OCR Text |
Show Chlorine and the resultant hydrogen chloride remain active components in the gas cooling stage of the plant. Thermodynamic calculations were run to examine the possibility of reaction between NOx and HCI at the intermediate temperatures (2200 to 450 OF) in the spray dryer vessel. The calculated effect does not appear to be significant except at very high levels of both HCI and NOx. Chlorine in the waste blend may reduce NOx formation in the combustion section but the resulting molecular chlorine and hydrogen chloride also affects post-combustion NOx reduction processes. For example the hydrogen chloride concentration needs to be considered in the design of gas-phase NOx reduction processes such as the Exxon Technologies Thermal de-NOx. (4) Operating Techniques to Control NOx When periods of high NOx emissions were observed during startup, the immediate goal was to develop operating procedures before the trial burn to maximize throughput while staying within all the permit limits. The operators were given training on the relationship of excess air to NOx formation. With a given feed and other burner settings, they were able to achieve about a 20 ppm reduction in NOx concentration by reducing excess air. However, the permit requires automatic waste feed cutoff at 3% oxygen and batch feeding of solids causes wide oxygen swings so there is a practical limit to using this technique. The operators were also asked to adjust the flow of aqueous waste to see if this would affect NOx formation. However, the aqueous waste spray nozzle is four feet from the liquid burner and the aqueous spray does not mix with the hottest part of the flame. Alternative ways of injecting the aqueous waste to more effectively cool the liquid burner flame have been proposed but have not been tested. Control of NO x emissions through control offuel NOx has been discussed but is of limited applicability in a commercial situation. Hazardous waste incinerators do not select their "fuels" but rather are operated to process the material presented by the customers. Also, measuring fuel nitrogen is difficult when a large portion of the feed consists of non-homogeneous solids, the conversion efficiency decreases with increasing nitrogen content, and the conversion efficiency of total nitrogen depends on the chemical form of the nitrogen (6). Conclusion The widely varying combustion conditions in a hazardous waste incinerator resulting from batch feeding of non-homogenous waste streams masks the effect of individual variables and makes correlation of NO x emissions difficult. During the trial burn the plant met the NOx limits while burning the highly chlorinated waste streams called for by the original test plan. Since the trial burn the operators have continued to meet NOx limits. Operating techniques include avoiding known problem conditions such as high firing rate with a narrow angle nozzle while on fuel oil. At times a particular waste |