OCR Text |
Show additional radicals which cause minimum NO emissions. Figure 15 compares NO and (NO+NH3) concentrations at T = 1255 K before OFA injection and the final NO concentration after OFA injection. In the OFA zone, some NO reacts with NH3 due to the Thennal DeNOx reactions, and the minimum level of NO at logA=10.5 after OFA injection is about 120 ppm Experimental data indicate that under similar conditions NO is reduced to 60-120 ppm by injection of N-agents with N~C03 into the rebuming zone (Figures 6 and 12). Hence, additional formation of radicals in the reburning zone can explain deeper NO reduction in the presence of Na-promoters. DISCUSSION In the rebuming zone, the initial NOx reduction reactions occur rapidly once the reburning fuel and furnace gases are contacted. Following an initially rapid NOx reduction, the subsequent NOx decay rate is slow since the radical pool has been depleted. If a N-agent is injected after a suitable delay time following the rebuming fuel, it can re-start the NOx reduction reactions and increase the overall level of NOx reduction beyond that of rebuming alone. The delayed introduction of N -agent into the rebuming zone can be accomplished either by physically separating the injection of N-agents from that of the reburning fuel, or by utilizing aqueous solutions of urea or arrnnonia injected in the form of a droplet cloud together with the reburning fuel. The evaporation t:irre of the aqueous droplets and N-agent decomposition time can provide the delay titre necessary f<?r the significant consumption of oxygen in the reburning zone. This delay time can be controlled by adjusting the droplet size to vary the rate of evaporation of the droplets. Sodium additives can be added along with solutions of N-agents to promote the chemical reactions between NO and N-agents in the reburning and burnout zones. As a result, the NOx control achieved with a given N-agent injection rate is increased. The promoters are sodium salts which are added in small quantities, in comparison with NO concentration. Sodium carbonate is one example which is non-toxic, readily available, and very effective. Most likely, promoters do participate in the process, but their role is to provide radical pool which will initiate the NO-NH3 interaction. Fuel-N and sodium compounds which are constituent parts of various fuels can serve as the precursors of ammonia, NHi radicals, and active sodium species in the reburning zone. For instance, coal, biomass, and various waste streams can contain nitrogen- and alkali compounds. These compounds can be released or fo~ed from different fuels by pyrolysis, gasification, physical or 9 |