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Show Prior literature1 surveyed the above combined technologies listing benefits and potential drawbacks of combining the technologies. It primarily reported from a technological feasibility viewpoint where a specific requirement for S C R is presumed. It is important to view the potential application of hybridized S N C R / S C R from an economic standpoint, particularly in the case where combustion modifications have already been employed. Besides assuming several physical configurations, hybrid SNCR/SCR can be operated in different ways. A m o n g the many considerations for the choice of designated hybrid operation are: • What is the desired level of NOx reduction? • What are the N H 3 slip and S 0 2 oxidation constraints? • What volume catalyst can fit in the existing ductwork where face velocity will be within catalyst manufacturer specifications? • What level of additional pressure drop is tolerable by the present fan? • Are N O x reduction requirements incremental? • What structural steel/ductwork changes must be made to support the catalyst? • What is the expected/guaranteed life of the catalyst? • What deviation from ideal reductant distribution is tolerable for the N O x limit? It is obvious that total capital requirement for the catalyst retrofit will increase as the catalyst size and retrofit complexity increase. The key to minimizing life cycle N O x reduction costs is to find the appropriate balance between annualized capital charges and operating costs for the remaining life of the system. The challenge for S C R retrofit is to minimize the capital requirement. The challenge for S N C R use is minimization of reagent required. Designing hybrid S N C R / S C R systems suggests optimization of these costs over the life cycle for a specific level of N O x reduction . Chemical Utilization In post-combustion NOx control processes, NOx reduction is achieved at a given Normalized Stoichiometric Ratio, or NSR. Simply put, N S R refers to the ratio of chemical reductant applied to the amount of N O x existing in the flue gas. With SCR, ammonia is typically the reductant and is typically applied at an N S R of one for deep reductions. In other words, one mole of N H 3 is applied per mole of N O x . If only a 7 5 % N O x reduction was required, the N H 3 N S R would be approximately 0.75. In non-catalytic systems, the reductant is applied in broader ranges of N S R because of relatively lower N O x reduction efficiency compared to catalytic systems. In commercial practice, the N S R ranges from 0.6-2.0. W h e n urea is used for S N C R systems, an N S R of 1.0 means 0.5 mole urea is applied for 1.0 mole N O x because urea has two nitrogen moieties for reaction with N O x. Chemical utilization is a quantification of NOx reduction efficiency expressed by: Utilization = NOx Reduction [%] / NSR In other words, if each lb-mole of injected urea or ammonia reduces NOx to the theoretical maximum amount2, utilization is 100%. One hundred percent chemical utilization is approached in S C R systems, but in S N C R system values range from 30-60%. In commercial post-combustion N O x control systems, maximizing utilization, all other things being equal, minimizes life cycle operating costs. Page 3 |