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Show 7 reduction can be expected to be less effective on commercial glass tanks because of the higher temperature, as well as the higher 02 content of the flue gases. Furthermore, this technique would have an adverse effect on furnace production rate and furnace thermal efficiency, similar to that with reduced air preheat -- a significantly simpler approach. Lower Furnace Temperature: Operating the furnace at lower-than-normal temperatures to increase heat transfer from the flame, thus reducing the peak flame temperature. This was tested on the IGT glass tank simulator during furnace heat-up. The results are similar to those found for the effect of combustion air temperature. This also, however, is not a viable option for glass melters, because a reduction in furnace refractory temperatures would reduce the furnace production rate or furnace thermal efficiency, or both, depending on the methods employed, for example, reduced firing rate, reduced furnace insulation, etc. Water or Steam Injection: Water or steam is injected into the flame to reduce the peakflame temperature. This technique can reduce NOx formation, however, it also increases heat losses through the flue gases thereby decreasing thermal efficiency. Oxygen Enrichment: Industrial oxygen used in certain ways to increase heat transfer from the flame and reduce peak flame temperature,' however, NOx is produced by the electric utility. Oxygen enrichment of combustion air, either by adding oxygen to the combustion air or by injection of oxygen directly into the flame (using lances or by using auxiliary oxy/natural burners), has several benefits including increased furnace production rate; however, generally NOx formation is also increased. The effect of 02 enrichment is opposite to that of flue-gas recirculation (9). 02 enrichment of combustion air was tested on the IGT glass tank simulator and showed a 500/0 increase in NOx upon enrichment to 270/0 oxygen (Figure 3). Significant increases in NOx formation with 02 enrichment are also reported by others (10). There are, however, ways of providing 02 enrichment to achieve a production-rate increase while reducing the NOx emission. These will be discussed in the next section. It should be noted that air separation requires a significant amount of electricity and that some additional NOx is produced by the electric utility supplying electricity for 02 production. Process Modification Process modifications, specific to glass melters, can reduce NOx emissions by increasing the specific production rate or decreasing the specific fuel requirement. These include -- Increased Cullet: Cullet requires less energy to melt than batch and, therefore, reduces specific fuel requirement. This method is well-known and is being practiced by the glass industry to achieve some reduction in NOx emissions. As discussed, NOx is reduced because of the reduced energy requirement for melting, increased furnace thermal efficiency, and increased furnace production rate. Electric Boost: Increases specific production rate without increasing natural gas consumption and without adding NOx to the furnace stack. This is a well-known method, which can provide NOx reduction of up to 300/0. It also helps in refining and increasing heat transfer to the glass by mixing the bath. Its use for additional NOx reduction is, however, questionable because of the higher costs of electricity and the potential for reduced furnace service life. It should be noted that some additional NOx is produced by the electric utility supplying electricity for the process. (P-002\1194Maui.doc |
