Glass Forming & Processing: A New Twist to Oxy-Fuel Melting

A new oxy-fuel glass melting technology can help glassmakers increase their furnace capacity, improve glass quality and/or redistribute furnace energy more effectively.

For a number of years, glass melting technologies have been based on heating glass with the radiant energy from flames fired horizontally above the melt. In this design, the heat transfer efficiency is generally limited by the size of the furnace and the temperature limit of the refractories. Even oxy-fuel combustion, which has been used for more than a decade, is restricted by these heat transfer dynamics.

Recently, however, a new system* was developed that overcomes these limitations. Rather than firing horizontally, the system directs the oxy-fuel flames almost vertically down onto the batch surface at the charging end of the furnace. By modifying conventional oxy-fuel melting technology, the new melting system provides significant improvements in melting rates and/or quality.

Heat Transfer in Glass Melting

Efficient heat transfer from the heat source to the batch is crucial in glass production. Total heat transfer can basically be described as the sum of the radiative and convective heat transfers, as shown in the equation:

QT = QR + QC

where QT is the total heat transfer to the batch, QR is the radiative heat transfer and QC is the convective heat transfer.

Both radiant (first term) and convective (second term) heat transfer depend on well-known heat transfer variables, such as the simplified equation shown in Equation 1, where Q is the heat to the surface (in watts), e is the emissivity, f is the radiation function, s is the Stefan-Boltzmann constant, g is the convection function, hc is the convection coefficient, A is the area of the batch/glass surface under consideration, Ts is the absolute temperature of the radiant source, Tg is the absolute temperature of the gases and Tb is the absolute temperature of the batch.

The typical heat transfer within a conventional glass melter is illustrated in Figure 1.

In conventional furnaces, the presence of relatively thick boundary layers and a low thermal driving force dictate that approximately 95% of the total heat transfer to the batch and glass bath comes from the combined radiation from the flame and superstructure. Therefore, the attainable heat transfer in conventional furnaces is a function of the melter area and the maximum temperature limit of the superstructure refractory. Since refractory materials have fixed maximum temperatures before failure, the only way to improve the radiative melting rate in fossil fuel glass furnaces is to increase the batch surface area. As a result, existing furnace technology constrains the unit melting rate of furnaces within a well-established range.

Conventional oxy-fuel firing can enhance radiative heat transfer, but its impact on convective transfer is small. Higher flame temperatures and the enhanced emissivity of the combustion products increase the radiation directly from the combustion space, but low velocities, thick boundary layers and the relatively low temperature of combustion products in contact with the batch and glass bath (compared to the temperature of the flame itself) diminish the impact on the convective component.

Firing vertically onto the batch, however, can significantly enhance convective heat transfer. As shown in Figure 2, the vertically oriented oxy-fuel flames in the new melting system actually impinge and flow radially over the batch and glass bath. Significant thinning of boundary layers occurs, leading to intimate contact between the extremely hot flame and the cooler batch and glass bath.

Oxy-fuel flames contain significant concentrations of partially reacted and partially dissociated species. As these species move toward the cool batch surface, they oxidize/recombine and liberate still more energy to the surface, further enhancing the convective heat transfer. This process also increases radiative flux because the burners in the new melting system are designed to produce the majority of the high-temperature combustion reactions near the batch, thereby increasing the radiation to the batch.

The increase in total heat transfer to the batch enables increased melting rates. Further, since the burners are installed in the crown rather than the side walls, fewer obstructions affect burner placement. Consequently, the new melting system can supply more energy per square foot of batch surface area without increasing refractory temperatures beyond normal operating limits. The result is a melting system that enables furnaces to melt more glass, and/or higher quality glass, in a furnace of a given size.

Proving the New Technology

Owens Corning’s Composites group provided the first opportunity to demonstrate this new technology in a production furnace. In 1996, BOC and Owens Corning converted an oxy-fuel furnace to the new melting technology without interrupting production. During a four-month trial, the furnace produced glass at significantly higher capacity than could be achieved using horizontal-fired oxy-fuel burners alone. This study demonstrated:

• A pull rate increase greater than 50% over conventional oxy-fuel capacity
• No increase in emissions on a per ton basis
• No change in analyzed glass chemistry
• No observable damage to the melter superstructure
• A reduction in glass defects

BOC and Owens Corning subsequently applied for a patent for the new technology, which was issued in 2001 (U.S. Patent 6,237,369).

Refining the Technology

The results of the Owens Corning collaboration supported the expectation that the new vertical melting system was commercially viable. To understand the relative contributions of convective and radiant heat transfer, BOC modeled the process with computational fluid dynamics (CFD). Of particular interest were the effects of flame shape, velocity and angles on the rate of heat transfer, the area of maximum heat transfer and batch carryover. Figure 3 illustrates how the heat transfer varies from the center of the flame and confirms that the total heat transfer is significantly higher than the radiant transfer provided by conventional melting technology. The green line represents the CFD calculation of heat transfer (primarily through radiation) to the batch from the background source—i.e. the furnace structure. The red line represents the CFD calculation of heat transfer to the batch directly from the flame. The direct flame heat transfer carries a large convective component, and it decays rapidly with distance from the axis of impingement. The blue line is the total heat transfer to the batch predicted by CFD—the sum of background and direct flame transfer.

To verify the implications of the CFD modeling, BOC teamed with Maxon Corp. to design and build a well-instrumented vertically firing test furnace at Maxon’s facility in Muncie, Ind. The furnace incorporated a vertically adjustable firing target to simulate the varying crown-to-batch distances found in real-world glass furnaces.

Trial firings of burners in the test furnace validated the CFD model predictions. As Figure 3 illustrates, a very good correlation was obtained between the direct flame heat transfer predicted by CFD (red line) and the direct flame heat transfer measured in the test furnace (black squares). The test furnace data also helped to optimize burner design and flow characteristics for installing the system in crowns of various heights.

Commercial Demonstrations

Increased Production of Soda-Lime-Silica Glass. The first trials on soda-lime-silica glass took place in mid-1998 in a three-port air-fuel tableware furnace. The furnace’s designed pull and actual maximum pull were 60 tons per day (TPD). Attempts to increase this pull rate with port firing resulted in unacceptable deterioration in glass quality.

The objectives of this trial were to determine the maximum pull rate attainable by the new melting technology with equivalent or better quality and color control; the ability and success of converting a regenerative furnace “on the fly” to 100% oxy-fuel and vertical melting; and the effect of the new melting technology on glass chemistry (if any).

This installation also provided the opportunity to demonstrate the operational flexibility of the new technology by using it both as a stand-alone melting technology (full conversion) and as a boost to the regenerative furnace. BOC engineers converted the furnace in stages from air-fuel to 100% with the new technology and ultimately back to air fuel—all without interrupting production.

Key observations from this trial included:

• Increased production to 85 TPD with no deterioration in glass quality or color
• No evidence of batch carryover from the vertical flames
• No change in glass chemistry
• Excellent color control
• More stable furnace operation

A time-lapse video analysis showed that the batch piles passing under the vertical flames melt away very quickly. Conventional wisdom suggests that the batch should flow away from the flame impingement area toward a colder surface, yet that does not occur. This phenomenon is consistent with observations in all installations with the new technology.

Melting Boost in a Large Regenerative Furnace. Later that same year, the capability of the new melting technology was demonstrated in a large regenerative furnace—a four-port, 750-square-foot flint container furnace equipped with approximately 1000 kw of electric boost. The plant’s management wanted to pull the furnace as hard as their forming capacity would allow, but attempts to increase pull resulted in glass temperatures at the throat that were too high for the forehearth to condition. Further increases in pull also resulted in unacceptable levels of batch stones. Therefore, the furnace was limited to pulling a tonnage equivalent to 82.5% of forming capacity.

The plant secured a 30-day variance from the production limits imposed by its environmental permit and installed the new technology. Port number one of the furnace was blocked off, and the vertical burners were installed in the port one region of the crown. At the end of the demonstration, the vertical burners were removed, and port one was returned to air fuel operation. Production was never interrupted.

The goals of this installation were to operate the new technology while reducing electric boost to achieve:

• A sustained pull rate up to full capacity of the forming line
• A throat temperature reduction
• Elimination of batch stones

The installation achieved all of these goals. At the increased production level, seed levels held steady or declined vs. baseline levels, with no observable deterioration in refractory material or changes in glass chemistry.

In addition to operating at higher pull and reduced electric boost levels, the hotspot temperature was reduced by an amount that would equate to another 10.5% increase in pull for a furnace of this size. Had the forming equipment been able to handle the load, the fuel in the new technology could have been increased to achieve a pull increase of approximately 30%. (Other potential applications of the new melting technology are discussed in the sidebar below.)

Observations in Operating Furnaces

Visual observation has provided evidence of the rapid melting achieved with the new technology. If a furnace is firing conventional horizontal burners only, and the new vertical burners are installed and operated to maintain the same pull level, the batch line retracts toward the charging end wall. This happens even when the total fuel input into the furnace is equal in both cases.

The new technology can also inject a disproportionate amount of energy into the charging end through crown-mounted burners. This redistribution of energy increases the temperature of the charging end, reducing the temperature differential between the charging end and the hot spot. Conventional wisdom suggests that reducing this differential below the range of 150-200 degrees F (depending on the industry segment) would short-circuit the furnace convection currents. However, no observable negative change in convection currents or in the basic operation of the melter occurs with the new technology.

Enhanced Profitability

In the face of increasing financial pressure, glassmakers are continually seeking ways to conserve capital and to improve the productivity of existing assets. The new melting system provides a potential solution. By using oxy-fuel technology in a new way, the system delivers a step-change improvement in the melting rate of fossil fuel furnaces, providing increases of 25% or more. Glassmakers can use this acceleration to melt more glass within a given furnace, reduce seed and stone counts, increase pack/melt ratio, reduce consumption of fossil fuel, and/or reduce or eliminate electric boost.

By providing glassmakers with a wide range of flexibility, the new system represents not only a revolution in glass melting technology, but also a promise of enhanced glass industry profitability.

Editor's Note

This article was adapted from a paper presented at the 62nd Conference on Glass Problems, October 16-17, 2001, Champaign, Ill.

For More Information

For more information about the new glass melting technology, contact BOC Glass Technologies, 1720 Indian Wood Circle, Bldg. E, Maumee, OH 43537; 866-262-4527 (866-BOC-Glass, toll-free worldwide); fax (419) 891-4008; or e-mail glasstechnologies@us.gases.boc.com.

SIDEBAR: Capabilities of the New Melting Technology

The new CGM technology can be used in several different ways. Though each installation is unique, it is possible to discuss the capabilities of each design in general terms.

Complete Conversions: Full CGM Furnaces
A “full CGM furnace” is an oxy-fuel furnace that incorporates vertical CGM burners in the charging end and horizontal oxy-fuel burners in the refining end. These installations are characterized by a high melting rate (T/ft2) and excellent fuel efficiency. Test data suggests that, as a rule, CGM provides capacity improvements of at least 25% over air-fuel and conventional oxy-fuel furnaces.

A greenfield (new) furnace designed for CGM operation offers glassmakers significant benefits, including:

• Capital reduction from elimination of regenerators and recuperators
• Capital reduction from smaller furnace footprint
• Capital and operating cost reduction from elimination of electric boost system
• Fuel reduction per ton with little to no efficiency decay over the life of the furnace
• Reduction of NOx emissions

Construction of a brownfield (rebuilt) CGM furnace offers the same benefits. As a practical matter, however, the cost of relocating structural supports and forming equipment lines may exceed the cost savings achieved by reducing the furnace size. Therefore, a glassmaker rebuilding an existing furnace as a CGM design is more likely to rebuild in the same footprint and take the benefit of CGM as an increase in tonnage or improvement in glass quality.

CGM Hybrid Furnaces
The hybrid design is of specific interest to the float glass industry. As its name implies, a hybrid furnace is a mixed design. The area of the first two or three ports looks like a CGM furnace; there are no air-fuel burners and no regenerators. The firing is provided by CGM burners in the crown. The remainder of the furnace looks like a standard regenerative float furnace, with conventional air-fuel burners, waist and working end. In this furnace, virtually all of the melting is done by the CGM burners, while the air-fuel zone provides refining capacity. This design provides many of the advantages of CGM firing, and it requires only 40-60% as much oxygen as a standard oxy-fuel furnace. Such a design offers float glass makers the following benefits in comparison to standard regenerative design:

• Reduced capital cost from the partial elimination of the regenerator packs
• Reduced batch carryover and reduced airflow demand through remaining regenerators (i.e., potential for furnace life extension)
• Reduced NOx emissions
• Greater capacity in the same furnace footprint

CGM-Boosted Furnaces
A CGM-boosted furnace is an air-fuel furnace in which CGM burners have been installed in the crown at the charging end. The number of CGM burners and the amount of fuel injected through them can vary widely, depending on furnace size and the amount of boost required. In some cases, the first one or several ports may be blocked off, with the fuel flowing instead through crown-mounted CGM burners in the affected port areas. In other cases, it will not be necessary to shut off any of the ports, but only to supplement them with CGM firing.

Though specific cases will differ, it is generally possible to use CGM to increase the capacity of an air fuel furnace by at least 25%. On furnaces in good working order, this may be taken as an increase above rated air-fuel capacity to achieve significant revenue increases with limited incremental cost.

Furnaces that have developed pull constraints (e.g., plugged checkers, deteriorating ports or port walls, hot spot refractory deterioration) are particularly attractive candidates for CGM boosting. The enhanced thermal efficiency of the CGM flames and the reduction of air-fuel relieve the strain on regenerator air-flow, extending regenerator life. Reduction of electric boosting also slows the process of sidewall wear. Through these mechanisms, CGM boosting not only recovers or increases the capacity of an air fuel furnace, but can extend the life of its campaign. More importantly, CGM can increase the tonnage (cumulative tons/square foot of melter area) melted over the furnace campaign.

SIDEBAR: Success at Owens Corning

The development of the CGM technology—the result of a close collaboration between BOC and Owens Corning—has led to a number of benefits for the Toledo, Ohio-based glass fiber manufacturer. “In 1997, following the initial series of trials, we installed the first CGM in our Guelph, Ontario, Canada, production facility, where we had begun making the new Advantex™ glass fibers,” said Dave Baker, project leader for the Glass & Melting Technology Group in the Owens Corning Granville Technical Center, Granville, Ohio. The Guelph facility had previously been using a combination of gas-air, gas-oxygen and electrically boosted operations to melt the glass.

“We replaced the eight existing horizontal burners with two vertical CGM burners and immediately began seeing results,” Baker said. “The furnace was easier to control because of the small number of burners and the sensitivity of the technology, and its faster melting speed enabled us to increase our throughput by 25%. But most importantly, the CGM lowered our emissions and increased our performance capabilities. Our particulate emissions have been reduced by 75%, while our NOx emissions have dropped by 80-90%, and we’re achieving better quality glass with the new system.”

Since the initial installation, Owens Corning has also begun using the CGM technology in four additional facilities, and it plans to install the technology in additional plants in the future. “It’s been a very good collaboration,” Baker said.