OCR Text |
Show other observations about this data include the fact that the spread in NOXST data is much lower than the spread in NOXUN values. For the AASB, the difference between the lowest and highest NOXUN values was 440 ppm while for the staged flames the difference was 140 ppm. The final column in the table lists the %Reduction in NOx through staging. Because of arguments made in the background section, it would expected that the %Reduction is higher for the coals with high VMIPfR values. This relationship is true for the AASB, but there is no clear trend for the EASB. The reason for increased difficulty to correlate the data relates to the degree of success attained by staging for each coal. That is, reaction of volatile nitrogen species to N2 is more complete for some coals than others. If the information presented in the background section is recalled, it can be seen that the theory indicates that coals with high VMIPfR values should have the highest NOXUN values and the lowest NOXsT values. This follows from the facts that NOx formation from volatile nitrogen species is very efficient in fuel lean conditions, and N2 formation from volatile nitrogen species is very efficient in fuel-rich conditions. However, the observed data do not indicate this trend. The problem arises when different coals are combusted with the same burner and the same operating conditions. In both burners, air is supplied on a basis of the coal requirements, however, the reactions of concern for NOx are with the volatile matter. Each coal emits a different fraction of volatile species and, therefore, has different stoichiometries based on the volatile matter. Assuming that the 02 requirement of the coal and the volatile matter are equal, a stoichiometry based on volatile matter can be calculated. For the EASB with a primary zone stoichiometry based on the coal of 0.5, the volatile matter of Peak Downs and Heinrich Roberts coal flames are exposed to fuel lean conditions while the volatile matter from Elk Creek and Scotts Branch flames are exposed to fuel rich conditions. This variation in primary zone gas composition with coal quality should result in lower NOXST values for coals with high VMIPfR values. Another problem is residence time. A finite residence time in the fuel rich zone is required for the reaction of volatile nitrogen species to N2 . High volatile matter coals have more volatile nitrogen species and, therefore, require longer residence times to react. The variation in requirements for residence time in the fuel rich zone should result in lower NOXST values for coals with low VMIPfR values. Residence time is a problem which is related to burner 9 |