The implementation of stringent nitrogen oxides (NOx) emissions reduction legislation in Europe and North America is driving the introduction of new exhaust aftertreatment systems, including those that treat NOx under the high-oxygen conditions typical of lean-burn engines.
One increasingly common solution, referred to as Selective Catalytic Reduction (SCR), comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). It is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea (CO(NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue® in Europe, and DEF - Diesel Exhaust Fluid - in the USA. The urea solution is injected into the exhaust and transformed to NH3 by various mechanisms for the SCR reactions.
Urea injection systems using AUS-32 are now in production and becoming a widespread mature technology on many on-road automotive and off-road vehicle applications. However, during the development phase of these systems, several drawbacks to the use of AUS-32 as the SCR reductant carrier were encountered, including component damage due to corrosion and freezing, as well as the unwanted spread of urea crystals which can be troublesome for vehicle electrical systems.
Aqueous guanidinium formate (GuFo) solutions have been identified as an alternative reductant carrier for SCR systems. In addition to potentially addressing these drawbacks of AUS-32, GuFo also could provide advantages in reductant carrier density and in certain configurations, overall SCR deNOx efficiency.
Testing was conducted to evaluate the compatibility of injection component technologies to guanidinium formate solutions. This paper presents the results of tests investigating corrosion resistance, freezing behavior, and injection spray quality performance with respect to AUS-32. A significant reduction in risk of corrosive attack was documented. Also, the freezing behavior of GuFo was shown to theoretically eliminate the risk of damage to system components due to expansion at phase change. Room temperature spray quality with certain sprays was reduced with GuFo. However, the spray structure improvements associated with flash boiling were documented and the concept of heated tip injection with GuFo solutions was introduced and its initial feasibility was demonstrated.