Encyclopedia of Energy Volume III (Encyclopedia of Energy Series)

836 Pages·2004·13.252 MB·English

by  Robert U Ayres, Robert Costanza, Jose Goldemberg, Marija D Ilic, Eberhard Jochem, Robert Kaufmann, Amory B Lovins, Mohan Munasinghe, R K Pachauri, Claudia Sheinbaum Pardo, Per Peterson, Lee Schipper, Margaret Slade, Vaclav Smil, Ernst Worrell, Cutler J. Cleveland

Glass and Energy
CHRISTOPHERW. SINTON
Alfred University
Alfred, New York, United States
1. INTRODUCTION
1. Introduction
2. Types of Glass The atomic or molecular structure of solid materials
3. Introduction to Commercial GlassManufacturing can generally be found in two states: ordered and
4. Energy disordered.Iftheatomsofamaterialareorderedina
repeated array throughout, the structure will be an
ordered three-dimensional pattern. This is a crystal-
Glossary line material. Solid materials that lack long-range
atomicorderarenoncrystallineoramorphous.When
container glass Thelargest glasssector; formed bya two-
a material is cooled from a liquid state to a solid, its
stageprocessofinitialformingbypressingfollowedby
formation into a crystalline or amorphous solid
blowingto obtainthefinishedhollowshape.
dependsonhoweasilytherandomatomicstructures
cullet Scrap or waste glass that is added to the batch
oftheliquidcanorganizeintoanorderedstatewhile
materials.
float glass Flat glass that is made by floatingmolten glass cooling. Factors such as composition and cooling
ona poolof moltentin. rate are important. Rapid cooling of a liquid
glass A material with a three-dimensional network of enhances the formation of an amorphous structure
atoms that forms a solid lacking the long-range since the material is given little time to reach an
periodicitytypical of crystallinematerials. ordered state. This is a glass. Figure 1 shows a
optical fibers Very high-purity silica glass fibers used to comparisonoftheorderedarrangementofcrystalline
transmit lightfor telecommunications.
silicon dioxide and the disordered arrangement of a
oxy-fuelfiring Thereplacementofthecombustionairwith
silicon dioxideglass. Acommon definition ofa glass
oxygenin a glassfurnace.
was given by the physicist W. H. Zachariasen, who
recuperator A continuous heat exchanger that preheats
stated that glass is an extended, three-dimensional
combustion airwith theheat fromthe exhaustgases.
networkofatomsthatformsasolidlackingthelong-
regenerator Astackofrefractorymaterialthatabsorbsthe
heatfromexhaustgases.Therearetworegeneratorsin range periodicity (or repeated, orderly arrangement)
a regenerative furnace that are cycled over a specific typical of crystalline materials.
timeperiod. Manufactured glass products are made by
melting crystalline raw materials into a molten
state. The melt is subsequently shaped into the
Glass is a material that enjoys widespread use in desired product and cooled to make a rigid form.
many commercial applications, including buildings, The composition of the glass depends on the
beverage containers, automobiles, and telecommuni- desired properties of the glass, which in turn define
cations. However, the manufacture of glass is a very which raw materials are needed. Conceptually,
energy-intensive undertaking and the industry is there are thousands of different combinations of
constantly working to reduce its energy consump- raw materials that will produce a glass, but in
tion.Thisarticledescribestheuseandflowofenergy practice there are a limited number of formulations
in commercial glass production. In order to under- that are used to make the common glass products
stand the relationship between energy and glass that are used on a daily basis. These limitations are
manufacturing,thisarticles firstdefines whatglassis typically dictated by practical and economic con-
and then describes how it is produced. siderations.
EncyclopediaofEnergy,Volume3.r2004ElsevierInc.Allrightsreserved. 1
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GlassandEnergy
Crystalline silicon dioxide (SiO ) as bottles and windows. The melting temperature
2
canbereducedtoapracticalrangebytheadditionof
Oxygen atom fluxes, the most common of which are alkali oxides
(Li O, Na O, and K O). However, the addition of
Silicon atom 2 2 2
too much alkali can lead to the degradation of
properties, such as the chemical durability of a glass
(i.e.,itwillbegintodissolvewhenexposedtowater).
This can be compensated for by the addition of
property modifiers, such as alkaline earth oxides
(CaO and MgO) and aluminum oxide (Al O ). The
2 3
addition of these glass modifiers changes the glass
structure, resulting in changes in the physical and
chemical properties of the glass.
The composition used for a specific glass product
Noncrystalline silicon dioxide (SiO2) depends on the desired final product performance,
manufacturing characteristics (e.g., melting and
forming), environmental considerations, and raw
material/fuel costs. Although there are many differ-
ent combinations of elements that will make a glass,
there are some general compositional types that are
widely used in commercial glass making. A glass
made of silica, alkali, and alkaline earth elements is
commonly called a soda-lime-silica glass and is the
most common type. It has a relatively low melting
temperature (begins melting at B8001C and is
usually processed up to 14001C), has the proper
viscosity characteristics for continuous production,
and uses raw materials that are relatively inexpen-
FIGURE 1 Schematic diagram showing the ordered arrange-
mentofcrystallinesilicondioxideandthedisorderedarrangement sive. It is the basis for making flat glass, containers,
ofasilicondioxideglass. and consumer ware (e.g., drinking glasses).
Borosilicateglassescontainboron,whichdecreases
2. TYPES OF GLASS thermalexpansion,lowersviscosity,andallowsfaster
fabrication speeds.Borosilicateglassesare used when
Glasses can be found in the natural environment, resistance to thermal shock is desired, such as for
mostlyinareasofvolcanismwhereeruptedmagmais cookware laboratory glass. Fiberglass products that
quickly cooled before crystallization can occur. are used for thermal insulation and structural
Perhapsthebestknownexampleisobsidian,ablack reinforcement of composites contain boron.
glass that consists mainly of oxides of silicon, Lead crystal and crystal glass are silicate glasses
aluminum, calcium, and sodium and/or potassium. with added lead. The lead decreases viscosity and,
Commercially produced glasses differ widely in hence, decreases the temperatures needed to work
chemical composition and in physical and optical the glass and enhances the dissolution of refractory
properties. Various materials can form a glassy or particles. Lead increases the refractive index of the
vitreous structure, such as the oxides of silicon and glass, imparting a brilliance to the finished piece.
boron. Most commercial glasses are based on silica Specialty glasses include a variety of products,
(SiO ), which is the most abundant material on the such as cathode ray tubes (CRTs), lighting glass
2
earth’s crust and is an excellent glass former. There (tubes and bulbs), laboratory and technical glass-
aresomenonsilicateglasses,buttheymakeuponlya ware, opticalfibers, ceramic glasses (cookware), and
very small fraction of commercially produced glass. glass for the electronics industry (e.g., LCD panels).
Pure vitreous silica glass can be made by melting Optical fibers are a specialty glass that is made of
quartz sand and cooling it quickly. Although silica extremely high-purity silica. Optical fiber glasses
makes an excellent glass for many applications, its cannot attain the needed purity levels produced by
high melting temperature (420001C) makes it too melting silica sand, so the glass is manufactured by
expensive for use in commodity glass products such precipitating chemically purified silica vapor.
3
GlassandEnergy
3. INTRODUCTION TO driven off as volatile CO , leaving the metal oxide
2
COMMERCIAL GLASS behind. Feldspar or nepheline syenite are aluminosi-
licate minerals that can be used in the batch to add
MANUFACTURING
alumina (Al O ) and some alkali. Borosilicate glass
2 3
batches can have 5–10% borate minerals, such a
Glass manufacturing is essentially a commodity
borax (Na O(cid:1)2B O (cid:1)5H O) and colemanite (Ca
industry, with the majority of product sold to other 2 2 3 2 2-
B O (cid:1)5H O)addedtothem. Mineralorrock wool
industries, such as beverage, construction, and 6 11 2
is often made from slag that is a by-product from
automobile manufacturers. In 1999, the U.S. glass
metal smelting.
manufacturing industry produced more than 18mil-
Recycled glass, or cullet, is added to the batch to
lion metric tons, worth approximately $17.6billion.
reuse waste glass from both within the plant and
In the European Union, 1996 production was
outsidesources.Culletalsoprovidesanearlymelting
approximately 29million metric tons (Table I).
liquid phase that enhances reaction rates. The effect
of cullet on energy use is discussed later. The main
problem associated with cullet is the potential for
3.1 Raw Materials
contamination, particularly from pieces of metal,
Theoverallgoalofglassmanufacturingistoconvert ceramic, or glass-ceramic. These contaminants can,
crystalline raw materials into a homogeneous, at best, lead to rejected product and, at worst,
flowing liquid that is free of visible defects that can damage the expensive refractory lining.
be formed into a final product. This needs to be Minor additives include colorants, reducing
accomplished as quickly and as economically as agents, and fining agents. Colorants are added to
possible and, at the same time, comply with all control or add color and are usually oxides of
environmental regulations. Figure 2 shows a sche- transition metals or rare earth elements, such as
matic diagram of the process of commercial glass- cobalt, cerium, iron, and chrome. Fining agents are
makingasdescribedinthissection.Thereareseveral used to promote the removal of bubbles from the
methods of making and forming glass, but all begin melt that, if not removed, will result in flaws in the
with the mixing of raw materials or ‘‘batching.’’ For finalproduct.Commonfiningagentsincludesulfates,
most glasses, the most important glass former is
silica.Therawmaterialforthisisquartzsandusually
derived predominantly from sandstone deposits. Limestone Minor
Sand Soda ash Feldspar
Such deposits must contain few impurities, such as dolomite materials
iron, which will affect the color of the glass. Glass
property modifiers are introduced with a mix of Batch mixing Cullet
limestone (CaCO ) and dolomite (CaMg[CO ] ).
3 3 2
The alkali flux is usually soda ash (Na CO ), which Melting
2 3 1500˚C
comes from either a mined mineral ore called trona
or the Solvay process of reacting sodium chloride
Fining
with limestone. The soda and lime materials are
Throat
carbonates so that upon heating the carbonate is
Conditioning
1300˚C
TABLEI
GlassSectorsandAnnualProduction(MillionMetricTons) Forming
800–1100˚C
Sector EuropeanUniona UnitedStatesb
Flatglass 6.4 4.5 Annealing
500˚C
Containerglass 17.4 8.7
Fiber 2.5 2.3
Speciality/Pressed/Blown 2.7 2.7 Cutting
packing
Total 29.0 18.3
FIGURE 2 Schematic diagram of the process of commercial
a1996estimates. soda–lime–silica glassmaking showing raw materials, process
b1999estimates. sections,andapproximateoperatingtemperatures.
4
GlassandEnergy
fluorides,nitrates,andoxidesofarsenicorantimony. toreduceparticlesegregationduringtransporttothe
The most common combination forsoda–lime–silica furnace.
glassesissodiumsulfateandareducingagentsuchas
carbon.
The batching process consists of transferring 3.2 Glass Melting Furnaces
raw materials directly from storage silos into a
There are several types of melting furnaces used in
hopper and subsequently mixing the raw batch in a
the glass industry depending on the final product,
large-scale mixer. Cullet may be added after mixing.
raw materials, fuel choice, the size of operation, and
The mixed batch is placed in storage hoppers
economic factors. A melter can be either periodic or
located directly adjacent to the ‘‘dog house,’’ or the
continuous. The latter is most widely used in large-
end of the glass furnace at which the batch is
scale operations. Continuous melters maintain a
introduced and melting commences. The batch may
constant level by removing the glass melt as fast as
be moistened with water or sometimes caustic soda
rawmaterialisadded.TableIIshowsthethreemajor
types of furnaces and their advantages and disad-
vantages. The energy for melting comes from either
TABLEII the combustion of fossil fuels or electricity. The
AdvantagesandDisadvantagesofDifferentFurnaceTypes temperature necessary for melting ranges between
1300 and 15501C. All furnaces are lined with high-
Furnacetype Advantages Disadvantages temperature refractory materials to keep the corro-
sive melt from escaping and to insulate the melter.
Regenerative Longfurnacelife HigherNO levels
x
Regenerative combustion furnaces recover heat
Long-termexperience Highercapitalcost
from the exhaust stream to preheat the incoming
Directfired Reductioninfuel Refractorycorrosion
(mostly NO reduction Short-termexperience combustion air by alternatively passing the exhaust
x
oxygen/fuel) and combustion air through large stacks of lattice-
Lowercapitalcost Costofoxygen
production work refractory brick (regenerators or checkers).
Particulatereduction There are two sets of regenerators, so that as one is
Electric Efficient Highelectricitycost being preheated by the exhaust gases the other is
Noone-sitepollution Shortfurnacelife transferring heat to the incoming combustion air
(Fig.3). The cycle is reversed approximately every
Glass melt
surface Throat Refiner
Bridge wall
Forehearth
Stack
MMeelltteerr
ccrr
oo
ww
Batch nn
feeder Port
Sidewall
Checkers
Burner
Combustion air
blower
FIGURE3 Diagramofacross-firedregenerativefurnace.Thistypeisoftenusedinlarge-volumeproductionofcontainer
andflatglass.FromU.S.DepartmentofEnergy(2002).‘‘EnergyandEnvironmentalProfileoftheU.S.GlassIndustry.’’
5
GlassandEnergy
20min. Most glass-container plants have either end- tions in demand without incurring the costs of
fired (burners at each end) or cross-fired (burners on operating a larger furnace. It is also used to enhance
each side) regenerative furnaces, and all flat glass the pull rate (the output at the forming end) of a
furnacesarecross-fired with fiveorsixports oneach furnace as it nears the end of its operating life.
side with two burners for each port. Combustion air
preheat temperatures of up to 14001C may be
3.3 The Melting Process
attained, leading to very high thermal efficiencies.
A variant of the regenerative furnace is the recup- Regardless of the furnace type used, there are four
erator,inwhichincomingcombustionairispreheated basic steps to melting glass:
continuously by the exhaust gas through a heat
1. Conversionofthebatchintoamoltenliquidfree
exchanger. Recuperative furnaces can achieve 8001C
ofundissolvedcrystallinematerial(calledstones)
preheated air temperatures. This system is more
2. Removal of bubbles, or fining
commonly used in smaller furnaces (25–100tons
3. Homogenizing the liquid so that it is chemically
per day). For large-capacity installations (4500tons
and thermally uniform
per day), cross-fired regenerative furnaces are almost
4. Conditioning the melt to bring it to a state in
always used. For medium-capacity installations
which it can be formed into the desired product
(100–500tons per day), regenerative end-port fur-
naces are most common. For a continuous furnace, these four steps occur
A direct-fired furnace does not use any type of simultaneously. The average residence time, from
heat exchanger. Most direct-fired combustion fur- introduction of the batch to forming, is on the order
nacesuseoxygenratherthanairastheoxidizer.This of24hforcontainerfurnacesandupto72hforfloat
is commonly called oxy-fuel melting. The main glass furnaces. Glass melting tanks can contain
advantages of oxy-fuel melting are increased energy 1500tons or more of molten glass.
efficiency and reduced emission of nitrogen oxides The melting of the batch consists of several steps
(NO ). By removing air, nitrogen is removed, which from the moment it is introduced into the tank. Due
x
reduces the volume of the exhaust gases by approxi- to the low thermal conductivity of the batch
mately two-thirds and therefore reduces the energy materials, the initial melting process is quite slow.
needed to heat a gas not used in combustion. This As the materials heat up, the moisture evaporates,
alsoresultsinadramaticdecreaseintheformationof some of the raw materials decompose, and the gases
thermal NO . However, furnaces designed for oxy- trapped in the raw materials escape. The first
x
gen combustion cannot use heat-recovery systems to reaction is the decomposition of the carbonates at
preheattheoxygen.Initially,furnacesthatuse100% approximately 5001C, which releases CO . The
2
oxy-fuel were used primarily in smaller melters fluxes begin to melt at approximately 7501C and
(o100tons per day), but there is a movement begin to react with the silica. At 12001C, the silica
toward using oxy-fuel in larger, float glass plants. sand and other refractory materials dissolve into the
An electric furnace uses electrodes inserted into moltenflux.Astheremainingsilicadissolvesintothe
the furnace to melt the glass by resistive heating as melt, its viscosity increases rapidly.
the current passes through the molten glass. These Bubbles are generated by the decomposition of
furnaces are more efficient, are relatively easy to raw materials, from reactions between the melt and
operate, have better on-site environmental perfor- the glass tank, and from air trapped between batch
mance,andhavelowerrebuildcostscomparedtothe particles. These are removed as they rise buoyantly
fossil-fueled furnaces. However, fossil fuels may be through the melt to the surface or as gases dissolve
neededwhenthefurnaceisstartedupandareusedto into the melt. Removal of the last few bubbles is the
provideheatintheworkingendorforehearth.These most difficult.Fining agents enlargethese bubbles to
furnaces are most common in smaller applications assist in their removal. Fining occurs as the melt
because at a certain size, the high cost of electricity moves forward through the throat of the melter to
negates the improved efficiency. the conditioner, where melt homogenization and
Some regenerative furnaces use oxygen enrich- cooling take place.
ment or electric boost to optimize the melting Without homogenizing the melt, linear defects
process. Electric boosting adds extra heat to a glass called cords (regions of chemical heterogeneity) will
furnacebyusingelectrodesinthebottomofthetank. beseeninthefinalproduct.Furnacesaredesignedto
Traditionally,itisusedtoincreasethethroughputof promote recirculating convective currents within the
a fossil fuel-fired furnace to meet periodic fluctua- melttohomogenizetheglass.Convectivemixingcan
6
GlassandEnergy
be enhanced by physically stirring the melt with mold and given its final shape by blowing com-
bubbles or mechanical stirring devices. pressed air into the container. During the forming
process,theglasstemperature isreducedbyasmuch
as6001Ctoensurethatthecontainersaresufficiently
3.4 Forming Processes
rigid when placed on a conveyor. The cooling is
After melting, fining, and homogenization, the melt achieved with high volumes of blown air against the
passes through the throat of the furnace (Fig.3) into molds.Afterforming,thecontainersareannealedby
a conditioning chamber, where it is cooled slowly to reheating to 5501C in a lehr and then cooled under
a working temperature between 900 and 13501C. It controlledconditions.Largeroperationscanproduce
is then ready to be delivered as a viscous mass 500bottles per minute.
through the forehearth to be formed into a specific Glass fibers can be found in nature where
product. The role of the forehearth is to remove fountaining lavas are wind blown to produce long
temperature gradients and transfer the molten glass strands known as Pele’s tears, after the Hawaiian
to forming operations. It consists of an insulated goddessoffire.Themanufactureofcommercialglass
refractorychannelequippedwithgas-firedburnersor fiber is similar, with some exceptions. Most fiber
electric heaters and an air cooling system. glass melts are produced with cross-fired recupera-
Flat glass is most commonly formed by the float tive furnaces, although there are several oxy-fuel
process, whereas patterned and wired glass are fired furnaces in Europe. Short-strand glass wool is
formed by rolling. Float glass and rolled glass are produced by spinning the melt in a crucible lined
produced almost exclusively with cross-fired regen- withholes.Asthemoltenglassisejectedthroughthe
erative furnaces. The float process was developed by holes, high-velocity air blows the glass into fibers.
thePilkingtonBrothersGlassCompanyofEnglandin For continuous glass fibers, the melt flows from the
the 1950s, and it is the technique of pouring the furnace through a series of refractory-lined channels
melted glass onto a pool of molten tin. The glass to bushings. Bushings have several hundred cali-
floats on the tin and spreads out to form a uniform brated holes or bushing tips that are electrically
thickness.Theglassisthenmechanicallystretchedor heatedtopreciselycontrolthetemperatureand,thus,
constrained to attain the desired ribbon width and the viscosity and flow rate of the melt. The melt is
thickness. The tin bath is surrounded by a reducing drawn through the bushing tips by a high-speed
atmosphere of nitrogen and hydrogen to keep the drum to form continuous filaments. Once drawn
bath from oxidizing. The glass ribbon is pulled and from the bushing, the filaments are quickly cooled
modulatedbywater-cooledpulleysandexitsthebath and then coated with a polymer binder.
into the annealing lehr. Annealing is the process of
heating and cooling the glass at a controlled rate to
remove any residual stresses that could cause 4. ENERGY
catastrophic failure while in use. After exiting the
lehr, the glass is automatically cut into the desired Based on the previous description of glassmaking, it
sizes. The float method uses much less energy than is evident that glass manufacturing is a very energy-
precedingplateandsheetglassformingduetotheuse intensive process. Energy accounts for 15–25% of
of more efficient furnaces and the elimination of the the cost of glass products, depending on the sector
need for surface polishing and grinding. Rolled glass and the country of manufacture. In 1999, the U.S.
isformedbysqueezingmoltenglassatapproximately glass industry spent more than $1.3billion on
10001Cbetweenwater-cooledsteelrollerstoproduce approximately 295 quadrillion joules (PJ) [280
a ribbon with a surface pattern or embedded wire. trillion British thermal units (Btu)], including pro-
Containerssuchasbottlesandjarsaremadeusing ducts from purchased glass. Taking into account
a two-stage pressing and blowing process. This electricitylossesduetogeneration,transmission,and
process is fully automated and begins with molten distribution, this figure increases to 417PJ(395tril-
glass flowing from the furnace forehearth to a spout lion Btu) of total energy use.
or set of spouts. The flow of glass is controlled by a By far the most energy-intensive components of
mechanical plunger that produces a piece or gob of glass manufacture are melting and refining, which
molten glass. The gob is then conveyed to the together account for 70–80% of the total energy
forming process, which produces a primary shape requirements. The remaining energy is used in the
in a blank mold using compressed air or a metal forehearths,tinbath(forfloatglass),forming/cutting
plunger. The primary shape is placed into the finish processes, annealing, and other plant services. For
7
GlassandEnergy
example, in container manufacturing, the furnace iron (25.4GJ/ton) and pulp and paper products
accountsfor79%ofthetotalenergyused.Theother (32.8GJ/ton).
areas of energy use are the forehearth (6%), In addition to the specific product, the type of
compressed air (4%), mold cooling (2%), lehr furnaceandthedesiredqualityoftheglassaffectthe
(2%), and others (7%). specific or per unit energy use. For instance, because
flat glass needs to be completely free of imperfec-
tions, it generally requires longer residence times in
4.1 Sources of Energy
the furnace than does container glass and therefore
The sources of energy used to produce glass are usesmoreenergypertonofglassproduced.TableIII
natural gas, electricity, and, to a lesser extent, fuel shows the range of specific energy values for several
oil. Fuel oil has been historically important, but different fossil fuel furnaces and glass products.
today natural gas is the dominant fossil fuel used in Efficiencies in large regenerative furnaces can be as
glassmaking. Natural gas has higher purity, is high as 55% and electric furnaces are 70–90%
relatively easy to control, and does not require on- efficient (not considering the 25–30% efficiency of
sitestoragefacilities.Thehigherpurityofnaturalgas off-site electricity generation). Electric melters are
results in a reduction of sulfur dioxide emissions more efficient because there are no heat losses in the
relative to fuel oil. Natural gas accounts for stack or regenerator. Table IV shows the range of
approximately 80% of energy use and more than efficiencies for several types of furnaces.
98% of purchased fuels in the U.S. glass industry. The large differences between the theoretical and
Although natural gas is mainly used to melt the raw theactualenergyusedcanbeattributedtoheatlosses
materials,itisalsousedinemission controleitherto and other inefficiencies. The majority of heat loss is
reduce NO in the exhaust stream or to incinerate
x
toxic emissions from glass fiber production. Electri- TABLEIII
city is used in smaller resistance melters, fiberglass
EnergyConsumptionforMeltingandRefining
bushings, tin bath heating, annealing, and for
powering motors to produce compressed air and U.S.average EUaverage
operate conveyers and blowers. specificenergy specificenergy
Furnacetype (GJ/metricton) (GJ/metricton)a
Flatglass
4.2 Theoretical vs Actual Energy
Regenerativeside-port 8.4 6.3
Use in Melting
Electricboostside-port 6.2 ––
The theoretical minimum energy required to trans- Containerglass
form the crystalline raw materials to a molten liquid Largeregenerative 7.5 4.2
isapproximately2.68GJ/metrictonforasoda–lime– Electricboost 5.3 ––
silica glass formulation. The calculation assumes Oxy-fuelfiredb 4.5 3.3
all available heat is fully utilized and has three Smallelectricmelter 2.7 ––
components: Pressedandblownglass
Regenerative 5.3 ––
* The heat of fusion to form the melt from the raw Oxy-fuelfiredb 3.5 ––
materials
Electricmelters 9.9 ––
* The heat, or enthalpy, required to raise the Recuperative –– 6.7
temperature of the glass from 20 to 15001C
RegenerativeTVtube –– 8.3
* The heat content of the gases released during Insulationfiberglass
melting.
Electricmelters 7.2 ––
Recuperativemelters 6.7 4.3
Theactualamountofenergyrequiredcanbefrom
Oxy-fuelfiredb 5.4 ––
2 to 15times the theoretical minimum, although for
Textilefiber
the majority of large-scale melters it is generally less
Recuperativemelters 10.1 ––
than 8GJ/ton. The amount of energy per unit of
Oxy-fuelfiredb 5.4 ––
product produced can be referred to as the specific
energy or energy intensity. As a comparison to other
aEUvaluesassume70%culletforcontainerglass,40%cullet
materials, the values for glass are similar to those of
fortelevisiontubeandtableware,and20%culletforflat.
cement(5.4GJ/ton)andlowerthanthoseofsteeland bDoesnotincludeenergyusedinoxygenproduction.
8
GlassandEnergy
TABLEIV by the other capital and operating costs, the desired
production rate, and environmental performance
ComparisonofEnergyEfficiencyCharacteristics
(although energy efficiency and environmental per-
Recuperativefurnaces 20–40% formance are related).
Regenerativefurnaces 35–55% Improvements in combustion efficiency have been
Electricfurnacesa 70–90% driven by the need to reduce NO emissions.
x
Oxy-fuelbfurnaces 40–55% Reengineeringoftheburnersandthecontrolsystems
Regenerativew/batch/culletpreheater 50–65% hasalsoresultedinoverallenergysavings.LowNO
x
Oxy-fuelw/batch/culletpreheaterb 50–65% burnersreducethevolumeofcombustionairtoclose
tostoichiometriclevels,suchthatonlytheamountof
aDoes not include 25–30% efficiency of off-site electricity air needed for combustion is introduced. Air that is
generation.
introduced but not used in the combustion process
bDoesnotincludeenergyusedinoxygenproduction.
would simply be heated and would carry that heat
away in the exhaust.
from the furnace structure and through the removal Themeltneedstime(residencetime)inthetankto
of exhaust gases for fossil fuel melters. As described produce a defect-free glass. However, the longer the
previously, modern melters that burn fuel employ a melt remains in the tank, the more energy is
regenerator or recuperator to recover heat from the expended in the process. Any reduction in the
exhaust gases and preheat the incoming combustion amount of time needed to melt, fine, or homogenize
air. This reduces the overall energy consumption by the melt will result in direct energy reductions.
50–70% compared to if it were direct-fired. The Residence times depend on the furnace temperature,
energy source, heating technique, and heat recovery composition of the batch, grain size of the batch
method are central to the design of the furnace. The ingredients, amount and grain size of cullet, and
same choices also affect the environmental perfor- homogeneity of the batch. Improved batching
manceandenergyefficiencyofthemeltingoperation. techniques include ensuring optimal grain sizes and
theadditionofcullet.Becauseofthelowviscosityof
theinitialmeltingfluxes,theremaybeatendencyfor
4.3 Energy Reduction
the batch to ‘‘unmix’’ and require excessive homo-
The glass industry is constantly striving to close the genizingtimes.Pelletizingorpre-reactingsomeofthe
gapbetweenthetheoreticalandactualenergyneeded batch components may reduce this effect. Furnace
tomeltrawmaterialsintoglass.Becausemeltingand temperature can be increased to reduce melting and
refiningusethemajorityofenergyinglassmaking,it refiningtimes,althoughthisisdoneattheexpenseof
is this part of the process that must be examined the life of the refractory lining. Electric boost can be
initially in any attempt to reduce per unit energy usedinfossilfuelmelterstoincreasethepullwithout
consumption. There are two general approaches changing the furnace size. In general, the electricity
toward reducing energy consumption: optimizing required to increase the pull rate by 1ton is
the existing system or completely replacing the approximately 22–28kW. The degree of homogene-
current methods with and advanced melting system. ity depends on the quality requirements, which vary
To reduce the amount of energy in conventional considerably between and within glass sectors.
melting furnaces, the following may be addressed: Bubbling is a method of decreasing homogenization
time in which air is injected through several nozzles
* Improvement of the combustion efficiency
installed at the bottom of the melting chamber,
* Reduction of the residence time in the tank
agitating the melt and promoting mixing.
* Improvement of the insulation around the
Asmuchas30%ofenergycanbelostthroughthe
structure
furnacestructure.Structuralheat lossesare inversely
* More effective use of the exhaust gas heat
proportional to the furnace size because of the
* Oxy-fuel firing
change in melter surface area-to-volume ratio. Thus,
* Increased cullet use
larger furnaces have inherently lower heat losses.
The melting technique chosen by a manufacturer Regardlessofthesizeofthemelter,improvementsin
is largely determined by economic considerations, the insulation of the structure will increase energy
and this can have a major effect on the energy efficiency. Most glass contact refractories that are
efficiency.Althoughenergyefficiencyisanimportant used to line the furnace are dense, fusion cast
aspectoftheoperatingcosts,thesemustbebalanced materials that have high thermal conductivity and
9
GlassandEnergy
needtobeinsulated.Theapplicationofinsulationto up to 4001C. Indirect preheating systems use a plate
afurnacedependsontheareaofthefurnaceandthe heat exchanger and may consist of blocks of
operating conditions, and not all parts can be horizontal waste gas flow and vertical material
insulated. Insulation increases the operating tem- funnels. This can lead to heating of the cullet or
perature of the refractory lining, which tends to batch to 3001C. The exhaust gases exiting the
shorten its life span. Overheated refractory material preheaters will be cooled by approximately 270–
can also degrade and shed material into the glass, 3001C. Potential problems are increased particulate
creating defects. Therefore, more efficient insulation emissions and size sorting, although this is not as
needstobebalancedwiththequalityoftheglassand important in preheating cullet.
lifetime of the refractories. As mentioned previously, oxy-fuel firing can be
Advances in refractory material engineering have more energy efficient than air-fuel furnaces. Most
allowed furnaces to operate longer between rebuilds glass companies have incorporated some oxy-fuel
at higher levels of insulation. For example, super- firing into their operations, either as oxygen-enriched
duty, high-purity silica and alumina–zirconia–silica air or as 100% oxygen. A furnace that is equipped
brickshavebeendevelopedforthecrowntoimprove with oxygen-enriched air can produce the same
insulation.Sprayedfiberinsulationcanbeappliedto amount of glass as with air combustion but at lower
the regenerator structure, resulting in up to a 50% fuel input because of the reduced exhaust volume.
reduction in regenerator structural heat losses. However,whenregenerativefurnacesareconvertedto
Up to 30% of the heat from melting is expelled oxy-fuel firing, the heat-recovery checkers are elimi-
through the exhaust gases. Furnaces equipped with nated. In addition, oxygen is expensive and to make
regenerators or recuperators recover some of this oxy-fuel systems economical, a cost-effective and
heat, with end-fired regenerative furnaces generally reliable source of oxygen is required. Several on-site
more efficient than cross-fired regenerative furnaces. systems can be used to fractionate oxygen from
However, combustion control and furnace size are ambient air. Liquified oxygen can beused, but its use
more limited for end-fired furnaces. The amount of islimitedbytransportationcosts.Theenergyrequired
energy recovered by regenerators may be increased touseoxygenrangesfrom250to800kWhpertonof
by increasing the quantity of refractory bricks in the oxygen.
checkers using enlarged regenerator chambers or in
separate but connected structures. However, as one
4.4 Cullet and Energy
approaches the physical maximum efficiency, there
are limitations on the cost of the extra refractory Cullet has a lower melting energy requirement than
bricks and the limitation of available space. Im- the virginraw materials becausetheheatoffusionof
proved materials and designs such as corrugated converting crystalline material to a liquid is not
cruciform bricks can improve heat transfer. needed and its mass is 20% lower than that of the
Gases exiting the regenerators or recuperators are equivalentbatchmaterials.Generalenergysavingsare
300–6001C, and there are other potential areas of 0.15–0.30% for each percent of cullet added to the
usefortheremainingwasteheat,althoughtoomuch batch.Forexample,amelterthatuses50%culletwill
heat loss in the exhaust stream will adversely affect use 7.5–15% less energy than a batch with no cullet.
the ability of the plant to vent the exhaust up the Most plants routinely recycle all internal cullet––
stack.Oneheat-recoverytechniqueistopassexhaust that is, waste glass produced within the production
gases through a boiler to generate steam. The steam process.Forflatglassproduction,approximately10–
canbeusedforheating,on-siteelectricitygeneration, 20% of the produced glass returns as internal cullet
or to drive air compressors. Potential problems are due to edge trimming, defects, product changes, and
fouling of the boiler tubes, high capital costs, and breakages.Externalsourcesofcullet(fromconsumer
interconnection with the utilities (in the case of orexternalindustrialsources)arealsoused,withthe
electricity generation). amount used depending on the product, specifica-
Another potential use for the heat is preheating tions, availability, and price. The composition of
the batch or cullet, which is normally fed cold into external cullet can vary and the presence of
the furnace. This can be accomplished by direct or impurities such as metal and ceramics may limit its
indirect exposure of the batch to the exhaust gases. use. Stringent final product quality requirements can
Direct preheating can be done by directing the hot restrict the amount of foreign cullet a manufacturer
exhaust gas opposite the batch as it falls from a canuse.Containerglassmanufacturingbenefitsfrom
feeder.Thiscanraisethebatch orcullet temperature bottle recycling schemes and, depending on the
10
GlassandEnergy
country,thissectorcanconsistentlyuse80%culletin these segment the furnace into distinct sections for
the batch. It is more difficult for flat glass manu- melting,fining,andhomogenization.Onesegmented
facturers to find external cullet, although they are melting concept takes advantage of the different
abletoobtainscrapfromtheindustriesthatpurchase melting requirements of virgin batch materials and
their product, such as architectural and automotive cullet. Virgin batch is charged into an all-electric
window manufacturers. All manufacturers keep a premelting furnace that converts 75% of the raw
store of cullet on site in the event that the raw material into glass. The premelted batch then moves
material batching system is offline and they need to into an enlarged doghouse where cullet is added
melt at 100% cullet. (cullet comprises at least 60% of the raw material).
The batch/cullet mix then enters the second melting
chamber that uses oxy-fuel burners. The potential
4.5 Advanced Melters
benefits are up to 25% improved energy efficiency
Thenextgenerationofglassmeltersneedstobemore and lower emissions. Another concept under devel-
energy efficient than current melters, but they also opment, referred to as the advanced glass melter,
mustoperateasrapidlyandeconomicallytoproduce injects all batch materials into the flame of a natural
the highest quality product. Currently, the most gas-fired furnace, heating them rapidly and dischar-
efficient glass melters are the large regenerative ging them into the melt chamber. Because of lower
furnaces,buttheyarealsothemostcapitalintensive, flametemperatures,thissystemhaspotentialforlow
have high rebuild costs, and are very inflexible in NO emissions.
x
terms of varying or reducing output.
There are potential technologies that can make
Acknowledgments
meltingmoreefficientbymakingheattransfertothe
batchmoreefficient.Submergedcombustionmelting C.W.S. thanks Jim Shelby (Alfred University), Tony Longobardo
usesnaturalgasandanoxidizerinjecteddirectlyinto (Guardian Industries), and Ernst Worrell (Lawrence Berkeley
themelt,whichimprovesheattransferandpromotes NationalLaboratory)fortheirassistanceandreviews.
rapidmixing.Plasma,whichisapartiallyionizedgas
that conducts electricity, can be very efficient at
transferring heat directly to batch materials, but it is SEE ALSO THE
difficult to apply on a large scale. Microwaves can FOLLOWING ARTICLES
heat without direct contact, but different compo-
nents of the batch have different susceptibilities to Aluminum Production and Energy (cid:2) Cement and
heating by microwaves. Energy (cid:2) Industrial Energy Use, Status and Trends (cid:2)
One obvious approach to reducing the energy Plastics Production and Energy (cid:2) Steel Production
needsofthecommercialglassindustryistoreducethe and Energy
residence time of the melt in the furnace. Rapid glass
melting systems would allow the raw materials to be
Further Reading
melted and homogenized without the need for the
relatively slow, convective mixing that is currently Glass Manufacturing Industry Council (2001). ‘‘Glass Melting
practiced.Theindustrywouldlikethecurrentaverage Technologies of the Future.’’ Glass Manufacturing Industry
24- to 48-h residence time of a melt reduced to 1h. Council,Westerville,OH.
Integrated Pollution Prevention Control (2000). ‘‘Reference
Combined melting and mixing can be achieved
DocumentonBestAvailablePracticesintheGlassManufactur-
using a variety of mechanisms. Higher temperatures
ingIndustry.’’EuropeanCommission,InstituteforProspective
reduce viscosity and decrease mixing times, but the TechnologicalStudies,Seville.
reduced time must be balanced by the increased Ruth,M.,andDell’Anno,P.(1997).Anindustrialecologyofthe
energy input. Higher temperatures also adversely U.S.glassindustry.Resour.Policy23,109–124.
Shelby, J. E. (1997). ‘‘Introduction to Glass Science and
affecttherefractoryliningandcanincreaseemissions
Technology.’’RoyalSocietyofChemistry,Cambridge,UK.
of NO and volatile components of the batch.
x U.S. Department of Energy (2002). ‘‘Energy and Environmental
Improved mechanical mixing will decrease homo- ProfileoftheU.S.GlassIndustry.’’U.S.DepartmentofEnergy,
genization times. Decreased fining times can be Washington,DC.
achieved by controlling the atmosphere surrounding Varshneya, A. K. (1994). ‘‘Fundamentals of Inorganic Glasses.’’
AcademicPress,SanDiego.
the melt or applying a vacuum.
Woolley, F. E. (1991). Melting/fining. In ‘‘Engineered Materials
There are several conceptual designs of fully
Handbook:CeramicsandGlasses,’’Vol.4,pp.386–393.ASM
reengineered methods of melting glass. Many of International,MaterialsPark,OH.