| OCR Text |
Show 6 of tertiaJy air. For the staged case, the over fire air fraction was increased, and consequently the burner stoichiometric ratio was constant The results show an increase of exit NOx concentration from 230 ppm to 310 ppm for the non-staged flame. No change was detected for the staged condition. For the latter case, the exit NOx concentration was found to be between 103 and 107 ppm, (corrected to 3% 02)' The air preheat was also increased at the staged condition (SR=O.92) from 500 K to 562 K but negligible change (96 to 98 ppm) was detected in the exit NOx concentration. Detailed Flame Characteristics Detailed flame measurements were performed in order to gain insight into the governing mechanisms of NOx formation. Only a few of those results will be discussed here. The characteristics of the three flames studied are given in Table 1. Two of these flames were staged PPM flames and one was an unstaged IVM flame. The study of staged flames would help determine the effect of staging on the NOx formation and the comparison between staged and unstaged flames would help explain the difference in the exit NOx concentrations for these flames. Flame 2 is the baseline staged flame discussed in the preceding section. VJSUal observation of the flames provided valuable qualitative information about the flame length, the radial flame stratification and the depth of intrusion of the transport air jet The visual features of the two staged flames are very similar. They are both characterized by a high degree of homogeneity (well mixed) and, in both cases, the quarl is filled with combustion products. There is no transport air/coal jet visIble on the center line of the flame. The IVM non-staged flame (Flame 3) had different attributes. The burner was operated with a different air distrIbution for this flame. There was a smaller amount of primary air, a higher amount of secondary air an especially high fraction of tertiary air (see Table 1.). The shape of the flame is different as it does not fill the quad and the tertiary air is clearly separated from the flame front (radially stratified). At a distance of about three burner diameters downstream of the burner the flow diverges as it approaches a stagnation zone in the flow. This stagnation zone marks the end of the radially stratified core region. Downstream of this, vigorous mixing between the tertiaJy air and the combustion products occurs. The different flame shapes indicate that the interaction of the coal laden transport jet with the primary air jet is very important For the PPM staged cases, the angular momentum of the primary air is much higher than that of the IVM non-staged stratified case. It follows that the interaction of the transport air jet is significantly different in the two cases. Both the visual observation and concentration measurements of solid samples along the axis support this conclusion. In the staged cases, the angular momentum of the primary air was sufficiently powerful to create a stagnation point within the primary barrel, which, in turn, resulted in a well mixed fuel rich primary air-coal flow to emerge from the burner. In the non-staged case, on the other hand, stagnation did not occur as early due to the weak angular momentum of the primary air (low swirl number and low air fraction). |
