A Review of Supplemental Oxygen Methods for Glass Melting
Increasing the amount of oxygen in the air from its initial 21% significantly increases the flame temperature achieved with any fuel. For example, natural gas burned in the air has a flame temperature of 3520 °F while the flame temperature of natural gas burned in 23% O2 is 3640 °F.
This effect is shown in the figure below.
Higher flame temperatures in the glass furnace improve the heat transfer to the batch and glass. This is due to the fact that all three heat transfer mechanisms, conduction, convection and radiation, are flame temperature dependent:
||Q µ (Tf - Tp)|
||Q µ (Tf - Tp)|
||Q µ (Tf - Tp)|
Tf = Flame Temperature
Tp = Product (Lime) Temperature
At glass melting temperatures radiation is the dominant mode of heat transfer. The heat transfer rates for conduction and convection are linear with the difference between the glass and the flame temperature. The heat transfer rate due to radiation is proportional to the difference between glass and flame temperature, each raised to the fourth power. Oxygen increases the flame temperature, which greatly increases radiation, the already dominant mode of heat transfer. Thus with oxygen enrichment, more heat is absorbed by the product, less heat is lost in the exiting combustion gas and the combustion process becomes more efficient.
With this technique, oxygen is injected into the main combustion air header well ahead of the delivery point to the furnace. This pre-mix of oxygen is most common on recuperative furnaces or unit melters that have many such delivery points (hot or cold air burners) or on regenerative melters where it is desirable to use the oxygen to enhance the entire combustion process in a consistent manner. Experience is needed to deliver the right amount of heat to the right zones and to ensure safe application of the oxygen.
This method has historically been the most cost-effective way to use oxygen to supplement air-fuel combustion. The strategic injection of oxygen beside, beneath or through air-fuel flames has allowed glass melters to reach campaign objectives in terms of pull rate, fuel efficiency and glass quality. The benefits of oxygen lancing accrue from having the oxygen mix with fuel where it is most needed; namely in oxygen-starved areas of the combustion space or in the underside (glass surface side) of the air-fuel flames where flame temperature has the greatest impact on heat transfer to the melt. Knowing how many lances, where to place them and flow rates to use, allows us to deliver the most cost-effective solution.
This method of using supplemental oxygen is relatively new to glass manufacturers and has been enabled by the emergence of superior oxy-fuel burner offerings developed for the 100% oxy-fuel conversion of melters. The boosting concept uses oxy-fuel burners positioned within the air-fuel melter to increase production, quality, efficiency and furnace stability. Depending on the needs of our customers, we can tailor the operation to deliver the benefit(s) desired. Oxy-fuel boosting is typically used to increase the pull rate on a furnace that is at capacity or that has been crippled due to a failure or loss of effectiveness of the air-fuel combustion system. Payback for the technology is often less than three months. The advantages of our boost technology are so significant that many furnaces, which used Air Products' boost at the end of the previous campaign to address furnace limitations, are rebuilt and come onstream using boost.
Here, the high temperature flames of oxy-fuel combustion are placed over the cold batch to create a tremendous amount of heat transfer. The result is early batch glazing and significantly enhanced melt run-off. This superior melt rate then allows for an increase in production or a reduction in overall fuel. Talk to us to find out which of these, or other, techniques is right for you. We continue to develop new technologies to improve glassmaking.