| OCR Text |
Show >» o c g> "o <D C g o _J •o CD Q_ o 50 25 u 50 25 0 50 25 0 50 25 i e Thoresby ft • •* 8 0 oo Ashfordby 0 • 8 Hunter Valley t 8"«8 La Jagua i o • 0 •• • o 8 9 • o 8 • 0 • 0 • 0 1 1 • • o o • • o o • o • 0 t 0 1 1 the effect of temperature and the spatial location of the evolved volatile matter. Effect Mixing of Temperature and 0.90 0.92 0.94 0.96 0.98 1.00 Stoichiometry, SR2 Fig. 11 Effect of temperature on NO reduction efficiency as a function of rebum zone stoichiometry. Open symbols : Tpr = 1723K ; closed symbols Tpr = 1573. RF=19%, SR1 = 1.03, SR3 = 1.08. The effect of temperature on the N O reduction for a selection of the coals tested at SR2=0.90 - 0.99 are shown in Fig. 11, where Tpr is the temperature of the combustion products leaving the primary zone. As SR2 is increased beyond 0.93, the N O reduction efficiency reduces for all the coals with increasing temperature. The detrimental effect of temperature can also be seen for in Fig. 12 for other values of RF for one of the coals. These results are consistent with those from other coal reburning experiments[l], but at a lower temperature (1200°C and 930°C) than in this study. Temperature can influence the rebum reactions from both the kinetic considerations of N O reduction by nitrogeneous intermediates and C H fragments [1,7] on the one hand and the physical effect of temperature on the coal pyrolysis and combustion behaviour. The temperatures in the rebum and bumout zone are above the optimum window for selective N O x reduction, so it may not be unreasonable to assume that reducing the temperature should result in increased N O removal by this route. The production of hydrocarbon fragments from the devolatilisation process is also increased with temperature and from consideration of the mechanism, the rate of the reaction of C H and N O in reaction (IX),y [(OH),T] is faster with increasing temperature because of increased O H radical production (equations (X) and (XII)). In the case reburning with natural gas, this leads to an increase in N O reduction with >» o c _" "o c o o Z3 _ 70 60 50 40 - 30 - 10 • . - • 0 • o ' • o 1 • o 1 0 • • • o o T =1450 °C F>r ^=1300 °C • i 15 20 25 Rebum Fraction 30 RF% 35 Fig. 12 Influence of temperature on NO reduction as a function of rebum fraction. (La Jagua, SR1=1.03, SR2 = 0.86-0.97, SR3=1.05-1.09). |
