Case Study: Improving the Melt

Furnace maintenance is paying dividends in energy savings and emissions reductions for one float glass plant.

Cardinal FG Co.'s Portage, Wis., plant, which makes float glass, has always tried to be proactive with maintenance. In early 2004, the plant decided to make a concerted effort to improve its furnace operation. The initial goals were to regain the combustion efficiency and firing conditions that were available when the furnace was at the start of its campaign.

Even though the furnace was over eight years old, the debris in the ports had not built up to the degree often seen in regenerative furnaces, especially in the first few ports. However, enough debris had collected to adversely affect the combustion airflow, and consequently the ability to set the burners to get the desired flame direction relative to the glass surface. From an operations perspective, the plant was no longer able to achieve the optimum flame configuration and fuel profile necessary to meet its high-quality glass requirements.

Along with these combustion problems, the expansion joints in the ports at the junction between the port snout and regenerator mouth were in need of repair. Additionally, the regenerator skews were beginning to open both inside and outside the furnace. As a result, cool ambient air was entering the furnace, and high-temperature exhaust gases were escaping and impinging on the skew support steel. Over time, such impingement can cause the supports to deteriorate and fail. Plant management knew that neither the escaping exhaust gases nor the cool air infiltration were conducive to a long and efficient furnace campaign.

Blasting the Port Debris

Cardinal approached Fuse Tech Inc. with these problems after hearing about the company's process for furnace cleaning and repair. The process uses ultra-high-pressure "water lasers" to remove debris from the ports, along with ceramic welding to repair expansion joints and skew lines.

To accomplish this work effectively and efficiently, the debris is first removed from the port floors. This ensures that the entire port sidewall expansion joint is clean and unobstructed from the port crown to the port floor so that the weld will achieve a good seal.

A hole is made in the side of the furnace, typically by removing a peephole in the regenerator target wall. The water-cooled lance is then inserted across the checker pack and through the port to the port's hot face. The lance is designed to blast the debris back out of the port onto the tops of the checkers. The high level of thermal and physical shock imparted by the water laser breaks the debris into very small particles that fall down through the checker pack and are removed from under the rider arches.

The ability of the water lasers to blast the debris into small particles is a significant advantage. If the debris is simply raked out of the port, it is usually in such large pieces that it plugs the checkers immediately behind the port and is very difficult to remove without disturbing the checker setting.

Once the lance is in place, the water laser is turned on low pressure (2000 to 3000 psi) to remove any loose debris first. The pressure is then slowly increased to the level necessary to cut the remaining debris. In some extreme cases, this pressure can be increased to as high as 10,000 psi to remove highly fused material. Even at such extreme pressures, the water volume is only in the range of two to three gallons per minute. As a result, the water immediately flashes to steam once it has done its work on the debris. Two lances are typically used, one on each side of the furnace, and the crew moves with the furnace reversal.

To reduce the risk of the incoming combustion air carrying airborne particles into the main melter and contaminating the glass batch, this work is only done on the exhaust portion of the combustion cycle. However, the process is being evaluated for use in other areas of the furnace.

Finishing the Maintenance

The amount of time required for port cleaning using the water laser varies, depending on the amount of debris in the ports and on the debris' chemical characteristics. Generally, upstream ports require more time to clean than downstream ports. In cases where the ports are very full of debris and highly fused, a full day might be required to complete a pair of ports.

At the Cardinal FG Portage plant, six ports on both sides of the furnace were cleaned in approximately 20 hours (10 hrs/day over two days). (A checker burnout had been conducted by the Portage plant a short time earlier to remove any sulfate buildup in the lower checker pack and ensure that the debris would easily fall through the clean checker pack openings.) After the ports were cleaned, the program continued with the port expansion joint welding. The regenerator skews on the hot face were sealed on the inside above the port entrances and on the outside above the target walls. The regenerator skews above the target walls were also welded on the cold face to complete the sealing program. The entire process, including port cleaning and welding, took 15 days to complete. The plant remained in production throughout the procedure.

Analyzing the Results

After the entire program had been completed, the Cardinal Portage Wisconsin team noted that NOx emissions decreased 6.6%, SO2 emissions decreased 3.3% and the gas set point decreased by 1%. Additionally, the air/fuel ratio decreased on the left side of the furnace by 0.25%, and on the right side by 0.15%.

The reduction in NOx emissions was achieved due to the ceramic welding, port cleaning and checker burnout, as well as the resulting decrease in air/fuel ratio and gas set point. According to plant personnel, the flame changed from a "rolling flame" to a "longer, cleaner flame" after the port cleaning was completed. "We had less turbulent air and gas mixing at the port neck area, which cleaned up the flames," noted one plant spokesman.

The ceramic welding, port cleaning and checker burnout also caused the change in the air/fuel ratio. Likewise, these factors, along with an increase in furnace temperatures and the ability to observe the melt line, were responsible for the change in the gas set point. "By cleaning up the port flames, we were able to increase the overall temperature of the furnace to a point where [our gas consumption] was reduced and temperatures were held well within our running parameters," said another plant representative. "Additionally, with the debris on the port pans at ports #3 and #4, it was difficult-and most of the time impossible-to see the melt line. This caused us to run our furnace a little warmer to ensure that we would not have batch extending too far down the tank. After the port cleaning, the observation of the melt line was and still is very easy. This visual observation of the melt line is a very large factor in our ability to cut our overall gas usage."

As a result of these maintenance procedures, the Cardinal FG Portage plant was able to improve its overall glass melting process. "The ability to run with sealed regenerator skew lines, clean port pans and a better burn in the tank were all factors that have allowed our facility to run more efficiently," said a plant spokesman.

Author's Acknowledgements

The author would like to thank the Cardinal FG Portage, Wis., plant personnel for their assistance with this article.

For more information about furnace repairs and cleaning, contact Fuse Tech Inc., 3400 Silica Rd., Sylvania, OH 43560; (419) 841-9323; fax (419) 842-0693; e-mail Dshamp8@aol.com; or visit http://www.fusetech.com.

Cardinal FG's Portage plant can be reached at 1650 Mohr Rd., Portage, WI 53901; (608) 742-1966; fax (608) 742-3688. Visit http://www.cardinalcorp.com for information about other companies and subsidiaries within Cardinal Glass Industries.

SIDEBAR: Pros and Cons

As with any process, the port cleaning and ceramic welding processes have both advantages and disadvantages that must be weighed by any plant that is evaluating furnace maintenance procedures. Following is a summary of the pros and cons observed by Cardinal FG's Portage, Wis., plant:

Ceramic Welding

Pros

• The regenerator skew lines and port neck expansion joints were sealed.
• Rat holes on the inside and outside regenerator vertical walls were sealed.
• The outside skew lines were cold-face welded.

Cons

• About 10% of the welding on the skew lines showed significant wear after six months. This situation might be improved by a thicker application of the weld.
• The inside skew lines were not cold-face welded; the plant would like to have ports #2 and #3 cold-face welded on the inside skew. This will be accomplished at a future date.
• The weld has fallen out of two port neck expansion joints and is now on the port pan. Fuse Tech is currently investigating the reason for this failure.

Port Neck Cleaning

Pros

• Cleaned up the flames and provided better mixing at the port necks.
• Decreased NOX/SO2 emissions and fuel consumption.
• Provided the ability to see the melt line, which led to more efficient furnace operation and a reduction in fuel consumption.

Cons

• Some quality problems were associated with the cleaning (i.e., defect density doubled during the port cleaning procedure), although saleable glass was still being produced. While quality issues cannot be completely avoided, especially when cleaning the downstream ports, the cleaning can be planned for times when defects will have less impact on the product.
• A loss in production occurred during the two days required for the cleaning. The plant will attempt to schedule future cleanings during times when production losses would be minimized.