| OCR Text |
Show different scales at otherwise comparable conditions are shown together, such as in the present SCALING 400 data in Fig. 8, it becomes clear that simple furnace residence time alone does not adequately scale NOx emissions. Evidently, NOx levels produced by industrial burners and furnaces of the broad class represented by the SCALING 400 tests are controlled by other, potentially more complex, physical processes. In the following section, fundamental consideration of the NOx formation mechanisms in burners and furnaces of this class will reveal several different sources for the NOx emissions produced. It will be seen that the relative importance of these NOx sources varies with burner scale and with burner operating conditions. Moreover, each of these sources will be found to yield different scalings for the NOx emissions produced. Together these N Ox sources and their respective scalings will be seen to account for most of the trends seen in Figs. 3-7, and produce a general NOx scaling model for this broad class of industrial burners and furnaces. 4. THE SCALING 400 MODEL The trends seen in the input-output data in Figs. 3-7 suggest that there is more than one source for the NOx emissions produced by the burners tested over this range of scales. In this section, the detailed in-flame data from the SCALING 400 test series are used together with a fundamental examination of NOx-formation mechanisms to identify possible NOx emissions sources in these burners. Physically-based scaling laws for the contribution of each of these sources to the total NOx emissions are then developed, which together comprise a general burner/furnace scaling model. Although the kinetic mechanisms of NOx formation in hydrocarbon combustion are by now fairly well understood, there remains disagreement in the technical community as to which of these are the dominant NOx-forming chemical route(s) in practical non-premixed combustion systems. NOx production mechanisms have been reviewed by Bowman [14], and more recently by Turns [15] for non-premixed flames. In the present scaling model, it will be assumed that NOx is produced entirely by the thermal (Zel'dovich) NO mechanism, since results will show that this approximation appears to suffice to account for the major trends seen in the present data. The further approximation of 0-02 equilibrium will also be introduced in the scaling model. Differences between predictions from the resulting scaling model and the data may be, in part, due to these approximations. However, it will be seen below that the resulting model predicts most of the trends seen in the SCALING 400 data, suggesting that these approximations might not be unrealistic in practical non-premixed combustion systems. The overall NOx emission in the flue gases is expressed in emission indexed NOx (EINOx), defined from the NOx mass flux in the flue gases and the input fuel mass flux to the burner as EINOXjlue = mjuel (1) Since NOx in the flue gases may originate from several different sources, each of these contributions to the total- NO.x· !!lass flux can be equ~ted under steady operating conditions 6 |
