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Show In comparison, the contribution due to char was only a marginal 3 % when the earlier data of Levy et al [10] was used, thus resulting in a significant under-prediction of the observed N O reduction. It is likely that the former rates are more applicable to the present study [26]. The dominant role part played by CH in reducing NO by coal reburning is supported by our earlier experimental work [3] which showed a strong correlation between N O reduction and the coal volatile matter content. In comparison, the N O reduction efficiency showed only a weak dependency on the fuel-N content. This is not unexpected since C H * is an extremely effective rebum fuel and it is also a major coal pyrolysis product. Figure 10 compares predictions and experimental values at various rebum fuel fractions (RF). Note that changing R F also changes the rebum zone stoichiometry, SR2. The predictions are in good quantitative agreement with the observed values, the latter lying within a band described by the predicted lines for R v = 1.32 and 1.1. Assuming a faster devolatilisation rate results in significant over-prediction at higher values of RF. Comparison of the predictions for Rv= 1.1 and 1.32 indicate that increasing the volatile yield (for a constant devolatilisation rate) improves the N O reduction efficiency, particularly at lower values of R F (or leaner rebum zone stoichimetries). Increasing the rate of devolatilisation is beneficial at higher R F values (>25% in this case) but not at lower values where the N O reduction efficiency falls sharply. If the rate of coal devolatilisation is too fast, so that volatiles are released early into the reaction zone, then, depending on the rebum-zone stoichiometry and availability of oxygen, these volatiles may be oxidised before they can be effective in reducing the primary N O . At higher values of R F (equivalent to richer rebum zone stoichiometrics), faster devolatilisation and dispersion into the surrounding NO-laden gases results in better N O reduction. The trends observed suggest that the reburning performance of a particular coal will depend on both the volatile yield and the devolatilisation rate, with high volatile coals being effective as already in Fig. 6 range of coals here. The latter more shown for the tested means that the coal pyrolysis behaviour, and operating factors which influence this, such as temperature, particle size and residence time, must be carefully matched to ensure that N O reducing agents are released at the locations where are likely to be most effective. The influence of pyrolysis behaviour is further demonstrated in the next section which considers o c <D O it c o T5 13 "O <D 70 60 50 40 30 20 in 0.96 0.92 SR2 0.88 0.84 1 • 1 • 1 • .-.- i , ^ --'•" Pittsburgh No. 8 ^ev ,.x _ ^ ^ y* * * jr / • _ .-•' y #• * * - y * * * i l l / 1 L_ L 1 15 20 25 30 Reburn Fraction, 35 Fig. 10 Comparison of measured and predicted N O reduction efficiency vs rebum fuel fraction for Pittsburgh No 8 coal for various volatile yields and devolatilisation rates. SR1=1.03, SR3= 1.08, Tpr =1300 C. |