OCR Text |
Show emissions performance of a broad class of non-premixed swirl-stabilized burners and furnaces representative of typical industrial combustion hardware over the range from 30 k W to 12 NIW. The emphasis placed in the design of these burner scaling tests on maintaining geometric, aerodynamic, and thermal similarity to the greatest extent possible, and the adherence to a uniform test matrix and measurement protocol, has allowed for meaningful comparisons of data among all scales. Results from the analysis portion of the study, described here, have provided a number of insights into the scaling of NOx emissions performance for this class of burners and furnaces, allowing several key conclusions to be drawn. 1. The burners and furnaces tested show a wide range of N Ox emissions performance trends with varying burner size, turndown, combustion air preheat, excess air dilution level, and fuel staging. These are suggestive of the presence of more than one NOx formation region, with potentially different scalings, in these flames. NOx scaling methods based on a single source for NOx production may be inherently incapable of accounting for many of these trends. 2. Simple furnace residence time scaling appears to correlate NOx emissions for any single burner scale, but fails to correlate emissions among different burner scales. This becomes evident only when data from different burner scales at otherwise identical conditions are compared. The previous lack of such scaling data appears to have contributed to the widespread use of furnace residence time scaling methods for NOx emissions. 3. A chemical mechanism based on thermal NOx alone appears sufficient to explain essentially all the major trends seen in the emissions performance data from this class of burners and furnaces. No accounting for prompt or other NOx-formation mechanisms was needed to adequately reproduce the performance of these burners and furnaces in the present scaling model. Similarly, the assumption of equilibrium oxygen atom concentrations was found to be sufficient to account for the N Ox emission data over the entire range of test conditions. 4. Fundamental consideration of potential NOx sources, guided by the in-flame data from these measurements, suggest four key sources of NOx production in this class of burners and furnaces. The underlying physics of each of these sources lead to scaling laws for their respective contributions to the overall NOx emissions performance. It is found that the relative importance of each source depends on the burner size and operating conditions. 5. At full-load conditions the near-burner region appears to be the dominant source for NOx production at the larger burner scales. At the smaller scales the contribution from the flame sheet region can be comparable. The contribution from the furnace region is negligible; this is consistent with the observed failure of simple furnace residence scalings to correlate N Ox emissions among different burner scales. 6. The widely used constant-velocity scaling principle fails to produce aerodynamic similari ty in th-e hearo-burner region that controls N Ox formation in these flames 19 |