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Show o c _ o e _ c o vo o _ •a <_ _: 50 45- 40- 35" 30- 25 20- 15" 10- 1.02 1.04 1.06 SR1 1.08 1.10 1.12 1.14 - t - 1.16 + 8 + • A 9 m • • • C> + Thoresby Gascoigne Wood Pittsburgh No.8 Koonfontein Hunter Valley La Jagua • 8 SR2 > 1.0 'A the thermal N O x is less than 20ppm, so this can also be discounted as contributing greatly to the predicted effect or the observations shown in Fig. 11. The effect of differences in pyrolysis behaviour on rebum performance is evident in Fig. 14 which shows the variability of six of the test coals for the same operating conditions, even though the coals are of the same rank and have similar proximate volatile matter contents ranging from around 32 to 3 7 % (dry, ash-free basis). The variations are more probably due to differences in the devolatilisation behaviour of the coals and highlight the need for more sophisticated and accurate devolatilisation models for incorporation into C F D codes [27]. Figure 14 also confirms that it is possible to reduce N O even when the rebum zone is operated overall fuel lean because of the production of locally fuel rich pockets in the vicinity of the coal injectors. SR2< 1.0 0.91 0.93 0.95 0.96 0.98 1.00 1.02 1.04 1.05 1.07 Rebum zone stoichiometry (SR2) Fig. 14 NO reduction vs rebum zone air/fuel ratio. SR3 =1.09 - 1.16, RF=22.0%, Tpr = 1250 - 1300C. 90 _J 75 >: o c _ ii 60 0) c o t3 -_ 45 30 - 15 10 4 Nozzles 2 Nozzles 15 20 25 Reburn Fraction (RF), % 30 35 Fig. 15 Variation of predicted and measured NO reduction efficiency vs rebum fuel fraction, RF, and coal injector configuration for Thoresby coal. SR1 = 1.03 ; SR2 = 0.78 - 0.92 ; SR3 = 1.08 ; Tpr = 1723K. Predictions : continuous line: two injectors ; broken line: four injectors. Figure 15 shows the influence of mixing on the N O reduction efficiency for Thoresby as a function of RF. As before, the reburn-fuel fraction is coupled with the rebum zone stoichiometry, SR2, the latter in the range 0.78-0.92. The data were obtained using two or four injector nozzles and confirm that improved mixing of the reburning coal into the rebum zone gases by the use of additional injectors increases N O reduction. This is consistent with the results of Knill et a/. [11]. For the four nozzle case, theoretical predictions have shown that the maximum volatile concentration along the rebum-jet central line is slightly lower, but the volatile distribution in the rebum zone is more uniform in comparison to the two nozzles, where the |