SPECIAL REPORT: GLASS MANUFACTURING: Advanced Melting
Glass manufacturers who need to increase pull and improve quality, consistency, and thermal efficiency while decreasing NOx emissions often have a few options depending on where they are in their furnace's campaign. Traditionally, float glass manufacturers have installed breastwall-mounted zero port oxy-fuel boost burners to extend the furnace life or gain an incremental 10% increase in pull on an existing operating furnace. At a major furnace repair, they can extend the charge wall, convert an existing air-fuel furnace to oxy-fuel or build a greenfield oxy-fuel furnace.
Patented convective glass melting (CGM) technology offers glassmakers another option.* CGM has been proven in multiple installations to deliver better heat transfer, improved quality, increased pull and decreased NOx emissions compared to conventional side wall boost. The CGM technology positions oxy-fuel burners in the crown of the furnace where unmelted batch material exists, rather than in the side walls, thus increasing the rate of heat transfer. One customer likened the process to "placing an object in front of a welding torch rather than to the side of it." The CGM flames have a much lower momentum than a welding torch, but the concept is similar.
Glass manufacturers such as LG.Philips LCD, Anchor Glass Container Corp. and Owens Corning have applied CGM technology to their furnaces. To date, most applications have been directed toward extending furnace life, but manufacturers are increasingly adapting the technology early in a furnace's campaign to improve quality and increase productivity.
*CGMTM, developed by BOC.
Meeting Increasing DemandsThe float glass sector is unique in the glass manufacturing industry because it is the one segment in which production demand-and the number of new furnaces needed to meet that demand-increases every year. An annual product demand growth of 3% in mature markets like the EU or North America is typical, and historically requires a new furnace to be constructed in those markets every year. In addition, the growth in developing markets, such as in Eastern Europe, Russia and India, for increased automotive and building products requires the construction of several new greenfield furnaces a year.
This growth places a large capital demand on float glass manufacturers, since even increasing the size of an existing glass furnace may require the relocation of batch bins and the batch delivery system, along with the excavation and construction of new foundations for the regenerators and melter.
CGM has been proven to increase the melting capacity of a conventional air-fuel float furnace by 20% over its design capacity, which cannot be achieved by other known technologies. The production line of an existing air-fuel furnace can be increased during scheduled repair without changing the footprint of the melter. This permits incremental regional expansion and can allow manufacturers to postpone the large capital investment of a new production line. CGM also permits a smaller greenfield furnace to be constructed, saving capital and improving energy efficiency.
A float furnace's unique design characteristic is the rather large distance (often 3-5 meters) between the first port and the charge end wall, often referred to as the zero port area. It has been common practice to install breastwall oxy-fuel burners in this area to increase pull or to extend the life of the furnace. This zero port area is also an ideal location for CGM technology, which relocates oxy-fuel burners from their more conventional breastwall location to the crown of a glass melter (see Figure 1).
Providing operational data is the ultimate achievement of a new technology, but to extrapolate and predict what may occur on a different furnace requires technical confirmation. In 2004, Glass Services of the Czech Republic was commissioned to independently model both conventional and CGM oxy-fuel burners for the Ford float furnace, whose operating data was in the public domain. The results of the modeling concluded that CGM provided a more effective method to transfer heat to the unmelted batch, since it provides a higher increase in pull rate with lower specific energy than conventional breastwall oxy-fuel burners.*
*The results of this modeling were presented by A.P. Richardson at the 8th International Seminar on Mathematical Modeling and Advanced Numerical Methods in Furnace Design & Operation, as coordinated by Glass Services in the Czech Republic, in 2005.
Case in PointSaint-Gobain has multiple float glass operations throughout the world, and one float line in its European operations had much more downstream capacity than the glass melter could supply. Conventional zero port boost had been previously applied to the healthy modern design float furnace, with a greater-than-anticipated 11% increase in melting capacity.
After appropriate confidence agreements were signed to permit full disclosure of the furnace design and operating parameters, BOC worked closely with both the plant and central engineering to arrive at a suitable CGM solution for the furnace that would provide the additional required glass melting capacity. As a result, Saint-Gobain agreed to allow BOC to install the CGM technology on the furnace's crown.
To ensure that any refractory debris did not cause operational issues, BOC drilled the CGM holes in the crown during a color change. Figure 3 illustrates the typical hole installation in an operating silica crown. BOC located the CGM burners in the zero port area of the crown, and, as it typically does with all burners, angled them slightly away from the charge end wall and all refractory surfaces to mitigate potential damage.
After the hole was drilled, a bonded alumina-zirconia-silica (AZS) sleeve coated with a zircon cement was installed to provide a barrier between the silica crown and the burner firing tube. A specially designed insertion tool improved the ease of installation and reduced the time required.
Once BOC and Saint-Gobain had established the furnace's baseline operations, the breastwall oxy-fuel boosts were removed and the CGM burners were commissioned at the same level of firing. The immediate result of this change was an increased amount of batch glazing and some retraction of the batch line. After a series of incremental pull increases and furnace energy distribution modifications from air-fuel to oxy-fuel, the plant reported a 9% increase in pull over its conventional zero port boost, without any detriment to the furnace or product quality.
The CGM installation increased the pull rate of the float furnace by 20% over its air-fuel design and 9% over that achieved with conventional side wall zero port boost. CGM has run continuously for nearly one year at this plant, enabling Saint-Gobain to increase production and substantially improve the plant's profitability by providing more glass than it has been able to historically produce.
"Our plant has never achieved the higher pull rates obtained by the use of CGM without damaging the superstructure of the furnace. CGM exceeded our expectations for increasing pull while maintaining our exacting product quality," said Jean-Pierre Bocquet, director, Saint-Gobain Conceptions Verrieres.
Glass Melting AlternativeCGM is an advanced glass melting technology that can be used to increase the pull rate of air-fuel or oxy-fuel glass furnaces. The operational and computational results show that CGM can provide a 20-25% increase in pull rate while minimizing capital costs, operating costs and repair downtime. In addition, the increase in pull rate can be obtained with both lower energy consumption and reduced NOx emissions per ton of glass melted.
SIDEBAR #1: Cost ConsiderationsThe two major capital cost items to consider when constructing or converting to oxy-fuel are the regenerators and the crown. An oxy-fuel greenfield glass furnace will require an estimated $3 million less in capital costs due to reduced excavation and the elimination of regenerators, while including the additional cost of the fused cast crown. The conversion of an air-fuel furnace to oxy-fuel may require an increase in capital cost due to:
- The demolition and disposal of the regenerators, plus the construction of new exhaust flues
- The replacement of the silica crown and its support steel with fused cast and its associated support steel
The financial justification of these two cases when considering a 20% energy savings from air-fuel furnaces supports CGM oxy-fuel for new construction. Other considerations-such as no annual reduction in energy efficiency (nominally 1% per year) and improved furnace stability (no reversal)-that can improve the energy efficiency by about 20% and improve product yield should be taken into account for conversion from air-fuel to conventional oxy-fuel.
SIDEBAR #2: Hybrid ModelThe CGM hybrid glass furnace combines the rapid melting benefits of CGM with the standard air-fuel refining process. The furnace can be adapted to a conventional crown design or a lower crown design. The CGM hybrid furnace supplies the following advantages over conventional 100% oxy-fuel conversion:
- Lower conversion capital cost since only the first crown section is made of fused cast materials
- Higher energy efficiency resulting from the use of oxy-fuel waste gases to provide higher combustion air preheat temperatures
- Lower operating cost due to lower oxygen consumption
- Lower process risk for those who have no oxy-fuel float glass experience