The method of storing and delivering ammonia to the SCR catalyst was a key topic at the Car Training Institute's (CTI) SCR Systems conference in Karlsruhe on 9-10 May 2007. With the advent of SCR-equipped passenger cars the space available for direct or indirect ammonia storage has become a critical issue. Proposed alternatives to AdBlue, the current industry standard for delivering ammonia, include a solid urea system and a solid ammonia storage material. The use of urea is classed as indirect ammonia storage because the chemical must be undergo a complex hydrolysis reaction to form ammonia upstream of the catalyst.

Prof. Dr Werner Mueller presented the use of solid urea as a space saving alternative to AdBlue. The mass of ammonia contained in solid urea is three times greater than in AdBlue, which is two-thirds water and therefore also affected by low temperatures. Disadvantages of the solid urea system include sensitivity of solud urea to moisture and the energy requirements of the thermal hydrolysis catalyst. This technology route is being developed by a consortium of OEMs and aftertreatment specialists including Audi, BMW, Bosch, Daimler, Emitec, Ford, MAN, Yara, Opel, Pierburg, Sudchemie, Umicor and VW.

Dr Tue Johannessen, of Amminex, delivered a presentation on the use of a metal amine ammonia storage system, with ammonia stored in magnesium chloride crystals. The storage of ammonia as a metal amine renders it safe to handle and the system has a higher ammonia capacity per unit volume and weight than AdBlue. The resulting chemical can be formed into high density tablets that undergo a process of thermal desorption to release ammonia gas directly into the exhaust system. The tablets are not damaged in this process and may be recharged with ammonia.

The Amminex metal amine system can store up to three times the volume of ammonia than the equivalent volume of AdBlue, and the tablet is unaffected by sub-zero temperatures. Dr Johannessen said that Amminex was unlikely to challenge the established European AdBlue infrastructure for heavy-duty SCR, and was instead looking towards passenger car applications, especially in the USA.