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4.1 Glass Manufacturing The glass industry consumes approximately 300 TBtu/yr,37 and some sources estimate that as much as 70% of this energy consumption is devoted to glass melting and refining processes in hightemperature furnaces.38 Furnaces vary widely in the energy required to melt a ton of glass. The theoretical minimum energy for melting glass is only about 2.2 million Btu per ton. However, some furnaces consume as much as 20 million Btu/ton.39 Furnaces used in large glass melting operations include directfired, recuperative, regenerative, unit melters, oxyfuel, and mixedfuel furnaces. In the United States, more than half of all glass furnaces are natural gasfired regenerative furnaces, which account for over 90% of the tonnage produced. Best practice furnaces have efficiencies of about 40%, with stack heat losses about 30% and structural losses accounting for another 30%.40 Regenerators and recuperators are the most frequently used systems for waste heat recovery in the glass industry. Glass melting is a hightemperature operation providing several opportunities for recovery of highgrade waste heat. Without heat recovery, stack exhaust temperatures typically exceed 2,400°F [1,315°C].41 Recuperators and regenerators for combustion air preheating are the most common methods for waste heat recovery. Regenerative furnaces employ two chambers with checker bricks. These chambers alternately absorb heat from exhaust gases and transfer heat to the incoming combustion air. The direction of airflow changes approximately every 20 minutes so that one chamber receives heat from the stack exhaust while the other one rejects heat to incoming air. Final exhaust temperatures vary between about 600 and 1,000°F [316 538°C] throughout the cycle.42 Recuperators are a lessefficient option more commonly employed in smaller operations that cannot afford the large costs of regenerative furnaces. A metallic recuperator is used to indirectly preheat combustion air. Preheat temperatures usually do not exceed about 1,470°F [800°C], and exhaust temperatures are reduced to about 1,800°F [982°C]. In addition to combustion air preheating, methods for waste heat recovery in glass manufacturing include preheating batch and cullet material and using waste heat boilers for electricity generation. However, these systems are most likely to be used in oxyfuel furnaces, where combustion air preheat is not used. Oxyfuel furnaces use oxygenenriched air or pure oxygen for combustion. This saves fuel by reducing the energy needed to heat nitrogen carried in atmospheric air. When furnaces are adapted to oxyfuel firing, the regenerators are removed, which can lead to higher exhaust temperatures around 2,660°F [1,460°C]. Although the waste heat is at a high temperature, the mass of exhaust gases is much lower, leading to lesser waste heat loss as a percentage of fuel input. Preheating batch material is used in one plant in the United States;43 it is more common in Europe, where energy costs are higher. About 13 new batch/cullet preheaters have been installed since the 1980s, nine of which were located in Germany.44 Challenges with batch preheating include the large amount of material that must be handled and the desire to maintain a homogeneous glass product. Fluid beds and special silos are used to agglomerate the batch and simplify heat transfer. Further improvements that reduce the capital costs and simplify operation of these systems may create opportunities for increased implementation of batch preheating. Analysis of glass furnaces in the United States shows that while heat recovery is a common practice, about 43 TBtu of medium to hightemperature waste heat provide additional opportunities for recovery (Table 10, see Appendix A – Documentation of Waste Heat Estimates). Waste heat losses from 34 PDF Image | Waste Heat to Energy Tech Opportunities in US Industry
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