For SCR (selective catalytic reduction) applications both aqueous ammonia and urea reagents are used for NOx reducing agents in exhaust systems. For both diesel engines and small boilers, the reagent injection systems often consist of a few, and in some cases, a single injector, located on the wall of the exhaust pipe or duct. Often numerical modeling is performed to determine the location and orientation of the injectors and to predict the NRMS (normalized root mean square) of the gas phase reducing specie distribution prior to the catalyst. Aqueous ammonia and aqueous urea have significantly different processes from the point of injection to the formation of the gas phase reducing species. Evaporation characteristics are important, but for urea, the molecule must also decompose into ammonia and isocyanic acid. For modeling purposes, a simplifying assumption is frequently made to treat the liquid reagent as liquid water and assume that the evaporation of liquid water satisfactorily emulates the processes of forming the gas phase reducing species for both aqueous ammonia and aqueous urea reagents. In reality, the reagent droplets are binary component mixtures and treating the droplets as a single component, namely water, may significantly depart from reality. Additionally, the evaporation processes for aqueous ammonia and aqueous urea have significantly different behaviors. This paper addresses the potential errors associated with using a single component water drop for emulating the evaporation of aqueous ammonia and aqueous urea. This is accomplished by analyzing binary component evaporation for both aqueous ammonia and aqueous urea. Additionally, the time for the gas phase chemical decomposition of urea into ammonia and isocyanic acid is evaluated for various conditions. Typical decomposition times are compared to droplet evaporation times. Finally, the paper attempts to provide guidelines for determining when treating the drops as a single component may be sufficiently accurate, and when the complexity of modeling binary component evaporation is necessary.

This content is only available via PDF.
You do not currently have access to this content.