OCR Text |
Show RESULTS The intent of these studies was to evaluate hybrid NO control schemes, including both combustion modifications and the addition of selective reducing agents. Some experiments were performed, however, under overall fuel lean conditions in order to evaluate differences in the mechanisms. Fuel Lean Injection Figure 1 summarizes typical results from studies to characterize the effect of adding selective NO reducing agents under excess air conditions. The low initial NO concentration is representative of that found in a source in which NO is already controlled by furnace modifications. The ratio of agent to NO (the -NINO ratio) was set to 1.5. In all cases the selective reducing agent was injected downstream of the main combustion zone, which was fired with natural gas at 15 kW. The optimum injection temperature for all compounds was between 950 and 10000 C, which is in agreement with the 967 0C optimum reported by Muzio et al. 10 The extent of reduction by NH3 appears to be consistant with that reported by Muzio et at. 10 The initial NO concentration of 240 ppm on a dry, 0% 02 basis corresponds to 158 ppm on the actual basis reported by Muzio. Under the optimum temperatue of 967 0 C, the NO removal based on Muzio's parametric study would be approximately 30%, which agrees relatively well with the NO minimum suggested by Fig. 1. Ammonium sulfate and ammonia gas produced similar reduction efficiencies; cyanuric acid was significantly less efficient and urea produced reduction efficiencies that were intermediate between the other compounds. This is consistent with probable decomposition products of HNCO and NH3. At higher initial NO levels (e.g., 600 ppm) all the compounds gave greater percentage reductions but the ranking remained essentially the same as shown in Fig. 1. The minimum NO emissions with cyanuric acid occur at a much higher temperature than that reported by Perry and Siebers. 14 The low reduction temperature reported by these authors was apparently due to catalytic activity involving the stainless steel reactor walls. I5 Also shown in Fig. 1 are the results of modeling calculations. The significance of these are discussed in a later section. Fuel Rich Injection In contrast to the overall lean injection results, very high NO reduction efficiencies were found if the reducing agents were added to the rich zone of a staged combustion configuration. These data provide support for a hybrid NOx control concept combining combustion modifications with the injection of selective reducing agents. The combustion modifications act to both reduce the amount of NO formed, and to provide a favorable environment for the reducing agents. The staged approach is attractive because it remains efficient, even on low initial NO concentrations. The effectiveness of this new NO control concept is strongly influenced by the temperature and stoichiometry at the point of selective agent injection, the temperature at which air is added to burn out any remaining fuel, and the amount of this burnout air. Detailed experimental and theoretical studies were conducted to evaluate each parameter. 6 |