OCR Text |
Show emissions were shown to not be a strong function of burner load, but were significantly influenced by flame temperature. In a specific boiler and burner configuration, the most significant parameter affecting flame temperature was the dilution of the premix with noncombustible gases, either additional excess air, flue gas, or any other diluent gas. NOx emissions (corrected to 3 percent dry stack 02) are plotted as a function of excess air for different levels of F G R in Figure 2. Note that a significant number of the points are at 9 ppm and below, which is consistent with the primary objective of the test. A secondary objective was to map out burner performance to determine operating conditions that would meet other levels of N 0 X emissions between 15 and 30 ppm, so these additional points were also recorded. Note that in general, sub-9 ppm NOx can be achieved at 50 percent excess air with no FGR. As F G R is added to the premix, the amount of excess air required to achieve a given level of N 0 X emissions is reduced. As an example, note that at nominally 20 percent excess air and 15-30 percent F G R sub-9 ppm emissions of N O x are achieved. F G R is defined in this report as the ratio of mass flow of recirculated flue gas to the mass flow of fresh combustion air added to the system. W h e n the excess combustion air and flue gas recirculated through the burner are combined as one parameter, total dilution, the dependence of N O x emissions on excess air and F G R becomes clearer. This dependence is shown in Figure 3. N 0 X levels drop as dilution is increased. Note that the data points shown in Figure 3 were recorded at a variety of excess air levels, the balance of the total dilution always coming from flue gas. The percentage F G R is defined as the volume percent of total flue gases recirculated into the burner flow relative to total combustion air, and therefore the percent F G R cannot be added directly to the excess air percentage. Mass flow of F GR is a function of both the F G R percentage and the total mass flow of combustion air. CO emissions have typical regulatory limits of 400 ppm and below, and commercial Alzeta burners are typically guaranteed to 200 ppm. Of the data presented in this report, only two data points had C O levels above 50 ppm, with C O emissions being between 50 and 70 ppm. Two other data points had C O emissions between 10 and 30 ppm. All other points had C O emissions less than 10 ppm. Because CO emissions in nearly all cases are well below our current targets, no test data are shown. 3 |