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Air-Fuel BenefitsThe biggest advantage of air-fuel combustion is that air is available at a low cost. As shown in Figure 1, the use of heat recovery allows air-fuel systems to be fairly efficient, considering that 80% of the air is inert.
Another key advantage to air-fuel systems is the operational experience that has been built up over many furnace campaigns. Recurring problems have been solved, and refractory performance and characteristics are well understood. As furnaces become larger and campaign targets get longer, this know-how plays an important role in minimizing risk.
Additionally, some glassmakers have reported increased foam in the refining area when using oxygen compared to using air-fuel. Whether more foam is formed with oxygen firing, or the higher velocities of the air-fuel combustion gases combined with the constant switching of firing from one side to the other simply break up the foam more effectively, is not clear. Foam acts as an insulator, which reduces heat transfer to the glass and potentially increases refractory temperatures and crown wear.
Oxy-Fuel BenefitsAs discussed previously, oxygen combustion is more efficient than air-fuel because of the elimination of nitrogen. Air-fuel efficiencies can be increased by using recuperators and regenerators, but this adds costs, maintenance requirements and complexity to the glassmaker's operation, and efficiencies still fall short of full oxy-fuel furnace operations. The higher efficiency of oxy-fuel leads to fuel savings, which can be a significant driver with today's high fuel costs.
Another advantage of oxy-fuel is the simplicity of the combustion system. Heat recovery is typically not used because of the much lower heat losses to be recovered. This eliminates problems with regenerators and recuperators such as blockage of ports (particularly near the batch charger), checker collapse, uneven flow rates and reduced campaign life. In addition, oxy-fuel systems fire at a continuous rate, controlled by the glassmaker without interruption. The benefit can be clearly seen from the stable batch patterns achieved in oxy-fuel furnaces when compared to air-fuel furnaces.
Oxy-fuel burner technology has delivered impressive gains in productivity and glass yields. Numerous articles have been published about production increases and yield improvements when converting a furnace from air-fuel to oxy-fuel combustion.1,2 State-of-the-art flat flame burners** have shown significant improvements in melt rates and glass quality. By maximizing the radiation achieved, spreading it over a large flame area and directing it down to the melt, the batch is melted more quickly, using less fuel. The size of the refining zone and the residence time for glass in that zone are both increased, with the net result being extra production and higher quality. In any economic evaluation, these two factors are vitally important and provide a significant advantage to the glassmaker.
The environmental benefits of oxy-fuel are often the main drivers for converting to this technology. By virtually eliminating nitrogen from the furnace, NOx levels can be exceedingly low with proper furnace design and burner selection. Reductions of 10-20 times compared to regenerator furnaces have been demonstrated. Additionally, the quicker melting and the lower gas velocities, combined with different flue arrangements, mean that far less batch is blown out of the furnace, thus reducing losses, handling costs and emissions. Batch carryover into the early ports of regenerative furnaces is one of the biggest ongoing challenges for glassmakers operating with air-fuel combustion systems.
Float furnaces offer a special case for oxygen supply. The tin bath used for forming the glass ribbon needs large quantities of nitrogen to prevent the tin at the surface from oxidizing. When the decision is made to convert the furnace to oxy-fuel, an air separation plant can be designed to produce both oxygen and nitrogen more efficiently than supplying each gas individually.
**The Cleanfire® HRTM burner, supplied by Air Products and Chemicals, Inc., is one example of the flat-flame technology.
Combining the Best of Both TechnologiesThe hybrid furnace technology combines the proven techniques of air-fuel melting and oxy-fuel melting in a way that delivers the benefits of each, while overcoming the inherent challenges of using both systems in the same furnace. The system uses oxy-fuel combustion in the batch charge end of the furnace and air-fuel with heat recovery in the downtank refining section of the furnace. Figure 3 shows one example of a furnace designed using this technology. Other arrangements make use of a recuperator(s), alternate methods of exhaust gas management and/or changes in geometry of the furnace construction.
Improved Glass Quality
Using oxygen in the melting zone maximizes the melt rate and fuel efficiency. The perennial problem of plugging uptank ports, or plugging and/or collapsing the uptank checker brick due to batch material carry-over in air-fuel operations, is also eliminated when oxy-fuel firing is used in this region of the furnace. The continuous firing in the melting area delivers stable batch patterns, and because the batch melts early, the refining section is effectively made larger. This expansion of the refining zone increases glass particle residence times, leading to higher-quality glass.
Additionally, because the downtank energy in the hybrid operating system is provided through air-fuel combustion, larger volumes of oxidizer and fuel are present in this region of the furnace. The higher volume of air-fuel combustion gases (required to deliver the same heat compared to oxy-fuel) can be used to more effectively spread the heat release in this critical area of the furnace, thereby minimizing one potential source of foam formation-localized overheating of the glass. While the stability of the oxy-fuel system in the melting area helps deliver good quality (via batch pattern and convection current control), paradoxically it can be the inherent instability of the regenerator system in the refining zone-with the variation of flame temperatures, periods with no firing and the reversal of the flames-that further minimizes foam and improves glass quality. Furnace trials conducted by Air Products have shown the effectiveness of air-fuel and air-oxy-fuel systems in breaking up foam in oxy-fuel furnaces.<
As outlined earlier, "available heat" is the energy that is available to the process when the energy lost in the flue gases is accounted for. The available heat curve shown in Figure 4 highlights one of the key drivers for the hybrid furnace concept. It is the first amounts of oxygen added that deliver the biggest increase in available heat and therefore deliver the biggest increase in efficiency. For this temperature, transitioning from air at 21% oxygen to a 60% oxygen enrichment level increases the available heat from 46 to 70%, providing 52% more heat to the process. Going from 60% enrichment to 100% oxygen increases the available heat from 70 to 75%, only adding 7% more heat for a similar change in enrichment level.
Often, solutions developed for one industry can be customized and fine-tuned to solve problems in another field. For example, the variation of available heat is widely understood in many industries such as aluminum melting, where highly efficient rotary furnaces use complete oxy-fuel combustion systems because they can absorb all the heat effectively. Within this same system and often at the same site, reverb (rectangular, stationary) type furnaces use air-oxy-fuel combustion systems because heat transfer to the bath is not quick enough to fully absorb the heat available from full oxy-fuel. By matching the heat input to the inherent efficiency of the process, these manufacturers use the optimal amount of oxygen. The reverb furnace operators achieve lower overall costs and use less oxygen using a combination of air-fuel and oxy-fuel combustion than they can achieve with full oxy-fuel.
In glassmaking, low levels of enrichment, such as with oxy-boost systems, are an established example of delivering significant benefits with only about 10% of the total furnace heat input. The operation is optimized to deliver the maximum glass production benefit, while minimizing the investment capital and power requirements of the oxygen supply. The hybrid concept is really a matter of using just the right amount of oxygen for the application-no more, no less.
Economic ConsiderationsA detailed economic model for various glass making technologies has shown excellent correlation with data from air-fuel, oxy-fuel and oxy-fuel boosted air-fuel furnaces. The results from the model indicate that significant savings can be achieved using the hybrid melting technology. The model includes capital costs such as refractories and labor for constructing the furnace and heat recovery system, as well as the costs associated with the time required for a rebuild, which is often shorter for a hybrid system. Operating costs, production levels, fuel usage and glass yields are also accounted for in detail.
The economic model has also shown that the hybrid melter provides big benefits when applied to a crippled furnace. The hybrid technology provides heat input where it is most effective-right above the glass-without modifications to the furnace walls or crown. The model projects savings of $5 to $12 per ton of glass for installations, assuming a production increase of 5-10% and improved yields of 1-2%. Greenfield site installations also show economic benefit when compared to air-fuel, and become all the more attractive when typical end-of-campaign issues are factored in.
Filling the GapHybrid glass melting might not be the right choice for every situation, but it represents another option to consider. Compared to air-fuel operations, the hybrid technology can deliver fuel savings, production increases, yield improvements, furnace life extension, emissions reduction and more consistent performance throughout the furnace campaign. Compared to either air-fuel or oxy-fuel operations, the technology can deliver lower-cost, higher-quality glass.
Furnaces have been operated with air-fuel combustion systems for thousands of years. This long track record has established familiarity and considerable operating know-how in its favor, compared to the relatively new oxy-fuel based technologies. The hybrid technology fills the gap between air-fuel and oxy-fuel operating systems, delivering the best of both technologies.
For more information about the hybrid glass melting furnace, contact Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195; (610) 481-7007; fax (610) 481-5136; e-mail firstname.lastname@example.org ; or visit http://www.airproducts.com .