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Show S N C R / S C R Hybrid Methodology and Theory NOxOUT CASCADE® is a combination of a redesigned SNCR and downstream SCR, hybridized to provide improvements in chemical utilization and overall N O x reduction. The two N O x reduction technologies each provide process strengths which make the hybrid combination more flexible and effective than the sum of its parts. Selective Non-Catalytic Reduction (SNCR) is typically applied in the furnace, where relatively high temperatures serve to initiate the breakdown of urea to form the transient species that lead to effective N O x reduction. The technology is limited to temperatures high enough to insure very low ammonia breakthrough. At very high furnace temperatures, however, performance can be lessened by competing reactions that either consume effective chemical or lead to N O x formation. Modified S N C R takes advantage of a downstream "ammonia sink" by injecting chemical in cooler regions where N O x reduction and chemical utilization improve dramatically. Selective Catalytic Reduction (SCR) is typically performed in much cooler flue gas passes where the oxidation potential of nitrogen species is minimized. The catalytic surface provides sites that permit the ammonia and N O x to react at nearly perfect utilization. The extent of N O x reduction is limited by the local ammonia to N O x ratio, the flue gas temperature, and the size of the catalyst reactor. The catalyst size is limited by the available space, an increase in pressure drop, the oxidation of S 0 2 to S 0 3 , and the cost of the precious metal components. Hybrid SNCR/SCR NOx utilizes lower temperature SNCR injection to provide substantially improved N O x reduction performance while generating somewhat higher ammonia slip. The ammonia slip feeds a small S C R reactor that removes the slip and reduces N O x while limiting the costs associated with a larger catalyst. For example, a C A S C A D E system which achieves 6 5 % overall N O x reduction ( 5 0 % reduction with S N C R and an additional 3 0 % S C R reduction ) requires less than one third the catalyst required for 6 5 % S C R reduction. The smaller catalyst converts proportionally less S 0 2 to S 0 3 and decreases the pressure drop by the same fraction. System Design Hybrid SNCR/SCR NOx reduction systems can be engineered in several forms. Clearly, it is possible to install a commercial S N C R system for furnace reductions of N O x , and install a commercial S C R system downstream of the economizer on the same unit for removal of the remaining N O x , and enjoy deep levels of N O x reduction with the combined system. For the purpose of semantic clarity, one might consider the foregoing system "combined SNCR/SCR" while reserving the "hybrid" description for units which utilize the ammonia slip from the S N C R process as the sole N O x reductant entering the downstream SCR. Hybridized SNCR/SCR can assume several configurations depending upon the level of overall N O x reduction desired and unit configuration. Both factors combined lead to differences in catalyst dimensions and, therefore, catalyst contributions to the total capital requirement. Various configurations for consideration would be S N C R with: • catalytic air heater • "in-duct" SCR-existing duct dimensions • "in-duct" SCR-expanded duct dimensions • reactor-housed S CR • combination of "in-duct" S C R with catalytic air heater Page 2 |