherein the oxidant combusted in
step (B) has an overall average oxygen content of at least 50 vol.
% oxygen
7. A method according to claim 1 wherein the oxidant combusted in
step (B) has an overall average oxygen content of at least 90 vol.
% oxygen
8. A method according to claim 1 wherein a portion of the heat that
flows from said hot combustion products to said barrier is absorbed
in a shadow wall in said first passageway and reduces the direct
radiative heat transfer from said hot combustion products to said
barrier.
9. A method according to claim 1 wherein said glass forming
material passes through said second passageway countercurrent to
the flow of said hot combustion products through said first
passageway.
10. A method according to claim 1 wherein said glass forming
material passes through said second passageway cocurrent with the
flow of said hot combustion products through said first
passageway.
11. A method according to claim 1 wherein before said glassmaking
material is fed into said second passageway it is heated in a
second heat exchange unit by indirect heat exchange.
12. A method according to claim 1 wherein said combustion products
after flowing out of said first passageway are cooled in a second
heat exchange unit by indirect heat exchange.
13. A method according to claim 1 wherein before said glassmaking
material is fed into said second passageway it is heated in a
second heat exchange unit by indirect heat exchange with said
combustion products that have flowed out of said first
passageway.
14. A method according to claim 1 wherein hot combustion products
withdrawn from said glassmelting furnace, before passing into said
first passageway, flow past a bed of said glassmaking material that
has passed through said second passageway, and exchange heat to
said bed of glassmaking material.
15. A method according to claim 1 wherein at least one offgas
stream is withdrawn from said second passageway and fed into said
first passageway.
16. A glassmelting method comprising (A) passing heated glassmaking
material into a glassmelting furnace; (B) combusting fuel with
oxidant having an overall average oxygen content of at least 35
vol. % oxygen to produce heat for melting said heated glassmaking
material in said glassmelting furnace and produce hot combustion
products having a temperature greater than 1800.degree. F.; (C)
withdrawing said hot combustion products from said glassmelting
furnace and feeding said hot combustion products into each of 2 to
10 first passageways of a heat exchange unit, wherein the
temperature of said hot combustion products entering said first
passageways is at least 1800.degree. F.; (D) flowing said hot
combustion products through and out of said first passageways; (E)
feeding glassmaking material into and through a plurality of second
passageways of said heat exchange unit that are separated from said
first passageways by barriers through which said glassmaking
material and said hot combustion products cannot pass and through
which heat from said hot combustion products passes to said
glassmaking material to form said heated glassmaking material; and
(F) maintaining the heat flux from hot combustion products in said
first passageways to said barriers sufficient that the temperature
of the surfaces of said barriers that are in contact with said
glassmaking material does not exceed 1600.degree. F. and that the
temperature of said glassmaking material does not reach or exceed
the temperature at which the glassmaking material becomes adherent.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the production of glass,
and more particularly to the heating of glassmaking material by
heat exchange with combustion products (flue gas) formed in the
combustion that is carried out to generate heat for melting the
glassmaking material.
BACKGROUND OF THE INVENTION
[0002] Conventional glassmaking methods require establishing in a
glassmelting furnace temperatures that are high enough to melt the
glassmaking material (by which is meant one or more materials such
as sand, soda ash, limestone, dolomite, feldspar, rouge, which are
collectively known as "batch" and/or broken, scrap and recycled
glass, known as "cullet"). The required high temperature is
generally obtained by combustion of hydrocarbon fuel such as
natural gas. The combustion produces gaseous combustion products,
also known as flue gas. Even in glassmaking equipment that achieves
a relatively high efficiency of heat transfer from the combustion
to the glassmaking materials to be melted, the combustion products
that exit the melting vessel typically have a temperature well in
excess of 2000.degree. F., and thus represent a considerable waste
of energy that is generated in the glassmaking operations unless
that heat energy can be at least partially recovered from the
combustion products. The prior art has addressed this problem by
using flue gas to air heat exchangers known as regenerators. In a
conventional air fired regenerative furnace, waste heat in the flue
gas is partially recovered in the regenerators by preheating the
incoming combustion air and the exit temperature of the flue gas
after passing through the regenerators is reduced to about 800 to
1000.degree. F.
[0003] Combustion of the hydrocarbon fuel with gaseous oxidant
having an average of at least 35 volume percent oxygen (known as
"oxy-fuel combustion") provides to the glassmaking operation
numerous advantages compared to combustion of the fuel with air.
Among those advantages are higher flame temperature, which affords
higher heat transfer and shorter melting times, and reduced overall
volume of the gaseous combustion products that exit the
glassmelting furnace, which affords a reduction in the size of the
gas-handling equipment that is needed. The gaseous combustion
products formed in combustion with oxidants having such higher
oxygen content can exhibit temperatures of 1800.degree. F. or
higher, even 2000.degree. F. or higher. Thus, the gaseous
combustion products of oxy-fuel combustion contain even more heat
energy, compared to the combustion products of conventional
air-fired combustion, which should be used to advantage to improve
the overall energy efficiency of the glassmaking operation.
[0004] While the glassmaking art is aware of using heat in the hot
gaseous combustion products from the glassmelting furnace to
preheat incoming glassmaking material which is to be melted in the
manufacture of the glass, the heretofore known technology has
believed that the temperature of the hot combustion products should
not exceed about 1000 to 1300.degree. F. as it is fed commences
heat exchange with the glassmaking material. This maximum
temperature is imposed by considerations of the capability of the
materials from which the heat exchanger is constructed to withstand
higher temperatures, and considerations of the tendency of the
glassmaking material to begin to soften and become adherent (or
"sticky") if it becomes too hot during the heat exchange step,
leading to reduced throughput and even plugging of the heat
exchanger passages. The temperature at which the glassmaking
material becomes adherent or sticky depends on the batch
composition and the material in contact with the glassmaking
material and is believed to be in a range between 1000 and
1300.degree. F. for a common batch to make soda lime glass for
bottles and windows. In a conventional air fired regenerative
furnace, the flue gas exit temperature after the regenerators is
about 800 to 1000.degree. F. and there is no need to cool down the
flue gas prior to a batch/cullet preheater.
[0005] When the gaseous combustion products are those obtained by
oxy-fuel combustion, the conventional belief has been that they
need to be cooled to the range of from 1000 to 1300.degree. F.
before heat exchange with the incoming glassmaking materials can
commence. Numerous examples exist showing the prior art's belief
that the temperature of the flue gas must be reduced before the
flue gas is used to heat incoming glassmaking materials. Such
examples include C. P. Ross et al., "Glass Melting Technology: A
Technical and Economic Assessment", Glass Manufacturing Industry
Council, August 2004, pp. 73-80; G. Lubitz et al., "Oxy-fuel Fired
Furnace in Combination with Batch and Cullet Preheating", presented
at NOVEM Energy Efficiency in Glass Industry Workshop (2000), pp.
69-84; U.S. Pat. No. 5,412,882; U.S. Pat. No. 5,526,580; and U.S.
Pat. No. 5,807,418.
[0006] However, reducing the temperature of this stream of
combustion products by adding to it a gaseous diluent such as air,
and/or spraying a cooling liquid such as water into the stream, is
disadvantageous as such approaches reduce the amount of recoverable
heat remaining in the gaseous combustion products, increase the
size of the gas handling equipment that is needed, and adds
additional equipment and process expense.
[0007] Thus, there remains a need in this field for method and
apparatus permitting practical and efficient heat exchange from the
gaseous combustion products of oxy-fuel combustion to glassmaking
material, which can be practiced even at the relatively higher
temperatures encountered when using oxy-fuel combustion in
glassmaking operations.
BRIEF SUMMARY OF THE INVENTION
[0008] One aspect of the invention is a glassmelting method
comprising
[0009] (A) passing heated glassmaking material into a glassmelting
furnace;
[0010] (B) combusting fuel with oxidant having an overall average
oxygen content of at least 35 vol. % oxygen to produce heat for
melting said heated glassmaking material in said glassmelting
furnace and produce hot combustion products having a temperature
greater than 1800.degree. F.;
[0011] (C) withdrawing said hot combustion products from said
glassmelting furnace and feeding said hot combustion products into
a first passageway of a heat exchange unit, wherein the temperature
of said hot combustion products entering said first passageway is
at least 1800.degree. F.;
[0012] (D) flowing said hot combustion products through and out of
said first passageway;
[0013] (E) feeding glassmaking material into and through a second
passageway of said heat exchange unit that is separated from said
first passageway by a barrier through which said glassmaking
material and said hot combustion products cannot pass and through
which heat from said hot combustion products passes to said
glassmaking material to form said heated glassmaking material;
and
[0014] (F) maintaining the heat flux from hot combustion products
in said first passageway to said barrier sufficient that the
temperature of the surface of said barrier that is in contact with
said glassmaking material does not exceed 1600.degree. F. and that
the temperature of said glassmaking material does not reach or
exceed the temperature at which the glassmaking material becomes
adherent.
[0015] As used herein, that glassmaking material is "adherent"
means that when 250 grams of the glassmaking material which is in
free-flowing particulate form at room temperature is heated to a
given temperature in a metal container made of the same material as
the barrier that the material is to flow past and is held at that
temperature for 30 minutes and the container is then inverted, at
least 1% of the material adheres to the surface of the container;
and the temperature at which the material "becomes adherent" is the
lowest temperature at which the material is thus "adherent" when it
is heated to that temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view of glassmaking apparatus with
which the method of the present invention can be practiced.
[0017] FIG. 2 is a cross-sectional view of a heat exchange unit
useful in the practice of the present invention.
[0018] FIG. 3 is a cross-sectional view of an alternative heat
exchange unit useful in the practice of the present invention.
[0019] FIG. 4 is a cross-sectional view of an alternative heat
exchange unit useful in the practice of the present invention.
[0020] FIG. 5 is a cross-sectional view of an alternative apparatus
useful in the practice of the present invention.
[0021] FIG. 6 is a cross-sectional view of an alternative heat
exchange unit useful in the practice of the present invention.
[0022] FIG. 7 is a cross-sectional view, seen from above, of an
alternate embodiment useful in the practice of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Referring to FIG. 1, fuel stream 1 and gaseous oxidant 2 are
fed to glassmelting furnace 3 and combusted therein to generate
sufficient heat to melt the glassmaking material present within
furnace 3. Stream 4 of molten glass can be recovered from
glassmaking furnace 3.
[0024] Suitable fuels include any that can be combusted with oxygen
to generate the required amount of heat of combustion. Preferred
fuels include gaseous hydrocarbons, such as natural gas.
[0025] The oxidant depicted as stream 2 can be fed as one stream to
a solitary burner within furnace 3, but is more often provided as a
plurality of streams to each of several burners within furnace 3.
Considered over the aggregate of all such gaseous streams, the
overall average oxygen content of all streams fed to and combusted
in furnace 3 should be at least 35 volume percent oxygen, and more
preferably at least 50 volume percent oxygen. That is, the oxygen
contents of the oxidant streams fed to different burners may differ
from one another, for instance if the operator desires to have some
burners (to which a higher oxygen content is fed) burn hotter than
other burners. The preferred manner of obtaining a gaseous oxidant
stream containing a desired oxygen content is to mix air and a gas
having an oxygen content higher than that of air (such as a stream
of 90 volume percent oxygen) either upstream from a particular
burner or at the burner outlets.
[0026] Combustion of the fuel and oxidant produces stream 5 of hot
gaseous combustion products which is removed from furnace 3 and fed
to heat exchange unit 7, which is described further hereinbelow,
from which stream 6 of cooled gaseous combustion products emerges.
Optional bypass stream 8 carries hot combustion products from
stream 5 to join exit stream 6 without passing through heat
exchange unit 7.
[0027] Stream 8 of heated glassmaking material to be fed to furnace
3 and melted in furnace 3 is obtained by passing glassmaking
material fed as stream 9 through heat exchange unit 7. Optional
bypass stream 10 denotes glassmaking material that is combined with
heated glassmaking material in stream 8, to be fed also to furnace
3, but which is not passed through heat exchange unit 7. Stream 9
and optional stream 10 typically receive the glassmaking material
from suitable bins and feeders of conventional design.
[0028] FIG. 2 illustrates one preferred embodiment of heat exchange
unit 7. Typically, the unit is cylindrical or rectangular in its
horizontal cross-section. In the embodiment shown in FIG. 2,
passageway 11 is surrounded by one or more passageways 12 which are
separated from passageway 11 by barrier 13. Considered in its
simplest form, this embodiment of heat exchange unit 7 is a heat
exchanger which enables heat to be exchanged from passageway 11
through barrier 13 to passageway or passageways 12 in indirect heat
exchange (by which is meant that heat can pass through barrier 13
without direct physical contact between the combustion products and
the glassmaking material, because gaseous, liquid or solid
materials cannot pass through barrier 13). The heat exchange unit 7
can have a horizontal cross-sectional shape which is circular,
rectangular, or any other geometric configuration, although
circular and rectangular, particularly square, are preferred. There
can be one passageway 12 completely surrounding passageway 11, or
passageway 12 can be divided into two or more such passageways by
appropriately positioned vertical dividers within the space
immediately surrounding passageway 11.
[0029] Stream 5 of hot combustion products from the glassmelting
furnace is fed through an inlet nozzle 14 in the bottom of unit 7
into the interior of passageway 11. Advantageously, stream 5 is
conveyed to the heat exchange unit 7 in a pipe that has a suitable
heat-resistant refractory interior lining that can withstand the
high temperature of this stream. Stream 5 as it enters passageway
11 is at a temperature of at least 1800.degree. F. and may be over
2000.degree. F. or even over 2200.degree. F. Thus, one advantage of
the practice of the present invention is that it can be carried out
without requiring any significant reduction in the temperature of
the hot combustion products before beginning to transfer heat from
the hot combustion products to the glassmaking material.
Significantly, no addition of dilution air or other cooling media
to stream 5, between the glassmelting furnace and unit 7, is
necessary.
[0030] As seen in FIG. 2, the stream 9 of incoming glassmaking
material to be preheated is fed to the passageway or passageways
12. Streams 9 can be fed into passageways outside the sides of the
unit 7, or can be fed on top of upper surface 17 if that surface is
sloped, so that the material moves along the sloped surface, toward
and then into passageway(s) 12 The glassmaking material is
preferably of a size, ranging from small pieces of cullet down to
finely divided particulate glassmaking material, such that the
glassmaking material is able to pass downwardly through the
passageway or passageways 12 under the influence of gravity. As the
glassmaking material passes through passageway or passageways 12,
its temperature increases by virtue of the flow of heat from the
hot combustion products in passageway 11 through barrier 13. The
thus heated glassmaking material exits heat exchange unit 7 as
stream 8 which can then be fed to the glassmelting furnace. An
alternative treatment of the heated glassmaking material is
illustrated in FIG. 5 and discussed hereinbelow. Stream 6 of cooled
combustion products exits the heat exchange unit 7 through top 17
at a temperature of typically 1400.degree. F. or less, although the
temperature at this point can be adjusted depending upon the
operational characteristics of heat exchange unit 7 and depending
on whether the operator wishes to pass this stream to another unit
from which additional heat can advantageously be drawn from stream
6, such as another heat exchange unit which passes heat to incoming
glassmaking material or to one or more streams of oxidant to be
employed in the combustion that is carried out in glassmaking
furnace 3. If desired, one or more offgas streams 21 are drawn from
the passageway(s) through which the incoming glassmaking materials
pass, such as passageways 12 in this embodiment, preferably being
drawn at the upper end, and are fed to stream 5, or to the
passageway through which the hot combustion gases pass (passageway
11 in this embodiment), or to an incinerator or other unit to
oxidize, decompose or otherwise remove undesirable components from
the off-gas (such as water vapor, organic fumes or byproducts that
were present on the incoming cullet materials).
[0031] Stream 9 can, as indicated above, be obtained from a storage
bin or similar apparatus which provides the glassmaking material,
or it can be obtained as a stream of heated material exiting
another heat exchange unit in which the glassmaking material is
preliminarily heated, for instance by heat exchange with hot
combustion products such as stream 6.
[0032] Heat exchange unit 7 can be constructed of any material that
is capable of withstanding the temperatures encountered in the
operation described herein. Preferably, barrier 13 is made of
metal, such as carbon steel, stainless steel, or other high
temperature alloys. The top and bottom of unit 7 should be made of
insulating ceramic materials. The top 17 may be flat as shown in
FIG. 2 or as shown in FIG. 2. The housing surrounding the exterior
of passageway or passageways 12 can be made of metal or refractory
bricks. Nozzle 14 is preferably constructed of ceramic material
that can withstand the temperature of the incoming hot combustion
product stream.
[0033] The glassmaking material can be fed through the passageway
or passageways 12 at a rate such that those passageways are
essentially filled by a packed, moving bed or a fluidized bed of
glassmaking material being heated. Preferably, however, to achieve
faster heat transfer and to reach greater uniformity of the
temperature to which the glassmaking material is heated, the
glassmaking material is fed in a fluidized bed of glassmaking
material being heated, or in a dispersed manner such that discrete
particles of material fall through the space in passageway or
passageways 12 as a "raining" flow of material. The efficiency of
heat transfer to the glassmaking material in the raining flow can
be enhanced even further by providing appropriate baffles such as
downwardly concave angle irons disposed in the path of the falling
particles, to deflect them from their paths thereby increasing
residence time and enhancing heat transfer even further. An example
of heat exchangers using such baffles is described in U.S. Pat. No.
5,992,041.
[0034] It has been determined that efficient heat transfer to the
glassmaking material can be obtained, without encountering the
problems of previous heat transfer devices, if the passageway into
which the hot combustion products are fed is configured such that
the temperature of the surface of barrier 13 that is in contact
with glassmaking material in passageway or passageways 12 does not
exceed 1600.degree. F. and the temperature of the glassmaking
material in the passageways 12 does not reach or exceed the
temperature at which the glassmaking material becomes adherent.
[0035] Typical components and ranges of the amounts thereof in
various types of glass can be determined from published sources and
from routine testing. For illustrative purposes, it can be
mentioned that many types of glass may contain 55 wt. % to 85 wt. %
silica (SiO.sub.2), a total of 4.5 wt. % to 20 wt. % of Na.sub.2O
and K.sub.2O, a total of 0.05 wt. % to 25 wt. % of CaO and MgO, and
0 to 15 wt. % of Al.sub.2O.sub.3, and optionally other components
such as Fe.sub.2O.sub.3, PbO (used in crystal glass and lead
crystal), B.sub.2O.sub.3 (in borosilicate glass), and/or compounds
that are or that contain oxides of Ti, S, Cr, Zr, Sb and/or Ba.
[0036] However, determination of the appropriate temperature at
which the present invention is carried out is based on the
properties of the mixture of ingredients of the glassmaking
materials that are fed through the passageways 11 or 12 on their
way to the glassmaking furnace. As is known in this field, those
ingredients need to contain, or be capable upon application of high
temperatures of being converted into, the desired glassmaking
components. Suitable ingredients may include not only the
aforementioned compounds but also precursors such as (but not
limited to) alkali silicates, carbonates and hydroxides, and
alkaline earth metals silicates, carbonates and hydroxides, as well
as hydrates of any of the foregoing. Lower adherent temperatures
(as that term is used herein) are generally associated with higher
amounts of alkali and alkaline earth metal oxides and
hydroxides.
[0037] For ingredients that become adherent at relatively lower
temperatures (such as the ingredients used to make common soda lime
glass or borosilicate glass), the temperature should not exceed
1300.degree. F., preferably not exceed 1200.degree. F. Since many
different ingredients are used in glass making and the adherent
characteristics of glassmaking materials not only depend on the
ingredients, but also on their particle size distributions and on
the metals used for barrier 13, the baffles or other metals that
come in contact with the heated batch materials, tests to determine
the maximum temperature to avoid sticking problems should be
conducted. A recommended test procedure is to heat 250 grams of the
glassmaking material, which is in free-flowing particulate form at
room temperature, to a given temperature in a metal container (or a
crucible) made of the same metal that as barrier 13 is to come in
contact with the heated batch materials, and hold the heated
material at that temperature for 30 minutes. The heated container
is then inverted to assess the flowability characteristics of the
material being thus tested. The lowest temperature at which at
least 1% of the material adheres to the surface of the container
after being subjected to these steps is defined as the "adherent
temperature" of the material for the metal used for the container.
The temperature to which the material is heated in unit 7 should
not exceed the adherent temperature, and preferably should not
exceed 100.degree. F. below the adherent temperature. Satisfying
these conditions ensure that glassmaking material will not become
so hot that it softens and becomes sticky and then begins to plug
the passageways or the openings through which heated glassmaking
material leaves passageways 12.
[0038] It has been determined that these conditions can be
satisfied for any given set of operating conditions, as described
below, by providing that the heat flux (in units of energy per area
of heat transfer surface at barrier 13 per unit of time) to all of
the heat transfer surface of barrier 13 remains sufficiently low
that the surface of barrier 13 that is exposed to the glassmaking
material does not reach a temperature above 1600.degree. F. and the
temperature of the glassmaking material in passageways 12 does not
reach or exceed the temperature at which it becomes adherent. The
heat flux and temperature distributions over the barrier 13 can be
estimated by radiative and convective heat transfer calculations
taking into account, among other things, the incoming temperature
and flow rate of the stream of hot combustion products, the
temperature and flow rate of the glassmaking material entering heat
exchange unit 7, the geometrical configuration of passageway 11,
and the thermal and physical properties (i.e., conductivity,
emissivity and thickness) of the barrier 13. Accurate prediction of
the temperature distribution, while achievable, is generally
difficult and requires an application of a detailed mathematical
heat transfer model for optimization. A practical way to achieve
the practice of the present invention is to provide a sufficiently
high transfer surface area and a sufficiently large space of the
passageway 11 into which the combustion products are fed. The
geometry of the passageway 11 is selected to allow good radiative
heat exchanges among all barrier walls and the hot combustion
products. A long narrow passageway 11 tends to make the area near
the inlet (nozzle 14) of the incoming hot combustion product stream
too hot.
[0039] For example the aspect ratio of a rectangular passageway,
defined as the ratio of the vertical length of the passageway to
the shorter side of the rectangle, is preferably less than 5 and
more preferably less than 3. A preferred method is to introduce the
combustion products near the center of the bottom 16 through which
nozzle 14 passes so that the distance of even the hottest portion
of the combustion products from the heat transfer walls is
sufficiently large that the heat flux to the barrier surfaces does
not become too high that the barrier surface temperature to which
the glassmaking material is exposed becomes too high. Thus, the
factors that can most readily be adjusted as determinative in
providing operation according to this invention are the total heat
exchange surface area of barrier 13, and the distance from the
point or points at which the combustion products are hottest as
they are fed into the heat exchange unit (typically this is at the
nozzle or nozzles 14 when the hot combustion products are fed into
the passageway 11 of the heat exchange unit through one or more
nozzles) to the nearest point or points on the inner surfaces of
barrier 13 which are exposed to the hot combustion products.
[0040] Without intending to be bound by any particular explanation
of the efficacy of this invention, it appears that the predominant
mode of heat transfer from the combustion products to the barrier
separating the combustion products from the glassmaking material is
radiative rather than solely convective. Thus, the calculations
that are carried out to determine a heat transfer surface area and
suitable location of the inlet nozzle or nozzles are those carried
out in the characterization of radiative heat transfer.
[0041] FIG. 3 illustrates another useful embodiment of the present
invention. In the embodiment of heat exchange unit 7 illustrated in
FIG. 3, stream 9 of glassmaking material to be heated is fed into
passageway 11 which is surrounded by passageway or passageways 12
through which hot combustion products 5 flow. The description above
with respect to the embodiment depicted in FIG. 2 is also
applicable to the embodiment depicted in FIG. 3, except that the
glassmaking material passes through a passageway 11 which is
centrally located with respect to the passageway or passageways 12
through with the hot combustion products flow. Preferably, 12
denotes a passageway completely surrounding central passageway 11,
although such a surrounding passageway 12 can be divided into
sectors by appropriately located vertical dividers. Whether or not
such passageway 12 is integral or subdivided, it is preferred to
feed the hot combustion products into passageway 12 as more than
one stream, and preferably as 2-16 streams spaced around the bottom
of passageway 12. Providing additional streams helps to provide
relatively uniform temperature conditions around passageway 11, at
any given elevation within passageway 12. The surface of barrier 13
that is exposed to the glassmaking material and whose temperature
should not be permitted to exceed 1600.degree. F. is in this
embodiment the inner surface of barrier 13. Accordingly,
observation of this condition is most effectively achieved by
suitably dimensioning not only the overall heat transfer surface
area of barrier 13, but also the geometry of the passageway 11 and
the location of the one or more inlet nozzles 14 and their
respective distances from barrier 13, so that again the heat flux
from the passageway or passageways 12 to the surface of barrier 13
to which the combustion products are exposed can be suitably
controlled so as to control the temperature of the barrier surface
to which the glassmaking material is exposed.
[0042] FIG. 4 illustrates another useful embodiment of the present
invention. FIG. 4 depicts the embodiment of FIG. 3, but to which
has been added "shadow wall" 15. Each shadow wall 15 is preferably
located between an inlet nozzle 14 and barrier 13, such that a
straight line drawn from the opening of an inlet nozzle 14 to
barrier 13 must pass through a shadow wall 15. The shadow wall is
made of suitable refractory material, such as
high-temperature-tolerant ceramic materials, that can withstand the
temperature of the incoming hot combustion product stream. Each
shadow wall has openings through it to only partially pass
radiative heat flux from the hot combustion product stream toward
barrier 13, thus reducing the radiative heat flux in a controlled
fashion. The openings can be circular or polygonal, or can be in
the form of elongated slots. Generally, the openings can occupy
from 10% to 90% of the surface of the shadow wall; the particular
percentage can readily be determined experimentally. The openings
can be uniformly spaced on the surface of the shadow wall, or one
may provide fewer openings nearer to the bottom (i.e. nearer to the
point where the hot combustion products enter the passageway) and
more openings further from the bottom. Shadow wall 15 may also
absorb heat from the hot combustion products, and reradiate the
heat toward the surface of barrier 13. These shadow walls 15 enable
the operator to reduce the overall size of heat exchange unit 7 by
reducing the heat flux from the hottest region of the passageway
through which the combustion product is flowing, which is usually
the region closest to where the hot combustion products enter that
passageway. The effective dimensions of any shadow walls 15,
especially the number of openings and their dimensions, can readily
be determined experimentally.
[0043] It should of course be appreciated that embodiments of the
type illustrated in FIG. 2, wherein the hot combustion products
flow through a central passageway surrounded by one or more
passageways through which glassmaking material passes, can also be
adapted by inclusion of one or more shadow walls located between
one or more of the inlets through which combustion products enter
the central passageway, and the inner surface of barrier 13.
[0044] FIG. 5 illustrates one manner of conveying the heated
glassmaking material to glassmelting furnace 3 after the
glassmaking material has passed through heat exchange unit 7. The
heated glassmaking material 8 descends onto a bed 18 from which the
glassmaking material passes into furnace 3. Bed 18 can be
horizontal or sloped, i.e. still having a horizontal component. The
material on bed 18 can move under the influence of gravity, but
preferably is moved with the aid of a moving conveyor belt, rotary
hearth, or similar equipment, such as a moving grate, that is
commercially available for moving beds of heated solids. In this
embodiment, the hot combustion products can flow into passageway 11
as a stream without the use of a nozzle. A dividing wall 20 can aid
in retaining the hot glassmelting atmosphere within furnace 3. The
hot combustion products exit furnace 3 past the upper surface of
bed 18 so that some heat exchange can occur even before the hot
combustion products enter passageway 11 to exchange heat to
material in passageway or passageways 12.
[0045] FIG. 6 illustrates another embodiment useful in the present
invention. In this embodiment, the hot combustion products and the
glassmaking material flow cocurrently rather than countercurrently
as illustrated in FIGS. 2, 3 and 4. The reference numerals employed
in both FIGS. 2 and 5 have the same meanings in FIG. 5 as they do
in FIG. 2. The difference, as can be seen, is that hot combustion
products are fed through inlet nozzle 14 into the top of passageway
11 and stream 6 of cooled combustion products exit from the bottom
of passageway 11. It should be recognized that the embodiments of
FIGS. 3 and 4 can also be adapted to provide cocurrent flow of the
heat-exchanging streams.
[0046] The present invention can also be carried out in embodiments
in which two or more, typically 2-10 and preferably 2-6,
passageways 11 each bounded by its own barrier 13 are situated
close enough to each other that passageways 12 are located between
two (or more) passageways 11. One such embodiment is shown in FIG.
7, in which four passageways 11 each receive through an inlet 14 a
portion of the hot combustion gases which then flow upward through
the passageways 11. The four passageways 11 are located with
respect to each other so that some passageways 12 are defined
between pairs of adjacent passageways 11. Heat flows through
barriers 13. Preferably, the hot combustion products pass through
passageways 11 and the glassmaking material flows through
passageways 12, in which case heat flows from passageways 11 into
passageways 12. The apparatus shown in FIG. 7 can also be used so
that hot combustion products pass through passageways 12 and
glassmaking material flows through the passageways 11, but this is
less preferred as the closer dimensions in the passageways 12 would
necessitate providing shadow walls or the equivalent to keep the
heat flux to the walls 13 from being excessive.
[0047] As noted above, one significant advantage of the present
invention is that more of the energy content of the stream of hot
combustion products can be used to advantage, even though its
temperature is higher as being obtained from oxy-fuel combustion,
without requiring any significant reduction in the temperature of
the stream such as by adding a diluent fluid stream. Other
advantages are inherent in the fact that the heat transfer between
the hot combustion products and the glassmaking material is
indirect, which means that there is no risk of entraining dust or
other particulates in the incoming glassmaking material, nor of
contaminating the exiting combustion product stream with entrained
dust and other particulate matter, nor of substantially oxidizing
the carbon content of the batch materials which is important to
make amber color glasses.
[0048] The fact that the present invention can take advantage of an
incoming combustion product stream having a higher temperature than
prior practice thought could be employed to heat incoming
glassmaking material also means that the temperature of the cooled
combustion product stream that exits the heat transfer unit 7 and
still be high enough that this stream can be used for additional
heat exchange. For instance, that exiting combustion product stream
6 can be fed to a conventional heat exchanger that exchanges heat
from a combustion product stream having a temperature on the order
of 1000.degree. F. or less, by convective heat exchange with
incoming glassmaking material, with oxidant or fuel to be
subsequently combusted in the glassmelting furnace, or with other
gaseous, liquid or solid material. As a further advantageous
embodiment, the glassmaking material that is fed as stream 9 can
have already been heated, for instance by passage through such a
conventional convective heat exchange unit, before it is fed as
steam 9 to the heat exchange unit described herein. The heat
exchange can be with cooled but still heat-bearing combustion
products, or with a stream of other hot material.
[0049] The stream of cooled combustion products emerging from heat
transfer unit 7, or from a subsequent heat exchanger, can if
desired be subjected to treatment steps that may be desirable or
necessary before the stream is discharged to the atmosphere or
employed as a feed stream to a chemical processing stage. For
instance, the stream can be passed through an electrostatic
precipitator or equivalent apparatus to remove fine particulate
contaminants. The stream can be treated to remove gaseous
atmospheric pollutants such as sulfur oxides, such as by contacting
the stream with a suitable absorbent or reactant such as
Ca(OH).sub.2 or sodium carbonate.
[0050] A sample set of calculations, based on a hypothetical set of
operating conditions that could be encountered in an actual
glassmaking operation, are described in the following example.
EXAMPLE
[0051] A 450 short tpd flint container glassmelting furnace is
equipped with a high temperature radiative batch/cullet preheater
and a conventional low temperature batch/cullet preheater,
installed in series. The furnace is fired with 47,000 SCFH of
natural gas and 105,000 SCFH of commercial oxygen (92% O.sub.2, 4%
N.sub.2 and 4% Ar). The total exhaust gas flow rate from the
melting furnace is about 192,000 SCFH which includes the gases
generated from the normal container batch materials and some air
infiltration. The temperature of exhaust gas as it leaves the
melting furnace is 2500.degree. F. An unheated batch/cullet mixture
(50/50 by weight) is first dried and heated to 316.degree. F. in
the conventional low temperature batch cullet preheater. A suitable
low temperature batch/cullet preheater is described in U.S. Pat.
Nos. 5,412,882 and 5,526,580. It takes in the cooled exhaust gas
from the radiative batch/cullet preheater. The preheated
batch/cullet mixture from the conventional low temperature
batch/cullet preheater is introduced into the radiative
batch/cullet preheater of the present invention and heated further
to 1050.degree. F. by heat exchange with the exhaust gas from the
melting furnace which is introduced through a refractory lined duct
to the bottom center of the counter-current radiative batch/cullet
preheater. The gas temperature at nozzle 14 is about 2325.degree.
F. due to 10,000 SCFH of cold air infiltration and wall heat losses
of about 0.5 MMBtu/hr after the flue gas left the furnace. In the
radiative batch/cullet preheater 7.2 MMBtu/hr of energy is required
to preheat batch and cullet from the aforementioned 316.degree. F.
to 1050.degree. F. Approximate radiative heat transfer calculations
show that an average heat transfer rate of about 6325
Btu/ft.sup.2/hr to the barrier 13 can be obtained by gas radiation
in passageway 11. Thus, the total heat transfer surface area of
barrier 13 required becomes about 1164 ft.sup.2. The average gas
and the average barrier surface temperatures may change from
1760.degree. F. and 1350.degree. F. at the hot end to 1000.degree.
F. and 700.degree. F. at the cold end. For example, the approximate
dimensions of a rectangular passageway of 25' W.times.10'
D.times.16.6' H may be built and tested. Due to the small aspect
ratios of the large rectangular passageway 11 in this example, the
actual gas temperature distribution along the height of the
preheater may become more uniform, for example 1600.degree. F. in
the hot end and 1100.degree. F. in the cold end, and resulting in
lower heat transfer. By increasing the height while keeping the
same total area, for example to the dimensions of 10' W.times.10'
D.times.29.1' H, the gas and barrier temperature distribution along
the height can be made closer to the desired design conditions. The
final more fully optimized determination of the optimum dimensions
is then preferably obtained by detailed radiative heat transfer
calculations using a three dimensional mathematical model and/or
pilot scale experiments.
* * * * *