OCR Text |
Show excess air. However, there was no direct relationship observed between NOx emissions and the coals' total N content; the extreme difference in NOx (640 ppm) for Beringen and Beckley, which have almost identical total N contents, testifies to this. A trend exists relating the NOx level of the non-staged flame with the volatile matter content of the coal (DIN standard); generally coals with a high volatile content yielding a higher NOx level (fig. 5(a)). By way of explanation, it has been observed that the volatile N release can vary considerably with coal type [2], N yield increasing with the total volatile matter content, and indeed table 1 shows that even under the low temperature dense phase conditions characterising the DIN standard method the trend remained true. Moreover, on combustion the high volatile coals realised higher and earlier peak flame temperatures (fig. 6) and consequently differences observed in volatile N yields under isothermal conditions for low and high rank coals will be accentuated on combusiton because of higher pyrolysis temperatures [ 2] for the former. Thus greater concentrations of volatile N compounds are obtained with the high volatile coals which leads to high NOx formation rates in the oxygen rich pool of the non-staged flame. Indeed, in agreement with the above, N release data from solids' samples collected in flame indicated greater N devolatilization for the high volatile coals. Undoubtedly, in estimating NOx emissions for a specific coal a more satisfactory correlating parameter than the total volatile matter content would be the volatile N fraction released on combustion. The latter coal property however is neither normally nor easily determined experimentally. It is a function of the time-temperature history of the coal particles. A less satisfactory but evidently useful, correlating parameter is the volatile N fraction released under DIN standard conditions (fig. 5(b)). An even better correlation (fig. 5(c)) is obtained with an empirical parameter comprised of the product of the coal volatile N content (DIN) and coal oxygen content (rank parameter). - 10 - |