OCR Text |
Show Variation of NOx reduction with changes in ammonia flow rate are shown in Figure 7. Maximum NOx reduction, shown at 85% with actual data recorded as high as 94%, was achieved at maximum ammonia injection levels. Argon flow was held steady at 80 sctb. NOx reduction did not increase linearly with ammonia flow but tended to follow an exponential. Ammonia effectiveness, a molar ratio of NOx reduced versus ammonia injected, was a maximum at the high flow rate. A local maximum in effectiveness also occurred at the lowest ammonia flow rate. Ammonia slip measurements revealed very little excess discharge. Slip data recorded were all below 30 ppm which was the detection limit of the tests performed. Argon flow rate variations had little effect on the reduction of NOx as shown in Figure 8. NOx reduction measurements recorded over the full span of argon flow rates do not vary by more than the 10% error bars accompanying them. The ammonia flow rate was held constant at approximately 10 sctb. If the effectiveness of ammonia radical generation depends largely on temperature, this suggests the heating of relatively small pockets of argon. The injection of hydrogen and methane in addition to the ammonia flow had little effect on NOx reduction at optimal reduction points. A number of regions were found that showed substantially less NOx reduction than optimal. A corresponding increase in ammonia slip was measured. At these points the addition of methane, and to a lesser extent hydrogen, was useful in restoring full effectiveness and reducing ammonia discharge. NOx reduction of less than 30% was improved to about 80% (Table 1 ). Methane and hydrogen provide an additional source of hydrogen atoms which react with ammonia (R2b) to enhance radical production. A small amount of methane, less than 25% of the corresponding ammonia, was used to achieve this effect. Additional methane injection has no further effect on NOx reduction. Table 1. P[kW] 11Ar [scth] 11 NH3 [sctb] 11CH4 [scth] %~ ef(NH3) 1.5 116 10.1 0.0 -29 0.10 1.5 114 9.7 2.3 -79 0.59 1.5 116 10.2 3.7 -79 0.57 1.5 116 9.7 6.1 -77 0.58 Injection of externally produced ammonia radicals at 1250 K was most successful at zero torch power. This temperature defines the optimal condition for current ammonia injection processes (e.g. SNCR). DISCUSSION Molecular radical injection depends on the effective generation of NOx reducing radicals and the ability of those radicals to efficiently remove NO. Kinetics studies of NH2 injection indicate that the externally generated molecules are capable of NOx reduction at temperatures well below that of selective non-catalytic reduction. At low temperature, and for conditions of excess radical injection, NOx concentrations in the test gases drop to well below 0.1 % of their initial value. This result reflects the fact that the chemistry of NOx reduction will not be a limiting factor. Other effects, such as mixing (e.g. mass transfer), local temperature gradients, and ability to introduce high levels of radical concentration into the flue will control reduction efficiency. 5 |