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Show In natural gas-fired regenerative glass furnaces, NOx is essentially fomled by theffi1al oxidation of nitrogen in combustion air (thennal NO,). Themlal NOx depends upon the time- temperature history of the flame and increases with increasing residence time and increasing peak flame temperatures. Furnace operating temperatures and flame temperatures-and, consequently, NOx generation-are quite high. NOx emissions over 10 lb/ton glass are not uncommon. Theffi1al NOx also increases significantly with increasing availability of oxygen in the high-temperature zone. To reduce NOx fornlation during natural gas combustion, both the peak flame temperatures and the oxygen availability must be reduced. California is the most aggressive in ternlS of regulating NOx emissions. Current regulations specify less than 4 lb NO/ton of glass produced. Some conversions to oxy-fuel sponsored by DOE have resulted in NOx emission of less than I Ib NO/ ton. At this time, the use of oxy-fuel firing is viewed by the glass industry as the leading melting technology to lower NOx emissions. Implementation of this technology for meeting future envirorunental compliance will initiate a significant driving force to integrate waste heat recovery schemes, such as batchlcullet preheating, co-generation, or gas refoffi1er technology. This technology is being rapidly accepted by the glass industry because it uses many similar operating principles as conventional furnaces, especially unit melters, and provides solutions to a variety of other requirements. Several NOx control methods, including air staging, were developed and successfully tested on a pilot-scale glass tank simulator in the early 1980s. The Gas Research Institute and DOE are presently continuing this effort for side-port-fired regenerative furnaces . Gas reburn technology and low excess air firing as a prerequisite to air staging are other methods of NO x reduction where some previous testing has been carried out. Several types of secondary oxidants required for staging have been considered in previous testing and current field tests. There is an interest to have modeling tools to better quantify various aspects of these conditions for combustion modifications. Approximately 1,200 - 1,500 lb of CO2 are emitted from combustion for each ton of glass melted. An additional 300 lb of CO2 come from raw material calcination for each ton of glass. Reduction of CO2 to address concerns with global wanning has not been of significant concern for the glass industry to date. Consequently, such fuels as hydrogen are not presently considered for study. 4.5.2 Suggested Research Opportunities Comparing the glass industry's vision of the future ''''ith the present situation provides insight on the important issues that must be addressed through improvements in teclmology. The following technologies associated with combustion issues have been identified. 4.5.2.1 Environmental Performance. The challenge is to meet the requirements of expected environment regulations (air staging, etc.) cost effectively. Combustion of fossil fuel is far from optimized when considering the need to balance heat transfer for melting and to minimize pollutants, especially NOx' Fuel or air staging and gas reburn have been identified as a potentially cost effective means of reducing NOx in conventional regenerative melters. Gas refomling as an integrated component of waste heat recovery may be used for increasing flame luminosity, including oxy-fuel fWllaces . 4.5.2.2 Improved Emissions Controls. Developing improved, more cost-effective air emissions control teclmology is necessary to meet the more stringent environmental regulations expected. Compliance using integrated process improvements are preferred to add-on devices. 4.5.2.3 Improved Process Control. In production processes where flame jet impingement is attractive, an understanding of the local distribution of heat flux, partitioning betvveen convective and radiative transfer, stability of the flame, optimum geometric configuration, pollutant formation and contTol, etc., is vital to the achievement of highest product quality and the minimization of energy consWllption. Improved comprehension of the transport mechanisms and generalization of heat fluxes to the load \\ ill also pennit advance of the modeling of the various fabrication processes. There is a need to characterize the fundamental transport mechanisms in flame jet impingement heat transfer. An experimental investigation of mechanisms governing heat transfer to the load (stock), rather than the detailed kinetics and mechanisms of the chemical reaction, is needed. Previous \ ork-related momentum-driven flame 15 |
