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Show (5) Reduction of NOy by ammoniacal liquor and urea Figure 7 compares NO reduction using NH3, liquor and urea. It can be seen that maximum NO removal from injection of ammonia (liq.) occurs at low NH3/NO molar ratios, whilst at a high NH3/NO molar ratio its NO removal efficiency decreased. Urea is also reasonably effective in reducing NO but only at lower urea flow rates. During urea injection it was observed that at a high flowrate of urea (mole ratio >4) sintering and agglomeration of the bed solid occurred, and this necessitated the bed solids being completely replaced. This problem may be serious in large scale combustors. A possible solution for this problem is to ensure that the urea is dispersed uniformly over the bed surface. (6) Emissions of CO and Ammonia Slippaie In conjunction with effective NO reduction by ammonia or ammonia-based compounds it is also imperative to minimise CO emissions and ammonia slippage so that unacceptable quantities of these secondary pollutants may be avoided. (a) CO emissions: Figure 8 illustrates the emissions of CO in the flue for injection of ammoniacal liquor and urea during staged combustion. For the purpose of comparison, the CO level during injection of ammonia(g) under equivalent operating conditions is also plotted. From this figure, it can be seen that CO level increases with the increase in the NH3/NO molar ratio for all three cases. However, the highest CO level was observed for ammonical liquor whilst the lowest level was obtained for ammonia (g). The highest concentration of CO in the flue for ammonical liquor was not unexpected due to the following reaction of water vapour with hot char: H20 (g) + Char -----+ CO + H2 The relatively high level of CO during urea injection could be attributed to the decomposition of some of the urea into CO via NHr CO-NH2 --=+-+ 2NH2 + CO (b) Ammonia sligpage: The ammonia slippage was determined by analysing the exit flue gas. A wet c emical method was adopted. Figure 9 presents the result of ammonia slippage during injection of ammonia (g) and ammonical liquor. It can be seen from this · figure that ammonia slippage rapidly increased during injection of ammonicalliquor when NH3/NO molar ratio was greater than 2. However, at low NH3/NO molar ration (05 to 1.0), where NO was reduced to over 60%, the slippage was very low and may be environmentally acceptable. High ammonia slippage during ammonicalliquor injection has one possible explanation. At high NH3/NO molar ratio i.e. at high flow rate of ammonicalliquor, the quantity of water entering the combustor would also be high which quenches the reacting region to a considerable extent This ·will cause a decrease in the rate of reaction between NO and NH3, releasing a large amount of unreacted ammonia into the flue. Figure 10 shows the NH3 emission (for NH3 gas injection) as the NH3/NO ratio is increased to the combustor for velocities of 2 m/s and 1.2 m/s. As mentIoned earlier, acceptable NOx reductions under staged conditions (50% or more) can be obtained with an NH3/NO mole ratio of only 1. Under such conditions the ammonia emissions is less than 30 ppm (Fig. 10) under either fluidising velocity which may be considered environmentally acceptable. 6. |