Glass melting process and furnace therefor with oxy-fuel...

Glass manufacturing – Processes – Fining or homogenizing molten glass

Reexamination Certificate

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C065S029210, C065S029150, C065S134600, C065S136300, C065S136200, C065S355000, C065S356000, C431S010000, C432S182000, C432S159000, C110S297000

Reexamination Certificate

active

06519973

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to a process and furnace for melting glass forming ingredients. In the typical glass melting furnace, or glass tank as it is commonly referred to, the raw glass making materials, termed batch, are charged into the melting zone of the furnace. Glass tanks are operated continuously and therefore there is an existing bath of molten glass, termed melt, in the melting zone onto which the raw material is placed. The molten glass and un-melted batch are collectively referred to as the charge. The raw batch may be charged into the tank by any of the well-known mechanical charging devices. In practice, the batch materials float on the surface of the molten bath forming a semi-submerged layer containing un-melted solids termed a batch blanket. The blanket sometimes breaks up to form separate batch piles or batch islands (also called rafts or logs). For the purposes of this disclosure, the section of the furnace containing significant unmelted batch solids floating on the surface of a molten glass bath is defined as the melting zone.
The glass tank usually consists of the melting zone and the fining zone. For the purpose of this disclosure, the fining zone is defined as that section of the furnace not containing significant un-melted batch solids floating on the surface of a molten glass bath. Foam or scum may be present on the surface of the molten glass bath in the fining zone or it may be clear, termed “mirror surface” glass. In the fining zone, glass is homogenized and defects, such as bubbles or “seeds” are driven out. Glass is continuously withdrawn from the fining zone. The melting zone and the fining zone of a glass tank may be present in a single chamber or the glass tank may consist of two or more connected and distinct chambers.
Glass has historically been melted in air-fuel furnaces where burners direct flames across molten glass and the exhaust gas from the flames is removed through heat recovery devices to improve the overall furnace efficiency, thereby reducing fuel consumption. Recuperators and regenerators are common heat recovery devices used in the glass industry. A recuperator is typically a metallic shell-and-tube-style heat exchanger that indirectly heats the combustion air with the heat removed from exhaust gases. In the case of regenerators, the exhaust gas passes through the regenerators transferring its heat to the checker packing or other heat storage media within the regenerator. The checker packing is generally constructed from refractory material. The regenerator may be a common chamber per each side of the furnace, a number of separate and distinct chambers attached to the furnace or may be incorporated into the burner supply ducting. The heated packing is used to preheat combustion air which is combined with fuel used to produce the flames during the firing cycle of the heating operation. These heat recovery devices are costly and sometimes limit the furnace life due to design limitations, failure caused by thermal shock to the refractory, corrosion, or plugging. Occasionally glass is melted in a unit melter which is a furnace without a heat recovery device to preheat combustion air.
In the case of regenerators, the thermal storage medium, i.e. the checkers, become plugged by condensed volatiles and particulates from the glass melting process, resulting in insufficient flow of combustion air to the ports. Consequently, glass manufacturers routinely clean out the checker packs to maintain air flow. The plugging problem is noticeably worse for the ports connected to the melting zone of the furnace. The buildup in the regenerator packing which is contacted by gases from the melting zone of the furnace is often viscous and difficult to remove. Controlling buildup of material on the checker packing of a regenerator is the subject of U.S. Pat. No, 5,840,093. The buildup in the down tank checkers which is contacted by gases from the fining zone of the furnace is drier and more powdery resulting in easier removal of the buildup. Because of the less aggressive attack, down tank checkers have been used for more than one furnace campaign.
Near the end of a furnace campaign, it is sometimes the case that the checkers become too badly degraded, at times even collapsing, and sufficient air flow is not possible even after a clean out. The problem usually manifests itself in the regenerator section receiving gases from the melting zone of the furnace. Oxygen enhanced combustion technologies have been used in these “crippled” air-fuel furnaces to extend the furnace life. While the oxygen enhanced combustion technologies do not prevent the checker plugging problem, they do provide a method to continue furnace operation, albeit sometimes with a higher operating cost.
Industrial oxygen has been used to enhance combustion in the glass industry for several decades. Oxygen enhanced combustion can be accomplished by (i) supplemental oxy-fuel burners, (ii) premixed oxygen enrichment of the combustion air, or (iii) lancing of oxygen to the port or burner. Supplemental oxy-fuel is the practice of installing one or more oxy-fuel burners into an air-fuel furnace. Premixed oxygen enrichment is the practice of introducing oxygen into the combustion air usually to a level of up to 30% total contained oxygen (i.e., 9% oxygen enrichment). The amount of oxygen enrichment is limited by materials compatibility issues in highly oxidizing environments. Lancing is the practice of strategically injecting oxygen through a lance into the combustion zone. These oxygen enhancing techniques are applied to furnaces with burners having standard air-fuel designs. The basic air-fuel furnace concept has not been significantly modified to apply the above mentioned oxygen enrichment technologies.
Supplemental oxy-fuel combustion has been applied to air-fuel glass furnaces and has shown benefits. One form of supplemental oxy-fuel combustion is commonly referred to as oxy-fuel boosting. Oxy-fuel boosting is a technology where oxy-fuel burners are added to an air-fuel furnace. Two locations for the oxy-fuel burners have been proposed: near the hot spot position and in the zero port position. Typically the oxy-fuel burners fire constantly, even during the reversal cycle of a regenerative furnace.
The rationale for putting the oxy-fuel burners in the hot spot position is to reinforce the hot spot with additional heat to positively influence the convective flow patterns in the glass melt and, as described in several patents, to affect the position of the batch-line. The overall glass flow pattern is strongly influenced by buoyancy driven flow and the temperature profile in the furnace is important for the buoyancy driven flow. Ultimately the glass quality is affected. This is why glass-makers control and monitor the temperature profile in a furnace.
Similar to the hot spot oxy-fuel boost, U.S. Pat. No. 3,592,623 discloses a process and furnace where at least part of the furnace heating is provided by an oxy-fuel flame from a position downstream of the hot spot. The combustion products of the flame impinge on the un-melted glass making materials (i.e. the batch) causing the un-melted materials to remain near the feed end of the tank until melted. An objective is to control the position of the un-melted batch material (batch-line) in the glass tank. The remaining heating is provided by air-fuel combustion as shown in the figures of '623.
U.S. Pat. No. 4,473,388 discloses an oxy-fuel boost process where the oxy-fuel flames cover substantially the whole width of the furnace and are directed at the batch-line.
U.S. Pat. No. 5,139,558 discloses a process where at least part of the furnace heating is provided by at least one flame from at least one oxy-fuel burner located in the roof of the furnace, the position of the burner being such that the tip of its flame is directed approximately at the batch-line. An objective of bo

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