| OCR Text |
Show respectively. The data for hot, cold, and radical injection in each column in Figure 5 were obtained for identical gas flow conditions. For each condition, the fraction of the total NO reduction attributable to each effect is indicated by the relative sizes of the shaded sub-regions compared to the overall column height. As shown, NH3 injection was more effective at lower levels of excess air. The effect of heating the NH3 before injection was negligible at 50% and 75% excess air, but became significant at 25% excess air and more so at 0%. In contrast, the contribution of the use of the plasma generator to NO reduction was largest at the higher levels of excess air. This plasma effect contribution was offset by the thermal effect at lower amounts of excess air, but was still significant in all cases. Discussion The hot injection experiment was designed to verify that the high NOx reduction achieved in the two demonstration studies of plasma-assisted ammonia radical injection was not due merely to the bulk thermal heating of the ammonia stream by the plasma systems used. The results show that this thermal effect is significant in some cases, particularly at lower levels of excess air, but it does .not account for the high NOx reductions achieved in the plasma process demonstrations. The advantage of externally generating the ammonia radicals via interaction with the plasma was significant at all conditions tested. At excess air conditions between 0-25%, typical of utility and industrial boilers, the plasma effect accounted for 15-35% additional NOx reduction beyond any thermal benefit, or about 50% of the NOx remaining in the system. Since the advantage of the plasma process has been verified with data from the hot injection experiment and the lab-scale demonstration project, further research is warranted. The demonstration projects identified several of the major factors affecting NOx reduction perfonnance, including ammonia and carrier gas flow rates, excess air conditions, and plasma power. However, these projects were narrow in scope in the sense that the potential of the plasma process to reduce other pollutants and to perform well using alternative reagent and carrier gas combinations over a wide range of operating conditions remains largely unexplored. In addition, the investigations in the demonstration projects were undertaken from a global perspective, exploring the effect varying the factors had on overall NOx reduction without being concerned with the more fundamental mechanisms associated with the process. Preliminary work by Boyle et. al. did study the basic chemical reactions that might lead to the NOx reductions observed, and concluded that amidogen (NH2) was the primary reducing agent according to the following reaction: 6 |
