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Show Figure 1 schematically depicts the enabling effect of downstream catalyst (down-sized or otherwise) on S N C R performance in a hybrid system. S N C R N O x reduction occurs in a defined temperature window, roughly bell-shaped with maximum S N C R N O x reduction occurring at the top, or plateau of the bell. In a commercial "stand-alone" S N C R system, performance is optimized while operating at the "right side of the slope" in the temperature window curve3. In this region, the hot side of the performance maximum, ammonia slip is very low or non-existent. This is often an operating constraint imposed by the source owner. In contrast, the S N C R component of the hybrid system operates best at the plateau, which is lower temperature. In this region, S N C R N O x reduction is higher and some ammonia slip is produced. The ammonia slip is available to reduce N O x in a catalyst system downstream. When operated in this manner, S N C R N O x reduction is maximized (compared to its stand-alone performance) and additional N O x reduction occurs in the catalyst portion, fueled by the generated ammonia slip. Hybrid systems can be designed to operate in the cooler zone (the "left side of the slope") which will produce more ammonia slip than the other regions. In this scenario, S N C R N O x reduction is less than maximal and S C R N O x reduction increases until limited by catalyst space velocity. Overall system N O x reductions beyond 7 5 % would typically require this type of operation and require catalyst reactor dimensions that would not be possible to fit in existing duct space. Hybrid systems can be designed to maximize SNCR performance while "existing duct" S C R controls the ammonia slip. Reagent utilization for N O x reduction can increase dramatically compared to stand-alone S N C R because of the reasons stated above. Therefore, reagent cost per unit of N O x reduced is lower with the hybrid system than with stand-alone S N C R . Current operators of S N C R systems consider these questions in the design stage for prospective hybrid systems: • What is the expected additional reduction of NOx for a constant urea (reagent) flow? • What is the expected reagent flow reduction for constant N O x reduction? Field Testing The NOxOUT SNCR/SCR Hybrid process was tested at Public Service Electric and Gas, Mercer Station4. The unit, which had an existing S N C R system, was partially retrofitted with an expanded duct catalyst as part of a study of SCR, combined SNCR-SCR, and Hybrid SNCR/SCR. In this preliminary work it was shown that deeper than design reductions in N O x were possible through modification of the S N C R system with less than design chemical (urea) flow rates. This was achieved by decreasing the effective chemical release temperature in the furnace. The by-product of this temperature shift, excessive ammonia slip, was utilized in the SCR reactor where further N O x reduction was achieved and ammonia slip levels were reduced to within acceptable limits. Although the S C R reactor was large enough to provide greater than 8 5 % N O x reduction on its own, it was shown that ammonia and N O x distributions to the catalyst were sufficiently uniform to allow for a substantial reduction in catalyst volume without adversely affecting the process. The next logical step in the development of SNCR/SCR hybridization is full-scale application to a utility boiler with a small catalyst used primarily for ammonia slip control. Page 4 |