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Show ( EFFECT OF ADDITIVE STOICHIOMETRY The effect of additive stoichiometry (NSR) on NOx capture and utilization is shown in Table 3 and illustrated in Figure 9. NOx capture by urea and ammonium carbonate increases with increasing NSR up to 1.7 to a maximum of 90% and 80% respectively, and by ammonia up to NSR - 2.0 to a maximum of 84%. Reagents utilization drops with increased stoichiometry for the three additives tested. The best utilization with urea was 55 - 56% at an NSR of 1.2 to 1.5, with ammonia, 56% at a stoichiometry of 1.0 and with ammonium carbonate, 41 to 42% at a stoichiometry of 1.5 to 1.7. EFFECT OF INITIAL NOx CONCENTRATION The NOx removal efficiency by urea as a function of initial NOx levels at 347, 550 and 867 ppm were studied, and the results are shown in Figure 10. The flue gas with 550 ppm was generated by burning the 1.7% S US coal, the 867 ppm was generated burning the US coal and doping the 550 ppm with bottled NO to raise the concentration to 867 ppm and the 347 ppm was generated by burning propane and again doping with bottled NO. The injection parameters were: T - 1100°C, NSR - 2.0, urea concentration - 13.5%. NOx capture was found to be effective at all NOx levels studied. Maximum capture being 90% for the 867 - 550 ppm NOx level and 70% at the 347 ppm NOx level. NOx content of the flue gas after urea injection for the 867,550 and 347 ppm NOx concentrations are: 60 ppm, 55 ppm and 60 ppm. ANALYSIS OF SLIP GASES The concentration of nitrogen containing species such as ammonia (NH3), hydrogen cyanide (HCN) and nitrous oxide (NzO) in the slip gases during additive injection is currently being investigated. Ammonia analysis is done by the Phenate method (colorimetric). HCN is also done colorimetrically whereby the distillate from auto-distillation is collected in sodium hydroxide and treated with sodium hypochlorite to form ammonia. The ammonia is then detected by the method described above. NzO is done "in situ" by the use of a gas chromatograph utilizing an electron capture detector. Preliminary results indicate that at optimum conditions while injecting urea (temp - 1100°C, NSR - 1.7 to 2.0) ammonia slippage was found to be between 7 to 26 ppm and HCN ranges between 1 and 4 ppm. Decomposition of urea has a side reaction which leads to formation of NzO. Preliminary analysis of the exit gas indicates that the amount of NzO so formed can be substantial, and further testing is underway to determine the extent of NzO formation as a function of injection temperature, type of additive and reagent stoichiometry. It should be noted that use of ammonia and ammonium carbonate does not result in NzO formation. BACK END STUDIES Ash resistivity and ESP performance measurements conducted with urea injection while burning the 1.7% S US coal and shown in Table 4 indicate that: ash resistivities remained unchanged from a baseline level of 107 ohm.cm and the |
