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.
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.
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.
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.