OCR Text |
Show o 1500 1000 500 0 24 16 8 0 100 75 50 25 0 (i) Temperature J i I i L (ii) Oxygen concentration iii) Total volatiles released I , I (iv) Unburned volatiles 600 - c_400 200 - (v) Cross-sectional averaged N O concentration increasing temperature [1] which is not the situation here for coal. In contrast to the volatiles effect considered above, N O reduction by char is enhanced at higher temperatures due to the increased rates of reaction. However, it has already been shown that char/NO reduction in the rebum zone provides only a marginal contribution to the total N O reduction. In order to interpret the experimental observations, consideration needs to be given to the effect of temperature on the devolatilisation rate and the resulting C H concentration with temperature and the prevailing oxygen concentration in the rebum zone. Figure 13 shows the pattern of oxygen consumption, volatiles production and the resultant average volatile and NO concentration for the two operating temperatures. The predictions data for Figs 13i to 13iv are plotted as a function of distance from the centre of a coal injector nozzle towards the central axis of the furnace, so as to follow the concentration gradients along the streamline of the coal jet, whereas Fig. 13v is plotted as a function of axial distance along the furnace. The predictions show that, even though the rate of temperature rise is more rapid at the higher initial temperature condition (Fig.l3i), the rate of oxygen consumption is not significantly different (Fig.l3ii) for both cases. At the higher temperature, volatiles are predicted to be more rapidly evolved (Fig.l3iii) and hence will have a higher average concentration in the corresponding upstream regions on the rebum zone where the oxygen concentration is highest as shown in Fig. 13iv. This produces more N O in this region and the net result is a higher average N O concentration along the furnace and reduced N O reduction (Fig.l3v). This effect will become increasingly more significant as the rebum zone is operated less fuel rich, as already shown by the data presented in Fig. 11. The magnitude of the effect will depend on the pyrolysis behaviour of a particular coal as already noted in the previous section. It could be argued that since thermal N O will increase with increasing temperature, this could explain the observed decrease in N O reduction efficiency. However, o.o 0.2 0.4 Distance, m 0.6 0.8 Fig. 13 Effect of rebum zone inlet temperature, Tpr, on the predicted (i) temperature, (ii) oxygen concentration, (iii) percentage volatile content released and (iv) volatile concentration as a function of distance from the coal injector, and (v) average N O concentration along the furnace axis. Continuous line Tpr = 1573K ; broken line Tpr = 1723K ;RF= 19% ;SR1 = 1.03 ;SR2 = 0.98. All predictions Thoresby coal assuming Rv = 1.1. |