In The Name OF God

Bahonar University Of Kerman

Materials Department

OXY-FUEL TECHNOLOGY & FURNACES

Alireza Mohamadizadeh

87425031

Metals Production1

Dr. A.Jafari

6/10/2010

Definition of Oxy-Fuel Term:

Oxy-fuel refers to technology that burns oxygen with gaseous fuel. As compared to air,

which contains 20.95% oxygen, higher temperatures can be reached using pure oxygen.

Approximately the same total energy is produced when burning a fuel with oxygen as compared

to with air; the difference is the lack of temperature diluting inert gases. The most common fuel

burned in a torch with oxygen is acetylene; even though it presents special handling problems, it

has the greatest heat output. The process has also been proposed as a method of capturing carbon

dioxide from coal-fired electric power plants because the output flue gases from combustion in

oxygen as opposed to air have a higher carbon dioxide content fraction. The combustion with

oxygen is called oxy-fuel combustion. I

With today’s high fuel costs, industrial gas suppliers often get asked if oxygen can save

their customers money. The economics depend on plant operations and particular business needs,

but usually oxygen makes sense when customers want to increase production or reduce

emissions of NOx and particulates. Although oxygen must be purchased (while air is available

for the cost of running a blower) its proven benefits can result in immediate cost savings.

Oxygen enrichment can increase production rates without the costly addition of another furnace,

thanks to increased thermal efficiency. Furnace consolidation is also possible—processing the

same amount of material in fewer furnaces offers plant managers more flexibility and reduced

costs. There are a variety of techniques for implementing oxygen enrichment. Oxygen may be

mixed with the combustion air stream, strategically injected through lances into the furnace, or

used in burners designed especially for higher oxygen concentrations. Choosing the optimal

technique depends on the type and size of furnace, operating benefits desired, capital cost

considerations, and supplier experience.

For a customer who can take advantage of 20 to 30% more production at 15 to 20%

lower cost per pound of product, oxygen technologies are a great fit. These customers often

realize paybacks of two to three months with doubled profit margins. However, if the only goal

is to reduce fuel costs, oxy-fuel combustion may not be the best approach.

Combustion is the chemical reaction between fuel and oxygen. When the oxygen

concentration is raised above the 20.9% present in air, the air is said to be oxygen-enriched. In

industrial heating applications reducing the amount of inert nitrogen gas flowing through the

combustion process makes the process more thermally efficient, since less energy is wasted to

heat the nitrogen, which is emitted through the stack. The reduction in nitrogen flow also has

environmental benefits: lower NOx emissions and lower particulate emissions.

Oxygen use in reverb furnaces

Recently both full oxy-fuel and air-oxy-fuel technologies have been applied to reverb

furnaces. Because reverb furnace design is based on air-fuel firing, the combustion space is often

large, allowing for the significant volume of the combustion gases. For furnaces that need

maximum production rates and that have molten metal pumps, full oxy-fuel can deliver the best

economics. However, if done incorrectly, 100% oxy-fuel can drastically reduce the gas flow

patterns and can lead to uneven heating. For furnaces with limited metal movement or significant

holding and casting times, air-oxy-fuel techniques often deliver the best results. Having

experience with both types of systems allows a supplier to custom fit the solution to the user.

Performance results using oxygen in reverb furnaces vary according to many factors. At a

typical operation, production increases range from 20 to 35%, along with fuel savings of 20% to

40%, reduced flue gas volume by up to 60%, and reduced total melting cost of 20%. II

OXY-FUEL COMBUSTION III

Oxy-fuel refers to the practice of totally replacing air as the source of oxidizer for

combustion with industrial grade oxygen. Industrial grade oxygen is defined as liquid oxygen

supply vaporized to a gas or on-site generated oxygen. Liquid oxygen supply generally has a

purity of over 99.99% whereas on-site generated oxygen purity is usually in the range of 90% to

93%.

The advantage of using on-site generated oxygen is lower cost as the product does not

need to be liquefied or transported and is delivered at lower pressure to minimize power

consumption.

The general advantage of replacing air with industrial grade oxygen is that the nitrogen

ballast brought to the combustion process with air is almost or completely eliminated. Reduction

of nitrogen in combustion allows for higher flame temperature and combustion efficiency as

lower combustion gas volume reduces the amount of heat taken from the flame and lost to the

exhaust.

Conceptual Figure of idealized industrial furnace

Thus, the benefits of using oxy-fuel as compared to air-fuel combustion are as follows:

Reduced Energy Consumption

Increased Heating Rate Resulting in Higher Production (with no increase in furnace

temperature set point)

Reduced Furnace Emissions

In addition to the benefits mentioned above, the option of using oxy-fuel combustion can

sometimes result in lower capital investment as compared to other methods of improving

efficiency such as recuperators or emissions control equipment. Also, in some cases conversion

to oxy-fuel combustion has resulted in less scale loss due to better control and shorter heating

time. The following sections discuss the benefits in more detail.

Available Heat of Combustion

The available heat of combustion is here defined as total energy input minus energy lost to

the exhaust. Percent available heat is the energy left after exhaust losses divided by the total

energy input as per the following calculation.

% Available Heat = (Net Heat Value of Fuel - Exhaust Energy)/Gross Heat Value of Fuel

The following graph compares calculated available heat for natural gas combustion with ambient

air-fuel and oxy-fuel assuming 2% oxygen content in the exhaust.

As illustrated in Figure 1, oxy-fuel results in substantial increases in available heat as

compared to air-fuel combustion. The increase in available heat is directly related to reductions

in energy consumption and increases in furnace throughput as discussed in the following

sections.

Energy Savings

An increase in available heat of combustion means that less heat is lost to the exhaust and

a larger percentage of the total energy input is left to do work in the furnace. Thus, when

available heat is increased the amount of total energy input required to do a constant amount of

work is decreased. Figure 2 shows the calculated energy savings with oxy-fuel versus air-fuel

using the available heat curves shown in Figure1.

The calculated energy savings shown in Figure 2 are based on theoretical comparisons of

calculated available heat of combustion. Actual energy savings will vary depending on fuel type,

existing combustion ratio and combustion air temperature. Other factors such as reduction in flue

port size and radiation loss can impact actual energy reduction results with oxy-fuel. The general

range of fuel savings shown in Figure 2 have been confirmed by actual installations.

Another impact of increased combustion efficiency with oxy-fuel is the ability to increase eating

rate and obtain more furnace throughput as discussed below.

Production Increase

The practical limit to production increase is dependent on the ability of the load to absorb

heat and the time and temperature at which the load is exposed to heat. AGA’s experience from

various oxy-fuel installations is that product throughput can be increased in most operations

without increasing furnace temperature set point with the exception of furnaces that are already

meeting a set temperature ramp limit. Besides the increase in available heat, the higher oxy-fuel

flame temperature and radiation potential of combustion gases have a positive impact on heating

capacity and production rate.

Oxy-fuel flame temperature is generally 1,000 oF to 1,500

oF higher than air-fuel flame

temperature. As radiation heat transfer is dependent on the temperature difference from the

source to the receiver to the fourth power, oxy-fuel combustion results in a large increase in

flame to load radiation potential. The combustion products from oxy-fuel are also better sources

of radiant heat transfer. This is because the majority of air-fuel combustion product is nitrogen

which is not as an efficient radiant heat transfer mechanism as carbon dioxide and water

vapor that make up the majority of oxy-fuel combustion products.

In some cases, furnace production is limited by the amount of gaseous emissions

permitted to exit the furnace. Oxy-fuel combustion can also be a means to reduce furnace

emissions and allow for increased production capacity within an allowable emission cap. Oxy-

fuel’s impact on furnace emissions is discussed in the next section.

Emissions

The volume of exhaust is substantially lower with oxy-fuel. Total exhaust volume with

oxy-fuel is generally 70% to 90% less than total air-fuel exhaust volume. In many cases the

exhaust volume reduction alone can be beneficial, especially where existing pollution control

equipment is limited and/or where particulate emissions are a concern. A more important result

with oxy-fuel combustion is lower emission of certain exhaust gas constituents.

The most obvious result of using oxy-fuel is that fuel consumption is reduced. With the

reduction in fuel consumption the emission of CO2 is lower over a given time or per unit of load

heated. While regulation of CO2 is not currently a major concern within industry, the political

climate indicates that more stringent regulation of CO2 emission may be forthcoming.

A more immediate concern of most furnace operators is NOx emission. With oxy-fuel

combustion the partial pressure of nitrogen in the combustion products is drastically reduced

lowering the potential for NOx formation even at elevated flame temperature. In a recent oxy-

fuel installation for box forge furnaces NOx emissions were 84% below allowable limits and

over 80% lower than NOx emissions from a similar air-fuel fired furnace. Many factors will

impact the NOx emission rate from oxy-fuel fired furnaces. Purity of the oxygen product is one

such factor. However, the major factor in minimizing oxy-fuel NOx emissions is furnace

pressure control. Secondary air leak combined with high oxy-fuel flame temperature can greatly

reduce the impact of oxy-fuel combustion as a NOx control technique. Good pressure control is

just one practical consideration among others for converting steel reheating furnace to oxy-fuel

firing.

CONSIDERATIONS OF USING OXY-FUEL FOR STEEL REHEATING

Oxy-fuel Burner Design and Location

Oxy-fuel flames have a higher temperature with less volume and length than air-fuel

flames. Figure 2 illustrates the general change in flame length as a function of percent oxygen in

the combustion air. The flame characteristic with oxy-fuel needs to be considered when

designing oxy-fuel burner systems for steel reheat applications. Generally, steel heating demands

even temperature distribution so that localized over heating or under heating in the product is

avoided. The type and placement of oxy-fuel burners depends on the type of furnace and the

proximity of flames to steel product.

In soaking pits and car bottom furnaces where the flame is in close proximity to the steel

product,AGA has utilized oxy-fuel burners designed to recirculate combustion gases into the

oxy-fuel flame. Recirculation of the combustion products promotes movement of gases in the

heating chamber, thereby minimizing temperature differences. Also, recirculation of combustion

gases into the oxy-fuel flame lowers peak flame temperature and promotes a more even flame

radiation profile to protect product closest to the burner from being over heated. AGA uses

several oxyfuel burner designs to obtain the recirculation effect including a patented nozzle

along with other designs that utilize oxy-fuel flame momentum to produce gas recirculation.

Figure * Volume Of Flue GasIV

Recirculation of combustion gases into the oxy-fuel flame is not necessary for steel

heating application in all cases. Many furnace designs offer combustion spaces where secondary

radiation from the furnace walls provides a large enough portion of the heat source so that the

effect of flame radiation profile is negligible. An example of such a furnace is a box type forge

furnace where all of the energy input is above the steel product. In this case, the major

consideration is to provide the number and location of oxy-fuel burners that will produce the

desired heat distribution. Once the proper design and location of the oxy-fuel burners has been

determined, control of the oxy-fuel combustion system is the next major consideration.

Oxy-fuel Combustion Control

While providing substantial benefits to efficiency, the low volume of combustion

products with oxy-fuel require some special attention when designing combustion control

systems. Proper control of combustion ratio is critical for steel heating processes as the products

of combustion make up the heating atmosphere and ultimately impact the rate and type of scale

formation. In air-fuel combustion systems, the high volume of nitrogen brought into the

combustion process with air provides a damper or safety factor against changes in air to fuel

ratio. With oxy-fuel this damper is almost completely eliminated. This means that a percent

change in oxygen to fuel ratio with oxy-fuel will have a larger impact on heating furnace

atmosphere than same change with air-fuel. Figure 3 compares the impact of combustion ratio

changes on furnace atmosphere with air-fuel and oxy-fuel.

Figure 3 illustrates that good control of furnace atmosphere with oxy-fuel requires a

higher level of accuracy for the control system than with air-fuel. Mass flow compensation is

usually required to meet the accuracy needed to maintain good furnace atmosphere with oxy-

fuel.

Another important variable for control of oxy-fuel combustion involves furnace exhaust

and pressure control systems.

Exhaust gas volume with oxy-fuel will be reduced in the range of 10% to 30% of air-fuel

exhaust volume depending on the amount of thermal efficiency gains. This means that existing

designs used to control furnace pressure with air-fuel are in most cases not adequate to maintain

good furnace pressure control when converting to oxy-fuel combustion. Specifically, the control

range of pressure control instrumentation with the larger flue port and damper sizes used with

air-fuel will be reduced to the point of being ineffective with oxy-fuel exhaust volume.

Therefore, it is necessary to reduce flue port size when converting to oxy-fuel or to compensate

for the lower exhaust volume when designing new furnace installations. The lack of good

furnace pressure control can result in tertiary air leak which can provide a source of nitrogen for

additional NOx formation thus minimize or eliminate the potential for reduction of emissions.

Air leak into the furnace will also influence furnace atmosphere and introduce complications for

steel surface quality control.

Steel Surface Quality

The rate and type of scale formation is an important consideration for steel reheating

furnace operations regardless of the type of combustion system. Besides steel alloy type, furnace

atmosphere along with heating rate are the major factors governing scale formation. As

previously discussed, good control of combustion ratio and furnace pressure are required to

maintain control of scale formation and steel surface quality. However, the atmosphere produced

by oxy-fuel combustion is different than the atmosphere with air-fuel combustion even under

ideal conditions as shown in Figure 4.

As evidenced by more than 35 separate installations of oxy-fuel for steel reheating by

AGA, the change in furnace atmosphere with oxy-fuel has no detrimental effect on scale

formation and in some cases has proven to be a benefit. The higher partial pressure of CO2 and

H2O in the products of combustion provide a more efficient heat transfer mechanism allowing

for increased heating rate which reduces the time factor for scale formation. Also, comparison of

air-fuel and oxy-fuel shows that the characteristics of scale formation change with oxy-fuel.

In one case where a stainless steel strip annealing furnace was converted to oxy-fuel

firing, the scale formed on the surface of the strip was a thinner layer than was observed with air-

fuel firing.

The reason for the change in scale characteristic is thought to be because the oxy-fuel

atmosphere quickly produces a thin and dense oxide layer which prevents further oxidation and

scale formation. The oxide layer was reduced to the extent that the time required for pickling was

substantially reduced. This particular project resulted in assignment of US patent number

5,783,000 and the European patent EP 0 804 622 ―Method for Heat Treatment of Steel, and

Products of Steel‖ to AGA Aktiebolag, Lidingo and Avesta Sheffield Aktiebolag, Avesta, both

of Sweden. The thin protective oxide layer observed in the strip annealing furnace mentioned

above has been observed in numerous installations and for various grades of steel.

Refractory/Insulation for Oxy-fuel Fired Reheat Furnaces

Experience from a number of installations has shown that with proper placement of

burners such that hot spots on or flame contact with furnace lining is avoided, there are no

special refractory modifications needed for oxy-fuel firing in reheat furnaces. As with any burner

system, attention needs to be paid to oxy-fuel burner orientation and the flow path of hot flame

gases to not only obtain good product heating but to also avoid possible damage to furnace

lining. As an example, in one soaking pit installation fired with one oxy-fuel burner at each end

of the pit, the opposing oxy-fuel flames created a hot gas flow pattern that resulted in accelerated

wear in the center of the pit roof lining. This problem was rectified by off setting the orientation

of the oxy-fuel. In this case the pit was insulated with fiber lining and the use of fiber insulation

with oxy-fuel firing has been an area of concern for many operators. AGA has made a number of

oxy-fuel installations in furnaces with fiber insulation. Some specifics from several such

installations are listed in Table 1.

Table 1: Oxy-fuel Burner Installation from Some Fiber Lined Furnaces

Table 1 illustrates just a few installations as reference. Experience in over 30 installations

has shown that while refractory and insulation design is a major issue regardless of the

combustion system, standard insulation and refractory types can be utilized with oxy-fuel

combustion. With proper design considerations in the areas previously discussed, oxy-fuel

combustion can be effectively utilized to improve performance of steel heating furnaces.

CASE STUDIES OF OXY-FUEL INSTALLATIONS

The following case studies are offered to illustrate the range of results with oxy-fuel

combustion in various steel reheating furnaces. Identification of specific users as well as some

key information is omitted for the sake of propriety.

Case 1

Furnace Type: Stainless Steel Strip Annealing

Furnace Length: 60 Feet

Furnace Width: 6 Feet

Fuel Type: LPG

Operating Temperature: 1,850 oF to 2,250 oF

Insulation: Ceramic Fiber

Number of Firing Zones: 6

Number of Oxy-fuel Burners: 48

Type of Oxy-fuel Burners: Standard - No Recirculation

Total Installed Oxy-fuel Energy: 31.5 mmBtu/Hr.

Results Of Oxy-fuel Conversion: Furnace throughput was increased by 40% with black strip.

Scale formation was greatly reduced resulting in less pickling time and cost.

Case 2

Furnace Type: Car Bottom Furnace, Forging 7-70 Ton Ingots

Furnace Length: ? Feet

Furnace Width: ? Feet

Fuel Type: LPG

Insulation: Ceramic Fiber

Number of Firing Zones: 3

Number of Oxy-fuel Burners: 8

Type of Oxy-fuel Burner High Velocity for Gas Recirculation

Total Installed Oxy-fuel Energy: 10 mmBtu/Hr.

Results Of Oxy-fuel Conversion: 35% increase in furnace throughput by improving

temperature gain from 75 oF/Hr. to 120 oF/Hr. Energy consumption reduced by 45% with lower

NOx emission. Some improvements in material yield were noted with oxy-fuel furnace

temperature uniformity of +-10 oF.

Case 3

Furnace Type: Roller Hearth

Furnace Length: ? Feet

Furnace Width: 13 Feet

Fuel Type: LPG

Insulation: Ceramic Fiber

Number of Firing Zones: 13 (7 top, 6 bottom)

Number of Oxy-fuel Burners: 60

Type of Oxy-fuel Burner Low NOx Staged

Total Installed Oxy-fuel Energy: 70 mmBtu/Hr.

Results Of Oxy-fuel Conversion: The furnace was a green field installation. Oxy-fuel was

selected as a means to lower capital investment and minimize NOx emissions with higher

heating

rate for the same hearth area

Case 4

Furnace Type: Soaking Pit

Furnace Length: 23 Feet

Furnace Width: 8.5 Feet

Fuel Type: Natural Gas

Insulation: Ceramic Fiber + Refractory

Number of Firing Zones: 1

Number of Oxy-fuel Burners: 2

Type of Oxy-fuel Burner Recirculation

Total Installed Oxy-fuel Energy: 20 mmBtu/Hr.

Results Of Oxy-fuel Conversion: Green field installation with energy consumption

reduced to between 30 and 33% of that for comparable air-fuel pits with 900 oF to 1,110 oF

combustion air preheat and hot charge steel. Capital investment was 10 to 15% lower that

comparable recouperated pits. NOx emissions were reduced by over 40%.

The above cases are just a few of over 30 AGA installations and are used to illustrate various

advantages of utilizing oxy-fuel combustion in steel reheating furnaces.

WHEN IS OXY-FUEL A GOOD FIT FOR STEEL REHEATING?

The simple answer to the above question is that using oxy-fuel for steel reheating is a

good fit when it results in economic benefit. The cases presented show that the economic

justification of oxy-fuel is usually based on a combination of benefits that oxy-fuel can provide.

Again, the benefits of oxy-fuel are as follows:

Reduced Energy Consumption

Increased Heating Rate Resulting in Higher Production Capacity

Reduced Furnace Emissions

Possibility of Lower Capital Investment as Compared to Other Methods to Improve

Efficiency

Reduced Scale Formation (In Some Cases)

The needs of the user will dictate which benefits of oxy-fuel can provide a bottom line

benefit. All of the possible benefits should be considered when evaluating installation of oxy-fuel

in steel reheating applications.

Proven oxy-fuel success with rotary furnaces

Rotary furnaces are much more efficient than reverb furnaces since they transfer heat both

via radiation and direct contact between the melt and the refractory as it passes beneath the

charge. The constant mixing and motion of the charge material maximizes the heat transfer

within the scrap. Experience with rotary furnaces has shown that the furnaces are efficient

enough to take full advantage of 100% oxy-fuel.

Aluminum dross and non-ferrous scrap (aluminum, copper, lead-tin solder, brass, etc.) have

traditionally been melted in gas fired rotary furnaces. The furnace arrangements come in two

variants: single pass and double pass. The single pass furnace has a burner at one end and a flue

at the other. The combustion gases flow straight through the furnace-melting chamber, resulting

in a lower thermal efficiency than double pass furnaces. The double pass furnace has the flue and

the burner at the same end. The combustion gases in a double pass furnace have a very uniform

residence time, which significantly improves efficiency. Having a furnace that tilts to pour the

metal and waste, speeds up the tap-to-tap cycle and increases overall productivity.

Many double pass furnaces do not have a furnace door, which limits the operator’s ability to

control the furnace atmosphere, pressure, and emissions, as well as allowing significant heat loss

via radiation. One advantage of these furnaces, however, is that the operators can see into the

furnace during the whole cycle. With a properly designed door and control system, the furnace

can be much more efficiently operated by monitoring flue gas emissions, temperatures, and

furnace rotation parameters. Adding a door and control system to an oxy-fuel double pass rotary

often increases production by 15%. Other advantages of a properly designed door include

reduced heat loss by radiation of almost 1 Million Btu/hr, improved control of free oxygen by

eliminating air entrainment, reduced dusting and noise and improved fuel efficiency of up to

20%. These benefits can cut processing cost by a further 10% compared to a furnace without a

door.V

Oxy-Fuel Fired

Glass Melting TechnologyVI

Introduction

In 1988 the U.S. Department of Energy awarded a program to Praxair, Inc. to demonstrate the use of

oxyfuel combustion in a large commercial glass furnace using an on-site vacuum-pressure swing

adsorption (VPSA) technology. A container glass furnace at Gallo Glass Company was rebuilt In 1991 as

the first large scale oxy-fuel fired furnace1. The successful conversion of the furnace and the

demonstration of substantial fuel savings (15%) and emissions reduction (80% NOx, 80% CO, and 30%

particulates) stimulated the glass industry to adopt the new technology at a rapid rate. By 1996 about 90

commercial glass furnaces were converted to oxy-fuel firing worldwide2. Today, about 200 commercial

glass furnaces of various glass types, including three float glass furnaces, are fired with oxygen3. A great

deal of experiences have been gained through these conversions and the design and operation of oxy-fuel

furnaces have improved substantially. Significant changes in the melting and fining behaviors were

observed under oxy-fuel firing, due to changes in the heat transfer characteristics and the chemical

changes caused by the higher water content of glassmelt under oxy-fuel firing. Most furnaces required

some batch modifications to optimize the glass fining chemistry. Extensive laboratory studies and

mathematical modeling have been conducted to investigate heat transfer, glass fining, alkali volatilization

and refractory corrosion mechanisms. Although accelerated silica crown corrosion was experienced,

especially in early conversions, improved burner and furnace designs made it possible to extended the life

of the silica crown close to that of a conventional air fired furnace. In the following sections the technical

understandings developed on the oxy-fuel glass melting technology are reviewed and potential future

improvements are discussed.

Oxy-Fuel Furnace Design and Burner Placement

Since 1970’s ―oxy-fuel boosting‖ was successfully applied to many types of air fired glass melting

furnaces for production increase. In this method typically a pair of auxiliary oxy-fuel are placed either in

the side walls or in the crown and angled down toward the unmelted batch areas in an air fired furnace to

accelerate the melting and fining rate. In most furnaces glass pull rate increases of about 10 to 30% were

achieved as compared with air only firing4. The experience from these partial conversions as well as

successful full oxy-fuel conversions of steel heating and aluminum melting furnaces provided important

technical background to engage in full conversion of large glass melting furnaces in the 1990’s. In most

full oxy-fuel furnace conversions, the furnace geometry (i.e., the melter area, the bath depth and the

crown height) of the original air fired furnace was maintained to keep the same melting and fining

characteristics and to avoid the extra costs of changing the structural steel beams. Yet, significant

production rate increases were achieved in many conversions. In an oxy-fuel fired furnace the

temperature profile or the heat flux distribution to the furnace load is relatively easily controlled by the

number and the placement of the oxy-fuel burners. This flexibility of placing a flame where it is needed is

a major reason enabling the production rate increase. With a proper burner selection and placement the

crown temperature is reduced at the same glass pull rate. Since the maximum glass pull rate is often

limited by the peak crown temperature, a lower crown temperature enables a higher firing rate and an

increased glass pull.

The crown height is an important design parameter. In general a higher crown height results in a

lower average crown refractory temperature and a flatter temperature profile along the furnace length. In

most conversions the original crown height was maintained. In one furnace conversion, the crown height

was reduced with fused cast alumina crown in order to increase the pull rate and also to reduce side wall

heat loss. In several recent conversions, the crown height was increased to reduce alkali corrosion of

crown. Further discussions of the effects of furnace height are provided in the refractory corrosion

section. Since the combustion and heat transfer conditions under oxy-fuel firing are substantially different

from those under air-fuel firing, retrofitting of an existing air fired furnace with oxy-fuel burners requires

a careful selection of the type and number of oxy-fuel burners and their proper placement on the glass

furnace walls. In general heat transfer is improved by placing a flame close to the glassmelt or angling a

flame toward the glassmelt5. However, it also increases volatilization of alkali species, which would cause

higher particulate emissions and increase the potential for refractory corrosion. Some refractory damages

have been observed in early oxy-fuel conversions due to flame impingement or excessive flame

momentum. Both the opposed and staggered burner arrangements have been successfully adopted. An

advantage of the opposed burner configuration is the symmetry of the heat flux profile. Although some

furnace designers believe that the symmetric design is critical for high quality glass production, CFD

analyses of furnace heat transfer conditions indicate that similar heat flux profiles can be achieved by

either method. In the symmetric burner arrangement two opposing flames meet in the center of the

furnace and create an upward flow toward the crown.

Accelerated corrosion of silica crown was observed in several furnaces with an opposed burner

arrangement under relatively high burner momentum. The problem can be managed either by increasing

the number of the burners, which reduces the individual flame momentum, or by using flat spreading

flames. Thus, the optimum number and the placement of burners depend on the furnace geometry and the

flame characteristics. The location and the number of flue ports are other design issues with different

opinions and experiences. In order to maximize the furnace energy efficiency, flue ports are almost

always located in the coldest area of the furnace, i.e., near the charge end of a furnace. Three different

flue arrangements have been successfully adopted; a single flue in the charge end wall, a single flue in a

side wall, and two symmetrically arranged flue ports on the side walls. The choice for the best flue ports

arrangement depends on the type of batch chargers used and the space available around the furnace.

Burner Type, Flame Characteristics and Heat Transfer

Traditional oxy-fuel burners are characterized by high flame temperature and intense localized

heat transfer, and used for special high temperature applications such as flame polishing of glass and

welding. For heating and melting applications in industrial furnaces, it is often desirable to provide

uniform heat transfer in a large hearth area. Many different burner types producing different flame shapes

(conical flame, flat flame, high momentum, low momentum, luminous and non-luminous flames) have

been developed for both natural gas and fuel oil, and have been successfully applied in glass melting

furnaces. More comprehensive discussions of industrial oxy-fuel burners are provided elsewhere6.

CFD simulation studies and actual measurements5 show that the local heat transfer from a flame is very

sensitive to the flame characteristics. A short intense flame produces a localized high flame temperature,

which has a tendency to create a hot spot on the adjacent glass or refractory wall surfaces. A high

momentum flame induces large furnace gas recirculation, reduces the flame temperature, and tends to

push the peak flame temperature zone away from the burner. A luminous flame, like an oil flame, has a

higher emissivity due to soot particles and transfers heat by flame radiation more efficiently, which

reduces the peak flame temperature. Thermal NOx emissions are reduced as a result. Furnaces fired with

oil flames often have lower NOx emissions than those fired with natural gas for this reason. Traditionally

long and wide luminous flames covering the entire glassmelt surface area are believed to be most efficient

and preferred. Oxy-fuel flames with high luminosity and wide flat flame shapes have been developed and

installed in many furnacesVII

.

Although the local heat transfer from a flame is very sensitive to the flame characteristics and the

burner placement, the overall heat transfer efficiency of the oxy-fuel fired furnaces is actually very

insensitive to the flame characteristics and the burner arrangement. Very seldom burner types can

influence the overall fuel efficiency of an oxy-fuel fired glass furnace by more than a few per cents8. This

somewhat counterintuitive statement can be understood by considering the effects of heat transfer on the

sensible heat loss to flue gas. A reduction in flame heat transfer results in an increase in the flue gas

temperature. Since the volume of the flue gas in an oxy-fuel fired glass furnace is reduced to 15 to 25% of

that of the equivalent air fired furnace, even an increase in flue gas temperature of 100 oC causes only a

few per cent increase in the sensible heat loss to flue gas.

A reduction in the flame heat transfer to a furnace load results in an increase in the local gas

temperature downstream of the flame. A relatively small increase in the bulk furnace gas temperature

causes a sharp increase in the gas-to-load radiative heat transfer due to the strong temperature dependence

of radiative heat transfer. Thus, any loss in local heat transfer from an oxy-fuel flame is naturally

compensated by an increase in the gas-to-load radiative heat transfer caused by a small increase in the

bulk furnace gas temperature after the flame zone. This is especially the case in an oxy-fuel combustion

because of the inherently efficiency in the radiative heat transfer, i.e., higher concentrations of CO2 and

H2O in the furnace atmosphere which increase the gas emissivity and the much longer gas residence time.

In the cross-fired regenerative furnace where the flame fired from a side wall port takes a straight path

and exhausts from the opposite wall. The gas residence time in the centerline of the flame is only about 1

second and the furnace volume average gas residence time is about 5 to 10 seconds. In the oxy-fuel fired

furnace the flue gas form oxy-fuel flames interact with other flames in the furnace and exhaust from the

charge end. The average gas residence time is on the order of 30 seconds. As the hot flue gas travels over

the cold batch toward the flue ports, the flue gas cools down by transferring heat to the cold batch. The

final flue gas temperature exiting the furnace is more strongly influenced by the geometry of the furnace

and the flue port locations in the charge area than the flame characteristics of the oxy-fuel burners in the

hotter zone of the furnace.

Glass Quality and Effects of Water

Glass quality and production rate have improved in most oxy-fuel furnace conversions. The main

reason for the quality improvement (less seeds) is the enhancement of the fining reactions with ―water‖

dissolved in glass. All commercial glasses contain water in the form of hydroxyls, which cause significant

impact on the fining behavior as well as the glass property such as viscosity. In some cases,

manufacturers are controlling the amount of water in glass to assure more consistent product quality9.

Oxy-fuel melting typically increases the water in glass by 30% or more, compared to air firing.

The solubility of water in glass is proportional to the square root of the partial pressure of water. It is

about 1100 ppm weight (expressed as H2O) with 100% water vapor in the furnace atmosphere at 1 atm. In

air natural gas fired furnace where the volume % of water vapor in the furnace atmosphere is about 16-

18%, the typical water content of the soda lime glass product is about 300 to 400ppm. In oxy-fuel firing,

the volume % of water vapor in the furnace atmosphere is about 50 to 55% and the typical water content

becomes about 500 to 600ppm. These measured water contents represent about 70 to 90% of the

saturation level. The high saturation level is surprising in view of the very low diffusivity of OH in glass,

but is believed to be promoted by the convective current of molten glass as well as by the mixing action

caused by rising bubbles from the batch reactions and fining reactions.

Furnace Energy Efficiency and Fuel Savings

The fuel efficiency of oxy-fuel fired furnaces is significantly better than that of the conventional

air fired furnaces. Most of the efficiency gains come from the elimination of nitrogen contained in air,

which constitutes about 78% by volume, and causes a major source of the sensible heat loss in the flue

gas. Another advantage of the oxy-fuel furnace is the stability of the specific fuel consumption over the

life of the furnace. With an air fired regenerative furnace the efficiency of the regenerators deteriorates

with the furnace life and the fuel consumption toward the end of the campaign typically increases by

about 10%.VIII

Fuel savings by oxy-fuel conversion depend on the conditions and the heat recovery system used in the

original air furnace. For container and float glass furnaces with efficient regenerators about 15 to 20%

fuel savings have been achieved. For fiber glass furnaces with metallic recuperators fuel savings are

typically in a range of 40 to 50%. For small speciality glass furnaces, which generally do not have

efficient regenerators or recuperators, fuel savings over 50% have been achieved.

Figure 1 shows the evolution of the oxy-fuel fired furnace performance on energy and emissions,

based on a few of the Praxair’s container glass furnace conversion projects. The first project was the cross

fired regenerative furnace at Gallo Glass and the air baseline data on natural gas consumption and

emissions of NOx and particulates in 1990 are compared with those after the conversion to oxyfuel in

1991.

CO2 REDUCTION FROM GLASS MELTING

FURNACES BY OXY-FUEL FIRING COMBINED

WITH BATCH/CULLET PREHEATINGIX

As the glass industry faces new regulations to reduce CO2 emissions, there are only a few

practical options, i.e., switching to less carbon-intensive fuel such as natural gas and fuel reduction

through furnace energy efficiency improvements. Oxy-fuel firing offers a practical option for

significant CO2 reduction through furnace efficiency improvements. Actual fuel savings and CO2

reduction achieved by oxy-fuel conversion depend on the type of the glass furnace and the conditions

of the heat recovery system in the original air furnace. For large container and float glass furnaces

with efficient regenerators, about 10 to 20% fuel savings were typically achieved. For fiber glass

furnaces with metallic recuperators fuel savings were typically in a range of 40 to 50%. For smaller

speciality glass furnaces, which are generally not equipped with efficient regenerators or

recuperators, fuel savings of 40-60% were achieved. Although a fuel reduction in a furnace

proportionally reduces CO2 emissions from combustion, CO2 emissions for the generation of oxygen

must be included to assess the global effects. The average CO2 emission to produce a ton of oxygen

corresponds to about 90-125 kg at the power plant and about 12-16% fuel savings are required to

break even on the overall CO2 emissions for natural gas fired furnaces [1]. Thus significant

reductions in CO2 emissions have already been achieved through the glass melting furnaces

converted to oxyfuel firing. The integration of a heat recovery system with oxy-fuel firing provides a

large potential for further reduction of CO2, as well as fuel and oxygen consumption.

DIFFERENCES IN RECOVERABLE WASTE HEAT

The batch/cullet preheaters available today were originally designed for a large volume of flue

gas from air fired regenerative furnaces. In order to use them for oxy-fuel fired furnaces, dilution air

is mixed into the flue gas to reduce the flue gas temperature from about 1450 oC to about 500 to 600

oC. The dilution of hot flue gas not only increases the volume of the flue gas, and hence the size of

the down stream gas handling equipment, but also reduces the amount of recoverable heat

substantially. Figure 1 compares the amount of recoverable heat from 410 tpd container glass

furnaces equipped with a batch/cullet preheater for (1) air fired regenerative furnace, (2) oxy-fuel

fired furnace with dilution air, and (3) oxy-fuel fired furnace without dilution air. The flue gas

temperature after the batch/cullet preheater is assumed to be 220 oC in all cases. For the air fired

regenerative furnace the recoverable heat corresponds to the enthalpy difference between 450 oC (i.e.,

assumed flue gas temperature after the regenerators) and 220 oC, or about 50% of 13.6 GJ/hr of waste

sensible heat. For the oxy-fuel fired furnace without dilution air, about 85% of 15.6 GJ/hr of waste

sensible heat is recoverable due to the high flue gas temperature of 1450oC. When dilution air is used

with oxy-fuel fired furnace to reduce the flue gas temperature down to 600 oC, flue gas volume is

roughly tripled and the recoverable heat is reduced to about 68% of the waste sensible heat. If cooled

flue gas is recirculated and used as the diluent, the problem of reduced recoverable heat could be

avoided. However, flue gas recirculation increases the complexity of the process and potentially

creates maintenance concerns.

Figure 1. Recoverable waste heat in flue gas – Air vs. Oxy with and without dilution air

Figure * Variation Of Fuel SavingIV

Clearly it is advantageous to use the hot flue gas from an oxy-fuel fired furnace without dilution

air or flue gas recirculation. Several options have been previously patented [5]. Recuperators can be

used directly to the hot flue gas to preheat oxygen and/or natural gas and to cool down the flue gas to

a temperature acceptable to a batch/cullet preheater. A shadow wall can be installed near the charge

end of a furnace to reduce flue gas temperature. These methods, however, add complexity and costs

to the overall heat recovery process. Praxair is currently developing a batch/cullet recovery system

that can take the hot flue gas without dilution air to simplify the flue gas handling design and to

improve the economics of oxy-fuel fired glass furnace.

I http://en.wikipedia.org/wiki/Oxy_Fuel

II Russell Hewertson, Manager of Combustion Technology,Air Products and Chemicals, Inc.2005

III ―Oxy-fuel Applications for Steel Reheating Furnaces‖ (AISE Iron and Steel Exposition & Annual Convention, September 27th-30th, 1999 -Cleveland, Ohio)

Authors: Chris Ebeling, Carl-Lennart Axelsson, Don Coe

IV High Efficiency Furnace with Oxy-Fuel Combustion and Zero-Emission by CO2 Recovery Ken Kiriishi , Tomoya Fujimine , Atsuhiko Hayakawa Industrial Technology Sect.

Industrial Gas Sales Dept. Tokyo Gas Co., Ltd V Russell Hewertson, Manager of Combustion Technology,Air Products and Chemicals, Inc.2005

VI Advances in Oxy-Fuel Fired Glass Melting Technology , Hisashi Kobayashi. Praxair, Inc., 39 Old Ridgebury Road, Danbury, CT 06810 USA

VII Snyder, W.J., ―Burner Fulfills Performance Promise in Service,‖ Glass Industry, July, 1999, pp. 23 to 24.

VIII Beerkens, R.G.C.,H. Van Limpt, ‖Energy Efficiency of Glass Furnaces‖ , Presented at GMIC Workshop on Evolutionary and Revolutionary Strategies for Keeping Glass Viable through

the 21st Century‖, July 30-31, 2003, Rochester, NY

IX H. KOBAYASHI, K.T. WU, L. H. SWITZER, S. MARTINEZ*, AND R. GIUDICI+ Praxair, Inc., Danbury, CT., U.S.A.